Intraoperative mobile-bearing movement in Oxford unicompartmental knee arthroplasty.

PURPOSE: Mobile-bearing Oxford unicompartmental knee arthroplasty has been used widely and successfully; however, there is no previous research on the intraoperative bearing movement. The purpose of this study was to characterise intraoperative bearing movement and investigate whether bearing movement relates to component positions, bearing size, intraoperative rotational kinematics and clinical results. METHODS: A trial tibial component, which was scaled every 2 mm, was used to measure the intraoperative movement of the mobile bearings. Bearing movements from 30° flexion to deep flexion were classified into two groups: 30 patients exhibited posterior bearing movement that contacted the lateral wall of the tibial component (Group W), and 37 patients exhibited posterior bearing movement, without contact, or with only transient contact, with the lateral wall (Group S). RESULTS: All mobile bearings moved posteriorly during knee flexion in the anteroposterior direction. In postoperative radiography with the knee flexed, the femoral component in Group W was significantly more laterally implanted than that in Group S. The width ratio between the bearing and tibial component in Group W was significantly greater than in Group S. There were no significant differences in other radiological measurements, intraoperative rotational kinematics or clinical results. CONCLUSION: During knee flexion, all mobile bearings moved posteriorly, and mobile bearings whose femoral components were set laterally, tended to move posteriorly while in contact with the lateral wall. However, there were no significant differences in clinical evaluation. LEVEL OF EVIDENCE: IV.