| Background:Skeletal class Ⅲ malocclusion is a common malocclusion which has a high incidence and a great impact on function and aesthetics.Maxillary protraction is the most often used treatment for skeletal class Ⅲ malocclusion with maxillary deficiency.However,substantial studies have shown that maxillary protraction is mainly used to compensate for maxillary deficiency through the inclination of the upper anterior teeth and correcting anterior crossbite.It has less skeletal effects and more likely induces upper arch constriction.In order to improve the skeletal remodeling and prevent constriction of the dental arch during maxillary protraction,some scholars have suggested the combination of maxillary expansion and maxillary protractionSome scholars have tried to stimulate the reconstruction of the maxillary suture to enhance the skeletal effect of maxillary protraction.Among them,Alternate Rapid Maxillary Expansions and Constrictions(Alt-RAMEC)combined with maxillary protraction is one of these treatments.In our previous study,Alt-RAMEC combined with maxillary protraction for skeletal Class Ⅲ malocclusion during the dentition stage and early permanent tooth stage received more significant improvement,shorter treatment duration,more skeletal effects and less tooth movement than single maxillary protraction.However,there are few biomechanical studies on Alt-RAMEC treatment,and the mechanism that produces this effect is not yet clear.Objective:In this study,a three-dimensional finite element model of the cranial-maxillary complex and spiral expander for maxillary protraction and Alt-RAMEC was established to simulate stress distribution and displacement changes caused by maxilla,maxillary dentition and various bone sutures under different conditions.From the perspective of mechanics,this study aimed to analyze the stress distribution differences between Alt-RAMEC combined with maxillary protraction and single maxillary protraction,and analyze its possible mechanism,in order to provide theoretical basis for orthodontic treatment in early treatment of skeletal classⅢ malocclusion.Methods:In this study,CBCT scans were used to obtain the original image data,Mimics 16.0,Geomagic Studio 2014,SolidWorks,and 3-matic software were used to establish the models,building the craniofacial complex,Hyrax expander,frontomaxillary suture,zygomaticomaxillary suture,zygomaticotemporal suture,pterygopalatal suture and midpalatal suture.ANSYS Workbench 15.0 software was used to assemble the created models.Then the mesh was divided,the properties,contact relationships and boundary constraints of each material were set according to previous studies.Three finite element models which includes maxillary protraction(model 1),maxillary expansion combined with maxillary protraction(model 2),and maxillary constriction combined with maxillary protraction(model 3)were established.In Model 1,500g of protraction force was applied to the maxillary bilateral canine in an oblique forward and downward direction and a plane of commissure at 30°.Under the same force and angle as Model 1,a horizontal forced displacement of 0.25 mm on each side of the Hyrax expander in Model 2 was simultaneously applied.In Model 3,a horizontal forced displacement of 0.25mm per side which in the opposite direction to Model 2 was applied to the Hyrax expander.Then the finite element method was used for calculation.This study observed and analyzed the three-dimensional displacements of certain landmarks of the maxillary dentition and the maxillary bone,and the Von-mises equivalent stress and the maximum principal stress of maxillary bone,maxillary dentition,bone suture,and the overall displacements tendencies of the maxilla.Results:Part Ⅰ.Establishment of the three-dimensional finite element model of the cranial-maxillary complex and spiral expanderEstablishing a relatively complete craniofacial complex including craniofacial related bones,maxillary dentition,periodontal ligament,frontomaxillary suture,zygomaticomaxillary suture,zygomaticotemporal suture,pterygopalatal suture and midpalatal suture,and Hyrax arch expander with a high mesh quality,good geometric similarity and biological similarity,this could accurately complete the biomechanical calculation requirements and was the basis for finite element analysis.Part Ⅱ.Three-dimensional finite element analysis of Alt-RAMEC combined with maxillary protraction and the unique maxillary protraction.Through the established three-dimensional finite element model under different working conditions,the finite element analysis was performed after the load is applied,and the results were as follows1.Stress distribution of craniofacial complex:Compared with model 1,the Von-mises equivalent stress distribution of model 2 and model 3 was more extensive.In addition to the anchorage teeth and alveolar bone,stresses in the frontal process of maxilla,orbital floor,the lateral margin of the orbit,around the piriform foramen,the pterygoid process of sphenoid bone,the posterior part of the upper palate,and the zygomatic area were all obviously observed.2.Von-mises equivalent stress and maximum principal stress of each bone suture:In model 2 and model 3,the average value of Von-mises equivalent stress of each bone suture was significantly greater than that in model 1,especially the midpalatal suture and pterygopalatal suture;In model 2,the equivalent stress of each suture is the midpalatal suture>the pterygopalatal suture>the zygomaticomaxillary suture>the frontomaxillary suture>the zygomaticotemporal suture;In model 3,the equivalent stress of each suture is the midpalatal suture>the pterygopalatal suture>the frontomaxillary suture>the zygomaticomaxillary suture>the zygomaticotemporal suture.3.The maximum principal stress of the sutures(frontomaxillary suture,zygomaticomaxillary suture,zygomaticotemporal suture,pterygopalatal suture)was mainly tensile stress,but the force value interval of model 1 was much smaller than that in model 2 and model 34.Displacement tendencies in the three-dimensional directions of the landmarks of the maxillary dentition:In the X-axis,Model 3 had obvious mesial tilting movement and posterior lingual movement compared with Model 1.Model 2 showed crown distal movement in anterior teeth and buccal inclination in posterior teeth;In the Y-axis,each landmark of the dentition in the three models had forwardly shift,especially the labial inclination in the anterior and the mesial inclination in posterior teeth in model 3;In the Z-axis,the crowns of the anterior teeth were all intruded,and the molars were extruded in the three models.5.Three-dimensional displacement of skeletal landmarks:In the X-axis,the displacement of point A and the anterior edge of the zygomatic arch were not obvious in the three models.The ANS points,zygomatic body points,and nasal cavity.the edge points displaced outward,and the PNS points were displaced inward in models 2 and 3,while the N point and the infraorbital edge point were obviously displaced outward in Model 3;In the Y-axis,except the PNS point,all points were displaced forward in the three models,and the degree of displacement in Model 2 and Model 3 was significantly greater than that in Model 1;In the Z-axis,there was no significant shift in the anterior edge points and zygomatic body points of the three models,but significant upward shift in the rest points in Model 3.6.The overall movement tendencies in the maxilla:all three models produced counterclockwise rotation of the maxilla,mainly in the front of the maxilla.Conclusion:1.Using CBCT data and related modeling software,a three-dimensional finite element model with high biological similarity and geometric similarity can be established for investigating the biomechanical mechanism of alternating maxillary expansions and constrictions combined with maxillary protraction.2.Compared with unique maxillary protraction,Alt-RAMEC combined with maxillary protraction can produce extensive and significant stresses on the bone surface and each bone suture of the craniofacial complex,suggesting that Alt-RAMEC can loosen the bone suture around the maxilla to a greater extent and strengthen the reconstruction of the sutures.3.Alt-RAMEC combined with maxillary protraction has more anterior displacement of the maxillary bone compared with unique maxillary protraction,the labial inclination of the upper incisor,the extruded of the molar and the counterclockwise rotation of the maxilla suggest that Alt-RAMEC combined with maxillary protraction is benefit for patients with deep overbite,but for patients with vertical growth patterns and open-bite,the use of the Alt-RAMEC combined with maxillary protraction treatment should be cautious.4.Both in the unique maxillary protraction or Alt-RAMEC combined with maxillary protraction,the stresses are concentrated on the anchorage teeth and its corresponding alveolar bone area,while the stress value of Alt-RAMEC combined with maxillary protraction is significantly enhanced,but the overall stress value is minor,suggesting that during the Alt-RAMEC combined with maxillary protraction treatment,there is less need to worry about the risk of anchor fracture or root resorption,but the possibility of bone resorption and bone crack of the alveolar bone caused by alternating expansions and constrictions should be prevented.5.This study further verified the results of our previous clinical research from the perspective of biomechanics,and revealed the biomechanical basis and mechanism of our treatment using Alt-RAMEC combined with maxillary protraction therapy. |
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