RaCe & Mtwo Rotary File Fatigue Resistance Study

Basic Research—Technology
Cyclic Fatigue Resistance of RaCe and Mtwo Rotary Files
in Continuous Rotation and Reciprocating Motion
Sekar Vadhana, BDS, Balasubramanian SaravanaKarthikeyan, MDS, Suresh Nandini, MDS,
and Natanasabapathy Velmurugan, MDS
Abstract
Introduction: The purpose of this study was to evaluate
and compare the cyclic fatigue resistance of RaCe (FKG
Dentaire, La Chaux-de-Fonds, Switzerland) and Mtwo
(VDW, Munich, Germany) rotary files in continuous rotation and reciprocating motion. Methods: A total of 60
new rotary Mtwo and RaCe files (ISO size = 25, taper
= 0.06, length = 25 mm) were selected and randomly
divided into 4 groups (n = 15 each): Mtc (Mtwo NiTi files
in continuous rotation), Rc (RaCe NiTi files in continuous
rotation), Mtr (Mtwo NiTi files in reciprocating motion),
and Rr (RaCe NiTi files in reciprocating motion). A cyclic
fatigue testing device was fabricated with a 60 angle of
curvature and a 5-mm radius. All instruments were
rotated or reciprocated until fracture occurred. The
time taken for each instrument to fracture and the
length of the broken fragments were recorded. All the
fractured files were analyzed under a scanning electron
microscope to detect the mode of fracture. The
Kolmogorov-Smirnov test was used to assess the
normality of samples distribution, and statistical analysis was performed using the independent sample t
test. Results: The time taken for the instruments of
the Mtr and Rr groups to fail under cyclic loading was
significantly longer compared with the Mtc and Rc
groups (P < .001). Scanning electron microscopic observations showed that the instruments of all groups had
undergone a ductile mode of fracture. The length of
the fractured segments was between 5 and 6 mm, which
was not statistically significant among the experimental
groups. Conclusions: Mtwo and RaCe rotary instruments showed a significantly higher cyclic fatigue resistance in reciprocating motion compared with continuous
rotation motion. (J Endod 2014;40:995–999)
Key Words
Cyclic fatigue, nickel-titanium files, reciprocation, rotation
N
ickel-titanium files (NiTi) are commonly used in current endodontic practice. NiTi
files offer many advantages over stainless steel files such as flexibility and elasticity
(1, 2). Despite these advantages, NiTi instruments appear to have a high risk of
separation (3). One of the reasons for the fracture of NiTi instruments is torsional
or cyclic fatigue (4, 5). Torsional fatigue occurs when the tip of the instrument
binds in the canal while the shank continues to rotate (3, 5). Cyclic fatigue occurs
when the instrument continues to rotate freely in a curvature, and at the point of
maximum flexure, tension/compression cycles are generated until fracture occurs
(5). Increasing the resistance to file separation has been the main goal for ensuring
safety during endodontic instrumentation.
Conventional NiTi rotary endodontic files are manufactured by machining starting
wire blanks that are in the superelastic austenitic phase (6). Under stress, it changes to
the martensitic phase. One of the unique properties of the martensitic phase is that it has
excellent resistance to fatigue (7). The stress-induced transformation to the martensitic
phase is reversible (8). The temperature at which the martensitic phase gets transformed to the austenitic phase is called the austenitic finish temperature. The higher
the austenitic finish temperature, the longer the file remains in the martensitic phase.
This phase transformational behavior and microstructure of NiTi can be optimized using thermomechanical processing. This ultimately increases the mechanical and fatigue
properties of the file (9). Another method of improving cyclic fatigue resistance is by
electropolishing the files. This surface treatment improves the surface smoothness,
thereby delaying the initiation of surface cracks (10).
An alternative method of increasing cyclic fatigue resistance is the use of rotary NiTi
instruments in reciprocating motion (11). Two reciprocating systems are currently
available: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland). Reciprocating NiTi files have a better cyclic fatigue resistance
when compared with that of continuous rotary NiTi files (12). Studies have been conducted to evaluate the use of various rotary NiTi files including Mtwo (VDW), K3, ProTaper (Dentsply Maillefer, Ballaigues, Switzerland), and Twisted Files (Sybron Endo,
Orange, CA) in reciprocating motion, and it has been proved that these files possess
better cyclic fatigue resistance (12–14). Mtwo files, which have a cross-section similar
to that of Reciproc files (15), have improved cyclic fatigue resistance in both continuous
and reciprocating motion (12). RaCe files (FKG Dentaire, La Chaux-de-Fonds,
Switzerland) have a triangular cross-section with distinct positive cutting angles
(16). RaCe files have improved cyclic fatigue resistance in continuous rotation when
compared with that of ProTaper and Helix rotary files (17). However, to date, the effect
of reciprocating motion on RaCe files has not been studied. The null hypothesis is that
there is no difference in the cyclic fatigue resistance of RaCe and Mtwo rotary files using
continuous rotation and reciprocating motion.
From the Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and
Research, Maduravoyal, Chennai, India.
Address requests for reprints to Dr Suresh Nandini, Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Meenakshi
Academy of Higher Education and Research, Alapakkam Main Road, Maduravoyal, Chennai 600 095, Tamil Nadu, India. E-mail address: [email protected]
0099-2399/$ - see front matter
Copyright ª 2014 American Association of Endodontists.
http://dx.doi.org/10.1016/j.joen.2013.12.010
JOE — Volume 40, Number 7, July 2014
Cyclic Fatigue Life of RaCe and Mtwo Files
995
Basic Research—Technology
Materials and Methods
A total of 30 new rotary Mtwo (VDW, Munich, Germany) and 30
RaCe instruments (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland)
(ISO tip size = 25, taper = 0.06, length = 25 mm) were selected. All
the instruments were previously inspected under an optical stereo microscope (Zoom Stereo Binocular Microscope [ZSM-111], Hicksville,
NY); with 20 magnification for any visible signs of deformation. None
of the instruments were discarded. All the files were then subjected to
cyclic fatigue testing.
Cyclic Fatigue Testing Device
A static cyclic fatigue testing device was custom fabricated for
this study (Fig. 1A and B). It consisted of a main metal frame
made of iron to which an artificial canal system and a support for
the handpiece were being attached. The canal system, which simulated a root canal, consisted of 2 adjustable metal frames made of
brass that can accommodate any instrument to its exact size and taper. It was constructed with a 60 angle of curvature. The curvature
started at 5 mm from the tip of the canal. The WaveOne handpiece
was mounted over the support, which also ensured the correct positioning and placement of files to the same appropriate depth for all
the samples.
Cyclic Fatigue Test
Sixty samples were randomly divided into 4 groups (n = 15) according to the type of rotary files and rotary motions used.
Group Mtc
Fifteen Mtwo instruments were allowed to rotate in continuous
rotation (CW) motion using a WaveOne motor set at continuous rotation
mode with recommended torque control settings and at a constant
speed of 300 rpm.
Group Mtr
Fifteen Mtwo instruments were allowed to rotate in reciprocating
motion (counterclockwise [CCW] = 170 and CW = 50) using a WaveOne motor set on the WaveAll mode with recommended torque control
settings and at a constant speed of 350 rpm (11).
Group Rc
Fifteen RaCe instruments were allowed to rotate in continuous
rotation motion using a WaveOne motor set on the continuous rotation
mode with recommended torque control settings and at a constant
speed of 600 rpm.
Group Rr
Fifteen RaCe instruments were allowed to rotate in reciprocating
motion (CCW = 170 and CW = 50) using a WaveOne motor set on
the WaveAll mode with recommended torque control settings and at
a constant speed of 350 rpm (11). Glycerin (Glycerin Pure; AB Enterprises, Mumbai, India) was used as a lubricant after the use of each file
during instrumentation in this study. Instruments were rotated or reciprocated until fracture occurred. To obviate human errors, cyclic fatigue
Figure 1. (A and B) The custom fabricated static cyclic fatigue testing device. (C) Stereomicroscopic image of a new RaCe file. (D) Stereomicroscopic image of a
fractured RaCe file depicting the fracture at the D5–D6 regions.
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Vadhana et al.
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Basic Research—Technology
testing was performed by viewing the rotating or reciprocating files
under a dental operating microscope (Seiler Microscope, St Louis,
MO) at a magnification of 25 to precisely determine the time of fracture. All files were tested by 1 operator while the other operator was
simultaneously operating the stopwatch. The length of the broken fragments was measured using a vernier caliper under a dental operating
microscope (25 magnification).
Scanning Electron Microscopic Analysis
The fractured fragments of each instrument were collected from
all the groups. The fractured surfaces of the files were examined under
a scanning electron microscope (FEI Quanta FEG 200; Hillsboro, OR)
to determine the modal characteristics of fracture (Fig. 2A–D).
Stereomicroscopic Analysis
A few samples were randomly selected from each group and
observed under an optical stereo microscope (40) before and after
the failure of the respective instruments.
independent sample t test (SPSS version .17; SPSS, Chicago, IL). A P
value of <.05 was used as a criterion for statistical significance.
Results
Descriptive statistics for the 4 experimental groups are listed in
Table 1. It was observed that the cyclic fatigue resistance of the RaCe
and Mtwo NiTi rotary files increased significantly when operated in
reciprocation compared with the continuous rotation mode. It was
also observed that Mtwo files showed significantly more resistance to
fracture under cyclic loading than RaCe files. The mean length of the
fractured segments was observed as 5–6 mm, which was not statistically
significant among the groups.
Scanning Electron Microscopic Analysis Results
Scanning electron microscopic analysis of fractured surfaces of all
the groups revealed craterlike formation along with numerous dimples
and microbubbles. These features indicate that the instrument had undergone a ductile mode of fracture, which is predominantly observed in
cyclic fatigue failure.
Statistical Analysis
Discussion
The mean and standard deviation of the time to fracture and the
fractured fragment length were calculated for all the experimental
groups. The Kolmogorov-Smirnov test was used to assess the normality
of sample distribution. Both intragroup and intergroup comparisons
of cyclic fatigue resistance of the samples were performed using the
This study compared the cyclic fatigue resistance of RaCe and
Mtwo rotary files in continuous and reciprocating motion and also assessed the mode of fracture under scanning electron microscopy. This
study rejects the proposed null hypothesis and has shown that both
Mtwo and RaCe rotary files have improved cyclic fatigue resistance in
Figure 2. Scanning electron microscopic images of fractured files showing microbubbles, craters, valleys, and dimples. RaCe file used in continuous rotation (A)
and in reciprocation motion (B). Mtwo file used in continuous rotation (C) and in reciprocation motion (D).
JOE — Volume 40, Number 7, July 2014
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Basic Research—Technology
TABLE 1. Intra- and Intergroup Comparison of Mean and Standard Deviation of Time Taken to Fracture of Instruments and Length of Fractured Instruments
Continuous
Groups
n = 15
Mtwo
RaCe
P value
Time ± SD
Reciprocating
Length ± SD
A
283.27 58.346
173.67 49.844A
<.001
Time ± SD
a
5.533 0.3994
5.467 0.4419a
.668
Length ± SD
B
815.73 82.893
337.67 39.811B
<.001
5.633 0.4419a
5.500 0.4629a
.426
A significant difference in P values is marked as A and B (P < .05). An insignificant difference in P values is marked as a and a (P > .05).
reciprocating motion when compared with continuous rotation. Mtwo
rotary files were selected because they have a cross-section similar to
Reciproc files (12), and RaCe files were chosen because there are no
studies using these files in reciprocating motion to date. Cyclic fatigue
testing can be performed using a static or dynamic model (15, 18).
In a static model, the instrument does not move axially. This creates
alternate compressive and tensile stresses in a particular area of the
instrument, leading to premature failure. The dynamic model
incorporates cyclic axial movement, which provides a better clinical
simulation and increases the lifespan of rotary files, but the
amplitude, speed of pecking motion, and axial movement are purely
subjective in clinical practice. The ability to constrain the files in a
precise trajectory is also difficult in dynamic testing (18).
There is no evidence for testing rotary files under standardized
specifications (3, 19). Hence, a nontooth static model of a
standardized artificial canal was used in this study, which minimizes
the influence of other variables of file separation other than cyclic
fatigue.
Earlier cyclic fatigue studies have been conducted using various
angles of curvatures such as 30 , 60 , and 90 . A canal of 30 does
not constrain the file properly, whereas a curvature of 90 incorporated
more stresses to the file (13, 20). The center of curvature in most of the
studies was between 5 and 7 mm from the tip of the instrument (4).
Thus, in this study, an artificial canal was fabricated with a 60 of curvature with the maximum point of curvature at 5 mm from the tip of the
instrument.
In this study, fatigue life of the instruments was evaluated by time
and not by the number of cycles to fracture (NCF). The time to fracture is
easier to record than NCF. In continuous rotation motion, the assessment of the number of cycles to fracture can be obtained by simply
multiplying the rotational speed by the time elapsed until fracture
occurred. However, in reciprocation motion, the NCF can be determined only by knowing the amplitude of the oscillating motion with a
constant time unit, which is not provided by the manufacturers (15).
Both Race and Mtwo rotary files showed better resistance to cyclic
fatigue in reciprocation compared with continuous rotation motion in
this study. The fatigue life is determined by the number of times the
crack opens and closes with each cycle. In continuous rotation motion,
a 360 rotation is completed in 1 cycle, which leads to increased stress
levels and more widening of surface cracks.
Tensile stresses are concentrated at a particular point of the instrument in continuous rotation, which culminates to the fracture of the instrument. However, in reciprocating motion using the WaveAll mode
(CCW = 170 and CW = 50), it takes 3 cycles to complete one 360 rotation (12). The stresses are distributed to 3 points around the working
portion of the file, minimizing the opening of surface cracks (15). In
our study, the cyclic fatigue life of RaCe files in reciprocation was 1.9
times more than that of continuous rotation, whereas Mtwo files performed 2.9 times better in reciprocation than in continuous rotation.
This increase in the cyclic fatigue life of RaCe files in reciprocation
compared with continuous rotation was not proportional compared
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Vadhana et al.
with that of Mtwo files. The reason could be attributed to the difference
in the rotational speeds of RaCe files when used in reciprocation (350
rpm) and continuous rotation (600 rpm). Further studies are required
to conclude the role of rotational speeds in the cyclic fatigue life of rotary
files in reciprocating motion.
In our study, Mtwo files performed better than RaCe rotary NiTi
files in both continuous and reciprocating motion. Previous studies
have shown controversial results regarding the cyclic fatigue life of
Mtwo and RaCe in continuous rotation when compared with other files
(17, 19–21). In reciprocating motion, Mtwo has been proven to have a
similar cyclic fatigue life to that of Twisted Files and Reciproc and
superior resistance compared with WaveOne (12).
Resistance to cyclic fatigue depends on various factors such as diameter, metal mass, flexibility, cross-sectional shape, and surface finish of rotary files (22). RaCe files have an electropolished surface finish and good
flexibility and thus are expected to possess a better cyclic fatigue life (23).
In our study, it showed a reduced fatigue life in comparison with Mtwo.
This could be caused by the presence of alternating cutting edges
in RaCe files. Although the alternating spiraled and nonspiraled segments of RaCe files increase its flexibility, maximum bending occurs
at these segment junctions. Because flexural stresses are confined to
the fluted portion of the working area and are not distributed evenly
throughout the length of the file, fatigue occurs more rapidly in RaCe
files (22). The stereomicroscopic analysis of the new RaCe files in
our study showed that the junction of the spiraled and nonspiraled segments exists at the D5–D6 region (Fig. 1C). The synergistic effect of the
presence of such a junction at 5 mm of the RaCe file (which corresponds to the maximum curvature of the artificial canal) and the static
mode of testing could have resulted in premature failure of RaCe files.
Figure 1D shows that the failure occurred at the segment junctions.
Electropolishing potentially removes the residual stresses on the instrument and increases the fatigue life (10). However, the role of electropolishing in the cyclic fatigue resistance of the instruments cannot be
considered in this study because the cross-sections of both the file
systems (RaCe and Mtwo) were different.
Another determining factor for the cyclic fatigue life of files is the
shape of the file at its circumference. The forces required to disrupt the
molecular attraction can be decreased by providing a greater mass of
rounded surface following the cutting edge of files at its circumference
(22). At the radial extremity, RaCe files have a more angular surface
because of their triangular cross-section in comparison with that of
Mtwo files. This might lead to increased stresses in a small area, which
ultimately resulted in cyclic fatigue failure with less tension.
Scanning electron microscopic observations have shown that all
specimens of both the groups depicted the presence of microbubbles.
Circular abrasion marks, dimples, and craterlike formation confirmed
that all instruments had undergone cyclic fatigue.
On analyzing the data regarding the length of the fractured fragments, there was no statistical difference in the mean length of all the
instruments tested. In this study, almost all the instruments fractured
between D5 and D6. The fractured length of the instruments ranged
JOE — Volume 40, Number 7, July 2014
Basic Research—Technology
from 4.5–5.5 mm, which confirmed the positioning of the files in a precise trajectory. Further studies may be required on the canal centering
ability and cutting efficiency of RaCe and Mtwo rotary NiTi files in reciprocating motion.
Conclusion
Within the limitations of this study, Mtwo and RaCe rotary files
were more resistant to cyclic fatigue in reciprocating motion when
compared with continuous rotation motion. Mtwo files performed better than RaCe files in both continuous and reciprocating motion.
Acknowledgments
The authors deny any conflicts of interest related to this study.
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