Finite Element Nonlinear Analysis Of Mandibular Molar Uprighting By Clear Aligner With Attachment

Objective:Unlike the traditional fixed orthodontic technique,the orthodontic force of clear aligner orthodontic technique is not only from the deformation force of the hot film,but also from the control of the tooth movement,which requires the help of attachments to realize the 3D movement.A mesially tipped mandibular molar is a frequent situation among orthodontic patients,which usually occurs after premature loss of adjacent teeth leading to the inclination of the molars.In this study,a three-dimensional model of mandibular dentition,periodontal ligament,alveolar bone,attachments and clear aligner was established to simulate molars uprighting with traditional rectangular attachments and optimized root-control attachments,to study the effects of traditional rectangular attachments and optimized root-control attachments on the stress distribution of tension-compression pattern at the periodontal ligament,the initial tooth displacement and the displacement quantity of the tipped molar,this study provides clinical guidance and theory for the selection of attachments for molars uprighting in orthodontic treatment.Method:Part I:The data of mandibular jaw bone and mesially tipped mandibular molars were obtained by random selection of CBCT scan of a patients with mesially tipped molars and stored in DICOM format.Three-dimensional finite element models of mandibular dentition,attachments,periodontal ligament,alveolar bone,and clear aligner were reconstructed by using Mimics,Geomagic,Solidworks and Ansys.The mesial tipped mandibular molar(right mandibular second molar)was randomly selected,then the traditional rectangular attachments:model 1 with a vertical rectangle attachment and model 2 with a horizontal rectangle were established.The mesially tipped mandibular molar(right mandibular first molar)was randomly selected and the optimized double rectangular attachments model 3 was established.The three-dimensional finite element nonlinear analysis of Models 1 and 2 was carried out by using AN SYS software to study the stress distribution of tension-compression pattern at the periodontal ligament,the initial tooth displacement and the displacement quantity of the tipped molar with the traditional rectangular attachment under the displacement-driven pattern of the clear appliance.Part II:MODEL 3 is carried out by ANSYS software.According to the different force modes,the model is divided into working condition 1-2 and working condition 1-3.Working conditions 1-2 load moment to the optimized double rectangular attachments;working conditions 1-3 load moment to attachments and mesial force.In this experiment,the stress distribution of tension-compression pattern at the periodontal ligament,the initial tooth displacement and the displacement quantity of the tipped molar with the buccal double rectangular optimized attachment under the force-driven pattern of clear aligners.The biomechanics mechanism of clear aligners for molar uprighting was analyzed.Results:1.When the clear aligner is used for molar uprighting,,we can see the tension-compression stress at the mesial root surface of the periodontal ligament:the compression stress distribution in meso-apical area and the tension stress distribution in meso-cervical areas.Displacement mode of all models:the crown displacement to the distal tipping and the apical displacement to the mesial tipping can be observed.The displacement trends of teeth are tipping movements.The distribution range of compression areas at the apex of model 3 is significantly wider and the center of rotation is closer to the tooth neck.2.Working condition 1-3 of the buccal double rectangular optimized attachment model 3 has a larger maximum apical displacement,and the minimum displacement point of working condition 1-3 is located at the level of the tooth neck,and the molar is uprighting,and the movement of the teeth is root-control movement approximately.Conclusions:1.The horizontal and vertical rectangular attachments of traditional attachments are driven by the displacement mode of the clear aligner system.The movement tendency of mesially tipped molars with the traditional rectangular attachments was uncontrolled tipping movement,and the rotation center is located under 1/3 of the root,and it can not achieve root-control movement mesially.2.When the clear aligner with the optimized double rectangular attachments is used for molar uprighting,the optimized double rectangular attachments increased the movement efficiency of the molar movement.3.When the clear aligner with the optimized double rectangular attachments is used for molar uprighting,a moment on the attachment while increasing the mesial driving force can generate a force system can approximately achieve the root-control movement of the molar.