Metal–Metal Bearing Surfaces in Hip Arthroplasty

There is no mystery regarding the allure of metal–metal bearings: high stability and low wear potential. The special risks associated with these bearings are coming into focus and include a macrophage response to excessive metal particles (metal reactivity) and a lymphocyte-dominated reaction (metal sensitivity). The most common presentation of an adverse local tissue reaction (ALTR) is persistent pain. The incidence of ALTR has not been completely defined, but the risk appears to be increased in resurfacing, in women, and in bilateral cases. The differential diagnosis includes septic and aseptic loosening, and the evaluation should include aspiration. The diagnosis is confirmed by histological examination. It can be difficult to assess the fixation status of monoblock cobalt chromium acetabular components. Both metal reactivity and metal sensitivity can cause periprosthetic bone resorption (osteolysis). The development of a large effusion may be a sign of metal sensitivity. Ultrasound is a cost-effective diagnostic tool but is operator dependent. A magnetic resonance imaging scan with metal artifact suppression can identify a periprosthetic fluid collection or cystic/solid mass (pseudotumor). Serum or whole blood ion levels elevated to >10 ppb are suggestive of a suboptimal wear mechanism and an increased risk of ALTR. The indications for revision include component loosening, progressive osteolysis, large effusion or pseudotumor, and unremitting pain. Correction of any identified mechanical aberrations with conversion to a nonmetal–metal bearing is recommended. A well-fixed cobalt chromium substrate (modular stem or modular acetabular shell) can be retained.