DIURNAL-VARIATION IN TOOTH MOVEMENT IN RESPONSE TO ORTHODONTIC FORCE IN RATS

The purpose of this study was to determine whether there is any differ ence in orthodontic tooth movement when the orthodontic force is appli ed at different times oi the day. Twenty-four rats were divided into t hree experimental groups based on the time of day that maxillary expan sive force was applied; i.e., the force was applied continuously throu ghout the entire experimental period of 21 days in animals in the whol e-day group; animals in the dark-period group and the light-period gro up received force only during the dark period (19:00-07:00) or the lig ht period (07.00-19:00), respectively. Maxillary expansion in the tigh t-period group was about 2 times greater than that in the dark-period group on day 21. There was no significant difference in expansion betw een the light-period group and the whole-day group. An experiment that used chronologic labeling with NTA-Pb revealed that there was greater formation of new bone on the tension side in the light-period group t han in the dark-period group. There was no significant difference in t he width of the palate among the three groups. These results indicate that there is a diurnal variation in tooth movement in response to ort hodontic external force and that the application of force during the a nimal's rest period may be more effective than that while it is active .