| Objective:To evaluate,by constructing a three-dimensional finite element model of the clear aligner with angel button using interradicular mini-implants(IRMIs),the biomechanical system and equivalent stress of teeth and Periodontal ligament(PDL)of molar distalization with clear aligner therapy(CAT)with different sagittal implantation position and varying elastic forces,and provide a theoretical reference for the design of implantation position and the magnitude of elastic force in clinical practice.Methods:1.Construction of three-dimensional finite element model: A patient conforming the indications of molar distalization was selected to take CBCT images.FE models including maxilla,complete maxillary dentition,PDL,composite attachments,mini-implants(MI),and dedicated orthodontic aligner,were constructed and optimized using Mimics 20.0(Materialise,Leuven,Belgium),Geomagic Studio 2014(Raindrop GEOMAGIC,North Carolina,USA),NX1911 software(Siemens,German)and other modeling software.2.Three-dimensional finite element analysis: Import the 3D FE model into ANSYS Workbench 2019(ANSYS,Pennsylvania,USA)software for load setup and 3D finite element analysis.Three working conditions are designed according to the implant position of the micro-implants:(1)Condition 1: control group,without MI and elastics,the simultaneous displacement of maxillary first and second molars by 0.2 mm was simulated(2)Condition 2: MI was placed buccally between the maxillary second premolar and the first molar(3)Condition 3: MI was placed buccally between the maxillary first molar and the second molar.Elastic forces(0 N,1 N,1.5 N,2 N)were applied to condition 2 and 3.The stress distribution,displacement trend of teeth and equivalent stress of PDL of anterior and posterior teeth under three conditions were analyzed.Results:1.In this experiment,the finite element 3D analysis model of "maxilla-maxillary dentition-periodontal ligament-clear aligners-micro-implants" was successfully constructed,which laid the foundation for the later load setting and displacement trend analysis.2.In the first condition,the upper anterior tooth exhibited mesial and labial tipping,and the crown moved more than the root.Maxillary central incisors teeth and maxillary canines showed extrusion,while maxillary lateral incisor teeth showed intrusion.For horizontal and vertical displacement,maxillary canines were largest,followed by maxillary central incisors and maxillary lateral incisor.As the traction force between the mini-implant and the angel buckle increased,the amount of movement in the mesial and labial tipping of the upper anterior teeth gradually decreases,and the tooth movement manner tends to bodily movement.3.The first molar showed distal inclination movement,and the crown movement was greater than the root movement;mesial of the first molar was intrusion and distal of the first molar was extrusion in vertical displacement;The crown moved to the buccal side and the root moved to lingual side in horizontal displacement.The second molar showed reverse movement,where the crown showed distal inclination and the root showed mesial inclination;The vertical upward both crown and root were depressed and the horizontal upward crown moved to the buccal side and the root moved to lingual side.With equal force loading in the mini-implants and the angel buckle,the distal movement of maxillary molars in condition three was greater than its in condition two,and vertical displacement also increased.4.Among all conditions,the PDL equivalent(von-Mises)stress of upper anterior tooth was the largest,whereas,the first molar and second molar were the smallest.As the miniimplantmoved backward,the PDL equivalent(von-Mises)stress of upper anterior tooth decreased,and the equivalent stress of upper molar increased.Maxillary exhibited same trend with increasing traction force.Conclusion:1.In the process of distal movement of maxillary molars with clear aligners,the upper anterior tooth exhibited labial tipping,and the center of rotation was located at the junction of the root tip and 1/3 of the root.The controlled distal inclination with mesial extrusion of first molar and uncontrolled distal inclination with intrusion of second molar were observed,and the distalization efficiency of the second molar was higher than that of the first molar.2.Mini-implant combined angel buckle loading traction force enhanced anchorage in the incisor,and the best effect of anchorage in the incisor area can observed when miniimplant located between the first and second molars.But clinically attention should be paid to vertical control to prevent the lateral appearance of patients with high angle due to the clockwise rotation of the jaw.3.With the increase of force loading in the mini-implants and the angel button,the upper anterior teeth more tend to bodily movement.While the risk of root resorption increased,which should be of clinical concern,especially for maxillary lateral incisors.Traction force within 1.5N was more conducive to periodontal tissue health. |
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