Orthodontic Tooth Movement
At its most basic level, orthodontic tooth movement is the application of forces to teeth resulting in bone resorption under pressure and bone deposition under tension. The entire process is based on tissue remodeling as both the teeth and surrounding tissues have active reparative mechanisms that adapt under orthodontic forces. Tooth movement is comprised of three distinct phases known as the displacement phase, the delay phase, and the acceleration and linear phase.
The displacement phase refers to the small amount of tooth movement that occurs immediately following force application (Huang et al., 2005). This initial movement is caused by physical compression of the viscoelastic periodontal ligament. Because the displacement phase is not the result of tissue remodeling, the amount of tooth movement is dependent on its biophysical limitations. Both the root length and surrounding alveolar bone height affect the amount of tooth displacement by altering the center of resistance and center of rotation (Tanne et al., 1991; Yoshida et al., 2001). In addition, the elasticity of the periodontal ligament (PDL) limits tooth movement in the displacement phase. Investigators (Tanne et al., 1998) have shown the Young’s modulus elasticity for the adult PDL is greater than in adolescents, correlating in slightly increased stress levels in the adult PDL. They have suggested this increased stress may cause delayed tooth movement in adults as a result of a decreased biological response of the PDL.
The second phase of the tooth movement cycle is the delay phase. During this phase there is an absence of clinical tooth movement as extensive remodeling occurs in the surrounding tissues (Huang et al., 2005). Depending on the amount of localized compression of the PDL, there will be either a partial or complete blockage of the local blood vessels. A partial blockage generally is preferred allowing the local blood vessels to adapt to the change in environment by undergoing angiogenesis to bypass the blocked areas. A complete blockage typically is undesirable as it results in a complete stop of vascular flow and tissue necrosis of the local area. It usually is the result of excessive forces and slows down the initiation of tooth movement by 1-2 weeks as the local area repairs itself.
Acceleration and Linear Phase
The final stage in the tooth movement cycle is the acceleration and linear phase. It is the result of the periodontium and alveolar bone adapting to the forces placed on them and is the phase that actually results in the majority of tooth movement (Huang et al., 2005). In fact, research has shown that if an orthodontic appliance is reactivated during this phase, a second cohort of osteoclasts is recruited resulting in even greater tooth movement with no greater risk of root resorption (King et al., 1998). It is likely that any modulation of osteoclast activity to regulate tooth movement would occur during this phase of tooth movement.
Dr. Matthew Dunn is a board-certified orthodontist in Phoenix, Arizona.
He completed both his dental and orthodontic training at the University
of Michigan. Dr. Dunn has won multiple national research awards for his
studies on the effects of pharmacological agents on tooth movement. He
lives in Goodyear, Arizona with his wife Courtney Dunn (also an
orthodontist) and their three children.