Biomechanical Analysis Of Repairing Lower Residual Crown Molars With Post-core Crown,Endocrown And Inlaycrown

Purpose:Using the finite element analysis to compare the fracture resistance of the dental tissues and retention effect of the restorations in the large area defects of mandibular first molars with clinical crown less than 3mm which repaired by post-core crown,endocrown and inlaycrown.In order to explore the optimized repair plan,and to provide theoretical guidance for their clinical applications.Methods:Scanning the dentition of a male volunteer with cone beam CT(CBCT)and selecting his healthy and integral left mandibular first molar as the experimental sample.Then a complete three dimensional geometrical model of the mandibular first molar was reconstructed by reverse engineering software such as Mimics,Geomagic and CATIA.On the basis of this tooth model,three groups low residual crown models with a large area defect were constructed,the clinical crown height of which were 1mm,2mm and 3mm respectively.Then we established three groups of restorations models namely post-core crown,endocrown and inlaycrown to repair three groups low residual crown models,a total of nine groups experimental models were generated.The experimental group models were meshed with Hypermesh software to establish finite element models.Then we applied the total load of 225 N static load from vertical direction which was parallel to the tooth axis on the occlusal surface of each experimental group model in the Abaqus software,simulating the intercuspal force.The vertical loading points were: the lingual inclines of mesial and distal buccal cusps,the lingual incline of distal cusp,as well as the buccal inclines of mesial and distal lingual cusps.And applied the total load of 225 N static load from angular direction which was 45°to the tooth axis on the buccal inclines of each experimental group models,simulating the mechanical effect of lateral movement.The angular loading points were: the buccal inclines of mesial and distal buccal cusps,the buccal incline of distal cusp,as well as the buccal inclines of mesial and distal lingual cusps.The stress peak and stress distribution cloud map of von Mise in dentin in two loads were calculated.In Abaqus software,forced rotational displacement load was applied to the lingual edge nodes of each experimental group restoration model.Making each restoration take a point on its buccal edge as the center of rotation and rotate counterclockwise by 0.05° to simulate rotational dislocation of restoration.Then the cohesive cohesion contact model of adhesive layer was set to simulate that adhesive damage occurred during restoration dislocating.Through calculation and analysis in Abaqus software,the non-axial retention force produced by restoration resisting rotational dislocation and stiffness degradation contours of adhesive layer were obtained.Results:1.The stress peak of von Mises stress in dentin restored by post-core crown,endocrown and inlaycrown was all lower than that in normal dentin.The von Mises stress peak results were as follow: inlaycrown>endocrown>post-core crown in vertical load and endocrown>inlaycrown>post-core crown in oblique load.2.According to the stress distribution cloud map,the stress maximum of post core crown in two loads were all located at the junction region of root middle 1/3 and apical 1/3,where contact with the post directly.The stress in dentin of the endocrown was concentrated at the pulp floor in vertical load and it was concentrated at the pulp floor,dental cervix and root apical 1/3 in oblique load.The dental cervix and root of inlaycrown could be observed stress concentrate obviously in two loads.3.In the group of 1mm residual crown,the non-axial retention force obtained by each restorations could be arranged in order: post-core crown > inlaycrown >endocrown,but in the groups of 2 and 3mm residual crown,it was inlaycrown>post-core crown>endocrown.4.According to stiffness degradation contours of resin adhesive layer,the cracked areas of adhesive during dislocation was inlaycrown>endocrown>post-core crown when each group restoration occurred the same angle rotation dislocating.Conclusion:1.The post-core crown,the endocrown and the inlaycrown all can be used for repairing low residual crown of molars,however,the tooth tissue fracture resistance of post-core crown is better than endocrown and inlaycrown.2.When repairing low residual crown molars,the non-axial retention force produced by the post-core crown and the inlaycrown to resist rotational dislocation was significantly greater than endocrown.And the cracked adhesive area of the post-core crown was smallest during dislocating.3.From the perspectives of protecting dental tissue and maintaining the stability of restorations,the post-core crown is an ideal restoration for repairing the lower residual crown of molars.