Biomechanical Analysis Of Repairing The Residual Root Of Mandibular First Molar With Different Post-core Systems

Purpose:To provide theoretical guidance for the choice of the clinical restoration methods for mandibular first molars without the ferrule,a residual root model was established in this study using a three-dimensional finite element method.Three cusp inclinations of the crown and three kinds of post and core materials were applied to the post-core crown restoration respectively.By comparing and analyzing the stress peaks and stress distributions of the remaining tooth tissues after repairing with different post and core systems,the adaptability of post and core materials to the inclination of the crown was evaluated.Methods:1.The angles of the proximal and distal tongue cusps of the established mandibular first molar model were set to 10°,20°,and 30° by UG NX software.The 3sets of models were considered as control groups.2.Establish the residual root model of the mandibular first molar without a ferrule whose edge is below the gingiva by CATIA V5R21 software.Then,the procedures of conventional root canal treatment and post preparation were simulated to establish the dental defect model after clinical treatment.Based on the control group models,the models of Co-Cr post and core,Ti post and core and fiber post resin core were established,each including three cusp inclinations.The 9 sets of models were considered as experimental groups.3.The control and experimental group were imported into ANSYS Workbench software.After meshing and setting boundary conditions,assumptions and material parameters,we mimicked the normal chewing behavior by loading a vertical static load of 200 N in total on the occlusal surface of each model.The loading site was at the midpoint of the lingual surface on the buccal cusp and buccal surface on thelingual cusp.The peaks and distributions of von Mises stress and maximum principal stress in the remaining tooth tissues after restoration in each group were compared and analyzed.Results:1.In this experiment,a model of mandibular first molar residual root repaired by different post and core systems was successfully constructed.The model structure is clear,accurate and clinically practical.2.The peak stresses of dental tissues after repairing with different post and core systems were lower than those of the control groups under the same cusp inclination and lower than the fracture strength of dental tissues.3.The stress peaks of the control group and the experimental groups both increased in varying degrees as the cusp inclination increased.Under the same conditions of post and core materials,the change of the stress distribution has certain regularity with the increase of cusp inclination.The stress distribution pattern of the Co-Cr group and the Ti group is similar-with the increase of the cusp inclination,the von Mises stress concentration area of the enamel tends to approach the mesiolingual region,while in the dentin to approach the mesiolingual side of the neck and apical area.Besides,the dentinal maximum principal stress is concentrated in the furcation area and have a tendency to concentrate in the apical area.When it comes to the fiber post resin core group,the von Mises stress concentration area of the enamel also tends to approach the mesiolingual region,and the maximum principal stress is concentrated in the mesial and distal regions of the lingual side.However,the dentinal von Mises stress tends to be concentrated in the neck and have a tendency to the middle and lower part of the root.The stress concentration area of the maximum principal stress is mainly located in the furcation area and the neck,and also tends to concentrate towards the middle and lower parts of the root.4.When the cusp inclination is at the same level,the peak stress of enamel from high to low is,fiber post resin core,Ti post and core,Co-Cr post and core;the peak stress of dentin from high to low is,Ti post and core,Co-Cr post and core,fiber post resin core.Conclusions:1.In this experiment,UG NX and CATIA V5R21 software were used to successfully construct the residual root model of the mandibular first molar without a ferrule.The stress analysis was performed by ANSYS Workbench software with good simulation.2.Co-Cr post and core,Ti post and core and fiber post resin core all can be used as restoration methods for the mandibular first molar without a ferrule,and the fiber post resin core provides more effective protection on the remaining dentin.3.No matter what kind of restoration material is selected,the risk of fracture in the tooth tissue after restoration increases along with the cusp inclination.It is recommended that the cusp inclination be appropriately reduced during clinical repair to improve the success rate.