Three-dimensional Finite Element Analysis Of Maxillary Protraction Combined With Miniscrew-assisted Rapid Maxillary Expansion

Background:Maxillary deficiency is one of the most common mechanisms of skeletal class III malocclusion,some of the patients suffer from developmental abnormalities in three dimensions.Maxillary protraction by facemask is well recognized as an effective protocol in treating maxillary retrusion.However,previous studies indicated that anterior crossbite was corrected mainly by incisors tipping in single maxillary protraction,there were limited skeletal modification,as well as insolubility of handling with maxillary transverse deficiencies(MTD).Due to the disadvantages,rapid maxillary expansion(RME)was recommended to combine with protraction in order to increase skeletal effects and treat MTD.Although the effectiveness was testified by substantial studies,significant dental effects were observed,which could influence the vertical pattern,endodontic tissues,periodontal tissues and even temporomandibular joint.Besides,tooth-borne RME and protraction require strict age indications,the efficacy and stability are decreased as the maturation of sutures increase,that is the reason why its application is limited in non-pubertal patients.Over the years,domestic and overseas orthodontists have been devoting themselves to break through the age limitation in orthopedic treatments and to discover protocols with more efficiency and safety.As the development of miniscrew-assisted rapid maxillary expansion(MARME),studies showed that it can not only expand the midpalatal sutures in post-pubertal adolescents or even young adults,but also stimulate the circummaxillary sutures.On this basis,scholars attempted to combine maxillary skeletal expansion(MSE)with facemask,patients receive significant improvements in sagittal and transverse dimension,as well as optimal control in vertical pattern.Our team complemented the same protocols in treating patients of early permanent dentition and the results were surprising.However,since the previous studies and long-term follow-ups are limited,our preliminary works require more sufficient theoretical supports,the therapeutic effects and mechanism of MARME or MSE were not explicit,we still need further biomechanical or biological studies to support the clinical application.Objective:The present study is designed to construct three-dimensional finite models based on CBCT data and by which to compare the differences between MSE with facemask,tooth-borne RME with facemask and single facemask,from perspectives of stress distribution and displacements tendencies on craniomaxillary bones,sutures,alveolar bones and teeth.The aim was to provide the theoretical foundation and instructions for treatments design,devices selection and efficacy prediction in clinical applications of maxillary protraction combined with MSE.Methods:The FEM models was created from CBCT of a 12-year old adolescent with maxillary retrusion.The tomography data was imported into MIMICS 20.0 to reconstruct the craniomaxillary model.3-matic Research 12.0 was used to build models of frontomaxillary,zygomaticomaxillary,zygomaticotemporal,pterygopalatal and midpalatal sutures.Then,3D entity models were established in Geomagic Studio 2014.Models of Hyrax expander,MSE expander and miniscrews was constructed in ANSYS Workbench 15.0.In the same software,all models were assembled,mesh was generated afterwards,and setting of material properties,connections and boundary conditions were all completed.The study was divided into three groups:FM,tooth-borne RME with facemask(Hyrax/FM)and MSE with facemask(MSE/FM)groups.In each group,a protraction force of 500 g was applied in mesial region of canine adjacent to alveolar ridge.The expanders in Hyrax/FM and MSE/FM groups were activated transversely by 0.25 mm at the level of coil-spring.FEM were completed in postprocessing phase,observation targets included stress distributions of craniomaxillary complex,dentition and alveolar bone;Von-Mises stresses and maximum principal stress of sutures;directional displacements of skeletal and dental landmarks;and dental root/crown ratios.Exporting the nephograms and summing up the results,charts were drawing by GraphPad Prism 8.Results:Part Ⅰ Establishment of three FEM models including maxillary protraction combined with MSETetrahedral elements with central nodes were used for mesh generation,the FEM models were successfully established on basis of precisive image acquisition and modeling technology.The FEM model of FM group was composed of 358858 elements and 601677 nodes.Hyrax/FM model was composed of 364319 elements and 604354 nodes.MSE/FM model was composed of 366963 elements and 609389 nodes.The mesh possessed high quality,suture structures were complete,connections were reasonable.Overall,the FEM models have biological similarities,which provide solid foundation for the subsequent biomechanical simulations and analysis.Part Ⅱ Comparative three-dimensional finite analysis of stress distribution and displacement in different maxillary protraction protocols1.Stress distribution of craniomaxillary complex:compared with FM and Hyrax/FM group,Von-Mises stresses were more widely distributed in MSE/FM group,the maximum stress located at area around miniscrews,no distinct stress were observed on anchorage teeth and alveolar bone.2.Von-Mises stresses on sutures:under the equivalent force magnitude and direction,MSE/FM generated the highest stress on all sutures.The force magnitude on sutures ordered as:midpalatal>pterygopalatal>zygomaticomaxillary>frontomaxillary>zygomaticotemporal.3.The maximum principal stresses of sutures:all groups presented the minimum tensile stress.MSE/FM group generated uniform and wide tensile stress in the rest sutures,force magnitude was also increased.4.Displacement of craniomaxillary complex:(1)Expansion patterns of palates:palatal expansion was more parallel in MSE/FM group compared with Hyrax/FM,the latter one showed significant "V-shape"pattern.(2)Displacements of skeletal landmarks:Transversely,inferior border of orbit,frontomaxillary and zygomaticotemporal sutures displaced less obviously,other landmarks were remarkably expanded in MSE/M group.Sagittally,all landmarks moved anteriorly except for the inferior border of nasal bone,no intergroup difference was showed in palatal landmarks,MSE/FM group showed the maximum displacements in other landmarks.Vertically,ANS and PNS both moved distinctly downward in MSE/FM group.(3)Rotation patterns:Maxilla rotated counterclockwise in FM and Hyrax/FM group,which displaced bodily towards forward and downward in MSE/FM group.Rotation patterns of palatal plane were similar to maxilla.5.Displacement of dental landmarks:transversely,less molar buccal movement was showed in MSE/FM.Sagittally,incisors flared indistinctly in MSE/FM,as well as molar tipping.Vertically,FM and Hyrax/FM showed incisors intrusion which were caused by flaring,molars extruded distinctly in Hyrax/FM,while both buccal and palatal cusps were intruded in MSE/FM.All R/C ratios were high in MSE/FM,which means the teeth are tended to move bodily.Conclusion:1.MSE with facemask can effectively conquer the resistance generated by midpalatal suture,sphenoid bone and zygomatic bone,causing significantly increased stress on sutures and superficial structures.It shows more favors in realizing parallel palatal expansion and enhancing the advancement of maxilla.2.In MSE with facemask,the maxilla bodily displaces forward and downward,with no distinct counterclockwise rotation of maxilla,as well as the significant decrease in incisors proclination and buccal tipping and extrusion of molars,which shows its advantage in treating patients with open bite tendencies.3.In MSE with facemask,MSE/FM produces stresses mainly in peri-miniscrews region,as well as little stresses on anchorage teeth.The root resorption and periodontal damage are prevented.But other perspectives should also be prevented as:the snap or deformation of mini screws,pains and marginal bone loss.