Extraskeletal para-articular osteochondroma of the posterior knee.

Extraskeletal para-articular osteochondromas are unusual osteocartilaginous lesions that arise in the soft tissues adjacent to the joint with no bone or joint continuity. This diagnosis should be considered in patients with a well-circumscribed, extraskeletal, mineralized mass without any direct continuity with adjacent bone or joint. Although the knee is a common location for extraskeletal para-articular osteochondroma, it has not been described arising in the posterior aspect of the knee. This article presents a case of extraskeletal paraarticular osteochondroma posterior to the knee joint. Differentiation from other extraskeletal mineralized lesions, particularly extraskeletal sarcomas and synovial osteochondromatosis, is essential to avoid unnecessary aggressive surgical procedures as marginal excision is adequate for these lesions. Correlation of clinical and radiographic features with pathology is essential for diagnosis. The lesion in our patient was marginally excised, and the postoperative course was uneventful with no recurrence at 2-year follow-up.