Influence Of Stress Distribution On Bone-Anchored Maxillary Protraction With Different Protraction Sites

Part one Establishment of the three-dimensional finite element model contained implant anchorages with the craniofacial region in childrenObjective: To build a three-dimensional finite element model of children's head which includes craniofacial skeletons,sutures,teeth and implant anchorages.Methods: A skull specimen from a completed 8-year-old boy corpse was selected.The original DICOM data of 2D image was obtained by spiral CT scanning.Constructed 3D models of craniofacial skeleton,sutures,teeth and implant anchorages by Mimics 10.0 and made them become solid models by Geomagic 9.0,then stored them as IGES format.All IGES format files were imported to Abaqus 12.0 on the base of the XYZ coordinate system and the model parameters were set.So far the three-dimensional finite element model of children's head which included craniofacial skeletons,nine sutures,eight teeth and implant anchorages has been built successfully.Results: 3D geometrical models of children's head which included craniofacial skeletons,nine sutures,eight teeth and implant anchorages were obtained.The FEM model consisted of 657,594 elements and 990,460 nodes and had good comparability with the morphology.Part two Influence of stress distribution on bone-anchored maxillary protraction with different protraction sitesObjective: To evaluate the influence of stress distribution on bone-anchored maxillary protraction with different protraction sites using biomechanical method,to search an optimal protraction sites.Methods: Simulated the boundary conditions of maxillary protraction with Abaqus12.0 and fixed the margin of the foramen magnum of occipitalbone.Designed the parameter of the model simulating the clinical and four position of implants were loaded separately.Condition I: the alveolar bone at the intersection of distal 2mm of primary lateral incisor crown distal surface and gingival cervical margin to 5mm;Condition II: the alveolar bone at the intersection of mesial 2mm of maxillary first primary molar crown mesial surface and gingival cervical margin to 5mm;Condition III: the alveolar bone at the intersection of mesial 2mm of maxillary first molar crown mesial surface and gingival cervical margin to 5mm;Condition IV: the alveolar bone at the intersection of distal 2mm of maxillary first molar crown distal surface and gingival cervical margin to 5mm.The FEM models were loaded as 500 g each side and 30° forward direction to the occlusal plane.Stress distribution and displacement tendency of craniofacial skeletons,sutures and teeth were analysed.Data of equivalent Von Mises stress of each simulation was calculated.Then the stress nephograms and displacement tendency were protracted.Results:1 The stress distribution characteristics of each suture under different protraction sites:(1)FMS: The stress distribution of FMS focused on the former part of 2/3 and the maximum stress value was reflected in the leading edge of all sutures.The maximum stress value was in condition II and the range was 2.819×10-2-1.477×10-3MPa.(2)NMS: The stress distribution of NMS focused on the upper edge and posterior-lower edge in condition I and II.The stress distribution was more uniform in condition III and IV.However,The maximum stress value all focused on the upper edge in all conditions.The maximum stress value was in condition I and the range was9.24×10-4-5.296×10-6MPa.(3)ZMS: The stress distribution of ZMS focused on the leading edge in condition I and II,but the area was larger in condition I.The stress distribution focused on the posterior-lower part in condition III as well as the posterior edge in condition IV.The maximum stress value was in condition IV and the range was 1.313×10-2-3.947×10-4MPa.(4)ZFS: The stress distribution focused around the edge,and the area was larger incondition I and II,but it was more uniform in condition III and IV.The maximum stress value of ZFS was in condition IV and the range was3.169×10-2-4.952×10-4MPa.(5)ZTS: The stress distribution of ZTS focused on the lower edge and outer edge,and the maximum stress was in the lower edge.The maximum stress value was in condition III and the range was1.587×10-2-1.148×10-3MPa.The second was in condition IV and the range was1.367×10-2-1.129×10-3MPa.(6)MPS: The stress distribution of MPS was uniform in all conditions,the maximum stress value was in condition I and the range was 7.3×10-4-6.479×10-6MPa,but it was smaller compared to other sutures.2 The stress distribution characteristics of teeth under different protraction sites:(1)Maxillary first molar: The stress distribution was uniform in condition I and II and the maximum stress value focused on the mesial surface of tooth cervix.The maximum stress was reflected in the junction of crown and root on mesial surface in condition III,while the apical1/3 on distal surface in condition IV.It was 2.03×10-1MPa in condition III so that became the maximum stress value.(2)Maxillary first primary molar: The maximum stress was in the apical 1/3 of palatal root in condition I.The maximum stress was in root furcation of palatal root in condition II.The stress focused on the distal buccal root,and the maximum stress was in the apical part in condition III and IV.The maximum stress value of maxillary first primary molar was in condition II and the range was 1.061×10-1-4.048×10-4MPa.(3)Central and lateral incisors: The stress was uniform in condition I,while focused on the apical 2/3 of central incisor in condition II,III and IV.The maximum stress value was in condition I and the range was1.815×10-2-2.649×10-7MPa.3 The stress distribution characteristics of whole skeleton under different protraction sites: The stress distribution focused on the area of the dorsum nasi,the bridge of the nose,the dextro-ala nasi and the frontonasal-frontonmaxillary suture in condition I.The stress distribution focused on the area of the two sides of dorsum nasi and the dextro-ala nasi in condition II,III and IV.Thestress concentration zone was condition I > II > III.While the stress concentration zone increased in condition IV.4 The displacement tendency characteristics of each suture under different protraction sites: The leading part of FMS,NMS,ZMS,MPS showed larger displacement tendency and no significant differences were shown between each condition,as well as the leading and upper part of ZFS.The upper part of ZTS showed larger displacement tendency and was relatively obvious in condition I,II and III.While the outer part of ZTS showed larger displacement tendency in condition IV.5 The displacement tendency characteristics of whole skeleton under different protraction sites: The forward displacement decreased gradually transformed from maxillary incisor to the top of skull,then became backward displacement at the sagittal view.The forward displacement of root nasi and dorsum nasi was decreased while the alveolar was increased gradually in the four conditions.The craniofacial region mainly showed upward displacement tendency at the verticle view.The upward displacement tendency of teeth and alveolar was smallest in condition I.Conclusions:1 3D geometrical models of children's head which included craniofacial skeletons,nine sutures,eight teeth and implant anchorages were obtained.The FEM model has good precision and comparability with the morphology.2 It is more benefite to improve the concave profile of the midface and decrease the counterclockwise rotation trend of maxilla in bone-anchored maxillary protraction.