The quantitative assessment of UHMWPE wear debris produced in hip simulator testing : the influence of head material and roughness, motion and loading.

Biomaterial wear particles are known to provoke a foreign-body reaction when released from total joint prostheses. Considerable effort is being invested in the search for materials which, by wearing less, will release fewer particles. In this research the role of joint wear simulators is paramount. The wear debris from 26 simulator tests of hip prostheses was extracted from the bovine serum lubricant and sized using a laser diffraction technique. The influence on particle size of a broad range of parameters was examined. The parameters considered included the bedding-in phenomenon, the femoral head material and roughness and physiological versus simplified load cycles and wear paths. The physiological wear simulators produced a similar size distribution of wear particles to that produced by implanted joints. Head material had no effect on this observation. Simplifying the loading cycle or wear path independently made no impact on this finding. This remained the case when simplified load and wear path were combined in one test. The effect of head roughness was pronounced with an increase in minimum particle size with increasing roughness. Joint simulators remain the optimal method for assessing new joint materials and designs. However, their use to characterise joints in terms of wear rate solely should be guarded against. Instead focus should be concentrated on a combination of the size and amount of the wear debris created. In this way the "loosening hazard" of a joint can be distilled