The tribological significance of the joint fluid analog in a hip joint simulator
Wear is the number one concern with regards to the longevity of THR (total hip replacement). Therefore, reliable in-vitro prediction of wear is necessary. Thus, the laboratory should first validate their hip simulators with known clinical materials. The limiting factor in hip wear simulation has been the joint fluid analog. Using 100% bovine serum as the joint fluid analog, UHMWPE (ultra-high molecular weight polyethylene) wear-rates have been continually underestimated and PTFE (polytetrafluoroethylene) wear has been overestimated. Therefore, this work investigated the effect of protein concentration in bovine serum on the wear of PTFE and UHMWPE in a biaxial hip joint simulator. Validation criteria were developed based on the clinical findings of: ball size effect of increased wear with increased head size, 6% increase in wear for each millimeter of increased head diameter, clinical wear magnitudes, PTFE/UHWMPE wear-rate ratio and debris morphology. Both materials duplicated the clinical criteria using bovine serum with 10mg/ml of protein concentration. As protein concentration went from 0 to 10mg/ml, wear of both materials increased, however with greater than 10mg/ml protein; a) the rate of increase for PTFE was reduced by 80% and b) the wear of UHMWPE reversed, thus, showing that proteins cause wear. Additionally as the volume of fluid was increased, wear increased. This change in wear with protein concentration and volume was due to a protection of protein precipitate. As protein concentration increased protein precipitation increased and wear was decreased due to a protective layer of precipitates. Furthermore, wear protection was dependent on the amount of protein precipitation which was in turn, dependent on the initial concentration, volume of fluid and time. Therefore, wear in-vitro was dependent on the joint fluid analog. This work proved that the laboratory could duplicate clinical findings using bovine serum with 10mg/ml of protein concentration as the joint fluid analog and thus increase confidence in wear evaluation; taking the first steps to showing reliability of in-vitro THR wear studies.