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ICD-9-CM  726.91
Patient UK  Exostosis
DiseasesDB  18621
MeSH  D005096

An exostosis (plural: exostoses) is the formation of new bone on the surface of a bone. Exostoses can cause chronic pain ranging from mild to debilitatingly severe, depending on the shape, size, and location of the lesion. It is most commonly found in places like the ribs, where small bone growths form, but sometimes larger growths can grow on places like the ankles, knees, shoulders, elbows and hips. Very rarely are they on the skull.


They normally form on the bones of joints, and can grow upwards. For example, if an extra bone formed on the ankle, it might grow up to the shin. Osteophytes are not always distinguished from exostoses in any definite way though in general there are a number of differences.

When used in the phrases "cartilaginous exostosis" or "osteocartilaginous exostosis", the term is considered synonymous with osteochondroma. Some sources consider the two terms to mean the same thing even without qualifiers, but this interpretation is not universal.

Fossil record

Evidence for exostosis found in the fossil record is studied by paleopathologists, specialists in ancient disease and injury. Exostosis has been reported in dinosaur fossils from several species, including Acrocanthosaurus atokensis, Albertosaurus sarcophagus, Allosaurus fragilis, Gorgosaurus libratus, and Poekilopleuron bucklandii.

Hereditary multiple exostoses

Hereditary multiple exostoses (HME), also called hereditary multiple osteochondromas (HMO), is a condition that is estimated to affect 1 in 50,000 individuals. Multiple benign or noncancerous bone tumors develop in the affected individuals. The number and location vary among affected patients. Most people seem unaffected at birth; however, by the age of 12 years, they develop multiple exostoses.

Types of exostosis

  • Surfer's ear (exostosis of the ear canal)
  • Hereditary multiple exostoses (HME)
  • Subungual exostosis
  • Buccal exostosis
  • Torus mandibularis
  • Torus palatinus
  • References

    Exostosis Wikipedia