Human tooth movement in response to continuous stress of low magnitude

Conventional orthodontic therapy often uses force magnitudes in excess of 1 00 g to retract canine teeth. Typically, this results in a lag phase of app roximately 21 days before tooth movement occurs, The current project was un dertaken to demonstrate that by using lower force magnitudes, tooth transla tion can start without a lag phase and can occur at velocities that are cli nically significant. Seven subjects participated in the 84-day study. A con tinuous retraction force averaging 18 g was applied to 1 of the maxillary c anines, whereas a continuous retraction force averaging 60 g was applied to the other. The magnitude was adjusted for each canine to produce equivalen t compressive stresses between subjects. Estimated average compressive stre ss on the distal aspect of the canine teeth was 4 kPa or 13 kPa. The moment -to-force ratios were between 9 and 13 mm. Tooth movement in 3 linear and 3 rotational dimensions was measured with a 3-axis measuring microscope and a series of dental casts made at 1- to 14-day intervals. The results showed a statistical difference in the velocity of distal movement of the canines produced by the 2 stresses (P = .02). The lag phase was eliminated and ave rage velocities were 0.87 and 1.27 mm/month for 18 and 60 g of average retr action force. Interindividual velocities varied as much as 3 to 1 for equiv alent stress conditions. It was concluded that effective tooth movement can be produced with lower forces and that because loading conditions were con trolled, cell biology must account for the variability in tooth velocities measured in these subjects.