Tibial Shaft Axis for Femoral Component Rotation

Abstract

Numerous authors have cited the importance of proper rotational alignment of the femoral component in total knee arthroplasty (TKA).1–4 Position of the patellar groove and flexion gap stability are determined by the femoral component rotation. Improper alignment can lead to abnormal articulation with the tibial component as well as altered patellofemoral tracking.5,6 Rotational malalignment may lead to an increase in the risk of a total knee clinical failure from the complications associated with the patellofemoral joint. Patellar subluxation, dislocation, patellar clunk, eccentric wear, and anterior knee pain have been described as complications of abnormal patellofemoral tracking.2,7 The axis of the posterior femoral condyles has normally been used as the reference for neutral rotation of the femur.2,6,8,9 This was thought to be a reasonable landmark because the goal of TKA is to reestablish alignment of the anatomic femoral condyles.10 However, recent biomechanical analyses have suggested that the transepicondylar axis (TEA) and not the posterior condylar axis parallels the primary center of rotation of the knee joint. Berger and associates2 found that the angle between the posterior condylar surface and the surgical epicondylar axis, defined as the line connecting the lateral epicondylar prominence and medial sulcus of the medial epicondyle, was variable dependent upon sex. Mantas and colleagues4 found a nearly constant relationship of 5 degrees external rotation of the TEA from the posterior condylar axis. The lateral epicondyle was closer to the joint line than the medial epicondyle in both extension and flexion. In a previous study from the author’s, laboratory measurements were made on 13 embalmed cadaver specimens to determine the relationship of the TEA to the longitudinal axis of the lower extremity in both extension and flexion.11 The purpose of that study was to confirm whether or not the TEA could be used to reliably determine posterior condyle resection in TKA. The anatomical study clearly defined a constant relationship of the TEA to the transverse flexion axis of knee rotation and as a line perpendicular to the mechanical axis of the lower extremity. Most importantly, when the knee moves from extension to flexion, a perpendicular relationship is maintained from the TEA to the lower extremity centered on the midline of the talus. The mean tibial angle comparing the TEA to the mechanical axis was 0.4 degrees varus in extension and 0.43 degrees varus in flexion with no significant difference. It was also noted that the center of the knee, defined in the study as the lowest point of the intercondylar notch at the TEA, was located virtually on the mechanical axis of the leg as was the center of the tibial eminence. Based on these anatomical relationships, a technique was developed that allowed the femoral component rotation to be determined by lining up the posterior condylar cuts perpendicular with the longitudinal axis of the tibial shaft. By using the intramedullary femoral rod now available in most instrumentation sets, the posterior condylar cutting guide is attached to this rod, and a long axis rod simply aligns with the center of the ankle joint with the knee flexed.