Osteochondromas (OC) are benign tumors that account for 20-50% of benign and 10% of all bone tumors. The incidence rate of OC in males is twice that of females and its peak is usually in the second decade of life (1). These lesions are inherited in an autosomal dominant manner and cause the formation of isolated lesions or multiple exostoses during the development of bones in the process of enchondral ossification in the long bones (2). Also, OC is associated with the mutation of some tumor suppressor genes, including EXT1 or EXT2 genes. OC have a periosteal origin and are formed in the active parts of bones, including the metaphysis of long bones and the cartilage at their ends. Studies show that OC can be caused after surgery/radiation-induced injury and hematopoietic stem cell transplantation (3).

Osteocartilaginous exostosis is mainly detected in childhood and in the form of palpable masses, with chronic pain and sometimes with edema (4). The presence of misplaced bone masses in the joint capsule and sometimes with cartilaginous coating in radiographic images is one of its common manifestations. The most common joints involved in OC are the hip and knee joints. Patellar OC is rare and mostly affects the patellar bursa. Moraes et al. (2014) reported a patellar OC measuring 8 × 6 × 3 cm anterior to the patella in a 60-year-old man who was painless and without limitation of flexion-extension in the knee joint (5). In the present case, rare retro-patellar OC was observed in the area of the patellar ligament.