Osteochondromas (exostoses) are the second most common benign tumor of bone after nonossifying fibromas.
The male-to-female ratio is 1.5: 1 to 2: 1. These are congenital lesions but are usually discovered between 10 and 20 years of age.
Multiple exostoses are uncommon, occurring only one tenth as frequently as a solitary exostosis, but they manifest earlier, usually before 10 years of age.
Ninety percent of osteochondromas originate from a long bone close to the metaphysis. The most common locations are around the knee (distal femur and proximal tibia) and the proximal humerus. The morphology of an osteochondroma is more important than the signal intensity in diagnosis.
The incidence of malignant transformation is approximately 1% for solitary osteochondromas and from 5% to 25% for hereditary multiple exostoses.
Both CT and MRI demonstrate the continuity of the cortex and medullary cavity of the osteochondroma with that of the parent bone. The perichondrium is well seen on T2-weighted MR images as an area of low signal intensity surrounding the outer surface of the high signal intensity of the cartilage cap. MRI measurements of cartilage cap thickness are also accurate, whereas CT measurements of maximal cartilage thickness are often imprecise. The thickness of this structure is important in distinguishing benign osteochondroma from an exostotic chondrosarcoma. According to most authors, the cartilage cap is usually thicker than 3 cm in a chondrosarcoma.
Other complications of osteochondromas include nerve injuries, vascular injury, and bursa formation. The formation of a bursa over an osteochondroma is common, and the bursa is usually asymptomatic; if it becomes inflamed and distended, it can be painful. On T2-weighted images, bursal fluid has high signal intensity similar to that of the cartilage cap, and it can be difficult to differentiate between the two. On gradient-echo sequences, cartilage has a lower signal intensity than that of fluid, and the diagnosis can be easily made. A rarer complication is the formation of a pseudoaneurysm adjacent to the exost~