Finite Element Analysis Of Complete Dentition During Distal Movement Of Maxillary Second Molar By Clear Aligners Under Different Working Conditions

BackgroundAngle class II malocclusion is a common type of malocclusion in clinic.Its commonly used correction methods can be divided into two categories:tooth extraction or non-extraction.Distant removal of maxillary molars is a commonly used strategy in non-extraction correction.With the development of non-bracket invisible correction(Clear aligners)system,it has been able to treat almost all malocclusion and has a good clinical effect in distal molar movement.Its unique wrapping design and facial force application determine its unique mechanical properties.At present,there is a lack of perfect biomechanical analysis of non-bracket invisible correction in distal movement of molars,especially in the process of distal movement of molars,and there is no report to describe the influence of appliance,attachment and Anchorage on the mechanical system of whole dentition.Therefore,we studied the mechanical characteristics of bracket-free invisible correction in the process of distal maxillary second molars by three-dimensional finite element method.MethodsThe CBCT data of an adult male were selected,and the three-dimensional finite element model of maxillary dentition-bracket-free invisible orthodontic system was established.Four working conditions of distal movement of maxillary second molars were set up,and four indexes of maxillary tooth displacement,tooth displacement and rotation center,appliance deformation pressure and periodontal ligament equivalent stress were observed and analyzed.The first condition has no attachments and no anchorage reinforcement design;the second condition is the addition of rectangular attachments from the canine to the first molars without anchorage reinforcement design;the third condition is the same attachments supplemented by type II traction;the fourth condition is the use of micro-implant traction to strengthen anchorage design on the basis of the same attachments.Results(1)The displacement of each tooth in group 1 was the smallest,the displacement from the central incisor to the first molar was the largest in group 2,and the displacement of the second molar in group 3 and group 4 was slightly larger than that in group 2.(2)The periodontal ligament stress from the central incisor to the first molar was greater in the distal plane than in the mesial plane,the stress concentration area appeared on the root lip surface of the central incisor to the canine,and the periodontal ligament stress of the second molar was concentrated near the neck edge of the distal adjacent surface.The periodontal ligament stress group 1 was the smallest,the periodontal ligament stress group 2 from the central incisor to the first molar was the largest,and the periodontal ligament stress group 3 and group 4 of the second molar was slightly higher than that of group 2.(3)The rotation center of the second molar is located at about 2mm from the root bifurcation to the root,and the rotation center of the first molar is located at the junction of the root near the middle buccal root and the root tip 1.The rotation center of the anterior teeth and premolars is located at the junction of the root and the root tip,and the rotation center of the anterior teeth is closer to the root tip.The position of the rotation center in the attachment group was significantly closer to the root tip than that in the non-attachment group.(4)The maximum appliance equivalent stress was observed at the connection between the second molar and the first molar,the minimum equivalent stress appeared in the anterior tooth area,and the appliance equivalent stress decreased gradually with the increase of the distance between the second molar and the first molar.In the first group,the equivalent stress decreased rapidly with the increase of the distance from the orthodontic position,and the equivalent stress distribution of the appliance with attachment group was more uniform than that of the non-attachment group.Conclusion(1)The application of attachments can improve the efficiency of molar distal movement.Within the range of our setting,the stress is positively correlated with the tooth displacement.(2)In clinic,it is still necessary to pay attention to the inclination of molars and the loss of anterior Anchorage.Implant Anchorage has a depressing effect on anterior teeth and seems to be more suitable for patients with higher vertical requirements of anterior teeth.(3)Even if a small distal molar displacement is preset,a large instantaneous periodontal ligament stress is still observed.although the periodontal ligament stress attenuates with time,it is still necessary to consider the feelings of patients and avoid related complications in clinic.