Introduction

Unidirectional mobile bearing knees (RP) were developed to optimise the tibio-femoral articulation in an effort to enhance function and reduce polyethylene wear. The self aligning bearing should also benefit the patello-femoral joint further improving outcome. This study was designed to assess whether these potential benefits are realised in the clinical setting.

Aim: The aim of the study was to assess the impact of a self aligning unidirectional mobile bearing TKR on lateral patella release rates within a knee system using a common femoral component for both the fixed and mobile variants.

Methods and Results: A total of 357 patients undergoing TKR were randomly allocated to receive either a Mobile Bearing (181 knees) or a Fixed Bearing (176 knees) PSTKR. Further sub-randomisation into patella resurfacing or retention was performed for both designs. The need for lateral patella release was assessed during surgery.

The lateral release rate was similar for fixed bearing (9%) and mobile bearing (9%) implants (p=0.95). Patella resurfacing resulted in lower lateral release rates when compared to patella retention (5.5% vs 13.5%; p=0.012). This difference was most marked in the mobile bearing group where the lateral release rate was 16% with patella retention compared to 3% with patella resurfacing (p=0.009).

Conclusion: The addition of a rotating platform tibial component has had no impact on the lateral release rate in this study. Optimising patella geometry by patella resurfacing appears more important than tibial bearing deisgn per se. The combination of a mobile bearing design and patella resurfacing appears the optimise patello-femoral tracking.

Aim: This study aims to establish whether or not mobile bearing TKR delivers the often stated benefits improved function and range of motion when compared to its fixed bearing equivalent.

Methods and Results: A total of 357 patients undergoing TKR were randomly allocated to receive either a Mobile Bearing (181 knees) or a Fixed Bearing (176 knees) PSTKR. Further subrandomisation into patella resurfacing or retention was performed for both designs. All knees were scored using standard tools (Oxford, AKSS and SF12) preoperatively and at intervals postoperatively by independent observers.

The range of motion increased from an average of 96 deg. (pre-op) to an average of 109 deg. at 1 year post-op for both the fixed and mobile bearing design. The management of the patella had no effect in either group.

The knee society and knee function scores increased equally for both the fixed bearing and mobile bearing knees with no differences noted.

Conclusion: There were no measurable differences in range of motion and clinical outcome scores at 1 year post-op for a mobile bearing design over its fixed bearing equivalent. It is likely that any potential advantages of a mobile bearing design will manifest in longevity rather than function.