The effect of component placement on knee kinetics after arthroplasty with an unconstrained prosthesis

The mechanical success of a total knee replacement demands stable patellar tracking without subluxation and, stable tracking, in turn, can depend largely on the medial-lateral forces restraining the patella. Patellar button medialization has been advocated as a means of reducing subluxation, and experimental evidence has shown femoral component rotation also affects medial-lateral forces. Surgeons have choices in femoral component rotation and patellar button medialization and must frequently make intra-operative decisions concerning component placement because of anatomical variations among patients. Thus, in seeking to minimize medial-lateral patellar force, we examined the effects of patellar button medialization and external femoral component rotation. The study used an unconstrained total knee system implanted in nine cadaveric specimens tested on a knee simulator operating through flexion angles up to 100 degrees. Tests included all combinations of external femoral component rotation of 0 degree, 2.5 degrees, and 5 degrees and patellar placement at the geometric center and at 3.75 mm medial to the geometric center. A video-based motion analysis system tracked patellar and tibial kinematics while a six-component load cell measured patellofemoral loads. Repeated measures analysis of variance revealed a statistically significant decrease in the average medial-lateral force with button medialization but no significant change with femoral component rotation. Neither femoral component rotation nor patellar button medialization had an effect on the normal component of the patellar reaction force. External femoral component rotation did cause significant increases in lateral patellar tilt, in tibial varus angle, and in external tibial rotation. Button medialization caused significant increases in lateral patellar tracking, lateral patellar tilt and external tibial rotation. The results in medial-lateral patellar forces quantify the benefit of patellar button medialization and discount any benefit of femoral rotation. The change in tibial kinematics with patellar button medialization and femoral component rotation cannot be measured in vivo with current technology, and the precise clinical implications are unknown.