Arthroplasty/Joint Replacement

In the United States, surgeons perform an estimated 200,000 anterior cruciate ligament reconstructions (ACLRs) each year. Over the past decade, there has been a surge in interest in defining anterior cruciate ligament (ACL) anatomy to guide ACLR. With this renewed interest in the anatomical features of the ACL, particularly the insertion site, many authors have advocated an approach for complete or near-complete “footprint restoration” for anatomical ACLR.^1,2 Some have recommended a double-bundle (DB) technique that completely “fills” the footprint, but it is seldom used. Others have proposed centralizing the femoral tunnel position within the ACL footprint in the hope of capturing the function of both the anteromedial (AM) and posterolateral (PL) bundles.^1,3,4 Indeed, a primary surgical goal of most anatomical ACLR techniques is creation of a femoral tunnel based off the anatomical centrum (center point) of the ACL femoral footprint.^3,5 With a single-bundle technique, the femoral socket is localized in the center of the entire footprint; with a DB technique, sockets are created in the centrums of both the AM and PL bundles.

Because of the complex shape of the native ACL, however, the strategy of restoring the femoral footprint with use of either a central tunnel or a DB approach has been challenged. The femoral footprint is 3.5 times larger than the midsubstance of the ACL.⁶ Detailed anatomical dissections have recently demonstrated that the femoral origin of the ACL has a stout anterior band of fibers with a fanlike extension posteriorly.⁷ As the ACL fibers extend off the bony footprint, they form a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick.^2,8 Within this structure, there is no clear separation of the AM and PL bundles. The presence of this structure makes sense given the anatomical constraints inherent in the notch. Indeed, the space for the native ACL is narrow, as the posterior cruciate ligament (PCL) occupies that largest portion of the notch with the knee in full extension, leaving only a thin, 5-mm slot through which the ACL must pass.⁹ Therefore, filling the femoral footprint with a tubular ACL graft probably does not reproduce the dynamic 3-dimensional morphology of the ACL.

In light of the discrepancy between the sizes of the femoral footprint and the midsubstance of the native ACL, it seems reasonable that optimizing the position of the ACL femoral tunnel may be more complex than simply centralizing the tunnel within the footprint or attempting to maximize footprint coverage. In this article, we amalgamate the lessons of 4 decades of ACL research into 5 points for strategic femoral tunnel positioning, based on anatomical, histologic, isometric, biomechanical, and clinical data. These points are summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension.

1. Anatomy Considerations

In response to study results demonstrating that some transtibial ACLRs were associated with nonanatomical placement of the femoral tunnel—resulting in vertical graft placement, PCL impingement, and recurrent rotational instability^10-16—investigators have reexamined both the anatomy of the femoral origin of the native ACL and the ACL graft. Specifically, a large body of research has been devoted to characterizing the osseous landmarks of the femoral origin of the ACL¹⁷ and the dimensions of the femoral footprint.³ In addition, authors have supported the concept that the ACL contains 2 functional bundles, AM and PL.^5,17 Several osseous landmarks have been identified as defining the boundaries of the femoral footprint. The lateral intercondylar ridge is the most anterior aspect of the femoral footprint and was first defined by Clancy.¹⁸ More recently, the lateral bifurcate ridge, which separates the AM and PL bundle insertion sites, was described¹⁹ (Figure 1A).

These osseous ridges delineate the location of the femoral footprint. Studies have shown that ACL fibers attach from the lateral intercondylar ridge on the anterior border of the femoral footprint and extend posteriorly to the cartilage of the lateral femoral condyle (Figure 1B).

ACL fibers from this oblong footprint are organized such that the midsubstance of the ACL is narrower than the femoral footprint. Anatomical dissections have demonstrated that, though the femoral footprint is oval, the native ACL forms a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick as it takes off from the bone.^8,20 There is a resulting discrepancy between the femoral footprint size and shape and the morphology of the native ACL, and placing a tunnel in the center of the footprint or “filling the footprint” with ACL graft may not reproduce the morphology or function of the native ACL. Given this size mismatch, strategic decisions need to be made to place the femoral tunnel in a specific region of the femoral footprint to optimize its function.

2. Histologic Findings

Histologic analysis has further clarified the relationship between the femoral footprint and functional aspects of the native ACL. The femoral origin of the ACL has distinct direct and indirect insertions, as demonstrated by histology and 3-dimensional volume-rendered computed tomography.²¹ The direct insertion consists of dense collagen fibers anterior in the footprint that is attached to a bony depression immediately posterior to the lateral intercondylar ridge.¹⁹ Sasaki and colleagues²² found that these direct fibers extended a mean (SD) of 5.3 (1.1) mm posteriorly but did not continue to the posterior femoral articular cartilage. The indirect insertion consists of more posterior collagen fibers that extend to and blend into the articular cartilage of the posterior aspect of the lateral femoral condyle. Mean (SD) width of this membrane-like tissue, located between the direct insertion and the posterior femoral articular cartilage, was found by Sasaki and colleagues²²  to be 4.4 (0.5) mm anteroposteriorly(Figure 2). This anterior band of ACL tissue with the direct insertion histologically corresponds to the fibers in the anterior, more isometric region of the femoral footprint. Conversely, the more posterior band of fibers with its indirect insertion histologically corresponds to the more anisometric region and is seen macroscopically as a fanlike projection extending to the posterior articular cartilage.⁷

The dense collagen fibers of the direct insertion and the more membrane-like indirect insertion regions of the femoral footprint of the native ACL suggest that these regions have different load-sharing characteristics. The direct fibers of the insertion form a firm, fixed attachment that allows for gradual load distribution into the subchondral bone. From a biomechanical point of view, this attachment is extremely important, a key ligament–bone link transmitting mechanical load to the joint.²³ A recent kinematic analysis revealed that the indirect fibers in the posterior region of the footprint, adjacent to the posterior articular cartilage, contribute minimally to restraint of tibial translation and rotations during stability examination.²⁴ This suggests it may be strategically wise to place a tunnel in the direct insertion region of the footprint—eccentrically anterior (high) in the footprint rather than in the centrum.

3. Isometric Considerations

Forty years ago, Artmann and Wirth²⁵ reported that a nearly isometric region existed in the femur such that there is minimal elongation of the native ACL during knee motion. The biomechanical rationale for choosing an isometric region of an ACL graft is that it will maintain function throughout the range of flexion and extension. A nonisometric graft would be expected to slacken during a large portion of the flexion cycle and not restrain anterior translation of the tibia, or, if fixed at the wrong flexion angle, it could capture the knee and cause graft failure by excessive tension. These 2 theoretical undesirable effects from nonisometric graft placement are supported by many experimental and clinical studies demonstrating that nonisometric femoral tunnel placement at time of surgery can cause recurrent anterior laxity of the knee.^26-28 Multiple studies have further clarified that the isometric characteristics of an ACL graft are largely determined by femoral positioning. The most isometric region of the femoral footprint is consistently shown to be localized eccentrically within the footprint, in a relatively narrow bandlike region that is proximal (deep) and anterior (along the lateral intercondylar ridge within the footprint)^19,29,30 (Figure 3).

A large body of literature has demonstrated that a tunnel placed in the center of the femoral footprint is less isometric than a tunnel in the more anterior region.^25,29,31,32 Indeed, the anterior position (high in the footprint) identified by Hefzy and colleagues²⁹ demonstrated minimal anisometry with 1 to 4 mm of length change through the range of motion. In contrast, a central tunnel would be expected to demonstrate 5 to 7 mm of length change, whereas a lower graft (in the PL region of the footprint) would demonstrate about 1 cm of length change through the range of motion.^31,32 As such, central grafts, or grafts placed in the PL portion of the femoral footprint, would be expected to see high tension or graft forces as the knee is flexed, or to lose tension completely if the graft is fixed at full extension.³²

Importantly, Markolf and colleagues³³ reported that the native ACL does not behave exactly in a so-called isometric fashion during the last 30° of extension. They showed that about 3 mm of retraction of a trial wire into the joint during the last 30° of extension (as measured with an isometer) is reasonable to achieve graft length changes approximating those of the intact ACL. Given this important caveat, a primary goal for ACLR is placement of the femoral tunnel within this isometric region so that the length change in the ACL graft is minimized to 3 mm from 30° to full flexion. In addition, results of a time-zero biomechanical study suggested better rotational control with anatomical femoral tunnel position than with an isometric femoral tunnel³⁴ placed outside the femoral footprint. Therefore, maximizing isometry alone is not the goal; placing the graft in the most isometric region within the anatomical femoral footprint is desired. This isometric region in the footprint is in the histologic region that corresponds to the direct fibers. Again, this region is eccentrically located in the anterior (high) and proximal (deep) portion of the footprint.

4. Biomechanical Considerations

Multiple cadaveric studies have investigated the relationship between femoral tunnel positioning and time-zero stability. These studies often demonstrated superior time-zero control of knee stability, particularly in pivot type maneuvers, with a femoral tunnel placed more centrally in the femoral footprint than with a tunnel placed outside the footprint.^34-37 However, an emerging body of literature is finding no significant difference in time-zero stability between an anteriorly placed femoral tunnel within the anatomical footprint (eccentrically located in the footprint) and a centrally placed graft.^38,39 Returning to the more isometric tunnel position, still within the femoral footprint, would be expected to confer the benefits of an anatomically based graft position with the advantageous profile of improved isometry, as compared with a centrally placed or PL graft. Biomechanical studies⁴⁰ have documented that ACL graft fibers placed posteriorly (low) in the footprint cause high graft forces in extension and, in some cases, graft rupture (Figure 4). Accordingly, the importance of reconstructing the posterior region of the footprint to better control time-zero stability is questioned.⁴¹

In addition to time-zero control of the stability examination, restoring the low tension-flexion pattern in the ACL graft to replicate the tension-flexion behavior of the native ACL is a fundamental biomechanical principle of ACLR.^15,33,42,43 These studies have demonstrated that a femoral tunnel localized anterior (high) and proximal (deep) within the footprint better replicates the tension-flexion behavior of the native ACL, as compared with strategies that attempt to anatomically “fill the footprint.”⁴⁰ Together, these studies have demonstrated that an eccentric position in the footprint, in the anterior (high) and proximal (deep) region, not only maximizes isometry and restores the direct fibers, but provides favorable time-zero stability and a low tension-flexion pattern biomechanically, particularly as compared with a tunnel in the more central or posterior region of the footprint.

5. Clinical Data

Clinical studies of the traditional transtibial ACLR have shown good results.^44,45 However, when the tibial tunnel in the coronal plane was drilled vertical with respect to the medial joint line of the tibia, the transtibially placed femoral tunnel migrated anterior to the anatomical femoral footprint, often on the roof of the notch.^10,14 This nonanatomical, vertical placement of the femoral tunnel led to failed normalization of knee kinematics.^46-50 Indeed, a higher tension-flexion pattern was found in this nonanatomical “roof” position for the femoral tunnel as compared with the native ACL—a pattern that can result in either loss of flexion or recurrent instability.^13,15,51

Clinical results of techniques used to create an anatomical ACLR centrally within the footprint have been mixed. Registry data showed that the revision rate at 4 years was higher with the AM portal technique (5.16%) than with transtibial drilling (3.20%).⁵² This higher rate may be associated with the more central placement of the femoral tunnel with the AM portal technique than with the transtibial technique, as shown in vivo with high-resolution magnetic resonance imaging.¹² Recent reports have documented a higher rate of failure with DB or central ACLR approaches than with traditional transtibial techniques.⁵³ As mentioned, in contrast to a more isometric position, a central femoral tunnel position would be expected to demonstrate 5 to 7 mm of length change, whereas moving the graft more posterior in the footprint (closer to the articular cartilage) would result in more than 1 cm of length change through the range of motion.^31,32 As such, these more central grafts, or grafts placed even lower (more posterior) in the footprint, would be expected to see high tension in extension (if fixed in flexion), or to lose tension completely during flexion (if the graft is fixed at full extension).³² This may be a mechanistic cause of the high failure rate in the more posterior bundles of the DB approach.⁵⁴

Together, these clinical data suggest that the femoral tunnel should be placed within the anatomical footprint of the ACL. However, within the footprint, a more eccentric femoral tunnel position capturing the isometric and direct region of the insertion may be preferable to a more central or posterior (low region) position.

Summary

Anatomical, histologic, isometric, biomechanical, and clinical data from more than 4 decades collectively point to an optimal position for the femoral tunnel within the femoral footprint. This position can be summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension (Figure 5).

In vivo and in vitro studies as well as surgical experience suggest a need to avoid both (a) the nonanatomical vertical (roof) femoral tunnel placement that causes PCL impingement, high tension in the ACL graft in flexion, and ultimately graft stretch-out with instability and (b) the femoral tunnel placement in the posterior (lowest) region of the footprint that causes high tension in extension and can result in graft stretch-out with instability.^13,15,39,40 The transtibial and AM portal techniques can both be effective in properly placing the femoral tunnel and restoring motion, stability, and function to the knee. Their effectiveness, however, depends on correct placement of the femoral tunnel. We think coming studies will focus on single-bundle ACLR and will be designed to improve the reliability of the transtibial and AM portal techniques for placing a femoral tunnel in keeping with the principles summarized by the I.D.E.A.L. acronym.

In the United States, surgeons perform an estimated 200,000 anterior cruciate ligament reconstructions (ACLRs) each year. Over the past decade, there has been a surge in interest in defining anterior cruciate ligament (ACL) anatomy to guide ACLR. With this renewed interest in the anatomical features of the ACL, particularly the insertion site, many authors have advocated an approach for complete or near-complete “footprint restoration” for anatomical ACLR.^1,2 Some have recommended a double-bundle (DB) technique that completely “fills” the footprint, but it is seldom used. Others have proposed centralizing the femoral tunnel position within the ACL footprint in the hope of capturing the function of both the anteromedial (AM) and posterolateral (PL) bundles.^1,3,4 Indeed, a primary surgical goal of most anatomical ACLR techniques is creation of a femoral tunnel based off the anatomical centrum (center point) of the ACL femoral footprint.^3,5 With a single-bundle technique, the femoral socket is localized in the center of the entire footprint; with a DB technique, sockets are created in the centrums of both the AM and PL bundles.

Because of the complex shape of the native ACL, however, the strategy of restoring the femoral footprint with use of either a central tunnel or a DB approach has been challenged. The femoral footprint is 3.5 times larger than the midsubstance of the ACL.⁶ Detailed anatomical dissections have recently demonstrated that the femoral origin of the ACL has a stout anterior band of fibers with a fanlike extension posteriorly.⁷ As the ACL fibers extend off the bony footprint, they form a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick.^2,8 Within this structure, there is no clear separation of the AM and PL bundles. The presence of this structure makes sense given the anatomical constraints inherent in the notch. Indeed, the space for the native ACL is narrow, as the posterior cruciate ligament (PCL) occupies that largest portion of the notch with the knee in full extension, leaving only a thin, 5-mm slot through which the ACL must pass.⁹ Therefore, filling the femoral footprint with a tubular ACL graft probably does not reproduce the dynamic 3-dimensional morphology of the ACL.

In light of the discrepancy between the sizes of the femoral footprint and the midsubstance of the native ACL, it seems reasonable that optimizing the position of the ACL femoral tunnel may be more complex than simply centralizing the tunnel within the footprint or attempting to maximize footprint coverage. In this article, we amalgamate the lessons of 4 decades of ACL research into 5 points for strategic femoral tunnel positioning, based on anatomical, histologic, isometric, biomechanical, and clinical data. These points are summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension.

1. Anatomy Considerations

In response to study results demonstrating that some transtibial ACLRs were associated with nonanatomical placement of the femoral tunnel—resulting in vertical graft placement, PCL impingement, and recurrent rotational instability^10-16—investigators have reexamined both the anatomy of the femoral origin of the native ACL and the ACL graft. Specifically, a large body of research has been devoted to characterizing the osseous landmarks of the femoral origin of the ACL¹⁷ and the dimensions of the femoral footprint.³ In addition, authors have supported the concept that the ACL contains 2 functional bundles, AM and PL.^5,17 Several osseous landmarks have been identified as defining the boundaries of the femoral footprint. The lateral intercondylar ridge is the most anterior aspect of the femoral footprint and was first defined by Clancy.¹⁸ More recently, the lateral bifurcate ridge, which separates the AM and PL bundle insertion sites, was described¹⁹ (Figure 1A).

These osseous ridges delineate the location of the femoral footprint. Studies have shown that ACL fibers attach from the lateral intercondylar ridge on the anterior border of the femoral footprint and extend posteriorly to the cartilage of the lateral femoral condyle (Figure 1B).

ACL fibers from this oblong footprint are organized such that the midsubstance of the ACL is narrower than the femoral footprint. Anatomical dissections have demonstrated that, though the femoral footprint is oval, the native ACL forms a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick as it takes off from the bone.^8,20 There is a resulting discrepancy between the femoral footprint size and shape and the morphology of the native ACL, and placing a tunnel in the center of the footprint or “filling the footprint” with ACL graft may not reproduce the morphology or function of the native ACL. Given this size mismatch, strategic decisions need to be made to place the femoral tunnel in a specific region of the femoral footprint to optimize its function.

2. Histologic Findings

Histologic analysis has further clarified the relationship between the femoral footprint and functional aspects of the native ACL. The femoral origin of the ACL has distinct direct and indirect insertions, as demonstrated by histology and 3-dimensional volume-rendered computed tomography.²¹ The direct insertion consists of dense collagen fibers anterior in the footprint that is attached to a bony depression immediately posterior to the lateral intercondylar ridge.¹⁹ Sasaki and colleagues²² found that these direct fibers extended a mean (SD) of 5.3 (1.1) mm posteriorly but did not continue to the posterior femoral articular cartilage. The indirect insertion consists of more posterior collagen fibers that extend to and blend into the articular cartilage of the posterior aspect of the lateral femoral condyle. Mean (SD) width of this membrane-like tissue, located between the direct insertion and the posterior femoral articular cartilage, was found by Sasaki and colleagues²²  to be 4.4 (0.5) mm anteroposteriorly(Figure 2). This anterior band of ACL tissue with the direct insertion histologically corresponds to the fibers in the anterior, more isometric region of the femoral footprint. Conversely, the more posterior band of fibers with its indirect insertion histologically corresponds to the more anisometric region and is seen macroscopically as a fanlike projection extending to the posterior articular cartilage.⁷

The dense collagen fibers of the direct insertion and the more membrane-like indirect insertion regions of the femoral footprint of the native ACL suggest that these regions have different load-sharing characteristics. The direct fibers of the insertion form a firm, fixed attachment that allows for gradual load distribution into the subchondral bone. From a biomechanical point of view, this attachment is extremely important, a key ligament–bone link transmitting mechanical load to the joint.²³ A recent kinematic analysis revealed that the indirect fibers in the posterior region of the footprint, adjacent to the posterior articular cartilage, contribute minimally to restraint of tibial translation and rotations during stability examination.²⁴ This suggests it may be strategically wise to place a tunnel in the direct insertion region of the footprint—eccentrically anterior (high) in the footprint rather than in the centrum.

3. Isometric Considerations

Forty years ago, Artmann and Wirth²⁵ reported that a nearly isometric region existed in the femur such that there is minimal elongation of the native ACL during knee motion. The biomechanical rationale for choosing an isometric region of an ACL graft is that it will maintain function throughout the range of flexion and extension. A nonisometric graft would be expected to slacken during a large portion of the flexion cycle and not restrain anterior translation of the tibia, or, if fixed at the wrong flexion angle, it could capture the knee and cause graft failure by excessive tension. These 2 theoretical undesirable effects from nonisometric graft placement are supported by many experimental and clinical studies demonstrating that nonisometric femoral tunnel placement at time of surgery can cause recurrent anterior laxity of the knee.^26-28 Multiple studies have further clarified that the isometric characteristics of an ACL graft are largely determined by femoral positioning. The most isometric region of the femoral footprint is consistently shown to be localized eccentrically within the footprint, in a relatively narrow bandlike region that is proximal (deep) and anterior (along the lateral intercondylar ridge within the footprint)^19,29,30 (Figure 3).

A large body of literature has demonstrated that a tunnel placed in the center of the femoral footprint is less isometric than a tunnel in the more anterior region.^25,29,31,32 Indeed, the anterior position (high in the footprint) identified by Hefzy and colleagues²⁹ demonstrated minimal anisometry with 1 to 4 mm of length change through the range of motion. In contrast, a central tunnel would be expected to demonstrate 5 to 7 mm of length change, whereas a lower graft (in the PL region of the footprint) would demonstrate about 1 cm of length change through the range of motion.^31,32 As such, central grafts, or grafts placed in the PL portion of the femoral footprint, would be expected to see high tension or graft forces as the knee is flexed, or to lose tension completely if the graft is fixed at full extension.³²

Importantly, Markolf and colleagues³³ reported that the native ACL does not behave exactly in a so-called isometric fashion during the last 30° of extension. They showed that about 3 mm of retraction of a trial wire into the joint during the last 30° of extension (as measured with an isometer) is reasonable to achieve graft length changes approximating those of the intact ACL. Given this important caveat, a primary goal for ACLR is placement of the femoral tunnel within this isometric region so that the length change in the ACL graft is minimized to 3 mm from 30° to full flexion. In addition, results of a time-zero biomechanical study suggested better rotational control with anatomical femoral tunnel position than with an isometric femoral tunnel³⁴ placed outside the femoral footprint. Therefore, maximizing isometry alone is not the goal; placing the graft in the most isometric region within the anatomical femoral footprint is desired. This isometric region in the footprint is in the histologic region that corresponds to the direct fibers. Again, this region is eccentrically located in the anterior (high) and proximal (deep) portion of the footprint.

4. Biomechanical Considerations

Multiple cadaveric studies have investigated the relationship between femoral tunnel positioning and time-zero stability. These studies often demonstrated superior time-zero control of knee stability, particularly in pivot type maneuvers, with a femoral tunnel placed more centrally in the femoral footprint than with a tunnel placed outside the footprint.^34-37 However, an emerging body of literature is finding no significant difference in time-zero stability between an anteriorly placed femoral tunnel within the anatomical footprint (eccentrically located in the footprint) and a centrally placed graft.^38,39 Returning to the more isometric tunnel position, still within the femoral footprint, would be expected to confer the benefits of an anatomically based graft position with the advantageous profile of improved isometry, as compared with a centrally placed or PL graft. Biomechanical studies⁴⁰ have documented that ACL graft fibers placed posteriorly (low) in the footprint cause high graft forces in extension and, in some cases, graft rupture (Figure 4). Accordingly, the importance of reconstructing the posterior region of the footprint to better control time-zero stability is questioned.⁴¹

In addition to time-zero control of the stability examination, restoring the low tension-flexion pattern in the ACL graft to replicate the tension-flexion behavior of the native ACL is a fundamental biomechanical principle of ACLR.^15,33,42,43 These studies have demonstrated that a femoral tunnel localized anterior (high) and proximal (deep) within the footprint better replicates the tension-flexion behavior of the native ACL, as compared with strategies that attempt to anatomically “fill the footprint.”⁴⁰ Together, these studies have demonstrated that an eccentric position in the footprint, in the anterior (high) and proximal (deep) region, not only maximizes isometry and restores the direct fibers, but provides favorable time-zero stability and a low tension-flexion pattern biomechanically, particularly as compared with a tunnel in the more central or posterior region of the footprint.

5. Clinical Data

Clinical studies of the traditional transtibial ACLR have shown good results.^44,45 However, when the tibial tunnel in the coronal plane was drilled vertical with respect to the medial joint line of the tibia, the transtibially placed femoral tunnel migrated anterior to the anatomical femoral footprint, often on the roof of the notch.^10,14 This nonanatomical, vertical placement of the femoral tunnel led to failed normalization of knee kinematics.^46-50 Indeed, a higher tension-flexion pattern was found in this nonanatomical “roof” position for the femoral tunnel as compared with the native ACL—a pattern that can result in either loss of flexion or recurrent instability.^13,15,51

Clinical results of techniques used to create an anatomical ACLR centrally within the footprint have been mixed. Registry data showed that the revision rate at 4 years was higher with the AM portal technique (5.16%) than with transtibial drilling (3.20%).⁵² This higher rate may be associated with the more central placement of the femoral tunnel with the AM portal technique than with the transtibial technique, as shown in vivo with high-resolution magnetic resonance imaging.¹² Recent reports have documented a higher rate of failure with DB or central ACLR approaches than with traditional transtibial techniques.⁵³ As mentioned, in contrast to a more isometric position, a central femoral tunnel position would be expected to demonstrate 5 to 7 mm of length change, whereas moving the graft more posterior in the footprint (closer to the articular cartilage) would result in more than 1 cm of length change through the range of motion.^31,32 As such, these more central grafts, or grafts placed even lower (more posterior) in the footprint, would be expected to see high tension in extension (if fixed in flexion), or to lose tension completely during flexion (if the graft is fixed at full extension).³² This may be a mechanistic cause of the high failure rate in the more posterior bundles of the DB approach.⁵⁴

Together, these clinical data suggest that the femoral tunnel should be placed within the anatomical footprint of the ACL. However, within the footprint, a more eccentric femoral tunnel position capturing the isometric and direct region of the insertion may be preferable to a more central or posterior (low region) position.

Summary

Anatomical, histologic, isometric, biomechanical, and clinical data from more than 4 decades collectively point to an optimal position for the femoral tunnel within the femoral footprint. This position can be summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension (Figure 5).

In vivo and in vitro studies as well as surgical experience suggest a need to avoid both (a) the nonanatomical vertical (roof) femoral tunnel placement that causes PCL impingement, high tension in the ACL graft in flexion, and ultimately graft stretch-out with instability and (b) the femoral tunnel placement in the posterior (lowest) region of the footprint that causes high tension in extension and can result in graft stretch-out with instability.^13,15,39,40 The transtibial and AM portal techniques can both be effective in properly placing the femoral tunnel and restoring motion, stability, and function to the knee. Their effectiveness, however, depends on correct placement of the femoral tunnel. We think coming studies will focus on single-bundle ACLR and will be designed to improve the reliability of the transtibial and AM portal techniques for placing a femoral tunnel in keeping with the principles summarized by the I.D.E.A.L. acronym.

In the United States, surgeons perform an estimated 200,000 anterior cruciate ligament reconstructions (ACLRs) each year. Over the past decade, there has been a surge in interest in defining anterior cruciate ligament (ACL) anatomy to guide ACLR. With this renewed interest in the anatomical features of the ACL, particularly the insertion site, many authors have advocated an approach for complete or near-complete “footprint restoration” for anatomical ACLR.^1,2 Some have recommended a double-bundle (DB) technique that completely “fills” the footprint, but it is seldom used. Others have proposed centralizing the femoral tunnel position within the ACL footprint in the hope of capturing the function of both the anteromedial (AM) and posterolateral (PL) bundles.^1,3,4 Indeed, a primary surgical goal of most anatomical ACLR techniques is creation of a femoral tunnel based off the anatomical centrum (center point) of the ACL femoral footprint.^3,5 With a single-bundle technique, the femoral socket is localized in the center of the entire footprint; with a DB technique, sockets are created in the centrums of both the AM and PL bundles.

Because of the complex shape of the native ACL, however, the strategy of restoring the femoral footprint with use of either a central tunnel or a DB approach has been challenged. The femoral footprint is 3.5 times larger than the midsubstance of the ACL.⁶ Detailed anatomical dissections have recently demonstrated that the femoral origin of the ACL has a stout anterior band of fibers with a fanlike extension posteriorly.⁷ As the ACL fibers extend off the bony footprint, they form a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick.^2,8 Within this structure, there is no clear separation of the AM and PL bundles. The presence of this structure makes sense given the anatomical constraints inherent in the notch. Indeed, the space for the native ACL is narrow, as the posterior cruciate ligament (PCL) occupies that largest portion of the notch with the knee in full extension, leaving only a thin, 5-mm slot through which the ACL must pass.⁹ Therefore, filling the femoral footprint with a tubular ACL graft probably does not reproduce the dynamic 3-dimensional morphology of the ACL.

In light of the discrepancy between the sizes of the femoral footprint and the midsubstance of the native ACL, it seems reasonable that optimizing the position of the ACL femoral tunnel may be more complex than simply centralizing the tunnel within the footprint or attempting to maximize footprint coverage. In this article, we amalgamate the lessons of 4 decades of ACL research into 5 points for strategic femoral tunnel positioning, based on anatomical, histologic, isometric, biomechanical, and clinical data. These points are summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension.

1. Anatomy Considerations

In response to study results demonstrating that some transtibial ACLRs were associated with nonanatomical placement of the femoral tunnel—resulting in vertical graft placement, PCL impingement, and recurrent rotational instability^10-16—investigators have reexamined both the anatomy of the femoral origin of the native ACL and the ACL graft. Specifically, a large body of research has been devoted to characterizing the osseous landmarks of the femoral origin of the ACL¹⁷ and the dimensions of the femoral footprint.³ In addition, authors have supported the concept that the ACL contains 2 functional bundles, AM and PL.^5,17 Several osseous landmarks have been identified as defining the boundaries of the femoral footprint. The lateral intercondylar ridge is the most anterior aspect of the femoral footprint and was first defined by Clancy.¹⁸ More recently, the lateral bifurcate ridge, which separates the AM and PL bundle insertion sites, was described¹⁹ (Figure 1A).

These osseous ridges delineate the location of the femoral footprint. Studies have shown that ACL fibers attach from the lateral intercondylar ridge on the anterior border of the femoral footprint and extend posteriorly to the cartilage of the lateral femoral condyle (Figure 1B).

ACL fibers from this oblong footprint are organized such that the midsubstance of the ACL is narrower than the femoral footprint. Anatomical dissections have demonstrated that, though the femoral footprint is oval, the native ACL forms a flat, ribbonlike structure 9 to 16 mm wide and only 2 to 4 mm thick as it takes off from the bone.^8,20 There is a resulting discrepancy between the femoral footprint size and shape and the morphology of the native ACL, and placing a tunnel in the center of the footprint or “filling the footprint” with ACL graft may not reproduce the morphology or function of the native ACL. Given this size mismatch, strategic decisions need to be made to place the femoral tunnel in a specific region of the femoral footprint to optimize its function.

2. Histologic Findings

Histologic analysis has further clarified the relationship between the femoral footprint and functional aspects of the native ACL. The femoral origin of the ACL has distinct direct and indirect insertions, as demonstrated by histology and 3-dimensional volume-rendered computed tomography.²¹ The direct insertion consists of dense collagen fibers anterior in the footprint that is attached to a bony depression immediately posterior to the lateral intercondylar ridge.¹⁹ Sasaki and colleagues²² found that these direct fibers extended a mean (SD) of 5.3 (1.1) mm posteriorly but did not continue to the posterior femoral articular cartilage. The indirect insertion consists of more posterior collagen fibers that extend to and blend into the articular cartilage of the posterior aspect of the lateral femoral condyle. Mean (SD) width of this membrane-like tissue, located between the direct insertion and the posterior femoral articular cartilage, was found by Sasaki and colleagues²²  to be 4.4 (0.5) mm anteroposteriorly(Figure 2). This anterior band of ACL tissue with the direct insertion histologically corresponds to the fibers in the anterior, more isometric region of the femoral footprint. Conversely, the more posterior band of fibers with its indirect insertion histologically corresponds to the more anisometric region and is seen macroscopically as a fanlike projection extending to the posterior articular cartilage.⁷

The dense collagen fibers of the direct insertion and the more membrane-like indirect insertion regions of the femoral footprint of the native ACL suggest that these regions have different load-sharing characteristics. The direct fibers of the insertion form a firm, fixed attachment that allows for gradual load distribution into the subchondral bone. From a biomechanical point of view, this attachment is extremely important, a key ligament–bone link transmitting mechanical load to the joint.²³ A recent kinematic analysis revealed that the indirect fibers in the posterior region of the footprint, adjacent to the posterior articular cartilage, contribute minimally to restraint of tibial translation and rotations during stability examination.²⁴ This suggests it may be strategically wise to place a tunnel in the direct insertion region of the footprint—eccentrically anterior (high) in the footprint rather than in the centrum.

3. Isometric Considerations

Forty years ago, Artmann and Wirth²⁵ reported that a nearly isometric region existed in the femur such that there is minimal elongation of the native ACL during knee motion. The biomechanical rationale for choosing an isometric region of an ACL graft is that it will maintain function throughout the range of flexion and extension. A nonisometric graft would be expected to slacken during a large portion of the flexion cycle and not restrain anterior translation of the tibia, or, if fixed at the wrong flexion angle, it could capture the knee and cause graft failure by excessive tension. These 2 theoretical undesirable effects from nonisometric graft placement are supported by many experimental and clinical studies demonstrating that nonisometric femoral tunnel placement at time of surgery can cause recurrent anterior laxity of the knee.^26-28 Multiple studies have further clarified that the isometric characteristics of an ACL graft are largely determined by femoral positioning. The most isometric region of the femoral footprint is consistently shown to be localized eccentrically within the footprint, in a relatively narrow bandlike region that is proximal (deep) and anterior (along the lateral intercondylar ridge within the footprint)^19,29,30 (Figure 3).

A large body of literature has demonstrated that a tunnel placed in the center of the femoral footprint is less isometric than a tunnel in the more anterior region.^25,29,31,32 Indeed, the anterior position (high in the footprint) identified by Hefzy and colleagues²⁹ demonstrated minimal anisometry with 1 to 4 mm of length change through the range of motion. In contrast, a central tunnel would be expected to demonstrate 5 to 7 mm of length change, whereas a lower graft (in the PL region of the footprint) would demonstrate about 1 cm of length change through the range of motion.^31,32 As such, central grafts, or grafts placed in the PL portion of the femoral footprint, would be expected to see high tension or graft forces as the knee is flexed, or to lose tension completely if the graft is fixed at full extension.³²

Importantly, Markolf and colleagues³³ reported that the native ACL does not behave exactly in a so-called isometric fashion during the last 30° of extension. They showed that about 3 mm of retraction of a trial wire into the joint during the last 30° of extension (as measured with an isometer) is reasonable to achieve graft length changes approximating those of the intact ACL. Given this important caveat, a primary goal for ACLR is placement of the femoral tunnel within this isometric region so that the length change in the ACL graft is minimized to 3 mm from 30° to full flexion. In addition, results of a time-zero biomechanical study suggested better rotational control with anatomical femoral tunnel position than with an isometric femoral tunnel³⁴ placed outside the femoral footprint. Therefore, maximizing isometry alone is not the goal; placing the graft in the most isometric region within the anatomical femoral footprint is desired. This isometric region in the footprint is in the histologic region that corresponds to the direct fibers. Again, this region is eccentrically located in the anterior (high) and proximal (deep) portion of the footprint.

4. Biomechanical Considerations

Multiple cadaveric studies have investigated the relationship between femoral tunnel positioning and time-zero stability. These studies often demonstrated superior time-zero control of knee stability, particularly in pivot type maneuvers, with a femoral tunnel placed more centrally in the femoral footprint than with a tunnel placed outside the footprint.^34-37 However, an emerging body of literature is finding no significant difference in time-zero stability between an anteriorly placed femoral tunnel within the anatomical footprint (eccentrically located in the footprint) and a centrally placed graft.^38,39 Returning to the more isometric tunnel position, still within the femoral footprint, would be expected to confer the benefits of an anatomically based graft position with the advantageous profile of improved isometry, as compared with a centrally placed or PL graft. Biomechanical studies⁴⁰ have documented that ACL graft fibers placed posteriorly (low) in the footprint cause high graft forces in extension and, in some cases, graft rupture (Figure 4). Accordingly, the importance of reconstructing the posterior region of the footprint to better control time-zero stability is questioned.⁴¹

In addition to time-zero control of the stability examination, restoring the low tension-flexion pattern in the ACL graft to replicate the tension-flexion behavior of the native ACL is a fundamental biomechanical principle of ACLR.^15,33,42,43 These studies have demonstrated that a femoral tunnel localized anterior (high) and proximal (deep) within the footprint better replicates the tension-flexion behavior of the native ACL, as compared with strategies that attempt to anatomically “fill the footprint.”⁴⁰ Together, these studies have demonstrated that an eccentric position in the footprint, in the anterior (high) and proximal (deep) region, not only maximizes isometry and restores the direct fibers, but provides favorable time-zero stability and a low tension-flexion pattern biomechanically, particularly as compared with a tunnel in the more central or posterior region of the footprint.

5. Clinical Data

Clinical studies of the traditional transtibial ACLR have shown good results.^44,45 However, when the tibial tunnel in the coronal plane was drilled vertical with respect to the medial joint line of the tibia, the transtibially placed femoral tunnel migrated anterior to the anatomical femoral footprint, often on the roof of the notch.^10,14 This nonanatomical, vertical placement of the femoral tunnel led to failed normalization of knee kinematics.^46-50 Indeed, a higher tension-flexion pattern was found in this nonanatomical “roof” position for the femoral tunnel as compared with the native ACL—a pattern that can result in either loss of flexion or recurrent instability.^13,15,51

Clinical results of techniques used to create an anatomical ACLR centrally within the footprint have been mixed. Registry data showed that the revision rate at 4 years was higher with the AM portal technique (5.16%) than with transtibial drilling (3.20%).⁵² This higher rate may be associated with the more central placement of the femoral tunnel with the AM portal technique than with the transtibial technique, as shown in vivo with high-resolution magnetic resonance imaging.¹² Recent reports have documented a higher rate of failure with DB or central ACLR approaches than with traditional transtibial techniques.⁵³ As mentioned, in contrast to a more isometric position, a central femoral tunnel position would be expected to demonstrate 5 to 7 mm of length change, whereas moving the graft more posterior in the footprint (closer to the articular cartilage) would result in more than 1 cm of length change through the range of motion.^31,32 As such, these more central grafts, or grafts placed even lower (more posterior) in the footprint, would be expected to see high tension in extension (if fixed in flexion), or to lose tension completely during flexion (if the graft is fixed at full extension).³² This may be a mechanistic cause of the high failure rate in the more posterior bundles of the DB approach.⁵⁴

Together, these clinical data suggest that the femoral tunnel should be placed within the anatomical footprint of the ACL. However, within the footprint, a more eccentric femoral tunnel position capturing the isometric and direct region of the insertion may be preferable to a more central or posterior (low region) position.

Summary

Anatomical, histologic, isometric, biomechanical, and clinical data from more than 4 decades collectively point to an optimal position for the femoral tunnel within the femoral footprint. This position can be summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension (Figure 5).

In vivo and in vitro studies as well as surgical experience suggest a need to avoid both (a) the nonanatomical vertical (roof) femoral tunnel placement that causes PCL impingement, high tension in the ACL graft in flexion, and ultimately graft stretch-out with instability and (b) the femoral tunnel placement in the posterior (lowest) region of the footprint that causes high tension in extension and can result in graft stretch-out with instability.^13,15,39,40 The transtibial and AM portal techniques can both be effective in properly placing the femoral tunnel and restoring motion, stability, and function to the knee. Their effectiveness, however, depends on correct placement of the femoral tunnel. We think coming studies will focus on single-bundle ACLR and will be designed to improve the reliability of the transtibial and AM portal techniques for placing a femoral tunnel in keeping with the principles summarized by the I.D.E.A.L. acronym.

Osteoarthritic (OA) knees with varus deformities commonly present with tight, contracted medial collateral ligaments and soft-tissue sleeves.¹ More severe varus deformities require more extensive medial releases on the concave side to optimize flexion-extension gaps. Excessive soft-tissue releases in milder varus deformities can result in medial instability in flexion and extension.^2-4 Misjudgments in soft-tissue release can therefore lead to knee instability, an important cause of early total knee arthroplasty (TKA) failures.^2,5,6 Some authors have reported difficulty in coronal plane balancing in knees with preoperative varus deformity of more than 20°.^4,7

Surgeons often refer to varus as a description of coronal mal­alignment, mainly with the knee in extension. In the surgical setting, however, descriptions are given regarding differential medial soft-tissue tightness in extension and flexion. Balancing the knee in extension may not necessarily balance the knee in flexion. Thus, there is the concept of extension and flexion varus, which has not been well described in the literature. Releases on the anterior medial and posterior medial aspects of the proximal tibia have differential effects on flexion and extension gaps, respectively.²

Intraoperative alignment certainly has a pivotal role in component longevity.⁸ Since its advent in the 1990s, use of computer navigation in TKA has offered new hope for improving component alignment. Some authors routinely use computer navigation for intraoperative soft-tissue releases.⁹ A recent meta-analysis found that computer-navigated surgery is associated with fewer outliers in final component alignment compared with conventional TKA.¹⁰

Increased use of computer navigation in TKA at our institution in recent years has come with the observation that knees with severe extension varus seem to have correspondingly more severe flexion varus. Before computer navigation, coronal alignment of knees in flexion was almost impossible to measure because of the spatial alignment of the knees in that position.

We conducted a study to evaluate the relationship of extension and flexion varus in OA knees and to determine whether severity of fixed flexion deformity (FFD) in the sagittal plane correlates with severity of coronal plane varus deformity. We hypothesized that there would be differential varus in flexion and extension and that increasing knee extension varus would correlate closely with knee flexion varus beyond a certain tibiofemoral angle. We also hypothesized that severity of sagittal plane deformity will correlate with the severity of coronal plane deformity.

Patients and Methods

Data Collection

After this study was approved by our institution’s ethics review committee, we prospectively collected data from 403 consecutive computer-navigated TKAs performed at our institution between November 2008 and August 2011. Dr. Tan, who was not the primary physician, retrospectively analyzed the radiographic and navigation data.

Each patient’s knee varus-valgus angles were captured by Dr. Teo, an adult reconstruction surgeon, in standard fashion from maximal extension to 0º, 30º, 45º, 60º, 90º, and maximal flexion. An example of standard data capture appears in Table 1. With varus-hyperextension defined as –0.5° or less (more negative), neutral as 0°, and valgus-flexion as 0.5° or more, there were 362 varus knees, 41 valgus knees, and no neutral knees.

Study inclusion criteria were OA and varus deformity. Exclusion criteria were rheumatoid arthritis, other types of inflammatory arthritis, neuromuscular disorders, knees with valgus angulation, and incomplete data (Table 2). Figure 1 summarizes the inclusion/exclusion process, which left 317 knees available for study. Cases of incomplete data were likely due to computer errors or to inadvertent movement when navigation data were being acquired during surgery.

In conventional TKA, the main objective is to equalize flexion-extension gaps with knee at 90° flexion and 0° extension. The ability to achieve this often implies the knee will be balanced throughout its range of motion (ROM). From the data for the 317 study knees, 3 sets of values were extracted: varus angles from maximal knee extension (extension varus), varus angles from 90° knee flexion (flexion varus), and maximal knee extension. All knees were able to achieve 90° flexion.

Power Calculation

Our analysis used a correlation coefficient (r) of at least 0.5 at a 5% level of significance and power of 80%. With 317 knees, the study was more than adequately powered for significance.

Surgical and Navigation Technique

All patients underwent either general or regional anesthesia for their surgeries, which were performed by Dr. Teo. Standard medial parapatellar arthrotomy was performed. Navigation pins were then inserted into the femur and tibia outside the knee wound. Anatomical reference points were digitized per routine navigation requirements. (The reference for varus-valgus alignment of the femur is the mechanical femur axis defined by the digitized hip center and knee center, and the reference for varus-valgus alignment of the tibia is the mechanical tibia axis defined by the digitized tibia center and calculated ankle center. The ankle center is calculated by dividing the digitized transmalleolar axis according to a ratio of 56% lateral to 44% medial with the inherent navigation software.) Our institution uses an imageless navigation system (Navigation System II; Stryker Orthopedics, Mahwah, New Jersey).

The leg was then brought from maximal knee extension to maximal knee flexion to assess preoperative ROM, which indicates inherent flexion contracture or hyperextension. Varus-valgus measurements of the knee were then generated as part of the navigation software protocol. These measurements were obtained without additional varus or valgus stress applied to the knee and before any bony resection. The rest of the operation was completed using navigation to guide bony resection and soft-tissue balancing. The final components used were all cemented cruciate-substituting TKA implants. After component insertion, the knee was again brought through ROM from maximal knee extension to maximal knee flexion to assess postoperative ROM before wound closure.

Extension and Flexion Varus

As none of the patients in the flexion varus dataset (range, –0.5° to –19°) had a varus deformity of more than 20° at 90° flexion, we used a cutoff of 10° to divide these patients into 2 subgroups: less than 10° (237 knees) and 10° or more (80 knees). The extension varus dataset ranged from –0.5° to –24°. Incremental values of –0.5° to –24° in this dataset were then analyzed against the 90° flexion varus subgroups using logistic regression. A scatterplot of the relationship between extension and flexion varus is shown in Figure 2. The probability function was then derived and a probability graph plotted.

FFD and Extension and Flexion Varus

Maximal knee extension, obtained from intraoperative navigation measurements, ranged from –9° (hyperextension) to 33° (FFD) and maximal knee flexion ranged from 90° to 146°. Ninety-two knees had slight hyperextension, and 6 were neutral. Of the 317 OA knees with varus deformity, 219 (69%) had FFD. This sagittal plane alignment parameter was analyzed against coronal plane alignment in maximal knee extension and 90° knee flexion to determine if increasing severity of FFD corresponds with increasing extension or flexion varus.

Statistical Analysis

Statistical analysis was performed with Stata 10.1 (Statacorp, College Station, Texas). Significance was set at P < .05.

Results

Extension and Flexion Varus

Patient demographic data are listed in Table 3. Univariate logistic regression analysis revealed that age (P = .110), body mass index (P = .696), and sex (P = .584) did not affect the association between preoperative extension and flexion varus.

Mean (SD) preoperative extension varus was –9.9° (4.80°), and mean (SD) preoperative flexion 90° varus was –7.02° (3.74°). Linear regression of the data showed a significant positive correlation between preoperative extension varus and flexion varus (Pearson correlation coefficient, 0.57; P < .0001). The probability function was determined as follows: Probability of having flexion varus of more than 10° = 1 / (1 + e–z), where z = –4.014 – 0.265 × extension varus. Plotting the probability graph of flexion varus against varus angles at maximal knee extension from the probability formula yielded a sigmoid graph (Figure 3). The most linear part of the graph corresponds to the 10° to 20° of extension varus (solid line), demonstrating an almost linear increase in the probability of having more than 10° flexion varus with increasing extension varus from 10° to 20°. For extension varus of 20° or more, the probability of having flexion varus of more than 10° approaches 1.

FFD and Extension and Flexion Varus

Mean (SD) preoperative maximal knee extension (analogous to FFD) was 4.41° (7.50°), mean (SD) extension varus was –9.9° (4.80°), and mean (SD) 90° flexion varus was –7.02° (3.74°). We did not find any correlation between preoperative FFD and preoperative flexion varus (r = –0.02; P = .6583) or extension varus (r = –0.11; P = .046) (Figure 4).

Postoperative Alignment

Of the 317 OA knees, 18 had incomplete navigation-acquired postoperative alignment data. The postoperative alignment of the other 299 knees at various degrees of knee flexion is illustrated with a box-and-whisker plot (Figure 5).

Knees With Severe Extension Varus

Fourteen of the 15 knees with severe extension varus (>20°) had flexion varus of more than 9° (range, –9° to –17.5°, with only 1 outlier, at –5°). For the 15 patients, maximal knee extension ranged from –9° hyperextension to 27.5° FFD. Six knees had slight hyperextension, and 9 had FFD demonstrating large variability in sagittal alignment. Despite severe preoperative coronal deformity, all 15 knees had satisfactory deformity correction. Preoperative and postoperative knee alignment data for these 15 knees appear in Table 4 and Figure 6, respectively.

Discussion

OA varus knees represent a majority of the cases being managed by orthopedic surgeons. Soft-tissue contractures involving the medial collateral ligament (MCL), posteromedial capsule, pes anserinus, and semimembranosus muscle are commonly encountered. Bone loss may also occur on the tibial and femoral joint surfaces in knees with severe angular deformity. In an OA varus knee, bone loss tends to be mainly on the medial tibial plateau and usually on the posterior aspect of the tibia because flexion contractures often are concomitant with these marked deformities.¹¹ Therefore, a varus deformity is apparent whether the knee is extended or flexed. Our results showed a correlation between extension and flexion varus in OA varus knees. In contrast, for a valgus deformity, as bone loss can occur on both the tibial and femoral surfaces,¹¹ a similar correlation may not be seen. For that reason, and because there were only 41 valgus knees in this study, they were excluded. For FFD, soft-tissue contractures often involve both the posterior capsule and the posterior cruciate ligament (PCL). Posterior osteophytes often cause tenting of the posterior capsule in knees with FFD. Anteriorly, growth of osteophytes at the tibial spine and intercondylar notch of the femur can result in bony causes of restricted knee extension.¹²

One would expect increased coronal plane angular deformity to correspond to more severe FFD in the sagittal plane because the same pathology affects soft tissue or bones in an OA knee in both planes. Interestingly, our study results proved otherwise. FFD did not correlate with degree of extension or flexion varus severity. This phenomenon has not been described in the literature likely because clinical measurements of flexion varus and FFD were difficult to perform because of the spatial alignment of the knee in flexion. In recent years, however, computer navigation technology has made such measurements possible.

Mihalko and colleagues² established that soft-tissue releases on different parts of the proximal tibia have different effects on soft-tissue balancing in flexion and extension. In knees with extension varus, more releases are required on the posterior medial aspect of the tibia (the posterior oblique fibers of the superficial MCL, the posteromedial capsule, and, sometimes, the semimembranosus), whereas knees with flexion varus require more releases on the anterior medial aspect of the tibia (the deep MCL, the anterior fibers of the superficial MCL, and, sometimes, the pes anserinus attachment).¹³ Consequently, soft-tissue stabilizers seem to have different functions in flexion and extension and cannot reliably be released solely in extension or flexion for optimal gap balancing during TKA.² Other authors, in cadaveric studies, have found that a larger amount of coronal deformity correction is achieved with more distal soft-tissue releases from the joint line.^9,14 Surgical techniques for correcting FFD include removal of prominent anterior and posterior osteophytes, posterior capsular releases, sometimes PCL sacrifices, and even gastrocnemius recession.¹²

In our study, all 14 patients with severe extension and correspondingly severe flexion varus needed not only modest posterior medial soft-tissue releases for the severe extension varus, but also modest anterior medial releases for the flexion varus. The respective soft-tissue releases were confirmed in real time with computer navigation sequentially after bony resection and osteophyte removal. With this method, we restored final postoperative alignment to within 3° of the mechanical axis (Figure 6). Our experience here led us to believe that, with these patients, modest anterior medial and posterior medial releases could be performed at the start of surgery, as severe extension varus (>20°) almost certainly equates to severe flexion varus (>10°). Therein lies the clinical relevance of our study. However, not all patients with severe coronal plane deformity have correspondingly severe sagittal plane deformity in the form of FFD, as illustrated in our study. Therefore, not all patients with severe varus knee deformity need aggressive posterior capsular release or PCL recession to correct FFD. Some patients have mild hyperextension, which can be attributed partly to the postanesthesia effects of soft-tissue laxity. It is unclear exactly how much anesthesia contributes to this difference in sagittal alignment, though the majority of our patients had FFD. It is not our intent here to discuss the surgical techniques of soft-tissue balancing or to advocate routine use of computer navigation.

Many factors (eg, medial femoral condyle bone loss, medial tibial plateau bone loss, femur or tibia bowing, medial soft-tissue contracture) can contribute to varus malalignment. Current navigation technology cannot isolate the causes of varus alignment, and we did not intend to investigate them in this study. Our primary aim was to assess for a correlation between overall extension varus alignment and expected flexion varus. We also wanted to analyze the correlation between FFD and the coronal plane alignment, in extension and flexion, contributed by the combined bony and soft-tissue components in OA varus knees.

The strengths of this study are that it was a single-surgeon series with knee data from consecutive patients who had computer-navigated TKA. Patient data were prospectively generated from the navigation software and retrospectively analyzed. All navigation alignment was performed by a single surgeon, thereby eliminating examination bias during the time knee alignment data were being obtained. The study was adequately powered and had a large number of patients for data analysis. The authors believe that this is the first study to analyze alignment in both the coronal and sagittal plane in varus OA knees.

We acknowledge a few limitations in our study. Although several investigators have found that navigation can be used to achieve accurate postoperative alignment,^10,15,16 subtle errors may be inadvertently introduced at different points of alignment measurement. These error points include identification of visually selected anatomical landmarks; kinematic registration of hip, knee, and ankle; and intraoperative changes in the navigation environment (eg, inadvertent movement of pins or rigid bodies). In addition, different surgeons have different techniques for kinematic registration. However, the surgeries in our study were performed by the same surgeon, so this confounding factor was effectively removed. Another limitation was that navigation alignment was obtained during surgery, when patients were under anesthesia and in a supine, non-weight-bearing position, whereas routine clinical weight-bearing radiographs are taken with nonanesthetized patients and this might overestimate the deformities intraoperatively. However, all parameters were measured in the same patient under the same anesthetic effects, so this should not have affected the analyses. Most surgeons would make an intraoperative assessment of the severity of any deformity before the surgery proper anyway. Nevertheless, some authors have found that knee alignment obtained with intraoperative navigation correlated well with alignment obtained with weight-bearing radiographs.^17,18

Conclusion

Our study results showed that, in OA varus knees, extension varus highly correlated with flexion varus. However, there was no correlation between FFD and coronal plane varus deformity.

Osteoarthritic (OA) knees with varus deformities commonly present with tight, contracted medial collateral ligaments and soft-tissue sleeves.¹ More severe varus deformities require more extensive medial releases on the concave side to optimize flexion-extension gaps. Excessive soft-tissue releases in milder varus deformities can result in medial instability in flexion and extension.^2-4 Misjudgments in soft-tissue release can therefore lead to knee instability, an important cause of early total knee arthroplasty (TKA) failures.^2,5,6 Some authors have reported difficulty in coronal plane balancing in knees with preoperative varus deformity of more than 20°.^4,7

Surgeons often refer to varus as a description of coronal mal­alignment, mainly with the knee in extension. In the surgical setting, however, descriptions are given regarding differential medial soft-tissue tightness in extension and flexion. Balancing the knee in extension may not necessarily balance the knee in flexion. Thus, there is the concept of extension and flexion varus, which has not been well described in the literature. Releases on the anterior medial and posterior medial aspects of the proximal tibia have differential effects on flexion and extension gaps, respectively.²

Intraoperative alignment certainly has a pivotal role in component longevity.⁸ Since its advent in the 1990s, use of computer navigation in TKA has offered new hope for improving component alignment. Some authors routinely use computer navigation for intraoperative soft-tissue releases.⁹ A recent meta-analysis found that computer-navigated surgery is associated with fewer outliers in final component alignment compared with conventional TKA.¹⁰

Increased use of computer navigation in TKA at our institution in recent years has come with the observation that knees with severe extension varus seem to have correspondingly more severe flexion varus. Before computer navigation, coronal alignment of knees in flexion was almost impossible to measure because of the spatial alignment of the knees in that position.

We conducted a study to evaluate the relationship of extension and flexion varus in OA knees and to determine whether severity of fixed flexion deformity (FFD) in the sagittal plane correlates with severity of coronal plane varus deformity. We hypothesized that there would be differential varus in flexion and extension and that increasing knee extension varus would correlate closely with knee flexion varus beyond a certain tibiofemoral angle. We also hypothesized that severity of sagittal plane deformity will correlate with the severity of coronal plane deformity.

Patients and Methods

Data Collection

After this study was approved by our institution’s ethics review committee, we prospectively collected data from 403 consecutive computer-navigated TKAs performed at our institution between November 2008 and August 2011. Dr. Tan, who was not the primary physician, retrospectively analyzed the radiographic and navigation data.

Each patient’s knee varus-valgus angles were captured by Dr. Teo, an adult reconstruction surgeon, in standard fashion from maximal extension to 0º, 30º, 45º, 60º, 90º, and maximal flexion. An example of standard data capture appears in Table 1. With varus-hyperextension defined as –0.5° or less (more negative), neutral as 0°, and valgus-flexion as 0.5° or more, there were 362 varus knees, 41 valgus knees, and no neutral knees.

Study inclusion criteria were OA and varus deformity. Exclusion criteria were rheumatoid arthritis, other types of inflammatory arthritis, neuromuscular disorders, knees with valgus angulation, and incomplete data (Table 2). Figure 1 summarizes the inclusion/exclusion process, which left 317 knees available for study. Cases of incomplete data were likely due to computer errors or to inadvertent movement when navigation data were being acquired during surgery.

In conventional TKA, the main objective is to equalize flexion-extension gaps with knee at 90° flexion and 0° extension. The ability to achieve this often implies the knee will be balanced throughout its range of motion (ROM). From the data for the 317 study knees, 3 sets of values were extracted: varus angles from maximal knee extension (extension varus), varus angles from 90° knee flexion (flexion varus), and maximal knee extension. All knees were able to achieve 90° flexion.

Power Calculation

Our analysis used a correlation coefficient (r) of at least 0.5 at a 5% level of significance and power of 80%. With 317 knees, the study was more than adequately powered for significance.

Surgical and Navigation Technique

All patients underwent either general or regional anesthesia for their surgeries, which were performed by Dr. Teo. Standard medial parapatellar arthrotomy was performed. Navigation pins were then inserted into the femur and tibia outside the knee wound. Anatomical reference points were digitized per routine navigation requirements. (The reference for varus-valgus alignment of the femur is the mechanical femur axis defined by the digitized hip center and knee center, and the reference for varus-valgus alignment of the tibia is the mechanical tibia axis defined by the digitized tibia center and calculated ankle center. The ankle center is calculated by dividing the digitized transmalleolar axis according to a ratio of 56% lateral to 44% medial with the inherent navigation software.) Our institution uses an imageless navigation system (Navigation System II; Stryker Orthopedics, Mahwah, New Jersey).

The leg was then brought from maximal knee extension to maximal knee flexion to assess preoperative ROM, which indicates inherent flexion contracture or hyperextension. Varus-valgus measurements of the knee were then generated as part of the navigation software protocol. These measurements were obtained without additional varus or valgus stress applied to the knee and before any bony resection. The rest of the operation was completed using navigation to guide bony resection and soft-tissue balancing. The final components used were all cemented cruciate-substituting TKA implants. After component insertion, the knee was again brought through ROM from maximal knee extension to maximal knee flexion to assess postoperative ROM before wound closure.

Extension and Flexion Varus

As none of the patients in the flexion varus dataset (range, –0.5° to –19°) had a varus deformity of more than 20° at 90° flexion, we used a cutoff of 10° to divide these patients into 2 subgroups: less than 10° (237 knees) and 10° or more (80 knees). The extension varus dataset ranged from –0.5° to –24°. Incremental values of –0.5° to –24° in this dataset were then analyzed against the 90° flexion varus subgroups using logistic regression. A scatterplot of the relationship between extension and flexion varus is shown in Figure 2. The probability function was then derived and a probability graph plotted.

FFD and Extension and Flexion Varus

Maximal knee extension, obtained from intraoperative navigation measurements, ranged from –9° (hyperextension) to 33° (FFD) and maximal knee flexion ranged from 90° to 146°. Ninety-two knees had slight hyperextension, and 6 were neutral. Of the 317 OA knees with varus deformity, 219 (69%) had FFD. This sagittal plane alignment parameter was analyzed against coronal plane alignment in maximal knee extension and 90° knee flexion to determine if increasing severity of FFD corresponds with increasing extension or flexion varus.

Statistical Analysis

Statistical analysis was performed with Stata 10.1 (Statacorp, College Station, Texas). Significance was set at P < .05.

Results

Extension and Flexion Varus

Patient demographic data are listed in Table 3. Univariate logistic regression analysis revealed that age (P = .110), body mass index (P = .696), and sex (P = .584) did not affect the association between preoperative extension and flexion varus.

Mean (SD) preoperative extension varus was –9.9° (4.80°), and mean (SD) preoperative flexion 90° varus was –7.02° (3.74°). Linear regression of the data showed a significant positive correlation between preoperative extension varus and flexion varus (Pearson correlation coefficient, 0.57; P < .0001). The probability function was determined as follows: Probability of having flexion varus of more than 10° = 1 / (1 + e–z), where z = –4.014 – 0.265 × extension varus. Plotting the probability graph of flexion varus against varus angles at maximal knee extension from the probability formula yielded a sigmoid graph (Figure 3). The most linear part of the graph corresponds to the 10° to 20° of extension varus (solid line), demonstrating an almost linear increase in the probability of having more than 10° flexion varus with increasing extension varus from 10° to 20°. For extension varus of 20° or more, the probability of having flexion varus of more than 10° approaches 1.

FFD and Extension and Flexion Varus

Mean (SD) preoperative maximal knee extension (analogous to FFD) was 4.41° (7.50°), mean (SD) extension varus was –9.9° (4.80°), and mean (SD) 90° flexion varus was –7.02° (3.74°). We did not find any correlation between preoperative FFD and preoperative flexion varus (r = –0.02; P = .6583) or extension varus (r = –0.11; P = .046) (Figure 4).

Postoperative Alignment

Of the 317 OA knees, 18 had incomplete navigation-acquired postoperative alignment data. The postoperative alignment of the other 299 knees at various degrees of knee flexion is illustrated with a box-and-whisker plot (Figure 5).

Knees With Severe Extension Varus

Fourteen of the 15 knees with severe extension varus (>20°) had flexion varus of more than 9° (range, –9° to –17.5°, with only 1 outlier, at –5°). For the 15 patients, maximal knee extension ranged from –9° hyperextension to 27.5° FFD. Six knees had slight hyperextension, and 9 had FFD demonstrating large variability in sagittal alignment. Despite severe preoperative coronal deformity, all 15 knees had satisfactory deformity correction. Preoperative and postoperative knee alignment data for these 15 knees appear in Table 4 and Figure 6, respectively.

Discussion

OA varus knees represent a majority of the cases being managed by orthopedic surgeons. Soft-tissue contractures involving the medial collateral ligament (MCL), posteromedial capsule, pes anserinus, and semimembranosus muscle are commonly encountered. Bone loss may also occur on the tibial and femoral joint surfaces in knees with severe angular deformity. In an OA varus knee, bone loss tends to be mainly on the medial tibial plateau and usually on the posterior aspect of the tibia because flexion contractures often are concomitant with these marked deformities.¹¹ Therefore, a varus deformity is apparent whether the knee is extended or flexed. Our results showed a correlation between extension and flexion varus in OA varus knees. In contrast, for a valgus deformity, as bone loss can occur on both the tibial and femoral surfaces,¹¹ a similar correlation may not be seen. For that reason, and because there were only 41 valgus knees in this study, they were excluded. For FFD, soft-tissue contractures often involve both the posterior capsule and the posterior cruciate ligament (PCL). Posterior osteophytes often cause tenting of the posterior capsule in knees with FFD. Anteriorly, growth of osteophytes at the tibial spine and intercondylar notch of the femur can result in bony causes of restricted knee extension.¹²

One would expect increased coronal plane angular deformity to correspond to more severe FFD in the sagittal plane because the same pathology affects soft tissue or bones in an OA knee in both planes. Interestingly, our study results proved otherwise. FFD did not correlate with degree of extension or flexion varus severity. This phenomenon has not been described in the literature likely because clinical measurements of flexion varus and FFD were difficult to perform because of the spatial alignment of the knee in flexion. In recent years, however, computer navigation technology has made such measurements possible.

Mihalko and colleagues² established that soft-tissue releases on different parts of the proximal tibia have different effects on soft-tissue balancing in flexion and extension. In knees with extension varus, more releases are required on the posterior medial aspect of the tibia (the posterior oblique fibers of the superficial MCL, the posteromedial capsule, and, sometimes, the semimembranosus), whereas knees with flexion varus require more releases on the anterior medial aspect of the tibia (the deep MCL, the anterior fibers of the superficial MCL, and, sometimes, the pes anserinus attachment).¹³ Consequently, soft-tissue stabilizers seem to have different functions in flexion and extension and cannot reliably be released solely in extension or flexion for optimal gap balancing during TKA.² Other authors, in cadaveric studies, have found that a larger amount of coronal deformity correction is achieved with more distal soft-tissue releases from the joint line.^9,14 Surgical techniques for correcting FFD include removal of prominent anterior and posterior osteophytes, posterior capsular releases, sometimes PCL sacrifices, and even gastrocnemius recession.¹²

In our study, all 14 patients with severe extension and correspondingly severe flexion varus needed not only modest posterior medial soft-tissue releases for the severe extension varus, but also modest anterior medial releases for the flexion varus. The respective soft-tissue releases were confirmed in real time with computer navigation sequentially after bony resection and osteophyte removal. With this method, we restored final postoperative alignment to within 3° of the mechanical axis (Figure 6). Our experience here led us to believe that, with these patients, modest anterior medial and posterior medial releases could be performed at the start of surgery, as severe extension varus (>20°) almost certainly equates to severe flexion varus (>10°). Therein lies the clinical relevance of our study. However, not all patients with severe coronal plane deformity have correspondingly severe sagittal plane deformity in the form of FFD, as illustrated in our study. Therefore, not all patients with severe varus knee deformity need aggressive posterior capsular release or PCL recession to correct FFD. Some patients have mild hyperextension, which can be attributed partly to the postanesthesia effects of soft-tissue laxity. It is unclear exactly how much anesthesia contributes to this difference in sagittal alignment, though the majority of our patients had FFD. It is not our intent here to discuss the surgical techniques of soft-tissue balancing or to advocate routine use of computer navigation.

Many factors (eg, medial femoral condyle bone loss, medial tibial plateau bone loss, femur or tibia bowing, medial soft-tissue contracture) can contribute to varus malalignment. Current navigation technology cannot isolate the causes of varus alignment, and we did not intend to investigate them in this study. Our primary aim was to assess for a correlation between overall extension varus alignment and expected flexion varus. We also wanted to analyze the correlation between FFD and the coronal plane alignment, in extension and flexion, contributed by the combined bony and soft-tissue components in OA varus knees.

The strengths of this study are that it was a single-surgeon series with knee data from consecutive patients who had computer-navigated TKA. Patient data were prospectively generated from the navigation software and retrospectively analyzed. All navigation alignment was performed by a single surgeon, thereby eliminating examination bias during the time knee alignment data were being obtained. The study was adequately powered and had a large number of patients for data analysis. The authors believe that this is the first study to analyze alignment in both the coronal and sagittal plane in varus OA knees.

We acknowledge a few limitations in our study. Although several investigators have found that navigation can be used to achieve accurate postoperative alignment,^10,15,16 subtle errors may be inadvertently introduced at different points of alignment measurement. These error points include identification of visually selected anatomical landmarks; kinematic registration of hip, knee, and ankle; and intraoperative changes in the navigation environment (eg, inadvertent movement of pins or rigid bodies). In addition, different surgeons have different techniques for kinematic registration. However, the surgeries in our study were performed by the same surgeon, so this confounding factor was effectively removed. Another limitation was that navigation alignment was obtained during surgery, when patients were under anesthesia and in a supine, non-weight-bearing position, whereas routine clinical weight-bearing radiographs are taken with nonanesthetized patients and this might overestimate the deformities intraoperatively. However, all parameters were measured in the same patient under the same anesthetic effects, so this should not have affected the analyses. Most surgeons would make an intraoperative assessment of the severity of any deformity before the surgery proper anyway. Nevertheless, some authors have found that knee alignment obtained with intraoperative navigation correlated well with alignment obtained with weight-bearing radiographs.^17,18

Conclusion

Our study results showed that, in OA varus knees, extension varus highly correlated with flexion varus. However, there was no correlation between FFD and coronal plane varus deformity.

Osteoarthritic (OA) knees with varus deformities commonly present with tight, contracted medial collateral ligaments and soft-tissue sleeves.¹ More severe varus deformities require more extensive medial releases on the concave side to optimize flexion-extension gaps. Excessive soft-tissue releases in milder varus deformities can result in medial instability in flexion and extension.^2-4 Misjudgments in soft-tissue release can therefore lead to knee instability, an important cause of early total knee arthroplasty (TKA) failures.^2,5,6 Some authors have reported difficulty in coronal plane balancing in knees with preoperative varus deformity of more than 20°.^4,7

Surgeons often refer to varus as a description of coronal mal­alignment, mainly with the knee in extension. In the surgical setting, however, descriptions are given regarding differential medial soft-tissue tightness in extension and flexion. Balancing the knee in extension may not necessarily balance the knee in flexion. Thus, there is the concept of extension and flexion varus, which has not been well described in the literature. Releases on the anterior medial and posterior medial aspects of the proximal tibia have differential effects on flexion and extension gaps, respectively.²

Intraoperative alignment certainly has a pivotal role in component longevity.⁸ Since its advent in the 1990s, use of computer navigation in TKA has offered new hope for improving component alignment. Some authors routinely use computer navigation for intraoperative soft-tissue releases.⁹ A recent meta-analysis found that computer-navigated surgery is associated with fewer outliers in final component alignment compared with conventional TKA.¹⁰

Increased use of computer navigation in TKA at our institution in recent years has come with the observation that knees with severe extension varus seem to have correspondingly more severe flexion varus. Before computer navigation, coronal alignment of knees in flexion was almost impossible to measure because of the spatial alignment of the knees in that position.

We conducted a study to evaluate the relationship of extension and flexion varus in OA knees and to determine whether severity of fixed flexion deformity (FFD) in the sagittal plane correlates with severity of coronal plane varus deformity. We hypothesized that there would be differential varus in flexion and extension and that increasing knee extension varus would correlate closely with knee flexion varus beyond a certain tibiofemoral angle. We also hypothesized that severity of sagittal plane deformity will correlate with the severity of coronal plane deformity.

Patients and Methods

Data Collection

After this study was approved by our institution’s ethics review committee, we prospectively collected data from 403 consecutive computer-navigated TKAs performed at our institution between November 2008 and August 2011. Dr. Tan, who was not the primary physician, retrospectively analyzed the radiographic and navigation data.

Each patient’s knee varus-valgus angles were captured by Dr. Teo, an adult reconstruction surgeon, in standard fashion from maximal extension to 0º, 30º, 45º, 60º, 90º, and maximal flexion. An example of standard data capture appears in Table 1. With varus-hyperextension defined as –0.5° or less (more negative), neutral as 0°, and valgus-flexion as 0.5° or more, there were 362 varus knees, 41 valgus knees, and no neutral knees.

Study inclusion criteria were OA and varus deformity. Exclusion criteria were rheumatoid arthritis, other types of inflammatory arthritis, neuromuscular disorders, knees with valgus angulation, and incomplete data (Table 2). Figure 1 summarizes the inclusion/exclusion process, which left 317 knees available for study. Cases of incomplete data were likely due to computer errors or to inadvertent movement when navigation data were being acquired during surgery.

In conventional TKA, the main objective is to equalize flexion-extension gaps with knee at 90° flexion and 0° extension. The ability to achieve this often implies the knee will be balanced throughout its range of motion (ROM). From the data for the 317 study knees, 3 sets of values were extracted: varus angles from maximal knee extension (extension varus), varus angles from 90° knee flexion (flexion varus), and maximal knee extension. All knees were able to achieve 90° flexion.

Power Calculation

Our analysis used a correlation coefficient (r) of at least 0.5 at a 5% level of significance and power of 80%. With 317 knees, the study was more than adequately powered for significance.

Surgical and Navigation Technique

All patients underwent either general or regional anesthesia for their surgeries, which were performed by Dr. Teo. Standard medial parapatellar arthrotomy was performed. Navigation pins were then inserted into the femur and tibia outside the knee wound. Anatomical reference points were digitized per routine navigation requirements. (The reference for varus-valgus alignment of the femur is the mechanical femur axis defined by the digitized hip center and knee center, and the reference for varus-valgus alignment of the tibia is the mechanical tibia axis defined by the digitized tibia center and calculated ankle center. The ankle center is calculated by dividing the digitized transmalleolar axis according to a ratio of 56% lateral to 44% medial with the inherent navigation software.) Our institution uses an imageless navigation system (Navigation System II; Stryker Orthopedics, Mahwah, New Jersey).

The leg was then brought from maximal knee extension to maximal knee flexion to assess preoperative ROM, which indicates inherent flexion contracture or hyperextension. Varus-valgus measurements of the knee were then generated as part of the navigation software protocol. These measurements were obtained without additional varus or valgus stress applied to the knee and before any bony resection. The rest of the operation was completed using navigation to guide bony resection and soft-tissue balancing. The final components used were all cemented cruciate-substituting TKA implants. After component insertion, the knee was again brought through ROM from maximal knee extension to maximal knee flexion to assess postoperative ROM before wound closure.

Extension and Flexion Varus

As none of the patients in the flexion varus dataset (range, –0.5° to –19°) had a varus deformity of more than 20° at 90° flexion, we used a cutoff of 10° to divide these patients into 2 subgroups: less than 10° (237 knees) and 10° or more (80 knees). The extension varus dataset ranged from –0.5° to –24°. Incremental values of –0.5° to –24° in this dataset were then analyzed against the 90° flexion varus subgroups using logistic regression. A scatterplot of the relationship between extension and flexion varus is shown in Figure 2. The probability function was then derived and a probability graph plotted.

FFD and Extension and Flexion Varus

Maximal knee extension, obtained from intraoperative navigation measurements, ranged from –9° (hyperextension) to 33° (FFD) and maximal knee flexion ranged from 90° to 146°. Ninety-two knees had slight hyperextension, and 6 were neutral. Of the 317 OA knees with varus deformity, 219 (69%) had FFD. This sagittal plane alignment parameter was analyzed against coronal plane alignment in maximal knee extension and 90° knee flexion to determine if increasing severity of FFD corresponds with increasing extension or flexion varus.

Statistical Analysis

Statistical analysis was performed with Stata 10.1 (Statacorp, College Station, Texas). Significance was set at P < .05.

Results

Extension and Flexion Varus

Patient demographic data are listed in Table 3. Univariate logistic regression analysis revealed that age (P = .110), body mass index (P = .696), and sex (P = .584) did not affect the association between preoperative extension and flexion varus.

Mean (SD) preoperative extension varus was –9.9° (4.80°), and mean (SD) preoperative flexion 90° varus was –7.02° (3.74°). Linear regression of the data showed a significant positive correlation between preoperative extension varus and flexion varus (Pearson correlation coefficient, 0.57; P < .0001). The probability function was determined as follows: Probability of having flexion varus of more than 10° = 1 / (1 + e–z), where z = –4.014 – 0.265 × extension varus. Plotting the probability graph of flexion varus against varus angles at maximal knee extension from the probability formula yielded a sigmoid graph (Figure 3). The most linear part of the graph corresponds to the 10° to 20° of extension varus (solid line), demonstrating an almost linear increase in the probability of having more than 10° flexion varus with increasing extension varus from 10° to 20°. For extension varus of 20° or more, the probability of having flexion varus of more than 10° approaches 1.

FFD and Extension and Flexion Varus

Mean (SD) preoperative maximal knee extension (analogous to FFD) was 4.41° (7.50°), mean (SD) extension varus was –9.9° (4.80°), and mean (SD) 90° flexion varus was –7.02° (3.74°). We did not find any correlation between preoperative FFD and preoperative flexion varus (r = –0.02; P = .6583) or extension varus (r = –0.11; P = .046) (Figure 4).

Postoperative Alignment

Of the 317 OA knees, 18 had incomplete navigation-acquired postoperative alignment data. The postoperative alignment of the other 299 knees at various degrees of knee flexion is illustrated with a box-and-whisker plot (Figure 5).

Knees With Severe Extension Varus

Fourteen of the 15 knees with severe extension varus (>20°) had flexion varus of more than 9° (range, –9° to –17.5°, with only 1 outlier, at –5°). For the 15 patients, maximal knee extension ranged from –9° hyperextension to 27.5° FFD. Six knees had slight hyperextension, and 9 had FFD demonstrating large variability in sagittal alignment. Despite severe preoperative coronal deformity, all 15 knees had satisfactory deformity correction. Preoperative and postoperative knee alignment data for these 15 knees appear in Table 4 and Figure 6, respectively.

Discussion

OA varus knees represent a majority of the cases being managed by orthopedic surgeons. Soft-tissue contractures involving the medial collateral ligament (MCL), posteromedial capsule, pes anserinus, and semimembranosus muscle are commonly encountered. Bone loss may also occur on the tibial and femoral joint surfaces in knees with severe angular deformity. In an OA varus knee, bone loss tends to be mainly on the medial tibial plateau and usually on the posterior aspect of the tibia because flexion contractures often are concomitant with these marked deformities.¹¹ Therefore, a varus deformity is apparent whether the knee is extended or flexed. Our results showed a correlation between extension and flexion varus in OA varus knees. In contrast, for a valgus deformity, as bone loss can occur on both the tibial and femoral surfaces,¹¹ a similar correlation may not be seen. For that reason, and because there were only 41 valgus knees in this study, they were excluded. For FFD, soft-tissue contractures often involve both the posterior capsule and the posterior cruciate ligament (PCL). Posterior osteophytes often cause tenting of the posterior capsule in knees with FFD. Anteriorly, growth of osteophytes at the tibial spine and intercondylar notch of the femur can result in bony causes of restricted knee extension.¹²

One would expect increased coronal plane angular deformity to correspond to more severe FFD in the sagittal plane because the same pathology affects soft tissue or bones in an OA knee in both planes. Interestingly, our study results proved otherwise. FFD did not correlate with degree of extension or flexion varus severity. This phenomenon has not been described in the literature likely because clinical measurements of flexion varus and FFD were difficult to perform because of the spatial alignment of the knee in flexion. In recent years, however, computer navigation technology has made such measurements possible.

Mihalko and colleagues² established that soft-tissue releases on different parts of the proximal tibia have different effects on soft-tissue balancing in flexion and extension. In knees with extension varus, more releases are required on the posterior medial aspect of the tibia (the posterior oblique fibers of the superficial MCL, the posteromedial capsule, and, sometimes, the semimembranosus), whereas knees with flexion varus require more releases on the anterior medial aspect of the tibia (the deep MCL, the anterior fibers of the superficial MCL, and, sometimes, the pes anserinus attachment).¹³ Consequently, soft-tissue stabilizers seem to have different functions in flexion and extension and cannot reliably be released solely in extension or flexion for optimal gap balancing during TKA.² Other authors, in cadaveric studies, have found that a larger amount of coronal deformity correction is achieved with more distal soft-tissue releases from the joint line.^9,14 Surgical techniques for correcting FFD include removal of prominent anterior and posterior osteophytes, posterior capsular releases, sometimes PCL sacrifices, and even gastrocnemius recession.¹²

In our study, all 14 patients with severe extension and correspondingly severe flexion varus needed not only modest posterior medial soft-tissue releases for the severe extension varus, but also modest anterior medial releases for the flexion varus. The respective soft-tissue releases were confirmed in real time with computer navigation sequentially after bony resection and osteophyte removal. With this method, we restored final postoperative alignment to within 3° of the mechanical axis (Figure 6). Our experience here led us to believe that, with these patients, modest anterior medial and posterior medial releases could be performed at the start of surgery, as severe extension varus (>20°) almost certainly equates to severe flexion varus (>10°). Therein lies the clinical relevance of our study. However, not all patients with severe coronal plane deformity have correspondingly severe sagittal plane deformity in the form of FFD, as illustrated in our study. Therefore, not all patients with severe varus knee deformity need aggressive posterior capsular release or PCL recession to correct FFD. Some patients have mild hyperextension, which can be attributed partly to the postanesthesia effects of soft-tissue laxity. It is unclear exactly how much anesthesia contributes to this difference in sagittal alignment, though the majority of our patients had FFD. It is not our intent here to discuss the surgical techniques of soft-tissue balancing or to advocate routine use of computer navigation.

Many factors (eg, medial femoral condyle bone loss, medial tibial plateau bone loss, femur or tibia bowing, medial soft-tissue contracture) can contribute to varus malalignment. Current navigation technology cannot isolate the causes of varus alignment, and we did not intend to investigate them in this study. Our primary aim was to assess for a correlation between overall extension varus alignment and expected flexion varus. We also wanted to analyze the correlation between FFD and the coronal plane alignment, in extension and flexion, contributed by the combined bony and soft-tissue components in OA varus knees.

The strengths of this study are that it was a single-surgeon series with knee data from consecutive patients who had computer-navigated TKA. Patient data were prospectively generated from the navigation software and retrospectively analyzed. All navigation alignment was performed by a single surgeon, thereby eliminating examination bias during the time knee alignment data were being obtained. The study was adequately powered and had a large number of patients for data analysis. The authors believe that this is the first study to analyze alignment in both the coronal and sagittal plane in varus OA knees.

We acknowledge a few limitations in our study. Although several investigators have found that navigation can be used to achieve accurate postoperative alignment,^10,15,16 subtle errors may be inadvertently introduced at different points of alignment measurement. These error points include identification of visually selected anatomical landmarks; kinematic registration of hip, knee, and ankle; and intraoperative changes in the navigation environment (eg, inadvertent movement of pins or rigid bodies). In addition, different surgeons have different techniques for kinematic registration. However, the surgeries in our study were performed by the same surgeon, so this confounding factor was effectively removed. Another limitation was that navigation alignment was obtained during surgery, when patients were under anesthesia and in a supine, non-weight-bearing position, whereas routine clinical weight-bearing radiographs are taken with nonanesthetized patients and this might overestimate the deformities intraoperatively. However, all parameters were measured in the same patient under the same anesthetic effects, so this should not have affected the analyses. Most surgeons would make an intraoperative assessment of the severity of any deformity before the surgery proper anyway. Nevertheless, some authors have found that knee alignment obtained with intraoperative navigation correlated well with alignment obtained with weight-bearing radiographs.^17,18

Conclusion

Our study results showed that, in OA varus knees, extension varus highly correlated with flexion varus. However, there was no correlation between FFD and coronal plane varus deformity.

Postoperative dislocation remains a common complication of primary total hip arthroplasties (THAs), affecting less than 1% to more than 10% in reported series.^1,2 In large datasets for modern implants, the incidence of dislocation is 2% to 4%.^3,4 Given that more than 200,000 THAs are performed in the United States each year,⁵ these low percentages represent a large number of patients. The multiplex patient variables that affect THA stability include age, sex, body mass index (BMI), and comorbid conditions.^6-8 Surgical approach, restoration of leg length and femoral offset, femoral head size, and component positioning are also important surgical factors that can increase or decrease the incidence of dislocation.^3,8,9 In particular, appropriate acetabular component orientation is crucial; surgeons can control this factor and thereby limit the occurrence of dislocation.¹⁰ Furthermore, acetabular malpositioning can increase the risk of liner fractures and accelerate bearing-surface wear.^11-14

To minimize the risk of postoperative dislocation, surgeons traditionally have targeted the Lewinnek safe zone, with its mean (SD) inclination of 40° (10°) and mean (SD) anteversion of 15° (10°), for acetabular component orientation.¹⁵ However, the applicability of this target zone to preventing hip instability using modern implant designs, components, and surgical techniques remains unknown. Achieving acetabular orientation based on maximizing range of motion (ROM) before impingement may be optimal, with anteversion from 20° to 30° and inclination from 40° to 45°.^16,17 Furthermore, mean (SD) native acetabular anteversion ranges from 21.3° (6.2°) for men to 24.6° (6.6°) for women.¹⁸ Placing THA acetabular components near the native range for anteversion may best provide impingement-free ROM and thus optimize THA stability,^16,19 but this has not been proved in a clinical study.

Early dislocation is typically classified as occurring within 6 months after surgery,⁹ with almost 80% of dislocations occurring within 3 months after surgery.¹⁰ Surgeon-specific factors, such as acetabular component positioning, are thought to have a predominant effect on dislocations in the early postoperative period.¹⁰ Computer-assisted surgery (CAS), such as imageless navigation, is more accurate than conventional methods for acetabular component placement,^20-23 but the clinical relevance of improving accuracy for acetabular component placement has not been shown with respect to altering patient outcomes.²³

We conducted a study in a large single-surgeon patient cohort to determine the incidence of early postoperative dislocation with target anteversion increased to 25°, approximating mean native acetabular anteversion.^16,19 In addition, we sought to determine the accuracy of imageless navigation in achieving target acetabular component placement.

Materials and Methods

After obtaining institutional review board approval for this retrospective clinical study, we reviewed 671 consecutive cases of primary THA performed by a single surgeon using an imageless CAS system (AchieveCAS; Smith & Nephew, Memphis, Tennessee) between July 2006 and October 2012. THAs were excluded if a metal-on-metal bearing surface was used, if an adequate 6-week postoperative supine anteroposterior (AP) pelvis radiograph was unavailable, or if 6-month clinical follow-up findings were not available (Figure 1). The quality of AP radiographs was deemed poor if they were not centered on the symphysis pubis and if the sacrococcygeal joint was not centered over the symphysis pubis. After exclusion criteria were applied, 553 arthroplasties (479 patients) with a mean (SD) follow-up of 2.4 (1.4) years remained. Perioperative demographic data and component sizes are listed in Table 1.

During surgery, the anterior pelvic plane, defined by the anterior-superior iliac spines and pubic tubercle, was registered with the CAS system with the patient in the supine position. THA was performed with the patient in the lateral decubitus position using a posterolateral technique. For all patients, the surgeon used a hemispherical acetabular component (R3 Acetabular System; Smith & Nephew); bearings that were either metal on highly cross-linked polyethylene (XLPE) or Oxinium (Smith & Nephew) on XLPE; and neutral XLPE acetabular inserts. The goals for acetabular inclination and anteversion were 40° and 25°, respectively, with ±10° each for the target zone. The CAS system was used to adjust target anteversion for sagittal pelvic tilt.²⁴ Uncemented femoral components were used for all patients, and the goal for femoral component anteversion was 15°. Transosseous repair of the posterior capsule and short external rotators was performed after component implantation.²⁵

On each 6-week postoperative radiograph, acetabular orientation was measured with Ein-Bild-Röntgen-Analyse (EBRA; University of Innsbruck, Austria) software, which provides a validated method for measuring acetabular inclination and anteversion on supine AP pelvis radiographs.^10,26 Pelvic boundaries were delineated with grid lines defining pelvic position. Reference points around the projections of the prosthetic femoral head, the hemispherical cup, and the rim of the cup were marked (Figure 2). EBRA calculated radiographic inclination and anteversion of the acetabular component based on the spatial position of the cup center in relation to the plane of the radiograph and the pelvic position.²⁶

Charts were reviewed to identify patients with early postoperative dislocations, as well as dislocation timing, recurrence, and other characteristics. We defined early dislocation as instability occurring within 6 months after surgery. Revision surgery for instability was also identified.

For the statistical analysis, orientation error was defined as the absolute value of the difference between target orientation (40° inclination, 25° anteversion) and radiographic measurements. Repeated-measures multiple regression with the generalized estimating equations approach was used to identify baseline patient characteristics (age, sex, BMI, primary diagnosis, laterality) associated with component positioning outside of our targeted ranges for inclination and anteversion. Fisher exact tests were used to examine the relationship between dislocation and component placement in either the Lewinnek safe zone or our targeted zone. All tests were 2-sided with a significance level of .05. All analyses were performed with SAS for Windows 9.3 (SAS Institute, Cary, North Carolina).

Results

Mean (SD) acetabular inclination was 42.2° (4.9°) (range, 27.6°-65.0°), with a mean (SD) orientation error of 4.2° (3.4°) (Figure 3A). Mean (SD) anteversion was 23.9° (6.5°) (range, 6.2°-48.0°), with a mean (SD) orientation error of 5.2° (4.1°) (Figure 3B). Components were placed outside the Lewinnek safe zone for inclination or anteversion in 46.5% of cases and outside the target zone in 17.7% of cases (Figure 4). Variation in acetabular anteversion alone accounted for 67.3% of target zone outliers (Table 2). Only 0.9% of components were placed outside the target ranges for both inclination and anteversion.

Regression analysis was performed separately for inclination and anteversion to determine the risk factors for placing the acetabular component outside the target orientation ranges. Only higher BMI was associated with malposition with respect to inclination (hazard ratio [HR], 1.059; 95% confidence interval [CI], 1.011-1.111; P = .017). Of obese patients with inclination outside the target range, 90.9% had an inclination angle of more than 50°. Associations between inclination outside the target range and age (P = .769), sex (P = .217), preoperative diagnosis (P > .99), and laterality (P = .106) were statistically insignificant. Only female sex was associated with position of the acetabular component outside the target range for anteversion (HR, 1.871; 95% CI, 1.061-3.299; P = .030). Of female patients with anteversion outside the target range, 70.0% had anteversion of less than 15°. Associations between anteversion outside the target range and age (P = .762), BMI (P = .583), preoperative diagnosis (P > .99), and laterality (P = .235) were statistically insignificant.

Six THAs (1.1%) in 6 patients experienced dislocation within 6 months after surgery (Table 3); mean (SD) time of dislocation was 58.3 (13.8) days after surgery. There was no relationship between dislocation incidence and component placement in the Lewinnek zone (P = .224) or our target zone (P = .287). Of the dislocation cases, 50% involved female patients, and 50% involved right hips. Mean (SD) age of these patients was 53.3 (7.6) years. Mean (SD) BMI was 25.4 (0.9) kg/m². Osteoarthritis was the primary diagnosis for all patients with early dislocation; 32- or 36-mm femoral heads were used in these cases. Two patients had acetabular components placed outside of our target zone. One patient, who had abnormal pelvic obliquity and sagittal tilt from scoliosis (Figures 5A, 5B), had an acetabular component placed outside both the target zone and the Lewinnek safe zone. Mean (SD) acetabular inclination was 39.8° (3.6°), and mean (SD) anteversion was 21.8° (7.3°) (Figure 5C). Two dislocations resulted from trauma, 1 dislocation was related to hyperlaxity, 1 patient had cerebral palsy, and 1 patient had no evident predisposing risk factors. Three patients (0.54%) had multiple episodes of instability requiring revision during the follow-up period.

Discussion

To our knowledge, this study represents the largest cohort of primary THAs performed with an imageless navigation system. Our results showed that increasing targeted acetabular anteversion to 25° using a posterolateral surgical approach and modern implants resulted in a 1.1% incidence of early dislocation and a 0.54% incidence of recurrent instability requiring reoperation. Of the patients with a dislocation, only 1 did not experience trauma and did not have a risk factor for dislocation. Only 1 patient with a dislocation had acetabular components positioned outside both the target zone and the Lewinnek safe zone. The acetabular component was placed within the target zone in 82.3% of cases in which the imageless navigation system was used. In our cohort, BMI was the only risk factor for placement of the acetabular component outside our target range for inclination, and sex was associated with components outside the target range for anteversion.

Early dislocation after THA is often related to improper implant orientation, inadequate restoration of offset and myofascial tension, and decreased femoral head–neck ratio.⁸ Although dislocation rates in the literature vary widely,^1,2 Medicare data suggest that the rate for the first 6 months after surgery can be as high as 4.21%.^3,4 Although use of femoral heads with a diameter of 32 mm or larger may decrease this rate to 2.14%,³ accurate acetabular component orientation helps prevent postoperative dislocation.¹⁰ Using an imageless navigation system to target 25° of anteversion and 40° of inclination resulted in an early-dislocation rate about 49% less than the rate in a Medicare population treated with similar, modern implants.³

Callanan and colleagues¹¹ found that freehand techniques were inaccurate for acetabular positioning in up to 50% of cases, and several studies have demonstrated that imageless navigation systems were more accurate than conventional guides.^20,21,27-29 Higher BMI has been implicated as a risk factor for acetabular malpositioning in several studies of the accuracy of freehand techniques¹¹ and imageless navigation techniques.^23,30 Soft-tissue impediment to the component insertion handle poses a risk of increased inclination and inadequate anteversion, regardless of method used (conventional, CAS). When the acetabular component is placed freehand in obese patients, it is difficult to judge the position of the pelvis on the operating room table. For imageless navigation, a larger amount of adipose tissue over bony landmarks may limit the accuracy of anterior pelvic plane registration.³⁰ Sex typically is not cited as a risk factor for inaccurate acetabular component positioning. We speculate that omitted-variable bias may explain the observed association between female sex and anteversion. For example, changes in postoperative pelvic tilt alter apparent anteversion on plain radiographs,^31-34 but preoperative and postoperative sagittal pelvic tilt was not recorded in this study.

The proper position of the acetabular component has been debated.^15,16,35,36 Although it is generally agreed that inclination of 40° ± 10° balances ROM, stability, and bearing-surface wear,^12,13,15,16 proposed targets for anteversion vary widely, from 0° to 40°.^35,36 Patel and colleagues¹⁶ formulated computer models based on cadaveric specimens to determine that THA impingement was minimized when the acetabular component was placed to match the native anteversion of the acetabulum.In their study model, 20° of anteversion paralleled native acetabular orientation. Tohtz and colleagues¹⁸ reviewed computed tomography scans of 144 female hips and 192 male hips and found that mean (SD) anteversion was 24.6° (6.6°) for women and 21.3° (6.2°) for men. Whether native anatomy is a valid reference for acetabular anteversion is controversial,¹⁹ and definitive recommendations for target anteversion cannot be made, as the effect of acetabular anteversion on the wear of various bearing materials is unknown.¹⁴ Yet, as with inclination, ideal anteversion is likely a compromise between maximizing impingement-free ROM and minimizing wear.

The present study had several limitations. A single-surgeon patient series was reviewed retrospectively, and there was no control group. We determined the incidence only of early dislocation, and 5.3% of THAs that were not metal-on-metal were either lost to follow-up or had inadequate radiographs. However, of the patients excluded for inadequate radiographs, none had an early dislocation. The effects of our surgical techniques on long-term outcomes, bearing wear, and dislocation are unknown. We were not able to comment on the direction of dislocation for any of the 6 patients with early dislocation, as all dislocations were reduced at facilities other than our hospital. Therefore, we cannot determine whether increasing acetabular anteversion resulted in a larger number of anterior versus posterior dislocations.¹⁵

We did not use CAS to place any of the femoral components. Therefore, we could not accurately target combined anteversion, defined as the sum of acetabular and femoral version, which may be an important determinant of THA stability.²⁸ Although restoration of femoral offset and leg length is important in preventing THA dislocation,⁸ the CAS techniques used did not influence these parameters, and they were not measured.

As an imageless navigation system was used, there were no preoperative axial images, which could have been used to assess native acetabular orientation. This limited our assessment with respect to matching each patient’s natural anteversion. Imageless navigation, which references only the anterior pelvic plane, may not be reliable in patients with excessive sagittal pelvic tilt.³⁷ Furthermore, changes in the functional position of the pelvis from supine to sitting to standing were not accounted for, and changes in sagittal tilt between these positions can be significant.³⁸ Changes in sagittal pelvic tilt affect measurement of acetabular anteversion on plain radiographs, with anterior tilt reducing apparent anteversion and posterior tilt increasing it.^32,34 Although postoperative computed tomography is the gold standard for assessing acetabular component orientation, EBRA significantly reduces errors of measurement on plain radiographs.¹⁰ Some variability in measured anteversion may be explained by our surgical technique. In particular, if the cup was uncovered anteriorly, additional anteversion was usually accepted during surgery to minimize anterior impingement and limit the risk of iliopsoas tendonitis.^16,39

Our study results suggested that increasing target acetabular anteversion to 25° may reduce the incidence of early postoperative instability relative to rates reported in the literature. Despite the higher accuracy of component placement with an imageless navigation system, dislocations occurred in patients with acetabular components positioned in our target zone and in the historical safe zone. These dislocations support the notion that there likely is no absolute safe range for acetabular component positioning, as THA stability depends on many factors. Ideal targets for implant orientation for acetabulum and femur may be patient-specific.^16,19 Investigators should prospectively evaluate patient-specific THA component positioning and determine its effect on postoperative dislocation and bearing-surface wear. As specific implant targets are further defined, tools that are more precise and accurate than conventional techniques will be needed to achieve goal component positioning. Our study results confirmed that imageless navigation is an accurate method for achieving acetabular orientation targets.

Postoperative dislocation remains a common complication of primary total hip arthroplasties (THAs), affecting less than 1% to more than 10% in reported series.^1,2 In large datasets for modern implants, the incidence of dislocation is 2% to 4%.^3,4 Given that more than 200,000 THAs are performed in the United States each year,⁵ these low percentages represent a large number of patients. The multiplex patient variables that affect THA stability include age, sex, body mass index (BMI), and comorbid conditions.^6-8 Surgical approach, restoration of leg length and femoral offset, femoral head size, and component positioning are also important surgical factors that can increase or decrease the incidence of dislocation.^3,8,9 In particular, appropriate acetabular component orientation is crucial; surgeons can control this factor and thereby limit the occurrence of dislocation.¹⁰ Furthermore, acetabular malpositioning can increase the risk of liner fractures and accelerate bearing-surface wear.^11-14

To minimize the risk of postoperative dislocation, surgeons traditionally have targeted the Lewinnek safe zone, with its mean (SD) inclination of 40° (10°) and mean (SD) anteversion of 15° (10°), for acetabular component orientation.¹⁵ However, the applicability of this target zone to preventing hip instability using modern implant designs, components, and surgical techniques remains unknown. Achieving acetabular orientation based on maximizing range of motion (ROM) before impingement may be optimal, with anteversion from 20° to 30° and inclination from 40° to 45°.^16,17 Furthermore, mean (SD) native acetabular anteversion ranges from 21.3° (6.2°) for men to 24.6° (6.6°) for women.¹⁸ Placing THA acetabular components near the native range for anteversion may best provide impingement-free ROM and thus optimize THA stability,^16,19 but this has not been proved in a clinical study.

Early dislocation is typically classified as occurring within 6 months after surgery,⁹ with almost 80% of dislocations occurring within 3 months after surgery.¹⁰ Surgeon-specific factors, such as acetabular component positioning, are thought to have a predominant effect on dislocations in the early postoperative period.¹⁰ Computer-assisted surgery (CAS), such as imageless navigation, is more accurate than conventional methods for acetabular component placement,^20-23 but the clinical relevance of improving accuracy for acetabular component placement has not been shown with respect to altering patient outcomes.²³

We conducted a study in a large single-surgeon patient cohort to determine the incidence of early postoperative dislocation with target anteversion increased to 25°, approximating mean native acetabular anteversion.^16,19 In addition, we sought to determine the accuracy of imageless navigation in achieving target acetabular component placement.

Materials and Methods

After obtaining institutional review board approval for this retrospective clinical study, we reviewed 671 consecutive cases of primary THA performed by a single surgeon using an imageless CAS system (AchieveCAS; Smith & Nephew, Memphis, Tennessee) between July 2006 and October 2012. THAs were excluded if a metal-on-metal bearing surface was used, if an adequate 6-week postoperative supine anteroposterior (AP) pelvis radiograph was unavailable, or if 6-month clinical follow-up findings were not available (Figure 1). The quality of AP radiographs was deemed poor if they were not centered on the symphysis pubis and if the sacrococcygeal joint was not centered over the symphysis pubis. After exclusion criteria were applied, 553 arthroplasties (479 patients) with a mean (SD) follow-up of 2.4 (1.4) years remained. Perioperative demographic data and component sizes are listed in Table 1.

During surgery, the anterior pelvic plane, defined by the anterior-superior iliac spines and pubic tubercle, was registered with the CAS system with the patient in the supine position. THA was performed with the patient in the lateral decubitus position using a posterolateral technique. For all patients, the surgeon used a hemispherical acetabular component (R3 Acetabular System; Smith & Nephew); bearings that were either metal on highly cross-linked polyethylene (XLPE) or Oxinium (Smith & Nephew) on XLPE; and neutral XLPE acetabular inserts. The goals for acetabular inclination and anteversion were 40° and 25°, respectively, with ±10° each for the target zone. The CAS system was used to adjust target anteversion for sagittal pelvic tilt.²⁴ Uncemented femoral components were used for all patients, and the goal for femoral component anteversion was 15°. Transosseous repair of the posterior capsule and short external rotators was performed after component implantation.²⁵

On each 6-week postoperative radiograph, acetabular orientation was measured with Ein-Bild-Röntgen-Analyse (EBRA; University of Innsbruck, Austria) software, which provides a validated method for measuring acetabular inclination and anteversion on supine AP pelvis radiographs.^10,26 Pelvic boundaries were delineated with grid lines defining pelvic position. Reference points around the projections of the prosthetic femoral head, the hemispherical cup, and the rim of the cup were marked (Figure 2). EBRA calculated radiographic inclination and anteversion of the acetabular component based on the spatial position of the cup center in relation to the plane of the radiograph and the pelvic position.²⁶

Charts were reviewed to identify patients with early postoperative dislocations, as well as dislocation timing, recurrence, and other characteristics. We defined early dislocation as instability occurring within 6 months after surgery. Revision surgery for instability was also identified.

For the statistical analysis, orientation error was defined as the absolute value of the difference between target orientation (40° inclination, 25° anteversion) and radiographic measurements. Repeated-measures multiple regression with the generalized estimating equations approach was used to identify baseline patient characteristics (age, sex, BMI, primary diagnosis, laterality) associated with component positioning outside of our targeted ranges for inclination and anteversion. Fisher exact tests were used to examine the relationship between dislocation and component placement in either the Lewinnek safe zone or our targeted zone. All tests were 2-sided with a significance level of .05. All analyses were performed with SAS for Windows 9.3 (SAS Institute, Cary, North Carolina).

Results

Mean (SD) acetabular inclination was 42.2° (4.9°) (range, 27.6°-65.0°), with a mean (SD) orientation error of 4.2° (3.4°) (Figure 3A). Mean (SD) anteversion was 23.9° (6.5°) (range, 6.2°-48.0°), with a mean (SD) orientation error of 5.2° (4.1°) (Figure 3B). Components were placed outside the Lewinnek safe zone for inclination or anteversion in 46.5% of cases and outside the target zone in 17.7% of cases (Figure 4). Variation in acetabular anteversion alone accounted for 67.3% of target zone outliers (Table 2). Only 0.9% of components were placed outside the target ranges for both inclination and anteversion.

Regression analysis was performed separately for inclination and anteversion to determine the risk factors for placing the acetabular component outside the target orientation ranges. Only higher BMI was associated with malposition with respect to inclination (hazard ratio [HR], 1.059; 95% confidence interval [CI], 1.011-1.111; P = .017). Of obese patients with inclination outside the target range, 90.9% had an inclination angle of more than 50°. Associations between inclination outside the target range and age (P = .769), sex (P = .217), preoperative diagnosis (P > .99), and laterality (P = .106) were statistically insignificant. Only female sex was associated with position of the acetabular component outside the target range for anteversion (HR, 1.871; 95% CI, 1.061-3.299; P = .030). Of female patients with anteversion outside the target range, 70.0% had anteversion of less than 15°. Associations between anteversion outside the target range and age (P = .762), BMI (P = .583), preoperative diagnosis (P > .99), and laterality (P = .235) were statistically insignificant.

Six THAs (1.1%) in 6 patients experienced dislocation within 6 months after surgery (Table 3); mean (SD) time of dislocation was 58.3 (13.8) days after surgery. There was no relationship between dislocation incidence and component placement in the Lewinnek zone (P = .224) or our target zone (P = .287). Of the dislocation cases, 50% involved female patients, and 50% involved right hips. Mean (SD) age of these patients was 53.3 (7.6) years. Mean (SD) BMI was 25.4 (0.9) kg/m². Osteoarthritis was the primary diagnosis for all patients with early dislocation; 32- or 36-mm femoral heads were used in these cases. Two patients had acetabular components placed outside of our target zone. One patient, who had abnormal pelvic obliquity and sagittal tilt from scoliosis (Figures 5A, 5B), had an acetabular component placed outside both the target zone and the Lewinnek safe zone. Mean (SD) acetabular inclination was 39.8° (3.6°), and mean (SD) anteversion was 21.8° (7.3°) (Figure 5C). Two dislocations resulted from trauma, 1 dislocation was related to hyperlaxity, 1 patient had cerebral palsy, and 1 patient had no evident predisposing risk factors. Three patients (0.54%) had multiple episodes of instability requiring revision during the follow-up period.

Discussion

To our knowledge, this study represents the largest cohort of primary THAs performed with an imageless navigation system. Our results showed that increasing targeted acetabular anteversion to 25° using a posterolateral surgical approach and modern implants resulted in a 1.1% incidence of early dislocation and a 0.54% incidence of recurrent instability requiring reoperation. Of the patients with a dislocation, only 1 did not experience trauma and did not have a risk factor for dislocation. Only 1 patient with a dislocation had acetabular components positioned outside both the target zone and the Lewinnek safe zone. The acetabular component was placed within the target zone in 82.3% of cases in which the imageless navigation system was used. In our cohort, BMI was the only risk factor for placement of the acetabular component outside our target range for inclination, and sex was associated with components outside the target range for anteversion.

Early dislocation after THA is often related to improper implant orientation, inadequate restoration of offset and myofascial tension, and decreased femoral head–neck ratio.⁸ Although dislocation rates in the literature vary widely,^1,2 Medicare data suggest that the rate for the first 6 months after surgery can be as high as 4.21%.^3,4 Although use of femoral heads with a diameter of 32 mm or larger may decrease this rate to 2.14%,³ accurate acetabular component orientation helps prevent postoperative dislocation.¹⁰ Using an imageless navigation system to target 25° of anteversion and 40° of inclination resulted in an early-dislocation rate about 49% less than the rate in a Medicare population treated with similar, modern implants.³

Callanan and colleagues¹¹ found that freehand techniques were inaccurate for acetabular positioning in up to 50% of cases, and several studies have demonstrated that imageless navigation systems were more accurate than conventional guides.^20,21,27-29 Higher BMI has been implicated as a risk factor for acetabular malpositioning in several studies of the accuracy of freehand techniques¹¹ and imageless navigation techniques.^23,30 Soft-tissue impediment to the component insertion handle poses a risk of increased inclination and inadequate anteversion, regardless of method used (conventional, CAS). When the acetabular component is placed freehand in obese patients, it is difficult to judge the position of the pelvis on the operating room table. For imageless navigation, a larger amount of adipose tissue over bony landmarks may limit the accuracy of anterior pelvic plane registration.³⁰ Sex typically is not cited as a risk factor for inaccurate acetabular component positioning. We speculate that omitted-variable bias may explain the observed association between female sex and anteversion. For example, changes in postoperative pelvic tilt alter apparent anteversion on plain radiographs,^31-34 but preoperative and postoperative sagittal pelvic tilt was not recorded in this study.

The proper position of the acetabular component has been debated.^15,16,35,36 Although it is generally agreed that inclination of 40° ± 10° balances ROM, stability, and bearing-surface wear,^12,13,15,16 proposed targets for anteversion vary widely, from 0° to 40°.^35,36 Patel and colleagues¹⁶ formulated computer models based on cadaveric specimens to determine that THA impingement was minimized when the acetabular component was placed to match the native anteversion of the acetabulum.In their study model, 20° of anteversion paralleled native acetabular orientation. Tohtz and colleagues¹⁸ reviewed computed tomography scans of 144 female hips and 192 male hips and found that mean (SD) anteversion was 24.6° (6.6°) for women and 21.3° (6.2°) for men. Whether native anatomy is a valid reference for acetabular anteversion is controversial,¹⁹ and definitive recommendations for target anteversion cannot be made, as the effect of acetabular anteversion on the wear of various bearing materials is unknown.¹⁴ Yet, as with inclination, ideal anteversion is likely a compromise between maximizing impingement-free ROM and minimizing wear.

The present study had several limitations. A single-surgeon patient series was reviewed retrospectively, and there was no control group. We determined the incidence only of early dislocation, and 5.3% of THAs that were not metal-on-metal were either lost to follow-up or had inadequate radiographs. However, of the patients excluded for inadequate radiographs, none had an early dislocation. The effects of our surgical techniques on long-term outcomes, bearing wear, and dislocation are unknown. We were not able to comment on the direction of dislocation for any of the 6 patients with early dislocation, as all dislocations were reduced at facilities other than our hospital. Therefore, we cannot determine whether increasing acetabular anteversion resulted in a larger number of anterior versus posterior dislocations.¹⁵

We did not use CAS to place any of the femoral components. Therefore, we could not accurately target combined anteversion, defined as the sum of acetabular and femoral version, which may be an important determinant of THA stability.²⁸ Although restoration of femoral offset and leg length is important in preventing THA dislocation,⁸ the CAS techniques used did not influence these parameters, and they were not measured.

As an imageless navigation system was used, there were no preoperative axial images, which could have been used to assess native acetabular orientation. This limited our assessment with respect to matching each patient’s natural anteversion. Imageless navigation, which references only the anterior pelvic plane, may not be reliable in patients with excessive sagittal pelvic tilt.³⁷ Furthermore, changes in the functional position of the pelvis from supine to sitting to standing were not accounted for, and changes in sagittal tilt between these positions can be significant.³⁸ Changes in sagittal pelvic tilt affect measurement of acetabular anteversion on plain radiographs, with anterior tilt reducing apparent anteversion and posterior tilt increasing it.^32,34 Although postoperative computed tomography is the gold standard for assessing acetabular component orientation, EBRA significantly reduces errors of measurement on plain radiographs.¹⁰ Some variability in measured anteversion may be explained by our surgical technique. In particular, if the cup was uncovered anteriorly, additional anteversion was usually accepted during surgery to minimize anterior impingement and limit the risk of iliopsoas tendonitis.^16,39

Our study results suggested that increasing target acetabular anteversion to 25° may reduce the incidence of early postoperative instability relative to rates reported in the literature. Despite the higher accuracy of component placement with an imageless navigation system, dislocations occurred in patients with acetabular components positioned in our target zone and in the historical safe zone. These dislocations support the notion that there likely is no absolute safe range for acetabular component positioning, as THA stability depends on many factors. Ideal targets for implant orientation for acetabulum and femur may be patient-specific.^16,19 Investigators should prospectively evaluate patient-specific THA component positioning and determine its effect on postoperative dislocation and bearing-surface wear. As specific implant targets are further defined, tools that are more precise and accurate than conventional techniques will be needed to achieve goal component positioning. Our study results confirmed that imageless navigation is an accurate method for achieving acetabular orientation targets.

Postoperative dislocation remains a common complication of primary total hip arthroplasties (THAs), affecting less than 1% to more than 10% in reported series.^1,2 In large datasets for modern implants, the incidence of dislocation is 2% to 4%.^3,4 Given that more than 200,000 THAs are performed in the United States each year,⁵ these low percentages represent a large number of patients. The multiplex patient variables that affect THA stability include age, sex, body mass index (BMI), and comorbid conditions.^6-8 Surgical approach, restoration of leg length and femoral offset, femoral head size, and component positioning are also important surgical factors that can increase or decrease the incidence of dislocation.^3,8,9 In particular, appropriate acetabular component orientation is crucial; surgeons can control this factor and thereby limit the occurrence of dislocation.¹⁰ Furthermore, acetabular malpositioning can increase the risk of liner fractures and accelerate bearing-surface wear.^11-14

To minimize the risk of postoperative dislocation, surgeons traditionally have targeted the Lewinnek safe zone, with its mean (SD) inclination of 40° (10°) and mean (SD) anteversion of 15° (10°), for acetabular component orientation.¹⁵ However, the applicability of this target zone to preventing hip instability using modern implant designs, components, and surgical techniques remains unknown. Achieving acetabular orientation based on maximizing range of motion (ROM) before impingement may be optimal, with anteversion from 20° to 30° and inclination from 40° to 45°.^16,17 Furthermore, mean (SD) native acetabular anteversion ranges from 21.3° (6.2°) for men to 24.6° (6.6°) for women.¹⁸ Placing THA acetabular components near the native range for anteversion may best provide impingement-free ROM and thus optimize THA stability,^16,19 but this has not been proved in a clinical study.

Early dislocation is typically classified as occurring within 6 months after surgery,⁹ with almost 80% of dislocations occurring within 3 months after surgery.¹⁰ Surgeon-specific factors, such as acetabular component positioning, are thought to have a predominant effect on dislocations in the early postoperative period.¹⁰ Computer-assisted surgery (CAS), such as imageless navigation, is more accurate than conventional methods for acetabular component placement,^20-23 but the clinical relevance of improving accuracy for acetabular component placement has not been shown with respect to altering patient outcomes.²³

We conducted a study in a large single-surgeon patient cohort to determine the incidence of early postoperative dislocation with target anteversion increased to 25°, approximating mean native acetabular anteversion.^16,19 In addition, we sought to determine the accuracy of imageless navigation in achieving target acetabular component placement.

Materials and Methods

After obtaining institutional review board approval for this retrospective clinical study, we reviewed 671 consecutive cases of primary THA performed by a single surgeon using an imageless CAS system (AchieveCAS; Smith & Nephew, Memphis, Tennessee) between July 2006 and October 2012. THAs were excluded if a metal-on-metal bearing surface was used, if an adequate 6-week postoperative supine anteroposterior (AP) pelvis radiograph was unavailable, or if 6-month clinical follow-up findings were not available (Figure 1). The quality of AP radiographs was deemed poor if they were not centered on the symphysis pubis and if the sacrococcygeal joint was not centered over the symphysis pubis. After exclusion criteria were applied, 553 arthroplasties (479 patients) with a mean (SD) follow-up of 2.4 (1.4) years remained. Perioperative demographic data and component sizes are listed in Table 1.

During surgery, the anterior pelvic plane, defined by the anterior-superior iliac spines and pubic tubercle, was registered with the CAS system with the patient in the supine position. THA was performed with the patient in the lateral decubitus position using a posterolateral technique. For all patients, the surgeon used a hemispherical acetabular component (R3 Acetabular System; Smith & Nephew); bearings that were either metal on highly cross-linked polyethylene (XLPE) or Oxinium (Smith & Nephew) on XLPE; and neutral XLPE acetabular inserts. The goals for acetabular inclination and anteversion were 40° and 25°, respectively, with ±10° each for the target zone. The CAS system was used to adjust target anteversion for sagittal pelvic tilt.²⁴ Uncemented femoral components were used for all patients, and the goal for femoral component anteversion was 15°. Transosseous repair of the posterior capsule and short external rotators was performed after component implantation.²⁵

On each 6-week postoperative radiograph, acetabular orientation was measured with Ein-Bild-Röntgen-Analyse (EBRA; University of Innsbruck, Austria) software, which provides a validated method for measuring acetabular inclination and anteversion on supine AP pelvis radiographs.^10,26 Pelvic boundaries were delineated with grid lines defining pelvic position. Reference points around the projections of the prosthetic femoral head, the hemispherical cup, and the rim of the cup were marked (Figure 2). EBRA calculated radiographic inclination and anteversion of the acetabular component based on the spatial position of the cup center in relation to the plane of the radiograph and the pelvic position.²⁶

Charts were reviewed to identify patients with early postoperative dislocations, as well as dislocation timing, recurrence, and other characteristics. We defined early dislocation as instability occurring within 6 months after surgery. Revision surgery for instability was also identified.

For the statistical analysis, orientation error was defined as the absolute value of the difference between target orientation (40° inclination, 25° anteversion) and radiographic measurements. Repeated-measures multiple regression with the generalized estimating equations approach was used to identify baseline patient characteristics (age, sex, BMI, primary diagnosis, laterality) associated with component positioning outside of our targeted ranges for inclination and anteversion. Fisher exact tests were used to examine the relationship between dislocation and component placement in either the Lewinnek safe zone or our targeted zone. All tests were 2-sided with a significance level of .05. All analyses were performed with SAS for Windows 9.3 (SAS Institute, Cary, North Carolina).

Results

Mean (SD) acetabular inclination was 42.2° (4.9°) (range, 27.6°-65.0°), with a mean (SD) orientation error of 4.2° (3.4°) (Figure 3A). Mean (SD) anteversion was 23.9° (6.5°) (range, 6.2°-48.0°), with a mean (SD) orientation error of 5.2° (4.1°) (Figure 3B). Components were placed outside the Lewinnek safe zone for inclination or anteversion in 46.5% of cases and outside the target zone in 17.7% of cases (Figure 4). Variation in acetabular anteversion alone accounted for 67.3% of target zone outliers (Table 2). Only 0.9% of components were placed outside the target ranges for both inclination and anteversion.

Regression analysis was performed separately for inclination and anteversion to determine the risk factors for placing the acetabular component outside the target orientation ranges. Only higher BMI was associated with malposition with respect to inclination (hazard ratio [HR], 1.059; 95% confidence interval [CI], 1.011-1.111; P = .017). Of obese patients with inclination outside the target range, 90.9% had an inclination angle of more than 50°. Associations between inclination outside the target range and age (P = .769), sex (P = .217), preoperative diagnosis (P > .99), and laterality (P = .106) were statistically insignificant. Only female sex was associated with position of the acetabular component outside the target range for anteversion (HR, 1.871; 95% CI, 1.061-3.299; P = .030). Of female patients with anteversion outside the target range, 70.0% had anteversion of less than 15°. Associations between anteversion outside the target range and age (P = .762), BMI (P = .583), preoperative diagnosis (P > .99), and laterality (P = .235) were statistically insignificant.

Six THAs (1.1%) in 6 patients experienced dislocation within 6 months after surgery (Table 3); mean (SD) time of dislocation was 58.3 (13.8) days after surgery. There was no relationship between dislocation incidence and component placement in the Lewinnek zone (P = .224) or our target zone (P = .287). Of the dislocation cases, 50% involved female patients, and 50% involved right hips. Mean (SD) age of these patients was 53.3 (7.6) years. Mean (SD) BMI was 25.4 (0.9) kg/m². Osteoarthritis was the primary diagnosis for all patients with early dislocation; 32- or 36-mm femoral heads were used in these cases. Two patients had acetabular components placed outside of our target zone. One patient, who had abnormal pelvic obliquity and sagittal tilt from scoliosis (Figures 5A, 5B), had an acetabular component placed outside both the target zone and the Lewinnek safe zone. Mean (SD) acetabular inclination was 39.8° (3.6°), and mean (SD) anteversion was 21.8° (7.3°) (Figure 5C). Two dislocations resulted from trauma, 1 dislocation was related to hyperlaxity, 1 patient had cerebral palsy, and 1 patient had no evident predisposing risk factors. Three patients (0.54%) had multiple episodes of instability requiring revision during the follow-up period.

Discussion

To our knowledge, this study represents the largest cohort of primary THAs performed with an imageless navigation system. Our results showed that increasing targeted acetabular anteversion to 25° using a posterolateral surgical approach and modern implants resulted in a 1.1% incidence of early dislocation and a 0.54% incidence of recurrent instability requiring reoperation. Of the patients with a dislocation, only 1 did not experience trauma and did not have a risk factor for dislocation. Only 1 patient with a dislocation had acetabular components positioned outside both the target zone and the Lewinnek safe zone. The acetabular component was placed within the target zone in 82.3% of cases in which the imageless navigation system was used. In our cohort, BMI was the only risk factor for placement of the acetabular component outside our target range for inclination, and sex was associated with components outside the target range for anteversion.

Early dislocation after THA is often related to improper implant orientation, inadequate restoration of offset and myofascial tension, and decreased femoral head–neck ratio.⁸ Although dislocation rates in the literature vary widely,^1,2 Medicare data suggest that the rate for the first 6 months after surgery can be as high as 4.21%.^3,4 Although use of femoral heads with a diameter of 32 mm or larger may decrease this rate to 2.14%,³ accurate acetabular component orientation helps prevent postoperative dislocation.¹⁰ Using an imageless navigation system to target 25° of anteversion and 40° of inclination resulted in an early-dislocation rate about 49% less than the rate in a Medicare population treated with similar, modern implants.³

Callanan and colleagues¹¹ found that freehand techniques were inaccurate for acetabular positioning in up to 50% of cases, and several studies have demonstrated that imageless navigation systems were more accurate than conventional guides.^20,21,27-29 Higher BMI has been implicated as a risk factor for acetabular malpositioning in several studies of the accuracy of freehand techniques¹¹ and imageless navigation techniques.^23,30 Soft-tissue impediment to the component insertion handle poses a risk of increased inclination and inadequate anteversion, regardless of method used (conventional, CAS). When the acetabular component is placed freehand in obese patients, it is difficult to judge the position of the pelvis on the operating room table. For imageless navigation, a larger amount of adipose tissue over bony landmarks may limit the accuracy of anterior pelvic plane registration.³⁰ Sex typically is not cited as a risk factor for inaccurate acetabular component positioning. We speculate that omitted-variable bias may explain the observed association between female sex and anteversion. For example, changes in postoperative pelvic tilt alter apparent anteversion on plain radiographs,^31-34 but preoperative and postoperative sagittal pelvic tilt was not recorded in this study.

The proper position of the acetabular component has been debated.^15,16,35,36 Although it is generally agreed that inclination of 40° ± 10° balances ROM, stability, and bearing-surface wear,^12,13,15,16 proposed targets for anteversion vary widely, from 0° to 40°.^35,36 Patel and colleagues¹⁶ formulated computer models based on cadaveric specimens to determine that THA impingement was minimized when the acetabular component was placed to match the native anteversion of the acetabulum.In their study model, 20° of anteversion paralleled native acetabular orientation. Tohtz and colleagues¹⁸ reviewed computed tomography scans of 144 female hips and 192 male hips and found that mean (SD) anteversion was 24.6° (6.6°) for women and 21.3° (6.2°) for men. Whether native anatomy is a valid reference for acetabular anteversion is controversial,¹⁹ and definitive recommendations for target anteversion cannot be made, as the effect of acetabular anteversion on the wear of various bearing materials is unknown.¹⁴ Yet, as with inclination, ideal anteversion is likely a compromise between maximizing impingement-free ROM and minimizing wear.

The present study had several limitations. A single-surgeon patient series was reviewed retrospectively, and there was no control group. We determined the incidence only of early dislocation, and 5.3% of THAs that were not metal-on-metal were either lost to follow-up or had inadequate radiographs. However, of the patients excluded for inadequate radiographs, none had an early dislocation. The effects of our surgical techniques on long-term outcomes, bearing wear, and dislocation are unknown. We were not able to comment on the direction of dislocation for any of the 6 patients with early dislocation, as all dislocations were reduced at facilities other than our hospital. Therefore, we cannot determine whether increasing acetabular anteversion resulted in a larger number of anterior versus posterior dislocations.¹⁵

We did not use CAS to place any of the femoral components. Therefore, we could not accurately target combined anteversion, defined as the sum of acetabular and femoral version, which may be an important determinant of THA stability.²⁸ Although restoration of femoral offset and leg length is important in preventing THA dislocation,⁸ the CAS techniques used did not influence these parameters, and they were not measured.

As an imageless navigation system was used, there were no preoperative axial images, which could have been used to assess native acetabular orientation. This limited our assessment with respect to matching each patient’s natural anteversion. Imageless navigation, which references only the anterior pelvic plane, may not be reliable in patients with excessive sagittal pelvic tilt.³⁷ Furthermore, changes in the functional position of the pelvis from supine to sitting to standing were not accounted for, and changes in sagittal tilt between these positions can be significant.³⁸ Changes in sagittal pelvic tilt affect measurement of acetabular anteversion on plain radiographs, with anterior tilt reducing apparent anteversion and posterior tilt increasing it.^32,34 Although postoperative computed tomography is the gold standard for assessing acetabular component orientation, EBRA significantly reduces errors of measurement on plain radiographs.¹⁰ Some variability in measured anteversion may be explained by our surgical technique. In particular, if the cup was uncovered anteriorly, additional anteversion was usually accepted during surgery to minimize anterior impingement and limit the risk of iliopsoas tendonitis.^16,39

Our study results suggested that increasing target acetabular anteversion to 25° may reduce the incidence of early postoperative instability relative to rates reported in the literature. Despite the higher accuracy of component placement with an imageless navigation system, dislocations occurred in patients with acetabular components positioned in our target zone and in the historical safe zone. These dislocations support the notion that there likely is no absolute safe range for acetabular component positioning, as THA stability depends on many factors. Ideal targets for implant orientation for acetabulum and femur may be patient-specific.^16,19 Investigators should prospectively evaluate patient-specific THA component positioning and determine its effect on postoperative dislocation and bearing-surface wear. As specific implant targets are further defined, tools that are more precise and accurate than conventional techniques will be needed to achieve goal component positioning. Our study results confirmed that imageless navigation is an accurate method for achieving acetabular orientation targets.

Total hip arthroplasty (THA) effectively provides adequate pain relief and favorable outcomes in patients with hip osteoarthritis (OA). However, leg-length discrepancy (LLD) is still a significant cause of morbidity,¹ including nerve damage,^2,3 low back pain,^2,4,5 and abnormal gait.^2,6,7 Although most of the LLD values reported in the literature fall under the acceptable threshold of 10 mm,⁸ some patients report dissatisfaction,⁹ leading to litigation against orthopedic surgeons.² However, lower extremity lengthening is sometimes needed to achieve adequate hip joint stability and prevent dislocations.^2,10

Several methods have been developed to help surgeons estimate the change in leg length during surgery in an attempt to improve clinical outcomes. Use of guide pins as a reference on the pelvis decreased LLD and improved outcomes in some published studies.^11,12 Preoperative templating of implant size, cup position, and level of femoral neck cut is very important in helping minimize clinically significant LLD after THA.^2,13,14 Computer-assisted THA has also been introduced to try to improve component positioning, restoration of hip center of rotation, and minimizing of LLD.^15-17 However, cost and increased operative time have prevented widespread adoption of computer-assisted surgery in THA.

Proponents of different surgical approaches have argued about the superiority of one approach over another. The posterior approach is the gold standard in THA because it is safe, easy to perform, and, if needed, extensile.¹¹ However, exact determination of the intraoperative 3-dimensional (3-D) orientation of the pelvis, and subsequently of LLD, is challenging when the patient lies in the lateral position. The anterior approach has gained in popularity because of its advantages in accelerating postoperative rehabilitation and decreasing hospital length of stay.¹⁸ Placing the patient supine is advantageous because it allows leveling of the pelvis and estimation of LLD (by comparing the positions of the lower extremities).¹⁹ The anterior approach also allows for radiographic measurements on the operating table.^19,20 However, this approach has a high learning curve²¹ and is not extensile.²¹ To date, no study has shown superiority of the anterior approach over either the conventional posterior approach or the robot-assisted posterior approach in minimizing LLD after THA.

We conducted a study to compare LLD in patients who underwent THA performed with a robot-assisted posterior approach (RTHA), a fluoroscopy-guided anterior approach (ATHA), or a conventional posterior approach (PTHA). We hypothesized that, compared with PTHA, both RTHA and ATHA would result in reduced LLD.

Materials and Methods

We reviewed all RTHAs, ATHAs, and PTHAs performed by Dr. Domb between September 2008 and December 2012. Study inclusion criteria were a diagnosis of hip OA and the availability of postoperative supine anteroposterior pelvis radiographs. Exclusion criteria were a diagnosis other than hip OA, missing or improper postoperative radiographs (radiographs with rotated or tilted pelvis),²² and radiographs on which at least one of the lesser trochanters was difficult to define. Of the 155 cases included in the study, 67 were RTHAs, 29 were ATHAs, and 59 were PTHAs.

All patients scheduled for THA underwent preoperative planning; plain radiographs were used to determine component size and position, level of neck cut, and amount of leg lengthening or shortening needed. In all RTHA cases, computed tomography of the involved hip was performed before surgery. The MAKO system (MAKO Surgical Corporation, Davie, Florida) was used to develop a patient-specific 3-D model of the pelvis and proximal femur, and this model was used to guide THA execution. The system was then used to detect patient-specific landmarks during surgery, to register the femur and the acetabulum, and to help determine the position of the pelvis and proximal femur during surgery. This system, which uses a haptic robotic arm that guides acetabular reaming and cup placement, provides feedback regarding cup placement, stem version, leg length, and global offset. Pelvic tilt and rotation were accounted for by the MAKO software, and all provided measurements were made on the coronal (functional) plane of the body, as described by Murray.²³ ATHA was performed with the patient in the supine position on a Hana table (Mizuho OSI, Union City, California) with fluoroscopic guidance. PTHA was performed in the conventional way, with the patient in the lateral position.

Radiographic measurements of LLD were made with TraumaCad software (Build 2.2.535.0; Voyant Health, Petah-Tikva, Israel). The accuracy of this software has been studied and reported in the literature.^24-26 Radiographs were calibrated using the known size of each femoral head as a marker. The reference on the pelvis was the interobturator line (line tangent to inferior border of obturator foramina), and the reference on the femurs was the most superior and medial aspect of each lesser trochanter. Two lines were drawn, each perpendicular to the interobturator line, starting from the previously defined reference point on each lesser trochanter. The difference in length between these 2 lines was recorded as the LLD. Values were recorded relative to the operative extremity. For example, if the operative extremity was longer than the nonoperative extremity, the LLD was given a positive value.

To eliminate bias and increase measurement accuracy, the study had each of 2 observers collect the LLD data twice, 2 months apart. These observers were blinded to each other’s results and to the type of surgery performed. (Neither observer was Dr. Domb, the senior surgeon.) IBM SPSS Statistics software (Version 20; IBM, Armonk, New York) was used for statistical analysis. Each patient’s 4 measurements were averaged into a single number for LLD, and the absolute LLD values were used in all statistical analyses. Means, standard deviations (SDs), and 95% confidence intervals (CIs) were calculated for LLD in each of the 3 groups. Pearson correlation coefficient was used to determine interobserver and intraobserver reliability. One-way analysis of variance (ANOVA) was used to compare group means for age, body mass index (BMI), and LLD. In each group, number of outliers was determined with outliers set at LLDs of more than 3 mm and more than 5 mm. Fischer exact test was used to compare number of outliers in each group. P < .05 was considered statistically significant.

Results

Table 1 lists the demographic data, including age, sex, and BMI, and compares the means. There were strong interobserver and intraobserver correlations for all LLD measurements (r > 0.9; P < .001). Mean (SD) LLD was 2.7 (1.8) mm (95% CI, 2.3-3.2) in the RTHA group, 1.8 (1.6) mm (95% CI, 1.2-2.4) in the ATHA group, and 1.9 (1.6) mm (95% CI, 1.5-2.4) in the PTHA group (P = .01). When LLD of more than 3 mm was set as an outlier, percentage of outliers was 37.3% (RTHA), 17.2% (ATHA), and 22% (PTHA) (P = .06-.78). When LLD of more than 5 mm was set as an outlier, percentage of outliers was 10.4% (RTHA), 6.9% (ATHA), and 8.5% (PTHA) (P = .72 to >.99). No patient in any group had LLD of 10 mm or more (Figure). Table 2 lists percentages of patients’ operated extremities that were longer, shorter, or the same size as their contralateral extremities. Six (9.0%) of the 67 RTHA patients, 4 (13.8%) of the 29 ATHA patients, and 3 (5.1%) of the 59 PTHA patients had a contralateral THA.

Discussion

Our study results showed that RTHA, ATHA, and PTHA were equally effective in minimizing LLD. There was a statistically significant difference in mean LLD among the 3 groups studied. The RTHA group had the largest mean (SD) LLD: 2.7 (1.8) mm. However, statistically significant differences do not always indicate clinical significance.²⁷ Therefore, comparison of the 3 groups’ means is not enough for drawing significant conclusions. The more important point to consider is the number of cases of LLD of 10 mm or more—a discrepancy that would be perceptible to patients and thus become a source of dissatisfaction with painless THA.²⁸ Patients perceive LLD when shortening exceeds 10 mm and lengthening exceeds 6 mm,²⁹ or when LLD is more than 10 mm.^16,19,20 Despite significant differences in means, all our cases came in under the 10-mm threshold. When the threshold was decreased to 5 mm (and to 3 mm), there was no statistically significant difference among the groups in the number of cases above the threshold.

LLD remains a source of significant post-THA comorbidity and patient dissatisfaction.^1-7,19 Despite surgeons’ efforts to minimize LLD, some patients can detect even a subtle LLD after surgery.^1,8,29 Most LLD values reported in the literature fall under the 10-mm threshold.^16,19,20 In some cases, however, postoperative LLD is more than 1 cm, enough to prompt litigation against orthopedic surgeons.² Surgeons have tried to improve LLD with use of multiple techniques, including use of intraoperative measuring devices,³⁰ patient positioning during surgery,²⁰ use of computer-assisted surgery,¹⁹ and use of intraoperative fluoroscopy.²⁰

Proponents of computer-assisted THA have argued that this technique improves accuracy in placing the acetabular cup in the safe zone,³¹ minimizes LLD, and restores femoral offset.^32,33 Manzotti and colleagues¹⁶ reported on 48 cases of computer-assisted THA matched to 48 cases of conventional THA using the posterior approach. Mean (SD) LLD was 5.06 (2.99) mm in the computer-assisted group and 7.64 (4.36) mm in the conventional group; there was a statistically significant difference in favor of the computer-assisted group (P = .04). However, 5 patients in the computer-assisted group and 13 in the conventional group had LLD of more than 10 mm, and the difference was statistically significant.¹⁶ Moreover, the study population was heterogeneous, with 12 patients in both groups having developmental dysplasia as a primary diagnosis.¹⁶ All the cases in our study had a diagnosis of OA, and no case had LLD of 10 mm or more.

Several advantages have been proposed for the anterior approach. The supine position (with direct comparison of leg lengths) and the use of fluoroscopy have been described as advantageous in minimizing LLD.^20,21 In their study of 494 primary THAs performed with the anterior approach, Matta and colleagues²⁰ reported mean (SD) postoperative LLD of 3 (2) mm (range, 0-26 mm) and concluded that the anterior approach was effective in restoring leg lengths and ensuring proper cup placement while not increasing the dislocation rate. However, they did not compare this approach with others or with computer-assisted THA with respect to LLD.

In another study, Nam and colleagues¹⁹ compared LLD after THA performed with 3 different approaches (anterior, conventional posterior, posterior-navigated) and found no statistically significant difference in LLD among the groups. However, LLD was more than 10 mm in 2.2% of anterior cases, 4.4% of conventional posterior cases, and 4.4% of posterior-navigated cases. When 5 mm was used as a cutoff, percentage of patients who were outliers was 31.1% (anterior), 20% (conventional posterior), and 23.3% (navigated-posterior). Our data showed superior results in using 5 mm as a cutoff, with percentage of outliers of 6.9% with ATHA, 8.5% with PTHA, and 10.4% with RTHA. However, Nam and colleagues¹⁹ used a larger patient cohort and different techniques for measuring LLD on anteroposterior pelvis radiographs.

The most likely reason that the groups in our study were comparable in terms of LLD accuracy and lack of outliers over the 10-mm cutoff was Dr. Domb’s high accuracy in minimizing LLD using each of the 3 techniques. For ATHA, mean (SD) LLD was 1.8 (1.6) mm (no LLD of ≥10 mm), better than the 3 (2) mm (0.9% with LLD of >10 mm) reported by Matta and colleagues²⁰ and the 3.8 (3.9) mm (2.2% with LLD of >10 mm) reported by Nam and colleagues.¹⁹ For PTHA, mean (SD) LLD was 1.9 (1.6) mm (no LLD of ≥10 mm), comparable to some of the best results reported in the literature—for example, the 1 mm (3% with LLD of >10 mm) reported by Woolson and colleagues.³⁴ For RTHA, mean (SD) LLD was 2.7 (1.8) mm (no LLD of ≥10 mm), superior to the 3.9 (2.7) mm (4.4% with LLD of >10 mm) reported by Nam and colleagues¹⁹ for posterior-navigated THA and the 5.06 (2.99) mm (10.4% with LLD of >10 mm) reported by Manzotti and colleagues¹⁶ for computer-assisted THA.

This study had several notable strengths. All patients had a diagnosis of hip OA and were operated on by a single surgeon. Radiographs were calibrated using the size of the implanted femoral head. Radiographic data were measured using the same technique in all cases and were collected twice by 2 observers (not the senior surgeon) to decrease bias and determine interobserver and intraobserver reliability. In addition, surgeon experience might have played an important role in minimizing LLD regardless of technique and approach used for THA.

Study limitations were different number of cases in each group, lack of matching, lack of clinical follow-up, and lack of long-term assessment of clinical outcomes and complications.

Conclusion

As performed by an experienced surgeon, RTHA, ATHA, and PTHA did not differ in obtaining minimal LLD. All 3 groups had a low frequency of outliers, using thresholds of 3 mm and 5 mm, and no patient in any group had LLD of 10 mm or more. All 3 techniques are effective in achieving accuracy in LLD.

Total hip arthroplasty (THA) effectively provides adequate pain relief and favorable outcomes in patients with hip osteoarthritis (OA). However, leg-length discrepancy (LLD) is still a significant cause of morbidity,¹ including nerve damage,^2,3 low back pain,^2,4,5 and abnormal gait.^2,6,7 Although most of the LLD values reported in the literature fall under the acceptable threshold of 10 mm,⁸ some patients report dissatisfaction,⁹ leading to litigation against orthopedic surgeons.² However, lower extremity lengthening is sometimes needed to achieve adequate hip joint stability and prevent dislocations.^2,10

Several methods have been developed to help surgeons estimate the change in leg length during surgery in an attempt to improve clinical outcomes. Use of guide pins as a reference on the pelvis decreased LLD and improved outcomes in some published studies.^11,12 Preoperative templating of implant size, cup position, and level of femoral neck cut is very important in helping minimize clinically significant LLD after THA.^2,13,14 Computer-assisted THA has also been introduced to try to improve component positioning, restoration of hip center of rotation, and minimizing of LLD.^15-17 However, cost and increased operative time have prevented widespread adoption of computer-assisted surgery in THA.

Proponents of different surgical approaches have argued about the superiority of one approach over another. The posterior approach is the gold standard in THA because it is safe, easy to perform, and, if needed, extensile.¹¹ However, exact determination of the intraoperative 3-dimensional (3-D) orientation of the pelvis, and subsequently of LLD, is challenging when the patient lies in the lateral position. The anterior approach has gained in popularity because of its advantages in accelerating postoperative rehabilitation and decreasing hospital length of stay.¹⁸ Placing the patient supine is advantageous because it allows leveling of the pelvis and estimation of LLD (by comparing the positions of the lower extremities).¹⁹ The anterior approach also allows for radiographic measurements on the operating table.^19,20 However, this approach has a high learning curve²¹ and is not extensile.²¹ To date, no study has shown superiority of the anterior approach over either the conventional posterior approach or the robot-assisted posterior approach in minimizing LLD after THA.

We conducted a study to compare LLD in patients who underwent THA performed with a robot-assisted posterior approach (RTHA), a fluoroscopy-guided anterior approach (ATHA), or a conventional posterior approach (PTHA). We hypothesized that, compared with PTHA, both RTHA and ATHA would result in reduced LLD.

Materials and Methods

We reviewed all RTHAs, ATHAs, and PTHAs performed by Dr. Domb between September 2008 and December 2012. Study inclusion criteria were a diagnosis of hip OA and the availability of postoperative supine anteroposterior pelvis radiographs. Exclusion criteria were a diagnosis other than hip OA, missing or improper postoperative radiographs (radiographs with rotated or tilted pelvis),²² and radiographs on which at least one of the lesser trochanters was difficult to define. Of the 155 cases included in the study, 67 were RTHAs, 29 were ATHAs, and 59 were PTHAs.

All patients scheduled for THA underwent preoperative planning; plain radiographs were used to determine component size and position, level of neck cut, and amount of leg lengthening or shortening needed. In all RTHA cases, computed tomography of the involved hip was performed before surgery. The MAKO system (MAKO Surgical Corporation, Davie, Florida) was used to develop a patient-specific 3-D model of the pelvis and proximal femur, and this model was used to guide THA execution. The system was then used to detect patient-specific landmarks during surgery, to register the femur and the acetabulum, and to help determine the position of the pelvis and proximal femur during surgery. This system, which uses a haptic robotic arm that guides acetabular reaming and cup placement, provides feedback regarding cup placement, stem version, leg length, and global offset. Pelvic tilt and rotation were accounted for by the MAKO software, and all provided measurements were made on the coronal (functional) plane of the body, as described by Murray.²³ ATHA was performed with the patient in the supine position on a Hana table (Mizuho OSI, Union City, California) with fluoroscopic guidance. PTHA was performed in the conventional way, with the patient in the lateral position.

Radiographic measurements of LLD were made with TraumaCad software (Build 2.2.535.0; Voyant Health, Petah-Tikva, Israel). The accuracy of this software has been studied and reported in the literature.^24-26 Radiographs were calibrated using the known size of each femoral head as a marker. The reference on the pelvis was the interobturator line (line tangent to inferior border of obturator foramina), and the reference on the femurs was the most superior and medial aspect of each lesser trochanter. Two lines were drawn, each perpendicular to the interobturator line, starting from the previously defined reference point on each lesser trochanter. The difference in length between these 2 lines was recorded as the LLD. Values were recorded relative to the operative extremity. For example, if the operative extremity was longer than the nonoperative extremity, the LLD was given a positive value.

To eliminate bias and increase measurement accuracy, the study had each of 2 observers collect the LLD data twice, 2 months apart. These observers were blinded to each other’s results and to the type of surgery performed. (Neither observer was Dr. Domb, the senior surgeon.) IBM SPSS Statistics software (Version 20; IBM, Armonk, New York) was used for statistical analysis. Each patient’s 4 measurements were averaged into a single number for LLD, and the absolute LLD values were used in all statistical analyses. Means, standard deviations (SDs), and 95% confidence intervals (CIs) were calculated for LLD in each of the 3 groups. Pearson correlation coefficient was used to determine interobserver and intraobserver reliability. One-way analysis of variance (ANOVA) was used to compare group means for age, body mass index (BMI), and LLD. In each group, number of outliers was determined with outliers set at LLDs of more than 3 mm and more than 5 mm. Fischer exact test was used to compare number of outliers in each group. P < .05 was considered statistically significant.

Results

Table 1 lists the demographic data, including age, sex, and BMI, and compares the means. There were strong interobserver and intraobserver correlations for all LLD measurements (r > 0.9; P < .001). Mean (SD) LLD was 2.7 (1.8) mm (95% CI, 2.3-3.2) in the RTHA group, 1.8 (1.6) mm (95% CI, 1.2-2.4) in the ATHA group, and 1.9 (1.6) mm (95% CI, 1.5-2.4) in the PTHA group (P = .01). When LLD of more than 3 mm was set as an outlier, percentage of outliers was 37.3% (RTHA), 17.2% (ATHA), and 22% (PTHA) (P = .06-.78). When LLD of more than 5 mm was set as an outlier, percentage of outliers was 10.4% (RTHA), 6.9% (ATHA), and 8.5% (PTHA) (P = .72 to >.99). No patient in any group had LLD of 10 mm or more (Figure). Table 2 lists percentages of patients’ operated extremities that were longer, shorter, or the same size as their contralateral extremities. Six (9.0%) of the 67 RTHA patients, 4 (13.8%) of the 29 ATHA patients, and 3 (5.1%) of the 59 PTHA patients had a contralateral THA.

Discussion

Our study results showed that RTHA, ATHA, and PTHA were equally effective in minimizing LLD. There was a statistically significant difference in mean LLD among the 3 groups studied. The RTHA group had the largest mean (SD) LLD: 2.7 (1.8) mm. However, statistically significant differences do not always indicate clinical significance.²⁷ Therefore, comparison of the 3 groups’ means is not enough for drawing significant conclusions. The more important point to consider is the number of cases of LLD of 10 mm or more—a discrepancy that would be perceptible to patients and thus become a source of dissatisfaction with painless THA.²⁸ Patients perceive LLD when shortening exceeds 10 mm and lengthening exceeds 6 mm,²⁹ or when LLD is more than 10 mm.^16,19,20 Despite significant differences in means, all our cases came in under the 10-mm threshold. When the threshold was decreased to 5 mm (and to 3 mm), there was no statistically significant difference among the groups in the number of cases above the threshold.

LLD remains a source of significant post-THA comorbidity and patient dissatisfaction.^1-7,19 Despite surgeons’ efforts to minimize LLD, some patients can detect even a subtle LLD after surgery.^1,8,29 Most LLD values reported in the literature fall under the 10-mm threshold.^16,19,20 In some cases, however, postoperative LLD is more than 1 cm, enough to prompt litigation against orthopedic surgeons.² Surgeons have tried to improve LLD with use of multiple techniques, including use of intraoperative measuring devices,³⁰ patient positioning during surgery,²⁰ use of computer-assisted surgery,¹⁹ and use of intraoperative fluoroscopy.²⁰

Proponents of computer-assisted THA have argued that this technique improves accuracy in placing the acetabular cup in the safe zone,³¹ minimizes LLD, and restores femoral offset.^32,33 Manzotti and colleagues¹⁶ reported on 48 cases of computer-assisted THA matched to 48 cases of conventional THA using the posterior approach. Mean (SD) LLD was 5.06 (2.99) mm in the computer-assisted group and 7.64 (4.36) mm in the conventional group; there was a statistically significant difference in favor of the computer-assisted group (P = .04). However, 5 patients in the computer-assisted group and 13 in the conventional group had LLD of more than 10 mm, and the difference was statistically significant.¹⁶ Moreover, the study population was heterogeneous, with 12 patients in both groups having developmental dysplasia as a primary diagnosis.¹⁶ All the cases in our study had a diagnosis of OA, and no case had LLD of 10 mm or more.

Several advantages have been proposed for the anterior approach. The supine position (with direct comparison of leg lengths) and the use of fluoroscopy have been described as advantageous in minimizing LLD.^20,21 In their study of 494 primary THAs performed with the anterior approach, Matta and colleagues²⁰ reported mean (SD) postoperative LLD of 3 (2) mm (range, 0-26 mm) and concluded that the anterior approach was effective in restoring leg lengths and ensuring proper cup placement while not increasing the dislocation rate. However, they did not compare this approach with others or with computer-assisted THA with respect to LLD.

In another study, Nam and colleagues¹⁹ compared LLD after THA performed with 3 different approaches (anterior, conventional posterior, posterior-navigated) and found no statistically significant difference in LLD among the groups. However, LLD was more than 10 mm in 2.2% of anterior cases, 4.4% of conventional posterior cases, and 4.4% of posterior-navigated cases. When 5 mm was used as a cutoff, percentage of patients who were outliers was 31.1% (anterior), 20% (conventional posterior), and 23.3% (navigated-posterior). Our data showed superior results in using 5 mm as a cutoff, with percentage of outliers of 6.9% with ATHA, 8.5% with PTHA, and 10.4% with RTHA. However, Nam and colleagues¹⁹ used a larger patient cohort and different techniques for measuring LLD on anteroposterior pelvis radiographs.

The most likely reason that the groups in our study were comparable in terms of LLD accuracy and lack of outliers over the 10-mm cutoff was Dr. Domb’s high accuracy in minimizing LLD using each of the 3 techniques. For ATHA, mean (SD) LLD was 1.8 (1.6) mm (no LLD of ≥10 mm), better than the 3 (2) mm (0.9% with LLD of >10 mm) reported by Matta and colleagues²⁰ and the 3.8 (3.9) mm (2.2% with LLD of >10 mm) reported by Nam and colleagues.¹⁹ For PTHA, mean (SD) LLD was 1.9 (1.6) mm (no LLD of ≥10 mm), comparable to some of the best results reported in the literature—for example, the 1 mm (3% with LLD of >10 mm) reported by Woolson and colleagues.³⁴ For RTHA, mean (SD) LLD was 2.7 (1.8) mm (no LLD of ≥10 mm), superior to the 3.9 (2.7) mm (4.4% with LLD of >10 mm) reported by Nam and colleagues¹⁹ for posterior-navigated THA and the 5.06 (2.99) mm (10.4% with LLD of >10 mm) reported by Manzotti and colleagues¹⁶ for computer-assisted THA.

This study had several notable strengths. All patients had a diagnosis of hip OA and were operated on by a single surgeon. Radiographs were calibrated using the size of the implanted femoral head. Radiographic data were measured using the same technique in all cases and were collected twice by 2 observers (not the senior surgeon) to decrease bias and determine interobserver and intraobserver reliability. In addition, surgeon experience might have played an important role in minimizing LLD regardless of technique and approach used for THA.

Study limitations were different number of cases in each group, lack of matching, lack of clinical follow-up, and lack of long-term assessment of clinical outcomes and complications.

Conclusion

As performed by an experienced surgeon, RTHA, ATHA, and PTHA did not differ in obtaining minimal LLD. All 3 groups had a low frequency of outliers, using thresholds of 3 mm and 5 mm, and no patient in any group had LLD of 10 mm or more. All 3 techniques are effective in achieving accuracy in LLD.

Total hip arthroplasty (THA) effectively provides adequate pain relief and favorable outcomes in patients with hip osteoarthritis (OA). However, leg-length discrepancy (LLD) is still a significant cause of morbidity,¹ including nerve damage,^2,3 low back pain,^2,4,5 and abnormal gait.^2,6,7 Although most of the LLD values reported in the literature fall under the acceptable threshold of 10 mm,⁸ some patients report dissatisfaction,⁹ leading to litigation against orthopedic surgeons.² However, lower extremity lengthening is sometimes needed to achieve adequate hip joint stability and prevent dislocations.^2,10

Several methods have been developed to help surgeons estimate the change in leg length during surgery in an attempt to improve clinical outcomes. Use of guide pins as a reference on the pelvis decreased LLD and improved outcomes in some published studies.^11,12 Preoperative templating of implant size, cup position, and level of femoral neck cut is very important in helping minimize clinically significant LLD after THA.^2,13,14 Computer-assisted THA has also been introduced to try to improve component positioning, restoration of hip center of rotation, and minimizing of LLD.^15-17 However, cost and increased operative time have prevented widespread adoption of computer-assisted surgery in THA.

Proponents of different surgical approaches have argued about the superiority of one approach over another. The posterior approach is the gold standard in THA because it is safe, easy to perform, and, if needed, extensile.¹¹ However, exact determination of the intraoperative 3-dimensional (3-D) orientation of the pelvis, and subsequently of LLD, is challenging when the patient lies in the lateral position. The anterior approach has gained in popularity because of its advantages in accelerating postoperative rehabilitation and decreasing hospital length of stay.¹⁸ Placing the patient supine is advantageous because it allows leveling of the pelvis and estimation of LLD (by comparing the positions of the lower extremities).¹⁹ The anterior approach also allows for radiographic measurements on the operating table.^19,20 However, this approach has a high learning curve²¹ and is not extensile.²¹ To date, no study has shown superiority of the anterior approach over either the conventional posterior approach or the robot-assisted posterior approach in minimizing LLD after THA.

We conducted a study to compare LLD in patients who underwent THA performed with a robot-assisted posterior approach (RTHA), a fluoroscopy-guided anterior approach (ATHA), or a conventional posterior approach (PTHA). We hypothesized that, compared with PTHA, both RTHA and ATHA would result in reduced LLD.

Materials and Methods

We reviewed all RTHAs, ATHAs, and PTHAs performed by Dr. Domb between September 2008 and December 2012. Study inclusion criteria were a diagnosis of hip OA and the availability of postoperative supine anteroposterior pelvis radiographs. Exclusion criteria were a diagnosis other than hip OA, missing or improper postoperative radiographs (radiographs with rotated or tilted pelvis),²² and radiographs on which at least one of the lesser trochanters was difficult to define. Of the 155 cases included in the study, 67 were RTHAs, 29 were ATHAs, and 59 were PTHAs.

All patients scheduled for THA underwent preoperative planning; plain radiographs were used to determine component size and position, level of neck cut, and amount of leg lengthening or shortening needed. In all RTHA cases, computed tomography of the involved hip was performed before surgery. The MAKO system (MAKO Surgical Corporation, Davie, Florida) was used to develop a patient-specific 3-D model of the pelvis and proximal femur, and this model was used to guide THA execution. The system was then used to detect patient-specific landmarks during surgery, to register the femur and the acetabulum, and to help determine the position of the pelvis and proximal femur during surgery. This system, which uses a haptic robotic arm that guides acetabular reaming and cup placement, provides feedback regarding cup placement, stem version, leg length, and global offset. Pelvic tilt and rotation were accounted for by the MAKO software, and all provided measurements were made on the coronal (functional) plane of the body, as described by Murray.²³ ATHA was performed with the patient in the supine position on a Hana table (Mizuho OSI, Union City, California) with fluoroscopic guidance. PTHA was performed in the conventional way, with the patient in the lateral position.

Radiographic measurements of LLD were made with TraumaCad software (Build 2.2.535.0; Voyant Health, Petah-Tikva, Israel). The accuracy of this software has been studied and reported in the literature.^24-26 Radiographs were calibrated using the known size of each femoral head as a marker. The reference on the pelvis was the interobturator line (line tangent to inferior border of obturator foramina), and the reference on the femurs was the most superior and medial aspect of each lesser trochanter. Two lines were drawn, each perpendicular to the interobturator line, starting from the previously defined reference point on each lesser trochanter. The difference in length between these 2 lines was recorded as the LLD. Values were recorded relative to the operative extremity. For example, if the operative extremity was longer than the nonoperative extremity, the LLD was given a positive value.

To eliminate bias and increase measurement accuracy, the study had each of 2 observers collect the LLD data twice, 2 months apart. These observers were blinded to each other’s results and to the type of surgery performed. (Neither observer was Dr. Domb, the senior surgeon.) IBM SPSS Statistics software (Version 20; IBM, Armonk, New York) was used for statistical analysis. Each patient’s 4 measurements were averaged into a single number for LLD, and the absolute LLD values were used in all statistical analyses. Means, standard deviations (SDs), and 95% confidence intervals (CIs) were calculated for LLD in each of the 3 groups. Pearson correlation coefficient was used to determine interobserver and intraobserver reliability. One-way analysis of variance (ANOVA) was used to compare group means for age, body mass index (BMI), and LLD. In each group, number of outliers was determined with outliers set at LLDs of more than 3 mm and more than 5 mm. Fischer exact test was used to compare number of outliers in each group. P < .05 was considered statistically significant.

Results

Table 1 lists the demographic data, including age, sex, and BMI, and compares the means. There were strong interobserver and intraobserver correlations for all LLD measurements (r > 0.9; P < .001). Mean (SD) LLD was 2.7 (1.8) mm (95% CI, 2.3-3.2) in the RTHA group, 1.8 (1.6) mm (95% CI, 1.2-2.4) in the ATHA group, and 1.9 (1.6) mm (95% CI, 1.5-2.4) in the PTHA group (P = .01). When LLD of more than 3 mm was set as an outlier, percentage of outliers was 37.3% (RTHA), 17.2% (ATHA), and 22% (PTHA) (P = .06-.78). When LLD of more than 5 mm was set as an outlier, percentage of outliers was 10.4% (RTHA), 6.9% (ATHA), and 8.5% (PTHA) (P = .72 to >.99). No patient in any group had LLD of 10 mm or more (Figure). Table 2 lists percentages of patients’ operated extremities that were longer, shorter, or the same size as their contralateral extremities. Six (9.0%) of the 67 RTHA patients, 4 (13.8%) of the 29 ATHA patients, and 3 (5.1%) of the 59 PTHA patients had a contralateral THA.

Discussion

Our study results showed that RTHA, ATHA, and PTHA were equally effective in minimizing LLD. There was a statistically significant difference in mean LLD among the 3 groups studied. The RTHA group had the largest mean (SD) LLD: 2.7 (1.8) mm. However, statistically significant differences do not always indicate clinical significance.²⁷ Therefore, comparison of the 3 groups’ means is not enough for drawing significant conclusions. The more important point to consider is the number of cases of LLD of 10 mm or more—a discrepancy that would be perceptible to patients and thus become a source of dissatisfaction with painless THA.²⁸ Patients perceive LLD when shortening exceeds 10 mm and lengthening exceeds 6 mm,²⁹ or when LLD is more than 10 mm.^16,19,20 Despite significant differences in means, all our cases came in under the 10-mm threshold. When the threshold was decreased to 5 mm (and to 3 mm), there was no statistically significant difference among the groups in the number of cases above the threshold.

LLD remains a source of significant post-THA comorbidity and patient dissatisfaction.^1-7,19 Despite surgeons’ efforts to minimize LLD, some patients can detect even a subtle LLD after surgery.^1,8,29 Most LLD values reported in the literature fall under the 10-mm threshold.^16,19,20 In some cases, however, postoperative LLD is more than 1 cm, enough to prompt litigation against orthopedic surgeons.² Surgeons have tried to improve LLD with use of multiple techniques, including use of intraoperative measuring devices,³⁰ patient positioning during surgery,²⁰ use of computer-assisted surgery,¹⁹ and use of intraoperative fluoroscopy.²⁰

Proponents of computer-assisted THA have argued that this technique improves accuracy in placing the acetabular cup in the safe zone,³¹ minimizes LLD, and restores femoral offset.^32,33 Manzotti and colleagues¹⁶ reported on 48 cases of computer-assisted THA matched to 48 cases of conventional THA using the posterior approach. Mean (SD) LLD was 5.06 (2.99) mm in the computer-assisted group and 7.64 (4.36) mm in the conventional group; there was a statistically significant difference in favor of the computer-assisted group (P = .04). However, 5 patients in the computer-assisted group and 13 in the conventional group had LLD of more than 10 mm, and the difference was statistically significant.¹⁶ Moreover, the study population was heterogeneous, with 12 patients in both groups having developmental dysplasia as a primary diagnosis.¹⁶ All the cases in our study had a diagnosis of OA, and no case had LLD of 10 mm or more.

Several advantages have been proposed for the anterior approach. The supine position (with direct comparison of leg lengths) and the use of fluoroscopy have been described as advantageous in minimizing LLD.^20,21 In their study of 494 primary THAs performed with the anterior approach, Matta and colleagues²⁰ reported mean (SD) postoperative LLD of 3 (2) mm (range, 0-26 mm) and concluded that the anterior approach was effective in restoring leg lengths and ensuring proper cup placement while not increasing the dislocation rate. However, they did not compare this approach with others or with computer-assisted THA with respect to LLD.

In another study, Nam and colleagues¹⁹ compared LLD after THA performed with 3 different approaches (anterior, conventional posterior, posterior-navigated) and found no statistically significant difference in LLD among the groups. However, LLD was more than 10 mm in 2.2% of anterior cases, 4.4% of conventional posterior cases, and 4.4% of posterior-navigated cases. When 5 mm was used as a cutoff, percentage of patients who were outliers was 31.1% (anterior), 20% (conventional posterior), and 23.3% (navigated-posterior). Our data showed superior results in using 5 mm as a cutoff, with percentage of outliers of 6.9% with ATHA, 8.5% with PTHA, and 10.4% with RTHA. However, Nam and colleagues¹⁹ used a larger patient cohort and different techniques for measuring LLD on anteroposterior pelvis radiographs.

The most likely reason that the groups in our study were comparable in terms of LLD accuracy and lack of outliers over the 10-mm cutoff was Dr. Domb’s high accuracy in minimizing LLD using each of the 3 techniques. For ATHA, mean (SD) LLD was 1.8 (1.6) mm (no LLD of ≥10 mm), better than the 3 (2) mm (0.9% with LLD of >10 mm) reported by Matta and colleagues²⁰ and the 3.8 (3.9) mm (2.2% with LLD of >10 mm) reported by Nam and colleagues.¹⁹ For PTHA, mean (SD) LLD was 1.9 (1.6) mm (no LLD of ≥10 mm), comparable to some of the best results reported in the literature—for example, the 1 mm (3% with LLD of >10 mm) reported by Woolson and colleagues.³⁴ For RTHA, mean (SD) LLD was 2.7 (1.8) mm (no LLD of ≥10 mm), superior to the 3.9 (2.7) mm (4.4% with LLD of >10 mm) reported by Nam and colleagues¹⁹ for posterior-navigated THA and the 5.06 (2.99) mm (10.4% with LLD of >10 mm) reported by Manzotti and colleagues¹⁶ for computer-assisted THA.

This study had several notable strengths. All patients had a diagnosis of hip OA and were operated on by a single surgeon. Radiographs were calibrated using the size of the implanted femoral head. Radiographic data were measured using the same technique in all cases and were collected twice by 2 observers (not the senior surgeon) to decrease bias and determine interobserver and intraobserver reliability. In addition, surgeon experience might have played an important role in minimizing LLD regardless of technique and approach used for THA.

Study limitations were different number of cases in each group, lack of matching, lack of clinical follow-up, and lack of long-term assessment of clinical outcomes and complications.

Conclusion

As performed by an experienced surgeon, RTHA, ATHA, and PTHA did not differ in obtaining minimal LLD. All 3 groups had a low frequency of outliers, using thresholds of 3 mm and 5 mm, and no patient in any group had LLD of 10 mm or more. All 3 techniques are effective in achieving accuracy in LLD.

Total knee arthroplasty (TKA) is a good surgical option to relieve pain and improve function in patients with osteoarthritis. The goal of surgery is to achieve a well-aligned prosthesis with well-balanced ligaments in order to minimize wear and improve implant survival. Overall, 82% to 89% of patients are satisfied with their outcomes after TKA, with good 10- to 15-year implant survivorship; however, there is still a subset of patients that are unsatisfied. In many cases, patient dissatisfaction is attributed to improper component alignment.^1-3 Over the past decade, computer navigation and robotics have been introduced to control surgical variables so as to gain greater consistency in implant placement and postoperative component alignment.

Computer-assisted navigation tools were introduced not only to improve implant alignment but, more importantly, to optimize clinical outcomes. Most studies have demonstrated that the use of navigation is associated with fewer radiographic outliers after TKA.⁴ Various studies have compared radiographic results of navigated TKA with results of TKA using standard instrumentation.^4-7 While long-term studies are necessary, short-term follow-up has shown that computer-assisted TKA can improve alignment, especially in patients with severe deformity.^8-10 Currently, there is no definitive consensus that computer-assisted TKA leads to significantly better component alignment or postoperative outcomes due to the fact that many studies are limited by study design or small cohorts. However, the currently published articles support better component alignment and clinical outcomes with computer-assisted TKA. While some argue that the use of computer-assisted surgery is dependent on the user’s experience, computer-assisted surgery can assist less-experienced surgeons to reliably achieve good midterm outcomes with a low complication rate.^8,11 Various studies have looked at computer-assisted TKA at midterm follow-up, with no significant differences in clinical outcome between navigated and traditional techniques. However, long-term studies showing the benefits of computer navigation are beginning to emerge. For example, de Steiger and colleagues¹² recently found that computer-assisted TKA reduced the overall revision rate for loosening after TKA in patients less than 65 years of age.

While surgical navigation helps improve implant planning, robotic tools have emerged as a tool to help refine surgical execution. Coupled with surgical navigation tools, robotic control of surgical gestures may further enhance precision in implant placement and/or enable novel implant design features. At present, robotic techniques are increasingly used in unicompartmental knee arthroplasty (UKA) and TKA.¹³ Studies have demonstrated that the robotic tool is 3 times more accurate with 3 times less variability than conventional techniques in UKA.¹⁴ The utility of robotic tools for TKA remains unclear. Robotic-driven automatic cutting guides have been shown to reduce time and improve accuracy compared with navigation guides in femoral TKA cutting procedures in a cadaveric model.¹⁵ However, robotic-enabled TKA procedures are poorly described at present, and the clinical implications of their proposed improved precision remain unclear.

Computer navigation and robotic tools in TKA hold the promise of enhanced control of surgical variables that influence clinical outcome. The variables that may be impacted by these advanced tools include implant positioning, lower limb alignment, soft-tissue balance, and, potentially, implant design and fixation. At present, these tools have primarily been shown to improve lower limb alignment in TKA. The clinical impact of the enhanced control of this single surgical variable (lower limb alignment) has been muted in short-term and midterm studies. Future studies should be directed at understanding which surgical variable, or combination of variables, it is most essential to precisely control so as to positively impact clinical outcomes. ◾

Total knee arthroplasty (TKA) is a good surgical option to relieve pain and improve function in patients with osteoarthritis. The goal of surgery is to achieve a well-aligned prosthesis with well-balanced ligaments in order to minimize wear and improve implant survival. Overall, 82% to 89% of patients are satisfied with their outcomes after TKA, with good 10- to 15-year implant survivorship; however, there is still a subset of patients that are unsatisfied. In many cases, patient dissatisfaction is attributed to improper component alignment.^1-3 Over the past decade, computer navigation and robotics have been introduced to control surgical variables so as to gain greater consistency in implant placement and postoperative component alignment.

Computer-assisted navigation tools were introduced not only to improve implant alignment but, more importantly, to optimize clinical outcomes. Most studies have demonstrated that the use of navigation is associated with fewer radiographic outliers after TKA.⁴ Various studies have compared radiographic results of navigated TKA with results of TKA using standard instrumentation.^4-7 While long-term studies are necessary, short-term follow-up has shown that computer-assisted TKA can improve alignment, especially in patients with severe deformity.^8-10 Currently, there is no definitive consensus that computer-assisted TKA leads to significantly better component alignment or postoperative outcomes due to the fact that many studies are limited by study design or small cohorts. However, the currently published articles support better component alignment and clinical outcomes with computer-assisted TKA. While some argue that the use of computer-assisted surgery is dependent on the user’s experience, computer-assisted surgery can assist less-experienced surgeons to reliably achieve good midterm outcomes with a low complication rate.^8,11 Various studies have looked at computer-assisted TKA at midterm follow-up, with no significant differences in clinical outcome between navigated and traditional techniques. However, long-term studies showing the benefits of computer navigation are beginning to emerge. For example, de Steiger and colleagues¹² recently found that computer-assisted TKA reduced the overall revision rate for loosening after TKA in patients less than 65 years of age.

While surgical navigation helps improve implant planning, robotic tools have emerged as a tool to help refine surgical execution. Coupled with surgical navigation tools, robotic control of surgical gestures may further enhance precision in implant placement and/or enable novel implant design features. At present, robotic techniques are increasingly used in unicompartmental knee arthroplasty (UKA) and TKA.¹³ Studies have demonstrated that the robotic tool is 3 times more accurate with 3 times less variability than conventional techniques in UKA.¹⁴ The utility of robotic tools for TKA remains unclear. Robotic-driven automatic cutting guides have been shown to reduce time and improve accuracy compared with navigation guides in femoral TKA cutting procedures in a cadaveric model.¹⁵ However, robotic-enabled TKA procedures are poorly described at present, and the clinical implications of their proposed improved precision remain unclear.

Computer navigation and robotic tools in TKA hold the promise of enhanced control of surgical variables that influence clinical outcome. The variables that may be impacted by these advanced tools include implant positioning, lower limb alignment, soft-tissue balance, and, potentially, implant design and fixation. At present, these tools have primarily been shown to improve lower limb alignment in TKA. The clinical impact of the enhanced control of this single surgical variable (lower limb alignment) has been muted in short-term and midterm studies. Future studies should be directed at understanding which surgical variable, or combination of variables, it is most essential to precisely control so as to positively impact clinical outcomes. ◾

Total knee arthroplasty (TKA) is a good surgical option to relieve pain and improve function in patients with osteoarthritis. The goal of surgery is to achieve a well-aligned prosthesis with well-balanced ligaments in order to minimize wear and improve implant survival. Overall, 82% to 89% of patients are satisfied with their outcomes after TKA, with good 10- to 15-year implant survivorship; however, there is still a subset of patients that are unsatisfied. In many cases, patient dissatisfaction is attributed to improper component alignment.^1-3 Over the past decade, computer navigation and robotics have been introduced to control surgical variables so as to gain greater consistency in implant placement and postoperative component alignment.

Computer-assisted navigation tools were introduced not only to improve implant alignment but, more importantly, to optimize clinical outcomes. Most studies have demonstrated that the use of navigation is associated with fewer radiographic outliers after TKA.⁴ Various studies have compared radiographic results of navigated TKA with results of TKA using standard instrumentation.^4-7 While long-term studies are necessary, short-term follow-up has shown that computer-assisted TKA can improve alignment, especially in patients with severe deformity.^8-10 Currently, there is no definitive consensus that computer-assisted TKA leads to significantly better component alignment or postoperative outcomes due to the fact that many studies are limited by study design or small cohorts. However, the currently published articles support better component alignment and clinical outcomes with computer-assisted TKA. While some argue that the use of computer-assisted surgery is dependent on the user’s experience, computer-assisted surgery can assist less-experienced surgeons to reliably achieve good midterm outcomes with a low complication rate.^8,11 Various studies have looked at computer-assisted TKA at midterm follow-up, with no significant differences in clinical outcome between navigated and traditional techniques. However, long-term studies showing the benefits of computer navigation are beginning to emerge. For example, de Steiger and colleagues¹² recently found that computer-assisted TKA reduced the overall revision rate for loosening after TKA in patients less than 65 years of age.

While surgical navigation helps improve implant planning, robotic tools have emerged as a tool to help refine surgical execution. Coupled with surgical navigation tools, robotic control of surgical gestures may further enhance precision in implant placement and/or enable novel implant design features. At present, robotic techniques are increasingly used in unicompartmental knee arthroplasty (UKA) and TKA.¹³ Studies have demonstrated that the robotic tool is 3 times more accurate with 3 times less variability than conventional techniques in UKA.¹⁴ The utility of robotic tools for TKA remains unclear. Robotic-driven automatic cutting guides have been shown to reduce time and improve accuracy compared with navigation guides in femoral TKA cutting procedures in a cadaveric model.¹⁵ However, robotic-enabled TKA procedures are poorly described at present, and the clinical implications of their proposed improved precision remain unclear.

Computer navigation and robotic tools in TKA hold the promise of enhanced control of surgical variables that influence clinical outcome. The variables that may be impacted by these advanced tools include implant positioning, lower limb alignment, soft-tissue balance, and, potentially, implant design and fixation. At present, these tools have primarily been shown to improve lower limb alignment in TKA. The clinical impact of the enhanced control of this single surgical variable (lower limb alignment) has been muted in short-term and midterm studies. Future studies should be directed at understanding which surgical variable, or combination of variables, it is most essential to precisely control so as to positively impact clinical outcomes. ◾

Ruptures of the patellar tendon usually occur in patients under age 40 years, with men having a higher incidence than women.¹ History of local steroid injection,^2,3 total knee arthroplasty,^4-8 anterior cruciate ligament reconstruction with central third patellar tendon autograft,^9-11 and a variety of systemic diseases are associated with an increased tendency to rupture.^12-15 Primary acute ruptures of the patellar tendon can be difficult to repair because of the quality of remaining tissues. In cases of chronic tendon ruptures subject to delayed treatment, additional complications such as tissue contracture and scar-tissue formation are likely to exist.^15-17

Complications after intramedullary (IM) nailing of the tibia include infection, compartment syndrome, deep vein thrombosis, thermal necrosis of the bone with alteration of its endosteal architecture, failure of the hardware, malunion, and nonunion.¹⁸ The most common complaint after IM nailing of the tibia is chronic anterior knee pain and symptoms similar to tendonitis; incidences as high as 86% have been reported.^18-20 Extensive review of the literature found only 2 reports of patellar tendon rupture after IM nailing of the tibia; both cases used a patellar tendon–splitting approach. The first report described patellar tendon rupture 8 years after IM nailing of the tibia during a forced deep-flexion movement.²¹ Radiographic examination showed the IM nail positioned proud relative to the tibial plateau, impinging upon the patellar tendon. An intraoperative examination confirmed the radiographic findings and found rupture of the patellar tendon to be consistent with the exposed tip of the IM nail. The second report described patellar tendon rupture 2 months postoperatively in a patient with Ehlers-Danlos syndrome, a hereditary disorder characterized by alterations to muscle/tendon tissue and hyperextensible skin.²²

Patellar tendon rupture after IM nailing of the tibia is a rare complication. Patellar tendon re-rupture after primary repair in a patient with history of IM tibial nailing has not been reported. This case outlines the progression of such a patient with a recurrent patellar tendon rupture that was successfully reconstructed using an Achilles tendon allograft. The patient’s surgical history of IM tibial nailing through a mid-patellar tendon–splitting approach 4 years prior to initial tendon rupture is noteworthy and potentially predisposed the patient to injury. The patient provided written informed consent for print and electronic publication of this case report.   

Case Report

A 44-year-old woman, 5 ft, 3 in tall, and weighing 129 lb (body mass index, 22.8), with a history of osteoporosis and transverse myelitis, presented with pain and persistent swelling about the left knee. Her baseline ambulatory status required crutches because of decreased sensation and strength in her lower extremity in conjunction with a foot drop; she had mild quadriceps and hamstring muscle weakness but otherwise normal knee function. The patient had been seen 4 years earlier at our facility for IM fixation of a distal tibia fracture through a patellar tendon–splitting approach. The fracture was well healed and showed no signs of complication or nail migration; the nail was not proud.

Initially, the patient was admitted to another hospital through the emergency department for swelling and pain about the left knee. She was believed to have an infection and was placed on antibiotics by the primary care team. An orthopedic evaluation showed induration, edema, and warmth in the patellar tendon region of the left knee. Magnetic resonance imaging (MRI) showed a full-thickness patellar tendon rupture. Aspiration of the knee was performed and cultures were negative; white blood cell, erythrocyte sedimentation rate, and C-reactive protein values were normal. The risks and benefits of various treatments were discussed, and surgical intervention was elected to repair the patellar tendon.

Intraoperative findings showed a massive midsubstance rupture of the patellar tendon, accompanied by medial and lateral retinacular tears and a quadriceps tendon partial rupture; the central aspect of the quadriceps tendon attaching to the patella remained intact. The patella was retracted proximally; no evidence of active infection was present. Good-quality tissue remained attached to both the tibial tuberosity and the inferior pole of the patella. A No. 2 FiberWire suture (Arthrex, Inc, Naples, Florida) was used to run whip stitches in the distal end of the patellar tendon and a second No. 2 FiberWire suture was used to run whip stitches in the proximal aspect of the patellar tendon rupture. The 4 ends of the sutures were tied together, thus re-approximating the distal and proximal ends of the ruptured patellar tendon. No bone drilling was used because the midsubstance tear was amenable to good repair with reasonable expectation of healing based on tissue quality. The quadriceps tendon, which was partially torn, was repaired with a No. 1 Vicryl suture (Ethicon, Somerville, New Jersey). The medial and lateral retinacula were also repaired with a No. 1 Vicryl suture. The suturing scheme effectively re-approximated the knee extensor mechanism, and the patient was placed in a knee immobilizer that permitted no flexion for 6 weeks postoperatively.

After 3 months of gradual improvement with physical therapy, the patient returned for a follow-up visit, concerned that her knee function was beginning to decline. Physical examination showed patella alta with a thinned and diminutive palpable tendon in the patellar tendon region. She was capable of active flexion to 90º and extension to 50º, but beyond 50º, she was unable to actively extend; she was capable of full passive extension. MRI showed a repeat full-thickness patellar tendon tear with retraction from the inferior pole of the patella; previous tears to the quadriceps tendon were healed. Because of the recurrent nature of the injury, the patient’s physical examination, MRI findings, and anticipated poor quality of remaining tendon tissue, patellar tendon reconstruction using a cadaveric Achilles tendon allograft was recommended. The patient chose surgery for potential improvement in knee range of motion, active extension, and ambulation.

The previous anterior midline incision was used and carried down through the subcutaneous tissues where a complete rupture of the patellar tendon was identified. A limited amount of good-quality tendon tissue remained at the medial aspect of the tibial tuberosity. The remaining tissue located at the patella’s inferior pole was nonviable for use in surgical repair. Retinacular contractures were released to bring the patella distally; the trochlear groove was used as the anatomic landmark for the patella resting position. During reconstruction, the knee was placed into 30° of flexion, with the patella located in the trochlear groove, and the cadaveric Achilles tendon was placed on the midline of the patella, where measurements were done to assess proper length and tension (Figure 1).

The patient’s remaining native tissue on the medial aspect of the tibial tuberosity was used to augment the Achilles tendon graft medially. The cadaveric Achilles tendon graft was primarily used to replace the central and lateral aspects of the patellar tendon. Additionally, the calcaneal bone segment at the end of the Achilles tendon graft was removed prior to use. Cadaveric and host tissues at the medial aspect of the tibial tuberosity were sutured together with a No. 1 Vicryl suture (Figure 2). The distal aspect of the cadaveric Achilles tendon was used to re-approximate the patient’s native patellar tendon insertion at the tibial tuberosity. To supplement the graft anchor, a Richards metallic ligament staple (Smith & Nephew, Memphis, Tennessee) was used to fix the distal aspect of the Achilles tendon graft into the tibial tuberosity.

Proper tensioning of the graft was performed by visualizing patella tracking during the arc-of-knee motion and properly suturing the graft to allow for functional range. The proximal aspect of the cadaveric Achilles tendon was sutured into host tissues surrounding the superior pole of the patellar and quadriceps tendon. The edges of the graft were sutured with supplemental No. 1 Vicryl sutures (Figure 3).

Before surgical closure, knee range of motion was checked and noted to be 0º to 100º.  The repaired construct was stable and uncompromised throughout the entire range of motion. Patella tracking was central and significantly improved; knee stability was normal to varus and valgus stress.

The patient was placed in a knee immobilizer for 6 weeks before range of motion was allowed. Seven months postoperatively, the patient returned for a follow-up visit, ambulating with 2 forearm crutches, which was her baseline ambulatory status. Physical examination revealed passive range of motion from 0º to 130º, an extension lag of 10º, and 4/5 quadriceps strength. It was recommended the patient continue physical therapy to improve strength and range of motion.

Conclusion

This is the first report in the literature documenting a recurrent patellar tendon rupture after primary repair in a patient with a history of IM tibial nailing. It is also the first report of a cadaveric Achilles tendon allograft used as a solution to this problem. Complete reconstruction of the patellar tendon using an Achilles tendon allograft is a method commonly used for ruptures after total knee arthroplasty.^4-7,23,24 This case report highlights the utility of a cadaveric Achilles tendon in the setting of a recurrent patellar tendon rupture with poor remaining tissue quality.

Ruptures of the patellar tendon usually occur in patients under age 40 years, with men having a higher incidence than women.¹ History of local steroid injection,^2,3 total knee arthroplasty,^4-8 anterior cruciate ligament reconstruction with central third patellar tendon autograft,^9-11 and a variety of systemic diseases are associated with an increased tendency to rupture.^12-15 Primary acute ruptures of the patellar tendon can be difficult to repair because of the quality of remaining tissues. In cases of chronic tendon ruptures subject to delayed treatment, additional complications such as tissue contracture and scar-tissue formation are likely to exist.^15-17

Complications after intramedullary (IM) nailing of the tibia include infection, compartment syndrome, deep vein thrombosis, thermal necrosis of the bone with alteration of its endosteal architecture, failure of the hardware, malunion, and nonunion.¹⁸ The most common complaint after IM nailing of the tibia is chronic anterior knee pain and symptoms similar to tendonitis; incidences as high as 86% have been reported.^18-20 Extensive review of the literature found only 2 reports of patellar tendon rupture after IM nailing of the tibia; both cases used a patellar tendon–splitting approach. The first report described patellar tendon rupture 8 years after IM nailing of the tibia during a forced deep-flexion movement.²¹ Radiographic examination showed the IM nail positioned proud relative to the tibial plateau, impinging upon the patellar tendon. An intraoperative examination confirmed the radiographic findings and found rupture of the patellar tendon to be consistent with the exposed tip of the IM nail. The second report described patellar tendon rupture 2 months postoperatively in a patient with Ehlers-Danlos syndrome, a hereditary disorder characterized by alterations to muscle/tendon tissue and hyperextensible skin.²²

Patellar tendon rupture after IM nailing of the tibia is a rare complication. Patellar tendon re-rupture after primary repair in a patient with history of IM tibial nailing has not been reported. This case outlines the progression of such a patient with a recurrent patellar tendon rupture that was successfully reconstructed using an Achilles tendon allograft. The patient’s surgical history of IM tibial nailing through a mid-patellar tendon–splitting approach 4 years prior to initial tendon rupture is noteworthy and potentially predisposed the patient to injury. The patient provided written informed consent for print and electronic publication of this case report.   

Case Report

A 44-year-old woman, 5 ft, 3 in tall, and weighing 129 lb (body mass index, 22.8), with a history of osteoporosis and transverse myelitis, presented with pain and persistent swelling about the left knee. Her baseline ambulatory status required crutches because of decreased sensation and strength in her lower extremity in conjunction with a foot drop; she had mild quadriceps and hamstring muscle weakness but otherwise normal knee function. The patient had been seen 4 years earlier at our facility for IM fixation of a distal tibia fracture through a patellar tendon–splitting approach. The fracture was well healed and showed no signs of complication or nail migration; the nail was not proud.

Initially, the patient was admitted to another hospital through the emergency department for swelling and pain about the left knee. She was believed to have an infection and was placed on antibiotics by the primary care team. An orthopedic evaluation showed induration, edema, and warmth in the patellar tendon region of the left knee. Magnetic resonance imaging (MRI) showed a full-thickness patellar tendon rupture. Aspiration of the knee was performed and cultures were negative; white blood cell, erythrocyte sedimentation rate, and C-reactive protein values were normal. The risks and benefits of various treatments were discussed, and surgical intervention was elected to repair the patellar tendon.

Intraoperative findings showed a massive midsubstance rupture of the patellar tendon, accompanied by medial and lateral retinacular tears and a quadriceps tendon partial rupture; the central aspect of the quadriceps tendon attaching to the patella remained intact. The patella was retracted proximally; no evidence of active infection was present. Good-quality tissue remained attached to both the tibial tuberosity and the inferior pole of the patella. A No. 2 FiberWire suture (Arthrex, Inc, Naples, Florida) was used to run whip stitches in the distal end of the patellar tendon and a second No. 2 FiberWire suture was used to run whip stitches in the proximal aspect of the patellar tendon rupture. The 4 ends of the sutures were tied together, thus re-approximating the distal and proximal ends of the ruptured patellar tendon. No bone drilling was used because the midsubstance tear was amenable to good repair with reasonable expectation of healing based on tissue quality. The quadriceps tendon, which was partially torn, was repaired with a No. 1 Vicryl suture (Ethicon, Somerville, New Jersey). The medial and lateral retinacula were also repaired with a No. 1 Vicryl suture. The suturing scheme effectively re-approximated the knee extensor mechanism, and the patient was placed in a knee immobilizer that permitted no flexion for 6 weeks postoperatively.

After 3 months of gradual improvement with physical therapy, the patient returned for a follow-up visit, concerned that her knee function was beginning to decline. Physical examination showed patella alta with a thinned and diminutive palpable tendon in the patellar tendon region. She was capable of active flexion to 90º and extension to 50º, but beyond 50º, she was unable to actively extend; she was capable of full passive extension. MRI showed a repeat full-thickness patellar tendon tear with retraction from the inferior pole of the patella; previous tears to the quadriceps tendon were healed. Because of the recurrent nature of the injury, the patient’s physical examination, MRI findings, and anticipated poor quality of remaining tendon tissue, patellar tendon reconstruction using a cadaveric Achilles tendon allograft was recommended. The patient chose surgery for potential improvement in knee range of motion, active extension, and ambulation.

The previous anterior midline incision was used and carried down through the subcutaneous tissues where a complete rupture of the patellar tendon was identified. A limited amount of good-quality tendon tissue remained at the medial aspect of the tibial tuberosity. The remaining tissue located at the patella’s inferior pole was nonviable for use in surgical repair. Retinacular contractures were released to bring the patella distally; the trochlear groove was used as the anatomic landmark for the patella resting position. During reconstruction, the knee was placed into 30° of flexion, with the patella located in the trochlear groove, and the cadaveric Achilles tendon was placed on the midline of the patella, where measurements were done to assess proper length and tension (Figure 1).

The patient’s remaining native tissue on the medial aspect of the tibial tuberosity was used to augment the Achilles tendon graft medially. The cadaveric Achilles tendon graft was primarily used to replace the central and lateral aspects of the patellar tendon. Additionally, the calcaneal bone segment at the end of the Achilles tendon graft was removed prior to use. Cadaveric and host tissues at the medial aspect of the tibial tuberosity were sutured together with a No. 1 Vicryl suture (Figure 2). The distal aspect of the cadaveric Achilles tendon was used to re-approximate the patient’s native patellar tendon insertion at the tibial tuberosity. To supplement the graft anchor, a Richards metallic ligament staple (Smith & Nephew, Memphis, Tennessee) was used to fix the distal aspect of the Achilles tendon graft into the tibial tuberosity.

Proper tensioning of the graft was performed by visualizing patella tracking during the arc-of-knee motion and properly suturing the graft to allow for functional range. The proximal aspect of the cadaveric Achilles tendon was sutured into host tissues surrounding the superior pole of the patellar and quadriceps tendon. The edges of the graft were sutured with supplemental No. 1 Vicryl sutures (Figure 3).

Before surgical closure, knee range of motion was checked and noted to be 0º to 100º.  The repaired construct was stable and uncompromised throughout the entire range of motion. Patella tracking was central and significantly improved; knee stability was normal to varus and valgus stress.

The patient was placed in a knee immobilizer for 6 weeks before range of motion was allowed. Seven months postoperatively, the patient returned for a follow-up visit, ambulating with 2 forearm crutches, which was her baseline ambulatory status. Physical examination revealed passive range of motion from 0º to 130º, an extension lag of 10º, and 4/5 quadriceps strength. It was recommended the patient continue physical therapy to improve strength and range of motion.

Conclusion

This is the first report in the literature documenting a recurrent patellar tendon rupture after primary repair in a patient with a history of IM tibial nailing. It is also the first report of a cadaveric Achilles tendon allograft used as a solution to this problem. Complete reconstruction of the patellar tendon using an Achilles tendon allograft is a method commonly used for ruptures after total knee arthroplasty.^4-7,23,24 This case report highlights the utility of a cadaveric Achilles tendon in the setting of a recurrent patellar tendon rupture with poor remaining tissue quality.

Ruptures of the patellar tendon usually occur in patients under age 40 years, with men having a higher incidence than women.¹ History of local steroid injection,^2,3 total knee arthroplasty,^4-8 anterior cruciate ligament reconstruction with central third patellar tendon autograft,^9-11 and a variety of systemic diseases are associated with an increased tendency to rupture.^12-15 Primary acute ruptures of the patellar tendon can be difficult to repair because of the quality of remaining tissues. In cases of chronic tendon ruptures subject to delayed treatment, additional complications such as tissue contracture and scar-tissue formation are likely to exist.^15-17

Complications after intramedullary (IM) nailing of the tibia include infection, compartment syndrome, deep vein thrombosis, thermal necrosis of the bone with alteration of its endosteal architecture, failure of the hardware, malunion, and nonunion.¹⁸ The most common complaint after IM nailing of the tibia is chronic anterior knee pain and symptoms similar to tendonitis; incidences as high as 86% have been reported.^18-20 Extensive review of the literature found only 2 reports of patellar tendon rupture after IM nailing of the tibia; both cases used a patellar tendon–splitting approach. The first report described patellar tendon rupture 8 years after IM nailing of the tibia during a forced deep-flexion movement.²¹ Radiographic examination showed the IM nail positioned proud relative to the tibial plateau, impinging upon the patellar tendon. An intraoperative examination confirmed the radiographic findings and found rupture of the patellar tendon to be consistent with the exposed tip of the IM nail. The second report described patellar tendon rupture 2 months postoperatively in a patient with Ehlers-Danlos syndrome, a hereditary disorder characterized by alterations to muscle/tendon tissue and hyperextensible skin.²²

Patellar tendon rupture after IM nailing of the tibia is a rare complication. Patellar tendon re-rupture after primary repair in a patient with history of IM tibial nailing has not been reported. This case outlines the progression of such a patient with a recurrent patellar tendon rupture that was successfully reconstructed using an Achilles tendon allograft. The patient’s surgical history of IM tibial nailing through a mid-patellar tendon–splitting approach 4 years prior to initial tendon rupture is noteworthy and potentially predisposed the patient to injury. The patient provided written informed consent for print and electronic publication of this case report.   

Case Report

A 44-year-old woman, 5 ft, 3 in tall, and weighing 129 lb (body mass index, 22.8), with a history of osteoporosis and transverse myelitis, presented with pain and persistent swelling about the left knee. Her baseline ambulatory status required crutches because of decreased sensation and strength in her lower extremity in conjunction with a foot drop; she had mild quadriceps and hamstring muscle weakness but otherwise normal knee function. The patient had been seen 4 years earlier at our facility for IM fixation of a distal tibia fracture through a patellar tendon–splitting approach. The fracture was well healed and showed no signs of complication or nail migration; the nail was not proud.

Initially, the patient was admitted to another hospital through the emergency department for swelling and pain about the left knee. She was believed to have an infection and was placed on antibiotics by the primary care team. An orthopedic evaluation showed induration, edema, and warmth in the patellar tendon region of the left knee. Magnetic resonance imaging (MRI) showed a full-thickness patellar tendon rupture. Aspiration of the knee was performed and cultures were negative; white blood cell, erythrocyte sedimentation rate, and C-reactive protein values were normal. The risks and benefits of various treatments were discussed, and surgical intervention was elected to repair the patellar tendon.

Intraoperative findings showed a massive midsubstance rupture of the patellar tendon, accompanied by medial and lateral retinacular tears and a quadriceps tendon partial rupture; the central aspect of the quadriceps tendon attaching to the patella remained intact. The patella was retracted proximally; no evidence of active infection was present. Good-quality tissue remained attached to both the tibial tuberosity and the inferior pole of the patella. A No. 2 FiberWire suture (Arthrex, Inc, Naples, Florida) was used to run whip stitches in the distal end of the patellar tendon and a second No. 2 FiberWire suture was used to run whip stitches in the proximal aspect of the patellar tendon rupture. The 4 ends of the sutures were tied together, thus re-approximating the distal and proximal ends of the ruptured patellar tendon. No bone drilling was used because the midsubstance tear was amenable to good repair with reasonable expectation of healing based on tissue quality. The quadriceps tendon, which was partially torn, was repaired with a No. 1 Vicryl suture (Ethicon, Somerville, New Jersey). The medial and lateral retinacula were also repaired with a No. 1 Vicryl suture. The suturing scheme effectively re-approximated the knee extensor mechanism, and the patient was placed in a knee immobilizer that permitted no flexion for 6 weeks postoperatively.

After 3 months of gradual improvement with physical therapy, the patient returned for a follow-up visit, concerned that her knee function was beginning to decline. Physical examination showed patella alta with a thinned and diminutive palpable tendon in the patellar tendon region. She was capable of active flexion to 90º and extension to 50º, but beyond 50º, she was unable to actively extend; she was capable of full passive extension. MRI showed a repeat full-thickness patellar tendon tear with retraction from the inferior pole of the patella; previous tears to the quadriceps tendon were healed. Because of the recurrent nature of the injury, the patient’s physical examination, MRI findings, and anticipated poor quality of remaining tendon tissue, patellar tendon reconstruction using a cadaveric Achilles tendon allograft was recommended. The patient chose surgery for potential improvement in knee range of motion, active extension, and ambulation.

The previous anterior midline incision was used and carried down through the subcutaneous tissues where a complete rupture of the patellar tendon was identified. A limited amount of good-quality tendon tissue remained at the medial aspect of the tibial tuberosity. The remaining tissue located at the patella’s inferior pole was nonviable for use in surgical repair. Retinacular contractures were released to bring the patella distally; the trochlear groove was used as the anatomic landmark for the patella resting position. During reconstruction, the knee was placed into 30° of flexion, with the patella located in the trochlear groove, and the cadaveric Achilles tendon was placed on the midline of the patella, where measurements were done to assess proper length and tension (Figure 1).

The patient’s remaining native tissue on the medial aspect of the tibial tuberosity was used to augment the Achilles tendon graft medially. The cadaveric Achilles tendon graft was primarily used to replace the central and lateral aspects of the patellar tendon. Additionally, the calcaneal bone segment at the end of the Achilles tendon graft was removed prior to use. Cadaveric and host tissues at the medial aspect of the tibial tuberosity were sutured together with a No. 1 Vicryl suture (Figure 2). The distal aspect of the cadaveric Achilles tendon was used to re-approximate the patient’s native patellar tendon insertion at the tibial tuberosity. To supplement the graft anchor, a Richards metallic ligament staple (Smith & Nephew, Memphis, Tennessee) was used to fix the distal aspect of the Achilles tendon graft into the tibial tuberosity.

Proper tensioning of the graft was performed by visualizing patella tracking during the arc-of-knee motion and properly suturing the graft to allow for functional range. The proximal aspect of the cadaveric Achilles tendon was sutured into host tissues surrounding the superior pole of the patellar and quadriceps tendon. The edges of the graft were sutured with supplemental No. 1 Vicryl sutures (Figure 3).

Before surgical closure, knee range of motion was checked and noted to be 0º to 100º.  The repaired construct was stable and uncompromised throughout the entire range of motion. Patella tracking was central and significantly improved; knee stability was normal to varus and valgus stress.

The patient was placed in a knee immobilizer for 6 weeks before range of motion was allowed. Seven months postoperatively, the patient returned for a follow-up visit, ambulating with 2 forearm crutches, which was her baseline ambulatory status. Physical examination revealed passive range of motion from 0º to 130º, an extension lag of 10º, and 4/5 quadriceps strength. It was recommended the patient continue physical therapy to improve strength and range of motion.

Conclusion

This is the first report in the literature documenting a recurrent patellar tendon rupture after primary repair in a patient with a history of IM tibial nailing. It is also the first report of a cadaveric Achilles tendon allograft used as a solution to this problem. Complete reconstruction of the patellar tendon using an Achilles tendon allograft is a method commonly used for ruptures after total knee arthroplasty.^4-7,23,24 This case report highlights the utility of a cadaveric Achilles tendon in the setting of a recurrent patellar tendon rupture with poor remaining tissue quality.