MOBILE BEARING KNEES: WHAT’S ALL THE FUSS ABOUT?

Abstract

Introduction and Aims: Dual surface articulation characterises mobile bearing knee (MBK) designs, thereby increasing the potential for polyethylene damage. This hypothesis was investigated for eight, contemporary MBK designs.

Method: Both joint simulator and computational evaluations were performed on contemporary MBK systems. As part of an FDA clinical investigation a knee joint simulator was designed that successfully cycled three pairs of a MBK design for five million walking cycles at room temperature under saline conditions. Additionally, a finite element analysis was developed to determine the potential for abrasion, delamination and pitting for eight designs during walking gait.

Results: For the MBK simulator evaluation, the mean wear volume was 125mm^3 (range 75–175mm^3). This result is comparable to evaluations performed on fixed bearing designs in contemporary knee simulators utilising bovine serum for lubrication. The surface and subsurface stress distributions measured displayed a wide variation in both magnitude and location. Several of the designs utilised their available articulating interface optimally, lowering the magnitude of the stresses, while others produced edge-loading scenarios with relatively high stresses.

Conclusion: Dual surface articulation between a polyethylene insert and metallic femoral and tibial tray components is a consequence of MBK designs. These studies determined that the potential for polymer damage in optimally aligned MBK systems is significantly less than their fixed plateau counterparts and remains so as long as edge-loading is avoided. Further, MBK designs require high-quality polyethylene and precision manufacturing of the metallic components to ach