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BACKGROUND: Intrusion of overerupted posterior teeth with clear aligner for adult patients with different bone densities, orthodontists mostly choose the diaphragm thickness by subjective experience, and finite element analysis is expected to provide a theoretical basis for choosing the appropriate thickness of diaphragms for patients with different bone densities. OBJECTIVE: To analyze the effects of diaphragm thickness and bone density on the intrusion of overerupted maxillary first molar in adult patients using clear aligners using finite element analysis. METHODS: Four groups of finite element models were created based on four bone densities (cancellous bone types I, II, III, and IV) and assembled with three clear aligner models with diaphragm thicknesses of 0.5, 0.75, and 1 mm, respectively. Finite element analysis was then performed using Ansys 2021 software. Clear aligners applied a 0.2 mm intrusion displacement to the maxillary first molar was simulated in each group. The von Mises stress and tooth displacement trends of root, periodontal ligament, alveolar bone of first molar and its adjacent teeth were compared between groups. RESULTS AND CONCLUSION: (1) The von Mises stress distribution of the first molar was the same in all groups, mainly at the root furcation; the second premolar was distributed in the proximal mid-cervical 1/3; the second molar was distributed at the buccal root furcation. (2) The displacement of the first molar and adjacent teeth was close to mesial displacement; the first molar displayed a buccal displacement trend, whereas adjacent teeth displayed a palatal displacement trend. At class II density of alveolar bone, the first molar X- and Y-axis displacement was the smallest, and the Z-axis displacement was the greatest; displacement of adjacent teeth increased with decreasing bone density. In the Z-axis direction, the displacement of the first molar and adjacent teeth was proportional to the thickness of the diaphragm. The smallest displacement was observed in the second premolar 1 mm and second molar 0.5 mm groups in the X- and Y-axis directions. (3) These results concluded that the use of clear aligners to lower the overerupted molars is less likely to cause pericemental fibrosis and root resorption. Decreased bone density does not increase the risk of root resorption. Class II bone density is optimal for molar intrusion. The maximum equivalent stress of the periodontal ligament and alveolar bone is proportional to the thickness of the diaphragm. [ABSTRACT FROM AUTHOR]