EFFECT OF ROTATIONAL MALPOSITION OF THE FEMORAL COMPONENT ON KNEE STABILITY KINEMATICS AFTER TOTAL KNEE ARTHROPLASTY

Excessive external rotation of the femoral component can cause an abno rmally tight popliteus tendon complex, which induces loss of rotationa l laxity of the knee in the late phase of knee flexion after total kne e arthroplasty. This study evaluated the effect of popliteus tendon re lease on rotational and varus-valgus laxity of implanted knees with an excessively externally rotated femoral component. Rotational and varu s-valgus laxity was measured with a knee kinematics testing device bef ore and after total knee arthroplasty. External rotational positions o f the femoral component of 5 degrees and 8 degrees were compared, and the effects of popliteus tendon release on rotational and varus-valgus laxity were evaluated. To further investigate this question, the effe ct of a conforming articular design was compared with that of a hat ti bial surface. External rotational position of 5 degrees did not change rotational or varus-valgus laxity of the knee. With an 8 degrees exte rnal rotational position, however, external rotational laxity signific antly decreased in knees with a conforming surface at angles of 30 deg rees, 45 degrees, 60 degrees, and 90 degrees. After popliteus tendon r elease, external rotational laxity significantly improved at 90 degree s flexion and was identical to that of the normal knee. Internal rotat ional range was similar before and after popliteus tendon release. Pop liteus tendon release did not affect the varus-valgus laxity (stabilit y) with either articular surface.