| With the development of medical technology and the continuous improvement of dental health care awareness,more and more serious teeth defect,roots and crowns in clinical work have been preserved and treated.Post-core crown restoration system has become one of the most important methods for the restoration of large-area tooth defects.It is mostly applied to perfect root canal therapy and cannot provide good retention.However,root fracture ofen occurs after post-core crown restoration.There are many reasons for root fracture,such as the types of post and core,the design of post and core,improper clinical operation,and so on.Therefore,in clinical choosing post-core crown restoration system,suitable post-core materials and reasonable post-core restoration design are of great clinical significance to reduce stress concentration of affected teeth and improve fracture resistance of teeth with tooth defect.After post-core crown restoration of teeth with tooth defects after root canal therapy,the stress magnitude and distribution of residual tooth tissue during functional movements such as chewing have been the focus of oral biomechanics research.At present,most studies have shown that the elastic modulus of fiber posts is closer to dentin than that of traditional cast metal posts,and it has strong buffering effect under sterss.Moreover,the clinical operation of the fiber post is simple,and the number of visits of patiens is reduced,so the fiber post is widely used in clinic.However,there are relatively single or less groups in the research on the diameter and length of the fiber post in the related research on the fracture resistance of finished fiber post for mandibular premolar restoration at current domestic and foreign.In this study,the length and diameter of the post are divided into experimental groups in detail.The finite element analysis method is used to systematically analyze the residual tooth structure of the affected tooth and the stress distribution at the interface between the post and dentin.Molars are used to achieve the best long-term effect,realize personalized treatment and reduce the occurrence of root fracture.ObjectivesBased on the influence of the length and diameter of finished fiber post on the fracture resistance of mandibular premolars after restoration by finite element analysis method,the optimal length and diameter design of finished fiber post for mandibular first premolar after restoration is obtained through fine analysis of residual tooth tissue and stress distribution law of post-dentin interface after restoration of finished fiber post with different lengths and diameters,which provides theoretical basis and guidance for the selection and design of the clinical fiber post-core restoration,with a view to providing a theoretical basis for personalized treatment.MethodsFirstly,one complete mandibular first premolar,which was extracted due to orthodontic and met Wang Huiyun's measurement average of Chinese teeth,was selected to complete root canal therapy in vitro.After retaining 2mm crown tissue,the remaining crown tissue was cut off and post canal preparation,resin core repair and full crown preparation were performed.The experimental teeth were embedded and fixed with epoxy resin and scanned with CBCT data to obtain three-dimensional dynamic images of the teeth.Secondly,CBCT scan data of mandibular first premolar is imported into Mimics software in DICOM format.In the Mimics software,the 3D model is generated through the selection and delineation of the thresholds in the software.Then the model is imported into Geomagic software in STL format,and finally a relatively smooth finite element model is obtained by fitting the curved surface.The smoothed model was imported into Abaqus in IGES format,and fiber posts with different lengths and diameters,alveolar bone and 0.2mm periodontal ligament were established in Abaqus software.Finally,all parts of the model were meshed to form finite element models including all-ceramic crown,finished fiber post,0.2mm periodontal ligament,alveolar bone,etc.The properties of each part of the experimental model are defined as continuous,homogeneous and isotropic.The length and diameter of fiber post were grouped,and the stress distribution of residual tooth tissue and the interface between the post and dentin was observed and studied.The stress loading method is static loading.The loading point is located at the lingual inclined plane of the functional tip of the mandibular first premolar.The loading force is 150N,and the loading is performed at an angle of 45°to the long axis of the tooth to simulate chewing force.The Von Mises stress peak value and stress distribution at the interface between root and the post and dentin were observed and compared.ResultA three-dimensional finite element model of mandibular first premolar post and crown restoration was established.The post-core crown model includes all-ceramic crowns,fiber posts,resin cores,gutta percha point,roots and other parts.Alveolar bone is a 30mm~3cube,including cortical bone and cancellous bone.The outer layer of parodontium is 2mm cancellous bone,and the rest is cortical bone.The height of the alveolar crest is 2mm below the enamel dentin boundary.For periodontal ligament,A3D model for restoration of mandibular first premolar with post crown of different lengths and diameters has been accurately established with a thickness of 0.2mm parodontium.When the same length of fiber post is used to repair mandibular first premolar:As the increase of fiber post diameter,the Von Mises peak stress of root shows not obvious changes.The diameter of fiber post is 1.4mm,which is about 1/3 of the root diameter width and Von Mises stress peak value of root is the lowest.Under the same diameter:As the increase of the length of the fiber post,the Von mises peak stress of the root shows a trend of increaseing first,then decreasesing,and finally stabilizing.The length is 12-15mm,about 3/5-4/5 of root length,and Von Mises stress peak value of root is the smallest.Fiber post with different lengths and diameters are used to repair mandibular first premolar,the stress distribution of residual tooth tissue is basically the same.Von Mises stress peak at the 1/3 shoulder of buccal-lingual neck is relatively high and concentrated.The stress distribution at the interface between the post and the dentin is relatively uniform.As the increase of the lengths and diameters of fiber post,the Von Mises stress peak value of the fiber pile increases continuously,but the Von Mises stress peak value of pile is obviously lower than root.And the stress is mainly concentrated at the bottom of the pile.Conclusion1.After the restoration of the fiber post and core crown,the stress distribution at the interface between the post and dentin of the affected tooth is relatively uniform,which can significantly reduce the occurrence of root fractures.2.Von Mises stress is mainly concentrated in the neck 1/3 of the buccal and lingual side of the root,so it is in the post-core crown,during the preparation process,keep as much of the buccal and lingual tissue as possible.3.The first premolar of mandibular defect restoration with fiber post repair canals,the optimal diameter is 1.4mm,which is about 1/3 of the root neck width.The optimal length is 12-15mm,and the root length is 3/5-4/5,On the basis of ensuring 4mm closure,the length of the post can be increased appropriately. |
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