Abstract: BACKGROUND: One of the advantages of clear aligner treatment is molar distalization. However, tooth tilting movement and loss of anterior anchorage may occur during treatment. There are few studies on whether these problems can be improved by selecting clear aligners with different thicknesses and edges to improve the clinical treatment effect.
OBJECTIVE: To analyze the control ability of clear aligners with different thickness and edges on the central incisor, lateral incisor, and second molar when pushing the maxillary second molar distally by three-dimensional finite element analysis.
METHODS: Three-dimensional finite element analysis models of bilateral maxillary second molar distalization with clear aligner, maxillary dentition, periodontal ligament, and alveolar bone with different thicknesses and margins were established by Mimics, Geomagic Wrap, 3-matic and SolidWorks software, respectively. There were 16 combinations of four thicknesses (0.4, 0.5, 0.625, and 0.75 mm) and four margins (scallop, straight, straight extension 2 mm and straight extension 4 mm). The data were imported into Ansys Workbench software for design and solution. The mean value, peak value and distribution of the periodontal ligament equivalent stress of the second molar, the equivalent stress and the maximum initial displacement of the second molar, and the control ability of each appliance on the second molar, central incisor, and lateral incisor were analyzed.
RESULTS AND CONCLUSION: (1) The mean equivalent stress of periodontal ligament of the second molar, the equivalent stress of the second molar and the maximum initial displacement of the second molar increased with the extension of the appliance edge and the increase of the thickness of the appliance in the 16 groups of models. (2) When the thickness of appliances was the same, the maximum equivalent stress of the second molar in the linear appliance group was the highest, and the maximum equivalent stress of the second molar in the linear extended appliance group was greater than that in the scallop appliance group. When the edge of the appliance was the same, the periodontal ligament equivalent stress peak of the second molar increased with the increase of the thickness of the appliance. The equivalent stress distribution in the periodontal ligament of the second molar in the linear extendable appliance group was more uniform than that in the scallop appliance group and the linear appliance group. (3) When the thickness of the appliance was the same, the scallop-shaped appliance had the worst control on the second molar. When the edge of the appliance was the same, with the increase of the thickness of the appliance, the control of the second molar by the linear extender appliance was gradually stronger than that by the linear appliance. The control of the central incisor was stronger and more stable with the linear extended 2 mm appliance, while the control of the lateral incisor was stronger and more stable with the linear appliance. (4) The results showed that when using clear aligners to push molars distally, extending the edge of the appliance could improve the control of the molars and reduce the tilting movement of the teeth. The design of a straight extension margin of 2 mm for the central incisor and a straight edge for the lateral incisor can enhance the control of the anchorage incisor and reduce the labial inclination of the anterior teeth.

Key words: clear aligner, clear aligner treatment, molar distalization, film thickness, edge design, cutting line design, margin extended, three-dimensional finite element analysis, engineered dental material