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The Evaluation of Metal-on-Metal Total Hip Arthroplasty
Total hip arthroplasty (THA) implants with metal-on-metal (MoM) bearings were introduced in 1938 and first became popular in the 1960s, with the McKee-Farrar prosthesis being the most common. Potential advantages of MoM bearings over conventional metal-onpolyethylene articulations include decreased volumetric wear, increased fracture toughness, and the ability to use larger femoral heads to decrease postoperative instability. Despite the popularity of MoM bearings in the 1960s and 1970s, they were quickly overtaken by metal-on-polyethylene designs because of higher loosening rates and concern regarding metal sensitivities. However, growing concerns regarding polyethylene wear and osteolysis sparked the introduction of a second-generation MoM THA implant in 1988. The popularity of MoM articulations continued to rise through the 1990s and 2000s. In 2005, 35% of all hip arthroplasties performed in the United States used MoM articulations. Estimates suggest that more than 1,000,000 MoM articulations have been implanted since 1996.
Although many early studies regarding MoM bearings were generally favorable, numerous unforeseen complications have been elucidated, including early osteolysis and adverse soft-tissue reactions (pseudotumors) likely related to a delayed-type hypersensitivity response. MoM bearings have a higher incidence of pain after THA or hip resurfacing than any other bearing type. Therefore, these implants are under close scrutiny and in some instances have been recalled from the market. Because of the large (and growing) number of complications surrounding these implants, most surgeons in North America have abandoned their use. Current indications for implantation of MoM bearings are limited. Their use should be restricted to hip resurfacing in men younger than 65 years.
Although many patients with MoM hip arthroplasties have asymptomatic hips in which the implant will likely last a lifetime, all patients should undergo surveillance to evaluate for indicators of potential impending failure of the implants or soft-tissue reaction to metal wear particles. Information gathered in the evaluation of these patients should be used to assess whether each patient is at low, intermediate, or high risk of implant failure. On the basis of this risk assessment, the surgeon will be able to determine whether each patient with a MoM THA or hip resurfacing should be offered routine follow-up, closer monitoring, or revision hip arthroplasty.