Bearing Separation From the Lateral Wall of the Tibial Component Is a Risk of Anterior Dislocation of the Mobile Bearing in Oxford Unicompartmental Knee Arthroplasty

Bearing dislocation is a serious complication after Oxford unicompartmental knee arthroplasty. Bearing separation from the lateral wall can cause it to spin (90° horizontal rotation) and eventually dislocate because there is just a 2 mm difference in height in both the lateral and medial sides from the bottom of the bearing, compared with the anterior (5 mm) and posterior (3 mm) sides. The details of this problem have not been previously examined.

Twenty-one dislocations in 12 patients were retrospectively analyzed. Bearing separation was defined as the bearing position being sufficiently distant from the lateral wall of the tibial component to allow spinning. We analyzed the incidence of separation, the direction and the recurrence of the dislocations, and their causes and treatments.

Five of the 12 patients had separation. Of the total of 21 dislocations, 11 occurred in cases of separation (52%). Seven of 11 anterior dislocations were found to have separation, whereas nine of 13 posterior dislocations occurred without separation (P = .0237). Three of 5 patients with separation had recurrence of dislocation, and eventually 2 underwent revision to fixed-bearing unicompartmental knee arthroplasty.

Bearing separation from the lateral wall of the tibial component can cause bearing dislocation, especially in an anterior direction. To prevent separation, the wall-bearing distance should be evaluated before the keel slot preparation, with manipulation as necessary. Conversely, posterior dislocation was predominant in our nonseparation cases.