To analyze the tooth movement distance and root resorption in rats under different orthodontic forces, to explore the effect of orthodontic force on the root resorption during orthodontic tooth movement.

64 male Sprague Dawley (SD) rats aged 10 months weighting 220-270 g in this study. 10 g and 30 g nickel-titanium closed-coil springs were applied mesially to the maxillary right first molars, while contralateral first molar was severed as a control. At day 3, 7, 14 and 28 after loading, the molars were extracted. The tooth movement distance and the volume on the surface area of root resorption were measured and scaned by Micro-CT.

Tooth movement gradually increased after loading. For 10 g group, 30 g group showed farther tooth movement distance with time throughout 14 days (P = 0.039), but there was a reverse result at day 28 (P＜0.05). There was a significant increased in the root resorption craters between the control group and the 10 g/ 30 g group after 28 days of loading (P = 0.004), and the deeply root resorption were observed on the 30 g group (P＜0.001).

Reliable evaluation and quantitative analysis in the tooth movement and the root resorption could be done by Micro-CT. Tooth movement in the 10 g group was more rapid than the 30 g group, nevertheless, the root resorption craters was less reduced after 28 days loading.