Weight-bearing condyle motion of the knee before and after cruciate-retaining TKA: In-vivo surgical transepicondylar axis and geometric center axis analyses.

An equal knee joint height during flexion and extension is of critical importance in optimizing soft-tissue balancing following total knee arthroplasty (TKA). However, there is a paucity of data regarding the in-vivo knee joint height behavior. This study evaluated in-vivo heights and anterior-posterior (AP) translations of the medial and lateral femoral condyles before and after a cruciate-retaining (CR)-TKA using two flexion axes: surgical transepicondylar axis (sTEA) and geometric center axis (GCA). Eleven osteoarthritis (OA) knee patients were studied during a weight-bearing single leg lunge, using a validated dual fluoroscopic imaging system (DFIS) based tracking technique. Eight healthy subjects were recruited as controls. The results demonstrated that following TKA, the medial and lateral femoral condyle heights were not equal at mid-flexion (15-45°, medial condyle lower then lateral by 2.4mm at least, p<0.01), although the knees were well-balanced at 0° and 90°. While the femoral condyle heights increased from the pre-operative values (>2mm increase on average, p<0.05), they were similar to the intact knees except that the medial sTEA was lower than the intact medial condyle between 0° and 90°. At deep flexion (>90°), both condyles were significantly higher (>2mm, p<0.01) than the healthy knees. Anterior femoral translation of the TKA knee was more pronounce at mid-flexion, whereas limited posterior translation was found at deep flexion. These data suggest that a well-balanced knee intra-operatively might not necessarily result in mid-flexion and deep flexion balance during functional weight-bearing motion, implying mid-flexion instability and deep flexion tightness of the knee.