Abstract: BACKGROUND: Clear aligners, due to their special material properties, are more prone to the “roller coaster effect” during the process of extracting the maxillary first premolar and closing the gap by retracting the anterior teeth, tilting and moving the teeth, leading to deepening of the anterior overjet and opening of the posterior teeth. There are fewer related studies at present concerning whether the thickness or stiffness of the aligner can be changed to reduce the “roller coaster effect” and better control the orthodontic tooth movement,.
OBJECTIVE: To analyze the differences in tooth movement control ability of clear aligners of different thicknesses and stiffness when internally retracting maxillary anterior teeth by means of the three-dimensional finite element method.
METHODS: Cone beam CT data of patients with convex profile were imported into Mimics and other software to simulate clinical cases. Three-dimensional finite element models were constructed to remove the first premolars, including the maxilla, teeth, and periodontal ligaments, while the diaphragms of clear aligners with thicknesses of 0.50 mm and 0.75 mm were assembled. Four sets of working conditions were created by loading the material properties of two Young’s modulus of 1 000 MPa and 2 000 MPa. According to the clinical design to close of the extraction gap, and analyze the distribution of periodontal ligament stress, the displacement trend and the initial displacement of the teeth after loading and calculating in Workbench 2023 R1 software.
RESULTS AND CONCLUSION: (1) The Von-Mises stress distribution pattern of periodontal ligament was consistent among the four working conditions, with the maximum value mainly distributed at the cervical part of teeth. The thicker diaphragm of the aligner, the higher the Von-Mises stress; the higher Young’s modulus of the aligner, the higher the Von-Mises stress. (2) Under the four working conditions, the overall displacement trend of the maxillary dentition was that the crowns of the anterior teeth moved towards the lingual side and the roots towards the labial side, while the crowns of the posterior teeth moved towards the proximal-medial side and the roots towards the distal-medial side, and it was observed that the crown-root displacements of the teeth were in the opposite direction in all the conditions, and that the displacement of the crowns was greater than the displacement of the roots. Except for canines, the thicker the aligner diaphragm, the lower the R/C value (root-crown displacement ratio); the higher the modulus of elasticity of the aligner diaphragm, the lower the R/C value. (3) The results showed that when the maxillary anterior teeth were internally retracted using invisible aligners, the teeth showed a tendency to tilt and move without incisor torque compensation being designed; the greater the thickness of the aligner and the elastic modulus, the greater the control of the teeth, and the tendency of the teeth to tilt and move decreased; and the cuspids had to be controlled by other auxiliary means.

Key words: clear aligners, anterior retraction, elastic modulus, aligner thickness, finite element analysis, biomechanics