Metal-on-metal total hip arthroplasty: known and unknown side effects.

The indications for hip replacement, especially in young patients with end-stage degenerative arthritis of the hip, have increased during the past 20 years. Because of the accelerated risk of polyethylene wear and subsequent prosthetic loosening due to osteolysis, new interest has focused on the metal-on-metal articulation couple. Failures of first-generation metal-on-metal total hip arthroplasties (THAs) are attributed to suboptimal surgical technique, excessive or negative clearance, poor fixation, and neck–socket impingement.1 Improvements in metallurgic and tribological properties (ie, sphericity and radial clearance) of metal-on-metal bearing couples has led to a renaissance of metal-on-metal resurfacing arthroplasties and THAs. Secondgeneration metal-on-metal wear rates are noted to be 20 to 100 times lower than metal-on-polyethylene wear rates.2 Good clinical and radiological results have been reported after metal-on-metal resurfacing arthroplasties in patients younger than 55 years, with survival rates of 99% and 98% at 10and 13-year follow-up, respectively.3 Also, good results have been described after a minimum 6-year follow-up (mean, 5.7 years) of cementless second-generation metal-on-metal THAs in prospective, randomized, controlled clinical trials, with a survival rate of 100%.4-6 Few prospective, randomized, controlled studies compare conventional metal-on-polyethylene THAs with second-generation metal-on-metal THAs. Zijlstra et al7 reported a 10-year survival rate of 95.5% for a metal-on-metal group and a 10-year survival rate of 96.8% for a metal-on-polyethylene group, with no difference in clinical or radiological outcomes. Small bearing couples of 28-mm femoral heads were used. Interpretation of the results of different prosthetic devices, as well as metal-on-metal resurfacing arthroplasties, is problematic because of differences in design, alloy, radial clearance, risk of head–neck impingement, edge wear, and other metallurgic properties. Besides better metallurgical and tribological properties, orthopedic companies also claim clinical advantages of largehead metal-on-metal THAs over conventional small-head metalon-polyethylene THAs, including a larger range of motion and a decreased risk of dislocation. However, in a randomized clinical trial, no clinically relevant difference in range of motion was observed between conventional 28-mm metal-on-polyethylene THAs compared with large-head metal-on-metal THAs.8 Until now, few prospective, randomized, controlled trials have documented the long-term clinical and radiological results of metal-on-metal resurfacing arthroplasties and metal-on-metal THAs, including survival analysis. No proven clinical advantage of metal-on-metal over metal-on-polyethylene THAs currently exists. Future research should focus on long-term follow-up of the new generation of hip replacements. Recently, concern has emerged about the development of pseudotumors in large-head metal-on-metal THAs and resurfacing arthroplasties. These pseudotumors, also known as aseptic, lymphocyte-dominated, vasculitis-associated lesions or adverse reactions to metal debris, are caused by a local metal allergy or sensitivity and a type IV, local, delayed-type hypersensitivity caused by T lymphocytes. Even with low wear rates, this type of allergy can develop. Whether aseptic, lymphocyte-dominated, vasculitis-associated lesions are specific to metal-on-metal implants is under investigation. More lymphocytes and plasma cells and fewer macrophages are found in tissue reactions in metal-on-metal THAs compared Metal-on-Metal Total Hip Arthroplasty: Known and Unknown Side Effects