Osteosclerosis is an elevation in bone density. It can be a pathology, normally detected on a radiograph as an area of increased opacity; that is, where more mineral is present in the bone to absorb or deflect the X-ray beam. Localized osteosclerosis can be caused by injuries that compress the bone, by osteoarthritis, and osteoma.
In the animal kingdom there also exists a non-pathological form of osteosclerosis, resulting in unusually solid bone structure with little to no marrow. It is often seen in in aquatic vertebrates, especially those living in shallow waters, providing ballast as an adaptation for an aquatic existence. It makes bones more heavy, but also more fragile. In those animal groups osteosclerosis often occurs together with bone thickening (pachyostosis). This joint occurrence is called pachyosteosclerosis.
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^Houssaye, A. (2009). "Pachyostosis" in aquatic amniotes: a review. Integrative Zoology 4(4): 325-340.