The lubrication of metal-on-metal total hip joints: A slide down the Stribeck curve

Abstract
Metal-on-metal joints can exhibit profoundly different modes of lubrication when tested in a hip simulator. A range of metal-on-metal joints of similar diameters differing only in diametral clearance can demonstrate differences in lubrication conditions during simulator testing, as can joints differing principally in diameter. Simulated walking cycles can similarly cause differences in lubricating conditions through the application of different motion and loading cycles. In this study, and work conducted previously by the authors, metal-on-metal joints were tested in 25 per cent bovine serum in a hip simulator. A simple resistivity technique was used to detect surface contact, or separation, in a lubrication study, while a wear study used a gravimetric protocol extending over at least 2000000 cycles duration. Simulated physiological and simplified walking cycles were used during the studies to investigate the effect of motion and load on lubrication and wear. Joints of 16, 22.225, 28 and 36 mm nominal diameter were tested to investigate the effect of diameter, while the effect of diametral clearance was studied using joints of 36 mm diameter with a range of diametral clearances from 97 to 170μm. The trend of an idealized Stribeck curve, plotted as coefficient of friction versus the dimensionless Sommerfeld parameter (viscosity × sliding velocity × radius of the femoral head/load), indicates the lubrication regime prevalent in a joint. Boundary, mixed and fluid-film lubrication, shown by the three classic regions on an idealized Stribeck curve, were all demonstrated using metal-on-metal joints in a hip simulator. The different modes of lubrication prevalent in the joints resulted in wear rates spanning two orders of magnitude between joints differing principally in diameter. Similarly, a reduction in diametral clearance caused a general increase in the proportion of surface separation per simulated walking cycle. Likewise, a simplified walking cycle generated more surface separation per walking cycle for the same joint subjected to a simulated physiological cycle. However, the two walking cycles generated no significant difference in simulator wear rates.

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