Abstract
Synovial sarcoma is a relatively common periarticular soft tissue malignancy, occurring mostly in the extremities of younger patients. Occasionally, diversity in its clinical features may lead to misdiagnosis and inappropriate management. The authors report herein a unique case of a patient who underwent a primary total knee arthroplasty to treat osteoarthritis. During the operation, a mass was discovered but was attributed to synovitis. Biopsy revealed a rare intra-articular synovial sarcoma. The patient underwent reoperation with wide excision and endoprosthesis placement and is disease-free at the 8-year follow-up.