Mobile bearing dislocation in lateral unicompartmental knee replacement.

Despite the theoretical advantages of mobile bearings for lateral unicompartmental replacement (UKR), the failure rate in the initial published series of lateral Oxford UKR's was unacceptably high. The main cause of failure was bearing dislocation. To address this problem we first modified the surgical technique and then introduced a new design with a convex domed tibial plateau. This paper presents the results of these changes. In the original series (n=53), implanted using a standard open approach, there were six dislocations, all of which occurred in the first year. Five of the dislocations were primary and one was secondary to trauma. In the second series (n=65), with the modified technique, there were three dislocations, all of which were primary and occurred in the second and third year. In the third series (n=101, 69 with a minimum 1-year follow-up), with the modified technique and the domed tibial plateau, there was one dislocation which was secondary to trauma and occurred in the second year. At 4 years the dislocation rates in the three series were 11%, 5% and 1.7% and the primary dislocation rates were 10%, 5% and 0%. Both the overall and the primary dislocation rates were significantly different (p=0.04 and p=0.03) in the different series. The combination of the modified surgical technique and new design with a domed tibial component appears to have reduced the early dislocation rate to an acceptable level.