Non-calcified Synovial Chondromatosis Masquerading as a Posterior Knee Cyst: A Diagnostic Challenge

Synovial chondromatosis (SC) is a rare, benign metaplastic disorder of the synovial membrane that typically presents with joint pain, swelling, and mechanical symptoms. Although the knee is the most frequently involved joint, diagnosis can be challenging in early or atypical cases, particularly when classical calcifications are absent on imaging.

We present the case of a young adult male with a 1-year history of insidious onset posterior knee pain, without any antecedent trauma. Clinical examination revealed a full range of motion with no joint instability. Radiographs were inconclusive, showing only a faint opacity, while magnetic resonance imaging (MRI) demonstrated a well-defined, encapsulated lesion in the posterior compartment adjacent to the posterior cruciate ligament. A provisional diagnosis of a cystic or neoplastic lesion was considered. The patient underwent open surgical excision via a posterior approach. Intraoperatively, multiple cartilaginous loose bodies were found within the lesion. Histopathological evaluation confirmed the diagnosis of SC. The patient recovered uneventfully with complete resolution of symptoms and full functional restoration at final follow-up.

This case highlights an uncommon presentation of non-calcified SC mimicking a posterior cystic or soft-tissue tumor, emphasizing the importance of MRI and histopathological evaluation in cases with atypical imaging features to avoid misdiagnosis and ensure appropriate treatment.