This study aims to analyze the mechanical behavior of two types of rotating endodontic instruments with diferent cross sections, both made by Coltene (Hyflex CM 40.04 of NiTi alloy and Hyflex EDM 40.04 of Heat-treated NiTi alloy), under cyclic fatigue or torsion. The study of the mechanical behavior combined the traditional approach, consisting in experimental investigations, with a finite element analysis under similar bending and torque conditions. The non-linear superelastic behavior of the NiTi alloy for fatigue and torsion, derived from experimental tests based on models in the scientific literature, was applied to the numerical analysis. A thermographic investigation was associated with the usual mechanical experimental tests to assess the dependence of the temperature on the deformations undergone by the endodontic files under stress, especially near the breakage. The comparison between experimental and numerical results was in good agreement for both types of files analyzed.
cycles.
Instead, the torsional failure is characterized by a
In endodontic therapy, rotary files made of nickel-tita- maximum torsional load and a critical angle of
rotanium alloy (NiTi) are widely used when processing tion. This reveals the capability of the file to twist
bethe root canal. This is due to the superelasticity and fore fracture [
Therefore, the NiTi alloy is able to recover its orig- ing the mechanical behavior of endodontic files: the
inal shape under a range of high deformations. How- Finite Element Analysis (FEA) [
Cyclic fatigue test results: mean ± SD of static bending force; number of rotations, dynamic bending and temperature of
the hotspot in middle of curvature at breakage. Courtesy of the authors [
Instrument
Angle of breakage
Torque at breakage
Time to breakage
Temperature of hotspot at breakage EDM 40.04 CM 40.04 376.32 ± 20.25 1.080 ± 0.052 31.365 ± 0.442 32.7 ± 1.3 241.44 ± 12.15 1.364 ± 0.29 20.127 ± 0.123 31.4 ± 0.2 Diferent superscript letters in the same column show statistically significant diference ( > 0.05) [deg] computing was adopted for both the two types of excitation. The geometrical shape of both rotary file types was replicated in CAD environment starting from microtomographic detections on relative commercial products.
Meshes of the CAD models were made according to the Voronoi-Delaunay algorithm with 16 Jacobian points per element.
analysis was implemented setting up a four-event cyclic calculation starting from the bending analysis previously described. The fatigue analysis leaded back to the meshes relative to the bending analysis. Moreover, each event was set as an impulsive load (R = 0) and and 8.841 for EDM 40.04, as a confirmation of the good imposed on the mesh model. The number of cycles re- quality of the models. Boundary conditions were apported agreements with the experimental tests. In fact, plied with a fixed constraint on the cutter surfaces relthe damage diagrams for both the models indicated a ative to the grip (5 mm from the file tip) and a hinge nodal distribution at 100% of material damage at 1,490 joint constraint to the file stem. Remote time-dependent cycles for CM 40.04 and 1,840 cycles for EDM 40.04, displacements through rigid beams were applied to the respectively (Fig. 9). last cross-section of the stem, at the drive connection, in order to simulate the torsion deformation. These displacements were consisted in a revolution of the 5. FEA on cyclic fatigue tested ultimate stem cross- section relative to the rotating samples driver around the file axis. Deformations were progressively applied with a linear time dependence to The mesh of CM 40.04 counted 42,897 nodes and 27,163 ensure a rotational velocity simulation of 2 rpm for tetrahedral elements (element size of 0.12 ± 0.005 mm), both the models. Results confirmed compliance with while that of EDM 40.04 consisted of 42,336 nodes and the model of the material constitutive curve. Simu26,809 tetrahedral elements (element size of 0.15 ± 0.01 lated time-to- failure durations returned strain values mm). The maximum Jacobian ratio was 4.98 for CM of 22.11 s for CM 40.04 and 30.55 s for EDM 40.04, rela40.04 and 7.21 for EDM 40.04, respectively. The max- tive to failure angles equal to 265.32 for CM 40.04 and imum aspect ratio returned was 10.306 for CM 40.04 366.624 EDM 40.04, respectively. It has to be noted how the breakage location was well predictable and recognized on both the models (Fig. 10). Analysis returned that the most stressed zone was computed at 0.93 mm from the grip for CM 40.04 and at 0.29 mm from the grip for EDM 40.04. Moreover, stress-strain curves relative to the most stressed element were extrapolated from the FEA. Figures 11a,b show the simulated phase transformation for rotation and strain of the most stressed element, respectively. It is clear that the failure occurs in the martensite phase for both the models.
tests on two diferent rotary endodontic files were performed in order to analyse both the cyclic fatigue and torsional behaviour. Simultaneous thermographic investigation was a helpful element of validation of results.