Introduction: Cementless cup designs in metal-on-metal (MoM) hip resurfacing devices generally depend on a good primary press-fit fixation which stabilises the components in the early post-operative period. Pressfitting the cup into the acetabulum generates non-uniform compressive stresses on the cup and consequently causes non-uniform cup deformation. That in turn may result in equatorial contact, high frictional torque and femoral head seizure. It has been reported that high frictional torque has the potential to generate micromotion between the implant and its surrounding bone and as a result adversely affect the longevity of the implant. The aim of this study was to investigate the effects of cup deformation on friction between the articulating surfaces in MoM bearings with various clearances.

Materials and methods: Six Birmingham Hip Resurfacing (BHR) devices with various clearances (80 to 306 μm) were tested in a hip friction simulator to determine the friction between the bearing surfaces. The components were tested in clotted blood which is the primary lubricant during the early post-operative period. The joints were friction tested initially in their pristine conditions and subsequently the cups were deflected by 25– 35 μm using two points pinching action before further friction tests were carried out.

Results and Discussions: It has been reported that reduced clearance results in reduced friction. However, none of the previous studies have taken cup deflection into consideration nor have they used physiologically relevant lubricant. The results presented in this study show that for the reduced clearance components, friction was significantly increased when the cups were deflected by only 30 μm. However, for the components with higher clearance the friction did not change before and after deflection. It is postulated that the larger clearances can accommodate for the amount of distortion introduced to the cups in this study.