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

The mechanical success of a total knee replacement demands stable patellar tracking without subluxation and, stable tracking, in turn, can depend larg ely on the medial-lateral forces restraining the patella. Patellar button m edialization has been advocated as a means of reducing subluxation, and exp erimental evidence has shown femoral component rotation also affects medial -lateral forces. Surgeons have choices in femoral component rotation and pa tellar button medialization and must frequently make intra-operative decisi ons concerning component placement because of anatomical variations among p atients. Thus, in seeking to minimize medial-lateral patellar force, we exa mined the effects of patellar button medialization and external femoral com ponent rotation. The study used an unconstrained total knee system implante d in nine cadaveric specimens tested on a knee simulator operating through flexion angles up to 100 degrees. Tests included all combinations of extern al femoral component rotation of 0 degrees, 2.5 degrees, and 5 degrees and patellar placement at the geometric center and at 3.75 mm medial to the geo metric center. A video-based motion analysis system tracked patellar and ti bial kinematics while a six-component load cell measured patellofemoral loa ds. Repeated measures analysis of variance revealed a statistically signifi cant 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 tibi al varus angle, and in external tibial rotation. Button medialization cause d significant increases in lateral patellar tracking, lateral patellar tilt and external tibial rotation. The results in medial-lateral patellar force s quantify the benefit of patellar button medialization and discount any be nefit of femoral rotation. The change in tibial kinematics with patellar bu tton medialization and femoral component rotation cannot be measured in viv o with current technology, and the precise clinical implications are unknow n. (C) 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd . All rights reserved.