Polyethylene in the form of a concave bearing surface at the hip wears very slowly, and clinical and histological evidence suggests that the volume of debris so released is well tolerated. However, significantly greater volumetric wear rates are produced when (1) polyethylene is used as the convex component of a joint and (2) abrasives such as PMMA or bone enter the joint. Further, large quantities of polyethylene debris lead to the replacement of bone at the bone-cement interface by soft tissue. Evidence is presented which strongly suggests that this mechanism can produce joint component loosening.