A tribological study of metal-on-metal total replacement hip joints

The resurgence of interest in hard-on-hard total replacement hip joints in recent years has promoted a number of studies on wear and lubrication in metal-on-metal implants. The wear of such implants is strongly influenced by joint geometry, particularly diameter, clearance and sphericity, materials of construction, and surface finish. In this Chapter the results from a number of laboratory investigations carried out over several years on a ProSim multi-station hip simulator will be reviewed. The laboratory studies fall into two categories: investigations of the role of joint geometry upon wear and a search for lubricating films by means of a voltage-drop technique. The latter studies have been supplemented by simple analytical predictions of elasto-hydrodynamic film thickness for representative mean values of the speed, load, and viscosity encountered in the walking cycle. The evidence gleaned from both the experimental and theoretical studies suggests that the severity of direct metal-to-metal interaction can be much relieved by effective lubrication of the joint during a substantial part of each loading cycle. Current understanding of the tribological characteristics of metal-on-metal joints will be reviewed. The understanding achieved provides an opportunity for the optimization of design and manufacture of metal-on-metal implants through the application of tribological principles. The provision of maximum relief from the severity of wear in metal-on-metal joints by effective lubrication in simulated walking cycles, on well controlled laboratory simulators, is a necessary, but by no means sufficient, step towards optimization of implant performance. Investigations of the effect of frequent interruptions to the loading and motion cycles, which represent more fully daily experience, are required. Further studies of the role of material composition and structure upon wear, and consideration of the behaviour of wear debris in the bearing clearance, are also required.