Para-articular osteochondroma of the infrapatellar fat pad

We report a rare case of para-articular osteochondroma of the infrapatellar fat pad. A 56-year-old woman presented with a 2-year history of mild anterior knee pain and a hard mass around the right knee. Plain radiographs and computed tomography revealed trabeculated ossification within the infrapatellar fat pad. We performed surgery through an open approach. The mass was located within the infrapatellar fat pad and did not communicate with the joint space or patellar tendon. The gross appearance of the tumor resembled an osteochondroma. On microscopic examination, it was composed of cartilaginous tissue with areas of multifocal endochondral ossification surrounded by fibrous connective tissue. There was no synovial tissue. We diagnosed it as a para-articular osteochondroma of the infrapatellar fat pad. In the literature, this entity is rare. Differential diagnosis includes a variety of bone-forming lesions including malignancy. Definitive diagnosis is made by combining clinical, radiologic, and histologic findings.