A synovial osteochondroma replacing the anterior cruciate ligament at the intercondylar notch

A woman presented with knee pain and locking. Pain was exacerbated at the end of the range of motion, especially during extension, with locking symptoms similar to those associated with a meniscus bucket handle tear. Ligamentous laxity was not definite. Plain radiographs showed multiple calcified loose bodies. Magnetic resonance imaging (MRI) showed a lobulated mass that was hypointense to muscle on T1-weighted sequences and hyperintense to muscle on T2-weighted sequences in the anterior cruciate ligament (ACL). Arthroscopically, multiple loose bodies were observed in the intercondylar notch and posterolateral compartment. A huge mass replaced the normal ACL and was caught in the intercondylar notch. The mass in the intercondylar notch caused loss of extension range of motion (ROM) because the piece caused a mechanical blockage. However, the loss of flexion ROM was likely caused by a loss of elasticity of the ligament rather than mechanical blockage. We resected the ACL mass, and removed the free bodies from the posterolateral corner. It was not possible to preserve the ACL fibers. Histological examination confirmed a diagnosis of osteochondromatosis. All symptoms resolved postoperatively. At 20 months postoperatively, the patient was pain free and had regained full knee motion without recurrence evidenced by follow-up MRI. However, ACL removal caused the knee instability. To date the patient has not undergone ACL reconstruction because she prefers conservative treatment and has experienced little discomfort in activities of daily living. To our knowledge, this is the first report to describe synovial osteochondromatosis wholly replacing the ACL fibers and causing mechanical blocking of both extension and flexion.