by Laura Iwasaki, DDS, MSc, PhD
OP: In your study, how fast did the average tooth move?
Iwasaki: Our research group here at the University of Nebraska Medical Center (UNMC) College of Dentistry has conducted studies of maxillary canine retraction by bodily tooth movement (ie, movement of the crown and the root) in humans. Orthodontic treatment in all cases involved extraction of the adjacent maxillary first premolars. The results of the first studies, based on the movement of 30 teeth in 15 subjects over 84 days, have been summarized in a recent publication.1 These results showed that the overall mean velocity of tooth movement was 3.8 mm/day, or about 1.1 mm/month.
OP: How did the amount of force applied affect the rate of movement?
Iwasaki: In our studies, we calculated the force needed to apply a desired pressure or stress (force per unit area) to a given tooth. This was a way of comparing results from teeth with roots of different sizes and shapes. Initially, we found that stress and velocity of tooth movement were positively related for applied stresses of 4 to 52 kPa. This range of stresses was associated with average applied forces of 18 to 240 g. The results of our most recent study2 suggest that 26 kPa (approximately 120 g) may approach an optimum stress for maximizing the speed of movement for maxillary canines. We are planning additional studies to investigate further this question of what is the optimal stress for fastest tooth movement.
In addition, we have shown that tooth translation can occur without a lag phase. In our studies to date, tooth movement was recorded throughout the 84-day study period for almost all teeth. At Day 1 after force application, there was on average about 0.3 mm of tooth movement, which likely represented squeezing of the periodontal ligament. After this initial movement, teeth generally moved at a steady velocity. We did, however, observe a classical lag phase in three of 12 teeth moved by the highest stress tested so far, 52 kPa (about 240 g).
OP: How much did the rate of movement differ among various people?
Iwasaki: We have found the rate of tooth movement for the same applied stress to vary by more than 5:1 among different people who were at approximately the same stage of development.
OP: What biological factors affected the speed of movement?
Iwasaki: We have begun to investigate whether the individual’s stage of development, secretion of biologically active substances during tooth movement, and genetics are correlated to the rate of tooth movement. Our results to date show that:
? On average, tooth movement in individuals who showed evidence of growth during their orthodontic treatment was about twice as fast as tooth movement in adult individuals who showed no growth during their treatment.
? Ratios of two cytokines measured in gingival crevicular fluids collected from the distal side of the tooth being moved and compared to a control site (a tooth not subjected to an orthodontic force) can account for 60% of the variance shown in the velocity of tooth movement. The cytokines measured were interleukin-1, (IL-1), a pro-inflammatory agent involved in many processes including bone resorption, and its naturally occurring competitive antagonist, interleukin-1 receptor antagonist (IL-1RA).
? Since the relative outputs of these cytokines (IL-1 and IL-1RA) have been linked to certain genetic characteristics called interleukin-1 gene cluster polymorphisms, we also measured the genotype of our recent subjects. These results have shown that specific interleukin-1 gene cluster polymorphisms are associated with faster tooth movement.
OP: Is there anything else about tooth movement that might be of practical help to orthodontists?
Iwasaki: It may become more important in the future to measure the forces and moments that we apply to teeth during orthodontic treatment. Thankfully, there are battery-operated force and moment gauges with components that can be sterilized available to measure these at chairside.
Laura Iwasaki, DDS, MSc, PhD, is a professor in the Department of Growth and Development at the University of Nebraska at Lincoln. She can be reached at [email protected]
1. Iwasaki LR, Crouch LD, Reinhardt RA, Nickel JC. The velocity of human orthodontic tooth movement is related to stress magnitude, growth status, and the ratio of cytokines in gingival crevicular fluid. In Biological Mechanisms of Tooth Movement and Craniofacial Adaptation. Davidovitch Z and Mah J (eds). Boston, Harvard Society for the Advancement of Orthodontics; 2004:133-143.
2. Gibson, CS, Crouch LD, Marx DB, Pandey JP, Iwasaki LR. Interleukin-1 gene cluster polymorphisms and velocity of orthodontic tooth movement. J Dent Res. 2005;84(Spec Iss A):255.