Analyze The Influence Of Elastic Modulus Of Posts On Root's Stress Distribution By PSA

Tooth root fold is the most severe complication after post-core-crown restoration, which is mostly caused by stress concentration, stress fatigue or sudden violence. Usually it's related to the root condition,abutment's preparation situation,post-core format,post-core material and etc. With the development of material science, there are many different post–core materials. In many scholars'opinion, dowel elastic modulus is closely related to the stress distribution of the root . So how to choose the suitable material of post-core becomes a big concern of the clinical workers. Some scholars analyzed the influence of different elastic moduli of posts on root's stress distribution by three dimentional finite element method, and it shows that the stress would tend to distribute on the root tip with the increase of elastic modulus of posts , which would lead to the fracture of the tooth root tip. This experiment method is a computational methodology, so its result is based on the computation and analysis of mathematic model, which requires the verification of other biomechanical analysis method. So this experiment plans to analyze the influence of elastic modulus of posts on root stress distribution by PSA, compare the result with 3-D FEM, and provide experimental foundation for the choice of posts in clinic.DesignAccording to the magnitude of elastic modulus, this experiment classifies the commonly-used posts into 3 different group:①Hypo-elastic-modulus posts: various kinds of fiber- resin posts (E≦5 0GPa);②moderate-elastic-modulus posts: titanium post, titanium alloy post, gold post, gold alloy post and so on. (50GPa﹤E≦ 150GPa);③homo-elastic-modulus posts: nickel-chromium alloy post, cobalt-chromium alloy, various kinds of ceramic posts. (E﹥150GPa)Relyx fiber post,cast titanium post and cast nickel-chromium alloy post are selected to represent the groups respectively. Analyze the characteristics of stress distribution of root which restored with the three posts above-mentioned by PSA, and then induce the change tendency of root stress when the post elastic modulus changes from low to high.①foundation of PSA models of upper mandible central incisor restored by Relyx fiber post,cast titanium post and cast nickel-chromium alloy post.②standardization of the stress-optic coefficient of epoxy resin③a nalysis of the influence of elastic modulus of posts on root stress distribution by PSA.Result1.In the PSA model of maxillary central incisor restored by Relyx fiber post,cast titanium post and cast nickel-chromium alloy post (annexal figure -1),the ratio of root dim to actual size is 1:1,and the ratio of elastic modulus of root,alveolar bone and periodontal membrane to the actual sizes is close to 1.35:1:0.0003. The model's structure and the ratio of elastic modulus meet the requirement of resemblance ,so the stress situation of actual assembly unit can be calculated by resemblance principle Exert 100N ,45°labial direction load on the PSA models , and undertake stress-freezing,cutting sheet and observation ,then obtain the stress fringe figure (annexal figure-2).According to annexal figure-2, we can see the limpid stress fringes appear in the epoxy resin root, and the the periradicular alveolar bone, and the color of stress fringe changes regularly. So the models prepared in the experiment are usable and they establish the foundation for the sequential stress analysis experiment.2. By means of stress-optic-coefficient -standardization test piece that has theory solution, the stress-optic coefficient of epoxy resin was measured by stretching method as f=12.28N/mm.3.Under the situation of 100N ,45°labial direction load, the stress fringe figures of each group show that :①In the Relyx fiber post group ,the stress fringes concentrate mainly in the labial and lingual part of dental cervix, and the color of fringe is mainly in higher order , but the root tip of tooth is contrary to it.②In the cast titanium post group and the cast nickel-chromium alloy post group, the stress fringes concentrate mainly in the labial part of dental cervix ,and the fringe color also ranks the highest in this part, then higher in the root tip of tooth .4. Under the situation of 100N ,45°labial direction load, compare the stress value(Tab-1) of each observation site (figure-1),it shows that :①the stress value of labial observational sites in dental cervix and root tip is higher than the one in lingual part②the highest stress value point is in the labial part of dental cervix ,while the lowest one is in the lingual part of root tip of tooth③while comparing the stress value of the same observational site of each group ,it is clear that ,especially in the root tip ,Nr-Cr posts'﹥Ti posts'﹥Relyx fiber posts.Conclusion:①the PSA model of maxillary central incisor restored by Relyx fiber post,cast titanium post and cast nickel-chromium alloy post have make a simulation of real situation .②the stress value of labial part of dental cervix and root tip is higher than the one of lingual, which corresponds to the clinic report that the fold of root of tooth after post-core restoration occurs more often in labial part;③the stress value of each observation site increases with the posts'elastic modulus, which can be explained by the mechanics principle that stress transfers form the high elastic modulus material to the low one, Scilicet it can Be comprehended that Relyx fiber post absorbs part of energy under bitingcondition for its low elastic modulus, so the stress value in the root of tooth is lower than the one of Nr-Cr post which has higher elastic modulus. Respecting root tip of tooth in anatomy is the weak part and abutment preparation impair its cervical part , while the result demonstrates the stress concentrate in them , and in order to avoid the fold of root of tooth , we should choose the suitable post which can absorb shock energy,decrease the conduction of stress with low elastic modulus . Fiber post meets such requirement, and there are other advantages. For example, it can be stripped after folding which is good for the salvage of root; it has a favourable aesthetic effect, and it could decrease the patient's visit frequency, and is very convenient to operate. So it's recommended to apply fiber post in clinic.