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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: In vitro wear of metal-on-metal hip replacements: a multifactorial problem.

Author: ANGADJI, Arash

Year: 2008

Supervisor(s): Julia Shelton

In this thesis, the wear of MoM hip prostheses has been evaluated under a range of testing parameters, evaluating the influence of clearance, loading conditions, material processing and cup position. The results have indicated that the in vitro test results for the influence of diameter and clearance may be predicted from theoretical models. However, the load parameters, cup orientation and material processing produced in vitro wear rates that were not anticipated.

Lubrication analysis indicated that the simulated jogging test (high load, increased sliding velocity) would lead to an increased λ value, compared to walking from 0.8 to 2.2; however, the experimental results showed jogging to produce the highest wear rates (34.88 mm3/106) amongst all activities, indicating that the assumptions used in the lubrication theory are not valid under such severe activities. The high clearance bearings (150 μm) displayed a high total wear volume of 0.33 mm3 over 6.3 million cycles, compared to the low clearance (50 μm) of 0.19 mm3 as a result of high wear during the run-in phase. Positioning of the acetabular component was shown to have a direct influence on a number of important clinical outcomes. The current study presented results from a hip simulator showing, for the first time, that at a high cup inclination angle of 60° in MoM hip bearing systems, the wear rates generated were 1.7 mm3/106 cycles, a 7-fold increase compared to the standard 35° cup inclination angle and a statistically higher value whilst also moving the position of the wear scar. Moreover, wear was found to be particularly sensitive to the metallurgical conditions in combination with the steep cup angles. The findings are very important in order to establish the critical parameters required for use in a more demanding patient population.