Clear Aligner Combined With Micro-implant Anchorage For En-mass Retraction Of Maxillary Anterior Teeth:A Three-dimensional Finite Element Analysis

Objective:The three-dimensional finite element model of clear aligner combined with micro-implant anchorage for en-mass retraction of maxillary anterior teeth was established to analyze the biomechanical effects of different micro-implant implantation height in posterior tooth area and different traction methods in canine area on the stress distribution,initial displacement trend and periodontal ligament stress distribution of teeth when the maxillary anterior teeth are retracted with clear aligner.To provide theoretical reference for the clinical application of micro-implant anchorage assisted clear aligner to retract maxillary anterior teeth.Method:Select an adult female volunteer with maxillary protrusion and extraction of the first premolars for orthodontic treatment.CBCT scans were performed with the informed consent of the patient,export the collected imaging data in DICOM format.A three-dimensional finite element model of clear aligner combined with micro-implant anchorage for en-mass retraction of maxillary anterior teeth was established by Mimics 20.0,Geomagic Studio 2015 and Unigraphics NX 1911.Five Configurations designed are analyzed by ANSYS Workbench 2019.Configuration A:The clear aligner is shortened by 0.25 mm in the distal part of the canine,to simulate the en-mass retraction of the anterior teeth with clear aligner;Configuration B: Based on configuration A,applying a traction force of 150 g between the micro-implant and clear aligner traction hook,the micro-implant is located at 4 mm above the alveolar crest between the first molar and the second premolar;Configuration C:Based on configuration A,applying a traction force of 150 g between the micro-implant and clear aligner traction hook,the micro-implant is located at 8 mm above the alveolar crest between the first molar and the second premolar;Configuration D: Based on configuration A,applying a traction force of 150 g between the micro-implant and Power arm,the micro-implant is located at 4 mm above the the alveolar crest between the first molar and the second premolar;Configuration E: Based on configuration A,applying a traction force of 150 g between the micro-implant and Power arm,the micro-implant is located at 8 mm above the the alveolar crest between the first molar and the second premolar.To analyze the biomechanical effects of different micro-implant implantation height and different traction methods on the stress distribution,initial displacement trend and periodontal ligament stress distribution of teeth when the maxillary anterior teeth are retracted with clear aligner.Result:A three-dimensional finite element model of clear aligner combined with micro-implant anchorage for en-mass retraction of maxillary anterior teeth was established,including maxillary bone,maxillary dentition,periodontal ligament,clear aligner,attchment,Power arm and micro-implant.In the five configurations,the roots of lateral incisor and canine are subject to large equivalent（von-Mises）stress,which is mainly concentrated in the labial cervix 1/ 3 and palatal cervix 1 / 3 of the lateral incisor’s root,the labial side of the canine’s root,the palatal middle 1 / 3 and palatal apical 1 / 3 of the canine’s root.The maximum equivalent（von-Mises）stress of periodontal ligament appeared on the canine,which is mainly concentrated in the distal cervix and apical region of the canine’s root.In the sagittal direction,if the traction methods are the same,the tendency of the incisors to move towards the palatal inclination and the tendency of the canine to move to the distal direction are smaller when the micro-implant is placed at a higher position than when the micro-implant is placed at a lower position.After the micro-implant applied traction to the canine area,the tendency of the posterior teeth to move obliquely to the mesial side decreased compared with that without traction,especially in configuration D and configuration E.When the implantation height of the micro-implant is the same,if the Power arm is used for traction,the tendency of lateral incisors to move obliquely to the palatal side increased,while the tendency of canines to move obliquely to the distal side and the posterior teeth to move obliquely to the mesial side decreased.However,the traction of Power arm increases the tendency of the posterior teeth’ crowns to twist mesially.In the vertical direction,when the traction hook of clear aligner is used,the high-positioned micro-implant decrease the tendency of the incisors extrusion,and increases the tendency of the canines intrusion.When micro-implant applies force to power arm,the incisors move from extrusion to intrusion,tendency of the canines intrusion and tendency of the second premolars intrusion was decreased obviously compared with that of the clear aligner traction hook is used,the first molar moves from extrusion to intrusion,tendency of the second molar extrusion was decreased significantly.Traction of the Power arm and high-positioned micro-implants increase the tendency of the incisor intrusion.In the horizontal direction,the implantation height of the micro-implant has no significant effect on the trend of the horizontal movement of each tooth.When no traction force is applied or the traction hook of clear aligner is used,the tendency of the canine to move towards the palatal inclination,the tendency of the second premolar to move towards the buccal inclination,the first molars and second molars to move towards the palatal inclination.When the Power arm was tracted,the tendency of the canines to move obliquely palatal decreased,the oblique movement trend of the second premolars changed from buccal to palatal,the tendency of the first molar to move obliquely palatal increased and the value of the buccal cusps of the first molar extrusion movement increased.However,the second molars were not affected by the traction methods,they all showed the tendency to move obliquely to the palatal,and the buccal cusps of the second molar showed the tendency to extrusion.Conclusion:1.During the process of retraction of anterior teeth with clear aligner,the stress is concentrated in lateral incisors and canines.We should be on the alert about the risk of root resorption of lateral incisors and canines.2.The implantation of micro-implant in the posterior teeth area can effectively protect the posterior teeth anchorage,especially the application of micro-implant and Power arm for traction can protect the posterior teeth anchorage more effectively,but at the same time,it will increase the mesial torsion trend of the posterior teeth crowns.The distal rotation of the posterior teeth can be actively designed to offset the mesial torsion of the posterior teeth.3.The traction of high-position micro-implant is more advantageous for controlling the torque of the anterior teeth and guiding the bodily movement of the en-mass retraction of maxillary anterior teeth.High-position micro-implant and Power arm can also effectively decrease the deepening trend of overbite when anterior teeth are retracted,and can also reduce the "roller coaster effect" of the dentition caused by posterior teeth to move obliquely to the mesial side and counterclockwise rotation of posterior teeth.4.The traction of the micro-implant and the Power arm can better control the root of the canine when maxillary anterior teeth are retracted,and can also reduce the stress on the periodontal ligament of the canine and reduce the risk of root resorption.5.The application of micro-implant and Power arm for traction is beneficial to control the torque of canines and second premolars to a certain extent,but it is not beneficial for molars.For molars,an appropriate amount of buccal torque of the crown should be added to prevent the molar torque from running out of control.