Para-articular osteochondroma of the knee.

We report a case of para-articular chondroma located in the infrapatellar fat pad of the knee of a 42-year-old woman with a history of anterior knee pain and restricted range of motion. On clinical examination, a solid palpable mass in the infrapatellar region was detected. Radiography and computed tomography of the knee revealed the presence of 3 different "popcorn"-like gross calcifications within Hoffa's fat pad. Magnetic resonance imaging showed areas of signal intensity in Hoffa's fat pad that were consistent with calcific nodules surrounded by chondral tissue. During arthroscopy, a dense extrasynovial mass was discerned within Hoffa's fat pad. The mass was entirely removed through an open approach medial to the patellar tendon. Resection specimens consisted of 3 well-circumscribed nodules of 5 x 2.5 x 1.5, 2 x 2 x 1, and 1.5 x 2 x 1 cm, respectively. Grossly, these nodules were surrounded by adipose tissue and presented a thin fibrous membrane. Histologic examination showed that the nodules were composed of cartilaginous tissue surrounded by fibrous connective and adipose tissue with multifocal endochondral ossification. The precise pathogenetic mechanism of these tumors remains unknown, but cartilaginous metaplasia of articular and para-articular connective tissue seems to be the primary cause. Patients with these lesions are currently treated with the use of marginal resection or excision.