Background: Range of motion is a crucial measure of outcome after total knee arthroplasty. In order for maximum flexion to occur, the femur must progressively shift posteriorly on the tibia, a movement that is known as femoral rollback. Mobile bearings with free anterior-posterior translation could improve knee flexion by allowing such motion. The purpose of the present study was to determine the influence of an anterior-posterior gliding mobile bearing on the postoperative range of knee motion in patients with an intact posterior cruciate ligament.
Methods: We performed a prospective, randomized clinical trial of fifty consecutive total knee arthroplasties that were performed with use of the LCS-Universal prosthesis. Participants were randomized to receive either a deep-dish rotating platform or a mobile bearing that allowed additional anterior-posterior translation, the latter of which requires an intact posterior cruciate ligament. The ranges of motion of the knees were assessed to detect a 15 degrees difference in the active non-weight-bearing range of motion with a power (1 - beta) of 20% and with the level of significance (alpha) set at 0.05. The translation of the mobile bearing was measured with use of a standardized ultrasound technique and was correlated with maximum knee flexion. The participants and the assessor were blinded throughout the study.
Results: At the time of the one-year follow-up evaluation, forty-eight knees were available for an intention-to-treat analysis. The mean active non-weight-bearing range of motion at one year was 113 degrees (95% confidence interval, 108 degrees to 118 degrees ) in the twenty-six knees that had received a rotating platform and 111 degrees (95% confidence interval, 115 degrees to 125 degrees ) in the twenty-two knees that had received an anterior-posterior gliding bearing (p = 0.57). In the latter group, a continuous rollback occurred in two knees. There was no significant correlation between knee flexion and anterior-posterior translation (r(2) = 0.015).
Conclusions: The use of a mobile bearing that allowed free anterior-posterior translation did not regularly restore femoral rollback and did not improve range of motion after total knee arthroplasty compared with the findings seen in association with the use of a rotating platform.