| Objective The six-axis force/torque transducer measurement system was used to simulate and measure the orthodontic force of the anterior teeth in the three-dimensional directions which had been affected by differential torque of maxillary posterior teeth of clear aligner treatment with the maxillary first premolar extraction,comparing the no movement of the anterior teeth and the anterior teeth underwent 0.25-mm activation for en-mass retraction to in order to provide the theoretical basis and reference for the orthodontic design of the cases of the clear aligner.Methods Maxillary models were designed for differential torque of posterior teeth on the basis on the three-dimensional mechanical test model with first premolar extraction.The stereolithography models were made with the same size as the standard model by three-dimensional printing.According to the torque of the six keys to normal occlusion of Andrews,group A1 and B1 were the standard torque of Andrews.The posterior teeth were lingual-crown torque of 5° in groups A2 and B2,and the posterior teeth were buccal-crown torque of 5°and 10°in groups A3、B3,and A4、B4,respectively.In the light of the activation of the anterior teeth,the anterior teeth of A1-A4 groups were not designed which did not move,and the anterior teeth of B1-B4 groups were designed 0.25-mm activation for en-mass retraction of the anterior teeth.Every group of clear aligners was fabricated with 0.75-mm-thick thermoplastic material.There were 12 pairs of clear aligners in each group.The six-axis force/torque transducer measurement system was used to measure the force applied by the clear aligners on the anterior teeth in the buccolingual,mesiodistal and vertical directions.The average force of anterior teeth in the three directions were calculated and the data was statistically analyzed.Results In groups A1-A4,even if there was no designed activation,forces were generated in the buccolingual and mesiodistal directions(P<0.05).Buccolingual direction: In the four groups,although the force of the anterior teeth in the buccolingual direction was not statistically significant(P>0.05),the left and right maxillary incisors and canines were subjected to the lingual force.Mesiodistal direction: The forces of bilateral canines were statistically significant in the mesiodistal direction(P<0.05).In groups A1-A3,the canines were subjected to the mesial force,and in group A4,the canines were subjected to the distal force.Except the right lateral incisor was subjected to the distal force,the other teeth were subjected to the mesial force.Vertical direction: In the four groups,the force direction of the anterior teeth was opposite.The force of bilateral canines was statistically significant(P<0.05).The forces of anterior teeth when underwent 0.25-mm activation for en-mass retraction of the anterior teeth,buccolingual direction: in group A1,the four incisors were applied to the lingual force of about 1N.In groups B2-B4,the anterior teeth were applied to the labial force,and the force of the central incisors was greater than the force of the lateral incisors.Mesiodistal direction: In groups B1-B4,four incisors were applied to the mesial force,and the force of the lateral incisors was greater than that of the central incisors.In groups B1-B4,the canines were applied to the distal force.The forces of left and right canines in group B4 were the largest of the four groups,and there was a significant difference between groups B2 and B3(P<0.05).In the B1-B4 groups,most of the teeth in the four groups were not statistically significant in the vertical direction.(P>0.05).Conclusion 1.Differential torque of maxillary posterior teeth had a significant effect on the force of anterior teeth in the buccolingual direction and mesiodistal direction,but the force of the vertical direction had no significant change.2.The superficial area of the crown and the position of the teeth would affect the force of the teeth.3.When en-mass retraction of the anterior teeth of clear aligners,attention should be paid to the torque of the incisors and the tip of the canines.When the posterior teeth were standard torque,the activation of labial-crown overcorrection of the anterior teeth should be increased.When the posterior teeth were lingual-crown torque or buccal-crown torque,the activation of labial-crown overcorrection of the anterior teeth should be appropriately reduced.Increasing the mesial overcorrection design of canines to avoid the extrusion of anterior teeth and the deepening of overbite due to the distal inclination of canines when en-mass retraction of the anterior teeth. |
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