A 3-dimensional Finite Element Analysis Of The Maxillary First Molar Which Palatal Root Entering Maxillary Sinus In Orthodontic Treatment

Objective:This study used the maxillary first molar as the experimental object and applied the method of finite element analysis to explore the differences of the tooth movement mode and the stress distribution between the normal tooth and the tooth whose roots were entering maxillary sinus.The aim of our study is to provide guidance and biomechanical basis for orthodontic treatment of maxillary first molar which roots into the maxillary sinus.Methods: A case of individual normal occlusion was chosen to collect CBCT data then the data was saved as DICOM files.Imported the DICOM files into the mimics17.0 and exported the STL(stereolithography)files of the right maxillary sinus area.Clinically,only palatal root into the maxillary sinus is the most common type in the all types of the roots of maxillary first molar into the maxillary sinus.We imported the STL files into the 3-matic 10.0 and we took the type above as experimentalprototype,and established different 3D models according to according to different embedding depth.(0,1.5mm,3.0mm,4.5mm,5.5mm,6.5mm).We regard the maxillary first molar whose root is not entering into the maxillary sinus as normal group and the tooth whose root is entering into the maxillary sinus as embedded group.Then we imported all 3D models into workbench17.0 and set load(Along the buccal tube,1.5N,acting on the midpoint of the inner side of the buccal tube),boundary condition and so on.Finally,we performed operation and obtained the displacement condition of the tooth and the stress distribution of roots and periodontal tissue.Result:1.Displacement condition of the tooth:under the load of mesial orthodontic force,the movement of the tooth in the normal group and the embedding groups showed mesial inclination and mesial lingual rotation.the angle of the mesial inclination and mesial lingual rotation of the embedding groups is larger than that of the normal group.And the angle increases with the increase of embedding depth.2.Stress condition of roots:The stress of the roots of embedding group increases with the increase of embedding depth.When the embedding depth of the palatal root is less than 3.0mm,the stress of the roots of the embedding group was even and lower than that of normal group.When the embedding depth of the palatal root is more than 3.0mm,the stress of the roots of the embedding group was concentrated obviously and higher than that of normal group.The stressof rootwas mainly concentrated in thelevel of the furcation area,but the stress in the area of root tip was small.The stress of normal group mainly concentrated in the palatal root,furcation area and dental cervix.After palatal root entering the maxillary sinus,the position of stress concentration was gradually transferred to the two buccal roots(especially on the distal buccal root).The region of furcation and dental cervix still had large stress distribution.3.stress distribution of periodontal tissue:The stress of the alveolar ridge in the normal group was mainly concentrated in the mesial side.The stress of the alveolar ridge in the embedding groups was mainly concentrated in the buccal and mesial.The stress of each region is: buccal side > mesial side>distal side>palatal side;In the normal group,the stress of the cortical bone of the sinus floor was even and small.The stress of the embedding group was significantly higher than that of the normal group and it increases with the increase of embedding depth.It was mainly concentrated in the contact area between the palatal root and the sinus floor;the overall stress condition of periodontal tissue is: root>cortical bone>cancellous bone>periodontal membrane.Conclusion:1.For the maxillary first molar whose palatal root was embedded into the maxillary sinus,the embedding depth of palatal root would change the rotational center of the tooth.And with the increase of the embedding depth of the palatal root,there was a tendency of the mesial inclination and the mesial lingual rotation.2.For the maxillary first molar whose palatal root was embedded into the maxillary sinus,the depth of palatal root affected the stress and stress distribution of roots.So the clinical treatment should be based on the different embedding depth of the palatal root.3.For the maxillary first molar whose palatal root was embedded or not embedded into the maxillary,the main concentration of stress was located at the root furcation level.