A Case Report of Synovial Osteochondromatosis of the Knee Joint with neither Calcification nor a Synovial Lesion on Imaging

Abstract

Synovial osteochondromatosis is a rare, benign condition of unknown etiology in which the synovium undergoes metaplasia, leading to cartilaginous nodules that ultimately break free, mineralize, and even ossify. The most commonly involved joint is the knee. Typically, radiographs can be diagnostic and mineralized nodules are pathognomonic. In as many as one-third of cases, however, no calcification or ossification of the cartilage occurs in the early stage of the disease because mineralization is time-dependent. In such cases, gadolinium-enhanced MRI can be useful. Unmineralized nodules are typically peripherally enhanced because they are attached to and derive a vascular supply from the synovium. We experienced an unmineralized case of synovial osteochondromatosis of the right knee joint, in which imaging diagnosis was difficult. Neither calcification nor ossification was observed, but all nodules were released from the synovium as loose bodies and there was no vascular supply. Therefore, MRI did not show a typical appearance. These findings suggest that synovial osteochondromatosis should be considered as a differential diagnosis in a case in which unmineralized loose bodies without a synovial lesion are found in an imaging examination.