Objective: To explore the effect of estrous cycle on tooth movement by establishing an animal model which mimic the orthodontic clinical operation. It will be provide a clue for orthodontist to choose the best time to treat their female patients. Methods:The sample contained 200 female Wistar rats which were 3-month-old. All of them had a stable estrous cycle with 5-day intervals. They were assigned to four groups randomly according to Haruyama's method. The specific phase of estrous cycle was determined by vaginal smear. The tooth movement appliances were worn and brought the first maxillary molar to bear 50 gram force. The measurements of each group were analyzed by t-test, one-way ANOVA and S-N-K test. Results: The average quantum of tooth movement in proestrous, estrous, metestrous and diestrous groups were 1.79±0.03,2.10±0.14,1.94±0.04,1.89±0.06 mm . The greatest amount of tooth movement was obtain in the estrous group and the lest amount was obtain in the proestrous. group. The differences among all the groups were significant (p<0.05) only except between the metestrous and diestrous group. Conclusion: The results of this study showed that orthodontic tooth movement was certainly correlated with the estrous cycle. It was suggested that there may be an optimal time for the female orthodontic treatment. More investigations are needed to determine which phase of female people is the best opportunity to apply orthodontic forces. This research was supported by National Nature Science of China through Research Fund No.30371560.

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