Biological Mechanisms of Tooth Movement

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.

Tooth movement by orthodontic force application is characterized by remodeling changes in dental and paradental tissues, including the dental pulp, PDL, alveolar bone, and gingiva. Studies during the early years of the twentieth century attempted mainly to analyze the histological changes in paradental tissues during and after tooth movement. Those studies have demonstrated that OTM causes inflammatory reactions in the periodontium and dental pulp. These reactions stimulate the release of various biochemical signals and mediators, causing alveolar bone remodeling and pain.

Although the orthodontic patient may feel periodic discomfort during treatment, the inflammation occurring along the entire duration of treatment is a crucial phenomenon because it is stationed in the heart of the remodeling process that facilitates tooth movement. Therefore, the control of inflammation and the efficiency of OTM are closely intertwined. Continuous unfolding of this intimate association will yield new information, which should enable orthodontists to provide ever improving treatment to their patients, based upon the acquisition of personal biological data of high diagnostic value. This approach to diagnosis and treatment planning draws its power from an increasing reliance upon meaningful personal information about ingredients and functions of cells affiliated with the nervous, skeletal, immune vascular, and endocrine systems. Such biological data, if included in the orthodontic diagnosis, should enable the orthodontist to tailor the treatment plan to fit the patient’s biological profile and minimize the risk of causing iatrogenic damage, such as root resorption.

Figure 4.5 (a) Schematic presentation of inflammation in the PDL cells resulting in cytokine release leading to pain, bone resorption, and possible root resorption. (b) Schematic presentation of inflammation of the dental pulp cells resulting in neuropeptide release leading to pain and possible tooth resorption.

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