**3. Orthodontic tooth movement and biological mechanisms of reshaping of the mechanic-sensitive dental tissue**

The role of cytokines in the orthodontic tooth movement is considered in the context of inflammation, which occurs at the very beginning of this process as a reaction to the mechanical pressure and represents necessary precondition for the realization of all its subsequent levels. In the conjunction of mechanical and biological mechanisms, which move the teeth during the orthodontic treatment, cytokines are given great importance for their feature of transmission of biochemical signals among numerous cells of various kinds reacting to orthodontic forces. Binding themselves to specific receptors at membranes of these cells, cytokines cause in them the biochemical changes responsible for the signal transmission to corresponding genes in these cells and, consequently, to the change of gene expression in them. This orthodontic tooth movement causes the features of unusually complex processes, whose different degrees—each individually and all together—are regulated by the network of positive and negative feedbacks, in which cytokine molecules act as mutual activators or inhibitors [3].

Orthodontic tooth movement is a biomechanical process initiated by the effect of mechanical

The process of orthodontic movement of teeth is based on the transformation (remodeling) of periodontal tissues and is initiated by external forces and differs from the processes that occur during normal jaw function (dentition, chewing) [5]. On the basis of remodeling of periodontium, there are mechanisms, which transform the physical effort into various cell responses within the periodontal system, which primarily leads to the disturbance and then to the establishment of the periodontal homeostasis on a different basis [6]. These mechanisms provide the adaptation of the biological system of periodontium to the changed conditions emerged

Biomechanical mechanisms of the orthodontic tooth movement, because of their complexity, have been explained by various, but not mutually exclusive theories. Orthodontic dogma is considered to be the one according to which the movement of the tooth in the periodontal space occurs by the effect of two dominant forces: pressure force (compression) and tensile strength (tension) [7]. As a result of the pressure, there is resorption (suction), whereas as a result of tension to apposition (addition) of alveolar bone, the movement of the tooth occurs as a direct outcome of the reshaping of the tissue around the tooth root caused by forces. On basis of this, processes are vascular, and consequently, cellular changes of the dental tissue are caused by chemical mediators, which are created and released under the influence of orthodontic forces. Even though, in the context of this, we must not neglect the theory, which emphasizes bending of the bones as the basis of the orthodontic tooth movement [8], as well as the theory of bioelectrical signals, which emphasizes the importance of electric potentials, which are created in the tissue as a response to the application of the mechanical force [9].

Orthodontic forces lead to the change of the structural features of dental tissues at the level of cells, molecules, and genes. Mutual activities of tooth cells, periodontal ligament, bone, and bioactive substances (cytokine, chemokine, hormone, growth factor, enzymes, neuropeptides, and ligands) are necessary because they provide that during these changes, the tooth and

**2. Orthodontic tooth movement and force effect**

as a result of the effect of orthodontic forces.

36 Current Approaches in Orthodontics

forces, which overpower the bio-elasticity of the support tissue [4].

The cells of alveolar bones and periodontal ligament, gingiva, and tooth pulp react to the effect of orthodontic forces after the remodeling of extracellular matrix (ECM), which surrounds them [11]. Dental and periodontal cell responses to the applied mechanical force comprise interactions of intracellular and extracellular structural elements and mutual influences of the effects of various biochemical structures. The nature of changes in the process of reshaping is determined by the combinatory of interactions, which is different at different levels of the tooth movement [12]. The scheme no. 1 presents the main events in dental tissues, which follow orthodontic movement of the tooth.
