Canine retraction with rare earth magnets: an investigation into the validity of the constant force hypothesis

The objective of this study was to test the hypothesis that a prolonged constant force provides more effective tooth movement than an impulsive force of short duration. Six human subjects were selected, the main criterion being a need for extraction of their upper first premolars. Canine retraction on these subjects was executed on one side with the application of a force rapidly declining in magnitude, produced by a vertical loop, and on the other side with the application of a relatively constant force. This type of force was achieved by a similar vertical loop which was constantly activated by three parylene-coated neodymium-iron-boron (Nd2Fe14P) block magnets. The vertical loop on the control side was reactivated 6 weeks after the initial activation. No reactivation was necessary on the experimental side for the duration of the experiment. The rate of tooth movement on the two sides was compared over a period of 3 months, on the basis of maxillary impressions taken at frequent intervals during the course of the study. The canines retracted with a constant force moved statistically significantly more than the control canines (p < 0.05) during the experimental period. The average differences in the mean rates of tooth movement between the two sides were in the order of 2:1 in favor of the experimental side. There were no statistically significant differences in the changes of angulation (tipping) or rotation about the y axis between the two sides. The duration of force application seems to be a critical factor in regulating rate of tooth movement. Conversely, magnitude of the applied force did not appear to be of primary significance.