Performance analysis of the RandomPOD wear test system

The type of relative motion between the bearing surfaces of prosthetic joints is known to strongly influence their wear behaviour. The previously validated 16-station wear simulator of the pin-on-disc type, called RandomPOD, was used to study the wear of a conventional, gamma-sterilized ultra-high molecular weight polyethylene (UHMWPE). The counterface was polished CoCr and the lubricant was diluted calf serum. Two test conditions were compared, random motion/random load and circular translation/static load. With random motion, the accumulated change of the direction of sliding was 2.8 times higher than that with circular translation. The test duration with both test conditions was 880 h. Random motion/random load resulted in a mean wear factor 23% higher than that produced with circular translation/static load. The difference was statistically significant. The wear mechanisms however were similar and in agreement with clinical observations. As earlier studies have shown that the type of load is of secondary importance, the present study confirms the earlier findings that the type of relative motion is tribologically of fundamental importance. In particular, the complex, yet biomechanically realistic non-cyclic motion, represented by the random track, resulted in a wear factor significantly higher than that produced by a fixed slide track shape.