| Objective:To analyze,using a three-dimensional finite element model(FEM)which included maxilla,maxillary dentition,periodontal ligament(PDL),dedicated orthodontic clear aligner,composite attachments and micro-implant,the initial displacement and equivalent stress of PDL of anterior teeth and molars created by aligners combined with different micro-implant insertion heights and sagittal positions in upper-molar simultaneous distalization,with different traction force loading.This study provides a reference for the selection of micro-implant insertion positions and traction force during the maxillary molars distalization treated with clear aligner.Methods:Part 1: CBCT scanning image of a volunteer with normal occlusion was adopted.Mimics 20.0,Geomagic Studio 2014,and Unigraphics NX 12 were used for establishing three-dimensional models of maxilla,maxillary dentition,periodontal ligament(PDL),dedicated orthodontic clear aligner,composite attachments and micro-implant,Ansys workbench 19.2 was utilized as finite element analysis tools to define material properities,mesh controls and contact settings for constructing a high-quality 3D finite element model.Part 2: On the basis of the 3D finite element model in the first part,the second part simulated maxillary molar simultaneous distalization treated with Clear Aligners combined with micro-implant with different positions loading 1.5N traction force.According to different positions of the micro-implant,four configurations were created:(1)Condition 1:The micro-implants(MIA)were positioned between the first premolar and the first molar with the head 4 mm from the alveolar crest,(2)Condition 2: The micro-implants(MIA)were positioned between the first premolar and the first molar with the head 6 mm from the alveolar crest,(3)Condition 3: The micro-implants(MIA)were positioned between the first and second molar with the head 4 mm from the alveolar crest,(4)Condition 4: The micro-implants(MIA)were positioned between the first and second molar with the head 6mm from the alveolar crest.The initial displacement of anterior teeth and molars under four configurations was analyzed.Part 3: On the basis of the 3D finite element model in the first part,the third part simulated maxillary molar simultaneous distalization treated with Clear Aligners combined with micro-implant anchorage with different traction force.The micro-implants(MIA)were positioned between the first and second molar with the head 4 mm from the alveolar crest.Four configurations were created to compare the aligner without micro-implant(no traction)with three traction force loading:(1)No traction force was loaded from micro-implant to clear aligner hook,(2)traction force of 1 N was loaded from micro-implant to clear aligner hook,(3)traction force of 1.5 N was loaded from micro-implant to clear aligner hook,and(4)traction force of 2 N from micro-implant to clear aligner hook.The initial displacement of anterior teeth and molars and equivalent stress of PDL under four configurations was analyzed.Results:1.The first part of the experiment showed that a 3D finite element model including maxilla,maxillary dentition,periodontal ligament(PDL),dedicated orthodontic clear aligner,composite attachments and micro-implant was successfully constructed to simulate maxillary molar simultaneous distalization treated with Clear Aligners combined with micro-implant anchorage.2.The second part of the experiment showed that in condition 3,the total displacement and the initial sagittal displacement of the anterior teeth were the least,that is,the anchorage loss was the least.Moreover,the sagittal displacement of the molars was the largest,that is,the molar distalization was the largest.With the same sagittal positions of the micro-implant and the increase of the micro-implant insertion heights,the initial mesial and labial displacement of the anterior teeth increased and the intrusion of anterior teeth became more pronounced.With the same micro-implant insertion heights and moving the micro-implant backward,the initial mesial and labial displacement and intrusion of the anterior teeth decreased.3.The third part of the experiment showed that anchorage loss of the anterior teeth expressed as mesial and labial tipping with intrusion.The initial mesial and labial displacement of anterior teeth decreased,but the intrusion became more pronounced with the increase of the traction force.Disto-lingual rotation of the canine was observed with the increase of the traction force.The controlled distal inclination with extrusion of the first molar and uncontrolled distal inclination with intrusion of the second molar were observed during the molar distalization with clear aligner combined micro-implant.The mesio-lingual rotation of the first molar was observed with the increase of the traction force.The maximum dentition initial displacement occurred on the crown of central incisor in condition 1 and 2,on the crown of second molar in condition 3 and condition 4.The maximum initial displacement of the second molar in condition 4 was greater than that in condition 3.The maximum model initial displacement occurred on the crown of central incisor in condition 1 and 2,on the crown of second molar in condition 3,and on the clear aligner in condition 4.The equivalent stress of the PDL of the central incisor in condition 1,2 and the molars in condition 4 was more than 0.026 MPa.The equivalent stress of the PDL of the anterior teeth and the molar in condition 3 was less than 0.026 MPa.Conclusion:1.The construction of the 3D finite element model including maxilla,maxillary dentition,periodontal ligament(PDL),dedicated orthodontic clear aligner,composite attachments and micro-implant provided the basis to study the biomechanical effects created by clear aligner combined with micro-implant during upper-molar distalization.2.Molar distalization and anchorage control seems to be the most efficient with the decrease of the micro-implant insertion heights and moving the micro-implant backward.The intrusion of anterior teeth became more pronounced with a posterior position and a higher implant height of the micro-implant.3.Anchorage loss of anterior teeth expressed as mesial and labial tipping with intrusion,and the controlled distal inclination with extrusion of the first molar and uncontrolled distal inclination with intrusion of the second molar were observed during the molar distalization with clear aligner combined micro-implant.Molar distalization and anchorage control seems to be the most efficient and the periodontal tissue is not easy to be damaged with 1.5N traction force loading.The disto-lingual rotation of the canine,mesio-lingual rotation of the first molar and clear aligner deflection should be considered by orthodontists when using a heavier traction force. |
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