| Objective:A three-dimensional finite element model of the lingual appliance,maxilla,upper dentition,periodontal ligament,power arms and mini-implant was constructed to establish a complex force system.The biomechanical characteristics,tooth displacement trend,periodontal ligament stress distribution characteristics and safety evaluation of maxillary anterior teeth under different retraction forces and intrusive forces were studied.This paper preliminarily discusses the mechanical system of the lingual appliance in the retraction of maxillary anterior teeth,which has better torque control of the anterior teeth and safer stress distribution of the periodontal ligament,and provides theoretical basis and guidance for rational clinical application of the lingual appliance system and reducing the risk of bone fenestration and bone dehiscence during retraction.Methods:1.Establishment of a three-dimensional finite element model of a double archwire lingual orthodontic system combined with different retraction forces and intrusive systems: A single normal volunteer was selected,and the data on the jaw and dentition were obtained by cone-beam CT scanning of the maxillofacial region.Three-dimensional models including the maxilla,upper dentition,periodontal ligament,lingual appliance,power arms and mini-implant nail were constructed by professional modeling software such as Mimics 17.0,Unigraphics NX 8.5 and Geomagic Studio 2013,and three-dimensional finite element processing was performed using Ansysworkbench 15.0 finite element analysis software.The material attributes of the three-dimensional model were defined,meshing was performed,and boundary constraints were determined to obtain a three-dimensional finite element model.2.To analyze the biomechanical characteristics and tooth displacement trend of the double archwire lingual orthodontic system combined with different retraction force and intrusive force systems: Two power armss were placed between lateral incisors and canines;two mini-implants were placed between maxillary first molars and second molars,and one mini-implant was placed between maxillary central incisors based on the three-dimensional finite element model constructed in the first part.According to the different applied retraction force and intrusive force mechanical systems,a total of nine working conditions were designed:(1)working condition 1:intrusive force 0N + retraction force 0.5N;(2)working condition 2: intrusive force 0N+ retraction force 1N;(3)working condition 3: intrusive force 0N + retraction force1.5N;(4)working condition 4: intrusive force 0.5N + retraction force 0.5N;(5)working condition 5: intrusive force 0.5N + retraction force 1N;(6)working condition 6: intrusive force 0.5N + retraction force 1.5N;(7)working condition 7:intrusive force 1N + retraction force 0.5N;(8)working condition 8: intrusive force 1N+ retraction force 1N;(9)working condition 9: intrusive force 1N + retraction force1.5N.The displacement trend of maxillary anterior teeth in nine working conditions was recorded and compared,and the experimental results were analyzed.3.Evaluation of stress distribution characteristics and safety of periodontal ligaments with a double archwire lingual orthodontic system combined with different retraction forces and intrusive systems: 3D FEM and working condition grouping were the same as in the second part.The stress distribution characteristics of periodontal ligament in nine working conditions were recorded and compared,and then safety was evaluated.The experimental results were analyzed.Results:1.A three-dimensional finite element model of lingual orthodontic treatment with lingual appliance,maxilla,upper dentition,periodontal ligament,power arms and mini-implant was established to simulate lingual appliance retraction of maxillary anterior teeth under different retraction forces and intrusive systems.2.Variable amounts of displacements like controlled tipping,uncontrolled tipping,lingual crown tipping,labial root tipping,extrusion and distal crown tipping were observed in all the models,and these tendencies increased as the magnitude of retraction force increased,and these tendencies decreased as the magnitude of retraction force increased.When the intrusive force was greater than or equal to the retraction force,the maxillary central incisors showed the trend of lingual crown tipping and labial root tipping,resulting in uncontrolled tipping movement.Regarding horizontal changes,the increasing width of bilateral anterior teeth was observed,and the least increasing trend of the width was observed in canines.3.The distribution of periodontal membrane stresses was basically the same in the nine working conditions.The periodontal membrane stress was concentrated in the cervical part of the root,the palatal side and the distal mesial adjacent surface,and the root stress decreased gradually from the cervical part to the root,in which the lateral incisors were subjected to the greatest periodontal membrane stress value.Under the same magnitude of intrusive force,the periodontal stress value of maxillary anterior teeth increased with the increase of internal retraction force.The periodontal membrane stress values in the A3 group were significantly higher than the rest of the groups,and the C1 group had the lowest periodontal membrane stress values.Conclusion:1.Various combinations of retraction force and intrusive force in doublearchwire lingual orthodontic system provide a new choice for torque control of the anterior teeth.Anterior mini-implants and elastics can achieve incisor intrusion and lingual root torquing,but still cannot achieve excepted torque without extra torque control methods.2.The periodontal membrane stress values were within the ultimate tensile strength range in all nine working condition models,which indicated that the nine working conditions were safe for clinical applications. |
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