**2. Biologic basis of orthodontic tooth movement**

Application of mechanical forces to teeth causes orthodontic tooth movement as a result of the biological responses of the periodontal tissues. Alveolar bone remodeling includes selective resorption in some areas and apposition in others. The biologic response to orthodontic therapy includes not only the response of the periodontal ligament and alveolar bone but also the response of growing areas which are distant from the dentition. However, it can be stated that tooth movement is primarily a periodontal ligament phenomenon because the alveolar bone response is also mediated by the periodontal ligament [13].

The first step of the biological response to orthodontic force is "tension and compression in the periodontal ligament" which constricts and deforms blood vessels damaging cells in the periodontal tissues. The initial aseptic acute inflammatory response is realized by releasing chemokines and cytokines from localized cells such as osteoblast, fibroblast, and endothelial cells. Most of these cytokines are pro-inflammatory providing the continuity of the inflammatory response by activating osteoclast precursors of periodontal ligament in extravascular range and inflammatory cells. Infiltration of inflammatory cells causes the increase of chemokine and cytokine levels. This enables differentiation of osteoclast precursors to multinucleated giant cells which will realize alveolar bone resorption that is required for tooth movement. The continuity of the existence of anti-inflammatory chemokines and cytokines is essential in order to suppress destructive pro-inflammatory and osteolytic processes [4]. Therefore; the pro- and anti-inflammatory responses of alveolar bone, periodontal ligament, and inflammatory cells to orthodontic force are required to be known in order to develop safe methods to shorten orthodontic treatment period.
