Purpose: The results of medial unicompartmental knee replacement have been reported as excellent over the intermediate term with both fixed and mobile bearing designs. However not all publications report equal success, with different outcomes also reported between mobile and fixed bearing designs. The purpose of this study was to compare the outcomes of mobile and fixed bearing medial unicompartmental arthroplasties.

Method: We evaluated the results of 230 consecutive medial unicompartmental knee arthroplasties (UKA) designs performed between 1989 and 2006 at a single academic centre (London Health Sciences Centre). Eighty mobile bearing UKA (63 patients, mean follow up3.2 years) and 160 fixed bearing UKA (117 patients, mean follow up 6.2 years) were reviewed. The mean age of the patients at the time of surgery was 66 years. Forty nine patients died at a mean of 12.7 years after the index arthroplasty. Patients were evaluated clinically using the Knee Society, WOMAC and SF12 outcome scores and radiographically using the Knee Society rating system.

Results: In the fixed bearing group 21 knees underwent revision at a mean of 6.9 years after the index procedure. In the mobile bearing group 7 knees underwent revision at a mean of 2.6 years. The mean Knee Society knee and function scores for the patients who had not had a revision improved in both groups, with the fixed bearing group mean function and Knee Society Scores improving from 53 and 48 points preoperatively to 80 and 93 points at the time of the most recent evaluation. In the mobile bearing group the mean function and Knee Society Scores improved from 55 and 44 points preoperatively to 85 and 91 points at the time of the most recent evaluation. The five year cumulative survival rates were 96% (SE +/− 0.18) and 89% (SE +/− 0.46) for the fixed and mobile bearing designs respectively, with ten year cumulative survival rates of 85% (SE +/− 0.35) in the fixed bearing group using an endpoint of revision to tricompartmental knee arthroplasty.

Conclusion: Both bearing designs of medial unicompartmental knee arthroplasty provided excellent pain relief and restoration of function in carefully selected patients and demonstrated durable implant survival over the midterm. In this study the fixed bearing designs demonstrated improved survivorship over the mobile bearing designs at 5 years. This may be related to the learning curve of the different bearing systems.