The Three-dimensional Forces Created By Premolars Extraction Treatment In Different Tips Of Posteriors With En-masse Retraction By Clear Aligners In Vitro

Objective To investigate the three-dimensional forces created by clear aligners on maxillary teeth during distal movement 0.25 mm of canine after 2 first premolars extraction treatment in the different tips of posteriors with en-masse retraction in vitro.Aligner forces were measured on a multi-axis force/torque transducer measurement system in real-time.Methods Four groups of maxillary models with the different tips of posteriors(T1-upright of posteriors,T2-Posteriors with distal inclination of 5°,T3-Posteriors with mesial inclination of 5°,T4-Posteriors with mesial inclination of10°)undergoing en-masse retraction treatment during distal movement 0.25 mm of canine using clear aligners were simulated on a multi-axis force/torque transducer measurement system.Horizontal rectangular attachments were bonded onto the first molars and the second molars,and vertical rectangular attachments were bonded onto the second premolars and canines with the size of 3mm×2mm×1mm.Four resin models were connected separately with the multi-axis force/torque transducers.Aligners were fabricated using thermoforming technology with thermoplastic materials.When each clear aligner was inserted,a computer connected to the transducer collected and recorded the buccolingual,mesiodistal and vertical forces of every tooth in real-time.Each group included 12 aligners.SPSS 26.0 statistical analysis software was used for data analysis.Results 1 、 Lingual and extrusive forces were observed on the incisors,and buccal forces were observed on the posteriors.In the mesiodistal direction,canines received distal forces,and mesial forces were observed on the posteriors.In the vertical direction,extrusive forces were observed on the incisors and the posteriors in 4 groups during en-masse retraction treatment with clear aligners,which is similar to the “bowing effect”.2、There are different forces of every tooth in three directions in the different tips of posteriors during en-masse retraction treatment with clear aligners.3、Incisors received smaller lingual and extrusive forces,canines received greater distal forces,smaller mesial forces were observed on the posteriors,and molars received greater buccal and extrusive forces,when posteriors incline distally.4、Incisors received smaller lingual and extrusive forces,canines received greater distal forces,greater mesial forces were observed on the posteriors,molars received greater buccal and extrusive forces,when posteriors incline mesially.5、The posterior inclined too much,with the posteriors with mesial inclination of 10°,the phenomenon of "dislocation" will occur,which did not facilitate the application of orthodontic forces with aligners.Conclusions 1、Posteriors with distal inclination contributed to the torque of the anterior and anchorage of posteriors.In group T2 posterior teeth experienced larger buccal forces and extrusive forces than that in group T1,which did design lingual root torque and intrusion overcorrection of molars.2 、 When the posteriors crown were inclined mesially,the anterior teeth were more prone to lingual tip and caused overbite,the anterior lingual root torque should be designed,the more inclined mesially the posterior teeth were,the greater the amount of overcorrection required.The posterior teeth were more prone to appear anchorage loss,to protect the anchorage of posterior teeth,anchorage preparation before the anteriors retraction.Also,lingual root torque and intrusion overcorrection of molars should be designed.3、The posterior inclined mesially too much,it was necessary to upright,which contributed to the torque of the anterior and anchorage of posteriors.And the retention should be enhanced.