Frictional heating of bearing materials tested in a hip joint wear simulator

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Abstract
In a hip simulator wear test using bovine serum as a lubricant, the heat generated by ball-cup friction may cause precipitation of the proteins from the lubricant. The resultant accumulation of a solid layer of precipitated protein between the ball and cup could artificially protect the bearing surfaces from wear, in a manner that does not occur in vivo. Alternatively, the gradual depletion of the soluble proteins could interfere with their ability to act as boundary lubricants on the bearing surfaces, thereby artificially increasing the wear rate. Because the rate of protein precipitation may depend on the maximum temperature at the bearing surfaces during sliding, rather than the mean temperature of the bulk lubricant, this study determined the transient surface temperatures using an array of thermocouples embedded in acetabular cups of GUR 415 ultra-high molecular weight polyethylene (UHMWPE) and femoral balls of metal or ceramic, in conjunction with a finite element model of the temperature distribution. The prostheses were tested at one cycle/s under a Paul-type, physiological load profile with 2030 N maximum force, with the load cycle synchronized to the motion cycle. The steady state temperatures of the bulk lubricant were 38°C for the zirconia balls, 36°C for the cobalt-chromium and 33°C for the alumina. However, the corresponding surface temperatures of the polyethylene, calculated with the finite element model, were 99°C with zirconia ceramic, 60°C with cobalt-chromium alloy, and 45°C with alumina ceramic. The rank order of the surface temperatures corresponded to the relative amounts of protein that were precipitated in the test chambers during wear tests with these materials.