Efficiency Of Maxillary Molars Distalization With Clear Aligners Assessed By Cone-beam Computed Tomography Superimposition

ObjectiveThe aim of this study is to evaluate the efficiency of maxillary molar distalization with clear aligners(CA)and three-dimensional changes of angle and displacements of maxillary molar,using cone-beam computed tomography(CBCT)superimposition pre and post treatment and root visualization of the molar at the Clincheck predicted position,to provide a theoretical basis for correction design in clinical treatment.Subjects and methods34 patients(5 males,29 females,25.97±7.23 years old,57.62±10.80 weeks treatment)were selected from the Department of Orthodontics,Hospital of Stomatology of Shandong University,and a total of 68 maxillary second molars and 67 maxillary first molars were included.All patients’ CBCT data were collected before and after the treatment,they were imported into Mimics 21.0 software for reconstructing,and segmentation of the craniomaxillary complex,maxillary stable structure and maxillary molars were completed.The posttreatment maxillary molars(M1)were registered to pretreatment maxillary molars(M0)with maxillary stable structure.The initial and target position dental crown models were obtained in Clincheck and defined as M0’ and M2’.M0’ and M2’ were imported into the pre-treatment CBCT data and global registration was carried out with M0 and M0’,M2’moved simultaneously.Then root visualization of M2’ was carried out by superimposing the copied separate molar of M0 with M2’,then the target position crown-root model was defined as M2.In this way,the molar’s crown-root models of the initial position(M0),the actual position(M1)and the target position(M2)were placed in the same measurement system.The three-dimensional planes and coordinate system were established to measure the linear and angular positions of M0,M1 and M2,and the movement and efficiency of maxillary molar distalization were analyzed.SPSS 26.0 software(IBM)was used to analyze the data.Depending on whether or not in line with a normal distribution,paired t-test or Wilcoxon signed rank test was used for the actual and predicted data of the same tooth,independent sample t-test or Mann-Whitney U test was used between crowns and roots and different teeth.P<0.05 was statistically significant.Results1.The efficiency and sagittal displacement of maxillary molar:The crown predicted distalization amount of maxillary second molar was(3.24±0.81)mm,the actual was(2.11±0.58)mm,the efficiency was(65.46±10.45)%,the root predicted distalization amount was(3.30±0.98)mm,the actual distalization amount was(2.088±0.72)mm,the efficiency was(63.38±12.34)%;the crown predicted distalization amount of maxillary first molar was(2.85±0.88)mm,the actual distalization amount was(1.77±0.55)mm,the efficiency was(61.25±12.76)%,the root predicted distalization amount was(2.38±0.85)mm,the actual distalization amount was(1.33±0.54)mm,the efficiency was(55.96±14.19)%.The actual distalization movement of crown and root was lower than the predicted.The efficiency of maxillary second molar root distalization was greater than that of first molar.The differences were statistically significant(P<0.01).2.Buccolingual displacement of maxillary molar:The predicted buccal displacement of the crown of the maxillary second molar was 0.26(-0.25,0.78)mm,the actual displacement was 0.55(-0.07,0.92)mm,the designed root movement was 0.49(-0.13,1.56)mm,and the actual displacement was-0.09(-0.44,0.62)mm;the predicted crown buccal displacement of maxillary first molar was(1.07±0.73)mm,the actual displacement was(1.25±0.65)mm,the predicted root displacement was(0.73±1.00)mm,and the actual displacement was(-0.06±0.58)mm.The actual crown buccal displacements of maxillary molars were greater than the predicted,and the actual root movement was less than the predicted movement.The buccal-lingual inclination of maxillary molars increased compared with the predicted.The above differences were statistically significant(P<0.01).3.Vertical displacement of maxillary molar:The predicted intrusion of maxillary second molar was 0.23(-0.32,0.62)mm,the actual intrusion was 0.56(-0.08,0.89)mm;the predicted intrusion of maxillary first molar was 0.25(-0.29,0.59)mm,the actual intrusion was 0.33(0.22,0.64)mm.The actual intrusion of maxillary molar was greater than the predicted.The differences were statistically significant(P<0.01).4.Three-dimensional angle changes of maxillary molar:The predicted buccal inclination of maxillary second molar increased by-1.11(-0.51,0.84)°,the actual increased by 1.84(-0.56,4.51)°;the predicted distal inclination of the maxillary first molar increased by 2.62(0.27,6.44)°,the actual increased by 3.25(1.44,4.79)°,the predicted buccal inclination increased by 0.27(-2.56,2.89)°,the actual increased by 1.63(-0.63,3.84)°,the distal inclination of the maxillary first molar increased compared with the predicted,and the buccal inclination of the maxillary molar increased compared with the predicted,the differences were statistically significant(P<0.01).The distal inclination of the maxillary second molar and the degree of maxillary molar rotation after distalization increased slightly compared with the predicted,but the differences were not statistically significant(P>0.05).Conclusions1.The efficiency of maxillary molar distalization with clear aligners needs improvement.Orthodontists should consider setting appropriate over-correction in treatment design based on efficiency of maxillary molar distalization.2.After maxillary molar distalization with clear aligners,the maxillary second molar tended to bodily movement,while the maxillary first molar tended to tipping movement in sagittal direction.There were extra intrusion and buccal crown tip in maxillary molar.The movement characteristics of maxillary molar distalization should be understood,thus facilitating a reasonable treatment design and the mesial-distal root tip control of maxillary first molar and buccal-lingual torque control of maxillary molars should be considered.