Numerical Stimulation Study Of Endodontically Treated Mandibular First Molars With DO Tooth Defect Rstored With Onlays

Purpose:To compare the stress distribution of the class II occlusal-distal(OD)cavity shape tooth defect restored with onlay I group and onlay II group and crown group,inlay group,composite resin group in endodontically mandibular first molars,using three-dimension FEM.The aim is to apply the FEM to check the fracture resistance of an endodontically treated molar and to evaluate possible cause of cracking failure.It could provide a theoretical basis for the selection of appropriate restoration methods of mandibular first molars defect after root canal therapy.Method:The extracted mandibular first molars in vitro are collected,and the isolated teeth are selected.The three-dimensional finite element models of mandibular first molars are established after CBCT scanning.A complete tooth(group F)is set as the control group.After stimulating root canal therapy and 5 models of different restorations,groups varied as follows: group A,onlay I restoration covering all occlusal surfaces,and group B onlay II restoration covering partial occlusal surface;group C,ceramic crown restoration;group D,inlay restoration;group E,composite resin restoration;group F,complete tooth.Then,numerical simulation is calculated and analyzed by ANSYS Workbench,a finite element software,and each part of the mesh is assigned,and resin-base,an adhesive cement,modeled with fixed-type contact elements,as well as restorative filling materials are considered.The materials are assumed to behave isotropic and elastic throughout the entire deformation.Then the numerical results performed by a preliminary static FEM model are obtained.Result:1.In this study,a three-dimensional finite element model of mandibular first molar,after complete root canal treatment,are established by using finite element software,as well as five other types of distal-occlusal(DO)defect models restored of different restorations of a defect model after root canal therapy.2.After vertical occlusal load applied,the group E shows the highest Von-Mises stress value,mainly distributes on the part of the lateral wall of the tooth tissue is obviously affected.While the distal-occlusal defect in group A shows lower peak Von-Mises stress values than that of the other 5 prosthodontic restorations.Under oblique loading,the peak Von-Mises stress values of group A decrease significantly,and the stress value of residual tooth part is the lowest,which is lower than that of other groups.Under vertical and lateral loading,compared with complete molar,the peak Von-Mises stress values in groups E rises up instead,and the peak Von-Mises stress values in groups A is significantly lower than that in other groups.3.The fatigue analysis tested by using a series CAD–FEM procedures,indicates that the peak concentration the groups A,B,and C concentrated on the cervix of the endodontically treated teeth,the stress of group D and E concentrate on the occlusal surface of residual tooth tissue,while the strength of the group A reduced,and disperse to remaining lingual and buccal wall dental tissue through the contact area between restoration and dentin,the fatigue damage of a restored molar and fatigue mechanic laws are evaluated.In the collection of experimental results,it is found that results of the models of onlay II,ceramic crown,onlay I restoration(design of cuspal coverage)under different loads,concentrated on the cervix of the teeth.On the other hand,the fatigue damage of inlay and composite resin restored models,mainly concentrated on the contact area between the restoration,and different restoration groups of the endodontically treated teeth may also accidentally be weaken.The strength of remained tooth is reduced in the onlay I group,and dispersed to dental tissue through the restoration,while the stress is transferred to remaining lingual and buccal wall on the tooth tissue in the inlay and resin groups.Conclusion:1.The three-dimensional finite element models of mandibular first molars are established after CBCT scanning,by using reverse engineering software and the finite element analysis software.It is found that the FEM model has good simulated and mechanical similarity.2.In a long-term study,the resin-repaired model after root canal therapy may not be sufficient to ensure the longevity of an endodontically treated tooth with a DO cavity,in which the equivalent stress is mainly concentrated on the enamel part of the tooth,and could lead to the leakage failure.Though inlay,resin group grinding tooth tissue as minimal as possible,but the restorations did not seem to protect the residual tooth structure effectively.On the other hand,onlay I restoration(designs of cuspal coverage)is applicable for mandibular first molar with class II occlusal-distal(DO)cavity shape tooth defect,and dispersed the vertical and lateral strength through the restoration,and had better protective effect for the tooth tissue.The onlay I group(design of cuspal coverage)is the most suitable one for the covered with high equivalent stress,that is,the performance of the restoration is reliable.The onlay prosthesis method(onlay design of cuspal coverage)protected tooth structures more effectively than other restoration of endodontically treated teeth.3.According to the research data,we can draw a new relationship among tooth preparation and defect restoration,and the reasonable selection of indications in clinical situation.It is shown that the biomechanical performance of the restoration had no relationship with the amount of tooth structure preparation.The selection of indications and restorative technique design are closely related to the stress distribution and biomechanical performance.