Giant cell tumor of the tendon sheath is an idiopathic benign tumor of the synovial membrane which is rarely seen in large joints. We report a case of a giant cell tumor of the tendon sheath in a knee which presented with persistent anterior knee pain and swelling after trivial injury to the knee with emphasis on the importance of considering such tumors in the differential diagnosis of patients presenting with knee pain and swelling. Case A 50-year-old woman normally fit and healthy presented to her general practitioner after a trivial injury to the left knee with persistent anterior knee pain. Physical examination revealed tenderness over the lateral aspect of the patella but with a full range of movement of the knee and a negative McMurray's test. X-rays were normal. She was treated with standard painkillers and quadriceps exercises. After 6 months of conservative management, the patient did not improve. An MRI scan was performed and showed a soft tissue mass, with a differential diagnosis of pigmented villonodular synovitis (PVNS) (fig1, 2). The patient was consequently referred to us for further evaluation and management. We performed an arthroscopy that revealed a bulging mass into the knee joint and a miniature arthrotomy was required to excise the mass completely. Morphologically it looked like a PVNS, but subsequent histology confirmed a giant cell tumor of the tendon sheath (GCTTS) (fig3). The post-operative period was uneventful and there was no evidence of local recurrence on follow up after one year.
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