Cyclic flexural fatigue resistance of NiTi Controlled Memory and Blue Technology instruments after torsional preloading

Abstract Objective The aim of this study was to evaluate the influence of torsional preloading on the cyclic flexural fatigue resistance of thermally treated NiTi instruments. Material and Methods Ten new instruments New Hyflex CM (HF 30.06; Coltene/Whaladent Inc.), Typhoon CM (TYP 30.06; Clinician's Choice Dental Products) and Vortex Blue (VB 30.06; Dentsply Tulsa Dental) were chosen, based on geometry and specific characteristics of the manufacturing process. The new instruments of each system were tested in a bench device to determine their fatigue resistance through mean value of number of cycles to failure (Nf) (Control Group – CG). Another group of 10 new HF, TYP and VB instruments were submitted to 20 cycles of torsional straining between 0° and 180° (Experimental Group – EG) and then submitted to fatigue until rupture under the same conditions of the CG. Tested instruments were examined by scanning electron microscopy (SEM). Data were analyzed using one-way analysis of variance and post hoc Tukey’s test (α=.05). Results Higher fatigue resistance was accomplished by HF instruments, followed by VB and TYP (p<0.05). During the torsional preloading, the lowest mean torque value was observed for TYP instruments (p<0.05). The torsional preload caused a significant reduction in the Nf values (p<0.05) of about 20%, 39% and 45% for instruments HF, VB and TYP, respectively. Longitudinal cracks, generated during the torsional preloading, were present in VB files, but were not observed in the CM instruments (HF and TYP). Conclusions In conclusion, the flexural fatigue resistance of thermally treated instruments is diminished after cyclic torsional loading.


Introduction
Thermomechanical manufacturing technologies have been developed to improve the mechanical properties of NiTi endodontic instruments. This approach allows adjusting transition temperatures of the NiTi alloy, aiming to control its microstructure and thus influence the mechanical behavior of the instruments. 1 The final goal of these specific thermal treatments is to produce instruments with higher flexibility and increased resistance to cyclic flexural fatigue. 2 1,2,7,8 In the clinical practice, fractures may happen under many circumstances, being more common during shaping curved and/or narrow root canals.  The selected geometry placed the maximum curvature at 3 mm from the tip. 15 The files rotated freely inside the canal until rupture, and the Nf was

Results
The mean values of the diameter at three     It has been reported in the literature that CM instruments contain a great amount of B19` and R-phase at room temperature. 1,14 Instruments containing large amounts of martensite can be easily deformed, yet they will recover their shape on heating above the transformation temperature. In addition, NiTi alloys present remarkable flexural fatigue resistance in the martensitic state. 13,17 Although VB instruments are not claimed to be produced with the CM technology, they also exhibit partial characteristic of shape memory at room temperature. 2,8 The cyclic flexural fatigue resistance of an instrument is determined by its Nf values. As mentioned before, instrument diameter (D) and radius of curvature of the root canal (R) have been identified as the most important factors controlling fatigue resistance of endodontic instruments. 15,[18][19][20] The critical parameter is the maximum tensile strain amplitude, ε T , which is given by the expression ε T =D/ (2R-D), when the canal radius is measured at the outer canal wall, as was done in this study. Considering the mean diameters at 3 mm from the tip (D3) of the instruments were the same as the artificial canal geometric standards, the differences in fatigue resistance of the instruments cannot be attributed only by the geometry, being the particularities in manufacturing process an influential factor. According loading. The load is higher whenever the dentine is hard and the canal diameter is small. Acting on the instrument surface, this torsional load can prevent its rotation to a greater or lesser extent. Although this is the principle by which dentine can be cut, in extreme cases, when the resistance is so high that it constrains the instrument, it may fracture. 21 As it was appreciated, VB instruments exhibited the highest torsional resistance in comparison with the other instruments, probably explained by a larger geometric factor such as A3 value.
However, only a few reports assessing these factors at the same time are available for CM instruments. [25][26][27] In this study, a specific torsional loading cycle was previously applied to rotary instruments before their final rupture through cyclic flexural fatigue testing. The number of cycles employed in the load-unload tests was assumed to be roughly equivalent to the use of the instrument in half of the recommended number of uses for rotary instruments. 12 Even though thermally treated files usually display a significantly higher cyclic flexural fatigue resistance than SE instruments, torsional preloads were able to significantly decrease the cyclic flexural fatigue resistance of the analyzed files. 2  previously in superelastic instruments. 12,28 but not in VB instruments. It is noteworthy that these longitudinal cracks did not interfere with the fatigue resistance of VB instruments, because the average decrease in Nf recorded for them (39%) was smaller than that geometrically similar to TYP instruments (45%), which did not develop longitudinal cracks. On the other hand, VB instruments showed a shape recover larger than the CM instruments after torsional loading, indicating that these instruments still conserve some characteristics of the conventional superelastic NiTi.

Conclusions
Within the limitations of the study, the conclusion is that torsional preloading decreases the resistance to cyclic flexural fatigue of thermal treated instruments.
In addition, Vortex Blue instruments exhibited some controlled memory behavior, displaying intermediate characteristics among CM instruments.