Abstract: Objective: To explore the influence of the time of fixed orthodontic treatment on root absorption. Methods: 36 patients (13-19 years old) with class II division 1 malocclusions were recruited, and treated by straight wire appliances after extraction of 4 first premolars without microimplants therapy. All patients were randomly assigned to short-term group (<18 months) and long-term group (>24 months). Teeth were classified by root morphology and analyzed using software to show their frequency distribution. To measure root length, panoramic radiographys were performed before and after treatment. To calculate distortion rates, a 5 mm ligature wire were cohered to the buccal sides of crown longitudinally before cone beam computed tomography (CBCT). The outline of 4 upper incisors was drew on CBCT, and the root length was measured and calculated according to the distortion rates. Results: Both groups showed more root resorption in level 1-2 after the treatment. Patients who were treated more than 2 years showed less level 3-4 dental root resorption than that of patients who were treated less than 1 year, and root uptake in the short term group was less than that of long term group (P< 0.01). Conclusion: The longer period of orthodontic treatment will lead to more root resorption, which will provide theoretical basis for clinical treatment.

Key words: Orthodontic, Root resorption, Period of the treatment