Structure and Features of cementum
Structure of cementum
Cementum has a similar structure to bone. It may be classified by the presence or absence of cells:
■ Acellular cementum: it is the first cementum to form and is sometimes termed primary cementum. It covers the root dentine from the cemento-enamel junction to near the root apex and does not contain cells.
■ Cellular cementum: this is found as a thin layer at the apical third of the tooth. It is sometimes termed secondary cementum. As cellular cementum develops, the cementoblasts which have created the cementum become embedded within the cementum matrix and become inactivated; these cells are termed cementocytes.
Cementocytes are contained in lacunae and their tiny processes spread along canaliculi to join up with other cementocytes. Their processes are directed towards the periodontal ligament, from which they obtain nutrients.
Features of cementum
The following features of cementum are significant:
■ Cemento-enamel junction: this can be variable. In approximately 60% of teeth the cementum overlaps the enamel; in approximately 30% of teeth the cementum and enamel meet exactly; and in approximately 10% of teeth the cementum and enamel do not meet, thus leaving an area of dentine exposed.
■ Functional changes of cementum: cementum formation continues throughout life. The attachment of the periodontal fibres in cementum can alter according to the functional needs of the tooth. Movement of teeth during orthodontic treatment or eruption can result in the periodontal fibres becoming rearranged and reattached in a new position.
■ Resorption of cementum: this is not fully understood; it can affect individual or groups of teeth. Resorption of cementum occurs when teeth are placed under excessive masticatory stress or orthodontic loading.
■ Hypercementosis: this is an increased thickening of cellular cementum. Chronic periapical inflammation around the apex of a root or excessive occlusal attrition may give rise to localised hypercementosis. Hypercementosis affecting all the teeth may be associated with Paget’s disease.
■ Concresence: this is used to describe when two teeth are fused together by cementum.