Multi-laboratory simulator studies on effects of serum proteins on PTFE cup wear

A multi-laboratory, simulator study investigated the wear of polytetrafluorethylene (PTFE) cups run in bovine serum. Each laboratory used its own test protocol with a variety of simulator types. Our wear model incorporated 32 mm dia CoCr heads matched to PTFE cups run with serum protein-concentrations in the range 17-69 mg/ml. The multi-lab data demonstrated that protein-concentration had the most significant effect on wear performance. Both inverted and anatomical cups followed the same trend with first a rapid increase in wear-rates apparent for the initially low-protein levels and then a wear-rate reduction effect becoming apparent beyond 17 mg/ml of proteins. The results showed that as the protein concentration increased from 17 to 69 mg/ml, the magnitude of the wear-rates increased 200% but the protein wear-rate gradient decreased 24-60% with "inverted" and "anatomical" cups, respectively. This effect was more pronounced with "anatomical" than "inverted" cups. Thus, the wear-trends with "inverted" cups were generally the more consistent, particularly at the low-protein levels. Increasing the serum volume by two-fold in one study increased the PTFE wear-magnitudes approximately 40% and the protein-wear gradient by 30%. These PTFE wear phenomena were consistent with the concept that low-concentrations of proteins promoted polymer wear but high-protein concentrations resulted in a protein-degradation phenomenon which progressively masked the actual polymer wear. In the selected protein range 17-69 mg/l, the multi-laboratory simulator data consistently overestimated the average clinical wear-rate by at least 50-100% depending on protein range. It would, therefore, appear clinically relevant to study PTFE wear with an inverted-cup model using a large volume of serum but only in low-protein concentrations. The protein-related wear phenomena observed with PTFE cups in this multi-laboratory project may also have relevance for wear-simulation of UHMWPE cups.