Bell, Carol Jane; (1999) Variability, oxidation resistance and wear of 'medical grade' ultra-high molecular weight polyethylene. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of this project was to examine factors affecting wear mechanisms in total knee replacement (TKR). The hypothesis of this work was that manufacturing, processing and sterilisation techniques for 'medical grade' ultra-high molecular weight polyethylene (UHMWPE) would result in a variability in material properties, oxidation resistance and wear performance. Examination of stock materials and retrievals showed there were differences in grain structure, extent of consolidation and resistance to oxidation. A novel staining technique was developed which Fourier Transform Infrared spectroscopy showed corresponded to regions of oxidation. Examination of retrieved TKRs and wear test specimens showed that the development of a sub-surface oxidised band (due to post irradiation damage) was related to delamination wear. It was also found that surface oxidation of UHMWPE was caused by articulation with the femoral component. The development of an artificial ageing regime allowed the effect of oxidation on the material and wear properties of polyethylene to be investigated. This showed that a critical level of oxidation was required before delamination occurred. Oxidation levels associated with five to ten years of shelf-ageing resulted in a reduction in fatigue life of two orders of magnitude and an increase in crack propagation rates by 10-400 times (depending on manufacturing method). Oxidation also resulted in a reduction in UTS, and brittle fracture of test specimens. Investigation of a modified polyethylene showed that annealing UHMWPE after irradiation resulted in a three times reduction in the wear rate compared to untreated polyethylene. Annealing also produced a reduction in the level of oxidation induced using artificial ageing and a wear rate which was ten times lower than standard UHMWPE. However this modified material was not totally resistant to oxidation. Many reports of excellent wear properties and long-term survivorship rates for directly moulded polyethylene components have been published and in my study this material was found to have a greater oxidation resistance than other materials, as well as lower wear rates. In a ten million cycle simulator test, artificial ageing of directly moulded polyethylene resulted in no significant increase in wear rate when compared with unaged material. Delamination wear did not occur in either shelf-aged or artificially aged directly moulded components, whereas delamination did occur in oxidised non- directly moulded components. My study demonstrated differences in 'medical grade' polyethylene manufactured by different techniques. This affected the resistance of UHMWPE to oxidation and to wear.

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