A hip simulator study of the influence of patient activity level on the wear of crosslinked polyethylene under smooth and roughened femoral conditions

Over the last three decades, tribological studies of polyethylene total hip replacements have been undertaken using a simplified model of normal walking. As hip prostheses are being implanted in younger and more active patients, coupled with the increased wear resistance of crosslinked polyethylene, such in vitro approximations in activity are very limiting. Using a hip joint simulator, the influence of a significant increase in patient activity was studied by applying a series of simulated walking, stumbling and jogging sequences, at varying cycle speeds, using crosslinked UHMWPE/CoCrMo components, with both smooth and roughened femoral heads. All tests were performed using 25% bovine calf serum, and all components were positioned physiologically. The effects on wear, frictional torque, counterface roughness, lubricant temperature, material deformation and particle morphology were measured and analysed. It was found that with smooth heads and non-constraining socket fixtures, the occurrence of excessive stumbling at 1 Hz (5 kN max) had a negligible effect on the wear rate of polyethylene, whilst simulated jogging at 1.75 Hz (4.5 kN max) only showed a median increase in wear volume of 40% compared to normal walking. Fast walking showed the largest wear rate, and was consistently greater than for simulated jogging, thus, suggesting that short periods of increased load and speed have a relatively small effect on polyethylene wear. However, increasing the femoral roughness Ra to 0.38 μm under simulated jogging, 1.75 Hz, led to a massive increase in wear and frictional torque, generating wear rates >3000 mm3/106 cycles for crosslinked polyethylene. Surface topography, sliding speed and the type of socket fixturing were shown to be the most influential factors when simulating increased patient activity.