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Infection is an uncommon but catastrophic complication of joint arthroplasty, usually requiring removal of the implant. In a 30-year-old woman a knee arthroplasty was infected with the rapidly growing mycobacterium Mycobacterium fortuitum. Review of other reports of arthroplasties infected with this organism illustrates the problem, diagnosis, and treatment. M. fortuitum is widely distributed in nature, and although usually of low pathogenecity, it can cause infection in conditions of reduced local tissue resistance, i.e. hypodermic abscesses, implant inflammations, and trauma. Only six cases of M. fortuitum prosthetic joint infection have been previously described. Persistent drainage characterized cases in which the prosthesis was left in place. Although antibiotic treatment temporarily suppressed the signs and symptoms of infection, cure required removal of the prosthesis, as in the present case. Diagnosis of M. fortuitum infection is difficult because acid-fast stains of the organisms are often negative. Routinely bacterial cultures are continued for less than about five days, a period not long enough for growth of M. fortuitum. M. fortuitum infections should be considered in draining prosthetic joints with negative bacterial cultures and in those that have had repeated glucocorticoid in-traarticular injections.

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