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Early failure of metal-on-metal hip arthroplasties is a current concern because tissue damage resulting from periprosthetic metallosis can compromise subsequent revision arthroplasty1,2. The United Kingdom’s Medicines and Healthcare products Regulatory Agency has advised that patients with metal-on-metal implants who have pain, prosthetic malposition, or implants that are known to have excessive failure rates should be evaluated with the measurement of serum cobalt. A serum cobalt level of >7 μg/L indicates possible periprosthetic metallosis3,4. A normal serum cobalt level is 0.19 μg/L, and 95% of those who are unexposed to cobalt have a value of <0.41 μg/L5. A serum cobalt level of >1 μg/L indicates excessive cobalt exposure, and levels of >5 μg/L are considered toxic6.In excess, cobalt blocks cellular metabolism and can damage multiple organs. Cobalt poisoning (cobaltism) from beer additives, industrial exposure, or medicinal use is well known. Cobaltism can result in tinnitus, vertigo, deafness, blindness, optic nerve atrophy, convulsions, headaches, peripheral neuropathy, cardiomyopathy, and hypothyroidism7,8. Including the two cases in the present report, there are at least six case reports of cobaltism related to arthroplasty implants (arthroprosthetic cobaltism)7,9-12. These four patients presented with combinations of deafness, blindness, cognitive decline, headaches, convulsions, fatigue, weakness, peripheral neuropathy, heart failure, and hypothyroidism. All of these patients had periprosthetic metallosis due to wear of steel femoral heads by ceramic, and all had a serum cobalt level of >60 μg/L. The two cases described in the present report were included in a summary paper by the author and are the first cases of arthroprosthetic cobaltism attributed to metal-on-metal implants that could be identified7.Both of the patients were fit, well, forty-nine-year-old men at the time of metal-on-metal arthroplasty with Articular Surface Replacement implants (ASR; DePuy, Warsaw, Indiana). In one of the patients (Case 1), the index hip arthroplasty was performed elsewhere and the revision arthroplasty was performed by one of the author’s partners. In the other patient, the index and revision arthroplasties were performed by the author. Both of these patients were briefly reported on in the State of Alaska Epidemiology Bulletin13, and are included in a review paper by the author on arthroprosthetic cobaltism7.CASE 1. Metal-on-metal hip replacement was performed for the treatment of osteoarthritis. The shell and femoral head diameters were 60 and 53 mm, respectively. Acetabular inclination measured 54° on a frontal hip radiograph, and anteversion appeared excessive on a lateral hip radiograph (Fig. 1-A). The patient demonstrated improvement initially but developed progressive hip pain with activity. At three months postoperatively, axillary rashes were noted, although skin patch testing and a lymphocyte transformation assay showed no reactivity to cobalt or chromium. Eleven months after surgery, dyspnea and progressive hip pain were noted, imaging studies showed a large periprosthetic fluid collection, and the serum cobalt level was 50 μg/L. At eighteen months, anxiety, headaches, irritability, fatigue, tinnitus, and hearing loss were noted; an audiogram demonstrated high-frequency hearing loss; and the serum cobalt level was 35 μg/L. At thirty months, pain at rest, hip creaking, hand tremor, incoordination, cognitive decline, and depression were noted. At thirty-six months, visual changes were noted, optic nerve atrophy was found, and the serum cobalt level was 122 μg/L.Forty-three months after the initial arthroplasty, revision arthroplasty was done. Laboratory findings included a serum cobalt level of 83 μg/L, a cerebrospinal fluid cobalt level of 2.2 μg/L, and a joint fluid cobalt level of 3200 μg/L. Diasystolic dysfunction was found on interval echocardiography. The pathologist noted metallosis, necrosis, and lymphocytic infiltrates.