| With the continuous development and progress of the society,the living standard of people has been improved significantly,and accordingly,the pursuit of beauty has become more and more intensely.Protrusive deformity is a common kind of malocclusion deformity,which has great influence on facial aesthetics.In the past,for the mild to moderate protrusive cases,the lateral appearance was improved mainly by extracting first premolar and using fixed orthodontic techniques to maximize retraction of the anterior teeth.In recent years,with the rapid development of clear aligner and the according technology,more and more orthodontic patients are interested in clear aligner.However,the clear aligner cannot control the three-dimensional movement of teeth accurately,especially for the cases with facial protrusion requiring tooth extraction and retraction of the anterior teeth,the control of anterior teeth torque and posterior teeth anchorage is always difficult.In the process of clinical practice,many clinicians have achieved good clinical results by using the combination of micro-implant anchorage and clear aligner.However,in the design process of the orthodontic project,the designer and producer of clear aligner will not take into consideration whether to combine the micro-implant nor not,and the current clinical application is mostly based on the experience of clinicians,without the support of scientific biomechanical evidence.Due to the precise modeling function and powerful data analysis function,three-dimensional finite element analysis has become one of the most widely used methods in biomech anics research area.At present,the research on three-dimensional finite element analysis of clear aligner mainly focuses on the periodontal stress,bodily movement of the canine,and there is no research on the combined use of clear aligner and miniscrew for en-masse retraction of maxillary anterior teeth.In this study,a three-dimensional finite element model of combined use of clear aligner and miniscrew for en-masse retraction of maxillary anterior teeth was established,and the initial displacement and local periodontal stress distribution of the teeth under different loading conditions were analyzed by relevant software,so as to provide some references for the clinical rational use of this technology.This study is divided into the following four parts:Experiment One: The construction of three-dimensional finite element modelObjective: To construct the three-dimensional finite element model for combined use of clear aligner and miniscrew for retraction of maxillary anterior teeth,which lays foundation and provides a mechanical platform for the subsequent experiments.Methods: Choose a healthy adult woman who has extracted maxillary first premolars for retraction of anterior teeth as a volunteer,she should undergo CBCT scanning after leveling and aligning,then the date was saved as DICOM.Through reverse engineering method,to construct and polish the three-dimensional model which contains maxillary,dentition,cortical bone,cancellous bone,and periodontal membrane by using Mimics and Geomagic Studio software.To construct the three-dimensional model of miniscrew,button and Power arm by using UG NX software.The three-dimensional model of attachments are the standard models of clear aligner Limited.To construct the three-dimensional models of clear aligner by using Atreat Manufacture software.Then,to assemble the models for necessary condition and define in Ansys software.Result:A three-dimensional finite element model of combined use of clear aligner and miniscrew for retraction of maxillary anterior teeth was constructed with high fidelity,which adopts triangular shell element and pentahedral solid element,and consist of 468725 elements and 234856 nodes.Conclusion: It is feasible to establish three-dimensional finite element model of combined use of clear aligner and miniscrew for en-masse retraction of maxillary anterior teeth via CBCT scanning,Mimics,Geomagic Studio,UG NX,Atreat Manufacture and Ansys software.Experiment Two: Study of the biomechanical effect on different manners and force values for en-masse retraction of anterior teethObjective: Study of the Biomechanical effect on different manners and force values for en-masse retraction of anterior teeth,provide reference for making clinical project.Methods: Miniscrew was implanted between the first molar and the second premolar,and elastic traction was performed with the miniscrew on the neck of canine or on the clear aligner at the canine with the force values of 0g,100 g,150g and 200 g,respectively.To analysis the initial displacement and the vonmises stress of the following seven loading conditions,the force was loaded by(1)clear aligner(2)clear aligner+100g force of retraction at clear aligner(3)clear aligner+150g force of retraction at clear aligner(4)clear aligner+200g force of retraction at clear aligner(5)clear aligner+100g force of retraction at canine(6)clear aligner+150g force of retraction at canine(7)clear aligner+200g force of retraction at canine.Result:In sagittal direction,the displacement difference of crown and root of the maxillary central incisor is 1.12E-02 mm,1.23E-02 mm,1.29E-02 mm,1.31E-02 mm,7.82E-03 mm,9.62E-03 mm,1.01E-02 mm,respectively;the displacement of the first molar is-2.49E-02 mm,-2.21E-02 mm,-2.09E-02 mm,-1.93E-02 mm,-2.09E-02 mm,-2.00E-02 mm,-1.91E-02 mm,respectively.The maximal vonmises stress of the central incisor,canine and first molar is 6.55E-03 MPa,6.60E-03 MPa,6.60E-03 MPa,6.73E-03 MPa,6.89E-03 MPa,7.05E-03 MPa,7.18E-03MPa;3.30E-02 MPa,3.30E-02 MPa,3.34E-02 MPa,3.39E-02 MPa,3.47E-02 MPa,3.53E-02 MPa,3.61E-02MPa;5.62E-03 MPa,4.94E-03 MPa,4.59E-03 MPa,4.26E-03 MPa,4.45E-03 MPa,4.24E-03MPa、4.13E-03 MPa,respectively.Conclusion: 1.It is more advantage to give traction on canine than clear aligner for controlling the torque of maxillary incisor,especially in the lower loading condition.2.It is more advantage to give traction on canine than clear aligner for saving the anchorage of maxillary posterior teeth,especially in the higher loading condition.3.the condition(5)is more advantage to control torque of maxillary anterior teeth and save anchorage of maxillary posterior teeth at the same time.4.In the two traction manners,the maximal vonmises stress of anterior teeth will increase and the stress of posterior teeth will decrease,in the same time the canine bears the most maximal vonmises stress in all conditions.Experiment Three: Study of the biomechanical effect on en-masse or two-step retraction of anterior teethObjective: Study of the Biomechanical effect on different methods for retraction of anterior teeth,one is en-masse retraction and the other is to move the canine firstly,provide reference for clinical practice.Methods: Attachments were attached to the canine,second premolar,first molar and second molar.To analysis the initial displacement of the following six loading conditions,(1)clear aligner moving the canine(2)clear aligner+150g force of retraction at appliance moving the canine(3)clear aligner+150g force of retraction at canine moving the canine(4)clear aligner retracting the anterior teeth(5)clear aligner+150g force of at appliance retracting the anterior teeth(6)clear aligner+150g force at canine retracting the anterior teeth.Result:In sagittal direction,when moving the canine distally,the crown displacement of the maxillary central incisor is-5.08E-03 mm,-9.67E-04 mm,-1.68E-03 mm,respectively,the crown displacement of the maxillary first molar is-1.53E-02 mm,-1.40E-02 mm,-1.51E-02 mm,respectively.When retracting the anterior teeth,the crown and root displacement difference of maxillary central incisor is 1.44E-01 mm,1.35E-01 mm,1.29E-01 mm,respectively,the crown displacement of the maxillary first molar is-2.93E-02 mm,-2.61E-02 mm,-2.33E-02 mm,respectively.Conclusion: 1.Combined the mini-screw in both methods can save the posterior teeth anchorage.2.When moving the canine firstly,the incisors will provide anchorage and retracting on the appliance has more advantage for saving anchorage.3.When retracting the anterior teeth,retracting on canine has more advantage for saving the anchorage of posterior teeth and controlling torque of anterior teeth.Experiment four: Study of the Biomechanical effect on Power arm for en-masse retraction of anterior teethObjective: Study of the biomechanical effect on Power arm for en-masse retraction of anterior teeth,provide reference for clinical application.Methods: Power arm,which is 6mm high,was attached to the canine or appliance.To analysis the initial displacement of the following three loading conditions,the force was loaded by(1)clear aligner+150g force of retraction at canine(2)clear aligner+150g force of retraction at Power arm at appliance(3)clear aligner+150g force of retraction at Power arm at canineResult:In sagittal direction,the crown and root displacement difference of maxillary central incisor is 1.29E-01 mm,1.29E-01 mm,1.33E-01 mm,respectively,the crown displacement of the maxillary first molar is-2.33E-02 mm,-2.35E-02 mm,-2.68E-02 mm,respectively.Conclusion: It can not be testified that retracting on the 6mm high Power arm at canine or appliance with 5mm high miniscrew has more advantage than on the canine directly for controlling the torque of incisors and saving anchorage of posterior teeth. |
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