Synovial osteochondroma originating from the synovium of the anterior cruciate ligament.

Primary synovial osteochondromatosis is a rarely seen disorder of the synovium in any joints. Among reported cases of knee osteochondromatosis, those originating from the anterior cruciate ligament (ACL) are very rare. We report an unusual case of primary synovial osteochondromatosis. A 27-year-old woman complained of a restricted range of right-knee motion and pain when walking. With locking symptoms similar to those associated with a meniscus bucket handle tear. Although plain radiograph and MRI showed normal appearance, diagnostic arthroscopy revealed the curious appearance of a huge mass seated on ACL. In addition, the sign of proliferation of synovium and cartilaginous tissues were apparent. The ACL mass has been removed while preserving the ACL fibres. Histological examination confirmed a diagnosis of osteochondromatosis. To the best of our knowledge, this is the first report to describe synovial osteochondromatosis originating from ACL fibres and has been resected while preserving the ACL.