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.