Giant Synovial Chondromatosis of the Knee Mimicking a Parosteal Osteosarcoma: A Case Report

Synovial chondromatosis is a benign nodular cartilaginous proliferation arising in the synovium of joints. Usually the major weight bearing joint is involved and approximately 70% of cases are the knee joint. The radiolographic features of this condition are variable. Classically, radiographs reveal multiple intraarticular calcifications of similar size and shape, distributed throughout the joint, with typical “ring-and-arc” chondroid mineralization. The classic appearance is not often encountered. Rarely, it would be confused with malignancy such as chondrosarcoma, osteosarcoma or synovial sarcoma. We report a case of primary synovial chondromatosis originating from the intra-articular tissues of the knee, mimicking a parosteal osteoarcoma with the clinical, radiographic and computed tomography (CT) findings.