Abstract: BACKGROUND: For patients with root curvature of maxillary central incisors, clinicians often ignore the impact of root morphology changes on orthodontic treatment in patients with root curvature. There are few studies in this area.
OBJECTIVE: To study the effects of different root distal curvature angles and diaphragm thicknesses with clear aligners on the impact of intruded maxillary central incisors by three-dimensional finite element analysis.
METHODS: A three-dimensional finite element model of the left maxillary central incisors with different angles of apical 1/3 distal curvature (no curvature, 20° curvature, 40°curvature, and 60° curvature) and three kinds of various thicknesses of clear aligners (0.5, 0.75, and 1 mm) were constructed. ANSYS software was used to perform finite element analysis to compare the stress distribution characteristics of teeth and periodontal ligament, and the initial displacement trends of teeth and clear aligners when intruding 0.2 mm along the long axis of the left maxillary central incisor for different thicknesses of clear aligners. 
RESULTS AND CONCLUSION: (1) The left maxillary central incisor showed a tipping movement trend during the intrusion, with the crown displacement more significant than the root displacement. The total displacement was inversely proportional to the root curvature angle and directly proportional to the diaphragm thickness. (2) As the root curvature angle increased, the crown showed an increase in displacement and extrusion in the mesiodistal direction, while the displacement in the labiolingual direction decreased. The root showed an increase in displacement in the mesiodistal direction, and both labiolingual displacement and intrusion decreased. The displacement of the crown and root in three dimensions was proportional to the thickness of the diaphragm. (3) The periodontal ligament and tooth Von-Mises stress was proportional to the root curvature angle and diaphragm thickness. The Von-Mises stress of the periodontal ligament was mainly distributed on the palatal side of the cervical tooth. Except for the thickness of the diaphragm of 0.5 and 0.75 mm, the root had no curvature, the thickness of the diaphragm was 0.5 mm, and the root was curvature by 20°, and the stress concentration of the root at the curvature occurred. (4) The displacement of the clear aligner was directly proportional to the angle of root curvature and inversely proportional to the thickness of the diaphragm, while the maximum displacement occurred at the labial edge of the depressed orthodontic appliance. (5) The root curvature of maxillary central incisors reduced the efficiency of tooth intrusion. When intruding a maxillary central incisor with a curved root, choosing a clear aligner with a thickness of 0.5 mm was recommended, and appropriately increasing the thickness of the appliance edge reduced the risk of the appliance coming off and lowered the probability of root resorption.

Key words: three-dimensional finite element, intrusion, curved root central incisors, diaphragm thickness, clear aligner, biomechanics, bending angle