Root resorption
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Dentistry
Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...

, root resorption is the breakdown or destruction, and subsequent loss, of the root structure of a tooth. This is caused by living body cells attacking part of the tooth. When the damage extends to the whole tooth, it is called tooth resorption. Severe root resorption is very difficult to treat and often requires the extraction of teeth.

Root resorption occurs as a result of differentiation of macrophages into osteoclasts in surrounding tissue which, if in close proximity to the root surface will resorb the root surface cementum and underlying root dentine. This can vary in severity from evidence of microscopic pits in the root surface to complete devastation of the root surface.

Deciduous root resorption is a natural process which allows exfoliation of the primary teeth to make way for the secondary teeth.
Deciduous root resorption is caused by osteoclast differentiation due to pressure exerted by the erupting permanent tooth.

Root resorption of secondary teeth can occur as a result of pressure on the root surface. This can be from trauma, ectopic teeth erupting in the path of the root, inflammation, excessive occlusal loading, aggressive tumours and growths. The most common cause in Western Society is orthodontic forces. (Weiland 2003)

Roots of teeth are covered with cementum
Cementum
Cementum is a specialized calcified substance covering the root of a tooth. Cementum is excreted by cells called cementoblasts within the root of the tooth and is thickest at the root apex. These cementoblasts develop from undifferentiated mesenchymal cells in the connective tissue of the dental...

. This is a structure that resembles bone. It is however more resistant to resorption than bone. There are a number of theories as to why this is the case. The most common hypothesis is that because cementum is harder and more mineralised than bone and has anti angiogenic properties, blood vessels are inhibited from forming adjacent to cementum which in turn prevents access to osteoclasts.
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