Cementum is the thin layer of calcified (tough calcium deposits) tissue covering the dentine of the root and is one of four tissues that support the tooth in the jaw (the periodontium). The others tissues that support the tooth are the alveolar bone the periodontal ligament and the gingivae. Cementum is the least understood one of these four tissues.

Cementum is pale yellow with a dull surface and is softer than dentine. The permeability of cementum varies with age and the type of cementum with the cellular variety being more permeable. In general cementum is more permeable than dentine. The relative softness of cementum combined with its thinness means that it is readily removed by abrasion when the root surface is exposed to the oral environment.

Very little is known about the origin and cell dynamics of the cementum-forming cells (cementoblasts). Although restricted to the root in humans cementum is present on the crowns of some mammals. Cementum varies in thickness at different levels of the root but is thickest at the root apex.

Cementum is adjacent with the periodontal ligament on its outer surface and is firmly fixed to dentine on its deep surface. Its primary function is to give attachment to collagen fibres of the periodontal ligament. It therefore is a highly responsive tissue maintaining the integrity of the root helping to maintain the tooth in its functional position in the mouth and being involved in tooth repair and regeneration.

Cementum is slowly formed throughout life and this allows for continual reattachment of the periodontal ligament fibres. Cementum is similar in chemical composition and physical properties to bone however cementum is avascular (not associated with or supplied by blood vessels) and is also less readily resorbed.

Cementum is volumetrically composed of 45% of inorganic material approximately 33% of organic material and 22% water. The principal inorganic components are hydroxyapatites which are thin and plate-like crystals similar to those in bone. They are on average 55 nm wide and 8 nm thick. The organic matrix is primarily composed of collagen. There are three main types of cementum. These are:

Acellular cementum

This is the cementum without cellular components that covers one-third to one-half of the tooth root adjacent to the cemento-enamel junction (the area where cementum and enamel meet).

Afibrillar cementum

This is a layer of cementum that sometimes extends onto the enamel of a tooth at the cemento-enamel junction.

Cellular cementum

This is the cementum covering the apical one-half to two-thirds of the tooth root.

A lack of cementum can lead to tooth loss as the tooth is held less firmly in place.

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