solitary benign tumour that consists partly of cartilage and partly of bone. Osteochondromas are common and may develop spontaneously following trauma or may have a hereditary basis. No treatment is required unless the tumour interferes with function, in which case it should be surgically removed. Rarely, a solitary osteochondroma will become malignant in adulthood, giving rise to a chondrosarcoma.

Osteochondromatosis (also called hereditary multiple exostosis or diaphyseal aclasis) is a relatively common disorder of skeletal development in children in which bony protrusions develop on the long bones, ribs, and vertebrae. If severe, the lesions may halt bone growth, and dwarfing will result. Pressure on tendons, blood vessels, or nerves may cause other disabilities. Normally, such lesions cease growing at the end of puberty; in rare cases, reactivation of growth in adulthood may indicate malignant changes.