The prediction of polyethylene wear rate and debris morphology produced bymicroscopic asperities on femoral heads

Counterface damage in the form of scratches, caused by bone cement, bone or metallic particles, has been cited as a cause of increased wear of ultra-h igh molecular weight polyethylene (UHMWPE) acetabular cups. It is known tha t high levels of particulate wear debris lead to osteolysis. Surface damage was characterized in a series of explanted Charnley femoral heads. The hea ds had a mean scratch height of 1 mu m with a mean aspect ratio (defined as height divided by half width) of 0.1. Wear discs were artificially scratch ed using these scratch geometries as a guide. In addition, the scratch geom etries were incorporated into a finite element model of a stainless steel a sperity repeatedly sliding over UHMWPE under conditions similar to those in an artificial hip joint. Wear tests showed a strong correlation between th e average cross-sectional area of the scratch lip above the mean zero line and the measured wear factor. The finite element model predicted increases in the area of UHMWPE suffering plastic strain with increases in the cross- sectional area of the asperity above the mean line. Analysis of the wear de bris showed the mode of the particle size was 0.01-0.5 mu m for all cases. The morphology of the particles varied with aspect ratio of the asperity, w ith an increased percentage mass of submicrometer-sized debris with increas ed scratch lip aspect ratio. The finite element results predicted that the maximum surface strains would increase with increasing asperity aspect rati o. Examination of the worn UHMWPE pin surfaces showed an association betwee n increased surface damage, probably due to high surface strains, and incre ased aspect ratio. The large areas of surface plastic strain predicted for asperities with high cross-sectional areas above the mean line offer an exp lanation for the positive correlation between wear rate and the average cro ss-sectional area of the scratch lip material. The higher surface strains p redicted for the higher aspect ratios may explain the increased percentage mass of biologically active submicrometer-sized wear particles found for sc ratch lips with higher aspect ratios. (C) 2000 Kluwer Academic Publishers.