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

The purpose of the study reported in this paper was to investigate experime ntally 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 wa s used to detect the extent of surface separation throughout a cycle of dyn amic loading simulating walking conditions. A metal-on-metal total joint re placement was tested in a single station of a ten-station hip joint simulat or in the presence of five different lubricants. Two proteinaceous serum-ba sed 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-bas ed fluids during the swing phase of the articulation and that an equilibriu m cyclic pattern of contact and separation was established after running th e simulator for some minutes. Furthermore, there was at times some evidence of separation during the severe conditions encountered during the stance p hase. Silicone fluid failed to promote surface separation, while water and the more viscous carboxymethylcellulose solution yielded only modest period s 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 surf ace separation and others of metal-to-metal contact. While the presence of protein was necessary for effective lubrication, the cyclic pattern of sepa ration appeared to be determined by elastohydrodynamic action. The bond str ength of the protein to the metals appeared to be weak. The partial film of lubricant between the boundary lubricating layers provided significant enh ancement to the combined protective action in this mixed lubrication system . The simple, cheap and effective electrical resistivity technique has reveal ed important new information on the tribological conditions in metal-on-met al hip joints which determine friction and wear. It can readily be adopted on most forms of total joint simulators. The technique and the findings rep orted here further advance the development of criteria for lubricants to be used in laboratory studies of the performance of total replacement joints.