Direct experimental evidence of lubrication in a metal-on-metal total hip replacement tested in a joint simulator

Abstract
The purpose of the study reported in this paper was to investigate experimentally the conditions of lubrication and contact between a metallic femoral head and a metallic acetabular cup in a hip joint simulator. An electrical resistivity technique previously developed for engine tribology studies was used to detect the extent of surface separation throughout a cycle of dynamic loading simulating walking conditions. A metal-on-metal total joint replacement was tested in a single station of a ten-station hip joint simulator in the presence of five different lubricants. Two proteinaceous serum-based fluids of concentrations 25 and 100 per cent were used as well as three lubricants devoid of protein but of quite different viscosities. It was observed that surface separation developed with both the protein-based fluids during the swing phase of the articulation and that an equilibrium cyclic pattern of contact and separation was established after running the simulator for some minutes. Furthermore, there was at times some evidence of separation during the severe conditions encountered during the stance phase. Silicone fluid failed to promote surface separation, while water and the more viscous carboxymethylcellulose solution yielded only modest periods of separation during the swing phase. It was deduced from experimental and analytical features of the study that the mode of lubrication was mixed, with some periods of very effective surface separation and others of metal-to-metal contact. While the presence of protein was necessary for effective lubrication, the cyclic pattern of separation appeared to be determined by elastohydrodynamic action. The bond strength of the protein to the metals appeared to be weak. The partial film of lubricant between the boundary lubricating layers provided significant enhancement to the combined protective action in this mixed lubrication system. The simple, cheap and effective electrical resistivity technique has revealed important new information on the tribological conditions in metal-on-metal hip joints which determine friction and wear. It can readily be adopted on most forms of total joint simulators. The technique and the findings reported here further advance the development of criteria for lubricants to be used in laboratory studies of the performance of total replacement joints.

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