Mobile-bearing prosthesis and intraoperative gap balancing are not predictors of superior knee flexion: a prospective randomized study

Purpose Range of motion is a crucial measure of the outcome of total knee arthroplasty. Gap balancing technique and mobile-bearing prosthesis can improve postoperative range of motion. The purpose of this study was to determine the factors that are predictive of the postoperative range of motion. Methods A total of 94 knees with varus osteoarthritis were prospectively randomized to receive either a posterior-stabilized mobile-bearing or a posterior-stabilized fixed-bearing prosthesis. All preoperative and postoperative protocols and operative techniques were identical in the two groups. Extension and flexion joint gaps were measured using a tensor device during the operation. Multiple regression analysis was conducted to determine the best predictors of the knee flexion angle 2 years after the operation. The independent variables were type of prosthesis (mobile-bearing or fixed-bearing), difference between flexion and extension joint gaps (mm), age, gender, body mass index (BMI), preoperative and intraoperative knee flexion angles, change in posterior condylar offset, and posterior tilt of the tibial plateau. Results The mean difference between flexion and extension joint gaps was 0.8 ± 1.3 (mean ± SD) mm for mobile-bearing and 0.8 ± 1.9 mm for fixed-bearing prosthesis. The mean flexion angle for mobile-bearing and fixed-bearing groups was 120 ± 16° and 116 ± 20° preoperatively (n.s.), 142 ± 9° and 141 ± 12° intraoperatively (n.s.), and 129 ± 10° and 128 ± 13° at 2 years postoperatively ( p = 0.773), respectively. Predictors were identified in the following three categories: (1) preoperative flexion angle, (2) intraoperative radiographic flexion angle, and (3) BMI ( R = 0.603, p