The initial clinical sign is a hard, painless mass fixed on the bone. Symptoms, when present, are usually due to irritation of the overlying soft tissues that may or may not be associated with a fluid-filled bursa. Variations in the fluid content in the bursa create a fluctuating, palpable mass.

In most patients, the lesion is solitary; however, polyostotic tumors occur in a small subset of patients. These multiple hereditary exostoses produce significant deformities, such as short stature and angular deformities. In multiple hereditary exostoses as with enchondromatosis there is an increased incidence of malignant degeneration.

Diagnostic Studies. The radiographic appearance of an exostosis is either a flat, sessile lesion or a pedunculated (stalklike) cortical-sharing process. The lesions are called cortical sharing because they share the same cortex as the adjacent normal cortical bone. It is usually a well-defined metaphyseal projection of bone with mottled density; the cartilaginous cap displays irregular areas of calcification. The radiographic hallmark is the blending of the tumor into the underlying metaphysis. Although a diagnosis is seldom problematic, the presence of a painful and enlarging lesion in an adult may necessitate staging studies to assess the risk of secondary malignant transformation to chondrosarcoma (see Plate 6-17).

Evidence of malignant transformation includes confirmation by CT or MRI of a soft tissue mass with cortical destruction and a sudden increase in size of the mass and pain. The radiographic differential diagnosis is periosteal chondroma (see Plate 6-5) and parosteal osteosarcoma (see Plate 6-16). In an adult, a symptomatic “exostosis” that increases in size could be a malignancy, although benign osteochondromas may develop a reactive bursa that increases in size, simulating tumor growth. About 70% of patients with multiple exostoses have germline EXT1 or EXT2 mutations.

Exostoses on gross examination reveal an overlying cartilaginous cap with no growth plate but separated from the underlying bone by an irregular, chalky white line (zone of calcification). The mass of trabecular bone that makes up the main portion of the exostosis merges into the underlying normal trabecular bone of the metaphysis.

On microscopic examination, the cartilaginous cap does not have the tidemark characteristic of articular cartilage.

Treatment/Prognosis. Marginal excision of an active exostosis, including the cartilaginous cap and the overlying perichondrium, minimizes the risk of recurrence. The deep bony base has minimal activity and may be removed piecemeal (in more than one piece). The prognosis for a solitary exostosis is excellent (<5% recurrence after marginal excision). The risk of sarcomatous transformation in solitary exostosis is about 1%, but in multiple hereditary exostoses, this risk approaches 10%.