Metallurgical considerations in the wear of metal-on-metal hip bearings

Metal-on-metal bearings offer one possible solution to problems of osteo-lysis associated with ultra high molecular weight polyethylene (UHMWPE) wear debris generated during articulation in total joint replacements. Designs using larger bearing diameters are also possible with associated improvements in both range of motion and stability. There has been considerable debate as to the optimum metallurgical structure of the cobalt chrome alloys used in these metal-on-metal bearings. This review addresses the main metallurgical design issues in metal-on-metal bearing design and reviews some recent studies from two independent academic centres. Cobalt chrome alloys with a low carbon content, when paired against themselves, have been shown to have poor wear properties under both simple configuration and simulator testing. Other cobalt chrome alloy couples with higher carbon content, whether wrought, cast or cast and heat-treated, appear to have similar performance in clinically relevant wear simulation studies. (Hip International 2004; 14: 1-10).