Osteochondromas (exostoses) are common benign bone tumors in children, with many being asymptomatic. The majority arise in childhood, and have a predilection for the metaphyses of the long bones, as seen in this case. Typically the lesions enlarge until the patient reaches skeletal maturity. They may be noticed as a bony, non-painful mass, or may be irritated by pressure from activity.

On plain films, the lesions appear smaller than by physical exam due to the presence of a cartilage cap, sometimes up to one cm thick, that is not seen. This cartilage cap serves as an epiphysis for the expanding lesion. The exostoses appear as stalks or broad projections from the bone, continuous with the cortex and marrow space. They consist of a cortical shell which surrounds cancellous bone. The lesion commonly projects in a direction away from the adjacent joint.

Hereditary multiple exostosis is an autosomal dominant condition characterized by presence of numerous osteochondromas. They are usually bilateral and symmetrical. In patients with severe involvement, short stature, limb length inequalities, and premature partial physeal arrest may accompany limb deformities. Recent genetic research into the disorder has suggested that the inactivation of tumor suppressor genes EXT1 and EXT2 on chromosome 8 lead to the cartilaginous growths. Indications for surgery include a rapidly enlarging mass, pain, mechanical symptoms or cosmetic concerns. Osteochondromas have low potential for malignancy, estimated at about 1%. In an adult, an osteochondroma that becomes enlarged or painful should be evaluated for sarcomatous degeneration.