An osteochondroma or exostosis is thought to be a cartilage covered osseous projection. The precise cause is not clear but it is clear that these lesions are derived in some manner form the growth plate. As the physis fuses, the osteochondroma stops growing. 80% of the lesions occur in patients younger than 20 years old. They are usually painless, non tender masses.

Osteochondroma involve the long tubular bones most frequently. 30% involve the femur and 20% involve the humerus. However, exostosis can involve any bone that develops by enchondral ossification centers including the iliac wing in this case. Exostosis can present in as a pedunculated outgrowth near the metaphysis of a long bone and pointing towards the diaphysis. They can also present as a sessile or broad base mass. The exostosis is covered by a hyaline cartilage cap. The average size that arises from tubular bones are about 4cms whereas the ones arising from flat bones are larger with some greater than 40cm in maximum dimension.

Complications can include fracture, mass effect on vessels and nerves and malignant transformation. Osteochondromas can transform into chondrosarcomas. Lesions with a cartilaginous cap of >1 cm, growth after skeletal maturation, irregular margins, or dispersed calcifications with in the cap are suspicious for chondrosarcoma. However, there is a wide variation in the size of the cartilage cap in exostosis and chondrosarcoma. In children, caps may be as thick as 3 cms.

Hereditary multiple exostoses is an autosomal dominant disorder. As the name implies, patients with this disorder has multiple osteochondromas which can exceed 100. They are bilateral and symmetrical and have similar types of complications.

REF: Resnick, Diagnosis of bone and joint disorder, 1st Edition, pp 3701-3718.