Synovial chondromatosis of the peroneal tendon sheath presenting as a painless ankle swelling: Diagnostic challenge with atypical radiographic findings

A man in his 30s presented with a progressively enlarging swelling over the lateral aspect of his right ankle for 8 months. The swelling was insidious in onset, gradually progressive and notably painless. Examination revealed a 6×4 cm lobulated mass over the lateral malleolus with preserved ankle motion. Ultrasonography showed a lobulated hypoechoic lesion with vascularity, and contrast-enhanced MRI revealed a multiloculated lesion with peripheral enhancement surrounding the peroneal tendons, suggestive of synovial chondromatosis (SC). Excisional biopsy demonstrated nodules of mature hyaline cartilage separated by fibrocollagenous septa, confirming SC. The patient recovered uneventfully after excision and remained asymptomatic at follow-up. This case highlights an unusual presentation of peroneal tendon sheath SC as a painless swelling, underscoring the importance of imaging and histopathology in diagnosis and the role of surgical excision as definitive treatment.