SEM evaluation
Scanning electron microscopy of the fracture surface showed similar and typical torsional and cyclic fatigue features for all instruments tested. After the torsional test, the instruments showed concentric abrasion marks and tear-shaped marks at the center of rotation (Fig. 1A–C and a–c). In the cyclic fatigue test, all the files presented fractured surfaces with microcracks, with morphologic characteristics of a ductile fracture (Fig. 2A–C and a-c).
DISCUSSION
Several factors may have been responsible for the results obtained in this study. The instruments performance in mechanical tests are also influenced by the type of NiTi alloy used and the heat treatment used. Flat File files are featured with gold heat treatment that has an austenite finish temperature approximately at 50ºC [9] , while Logic files are featured with a controlled memory alloy (CM wire) that has an austenite finish temperature around 55ºC [10,11]. So, when the cyclic fatigue test is performed with a controlled temperature (37ºC) simulating a body temperature [12,13] ,CM Wire instruments present more martensite phase during the test than gold heat treatment. This austenitic finishing temperatures influence properties during root canal preparation at body temperature [14]. So far, with a crystalline structure predominantly martensite that allows the control of the shape memory of the instrument, being extremely flexible and more resistant to cyclic fatigue, for example, when compared to ”Gold” instruments [2] .These aspects justify the higher fatigue cyclic values presented by Logic instruments compared with Flat File.
Core mass and cross-sectional design also can influence the results of mechanical tests. A smaller metal volume is associated with more excellent cyclic fatigue resistance [15]. This fact could be explained when calculating the bending section modulus, the ratio of moment of inertia, and the distance from the neutral axis in the instrument center to the instrument surface and geometrical shape [14]. In short, flat cross-section designs such as S-Shape are associated with an increased cyclic fatigue resistance [15, 16]. Also, bigger cross-sections are associated with a stiffer instrument, with great flexural strength and torsional resistance [17]. Logic instruments have a double helix cross-section, while Flat File instruments have a modified S-section. However, the reduction of the metallic mass of Flat File instruments to generate only one side with active spirals can negatively influence the cyclic fatigue values compared to Logic instruments. Besides, the manufacturing process (surface treatment) of Flat File instruments to provide a flat surface may also influence their structural characteristics, generating irregularities in this portion [18]. Possible microcracks can lead to instrument fractures under clinical or laboratory stress, especially in curvatures when the alloy is subjected to tension and compression [19].
Higher cyclic fatigue values on both kinematics (continuous and reciprocating) for the Logic 25.03 compared to the Logic 25.05 can be justified by the taper of the instruments. Instruments with the same heat treatment and structural characteristics but high taper will present lower resistance to cyclic fatigue than those instruments with low taper [20]. The static cyclic fatigue test was performed as described in previous studies [8,21]. In this model, the instruments are mounted in a stabilized handpiece that provides a free rotation in an artificial canal under specific conditions until the instruments fracture [22]. This condition leads the NiTi instruments to the maximum stress, mainly because they remain in a static position inside the simulated canal, generating areas of stress concentration (tension and compression) of the alloy in a specific region of the instrument. This method reduces some biases and induces less localized mechanical stress, increasing the time and number of cycles to fatigue [17,23].
Cyclic fatigue occurs through the formation of microcracks in surface irregularities that extend through the instrument during cycles until the fracture occurs [24]. Studies showed that reciprocating motion increases resistance to cyclic fatigue compared to continuous motion [25-27]. During reciprocating motion, the instrument disengages from the canal walls at the moment of higher tension, delaying the formation of microcracks on the surface of the endodontic instrument [28]. The use of rotary instruments in conventional reciprocating motion (150º reverse, 30º forward) had not been encouraged because their spirals were designed to cut in a clockwise direction. However, with the development of programable motors that allow modifications of the angle and direction of rotation, reciprocating kinematics also can be employed for rotary instruments (150º forward, 30º reverse) [29]. These programmable endodontic motors allow reciprocating motion to both sides (i.g. clockwise or counterclockwise)[29].
After the torsional resistance test, the Flat File 24.04 and Logic 25.05 showed higher values than the Logic 25.03 instrument (P < .05). This finding could be explained because it may be associated with its greater taper than the 0.3 instruments, which results in more torsional resistance than instruments with smaller tapers [14,17]. Higher values of torsional resistance are associated with instruments with greater mass, either as a function of their cross-section, tip diameter, or taper [30]. Also, torsional stiffness could be increased by files with reduced pitch and increased cross-sectional areas. For example, rectangular cross-sections had superior torsional stiffness compared to triangular cross-sections [31]. Specific cross-sectional configurations are more prone to failure by torsional overload than others by the principle of mechanics when an instrument with a small core diameter is more susceptible to failure by torsional overload [4]. The investigation of this property is essential to estimate the torque value necessary for the instrument to fracture inside the simulated canal contributing to the possibility of working with these instruments at a torque value lower than that necessary to cause a fracture.
Scanning electron microscopic analysis showed a typical fractographic appearance of cyclic fatigue and torsional fracture, presenting similar images among the three analyzed instruments. The fragments showed crack initiation areas on the cyclic fatigue test image, with numerous dimples on the fractured surface. After the torsional test, the instruments demonstrated the typical features of shear failure, including concentric tear marks and fibrous microscopic dimples at the center of rotation [21,32,33].
Finally, angular deflection makes it possible to identify how much the instrument can deform when attached to the root canal prior to its fracture by torsion. The angular deflection values obtained in this study differed for all instruments (P < .05). Flat File showed the lowest angular deflection values. This result can be attributed to its gold heat treatment since the CM treatment present in Logic files, provides greater flexibility and ductility [2,5]. In addition, Flat File instruments have a modified S section that is more irregular than the double helix section of Logic instruments. Asymmetrical cross-sections such as Flat File instruments generate worse stress distribution when used, making instruments of this type more susceptible to fracture [30].
CONCLUSIONS
Geometric characteristics, Ni-Ti alloy, heat treatment and manufacturing process influence the mechanical properties of the Ni-Ti instruments. Flat File 25.04 instrument presented a good resistance to cyclic fatigue, torsional loads, and angular deflection. Reciprocating motion improved the cyclic fatigue resistance of Flat File instrument and can be considered when using programmable endodontic motors.
ACKNOWLEDGMENTS
The authors deny any financial support and conflicts of interested related to this study.