Orthodontic tooth movement occurs as a result of a force being placed on a tooth. It is composed of three phases: initial tipping, lag phase and progressive tooth movement. When the force is placed on the crown of the tooth, initial tipping occurs. The periodontal ligament (PDL) is compressed adjacent to the alveolar bone on the side toward which the force is directed. On the opposite side, away from the force direction, the PDL is widened, experiencing tension. The center of resistance of the tooth is defined as the point at which a direct force would cause the tooth to move completely linearly in the direction of the applied force. Because the force is applied at the crown of the tooth, away from this center of resistance, the tooth tips. The location of the center of resistance changes depending on the length of the root and amount of periodontal bone support, thus changing the exact type of movement that occurs. These factors, in addition to the PDL width and force magnitude, affect the amount of initial tipping that occurs.23
The lag phase represents a delay in movement, which reflects recruitment of cells and the establishment of a microenvironment that will allow the PDL and bone to remodel. This is when osteoclasts are recruited to the area and osteoblasts are activated. The length of this phase is partially dependent on the amount of force applied. If excessive forces are applied, the root approaches the alveolar wall more closely on the compression side, and the vasculature to the area is compromised. As a result, a cell-free zone or hyalinized area is formed. The hyalinized tissue must be removed for tooth movement to occur. This occurs via undermining resorption, where osteoclasts present within the adjacent bone marrow spaces and resorb bone adjacent to the cell free area. This lag phase can last from several days to several weeks. The use of light forces can minimize the appearance of hyalinized tissue and therefore reduce the length of this phase.
The final phase represents the actual remodeling of bone, consisting of bone formation in the areas of tension and resorption in the areas of compression. This process results in the movement of the tooth, reduction of the applied strain, and appliance deactivation. In summary, bone resorption occurs on the side of compression in the PDL while formation occurs on the side of tension.
An acute inflammatory response is typically present in the early phase of orthodontic tooth movement. Cytokines, which are secreted by mononuclear cells, are chemical mediators that may interact directly or indirectly with bone cells. Cytokines, such as IL-1, can evoke the synthesis and secretion of numerous substances, including prostaglandins and a variety of growth factors. Prostaglandins have been shown to stimulate bone resorption and increase the rate of orthodontic tooth movement.23