Varus tibial component alignment enhances bearing orbit safety in Oxford unicompartmental knee arthroplasty

Abnormal bearing movements, including bearing contact with or separation from the vertical wall (VW) of the tibial component, can cause bearing dislocation or spin-out in Oxford mobile-bearing unicompartmental knee arthroplasty. This study aimed to evaluate the influence of varus placement of the tibial component on the bearing orbit in Oxford unicompartmental knee arthroplasty. This retrospective study included 75 knees that underwent Oxford unicompartmental knee arthroplasty. The conventional group comprised 43 knees where tibial cuts were made orthogonally to the tibial axis. The varus group included 32 knees where tibial cuts were made slightly varus (targeting 3°). Intraoperative bearing movement was assessed at various knee flexion angles and compared between the groups. In both groups, the mobile bearing shifted posteriorly during knee flexion, tracing a rough C-shaped orbit that was closest to the VW at 60° of flexion. The varus group bearings had shorter mediolateral (ML) movements from 0° to 60° of flexion (1.75 ± 1.10 mm vs. 0.93 ± 1.98 mm, p = 0.008) and were less likely to impinge against the VW (odds ratio = 3.20, p = 0.034). The incidence of bearings separating from the VW (>4 mm) did not differ between groups. Varus placement of the tibial component reduces intraoperative bearing ML translation during 0-60° of knee flexion and decreases the risk of impingement with the VW, compared with neutral placement along the mechanical axis. Level IV.