Exostosis

General descriptive term for any entity with bony outgrowth.

Osteochondroma is most common and clinically important subtype of exostosis. The term should be used instead of generic exostosis if it exhibits usual characteristics including:

• Cortico-medullary continuity
• Typical metaphyseal location
• Cartilage cap (especially on MRI)

Other types of exostoses

• Subungual exostosis: beneath toenail/fingernail, often reactive
• Traumatic exostosis: e.g. surfer’s ear (external auditory canal)
• Turret exostosis: post-traumatic bone spur
• Reactive exostosis: at insertion sites (chronic traction)