Abstract: BACKGROUND: The advantages of bracketless invisible orthodontic technology in patients with cleft palate are gaining attention, but there are fewer studies on its control of teeth and the effect of bracketless invisible orthodontic appliance on arch expansion in patients with cleft palate during dental replacement.
OBJECTIVE: Through three-dimensional finite element analysis, to evaluate the effect of arch expansion on tooth displacement pattern and root control efficiency in unilateral cleft lip and palate prosthesis repair with invisible appliance without groove, and to reduce the buccal tilt of teeth and improve the arch expansion effect of appliance through optimization design. 
METHODS: CT scan data of a 10-year-old male with unilateral complete cleft palate were selected to establish the finite element model of maxillary arch expansion in patients with unilateral complete palate cleft with bracketless invisible orthodontics. The model was imported into the finite element solver ANSYS workbench software to analyze the first premolar, first molar displacement and periodontal membrane stress distribution after arch expansion under various working conditions. Condition 1: Only the arch expansion of the invisible appliance was simulated; Condition 2: To simulate the arch expansion of the invisible appliance without brackets, and add 1° root buccal torque of the first molar and the first premolar; Condition 3: On the basis of Condition 2, the first premolar and the pressure point of the first molar were added by computer CAD and CAE combined optimization design; Condition 4: On the basis of Condition 2, the first premolar and a pair of pressure point sets of the first molar were added through optimal design. 
RESULTS AND CONCLUSION: The total displacements of the first premolars and first molars moved slightly to the middle in the four conditions. The root displacements of the first premolars and first molars were the largest in Condition 4, the difference of displacements was the smallest in Condition 4, and the periodontal membrane stress was the smallest in Condition 4. The expansion with bracketless invisible orthodontic appliance can effectively reduce the mesial inclination of the first premolar and first molar, improve the overall movement effect, and optimize the internal stress of periodontal ligament to achieve a more balanced and orderly distribution of stress, reducing the situation of excessive local stress concentration.

Key words: bracketless invisible orthodontic appliance, arch expansion, dental replacement period, cleft palate, tooth, finite element analysis, optimization design