Abstract: Objective: To investigate the tooth movement characteristics in the treatment of different anterior tooth lip inclination and different adduction movement step with the clear gligner by finite element analysis. Methods: The finite element model of maxillary anterior adduction with the step method of clear aligner was established. According to the lip inclination of central incisor and lateral incisor, it was divided into working condition 1: U1-SN=105°, working condition 2: U1-SN=115°, and working condition 3: U1-SN=125°. The total step distance of tooth movement was 0.2 mm, including the adduction along the occlusion plane and the compression along the long axis of the tooth body. The internal adduction a: 0.18 mm, b: 0.14 mm, c: 0.10 mm in each group of working conditions were divided into groups to analyze the tooth movement characteristics under the above different conditions. Results: Under different anterior lip inclination and adduction modes, the crown of the central incisor and lateral incisor were inclined to move to the lingual side and the root to the labial side, while the crown of the second premolar to the second molars was inclined to move to the proximal center. The greater the adduction step distance, the greater the lingual displacement of the anterior tooth crown, the greater the torque change of the anterior tooth, and the greater the mesial movement of the crown from the second premolar to the second molar. Conclusion: When the anterior teeth are adducted by the invisible orthodontic device without bracket, the lingual inclination and elongation of the anterior teeth and the mesial inclination of the posterior teeth are caused by the displacement of the anterior teeth, which is proportional to the distance of the adduction step. The pre-designed absolute compression of the anterior teeth cannot offset the relative elongation caused by the tooth adduction.

Key words: three-dimensional finite element study, clear aligner, stepwise adduction, extraction orthodontics