The effects of early rollback in total knee arthroplasty on stair stepping

We investigated the effects of early compared with late (ie, in flexion) rollback on quadriceps efficiency during stair stepping. We studied 10 patients with the IB II (Zimmer, Warsaw, IN) total knee arthroplasty (TKA), designed to enforce rollback at 73 degrees of knee flexion; 9 patients with the Maxim PS (Biomet, Inc, Warsaw, IN) TKA, designed to enforce rollback between 20 degrees and 30 degrees of flexion; 8 patients with the TRAC PS (Biomet, Inc, Warsaw, IN) mobile bearing TKA, designed to enforce rollback at 8 degrees of flexion; and 21 healthy control subjects during stair stepping. We measured the external knee flexion moments, which must be largely balanced by quadriceps force acting over the quadriceps lever arm, as indicators of quadriceps efficiency. The peak external knee flexion moment generated by the IB II patients during stair stepping was 12.4% and occurred at 65 degrees of knee flexion. This moment was significantly less (P=.006) than the peak moment, 17.6%, generated by the healthy controls. Knee flexion for the IB II patients did not reach 73 degrees, and rollback was not enforced until after the peak moment (ie, maximum demands on the quadriceps) had been attained. The peak moments generated by the TRAC PS patients, 14.2%, and Maxim patients, 14.8%, were not significantly different from that of the controls. These results suggest that early as compared with late rollback returns more normal quadriceps efficiency during stair stepping.