Arthroscopic Treatment of Atypical Synovial Osteochondromatosis of Infrapatellar Fat Pad: A 4-Year Follow-up Case Report and Literature Review

Primary synovial osteochondromatosis, also known as synovial chondromatosis, is a rare benign arthropathy caused by synovial cartilage metaplasia. The tumor is rarely seen outside the knee joint cavity, especially in the infrapatellar fat pad (IFP). This case presents an older woman who complained of left knee joint pain with a growing isolated mass for 2 years. Physical examination showed that a hard mass was palpable below the patella. X-ray and computed tomography examination of knee joint revealed cluster calcification foci under the patella. Under arthroscopy, the mass was found in the IFP and removed, with pathological examination indicating synovial osteochondromatosis. The patient recovered well after the operation, and no sign of recurrence was seen at a 4-year follow-up. This case report proposes that synovial osteochondromatosis in atypical sites, such as IFP, should heighten our attention and could be treated by arthroscopy.