Purpose of study There is renewed scientific interest in the use of metal-metal bearings for hip replacements. Such bearings have lower volumetric wear rates compared to metal or ceramic on polyethylene bearings. They permit the use of large diameter bearings which potentially have the benefit of reduced dislocation. They also allow the use of thin components without the risk of fracture associated with similar ceramic-ceramic components. However, there remain concerns about the long-term effects of nanometre sized debris and the release of metal ions. It is therefore critical to understand which parameters are important in minimising the amount of debris generated. This study investigated the effect of design and materials on the wear rates in a hip simulator.

Methods Wear studies were carried out in a 10 station ProSim hip simulator in 25% newborn calf serum. A Paul type load curve was applied (maximum load 3000N, minimum 300N) in an anatomical configuration. The extent of a fluid film between the bearing surfaces was determind by measuring the voltage drop between the components. Test samples were made from low-carbon (< 0.05%) and high-carbon (> 0.20%) CoCrMo alloys in various conditions. These samples had bearing surface diameters of 16–54.5mm. The diametral clearance between the femoral head and acetabular cups were from 50–300um.

Results The results of this study were that the low-carbon material wears more than high-carbon materials, there is no significant difference in wear performance of the various forms of high-carbon material tested (wrought, cast, and cast and heat treated), and wear decreased with reduced clearances and increased component diameter. Voltage changes indicated that reduced clearances resulted in component separation and fluid film lubrication

Conclusions These results are consistent with the hypothesis that large diameter metal-on-metal bearings with optimized bearing surface geometry operate in the mixed and/or fluid film lubrication regime.