Extra-articular synovial osteochondroma of the Hoffa's fat pad involving the patellar tendon: A case report and literature review

Synovial osteochondroma (SO) is a relatively uncommon tumor affecting the synovial joints that is characterized by the development of hyaline cartilage from the synovial membrane. The basic pathophysiology of SO is thought to be the metaplastic change of the synovium to hyaline cartilage. Large extra- or para-articular lesions in the Hoffa's fat pad are relatively uncommon. A 56-year-old woman complained of knee pain associated with a large lesion on the anterior of the right knee. She also had restricted range of motion in the knee. Examination revealed an elastic hard 8x9-cm lesion involving the patellar tendon. Computed tomography and magnetic resonance imaging demonstrated a mosaic-intensity mass lesion in the inferior side of the Hoffa's fat pad as well as the patellar tendon. The patient underwent needle biopsy. Histological analysis revealed a solitary SO. There were no atypical features suggestive of malignancy and the mass was resected marginally. The patient had no recurrence or knee functional disorder 3 years postoperatively. There are no reports of SO involving the patellar tendon with restriction of range of motion. Removal of the mass lesion for management for SO, as in the present case, is unusual. To the best of our knowledge, this is the first case of extra-articular SO in the Hoffa's fat pad involving the patellar tendon causing a restriction of range of motion of the knee. Marginal surgical resection improved knee function.