Patellar position after total knee arthroplasty: influence of femoral component malposition.

Patellar shift, tilt, and rotation were analyzed in 7 cadaveric knee specimens during simulated quadriceps loading, in the intact knee, and after implant reconstruction. Femoral component medialization, lateralization, and external rotation were also investigated. Relative motion of the patella with respect to the femur was measured using an electromagnetic tracking system. The spatial position of the patella did not change with standardized total knee arthroplasty (P <.05). After malpositioning of the femoral component, patellar rotation also did not change (P >.05); however, patellar tilt was altered by femoral component external rotation malposition (P <.05), and patellar shift was affected by all femoral component malpositions (P <.05). The spatial position of the patella relative to the femoral shaft was changed with any femoral component malposition, suggesting that the soft tissues were abnormally tensioned. This could result in subsequent wear on the patellar component and, therefore, early failure.