The rationale for metal-on-metal total hip arthroplasty.

Metal-on-metal total hip arthroplasty has the longest clinical history of any of the currently used articular couples. Long-term followup of what are now considered suboptimal designs (eg, McKee Farrar, Ring, Sivash) has produced a wealth of knowledge regarding the safety and efficacy of this articular combination. Retrieval analysis of metal-on-metal total hip arthroplasty articular couples has shown wear between 1 and 5 microns per year after initial wear-in, in comparison with 100 to 200 microns per year associated with metal-on-polyethylene wear. The constituent metal ions released through wear of the metal-on-metal total hip arthroplasty are excreted primarily in the urine; serum levels have been 3 to 5 times higher in patients who have had metal-on-metal total hip arthroplasties than in control subjects. No adverse physiologic effects have been identified in the long-term followup of patients exposed to cobalt-chromium implants. The clinical results of metal-on-metal total hip arthroplasties equal or exceed those of conventional articular couples and rarely are associated with osteolysis compared with conventional couples. Additional advantages of the metal-on-metal combination are the ability to use larger-diameter femoral heads for enhanced stability and the absence of concern over possible fracture of the articular components. The long-term experiences with metal-on-metal total hip arthroplasty make this combination of implant material the conservative choice for success.