Lilley, Peter A. (1994) An investigation of wear in lower limb endoprostheses. (PhD thesis), Kingston University, .

Abstract

This dissertation investigates the friction and wear behaviour of the predominant materials used in lower limb prostheses. plastic debris is common as a result of articulation at heavily loaded joint surfaces but metal debris can also be generated at the soft-tissue interface where the contact forees are much lower. The resultant tissue staining, and the wear products in general, are a matter of great concern: the plastic particles migrate between the bone-implant interface and have been implicated in the process of bone resorption with subsequent loosening of the implant and there is some evidence that release of corrosion and wear particles from permanent metal prostheses could constitute a carcinogenic hazard. The hypothesis for the phenomenon of soft-tissue wear of the body of massive prostheses was that there were particular combinations of physical contact conditions which led to high metal wear rates. Part of the experimental programme was an attempt to reproduce this wear phenomenon in the laboratory and to determine the conditions which influence soft-tissue wear. The conditions governing the wear of titanium by soft-tissue have not hitherto been investigated. The findings from the investigation were surprising and non intuitive in that metal was severely damaged by soft-tissue attrition. The findings contribute to the understanding of the causes of this type of wear. There is evidence that the hypothesis suggesting low contact loads create higher coefficients of dynamic friction, and subsequent wear, than that occurring with high contact loads was demonstrated and explained by reference to the phenomenon of a stick-slip mechanism. This would offer an explanation of why the explanted prostheses of elderly, low body weight patients, with reduced activity levels, often exhibit more severe wear from the shaft of prostheses than younger, heavier, and more active patients. The experimental findings could also help explain the reasons for patients with apparently similar sets ofin vivo biochemical conditions exhibiting extreme differences in debris production and tissue staining. Laboratory studies have also been carried out which investigate some of the predominant parameters which influence wear in prostheses such as material quality, conformity, thickness, temperature distribution, and the relative effects of sliding and twisting in the knee. The objective here was to effectively simulate the wear observed on retrieved plastic tibial components. Emphasis has been given to the development of wear test methodology and strict protocols for in vitro testing have been proposed and employed in an attempt to identify the causes of wear and aetiology of failure. Pin-on-disc, pin¬on-plate, and simulators have been used to reproduce the conditions associated with knee joint replacements. The experimental work examines the factors that cause damage and evaluates the relative merits of various materials, surface coatings, and surface treatments which could be used to reduce surface degradation and wear of prostheses at both the articulating surface and at the soft-tissue interface. The precise wear rate of titanium by soft-tissue has been determined and the work offers an increased understanding of this wear phenomenon. Minimising the rate of production of debris will reduce adverse cellular response and the potential revision of the implant. The investigation demonstrated that inherent defects in polyethylene lead to enhanced wear and quantifies the benefit offered by treatments to both plastic and metal components for in vivo applications. It is recommended that the metal faee of the femorotibial joint is nitrogen ion-implanted and that the titanium shaft of massive endoprostheses are diamond-like carbon coated. This research results in a better understanding of some of the significant causes and mechanisms of wear in lower limb endoprostheses and offers some proposals for reducing this wear.

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