Effects of hip-loading input on simulated wear of Al₂O₃-PTFE materials

The objective of this study was to compare the effects of static, sinusoidal and physiological load-profiles on wear of Al₂O₃-PTFE materials. This was an accelerated wear model of clinical relevance. In nine experiments, the peak load-levels were varied from 1 to 4 kN in a hip simulator with multi-directional kinematics and with bovine serum used as the lubricant. Systematic wear differences were checked using three sizes of femoral heads in each experiment. The Paul load-profile used was found to be more aggressive than sinusoidal, raising the polytetrafluoroethylene (PTFE) wear-rates by 28%. The PTFE cups showed a very mild response to increased load magnitudes. only 11-20% increase evident in volumetric wear per 1 kN increase in load. One recommendation was that simulator wear-studies adopt a 0.25-2.5 kN Paul load-profile as their standard. An experiment with 0.84 kN constant-load also performed satisfactorily, with PTFE wear-rates actually higher than with the 1 kN sine and Paul load-profiles. Some wear anomalies were encountered due to the use of serum lubrication. Combinations of large head size, high load-magnitudes, the Paul load-profile and the high serum protein concentrations used in this study were at times contributing factors. Use of low-protein serum solution may be advisable for wear studies, not only to properly simulate the polymeric wear characteristics but also to minimize the degradation artifacts more prevalent in higher protein-concentrations.