The effect of femoral component rotation on the kinematics of the tibiofemoral and patellofemoral joints after total knee arthroplasty

Purpose Complications after total knee arthroplasty (TKA) often involve the patellofemoral joint, and problems with patellar maltracking or lateral instability have sometimes been addressed by external rotation of the femoral component. This work sought to measure the changes of knee kinematics caused by TKA and then to optimise the restoration of both the patellofemoral and tibiofemoral joint kinematics, by variation of femoral component internal-external rotation. Methods The kinematics of the patella and tibia were measured in eight cadaveric knees during active extension motion. This was repeated with the knee intact, with a Genesis II TKA in the standard position (3A degrees of external rotation) and with the femoral component at +/- 5A degrees rotation from there. Results Both patellar and tibial motions were significantly different from normal with the standard TKA rotation, with 3A degrees tibial abduction at 90A degrees flexion and reversal of the screw-home from 5A degrees external rotation to 6A degrees internal rotation. The patella was shifted medially 6 mm in flexion and tilted 7A degrees more laterally near extension. Femoral rotation to address one abnormality caused increased abnormality in other degrees of freedom. Internal and then external rotation of 5A degrees caused tibial abduction and then adduction of 5A degrees at 90A degrees flexion. These femoral rotations also caused increased patellar lateral tilt of 4A degrees with femoral external rotation and decreased tilt by 4A degrees with internal rotation. Thus, correction of tibial abduction in flexion, by external rotation of the femoral component, worsened the patellar lateral tilt near extension. Conclusions It was concluded that femoral rotation alone could not restore all aspects of both patellar and tibial kinematics to normal with this specific implant. The clinical relevance of this is that it appears to be inadvisable to reposition the femoral component, in an attempt to improve patellar tracking, if that repositioning may then cause abnormal tibiofemoral kinematics. Further, the pattern of patellar tracking, with the type of TKA used in this study, could not be adjusted to normal by femoral component rotation.