The Stress Analysis Of The Anchorage Anterior Teeth Of The Molar Distalization With Clear Aligner

Objective: Use the three-dimensional finite element method to simulate molar distalization with clear aligner,observe the biomechanical effects of the anterior teeth during the process of molar distalization by applying different anchorage values,and discuss the needs of anchorage for molar distalization.The results provide theoretical basis and scientific guidance for clinicians’ plan design and choice of anchorage.Methods: Obtain a CBCT scan image of an individual normal occlusion patient.In Mimics,Geomagic Wrap,and Unigraphics NX software,the difference in gray thresholds of different tissues is used to establish a three-dimensional model of the maxillary dentition and alveolar bone.The tooth root is separated by running Boolean operation,and after extending 0.25 mm outward along the normal phase,a threedimensional model of the periodontal ligament is obtained.After adjusting the dentition according to the corresponding distance of the experimental group,the crown is separated by running the Boolean operation,and the three-dimensional model of the clear aligner is obtained after the crown is expanded 0.75 mm outwards along the normal phase.According to the different forms of the molar distalization,it is divided into groupⅠ(second molar moved only)and groupⅡ(first molar and second molar moved at the same time).The groupⅠis divided into four groups according to the value of the anchorage applied to the canine:ⅠA(0g),ⅠB(100g),ⅠC(150g),andⅠD(200g).The groupⅡis divided into four groups according to the value of the anchorage applied to the canine: Ⅱ A(0g),Ⅱ B(100g),Ⅱ C(200g),Ⅱ D(300g).After the corresponding load is given,the stress of the periodontal ligament and the displacement trend of the anterior tooth are compared and analyzed in the finite element analysis software.Results: The forms of molar distalization and the loading force value designed by each group did not produce stress that is not conducive to the health of the periodontal tissue.When using clear aligner to move molars far away,the anchorage anterior teeth will show the labial and intrusion movement,as well as the tendency of mesial oblique movement.When using clear aligner to move molars far away,groupⅡmoving two molars at the same time will cause greater periodontal stress than groupⅠmoving a molar alone.And the maximum stress is mainly concentrated in the labial cervical and lingual apical areas.GroupⅡmoving two molars at the same time will show more labial displacement than groupⅠmoving a molar alone.When groupⅠusing clear aligner to move a molar far away,applying 100 g anchorage to the canine can effectively reduce the anterior teeth labial displacement,while the periodontal ligament stress is the smallest.When groupⅡusing clear aligner to move two molars far away,applying 300 g anchorage to the canine can effectively reduce the anterior teeth labial displacement,while the periodontal ligament stress is the smallest.Conclusion: 1.When using clear aligner to move molars far away,the forms of the molar distalization should be chosen according to the anchorage of the anterior teeth.If the anterior teeth need the labial movement,the "V" mode should be chosen.If the anterior teeth don’t need the labial movement,the "A" mode should be chosen.2.Regardless of moving one molar far away only or moving two molars far away at the same time,applying a proper distal force to the canine can offset the anterior teeth labial force generated by the molar distalization.Therefore,increase an appropriate anchorage force to the canine,the "V" mode that move two molars far away at the same time can be chosen.3.It is important to choose the size of the anchorage force.If the anchorage force is too small,the anterior teeth will show labial movement.If the force is too large,the stress of the periodontal ligament of the anterior teeth will be too concentrated.Choosing different forms of molar distalization,the anchorage of the corresponding force value can be increased.When the "A" mode to push one molar away was chosen,100 g anchorage force can be applied.When the "V" mode to push two molars away at the same time was chosen,300 g anchorage force can be applied.