Comparing ceramic-metal to metal-metal total hip replacements - A simulator study of metal wear and ion release in 32- and 38-mm bearings

Our 32 and 38 mm alumina ceramic-on-metal (COM) bearings were run in a hip simulator study for comparison with 32 mm metal-on-metal (MOM) controls. The 32 mm MOM bearings demonstrated an overall wear rate of 1.58 mm₃/million cycles (Mc) that was comparable to previous simulator studies. The peak run-in MOM wear-rates (10, 15.7 mm₃/Mc) were higher than in previous simulator studies. There was a noticeable graying in color of serum lubricants with MOM wear rates of 2-3 mm³/Mc and with wear rates of 10-15 mm³/Mc the serum became much darker. The COM lubricants darkened during two "break-away" wear events with wear-rates 5.8-6.7 mm ³/Mc. The 32 and 38 mm COM bearings demonstrated overall wear-rates of 0.38 and 0.29 mm³/Mc, approximately four-fold reduced compared to MOM controls. The COM wear-rates were also much higher than in the one previous COM study. There may be methodological reasons that could explain this discrepancy. Our ion concentrations assessed from serum lubricants had Cobalt (Co) 68% and Chromium (Cr) 32% for average ratio of metal ion composition (i.e. averaging Co/Cr ratios: 2.26) in the parent alloy. Comparing Co ion concentrations during run-in, the COM bearings represented a 35-fold reduction compared to MOM. At 3.0 Mc, the COM represented a 33-fold reduction compared to MOM. Overall, our simulator study confirmed previously published advantages of low wear and reduced metal ions with the ceramic-metal coupling compared to standard metal-metal bearings.