| Summary: | The consequences of irradiation on the molecular structure, crystal morphology and mechanical properties of UHMWPE, have recently been implicated as a potential major factor influencing the wear, prompting several manufacturers to consider alternative sterilisation methods. The current work has contributed to this area through investigation of the effects of irradiation dose (representative of current orthopaedic practice) and post-sterilisation ageing on subtle changes in the microstructure of UHMWPE using a wide range of experimental techniques, including X-ray diffraction, scanning electron microscopy and differential scanning calorimetry to probe the microstructure. In particular, an analysis of retrieved human knee tibial implants of both earlier designs and a recently introduced total knee system design, has been conducted using the above techniques in order to understand the fundamental wear mechanisms leading to premature failure of these components. This has been complemented with laboratory simulation tests to investigate the effects of the above physical and chemical changes on the abrasive and fatigue wear resistance of sterilised, aged and unaged UHMWPE. A new micro-scale abrasion test and a new rolling contact fatigue test have been used to achieve this. The <I>in vivo</I> wear mechanisms observed under these test conditions have been compared with those seen in the retrieved tibial implant material to emphasise the clinical relevance of this study. This work has shown that gamma sterilisation in air, followed by long-term shelf and/or <I>in vivo</I> ageing, significantly reduces the fatigue and abrasive wear resistance of UHMWPE, associated with notable changes in its microstructure. Alternative sterilisation methods which eliminate long term free radical activity have been recommended.
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