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Hereditary multiple exostoses (HME) is an autosomal dominant condition resulting in multiple osteochondromas. In western populations the prevalence is approximately 1:50,000-100,000. Common clinical presentations include pain and cosmetic deformity.

Osteochondromas may be sessile or pedunculated and are the result of a separated cartilaginous fragment of the epiphyseal growth plate, which has herniated through the normal periosteal bone cuff (notch of Ranvier). This fragment continues to grow, resulting in an osseous protuberance with direct medullary continuity and a cartilaginous cap. Consequently, these lesions are typically found in a juxta-epiphyseal distribution.

Complications of osteochondromas include fracture, mass effect (vascular compromise and neurologic sequelae), bursa formation/inflammation (secondary to abnormal frictional forces), and malignant transformation. Malignant transformation occurs in approximately 1% of solitary osteochondromas and has been reported in approximately 3-5% of HME patients. The most commonly associated malignancy is chondrosarcoma but osteosarcoma is another possibility, albeit less likely.

There are several concerning radiographic findings which may suggest malignant transformation:
• Continued growth in a skeletally mature patient
• Irregular/indistinct surface
• Focal radiolucency
• Erosion/destruction of adjacent bone
• Soft tissue mass (with scattered irregular calcifications)

Finally, and perhaps the most important criterion, is a cartilaginous cap thickness greater than 1.5cm. This can be measured with ultrasound, CT and MRI.