A Computational Analysis Of Bending And Torsional Properties Of Three Nickel-Titanium Endodontic Instruments With Different Cross-sections

BackgroundThe superelasticity of the material coupled with the design of the blades allows the Nickel-Titanium endodontic instruments to be used in continuous rotation, even in curved root canals, to produce a desirable, tapered root canal form, with a low risk of transporting the original canal lumen. However, there is a general perception of a potential risk of instruments fracture. In recent years, many publications have been devoted to study the factors influencing the fracture of Nickel-Titanium endodontic instruments and to compare bending and torsional properties, cyclic fatigue resistance of different Nickel-Titanium instruments in vivo or in vitro. Although these experiments are essential, computational studies may add important information and theory support. Indeed, numeric analyses allow us to quantify parameters responsible for the instruments'failure as, for example, maximum stress and strain, which are difficult to measure in vivo or in vitro, and to easily compare the strain and stress levels by simply changing the model boundary and load conditions, removing the operator dependency factor. There are many factors which are related to Nickel-Titanium endodontic instruments fracture.Many studies have proved that the cross-sectional designs greatly affect the mechanical behaviors of Nickel-Titanium endodontic instruments and affect the probality of files fracture as well. In order to compare the bending and torsional properties of three Nickel-Titanium endodontic instruments of different cross-sections, and to provide reference evidences for clinical applications, the following researches are designed and conducted using the finite element analysis on the basis of biomechanics.ObjectivesThe experiment combines the Micro-CT scanning technology with Hyperworks 10.0,Pro/E 3.0,ANSYS 12.1 to construct the three-dimensional(3D) solid and finite element model(FEM)of three Nickel-Titanium endodontic instruments of different cross-sections, geometry and 3DFEM of eight curved canals. The stress that the instrument (i.e. ProTaper F3) undergoes during insertion in eight curved canals is studied to evaluate the effects of the curvature parameters on the stress state generated in the instrument to demonstrate the usefulness of the FE method in improving the knowledge of the mechanical behaviour of these instruments during root canal preparation and to provide the most critical and the best operating curved canals for the following studies. The stress that the three instruments undergo in bending and during insertion in the most critical curved canal is studied to compare the bending behavior. The stress that the three instruments undergo in sudden torsion at the end of the insertion step of the best operating curved canal is studied to compare the torsional behavior. Therefore, the influence of cross-section profile on the mechanical behaviour with different Nickel-Titanium instruments is studied to provide reference evidences for clinical selection and applications. Materials and MethodologiesChapter I Establishment of 3D solid and FEM of the three Nickel-Titanium endodontic instruments with different cross-sections, geometry and FEM of eight curved canalsPart I Establishment of 3D solid and FEM of the three Nickel-Titanium endodontic instruments with different cross-sectionsProFile.06/#30,ProTaper F3,ProTaper Universal F3 are selected and scanned at 14um intervals in a micro-CT scanner. Then, the outline of the instruments is imported into Hyperworks 10.0 in the format of IGES. Finally, a mesh of linear, tetrahedron elements is overlaid onto the rendered 3D image with the element type of solid 185. This numerical model of each instrument is entered into ANSYS 12.1 in the format of cdb and the nonlinear, stress-strain behavior of Nickel-Titanium material is entered.Part II Establishment of geometry and FEM of eight curved canalsThe complex curved root canal is simplified to a solid canal composed of a line segment and a curved segment which is discriminated by the angle, the radius and the position. The geometry models of eight canals which are supposed of smooth linear elasticity dentine, with the whole length of 12.9mm, thickness of 0.1mm, and 0.02mm wider than the diameter of the file(i.e. ProTaper F3) are constructed in Pro/E 3.0. Then, the outline of the canals is imported into Hyperworks 10.0 in the format of IGES. Finally, a mesh of linear, hexahedral elements is overlaid onto the rendered 3D image with the element type of solid 185. This numerical model of each canal is entered into ANSYS 12.1 in the format of cdb and the geometry and material parameters are entered.Chapter II The effects of the curvature parameters on the stress state generated in the instruments Take ProTaper F3 for example. The modelled ProTaper F3 is moved into the root canal until the apex is reached. The maximum von-mises stress(Smax) that the instrument undergoes at the apex of each canal is studied to evaluate the effects of the curvature parameters on the stress state generated in the instrument and to provide the most critical and the best operating curved canals for the following studies.ChapterⅢA computational analysis of bending and torsional properties of three Nickel-Titanium endodontic instruments with different cross-sectionsPart I A computational analysis of bending properties of three Nickel-Titanium endodontic instruments with different cross-sections1,The tip of the file is deflected for a distance of 3.5mm with the shaft rigidly held in the cantilever place. The von Mises stress distribution Smax and bending moment are examined.2,The modelled instrumentis moved into the most critical curved canal until the apexis reached. The von Mises stress distribution and Smax are examined.Part II A computational analysis of torsional properties of three Nickel-Titanium endodontic instruments with different cross-sections1,A 2.5Nmm moment of force is applied to the shaft in a clockwise direction normal to the long axis of the instrument, whilst 3mm of the tip is rigidly constrained. The von Mises stress distribution,Smax and angular deflection are examined.2,The instrument is inserted into the best operating curved canal up to 1 mm from the apex, and a counter-clockwise torque (2.5Nmm) is imposed to the shaft once the tip is blocked. The von Mises stress distribution and Smax are examined.Results1,3D solid and FEM of the three Nickel-Titanium endodontic instruments with different cross-sections, geometry and FEM of eight curved canals were established successfully.2,The stress distribution indicated:the greater angle, the smaller radius and the bigger instrument resulted in the lower stress level. The radius of curvature was the primary curvature parameter influencing the stress level of the instrument.3,The stress distribution indicated:the U-like triangle ProFile showed the lowest bending stress, while the convex-triangle ProTaper showed the lowest torsional stress. The Concentration of stresses were mainly observed at the bottom of the flutes in ProFile and the cutting edge in ProTaper and ProTaper Universal during cantilever bending tests while at the bottom of the flutes in ProFile and ProTaper Universal and convex in ProTaper during torsion tests.Conclusions1,It confirmed the usefulness of FEA method in improving the knowledge of the mechanical behaviour of Nickel-Titanium endodontic instruments in different boundary and load conditions.2,The curvature parameters greatly influence the stress level of the instrument in curved canals. The most demanding working conditions were observed in canals with sharp curves, especially in areas where the instruments had larger diameters. To prevent possible damage to instruments and fracture, it is advised that the instruments should be discarded following their use in such canals.3,The cross-sectional profile has a significant influence on the mechanical behavior of Nickel-Titanium endodontic instruments. ProTaper and ProTaper Universal might be more suitable for preparing the constricted canals, while ProFile might be more suitable for use in the severely curved canals. The use in the constricted canals is not recommended for ProFile. Incorporating a U-shaped groove design increases the flexibility but slightly decreases the rigidity of ProTaper Universal.