A predictive analysis of long-term friction and wear characteristics of metal-on-metal total hip replacement

Abstract

There has been a major revival of interest in metal-on-metal total hip replacements in recent years, particularly for younger and more active patients [1, 2]. Theoretical and experimental studies have confirmed that wear in both the running-in and steady-state regimes can be reduced considerably through careful design and manufacture. It has been amply demonstrated that large diameter femoral heads are advantageous, primarily because they are tribologically beneficial through improvements in lubrication and hence reduced wear in a mixed lubrication regime. This has permitted major improvements to be introduced and it has also facilitated the introduction of satisfactory surface replacement as well as large diameter monolithic implants with greater resistance to dislocation. Design guides have been emerging [3] as lubrication and structural analyses have advanced. The main driver has been the need to minimize wear in both running-in and steady-state phases of implant function and hence to reduce as much as possible the total volumetric wear and the number of tiny, nanometre sized wear particles. The achievable wear rates in well designed metal-on-metal implants are now so low that the tribologically predicted wear lives are exceedingly attractive. Attention is therefore turning to the question of long-term bearing performance and the possibility that gradual extension of the nominal contact patch will result in large frictional torques capable of disrupting the implant fixation. The potential longevity of metal-on-metal hip replacements implies that clinical experience of such effects will take a long time to emerge. Likewise laboratory simulator tests would have to extend over, say, at least 10-50 million cycles. This is expensive and generally limited to simulation of a repetitive walking cycle of operation, rather than a more representative reflection of random daily cycles of function. The alternative, but clearly tentative approach is to attempt a predictive analysis based upon developing tribological analysis and information on the wear performance of implants in simulators over much shorter periods, typically 5 million cycles. In this paper, a simple, but inevitably tentative predictive analysis of the volumetric wear; diameter of the nominal wear patch; head penetration, and frictional torque are all estimated over timescales of several tens of millions of loading cycles, equivalent to many decades of service in the body. Both boundary and fluid-film lubrication conditions are considered.