Tenosynovial Giant-Cell Tumor Presenting as Septic Arthritis of the Knee.

Tenosynovial giant-cell tumor (TGCT) is an intraarticular giant-cell tumor of the synovial tissue and tendon sheaths which often mimics multiple conditions on presentation. This case report describes a previously asymptomatic 67-year-old man with preliminary clinical and laboratory evaluation suggestive of septic arthritis; however, arthroscopy revealed diffuse synovitis, and biopsy confirmed TGCT. To our knowledge, this is the first report of TGCT presenting as septic arthritis in an adult patient. This diagnosis should be considered in evaluation of acute, atraumatic knee pain with associated inflammatory marker elevation.