Radiographic analysis of low contact stress meniscal bearing total knee replacements

Background: Meniscal bearing total knee replacements were developed to decr ease the contact stresses on polyethylene and to reduce polyethylene wear. The kinematics of meniscal bearing knee replacements is poorly understood. The present study was designed to evaluate, with radiographic analyses, the motion of the meniscal bearings and the femoral rollback of the Low Contac t Stress meniscal bearing knee replacement during knee flexion. Methods: Eighty-one Low Contact Stress meniscal bearing total knee replacem ents in seventy-six male patients were assessed on fluoroscopically centere d lateral radiographs made with the knee in full extension and in full flex ion at an average of six years (range, twenty-four to 147 months) after the operation. The distance and direction of motion of the meniscal bearings a nd the center contact position of the femoral condyles were measured. Knee evaluations were performed with use of the Knee Society rating system. Results: The average range of motion of the knees, measured on lateral radi ographs, was 90 degrees (range, 45 degrees to 136 degrees). As they moved f rom terminal extension to terminal flexion, thirty-nine knees (48%) exhibit ed anterior motion of both bearings and sixteen (20%) demonstrated posterio r motion of both bearings. Ten knees (12%) had reciprocal motion of the two bearings (one bearing moving anteriorly and one bearing moving posteriorly ) with flexion, nine knees (11%) had motion of only one bearing, and seven knees (9%) had no motion of either bearing. When moving from full extension to full flexion, eighteen knees (22%) demonstrated femoral rollback, six k nees (7%) showed no change in the position of femoral contact, and fifty-se ven knees (70%) exhibited anterior sliding of the femoral condyles, Flexion of the knees demonstrating femoral rollback averaged 104 degrees (range, 7 6 degrees to 128 degrees), and flexion of the knees demonstrating anterior sliding averaged 94 degrees (range, 45 degrees to 125 degrees). The differe nce was significant (p = 0.03). According to the Knee Society rating system , the average clinical score for the entire group was 76 points (range, 27 to 100 points) and the average functional score for the entire group was 72 points (range, 30 to 100 points). The average clinical score was 79 points (range, 27 to 98 points) for the knees that exhibited anterior sliding of the femoral condyles and 87 points (range, 52 to 100 points) for those exhi biting femoral rollback (p = 0.09). The average functional scores were 64 p oints (range, 30 to 100 points) and 72 points (range, 45 to 100 points), re spectively (p = 0.15). Conclusions: Radiographic analysis of meniscal bearing total knee replaceme nts demonstrated an average anterior motion of both the medial and the late ral meniscal bearing of 4.7 mm (range, 1 to 14 mm) in thirty-nine knees (48 %) as they moved from terminal extension to terminal flexion. Sixty-three k nees (78%) demonstrated no femoral rollback as they were flexed. Knees with anterior sliding of the condyles had a significantly smaller average range of flexion (p = 0.03) and a lower average Knee Society score than did knee s demonstrating femoral roll-back. We believe that lack of rollback indicat es a functional insufficiency of the posterior cruciate ligament.