Effect of femoral head surface roughness on the wear of ultrahigh molecular weight polyethylene acetabular cups.

We studied the effect of femoral head surface roughness on the wear of ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups using a hip joint simulator and a reciprocating wear tester. Compared with the hip simulator, the reciprocating wear tester severely exaggerates the effect of counterface roughness on UHMWPE wear and drastically underestimates the wear rate of the UHMWPE against smooth undamaged counterfaces. According to the hip simulator test results, the wear rate of the UHMWPE cups is approximately proportional to the square root of the femoral head roughness Ra (center-line-average roughness) rather than to Ra raised to a power greater than one as predicted by pin-on-disk studies. Roughening of the femoral heads by an order of magnitude results in a 2- to 3-fold increase in the wear rate. Therefore, the much wider clinical variations of wear cannot be fully explained by variations in surface roughness of the femoral heads.