[Orthodontic treatment and mechanical stress]

In orthodontic treatment, the role of periodontal ligament is essential for the teeth to move within the alveolar process against the mechanical stress. Mesenchymal stem cells that differentiate to chondrocytes, adipocytes, and nerve cells is known to exist in the PDL. In both process of physiological and orthodontic tooth movement, bone resorption mainly occurs at the anterior region toward the direction of tooth movement, and bone apposition/formation at the posterior region to the direction of the tooth movement, and bone turnover occurs locally and site specifically, which results in constant reconstruction of the alveolar process in 3 dimension. Among the bone cells, osteocytes act as a major role in bone remodelling during orthodontic tooth movement against mechanical stress. Moreover, the side effect during tooth movement is known to be the pain and external apical root resorption.