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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Accelerated wear testing methodologies for total hip replacements.

Author: BOWSHER, J

Year: 2002

Supervisor(s): Julia Shelton

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 (Ra of 0.38 mm), high joint forces (4.5 kN max) and high sliding speed (1.75 Hz) showed to generate 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 showed to produce 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.