The influence of stress conditions on the wear of UHMWPE for total joint replacements.

In vitro studies of the effect of contact stress on the wear of ultra high molecular weight polyethylene (UHMWPE) in orthopaedic applications have produced contradictory results which predict both increased and decreased wear with increasing contact stress. In vivo studies of functioning hip prostheses have reported that 22 mm femoral heads generate lower linear and volumetric wear rates than 32 mm femoral heads. The effect of decreasing the head size will increase the contact stress but decrease the sliding distance per motion cycle. The present study consists of wear experiments under a range of contact stress magnitudes and application conditions in order to simulate the wear processes occurring in vivo. The results from these tests indicated that the wear factor actually decreases with increasing contact stress if the stress was not varied with time. If a time dependent or spatially varying stress was applied, the wear factor can increase greatly when compared to similar magnitude constant contact stress. This effect may be due to the complex relationship between the rate of wear particle generation and the rate at which the particles are released from the interface. The results of these wear experiments are discussed in terms of the influence of the stress conditions upon potential wear processes in total hip and knee prostheses.