Effect of frictional heating on the serum lubricant and wear of UHMWPE cups against cobalt-chrome or zirconia balls

摘要

Hip simulators have long been used in creating an in vitro environment for testing wear of materials for artificial joints. The ultimate goal of hip simulation is to produce wear that is comparable to that which occurs clinically. In a previous hip simulator wear test, the wear of polyethylene (PH) cups against zirconia (Zr) balls was about 50% less than with cobalt-chrome (CoCr) or alumina balls [1], indicating zirconia might have advantages in clinical use for prosthetic joints. However, there was substantial precipitation of proteins from the serum lubricant in the tests with zirconia balls. The protein precipitate appeared dense and granular, as compared to the fluffy precipitate that typically sticks to the chamber wall in tests of CoCr/PE, suggesting the temperature of the bearing surface was well above 60 °C during the test due to frictional heating [2]. The precipitate also formed a layer (approximately 0.5 mm thick) between the zirconia ball and the polyethylene cup with higher bulk lubricant temperature, which may have been due to the lower thermal conductivity of the zirconia than CoCr [3]. This precipitate layer may have partially separated the components, artificially reducing the wear of the cups running against zirconia balls [2,4]. Conversely, the depletion of the proteins might have artificially reduced the lubricating ability of the serum, increasing the wear rate of the cups. The purpose of the present study was to use a hip simulator to investigate the effect of frictional heating and forced cooling on the implant interface temperature, the amount of protein precipitation, and the wear of UHMWPE cups, and to relate results to clinical conditions.