• Lesions/Condition: Osteochondroma

Tumor Growth: Lesions grow by enchondral ossification of proliferating cartilage cells in its cap. The tumor will continue to enlarge during skeletal growth, but typically becomes latent at skeletal maturity.

• Synonyms: Osteocartilaginous Exostosis

• Common Locations: The most common site of origin for an osteochondroma is the metaphysis at bony sites of tendon and ligamentous attachments. Common sites include distal femur, proximal humerus, proximal tibia but also may occur less frequently in the pelvis, scapula, spine or any bone preformed in cartilage.

• Demographics: Typically occur in first to third decades with prevalence greater in males than in females. It is the most common skeletal tumor in children with an occurrence of 1 in 200. They may be solitary or multiple and they may arise spontaneously.

• Gross Morphology: Osteochondromas vary in size. The average lesion arising from tubular bones is approximately 4cm. Osteochondromas arising from flat bones tend to be larger with a higher likelihood of malignant transformation.

• Histology: Pathologic sections show osteochondromas to have a cartilaginous cap. Histologically, the cartilaginous cap is identical to the physeal growth plate. During active growth, the cap is composed of hyaline cartilage. The thickness of the cap is correlated with the age of the patient, and the cap decreases in size as patient’s age. In children and adolescents, the cap may be as thick as 3cm, whereas in older patients, it may be thin. A thick cartilaginous cap (>1cm) in an adult should raise the possibility of malignant transformation (usually chondrosarcomas)

• Radiology: Plain radiographic appearances of an osteochondroma are those of a pedunculated or sessile bony excrescence with well-defined margins. In adults, the cartilage cap often contains flecks of calcification. Continuity of the osteochondroma with the medullary canal is a characteristic finding, particularly obvious in long bones. MR imaging is able to detect continuity of the cortical and medullary bone in the outgrowth with that of the parent bone. This finding is diagnostic of an osteochondroma. Of greatest utility in the functionality of the MR is the ability to precisely identify and measure the cartilaginous cap of an osteochondroma using T2 weighted spin echo images. These measurements can be used to supply information regarding the likelihood of malignant changes.

• Prognosis and Treatment: Most solitary osteochondromas are discovered in children and adolescents and are painless, slow-growing masses and require no treatment. Significant symptoms may occur as a result of complications such as fracture, bony deformity, mechanical joint problems, and vascular or neurologic compromise. Appreciable morbidity is related to resection of osteochondromas and corrective surgery on the osseous deformity. Resections are typically only performed on mature bone due to the risk of damage to the epiphyseal growth plate resulting in severe growth deformity in the affected bone. Serial radiographs are performed to monitor for malignant transformation which is estimated at 1% for solitary osteochondromas. The most common resulting malignant tumor is a chondrosarcoma.