Total hip replacements with metal-on-polyethylene bearings

Introduction: Polyethylene has been used as a bearing material for more than fifty years. It represents a golden standard in orthopaedics. Today the most important processes for manufacture of polyethylene are compression moulding and ram extrusion. As sterilization methods, irradiation with gamma rays in vacuum or inert gas and sterilization in plasma or ethylene oxide are used. Characteristics: Wear of polyethylene and consequent formation of wear particles are the key factor in the mechanism of aseptic loosening of total hip replacements. Pathophysiology of loosening is a complex process. Osteoclasts and their precursors have a crucial role in bone resorption. The differentiation of these multinucleated cells and their activation is regulated RANK/RANKL/OPG regulatory axis. Clinical results: Long-term results of the use of bearings with a metal femoral head and a polyethylene acetabular cup are good although dependent on the size of the used femoral head and the quality of polyethylene. However, these results are inferior to those obtained with a ceramic femoral head and a polyethylene cup, as well as those obtained with cross-linked polyethylene and ceramic-on-ceramic bearings. Conclusions: In-vitro simulation cannot fully predict the effect of various factors which in vivo affect the physical and chemical properties of polyethylene. The development of polethylene has thus not been completed yet. Currently, the research is targeted at highly cross-linked polyethylene and polyethylene enriched with alpha-tocopherol.