Varus/valgus angle of the femoral component relative to the tibial component increases risk of bearing dislocation in medial mobile bearing unicompartmental knee arthroplasty.

Background: In medial mobile-bearing unicompartmental knee arthroplasty (MB-UKA), the position of the bearing does not correspond to the planned position which will increasing the risk of bearing dislocation. This study aimed to explore the relationship between the malposition of the femoral and tibial components and the phenomenon of bearing deviation using postoperative radiological measurements.

Methods: One hundred twenty patients who underwent mobile-bearing uni-compartmental knee arthroplasty (MB-UKA) at our hospital between January and August 2023 were enrolled in this retrospective study. Standard anteroposterior radiographs were obtained postoperatively using C-arm fluoroscopy. A 3D-2D image matching method was used to measure the femoral component varus/valgus angle and internal/external rotation (IR/ER) relative to the tibial component. The bearing position parameters were measured. Patients were divided into varus/valgus and IR/ER groups according to the femoral component varus/valgus and IR/ER angles. The bearing position parameters of each group were compared separately.

Results: There were 57 patients in the varus group, 63 in the valgus group, 59 in the IR group, and 61 in the ER group. The distance between the bearing lateral edge and the lateral wall of the tibial component (W-BED) was significantly lower in the varus group than the valgus group (1.9 [0-6.4] vs. 2.8 [0.3-7.7] mm; P < 0.001). No differences were detected in any bearing position parameters. The varus/valgus angle of the femoral component showed a significant negative correlation with the W-BED. The IR/ER angle of the femoral component showed no significant correlation with any of the positional parameters.

Conclusions: Implantation of femoral component with varus/valgus angle relative to tibial component can cause the bearing to deviate from the planned position. If the varus/valgus angle exceeds 8° it can cause the bearing to hit or away from the lateral wall of the tibial component, resulting in an increased risk of dislocation of the bearing.