| Root canal therapy (RCT) is the main treatment for the pulposis and the periapical disease. The treatment divides into three major steps: the root canal preparation, the root canal disinfection and the root canal filling three major steps. A successful treatment is based on good root canal preparation. Currently, more and more clinicians tend to use large taper nickel titanium instruments in preparation of dental root canal system, because the high cutting efficiency of nickel-titanium instruments, good flexibility, better matching crown expansion and filling material makes the operation simpler. The taper of root canal crown increase can improve the efficiency of irrigation and filling, but makes the teeth become weaker. In order to investigate the effects of different taper of root canal on the tooth structure changes in the stress distribution and preparation with different instruments and repair, three-dimensional finite element method was adopted so as to provide a theoretical basis for clinical rational use of root canal preparation equipments.Objective and method:1. To reconstruct 3-dimension (3D) model of the human first mandibular molar for biomechanics analysis. The scanned Micro-CT images were input into Mimics v10.0 software and prefabricated the 3D model. Then, after improvement, in the Geomagic 10 software, the model was established.2. To reconstruct the 3-Dfinite element model of teeth with different root canal taper. Using Pro/Engineer Wildfire 4.0 software and the Workbench software, established the 3-D finite element model of teeth with different root canal taper and filling. The taper are 0.02, 0.06, and 0.1 in the models.3. Through finite element analysis software Workbench, the same central vertical load was applied to different models and stress distributions in dental roots were calculated.4. Stress analysis on the 3-D finite element model of different instruments in preparation of the mandibular first molar after filling.Result:1. The 3D model reconstructed by Micro-CT scan data objectively has a good similarity with the real teeth specimen .The surface and internal structure of the model in excellent shape to simulate the physical characteristics of the real teeth.2. Established 3-D finite element model of the first mandibular molar with different taper root canal after filling. Mesh after the 3-D finite element model of the teeth: The tooth of root canal taper is 0.02 has 53,992 tetrahedral elements and root system has 6971 tetrahedral elements. The tooth of root canal taper is0.06 has 55,030 tetrahedral elements and root system has 7705 tetrahedral elements. The tooth of root canal taper is 0.1has 55,917 tetrahedral elements and root system has 8179 tetrahedral elements.3. The dental root canal of different taper has different stress levels. Which root canal taper of the teeth models are 0.1 in this research suffered the greatest stress. In three different forms of dental anatomy models, the biggest stress of root canal orifice occurred in the tooth with double-root and three-root canal, to 14.93 Mpa. 0.06 taper of dental root canal are the second highest, 0.02 taper group are the smallest. With the increased of the taper of dental root canal, the stress of dental root canal wall increased.4. Although the stress level in three different forms of tooth anatomy model is different: the stress of root canal orifice in # 35 K file group is 11.68MPa , higher than that of Mtwo or ProTaper group; the stress of ProTaper group is the second highest; Mtwo group is the lowest. There is no more change in stress distribution in middle and apical of root canal. The stress values are much lower than ultimate compressive strength of dentin, indicating that the three commonly used root canal preparation methods are safe and feasible.
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