Acetabular Cups in 60 mm Metal-on-Metal Bearings Subjected to Dynamic Edge-Loading with 70° Peak-Inclination in 10-Million Cycle Simulator Study

Wear simulation of total-hip arthroplasty (THA) involves hip biomechanics, tribology, bearing designs and cup wear-patterns. This is the first demonstration of cup edge-loading using the “Inverted-cup” test mode. Benefits included, (i) clinically relevant wear-patterns, and (ii) cup inclinations varying from ideal to edge-loaded during each 1-s simulator cycle. The 60 mm head and cup bearings in metal-on-metal (MOM) hip joints showed run-in and steady-state wear phases to 10-million cycles (Mc). MOM edge-wear was not unduly high at 1.7 mm3/Mc overall, this 3-fold higher than 60 mm MOM study without edge-loading. One MOM outlier averaged 2.7 mm3/Mc, this representing the break-away wear (BAW) phenomena. A surprising result was that cups contributed 75–93% of total wear. The most disturbing conclusion from review of laboratory studies was that MOM wear-rates varied 1 to >30 mm3/Mc for reasons not understood. These data suggested a new hypothesis, that MOM bearings were very sensitive to external stimuli, be they simulator artifact or patient related