Abstract: BACKGROUND: Distalization of maxillary dentition is a commonly used non-extraction correction strategy in clinical practice, but the clinical implementation efficiency is low. The traditional method is prone to anchorage loss and reciprocating of molars. Therefore, domestic and foreign scholars continue to explore new treatment approaches and use finite element analysis to study the mechanical mechanism of distalization. 
OBJECTIVE: To establish a three-dimensional finite element model of the maxillary complex and orthodontic appliance, and compare the initial displacement of teeth and the stress changes of periodontal ligament in two distalization methods of maxillary dentition. 
METHODS: One adult volunteer with normal occlusion was selected, and maxillofacial cone beam CT images were taken. Based on this, a three-dimensional finite element model of maxillary bone-upper dentition-periodontal ligament-archwire-bracket-traction hook-micro-implant anchorage nail was established. Based on this model, two groups of loading modes were set, namely group A (overall distalization 2 N group) and group B (step-by-step distalization 2 N group with push spring). In each group, the traction hook height was set to 3, 5, and 7 mm, respectively, that is, A1, A2, A3, B1, B2, and B3, a total of 6 working conditions. The models in group A (A1, A2, and A3) simulated the overall distalization of the anchor pin, and the models in group B (B1, B2, and B3) simulated the step-by-step distalization of the anchor pin and the push spring. The loading force of the neck of the micro-implant anchor pin in groups A and B was 2 N. The initial displacement changes of the teeth in the horizontal, sagittal, and vertical directions and the stress distribution of the periodontal membrane were analyzed and calculated using finite element software.  
RESULTS AND CONCLUSION: (1) Both loading methods achieved the distalization of the maxillary dentition, but the movement amount and efficiency were different. Under the same traction hook height, the loading method of the push spring step-by-step distalization in group B achieved a greater distalization of the molars compared with the loading method of the overall distalization in group A. During the overall distalization, the molars moved distally with an inclination, and the displacement was the largest when the traction hook height was 5 mm; during the step-by-step distalization of the push spring, the molars showed an overall distalization trend, and the displacement was the largest when the traction hook height was 7 mm, but the incisors had a labial inclination trend. Under the same traction hook height, the distal displacement of the molars in group B was higher than that in group A, and the equivalent stress value of the periodontal ligament paradigm in group B2 was the largest, which was 31 kPa. (2) When the micro-implant support was used to assist the overall distal displacement of the maxillary dentition, the distal displacement efficiency was highest at a traction hook height of 5 mm; when the micro-implant support-push spring was used to step-by-step distal displacement of the maxillary dentition, the distal displacement efficiency was highest at a traction hook height of 7 mm; at different traction hook heights, the constructed buccal micro-implant-push spring-traction hook system was more efficient than the overall distal displacement method, and the molars tended to be distalized as a whole. 

Key words: maxillary dentition, distalization, temporary anchorage, three-dimensional finite element analysis, biomechanical research, periodontal ligament