Development of a Stop-Dwell-Start (SDS) Protocol for In Vitro Wear Testing of Metal-on-Metal Total Hip Replacements

The effect of stop-start motion on the wear of metal-on-metal (MoM) bearings was investigated. Thirty-six-millimeter MOM bearings were tested using a ProSim hip wear simulator. The test protocol was adapted from ISO 14242. The effect of different lengths of dwell periods and the effect of the number of steps between dwell periods were investigated. The standard ISO 14242 walking cycle was interrupted with dwell periods during which motion was held, and load was applied at 1250 N. Different dwell periods of 5, 30, and 60s were evaluated, with the dwell applied every 10 walking cycles. Under standard walking conditions, a mean wear rate of 0.11 +/- 0.08 mm(3)/motion cycle (mc) was recorded. When dwell periods of differing duration were introduced, no significant increases in wear were measured for any of the groups, with wear rates for the 5s, 30s, and 60s dwell groups of 0.15 +/- 0.15 mm(3)/mc, 0.18 +/- 0.11 mm(3)/mc, and 0.09 +/- 0.03 mm(3)/mc, respectively. The effect of the number of walking cycles between dwells was also evaluated. A standard dwell period of 10 s was used and alternated with walking cycles; one test had two steps between dwell periods, and the other had a single step between dwell periods. Significant increases in wear were observed in both groups relative to standard walking conditions. The mean wear rate for the two-step test was 0.53 +/- 0.38 mm(3)/mc, and it was 1.69 +/- 0.33 mm(3)/mc for the single-step test. This represents approximately 5-fold and 15-fold increases, respectively, in wear rate. These results indicate changes in bearing lubrication caused by the stop-start motion; however, the effect is limited to the first few cycles of walking after starting, and thus a shorter period of walking between pauses has a more pronounced effect. Further investigation of these parameters is necessary to determine the significance of these wear test methods.