Diurnal variation in tooth movement in response to orthodontic force in rats

The purpose of this study was to determine whether there is any difference in orthodontic tooth movement when the orthodontic force is applied at different times of the day. Twenty-four rats were divided into three experimental groups based on the time of day that maxillary expansive force was applied; i.e., the force was applied continuously throughout the entire experimental period of 21 days in animals in the whole-day group; animals in the dark-period group and the light-period group received force only during the dark period (19:00-07:00) or the light period (07:00-19:00), respectively. Maxillary expansion in the light-period group was about 2 times greater than that in the dark-period group on day 21. There was no significant difference in expansion between 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 than in the dark-period group. There was no significant difference in the width of the palate among the three groups. These results indicate that there is a diurnal variation in tooth movement in response to orthodontic external force and that the application of force during the animal's rest period may be more effective than that while it is active.