Abstract
Introduction
Despite the theoretical advantages of mobile bearings for lateral unicompartmental knee replacement (UKR), the failure rate in the initial published series of the lateral Oxford UKR's was unacceptably high. The main cause of failure was early dislocation. In contrast, dislocations of bearings in medial UKR's are rare. The lateral compartment present a higher laxity in flexion than the medial. An adaptation of the lateral design by introducing a convex tibial component and biconcave bearing should tackle this difference in kinematics.
The risk of dislocation increased substantially if the lateral tibial joint line was elevated, quantified by the proximal tibial varus angle. This angle had a significant relationship to dislocation.
A recent kinematic study identified roughly 3 times as much posterior translation of the tibia during deep knee bend activities after lateral UKR compared to the normal knee, possibly also resulting in a higher incidence of bearing dislocation.
With the exception of dislocation, the overall early complication ratio in the initial published series of lateral Oxford UKR was also rather high compared to the last published series. Is there a learning curve?
Materials and Methods
Between January 2009 and April 2010, 16 domed lateral Oxford unicompartmental knee replacements were implanted by the senior author. The valgus deformity was in 2 cases not completely correctable. All femoral components were positioned anatomically. In no case the popliteus tendon was divided. A partial iliotibial band (ITB) release was done in 2 cases. The most common tracking deviation of the bearing peroperatively was a small lift off in deep flexion, seen in 6 cases.
Results
Dislocation: no.
Clinical outcome
Twelve of our patients (75%) have already good or excellent results with no pain in rest, no or mild pain with activity and good restoration of function. One patient feels some pain in deep flexion during work as a carpenter. In only one patient there is still a flexion of less than 100°. Small extension deficits are seen in 4 patients.
Radiographic outcome
The full-leg radiographs showed a valgus axis of 1,2° (-1° to 7°) compared to preoperative 5,8° (0° to 14°). The assessment of the proximal tibial varus angle resulted in an angle of 3,8° (1 to 7°). The measurements on deep flexion radiographs are not yet available.
Discussion and Conclusion
Until now we had no dislocation of the bearing in our series, but further follow-up is needed. The preliminary clinical results are already promising and display no early complications needing further operations. By anatomical placement of the femoral component the height of the lateral joint line seems to be respected, confirmed by a nearly correct proximal tibial varus angle in all cases. An increased proximal tibial varus angle can also be avoided by minimizing damage to the lateral soft tissues during surgery and so not over-tighten the knee. Therefore the popliteus tendon should stay intact. The elevated posterior translation, as seen in the recent kinematic studies of the lateral Oxford UKP can perhaps also be reduced by respecting those soft tissues.
