Wear evaluation of cobalt-chromium alloy for use in a metal-on-metal hip prosthesis.

Wear of the polyethylene in total joint prostheses has been a source of morbidity and early device failure, which has been extensively reported in the last 20 years. Although research continues to attempt to reduce the wear of polyethylene joint-bearing surfaces by modifications in polymer processing, there is a renewed interest in the use of metal-on-metal bearing couples for hip prostheses. Wear testing of total hip replacement systems involving the couple of metal or ceramic heads on polymeric acetabular components has been performed and reported, but, until recently, there has been little data published for pin-on-disk or hip-simulator wear studies involving the combination of a metallic femoral head component with an acetabular cup composed of the same or a dissimilar metal. This study investigated the in vitro wear resistance of two cobalt/chromium/molybdenum alloys, which differed primarily in the carbon content, as potential alloys for use in a metal-on-metal hip-bearing couple. The results of pin-on-disk testing showed that the alloy with the higher (0.25%) carbon content was more wear resistant, and this alloy was therefore chosen for testing in a hip-simulator system, which modeled the loads and motions that might be exerted clinically. Comparison of the results of metal-on-polyethylene samples to metal-on-metal samples showed that the volumetric wear of the metal-on-polyethylene bearing couple after 5,000,000 cycles was 110-180 times that for the metal-bearing couple. Polyethylene and metal particles retrieved from either the lubricant for pin-on-disk testing or hip simulator testing were characterized and compared with particles retrieved from periprosthetic tissues by other researchers, and found to be similar. Based upon the results of this study, metal-on-metal hip prostheses manufactured from the high carbon cobalt/chromium alloy that was investigated hold sufficient promise to justify human clinical trials.