Proper Design Of Torque For Posterior Teeth In Arch Expansion With Clear Aligners:A Typodont Research

Objective:The aim of this research is to investigate the type of teeth movement during maxillary arch expansion with clear aligners using Heat Induced Typodont System(HITS).In this research,different amount of expansion and root buccal torque were set for maxillary posterior teeth in order to find the proper design for posterior teeth during arch expansion.Methods:This research used clear aligners from Anglealign,the HITS and a standard model.According to the preset amount of expansion and root buccal torque the experiment was set for eight groups:A1(1 mm,0°),A2(1 mm,10°),B1(1.5 mm,0°),B2(1.5 mm,10°),C1(2 mm,0°),C2(2 mm,5°),C3(2 mm,10°),C4(2 mm,13°).Polyvinyl siloxane impressions(PVS)were made before(TO)and after(T1)expansion to make stone casts of the models.After scanning the casts,digital models were exported as.stl and imported to Geomagic studio 2013 to measure the width of cusp tip of canines,buccal and lingual cusp tip of premolars,mesial buccal and lingual cusp tip of molars,midpoints of palatal gingival margin of premolars and molars,then calculated the width difference between TO and T1.Besides,preset amount of expansion was also measured in iOrtho to calculate the efficiency of expansion.Through the measurement of height difference of buccal and lingual cusps of molars and premolars in TO and T1,the buccal or lingual inclination of posterior teeth after expansion was estimated.The statistical analysis of data was performed using SPSS 19.0.Results:1.Torque Variation of posterior teethWhen preset 0° torque,group A1,B1 and C1 were all buccally inclined in T1,and C1>B1>A1(P<0.05).When preset 10° root buccal torque,for the first premolars,group A2 and B2 had more buccally controlled root movement than group C3(P<0.05).Preset amount of expansion has a negative correlation with buccolingual inclination of posterior teeth.The more the preset amount of expansion is,the more the posteriors will buccally incline.When 2 mm expansion per quadrant was preset,the height difference of buccal and lingual cusps of posterior teeth in C1 were all negative,while C4 were all positive(P<0.05).After expansion,posterior teeth in C1 and C2 buccally inclined,C3 had bodily movement,and C4 had buccally controlled root movement.Preset buccal root torque has a positive correlation with buccolingual inclination of posterior teeth.The more the preset buccal root torque is,the more the posteriors will have buccally controlled root movement.2.The change of canine widthIn Stage T1,the width of canine became narrower in every group.Width difference of canine of Group A1,B1,C1 were negative correlated(P<0.05),the more the preset posterior expansion is,the narrower the width of canine will be.3.Expansion efficiencyIn each group,the teeth position in the arch has a negative correlation with expansion efficiency(P<0.05),the more distal the teeth in the arch,the lower the expansion efficiency will be.For group A1,B1,C1,there was a low positive correlation among their expansion efficiency of the same tooth.And there was not a statistical significant correlation among group A2,B2,C3(P>0.05).Thus,it is hard to conclude that there is a significant correlation between preset amounts of expansion with expansion efficiency.Conclusion:1.For the maxillary arch,the teeth will show a lower expansion efficiency with a more distal position in the arch.2.The more the designed expansion for the posterior teeth is,the narrower the width of the canines will be.Thus,orthodontists should consider maintaining the width of canines when making the treatment plan.3.To avoid fenestration,dehiscence and gingival recession after expansion,individual amount of expansion and torque should be decided according to the buccal bone thickness of different teeth.4.When expanding maxillary arch with the clear aligners,the posterior teeth will buccally incline during expansion,and the more the designed expansion is,the more buccal inclination the posterior teeth will show.Once the designed buccal root torque is optimal,teeth will show bodily movement or even buccally controlled root movement.This research shows that it is proper to select a 10° to 13° root buccal torque for the posterior teeth if they are in upright positions.Besides,when presetting a relatively large designed expansion,a relatively large designed root buccal torque should be selected to prevent the buccal inclination after expansion.