Three-dimensional Finite Element Analysis Of Maxillary Molar Mesial Movement By Clear Aligners

Objective:The three-dimensional finite element model of maxillary molar mesial movement by clear aligners was established.In this study,clear aligners were designed with different distal tilt angles or long-arm traction hooks to pull with varying values of force during a conventional stride of 0.25 mm in molar mesial movement,and their effects on the stress distribution and displacement trend of target molars were analyzed.The results of this paper can provide a valuable reference for orthodontists to apply clear aligners in the clinical application of molar mesial movement.Method:Select an adult volunteer with an individual normal bite,in good health,no genetic or systemic diseases history,and no obvious abnormalities in dental examination and periodontal condition.The volunteer took CBCT scans,applied Mimics 20.0 for raw data extraction,and then used Geomagic Studio 2015 and Unigraphics NX1911 to build a 3D model of maxillary dentition,maxilla,periodontal membrane,and orthodontic device.Virtually remove 16 tooth bodies and periodontal membrane structures,and fill the gaps with maxillary tissue structures to form a three-dimensional model of using clear aligners to move molars over long distances.Ansys Workbench 2019 was used for 3D finite element model analysis,simulated clinical realities of applied loads,and calculated the deformation of the structure under load through mechanical analysis to obtain the stress distribution of the tissue structure and the initial displacement trend of the teeth.The step distance of this experiment was designed to be 0.25 mm,and seven configurations were designed.Configuration A: The second molar was upright in the alveolar bone,the buccal side was designed with an attachment,and the length of the clear aligner was shortened by0.25mm;Configuration B: Based on configuration A,the initial position of the second molar was changed to a distal inclination of 2°;Configuration C: Based on configuration A,the initial position of the second molar was changed to a distal inclination of 5°;Configuration C: Based on configuration A,the initial position of the second molar was changed to a distal inclination of 8°;Configuration E: The second molar was upright in the alveolar bone,the lingual side was designed with an attachment,and the length of the clear aligner was shortened by 0.25 mm.A long-arm traction hook was designed on the buccal surface of the teeth,and the end of the metal arm was pulled by a force of 150 g downward;Configuration F: Based on configuration E,the force value was set to 250g;Configuration G: Based on configuration E,the force value was set to 300 g.The root,periodontal membrane stress distribution,and initial displacement trend of the target molar under different configurations were observed and compared.Result:A three-dimensional finite element analysis model of the maxillary first molar missing and using clear aligners to complete the maxillary second molar mesial movement was successfully established,including maxillary dentition,maxilla,periodontal membrane,invisible appliance,attachment,and long-arm traction hook.Among the seven configurations,the maximum equivalent stress of the root and periodontal membrane appeared in the second molar,which was concentrated in the mesial side 1/3 of the distal buccal root and the neck position of the distal buccal side of the teeth.In the sagittal direction,the second molar showed a mesial inclination movement trend in all configurations,and the second premolar showed a tendency of displacement of distal inclination.The distal inclination resistance had no obvious effect on the molar displacement trend,but the horizontal bonding traction hook design could effectively reduce the mesial inclination displacement trend of the molar,and the effect was positively correlated with the traction value.In the horizontal direction,distal tilt resistance had no effect on the tooth movement tendency,there was only a slight shift of horizontal direction to the initial displacement of the teeth.The horizontal bonding traction hook design increases the trend of palate tilt of the second molar and the second premolar to the initial displacement,and the trend becomes more pronounced with the increase of the force.In the vertical direction,the teeth with distal tilt show a clear tendency to elongate,which is explained by the mesial erection of the second molar.The horizontal bonding traction hook of the second molar’s buccal surface elongates the tooth’s buccal side,and the tooth has a tendency with reduced torque.The rest of the teeth in the quadrant also show the same type of displacement,with the palate tip depressed and the buccal tip elongated.Conclusion:1.During the use of clear aligners to complete the mesial movement of a maxillary molar,the maximum principal stress of the root is concentrated on the mesial surface of the distal buccal root of the molar.When the traction force is within a suitable range,it can make the teeth evenly stressed and reduce the maximum stress.When the force value is high,the stress is concentrated in the buccal neck of the teeth,the risk of root absorption should be considered.2.The research data show that the orthodontic design of distal tilting the teeth before mesial movement only shows a slight improvement effect of tooth control ability in the initial position,but it may have important application value in the complete clinical correction process.3.The design of horizontal bonding of long-arm traction hook can effectively control the inclination of the molars shaft,which has the advantages of no implantation nails,can be bonded later and adjusted at any time,but will greatly increase the anchor consumption.4.The traction design will produce additional tooth movement,which is applied to maxillary second molars in this experiment,and there is a tendency of displacements such as the molar and premolars buccal elongation,palate tilt,etc.There are risks such as bone fenestration,bone cracking,and narrowing of the width of the dental arch,and the doctor should consider the impact of this process.5.In order to ensure that the force is applied in a safe range and to the movement effect of the teeth can be achieved,rigorous follow-up examination is essential.When long-distance translational molars are required,consideration should be given to reducing the step distance or designing step-by-step movement.