Inactivated periods of constant orthodontic forces related to desirable tooth movement in rats.

AIM: To examine the effects of inactive periods of force on the amount of tooth displacement and root resorption during experimental tooth movement in rats. SAMPLE: Sixty 11-week-old male Sprague-Dawley rats. METHOD: The maxillary first molar (M1) was moved mesially using a removable titanium-nickel alloy closed coil spring for 14 days. The rats were divided into four groups with, 0, 1, 4, and 9 hours of inactivation per day. RESULTS: Tooth displacement in the 0- and 1-hour groups was significantly greater than that in the 9-hour group. The area of root resorption in the 4- and 9-hour groups was significantly less than that in the 0- and 1-hour groups. There was no significant difference in root resorption between 0- and 1-hour groups, and also between 4- and 9-hour groups. CONCLUSION: The distance of tooth displacement gradually decreased as the inactive period increased, whereas root resorption suddenly decreased between 1 and 4 hours of inactive orthodontic force.