Ultra-low wear rates for rigid-on-rigid bearings in total hip replacements

With the increased clinical interest in metal-on-metal and ceramic-on-ceram ic total-hip replacements (THRs), the objective of this hip simulator study was to identify the relative wear ranking of three bearing systems, namely CoCr-polyethylene (M-PE), CoCr-CoCr (M-M) and ceramic-on-ceramic (C C). Vo lumetric wear rates were used as the method of comparison. The seven THR gr oupings included one M PE study, two M-M studies and four C-C studies. Spec ial emphasis was given to defining the 'run-in' phase of accelerated wear t hat rigid-on-rigid bearings generally exhibit. The hypothesis was that char acterization of the run-in and steady state wear phases would clarify not o nly the tribological performance in vitro but also help correlate these in vitro wear rates with the 'average' wear rates measured on retrieved implan ts. The implant systems were studied on multichannel hip simulators using t he Paul gait cycle and bovine serum as the lubricant. With 28 mm CoCr heads, the PE (2.5 Mrad/N-2) wear rates averaged 13 mm(3)/1 0(6) cycles duration. This was considered a low value compared with the cli nical model of 74 mm(3)/year (for 28 mm heads). Our later studies establish ed that this low laboratory value was a consequence of the serum parameters then in use. The mating CoCr heads (with PE cups) wore at the steady state rate of 0.028 mm(3)/10(6) cycles. The concurrently run Metasul(TM) M-M THRs wore at the steady state rate of 0.119 mm(3)/10(6) cycles with high-protein serum. III the second Metasul M- M study with low-protein serum, the THR run-in rate was 2.681 mm(3)/10(6) c ycles and steady state was 0.977 mm(3)/10(6) cycles. At 10 years, these dat a would predict a 70-fold reduction in M-M wear debris compared with the cl inical PE wear model. All M-M implants exhibited biphasic wear trends, with the transition point at 0.5 x 10(6) cycles between run-in and steady state phases, the latter averaging a 3-fold decrease in wear rate. White surface coatings on implants (coming from the serum solution) were a confounding f actor but did not obscure the two orders of magnitude wear performance impr ovement for CoCr over PE cups. The liners in the alumina head-alumina cup combination wore at the steady s tate rate of 0.004 mm(3)/10(6) cycles over 14 x 10(6) cycles duration (high -protein serum). The zirconia head-alumina cup THR combination wore at 0.17 4 and 0.014 mm(3)/10(6) cycles for run-in and steady state rates respective ly (low-protein serum). The zirconia head and cup THR combination wore slig htly higher initially with 0.342 and 0.013 mm(3)/10(6) cycles for run-in an d steady state rates respectively. Other wear studies have generally predic ted catastrophic wear for such zirconia-ceramic combinations. It was noted that the zirconia wear trends were frequently masked by the effects of tena cious white surface coatings. It was possible that these coatings protected the zirconia surfaces somewhat in this simulator study. The experimental c eramic Crystaloy THR had the highest ceramic run-in wear at 0.681 mm(3)/10( 6) cycles and typical 0.016 mm(3)/10(6) cycles for steady state. Since thes e implants represented the first Crystaloy THR sets made, it was likely tha t the surface conditions of this high-strength ceramic could be improved in the future. Overall, the ceramic THRs demonstrated three orders of magnitu de wear performance improvement over PE cups. With zirconia implants, while the cup wear was sometimes measurable, head wear was seldom discernible. T herefore, we have to be cautious in interpreting such zirconia wear data. I dentifying the run-in and steady state wear rates was a valuable step in pr ocessing the ceramic wear data and assessing its reliability. Thus, the M-M and C-C THRs have demonstrated two to three orders of reduction in volumet ric wear in the laboratory compared with the PE wear standard, which helps to explain the excellent wear performance and minimal osteolysis seen with such implants at retrieval operations.