The Three-Dimension Finite Element Analysis Of Stress In Maxillary First Molar Residual Root Restored With Post-Core Crown

With the extensive development of root canal therapy, most of teethwith all kinds of serious teeth defect, including all kinds of residual crownand residual root, can be reserved through post-core crown restoration.However, the teeth restored with post-core crown are more prone to teethfracture than healthy teeth. Once root facture happens in teeth restored withpost-core crown, the result is usually disastrous. The teeth will finally beextracted and the subsequent prosthetic treatment will become moredifficult. For this reason, with regard to teeth with serious teeth defect, themost important consideration when choosing post-core crown restorationafter root canal therapy, is how to increase frature resistance. Whendesigning post-core crown restoration, the clinic prosthetists should chooserational design and material to make stress distribute uniformly anddecrease stress concentration. This is very important for increasing fractureresistance and prolonging useful life of the restoration.The research about the stress magnitude and distribution of post-corecrown restoration are always the hotspot of oral prosthesis and oralbiomechanics. In the stress analysis of anterior teeth post-core crownrestoration, multiple factors including post-core length, post-cors diameter,post-core material, post-core bonding material and occlusal forcemagnitude and direction are found to be influential to stress magnitude anddistribution on remaining dentin. The external form and root canal structureof molar is more complex than anterior teeth. It is always difficult to build up finite element model of molar post-core crown restoration with highgeometric similarity because it has small volume, numerous hierarchicalstructure and complicated external form. Thus the biomechanic researchabout molar post-core crown restoration is rarely reported at home andabroad. In the recent years, along with rapid development of computertechenology, a new modeling method which builds up finite element modeldirectly using DICOM data from CT or MRI, is used extensively. Usingthis modeling method, finite element model of molar post-core crownrestoration with high geometric similarity can be built up and it make thestress analysis of molar post-core crown restoration possible.In this study, we build up finite element model of maxillary first molarresidual root post-core crown restoration through CT scanning and Mimicsand Abaqus dispose. Using this model we make stress analysis of maxillaryfirst molar residual root post-core crown restoration on condition ofdifferent post-core design, different post-core material, different post-corebonding material and different occlusal force direction. The three kinds ofpeak stress(Von Mises stress, most principle stress and shear stress) andstress distribution of different conditions are explored. The results of ourstudy will provide biomechanic basis for clinic prosthetists to chooserational maxillary molar post-core design, post-core material, post-corebonding material and crown form.The results of this study are as follows.1. As the number and length of posts changes, the peak stress onremaining dentin changes irregularly and the Von Mises peak stress locateson the shoulder of palatal root and mesialbuccal root. There is nosignificant difference of peak stress on post-core.2. As elastic modulus of the post-core material increases, the peakstress on remaining dentin decreases slightly, while the peak stress on post-core increases gradually and high stress area at the end point of palatalroot post increases evidently.3. As elastic modulus of the post-core bonding material increases, thepeak stress on remaining dentin increases slightly, and the peak stress onbonding layer increases evidently. The bonding layer near palatal cuspid,mesialbuccal cuspid and mesialbuccal root are high stress area.4. As the angle between occlusal force and teeth long axis increases,the peak stress on remaining dentin increases evidently and high stress areaat palatal side of palatal root and mesial side of mesialbuccal rootnear cervical region increases greatly. The peak stress on post-coreincreases slightly.5. Although the post-core design, post-core material, post-corebonding material and occlusal force direction all have influence on themagnitude and distribution of peak stress on remaining dentin, theinfluence of occlusal force direction is most apparent.According to these results, the suggestions of this study are as follows.1. On condition of normal occlusal force magnitude and verticalloading direction, the incidence of root fracture or post-core fracture ofmolar post-core crown restoration is very small regardless the post-coredesign, post-core material and post-core bonding material.2. In clinic practice, we may increase the number and length of postsof maxillary molar residual root post-core crown restoration to increaseretention force provided by post.3. When zinc-phosphate is used as bonding material for maxillarymolar residual root post-core crown restoration, the restoration is verylikely to fall off and fail. The resin luting agent may be a better choice forthe molar residual root post-core crown restoration in clinical practice.4. As the angle between occlusal force and teeth long axis increases, the possibility of teeth fracture will increase. In clinic practice, the cuspidinclination of the crown should be decreased as possible as we can todiminish the frature risk.Of course, this study is only the static stress analysis for threedimensional finite element model of the post-core crown restoration ofmaxillary first molar residual root. In clinic practice, the occlusal force willchange and repeat continually and this will diminish the teeth fractureresistance greatly. On the other hand, all the result of this study is obtainedon the premise that the remaining dentin and post-core will not fracture andthe bonding material will not lose retension or break after loading. Onceany part of these structures is damaged, the stress of the whole restorationwill redistributed, and lead to more serious results. Therefore, when makethe post-core crown restoration of molar residual root, clinic prosthetistshould comprehensively consider the results of some clinic study and stressanalysis, to provide most rational treatment plan for patients.