What does exostosis mean?
An exostosis is an abnormal, typically benign outgrowth of bone that projects from the surface of an existing bone, forming a firm lump or spur. Such growths may arise for no obvious reason, develop near joints during childhood and adolescence, or result from repeated local irritation — cold-water surfers and divers, for example, often develop exostoses in their ear canals, a condition popularly called surfer's ear. Most exostoses cause no symptoms and are discovered incidentally on X-rays, though some press on nerves, tendons, or nearby tissue and require surgical removal. The word derives from Greek roots meaning essentially 'outward bone formation', and its plural is exostoses. In zoology the term is extended to similar bony or calcified growths on the shells or skeletons of fish and molluscs. Precise and clinical in register, exostosis belongs mainly to medical writing rather than casual conversation.
nounAn abnormal benign outgrowth of bone, typically forming a bony lump or spur on the surface of a bone; also, more broadly in marine biology, an abnormal bony or calcareous growth on the shell or skeleton of aquatic animals such as fish and molluscs.
- An abnormal benign bony outgrowth on the surface of a bone, as in osteochondroma or surfer's ear.
- In zoology, a similar abnormal bony or calcified growth on the shell or skeleton of an animal such as a fish or mollusc.
"The X-ray revealed a small exostosis growing from the patient's heel bone."
"The orthopaedic surgeon removed the exostosis that had been pressing against the tendon behind her knee."
"Repeated exposure to cold water caused a painful exostosis to form in the diver's ear canal."
The plural retains the Greek-style '-es' ending, pronounced /ˌɛksɒˈstoʊsiːz/. The singular is standard in clinical description; the plural appears most often in the name of the inherited condition hereditary multiple exostoses.
"The radiographs showed several exostoses along both forearms, consistent with the hereditary form of the condition."
Surfers get them so often that the bony lumps growing in their ear canals have their own name: 'surfer's ear' is actually exostosis.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of exostosis
Exostosis comes from New Latin, itself borrowed from Greek elements: 'exo-' meaning 'outside' or 'outward', 'osteon' meaning 'bone', and the suffix '-osis' denoting an abnormal condition or process. It entered English in the early modern period as anatomical terminology. The same Greek root 'osteon' underlies a wide family of English words including osteoporosis, osteoarthritis, and osteopathy, while the prefix 'exo-' appears in exoskeleton and exotic.
Related word forms
How exostosis is actually used
A clinical and technical term used chiefly in medicine, pathology, and zoology; it is rare in everyday speech. The plural 'exostoses' follows Greek-style declension. When multiple hereditary exostoses occur, the condition is known as hereditary multiple exostoses (HME).
Easily confused with exostosis
An exoskeleton is the hard external supporting structure of animals like insects and crabs, whereas an exostosis is an abnormal bony growth arising from a bone.
Exposure means the state of being uncovered or subjected to something, while an exostosis is a bony outgrowth — the two share only a similar beginning.