Surgical epicondylar axis vs anatomical epicondylar axis for rotational alignment of the femoral component in total knee arthroplasty.

The anatomical epicondylar (AEpi) axis and the surgical epicondylar (SEpi) axis have been widely used as the epicondylar axis, one of the most commonly used axes for rotational alignment of the femoral component in total knee arthroplasty. The purpose of this study was to evaluate the differences and reliability between these two axes. Computerized tomography scan of the distal femur was done in 55 osteoarthritic knees. Thirty-two knees were varus and 23 knees were neutral in alignment. Axes for rotational alignment of the femoral component were lined including posterior condylar (PC), anteroposterior (AP), AEpi, and SEpi axes. Angles between each pair of axes were measured including PC-AEpi, PC-SEpi, AP-AEpi, AP-SEpi and AP-PC. The average PC-AEpi angle was 5.7 degrees +/- 1.7 degrees. The average PC-SEpi angle was 1.5 degrees +/- 2.1 degrees. The average AP-AEpi angle was 90.2 degrees +/- 1.0 degrees. The average AP-SEpi angle was 94.5 degrees +/- 1.3 degrees and the average AP-PC angle was 95.9 degrees +/- 2.0 degrees. Twenty-nine per cent of knees had prominent medial epicondyle (a landmark for AEpi axis) and 5 per cent had prominent medial sulcus (a landmark for SEpi axis). The lateral epicondyle was prominent in all knees. There were no significant differences of all angles of referencing axes between men and women (p>0.05). There were no significant differences between varus and neutral knees in terms of PC-AEpi angle and PC-SEpi angle (p>0.05). The AEpi axis was more perpendicular to the AP axis and more external rotated to the PC axis than the SEpi axis. Because the perpendicular line to the AEpi axis was closer to the AP axis than that of the SEpi axis and the AEpi axis provided appropriate external rotated to the PC axis, the AEpi axis was more reliable for rotational alignment of the femoral component than the SEpi axis. On the other hand, SEpi axis, providing less external rotated to the PC axis, may be difficult to define and could cause patellofemoral tracking problems in total knee arthroplasty.