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Exploitable results

More than 500 000 total hip replacements are used every year. The overall survivorship of hip replacements is now thought to be approximately 95 % at 10 years. The majority (80 %) of total hips are, however, implanted in the middle aged and elderly (>54 years old). The success rate of the procedure is considerably less in the younger (or indeed older) active patients. One of the limitations of joint replacement in this more challenging group is wear to the articulating surfaces. The approach explored in this project was the use of soft or compliant bearings in the acetubular component in order to improve joint lubrication (hence friction and wear) of the metal or ceramic/polymer couple. The incorporation of impervious elastomeric compliant layers onto the surface of acetubular cups was known to generate fluid lubrication conditions. Friction levels comparable to those of healthy human joints have been observed, with the surfaces of the joint remaining separate under a wide range of simulated human activities. Long term wear resistance of the artificial hip joints could therefore be substantially improved, increasing the average prosthesis lifetime from 10 years to 25 years or more. However, poor adhesion between (fully dense) stainless steel substrates and the elastomer prevented any further exploitation of this system. The work investigated the fixation of biocompatible elastomers to porous metal substrates produced by powder metallurgy and the development of heterogeneous polymeric systems with compliant surfaces. Powders of approved biocompatible metals were processed to give prototype porous components, enhancing adhesion by the mechanical interlocking of the elastomeric layers, whilst a parallel route investigated the potential of using polymeric substrates by developing a novel bonding technique. A multi-station hip function simulator, specifically designed for this project, was then used to study the friction and wear behaviour of prototype acetubular cups using the compliant layered bearing systems developed.