Introduction: The Oxford medial unicompartmental arthroplasty (UCA) is now routinely performed through a short incision without Dislocation of the patella. The aim of this study was to assess the one-year results of this new technique to determine whether it enhances the quality of outcome, as well as the speed of its achievement.

Method: The first 88 consecutive Oxford UCA’s (Phase 3) implanted into 75 patients by two consultant surgeons were scored pre-operatively and at review with the American Knee Society Score. The average age of the patients was 68.1 years; the male to female ratio was 0.9 to 1.

Results: At review, one patient (one knee) had died and one knee had been revised for infection. The other patients (86 knees) were examined at a mean of 1.3 years from surgery. The average ‘knee score’ improved from 37 points to 95 and the average ‘functional score’ from 51 points to 93. Average maximum flexion improved from 117° to 132°. The ‘knee score’ outcome categories at review were 87% excellent, 8% good, 2.5% fair and 2.5% poor.

Conclusions: Historical comparison of these results with the best results of the Oxford UCA, performed through an open approach with dislocation of the patella, suggest that the minimally invasive approach, in addition to faster recovery, improves the quality of outcome at one year.

When the Oxford unicompartmental meniscal bearing arthroplasty is used in the lateral compartment of the knee, 10% of the bearings dislocate. A radiological review was carried out to establish if dislocation was related to surgical technique.

The postoperative radiographs of 46 lateral unicompartmental arthroplasties were analysed. Five variables which related to the position and alignment of the components were measured. Dislocations occurred in six knees.

Only one of the five variables, the proximal tibial varus angle, had a statistically significant relationship to dislocation. This variable quantifies the height of the lateral joint line. The mean proximal tibial varus angle for knees the bearings of which had dislocated was 9° and for those which had not it was 5°. In both groups it was greater than would be expected in the normal knee (3°).

Our study suggests that a high proximal tibial varus angle is associated with dislocation. The surgical technique should be modified to account for this, with care being taken to avoid damage to or over-distraction of the lateral soft tissues.