Lubrication and Wear of Alumina-Alumina Hip Bearings

摘要

The majority of alumina-alumina hip simulator studies in the literature have been unable to reproduce the wear rates and surface features found in most implant retrievals. The present study investigated simulator wear with two novel tests regimes. The first test investigated the simulator wear of six alumina-alumina implants in the presence of different serum concentrations (30% and 90%) under continuous motion conditions for 3 million cycles (Mc). The serum concentration did not influence the wear significantly. The implants were grouped together to give an initial run-in wear rate of 0.98+-0.12 mm~3/Mc (for 0 - 0.25 Mc) that decreased to a steady-state wear rate of 0.03210.006 mm~3/Mc (for 0.25 - 3 Mc). The second test subjected the same implants to stop-start motion in the presence of a 30% serum solution for an additional 1 Mc, in order to elucidate the influence of more intimate surface contact. As a result, the implants went through a second, less dramatic run-in phase over the first 0.5 Mc, then the wear rate decreased to a steady-state value of almost zero. The implants were examined after the first test with Atomic Force Microscopy (AFM) and demonstrated a relief polishing on the femoral head that extended longitudinally about 5.5 mm from the apex, where a gradual transition to the original polished surface occurred. The root-mean square roughness in the worn region was found to be about twice that of the polished region. A similar relief pattern was observed on replicas of the acetabular cups, in an area that extended longitudinally in a zone 2.9 - 5.6 mm longitudinally from the apex of the cup. Using the initial geometry and roughness of the implants, lambda values were predicted that suggested mixed or full fluid film lubrication. Using the measured gravimetric wear, it was possible to predict a wear zone size on the head that was consistent with the AFM observation and to estimate the total linear wear. The low wear rates demonstrate the excellent wear resistance of the alumina-alumina. However, these simulator tests continue to under-predict the wear in-vivo, indicating that even stop-start simulator testing with a 30% concentration of serum did not provide severe enough conditions to produce the higher in-vivo wear rates.