Three-dimensional Analysis of Distally Maxillary Molars with Clear Aligners under Different Movement Designs
Authors: Hu Yirong , Song Baolong , Li Bo , Shi Ruiying , Gu Zexu 
Published: June 2023

Objection: This vitro experiments investigated the orthodontic force of different distalization designs of molars and the reaction on the other teeth, in order to provide a theoretical basis for clinical protocol design.
Method: Different molar distalization were designed in the three-dimensional force measurement device. There were four groups, group A: simultaneous distal movement of teeth 16, 17, 26, and 27; group B: distal movement of teeth 17 and 27; group C: distal movement of teeth 16 and 26 after teeth 17 and 27 in place; group G without any design movement as a control group. All the amount of these movements were designed to be 0.2 mm. Then, a three-dimensional mechanical model was established by a Nano17 mechanical sensor. The data of the whole dentition of the four groups were measured and obtained for comparative analysis, and an independent sample t-test was used to investigate the relationship between different molar distalization designs with forces.
Result: For the Group A, in which the second premolar, the first and second molars were in the simultaneous distal movement. The sagittal force values were − 4.76 ± 0.71 N, 5.61 ± 0.94 N, and 5.82 ± 1.08 N, respectively. The transverse force values were − 2.17 ± 1.06 N, 2.98 ± 1.14 N, and 1.04 ± 0.60 N, and the vertical force values were 1.52 ± 0.81 N, − 0.09 ± 1.06 N, and − 0.025 ± 1.60 N, respectively; For group B, the sagittal force of the second premolar, first molar and second molar force values were -2.46 ± 0.50, -2.06 ± 0.82 N and 12.39 ± 1.85 N, respectively. The transverse force values were -0.13 ± 0.69 N, -2.06 ± 0.82 N and 3.64 ± 0.96 N, respectively.The vertical force values were 0 ± 0.90 N, 1.30 ± 1.20 N and -1.51 ± 1.75 N, respectively. For group C, the sagittal force of the second premolar, first molar and second molar force values were − 1.62 ± 0.98 N, 11.28 ± 1.32 N, and − 6.73 ± 1.99 N, respectively. The transverse force values were − 1.85 ± 0.74 N, 4.83 ± 0.84 N, and − 2.85 + 0.57 N, respectively. The vertical force values were 1.46 ± 0.81 N, − 3.3 ± 1.29 N, and 1.32 ± 1.06 N, respectively. In the three groups, the first premolars to the canines was got slight force in the three-dimensional direction and almost negligible in clinical practice.
Conclusion: (1) Molars are subjected to different orthodontic forces in the sagittal direction during different distalization designs, and the orthodontic forces are greatest when the second molar is distally moved alone; and the orthodontic forces are least when the first and second molar are distally moved at the same time; When the second molars move far away in a right place, the anchorage of the second molars needs to be strengthened because a large reaction force from first molar. (2) The buccal displacement of adjacent anchorage teeth should be designed to resist the transverse reaction forces produced by during molar distalization; (3)The retention accessories need to be designed on adjacent anchorage teeth during molar distalization to resist tooth extrusion caused by the reaction force of molars distalization.