Osteochondromas, also called cartilaginous exostoses, are the most common benign bone tumor in children and adolescents.5 Most osteochondromas occur in the metaphysis of long bones, particularly the proximal tibia, proximal humerus, and distal femur. When they occur in flat bones, the ilium and scapula are most commonly affected. They are cartilaginous in growing children, enlarge during skeletal growth, and ossify at skeletal maturity. It likely arises as a result of physeal cartilage displacement onto the bone surface.2 Osteochondromas are usually discovered between 5 and 15 years of age and present as a bony, nonpainful mass. If pain is present, it is usually owing to friction or pressure on the adjacent structures. Occasionally, exostoses have been associated with neurovascular impingement, fracture, pseudoaneurysms, vascular obstruction, and nerve damage.
Although most exostoses occur singly, rarely some patients have autosomal dominant multiple hereditary exostoses (MHE) resulting in multiple osteochondromas.6 Although the degree of involvement tends to run in families, phenotypic expression is variable. Severely affected individuals may have short stature, limb-length discrepancies, premature partial physeal arrests, extremity deformities, and deterioration of the hip joint.
Radiographs demonstrate sessile or pedunculated projections arising from the surface of the bone usually directed away from the joint (Fig. 111-2). The lesion's cartilage cap, which can be more than 2-cm thick in childhood, is not seen on x-ray, making the osteochondroma appear smaller on radiographs than it is on physical examination. The cortex and marrow space of the involved bone are continuous with the lesion. CT or MRI may be needed to demonstrate it clearly. The radiographic appearance is usually pathognomonic, and biopsy is rarely needed to establish the diagnosis.6 Resection of osteochondromas may be considered for cosmetic reasons; pain relief; or if they cause growth retardation, vascular obstruction, or pseudoaneurysms.
Osteochondroma of the proximal humerus.
Malignant transformation of a single lesion is exceptionally rare. In families with MHE, however, there is a 1% to 8% lifetime risk of developing chondrosarcomas in the exostoses. Malignant transformation occurs in adulthood.7 Growth of the cartilage cap or a cap that is thicker than 2 cm after skeletal maturity is worrisome for malignant transformation.2