Three-dimensional Finite Element Analysis Of Retraction Maxillary Anterior Teeth In Clear Aligners

Purposes: To establish a three-dimensional finite element model of the maxillary bone,maxillary dentition,periodontal membrane and clear aligner,for simulating the use of clear aligner in the extraction of maxillary anterior teeth cases.The simulation analyzes the difference between two treatment designs in terms of stress distribution and displacement trend of teeth,stress distribution in periodontal membrane and changes in the center of rotation during the stepwise internal extraction of anterior teeth,investigates the biomechanical effects of the maxillary anterior teeth under different forces to provide clinical guidance for designing invisible orthodontic appliances in tooth extraction cases.Methods and Materials: Cone Beam Computerized Tomography(CBCT)was used to scan the heads of the volunteer,who was selected according to the set condition.The data obtained from the scanning were imported into Mimics 17.0 in Dicom format for generating an initial complete solid model of their maxilla and maxillary dentition,before using finite element analysis software such as Geomagic Studio 2013,Solidworks,Anays Workbench 16.0 to design four sets of finite element models of bracketless orthodontic as follows: Model A: with 0.2 mm adduction in the maxillary anterior teeth;Model B: with0.2 mm adduction and 0.10 mm intrusion in the maxillary anterior teeth;Model C: with 0.2mm adduction and 0.10 mm intrusion in the maxillary anterior teeth,and 10 degrees of lingual root torque;Model D: with 0.2 mm adduction and 0.25 mm intrusion in the maxillary anterior teeth,and 10 degrees of lingual root torque.The global coordinate system was set at the intersection of the two maxillary mesial incisors in the proximal-central plane,with the X-axis as the horizontal direction and its right direction as the positive direction,the Y-axis as the sagittal direction and its back direction as the backward direction,and the Z-axis as the vertical direction and its downward direction as the positive direction.The four model sets were meshed,given material properties and constraints to simulate stress distribution and displacement trend of teeth,and stress distribution in periodontal membrane during the stepwise internal extraction of anterior teeth.Results: 1.By calculating the centers of rotation of the four model sets,it can be found that the in terms of the center of rotation of the teeth,model D was closest to the root tip,while model C was farthest from it.2.In terms of initial displacement of maxillary anterior teeth in the sagittal direction,all models showed a displacement trend of lingual movement.The crown displacement of model A and model B were much larger than the root displacement of them,but the difference between two displacements of model B was smaller than that of model A;the two displacements of model C are same in direction and almost similar with each other;in model D,compare with the crown displacement,its root displacement was a basically negligible relative amount,(the tooth movement direction)under this situation,the teeth showed a controllable tilt movement trend.3.In the observation of the initial displacement of the maxillary anterior teeth in the vertical direction,it can be found that the anterior teeth in all four models showed crown elongation and root depression(not yet visible);the elongation of the central incisors was greater than that of the lateral incisors.In model A,the maximum displacement happened at the crown marker point and root tip;In model D,the maximum displacement happened at the crown marker point and root tip.4.All the anchoraged tooth showed a displacement tendency toward mesial inclination.In the nephogram of displacement,the anchoraged teeth in all four groups of models have a tendency towards the proximal center,and their size decreases from the second premolar in descending order.5.In terms of the stress distribution in the periodontium of maxillary anterior teeth,the stress concentration areas in all models were mainly located in the apical and cervical regions.By comparison with other three models,model A had more obvious stress concentration and higher maximum stress in the cervical and apical regions.Conclusion: Different stress distribution methods have different impacts on the tooth movement.During inward retraction of maxillary anterior teeth,the anterior tooth torque(labiolingual tilt)can be precisely controlled by adjusting the amount of pressure in the vertical direction as well as the control of torque.This suggests that,in the clinical practice using clear aligner for internalizing maxillary anterior teeth,the tendency of tilted tooth movement and elongation can be moderated by different methods such as adding rootlingual torque or vertical control.