Where Is The Natural Flexion-Extension Axis Of The Knee?

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1 Where Is The Natural Flexion-Extension Axis Of The Knee? Daniel Boguszewski 1, Paul Yang 2, Keith Markolf 1, Frank Petrigliano 1, David McAllister 1. 1 University of California Los Angeles, Los Angeles, CA, USA, 2 University of Miami, Miami, FL, USA. Disclosures: D. Boguszewski: 5; MTF. P. Yang: None. K. Markolf: None. F. Petrigliano: None. D. McAllister: 1; DJO. 2; Conmed/Linvatec. 3B; MTF. 3C; Smith and Nephew. Introduction: Advancements have been made in total knee arthroplasty (TKA), but complications still exist. Problems after TKA include patellofemoral maltracking and anterior knee pain that may be related to improper alignment of the components with respect to the natural flexion-extension axis of the knee (FEA). Clinically the transepicondylar axis (TEA), the axis connecting the medial and lateral epicondyles, is often used to represent the FEA during TKA. However, literature shows high intra- and interobserver variability establishing the TEA [1], as well as ambiguity due to multiple methods and definitions [2]. Previous investigations have attempted to establish a more accurate definition of the FEA [3-6], but there is controversy related to its position and orientation relative to the TEA. Using a new methodology, we sought to more accurately define the relative position and orientation of the FEA to the TEA, while examining the coupled tibial motions resulting from flexion-extension about each axis. In theory, minimizing coupled tibial motion during flexion-extension better defines the natural axis of the knee. Methods: Thirty-seven fresh-frozen knees were used for this study (22 right, 15 left, 18 male, and 19 female). The mean age was 33 years. The femur and tibia of each specimen were sectioned 12 inches from the joint line and potted in polymethylmethacrylate. First the TEA was established via manual palpation of the femoral condyles at the lateral epicondylar eminence and the medial epicondylar sulcus, and marked at the surface. Using a custom built test apparatus, the knee was then aligned such that flexion-extension occurred about the TEA (Fig. 1). Additional markers were placed along the tibial shaft and at the distal end of the tibia. Using a three-dimensional coordinate-measuring machine (CMM; Faro Gage, FARO Technologies Inc., Lake Mary, FL), the position and orientation of these markers were measured and recorded during 0-90 of passive flexionextension about the TEA. These measurements were used to examine coupled tibial motion, defined as the resultant displacement from medial-lateral, antero-posterior, and proximal-distal motion of the distal tibia during passive flexionextension (Fig. 1). Next the FEA was established in the knee by repositioning the femur within the test apparatus through a series of adjustments which resulted in minimized coupled tibial motion, accepted as < 10 mm of resultant distal tibia displacement as measured by the CMM. Once the final femoral position was determined, the position and orientation of the markers were measured and recorded during 0-90 of passive flexion-extension about the FEA. Comparisons were made between TEA and FEA coupled tibial motions resulting from flexion-extension about each axis. Statistical analysis included a twosample t-test for axis location and a paired t-test for coupled tibial motion comparisons. The significance level was set at p < Results: Flexion-extension about the FEA showed significantly less coupled tibial motion than about the TEA, producing 6.9 ± 3.3 mm (mean ± SD) of resultant displacement at the distal end of the tibia compared to 19.1 ± 13.4 mm (p < 0.01) for the TEA. The orientation of the FEA relative to the TEA was 1.4 ± 0.7, as the lateral aspect of the FEA was more posterior and superior than the medial aspect relative to the TEA. On the lateral femoral condyle, the FEA location relative to the TEA was 7.4 ± 3.7 mm posterior and 9.2 ± 3.6 mm inferior (Fig. 2). On the medial femoral condyle, the FEA location relative to the TEA was 5.3 ± 3.4 mm posterior and 8.6 ± 4.0 mm inferior (Fig. 2). Right-left and male-female comparisons of the FEA showed no significant differences in coupled tibial motions or the relative medial and lateral locations femoral condyle locations. Discussion: The natural flexion-extension axis of the knee, defined as our FEA, is significantly more posterior and closer to the joint line than the transepicondylar axis, defined as our TEA. While the FEA and TEA axes were oriented only 1.6 apart, the FEA defined in this study produced significantly less coupled tibial motion than the clinically used TEA. This is an important distinction and could aid in optimizing the alignment of TKA components. Greater tibial motion due to flexion-extension about an unnatural axis could lead to progressive wear on TKA components and may increase the risk of patellofemoral maltracking. Additionally, a more accurate location of the FEA could have important implications in joint biomechanics research, and aid in future cadaveric kinematic studies ranging from investigating TKA to studying possible knee injury mechanisms. Significance: A more precise flexion axis of the knee, found posterior and inferior to the transepicondylar axis, could help optimize TKA alignment and reduce incidence of anterior knee pain and patellofemoral maltracking. Acknowledgments: This study was supported by the Musculoskeletal Transplant Foundation (MTF Grant # ). References: 1. Stoeckl et al. J Arthroplasty (6): Akagi et al. Clin Orthop Relat Res (388): Hollister et al. Clin Orthop Relat Res (290): Churchill et al. 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2 6. Asano et al. J Arthroplasty (8):

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4 ORS 2014 Annual Meeting Poster No: 1687

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