The outcome and survivorship of osteotomy for medial compartment osteoarthritis are closely correlated to the changes in the weight bearing axis. Questions remain over the optimal correction when undertaking medial unicompartmental knee replacement (UKR).

Prospective data was collected on 50 patients (30F:20M) undergoing fixed bearing medial UKR which included pre-operative and 12 month Oxford Knee Scores and pre and post-operative weight-bearing long-leg radiographs.

The weight bearing axis was measured from the centre of the femoral head to the mid-point of the talus. The point at which this axis crossed the tibial plateau was expressed as a percentage of the width of that plateau - 0 (medial cortex) to 100% (lateral cortex).

Regression method and correlation coefficients were used to assess the relationship between the response and variables.

A significant correlation was seen between the 12 month score and the change in axis, which was maintained when the pre-operative score was adjusted for (p = 0.043 and 0.046 respectively). Larger changes in scores were seen with larger changes in axis (p = 0.046) when the pre-operative axis was adjusted for.

Higher BMIs reported worse scores at 12 months (p = 0.022) and a smaller overall change in score one year post-operatively (p = 0.037). This significance was improved when the pre-operative scores were adjusted (p = 0.017 and 0.017 respectively).

Proximity of correction of axis to the assumed contralateral normal was weakly correlated (p = 0.049) to the 12 month score, especially when BMI was corrected for.

These results suggest that the weight bearing axis and BMI do play a significant role in early patient outcomes following fixed bearing unicompartmental knee replacement.