Cyclic Fatigue Testing of NiTi Rotary Instruments Review

Review Article
A Review of Cyclic Fatigue Testing of Nickel-Titanium Rotary
Instruments
Gianluca Plotino, DDS, PhD,* Nicola M. Grande, DDS, PhD,* Massimo Cordaro, MD, DDS,†
Luca Testarelli, DDS,* and Gianluca Gambarini, MD, DDS†
Abstract
Introduction: Fractured rotary nickel-titanium (NiTi)
instruments have been classified into those that fail as
a result of cyclic flexural fatigue or torsional failure or
a combination of both. Clinically, NiTi rotary instruments
are subjected to both torsional load and cyclic fatigue,
and ongoing research aims to clarify the relative contributions of both factors to instrument separation.
Methods: To date, there is no specification or international standard to test cyclic fatigue resistance of
endodontic rotary instruments. As a consequence,
several devices and methods have been used to investigate in vitro cyclic fatigue fracture resistance of NiTi
rotary endodontic instruments. In nearly all studies reported in the endodontic literature, the rotating instrument was either confined in a glass or metal tube, in
a grooved block-and-rod assembly, or in a sloped metal
block. Results: There has been no mention of the ‘‘fit’’
of the instrument in the tube or groove. As the instrument is likely to be fitting loosely, the description of
the radius of curvature in those studies is likely to be
overstated (ie, the file was actually bent less severely
than reported, adding a variability in the amount of flexural stress). Conclusions: This review analyzed several
devices that have been used in endodontic literature for
cyclic fatigue testing and found that differences in the
methodology affected the fatigue behavior of rotary
instruments and, consequently, the outcome of these
studies. An international standard for cyclic fatigue
testing of NiTi rotary instruments is required to ensure
uniformity of methodology and comparable results.
(J Endod 2009;35:1469–1476)
Key Words
Cyclic fatigue, fracture, nickel-titanium, rotary instruments
From *La Sapienza University, Rome, Italy and †Catholic
University of Sacred Heart, Rome, Italy.
Address requests for reprints to Dr Gianluca Plotino, Via
Eleonora Duse, 22–00197 Rome, Italy. E-mail address:
[email protected]
0099-2399/$0 - see front matter
Copyright ª 2009 American Association of Endodontists
doi:10.1016/j.joen.2009.06.015
JOE — Volume 35, Number 11, November 2009
F
racture of instruments used in rotary motion occurs in two different ways: fracture
caused by torsion and fracture caused by flexural fatigue (1–3). Torsional fracture
occurs when an instrument tip or another part of the instrument is locked in a canal
while the shank continues to rotate. When the elastic limit of the metal is exceeded
by the torque exerted by the handpiece, fracture of the tip becomes inevitable (4).
Instruments fractured because of torsional loads often carry specific signs such as
plastic deformation (1).
Fracture caused by fatigue through flexure occurs because of metal fatigue. The
instrument does not bind in the canal, but it rotates freely in a curvature, generating
tension/compression cycles at the point of maximum flexure until the fracture occurs
(5, 6). As an instrument is held in a static position and continues to rotate, one half of
the instrument shaft on the outside of the curve is in tension, whereas the half of the shaft
on the inside of the curve is in compression. This repeated tension-compression cycle,
caused by rotation within curved canals, increases cyclic fatigue of the instrument over
time and may be an important factor in instrument fracture (5, 6).
The impact of torsional fracture, metal fatigue, or fracture of nickel-titanium
(NiTi) rotary instruments caused by a combination of torsional stress and accumulation
of fatigue is still debated (6). Material fatigue appears to be an important reason for the
separation of rotary instruments during clinical use. Peng et al (7) classified most of the
fractured instrument analyzed as flexural failure, implying fatigue being the predominant mechanism for material failure. In a related study, Cheung et al (8) reported
that the great majority (93%) of instruments appeared to have failed because of flexural
fatigue. This might be explained as follows (8): first; fatigue-crack growth rates in NiTi
alloys have been reported to be significantly greater than in other metals of similar
strength (9). Thus, once a microcrack is initiated, it can quickly propagate to cause
catastrophic failure.
In comparison, Sattapan et al (1) reported that torsional fracture occurred in
55.7% of all fractured files, whereas flexural fatigue occurred in 44.3%. These results
indicated that torsional failure, which may be caused by using too much apical force
during instrumentation or by other contributing factors such as the pre-existing size
of the canal, occurred more frequently than flexural fatigue, which may result from
use in curved canals. Alapati et al (10) hypothesized that the clinical fracture of
nickel-titanium rotary instruments was caused largely by a single overload incident
(that causes ductile fracture) during instrumentation, rather than the result of significant alloy fatigue after a large number of loading cycles. Cheung et al (8) have questioned the macroscopic or lateral view examination of a separated file used in these
studies, suggesting that more advanced fractography techniques are required in order
to evaluate precisely the type of fracture.
American National Standards Institute/American Dental Association (ANSI/ADA)
specification. No. 28 (11) prescribes tests to measure strength under torsion and flexibility of stainless steel hand files. The same tests are adopted by ISO 3630/1 (12), which
is designed for instruments having .02 ISO taper. To date, there is no specification or
international standard to test cyclic fatigue resistance of endodontic rotary instruments,
even if ISO and ADA are currently working to develop a new standard for NiTi
endodontic instrument of greater taper. Such a new standard is required by manufacturers, scientists, and clinicians to define suitable mechanical properties of NiTi rotary
instruments for a safe, efficient clinical use and to introduce universally accepted testing
devices for experimental evaluation of products or prototypes.
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Figure 2. The method described by Pruett et al (14) for describing canal
geometry using two parameters: radius of curvature and angle of curvature.
(Reprinted with permission [14]).
Figure 1. The degree of root canal curvature obtained using the method
described by Schneider for determining canal curvature using only one parameter to define the angle. A has an angle of 43 and B has an agle of 52, even
though both angles measured according to the method of Pruett et al. equaled
60 degrees. Location of the curve along the canal will also change the
measured angle. (Reprinted with permission [14]).
An ideal model would involve instrumentation of curved canals in
natural teeth. However, in such tests, a tooth can only be used once and
the shape of the root canal will change during instrumentation, making
it impossible to standardize experimental conditions. As a result, several
devices and methods have been used to investigate in vitro cyclic fatigue
fracture resistance of NiTi rotary endodontic instruments.
The aim of this review of the literature was to summarize and
analyze all the devices that have been used in endodontic literature
for cyclic fatigue testing, explaining how differences in the devices
may affect the fatigue behavior of rotary instruments and, consequently,
the outcome of these studies.
Cyclic Fatigue Testing Devices
The rotational bending is the test used in endodontic literature for
fatigue testing of NiTi rotary instruments. The devices used to test cyclic
fatigue resistance of NiTi rotary instruments permit instruments to rotate
until fracture using different geometric curvatures.
In 1986, Dederich and Zakariasen (13) underlined that a potential
problem with the use of 360 rotary engine files in curved canals was
metal fatigue and subsequent breakage. This study analyzed the effect
of cyclical axial motion on engine-driven K-type stainless steel instrument failure. A thick-walled Pyrex capillary with a 1-mm diameter
lumen was heated and bent to a curvature representative of a moderately
curved root canal, without precise curvature parameters. The angle and
radius of a circumference has been subsequently established by Pruett
et al (14) as the benchmark parameters widely accepted to define the
characteristics of a curvature. This study defined the parameters of
canal curvature in a more exact manner then generally used in
endodontic research. Canal curvature was historically defined using
the method introduced by Schneider in 1971 (15). This method
used only a single parameter to define an angle in degrees. To determine
the degree of root curvature, Schneider drew a line parallel to the long
axis of the canal. A second line was drawn from the apical foramen to
intersect with the first line at the point where the canal began to leave the
long axis of the canal. The acute angle formed was defined as the degree
of root curvature (Fig. 1). The shape of any root canal curvature was
more accurately described by Pruett et al (14) using two parameters:
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Plotino et al.
angle of curvature and radius of curvature (Fig. 2). To determine these
parameters, a straight line is drawn along the long axis of the coronal
straight portion of the canal. A second line is drawn along the long
axis of the apical straight portion of the canal. There is a point on
each of these lines at which the canal deviates to begin or end the canal
curvature. The curved portion of the canal is represented by a circle
with tangents at these two points. The angle of curvature is the number
of degrees on the arc of the circle between these two points. Angle of
curvature can also be defined by the angle formed by perpendicular
lines drawn from the points of deviation that intersect at the center of
the circle. The length of these lines is the radius of the circle and defines
the radius of the canal curvature defined in millimeters. This parameter
represents how abruptly a specific angle of curvature occurs as the
canal deviates from a straight line. The smaller is the radius of curvature,
the more abrupt is the canal deviation. These two parameters are independent of each other.
Several studies have used artificial canals that were constructed by
bending glass (16, 17) or metal (14, 18–25) cylindrical tubes with
different inner diameters and point of maximum curvature and using
different radii and angles of curvature (Fig. 3). A glass tube of internal
diameter 1.2 mm was used by Anderson et al (16). It was curved by
heating over a flame and curving over a metal cylinder, which gave
a radius of curvature of 5 mm. They have used 45 and 90 angles of
curvature, and the point of maximum curvature was 5 mm from the
tip of the instrument. A small glass tube with an angle of 45 and
a 5-mm curvature radius was used by Barbosa et (17) without specifying the inner diameter of the tube. The files were submitted to curvature between the third and the seventh millimeter from the tip.
Artificial canals used by Pruett et al (14) and Mize et al (18) were
fabricated from 18-G, stainless steel needles having an internal diameter
of 0.83 mm. A 2-mm and 5-mm radius of curvature measured to the
inner aspect of the curve of the guide tubes. They have used 30 ,
45 , and 90 angles of curvature, and the point of maximum curvature
was 7 mm from the tip of the instrument. Yared et al (19, 20) used a 90
metal tube with an internal diameter of 2 mm without specifying radius
of curvature and where the point of maximum curvature was located. To
construct the artificial canals, Melo et al (21) used stainless steel needles, with an external diameter of 1.6 mm and 40-mm long that were
bent with the help of a gauge to provide a 5-mm curvature radius
and 45 curvature angle. The maximum curvature region was located
at approximately 4.5 mm from the tip of the files. A 2-mm inner-diameter stainless steel tube was also used by Yao et al (22). They used an
angle of 60 and a radius of curvature of 5 mm. The author used
JOE — Volume 35, Number 11, November 2009
Review Article
Figure 3. A schematic drawing of curved glass or metal tubes used for fatigue
testing of NiTi rotary instruments.
JOE — Volume 35, Number 11, November 2009
a dynamic model without specifying at what distance from the tip of the
instrument the point of maximum curvature was located. Stainless steel
tubes with an inner diameter of 1.04 mm and a radius of curvature of 6
mm were used by Lopes et al (23). The authors used two different arcs
lengths because in the curved canals with the same radius, it is possible
that there are arcs (curved segments) with different lengths represented
by angles with different degrees. They used an arc of 9.4 mm corresponding to an angle of 90 and an arc of 14.1 mm, corresponding
to an angle of 135 when using a 6-mm radius of curvature. They
used a straight coronal part of 10.6 mm and 5.9 mm, respectively, so
that the total length of the curved and straight parts was 20 mm. Bui
et al (24) used artificial canals that were constructed by bending
a 16-mm gauge stainless steel Monojet blunt needle to a 5-mm radius
on curvature and angles of curvature of 30 , 45 , and 60 . The point of
maximum curvature was 7 mm from the tip of the instrument.
Similar to Pruett et al (14), Kramkowski et al (25) constructed
artificial canals by bending stainless steel tubing. Two canals were
bent to a 5-mm radius of curvature with angles of curvature of 45
and 60 . The center of the radius in the curved section of the canal
was 7 mm from the tip of the file. The artificial canals were inserted
into predrilled acrylic blocks for mounting in a fixed jig on the platform
of the cyclic fatigue instrument. The jig was placed at the opposite end of
the rotary handpiece fixed at a distance so that the files protruded
approximately 2 mm out of the end of the tube. Instruments were
rotated in the artificial canal with a consistent insertion and withdrawal
of 8 mm. Silicone spray (CRC Industries Inc, Warminster, PA) as a lubricant and a debris-clearing agent was applied between each file tested.
Cylindrical tubes did not sufficiently restrict the instrument shaft,
which spring back into its original straight shape, aligning into a trajectory of greater radius and reduced angle, as it has been speculated in
previous articles that have used this type of methodology (19–21,
26). Because of the inner diameter of the tubes (glass and metal) is
greater than that of the instruments, an instrument rotated in the tube
will follow a trajectory that is not predictable and without the parameters
of radius and angle of curvature and point of maximum curvature that
were established when constructing the artificial canals. Furthermore,
each instrument, depending on tip size, taper, design, pitch length,
and morphologic and geometric features, will follow its own trajectory
in tubes that do not sufficiently constrain the shafts of the instruments,
especially the smaller ones.
If instruments of the same dimensions follow different trajectories
in the test apparatus, a direct comparison between instruments of
different brands may be difficult to establish and the results obtained
may be unreliable and not consistent. Furthermore, it is unclear what
the predictability of these parameters of radius and angle of curvature
and point of maximum curvature obtained by bending a straight metal
or glass tube. Another problem with a loose-fitting canal is that the file
may ‘‘walk’’ or vibrate in that space, leading to a change in the magnitude of stress and possibly leading to variations in the results.
Ounsi et al (27) have used a custom-designed stainless steel model
mimicking a 2-mm-wide artificial canal space. The constant diameter of
the cavity that reproduced the curved canal presents the same problems
of the tube-like devices in lacking the reproducibility of the actual
trajectory followed by different files.
In other studies, the curvature of a rotary instrument was produced
when worked against a sloped metal block using a groove machined into
the face of the block to keep the file in place during testing (28–31)
(Fig. 4). The block had sufficient hardness to resist the operation of
an instrument. The different angles of curvature used in these studies
were determined according to Schneider’s method (15).
Li et al (28, 29) used a sloped carbon-steel block and calculated
only the angle of curvature by Schneider’s method (15), without
Cyclic Fatigue of Rotary Instruments
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Figure 5. A schematic drawing of the three stainless steel pins that constrained the instrument into the curvature in the studies by Cheung et al
(32–36).
Figure 4. The inclined plane used in some studies to produce the curvature
of a rotary instrument working against a sloped metal block. A groove
machined into the face of the block keeps the file in place during testing. (Reprinted with permission [28]).
considering the radius of curvature as done by Pruett et al (14). They
used four angles of curvature: 37 , 40.5 , 45 , and 48 . In the study by
Kitchens et al (30), a 2-mm-wide groove was machined into the face of
a hardened 316 stainless steel block with polished chrome plating to
keep the file in place during testing. Three angles of curvature were
used and measured using Schneider’s method (14): 25 , 28 , and
33.5 . Ray et al (31) used a highly polished area of a stainless steel
block with an incline of 15 to the horizontal plane, similar to Li et
al (28, 29). At a maximum flexure, all files produced an angle of 28
determined according to Schneider’s method (15).
Despite the radius of curvature having been recognized as the most
important factor influencing cyclic fatigue, these studies measured file
curvature according to Schneider’s method, which takes into consideration only the angle of curvature and not the more important radius of
curvature. Furthermore, it is unclear if a 2-mm-wide groove is able to
constrain the tiny tip of an endodontic instrument, thus keeping the file
in place during the test. Furthermore, using the previously described
device, the point in which the instrument begins to leave the established
long axis of the instrument is not predictable and depends greatly on the
physical and geometric properties of each instrument. It is really difficult to establish this point precisely; if the instrument is not sufficiently
constrained in a precise trajectory, the part of the instrument coronal to
1472
Plotino et al.
the beginning of the curvature will gradually move away from the long
axis of the shaft. For this reason, the choice of this point may vary greatly,
and the calculated angle may present great variability. Furthermore, it is
not possible to establish exactly the point of maximum curvature
because the physical and geometric features of the different instruments
may determine different bending properties, so that the point of
maximum curvature may lie at different points and at a different distance
from the tip of each file.
As mentioned earlier, bending properties of different files may
determine a different trajectory if the file is not constrained in a precise
trajectory. If testing is completed for all different files at a given angle to
ensure consistency, the bending properties of the different files determining different angles of curvature, thus biasing the results and the
comparisons.
To limit these problems, Cheung et al (32–36) constrained the
instrument into a curvature using three stainless steel pins (Fig. 5).
They used three smooth cylindrical pins of 2-mm diameter from
a high hardness stainless steel mounted in acrylic shims, which were
adjustable in the horizontal direction; the position of the pins determines the curvature of the instrument. A small V-shaped groove
prepared on the lowest pin maintained the position of the tip of the
instrument during rotation. It has been reported in a three-point
bending test of NiTi wires that such constraints will produce a curvature
that is circular (37). The authors affirmed that although this cannot
actually be true, the approximation should be reasonable. Unfortunately, NiTi endodontic files are tapered and with different crosssectional design. The different bending properties of the different files
JOE — Volume 35, Number 11, November 2009
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Figure 7. (A) A grooved block-and-rod showing the relationship of the
tempered steel cylinder to the radius form to guarantee the curvature of the
instruments. (B) The radius form with the V-groove for guiding the instrument.
(Reprinted with permission [39]).
Figure 6. A schematic drawing of the two hard-steel pins that constrained the
instrument in the trajectory chosen in the study by Zinelis et al (37).
and the different bending properties between the coronal and apical
portion of the same file may determine a different trajectory between
the pins if the file is not constrained precisely.
Zinelis et al (38) tested instruments for fatigue in a similar device
that allowed the instruments to be tested in a rotating bending position
inside a guide that had the form of an artificial root canal engraved on
the surface of only two hard-steel pins to constrain the instruments in
JOE — Volume 35, Number 11, November 2009
the trajectory chosen (Fig. 6). The instruments were rotated inside
the artificial canal with a 5-mm bending arc of curvature at a constant
speed of 200 rpm without specifying the angle and radius of the curvature.
Other studies (3, 26, 39–50) have used an artificial canal that
consisted of a grooved concave tempered steel block and a convextempered steel cylinder, which when held and fixed together, guaranteed the curve of the instruments (Fig. 7A). The concave radius block
incorporated a notched V-form for guiding the instruments and was
constructed with different radii and angles of curvature (Fig. 7B).
The concave grooved block of the assembly used in these studies
represents the portion of the artificial canal that guides the instrument.
This trajectory is given to the external surface of the instrument and not
to its axis, resulting in a different actual trajectory for instruments with
different taper. Moreover, there has been no mention of the fit of the
instrument in the block-and-rod assembly. The convex cylinder
contacts the tapered instrument in a nonpredictable way, so that the
instrument may fit loosely and radius and angle of curvature may not
be repeatable. Furthermore, it is difficult to control exactly the depth
of the instrument in these devices so that the point of the instrument,
which lies in the center of the curvature, may vary consistently.
Larsen et al (51) tested instruments in a metal block with a simulated canal with a 60 angle of curvature and a 3-mm radius of curvature
to the center of the 1.5-mm wide canal. The block was made from 300
series stainless steel. A swivelling acrylic top face cover allowed for
Cyclic Fatigue of Rotary Instruments
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Figure 9. Artificial root canals constructed with different type of curvatures
permit to test instruments in different conditions to obtain a complete picture
of the fatigue behavior of an instrument.
Figure 8. The experimental model used by Larsen et al (51). (Reprinted with
permission [51]).
visualization of the files rotating in the canal and the removal of broken
instruments between tests. A marker of permanent red ink was placed at
19 mm on the metal block to standardize instrument placement. From
the figure published in the article (Fig. 8), it is clear that the cylindrical
canal grooved in the metal block was not able to constraint the instrument in a precise trajectory and that to insert the instruments at the same
depth may be difficult.
Other studies (52–55) tried to overcome this problem by
manufacturing an artificial canal that reproduced instrument size and
taper, thus providing the instrument with a suitable trajectory. To ensure
the accuracy of the size of each canal, a copper duplicate of each instrument was milled increasing the original size of the instrument by 0.1 mm
using a computer numeric control machining bench (Bridgeport VMC
760XP3; Hardinge Machine Tools Ltd, Leicester, UK). The copper duplicates were constructed according to the curvature parameters that were
chosen for the study. With these negative molds, the artificial canals
were made using a die-sinking electrical-discharge machining process
(Agietron Hyperspark 3; AGIE Sa, Losone, Switzerland) in a stainlesssteel block. The blocks were hardened through annealing. The depth
of each artificial canal was machined to the maximum diameter of
the instrument +0.1 mm, allowing the instrument to rotate freely inside
the artificial canal. A simulated root canal with a precise angle and
radius of curvature can be constructed on the dimensions of every
instrument to be tested. They are milled in stainless-steel blocks with
a precision milling machine, and then the block is tempered. All the
parameters (ie, angle and radius of the curvature and position of the
center of the curvature) may be chosen by the researcher before to
construction of the artificial canal so that every type of trajectory may
be used to test the instruments (Fig. 9). In fact, it should be of somewhat
importance to test instruments at different curvatures to obtain
a complete picture of the fatigue behavior for an instrument.
The artificial canal was milled on a stainless-steel block that was
connected to a main frame to which a mobile plastic support for the
handpiece was connected. The dental handpiece was mounted on
a mobile device that allowed for precise and simple placement of
each instrument inside the artificial canal, ensuring three-dimensional
alignment and positioning of the instruments to the same depth
(Fig. 10). The artificial canal was covered with a tempered glass to
prevent the instruments from slipping out and to allow for observation
of the rotating instrument. Fracture was easily detectable because the
instruments were visible through the glass window. To reduce the fric1474
Plotino et al.
tion of the file as it contacted the artificial canal walls, a special high-flow
synthetic oil designed for lubrication of mechanical parts (Super Oil;
Singer Co Ltd, Elizabethport, NJ) was applied. Because environmental
conditions have been shown to significantly affect the fatigue behavior
of NiTi rotary instruments and fatigue testing has been suggested to
be in a service-like environment (36), sodium hypochlorite may be
used as lubricant in such tests.
The present device sought to overcome the limitations of other
experimental designs in terms of the models used for testing. Each artificial canal was specifically designed for each instrument in terms of size
and taper, giving it a precise trajectory. If the artificial canal is not identical (in shape and size) to the instrument, its trajectory will not respond
to the established parameters, thus having a reduced curvature during
the test. This can influence the results of cyclic fatigue tests. A study
submitted for publication has shown that if the artificial canal does
not sufficiently restrict the instrument shaft, it would tend to spring
back into its original straight shape, aligning into a trajectory of greater
radius and reduced angle. The study reported that the less the adaptation of the instrument to the artificial canal is, the more the variation in
the parameters of the curvature investigated is (Fig. 11).
An artificial canal manufactured as described previously to reproduce instrument size and taper seems to guarantee that different NiTi
rotary instruments may follow a precise and repeatable trajectory in
terms of radius and angle of curvature (56) (Fig. 12). This may ensure
comparable results between different instruments tested for fatigue life.
The results of another study submitted for publication have shown that
even small variations in the parameters of the curvature followed by an
instrument subjected to flexural fatigue resulted in a significant influence on the results of the cyclic fatigue test.
Figure 10. A dental handpiece mounted on the mobile block with the instrument inserted in an artificial root canal.
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Review Article
Figure 11. An example of three different artificial canals showing the different trajectory that the instrument follows.
Discussion
To date, several devices and methods have been used to investigate
in vitro the cyclic fatigue fracture resistance of NiTi rotary endodontic
instruments. In nearly all studies reported in the endodontic literature,
the rotating instrument was either confined in a glass or metal tube, in
a grooved block-and-rod assembly, or in a sloped metal block. There
has been no mention of the ‘‘fit’’ of the instrument in the tube or groove.
Because the instrument is likely to be fitting loosely, the description of
the radius of curvature in those studies is likely to be overstated (ie, the
file was actually bent less severely than reported). Furthermore, each
different instrument that fits loosely inside the device may follow
a more or less severe curvature depending on the stiffness of the file.
That would explain the wide variation in the reported fatigue life. The
large scatter generally encountered in various forms of fatigue tests
would add to the variation in the result.
These devices permit the instruments to rotate until fracture using
different curvature. The resistance of rotary instruments to cyclic fatigue
decreases with increasing instrument diameters (5, 6), and it is specifically related to the metal mass of the instrument in the point of
maximum stress (53). Moreover, the increased severity of the angle
and radius of the curve, around which the instrument rotates, decreases
instrument lifespan in vitro and clinically (5, 6, 53). If instruments of
the same dimensions follow different trajectories in the test apparatus,
a direct comparison between instruments of different brands may be
difficult to make, and the results obtained may be unreliable and inconsistent.
It is evident from this review of the literature that an international
standard is needed to validate a device for cyclic fatigue tests of NiTi
rotary endodontic instruments. Ideally, such a device should allow
testing of all instruments with a precise trajectory in terms of radius
and angle of the curvature and point of the center of the curvature,
allowing comparison of different instruments in different canals. This
is a way of testing cyclic fatigue that could simulate more precisely
the clinical accumulation of flexural fatigue.
Conclusions
This review analyzed several devices that have been used in
endodontic literature for cyclic fatigue testing and found that differences
in the methodology affected the fatigue behavior of rotary instruments
and, consequently, the outcome of these studies.
Cyclic fatigue tests investigate the in vitro resistance to fracture
caused by the accumulation of metal fatigue, which is determined by
the tension/compression cycles at the point of maximum flexure. The
clinical relevance of the results of such tests is difficult to assess because
this condition differs from intracanal instrumentation in which the fracture occurs because of several factors that act together at the same time,
including torsional stress. This represents a pure mechanical test to
extrapolate only one characteristic of the instruments (bending failure)
in the same way as torsional resistance is defined and measured by an
international standard.
Because NiTi rotary instruments are widely used, the need for
a standardization of testing of their properties including cyclic fatigue
Figure 12. An example of several instruments that follows the same trajectory in the artificial canal constructed to test cyclic fatigue. From the left: (A) ProTaper F2,
(B) ProFile 25/.06, (C) Race 25/.06, (D) EndoSequence 25/.06, (E) Twisted File 25/.06, (F) V Taper 25/.08, (G) Mtwo 25/.06.
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is required to ensure the uniformity of methodology and comparable
results for a safer, efficient clinical use. This is not easy and explains
why after more than 15 years since the introduction of nickel-titanium
rotary instruments, no specification or standard have yet been published.
Acknowledgments
We are very grateful to Maria Carola Rossini and Giulia Rossini for the drawings of Figures 3, 5, and 6.
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JOE — Volume 35, Number 11, November 2009