Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 78

Cementum contains 65% inorganic material and 12% water on a wet‐weight basis. By volume, inorganic material comprises approximately 45%, organic material 33%, and water 22%. Cementum is less densely mineralized than dentine and enamel, contains no blood vessels, and does not undergo physiological remodelling (Selving et al., 1962; Neiders et al., 1972; Cohen et al., 1992). The chemical composition of cementum may vary by individual, and morphologically, cementum is classified as both cellular and acellular (Foster, 2017).

Chutimanutskul et al. (2005) examined the physical properties of the cementum on the buccal and lingual surfaces of the roots at the cervical third, middle third, and apical third. The authors reported a decreasing gradient in the hardness and elastic modulus of cementum in both surface groups, from the cervical to apical thirds. Apical cementum is predominately cellular, less densely mineralized, and has lower hardness and elastic modulus values than the more densely mineralized acellular cementum found in the middle and cervical thirds of the root (Foster, 2017) . Therefore, the hardness and elastic modulus of cementum depend on the direction of the structural arrangement and the mineral content of the cementum (Henry and Weinmann, 1951; Jones and Boyde, 1972; Rex et al., 2005). Several studies have reported that the hardness of mineralized tissues was positively correlated with the extent of mineralization (Brear et al., 1990; Mahoney et al., 2000; Malek et al., 2001). Some have suggested that the mineral content of cementum might influence the resistance or susceptibility to root resorption.

Yamaguchi et al. (2016) examined whether there was individual variation in the Vickers hardness value of the cementum at the surface of the crown and root at three locations (cervical third, middle third, and apical third) of human first premolar teeth. The results of the study demonstrated that the hardness of the cementum decreased from the cervical to apical regions of the root surfaces. Furthermore, individual variations were observed in the hardness of the cementum, and the Vickers hardness value of the hard group was approximately two times higher than that of the soft group.

Yao‐Umezawa et al. (2017) investigated whether individual variation in the hardness and chemical composition of the root apex of the cementum affects the degree of root resorption. In a pit formation assay, the resorbed area in the soft group showed a greater increase than the moderate and hard groups. A correlation was noted between the Vickers hardness and the resorbed area of the cementum in the apical cementum. The Ca/P ratio of the cementum in the soft and moderate groups showed greater decreases than the hard group. A correlation was noted between the Vickers hardness and the Ca/P ratio of the cementum in the apical cementum. These results suggested that the hardness and Ca/P ratio of the cementum may be factors in the occurrence of root resorption caused by orthodontic forces. Furthermore, Iglesias‐Linares and Hartsfield (2017) suggested regulatory mechanisms of root resorption repair by cementum at the proteomic and transcriptomic levels.