Accelerated wear testing methodologies for total hip replacements
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 patient activity are limiting. Therefore an alternative wear testing methodology for total hip replacements has been proposed, measuring the influences of fast walking, stumbling and simulated jogging sequences, all at varying cycle speeds with both smooth and roughened femoral components. This hip simulator study has shown that the influence of femoral roughness on the wear of crosslinked polyethylene becomes significantly greater under increased patient activity, demonstrating that roughness may be a more influential factor than previously ascribed. The combined effects of high roughness (Re of 0.38 μm), high joint forces (4.5 kN max) and high sliding speed (1.75 Hz) generated excessive crosslinked polyethylene wear and high joint torque, with wear rates exceeding 3000 mm3/106 cycles (k = 50 x10-6 mm3/N m). Thus for more active patients, implant survival can be greatly increased by using harder and more damage resistant femoral heads compared to CoCrMo. Under smooth conditions however, the overall influence of a significant increase in patient activity showed a much weaker effect. 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 of crosslinked 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 produced the largest wear rate (53 mm3/106 cycles), and was consistently greater than for simulated jogging. Ignoring fixation and other factors, these results suggest that whilst preserving polished surfaces, short periods of increased patient activity, for example, aerobics, tennis etc, will not greatly reduce the survival of crosslinked polyethylene/metal implants. Sliding speed and the degree of socket clamping were shown to be the most influential factors under smooth conditions, with the results showing no significant differences in wear rate when testing in varying quantities of bovine serum, or using an inverted or physiological specimen orientation.