Advances in Polyethylene

Ultra high molecular weight polyethylene (UHMWPE) is a tough, wear resistant, biocompatible polymer that has been used for over four decades as a bearing material in total joint replacement prostheses. Despite its superior wear properties, osteolysis associated with particulate wear debris of UHMWPE released during in vivo use has been a major factor determining the longevity of hip replacements utilizing UHMWPE components. In the case of knee components, fatigue damage wear mechanisms, such as delamination wear have led to loosening of the implants leading to premature revision surgery. Aging of g-irradiated components is the primary factor promoting delamination wear of UHMWPE tibial components. In this chapter, various aspects of UHMWPE synthesis, morphology, processing, and their effects of mechanical and wear properties of clinical relevance have been outlined. The role of chain entanglements, radiation sterilization, radiation cross-linking, and crystallinity in controlling wear resistance and mechanical properties are also discussed. Currently, processing techniques and methods of chemical modification by radiation are being developed and optimized to maximize the mechanical and tribological performance of UHMWPE, as it varies for each joint. We have identified that chemical

factors, such as the number of chain entanglements and degree of cross-linking, are beneficial for wear resistance while physical factors such as density or degree of crystallinity lead to an increase in several clinically relevant mechanical properties such as resistance to creep deformation and resistance to fatigue crack propagation. All of these factors may be affected in the long term if free radicals associated with g-sterilization or radiation cross-linking remain in the UHMWPE component after processing and packaging. Longterm oxidative degradation can be decreased by packaging of UHMWPE components in reduced-oxygen environments but it remains to be seen whether in vivo oxidation will affect their mechanical and wear performance in the long term.