PATELLAR TRACKING AFTER TOTAL KNEE ARTHROPLASTY - THE EFFECT OF TIBIAL TRAY MALROTATION AND ARTICULAR SURFACE CONFIGURATION

The effect of total knee arthroplasty (TKA) with neutrally aligned and malrotated tibial trays was studied in five fresh anatomic specimen k nees. Patellar shift, tilt, and rotation, and the rotational position of the tibia were measured in normal knees and after TKA with the Orth oloc Modular knee system. Both semiconstrained and unconstrained artic ular surfaces were assessed in the neutral position and at anatomic, 1 5 degrees internal, and 15 degrees external rotation of the tibial tra y. After TKA, the patellae shifted slightly medially in the early phas e of knee flexion because the anterior lateral flange of the femoral c omponent was longer than the lateral trochlea of the femur and because the tibia rotated internally. The raised lateral flange on the femora l component tilted the patella medially at full extension after TKA. T he semiconstrained tray allowed minimal tibial rotation because of its articular configuration. As much as 15 degrees malrotation of the unc onstrained tibial tray did not affect patellar tracking. The semiconst rained tibial tray in the neutral position had almost the same patella r tracking as the unconstrained tray, but at 15 degrees external rotat ion, the semiconstrained tray internally rotated the tibia, leading to medial shift of the patella. Although 15 degrees internal rotation ca used external rotation of the tibia, the patella did not shift as much laterally, despite the increase in the Q angle.