Abstract: Objective: To analyze the stress value, distribution characteristics of periodontal ligament, and the distance between the center of rotation and the root tip when using invisible appliance with different material stiffness to expand upper dental arch. Methods: Three-dimensional finite element models with elastic modulus at 415.6, 816.31, and 2400MPa were established. The upper premolars and molars were designed to buccally move 0.3mm to analyze the stress distribution. Results: Under experimental condition, the stress distribution tendency of group A, group B, and group C was similar. Specific performances: buccal cervical 1/3 was compressive stress, toward the apex gradually transiting to the tensile stress. The stress distribution on the lingual side was contrast. Specific performances: the lingual cervical 1/3 was tensile stress, toward the apex gradually transiting to the compressive stress. In three models, stress concentrated in the areas of apex, root bifurcation, and neck. The area of stress distribution was: Group A>Group C>Group B. In these groups, center of teeth rotation was located in the area of the junction of middle 1/3 and apex 1/3. Expanding upper dental arch, the elastic modulus of the appliance increased and the stress increased. Conclusion: The bigger the hardness of clear aligners, the bigger the stress in periodontal ligament and aligners. Upper premolars and molars tended to move buccally.

Key words: three-dimensional element analysis, Clear removable orthodontic appliances, periodontal ligament, stress analysis, elastic modulus, stiffness