Biomechanical Analysis Of Repairing Large Area Defects Of Mandibular First Molar With Endocrown And Post-core Crown

Objective:In order to provide dentists with biomechanical rationale in clinical practice,this study compares the two methods of rehabilitation for the large-scale defect of mandibular first molar,by means of finite element method.To achieve this goal,six models were established which were designed to be the first molar after root canal treatment and with three kinds of defects that were restored by endocrown and post-core crown.In order to understand the differences in the fracture resistance of dental tissue,the following indicators were observed: the stress value and distribution of the dental tissue.Methods:1.An adult male volunteer's dentition was scanned by CBCT,whose dentition is intact,periodontal is health,mandibular first molar anatomical is clear and complete.According to the data obtained by scanning,the model of tooth and periodontal tissue of mandibular first molar was established by reverse engineering software such as Mimics,Geomagic and CATIA.This model serves as the control group.2.On the basis of the control group,models of different degrees defect of mandibular first molar were established(MOD defect,BOM defect,BOMD defect).Next,the endocrown and the post-core crown models including the prosthesis and the adhesive layer were established and were used as the experimental group.3.The HyperMesh software is used to transform the above models into finite element models.Force exerting the total load of 200 N in the vertical direction on the central fossa,mesial and distal marginal ridge,mesial and distal buccal cusp,simulating intercuspal position.Force exerting the total load of 200 N in the oblique direction on the buccal incline of mesial and distal buccal cusp,simulating lateral movement.After calculation by Abaqus software,the equivalent stress and maximum principal stress distribution of the dental tissue under both loads were output.Results:1.The model of tooth and periodontal tissue of mandibular first molar was established successfully,its anatomical structure is real and clear.On the basis of this model,the models of molar with different degrees of defect and repaired by different prosthesis were created,which are conform to clinical practice.2.The stress values of dental tissue in the experimental group ware lower than that in the control group.Compared to the application of endocrown,the values of the von Mises stress and the maximum principal stress in the post-core crown were both lower when loaded by both kinds of stress.3.The stress distribution of the experimental group is slightly different from that of the control group.Under the vertical load,the von Mises stress peak values of the dental tissue restored by endocrowns were found at the bottom of the pulp chamber,the maximum principal stress peak values were located in the root furcation region;post-core crown group's dental tissue von Mises stress peak values were located in distal apical 1/3,the maximum principal stress peak values were also located in the root furcation region.When oblique loading,the von Mises stress peak values of the dental tissue restored by endocrowns were located at the bottom of the pulp chamber,the maximum principal stress peak values appeared on the buccal side of the distal apical1/3;the peak values of von Mises stress and maximum principal stress in the dental tissue restored by post-core crown all appeared at distal apical 1/3.Conclusions:1.It is highly efficient and accurate that the method of CBCT scanning combined with reverse engineering software to creat models.The models of mandibular first molar and its defects and the corresponding prosthesis have good structural and mechanical similarity.2.The endocrown and the post-core crown both can be used as the restoration method for the mandibular first molar's large-scale defect,however,it is necessary to try to reduce the lateral force applied to the teeth so as to ensure the long-term effects of the restoration.3.In order to protect the mandibular first molar with large-scale defect,the post-core crown has several advantages over the endocrown,but it needs to be considered in combination with the actual situation.4.For the first molar of the mandible with non parallel wall defect,it is suggested that the post-core crown restoration be adopted.