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

Orthodontic tooth movement is a biomechanical process initiated by the effect of mechanical forces, which overpower the bio-elasticity of the support tissue [4].

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 as a result of the effect of orthodontic forces.

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 periodontal ligament remain clinically intact and surrounding bone is reorganized. The final outcome of these activities (the speed of orthodontic movement of teeth) may be defined as a phenotypic expression of numerous gene-controlled mechanisms, which connects the orthodontic tooth movement with hereditary basis, i.e., hereditary variations of factors which participate in this process [10].
