Large diameter metal-metal bearings possess several clinical advantages over small bearings including greater joint stability, improved range of motion, and lower wear due to improved lubrication. Simulator wear tests were conducted to assess the effects of thermal processing on the wear behavior of large diameter metal-metal hip bearings.

Three groups of high carbon, cast 54 mm hip bearings with different thermal processing histories were tested. Two groups of bearings were manufactured to identical specifications and subjected to either no heat treatment(as-cast) or to typicall thermal processing prior to testing. The third group was comprised of commercially available as castbearing systems. Wear tests were performed on a Shore Western orbital bearing wear test machine. A simulated gait profile (triple-peak Paulprofile) with a maximum force of 2000N was applied to the bearings at a frequency of 1 Hz. The bearings were tested in the inverted position (headabove, shell below).

The general wear behavior of all three groups of bearing couples was similar to that previously reported for metal-metal bearings. All couples exhibited a run-in wear phase followed by a low-wear steady-state phase. For all bearing couples tested the heads demonstrated more wear than the shells. The appearance of the worn surfaces of all the bearing couples tested in this study were consistent with that of previously reported in-vitro wear testing as well as metal-metal hip bearing retrieval studies. There was no statistical difference among the three groups tested in the run-in or steady-state wear rates, although the heat treated bearings tended to wear less on average.

The results of this study indicate that thermal processing has no adverse effect on the wear of large diameter metal-metal hip bearings.