10.59987/ads/2012.3-4.119-122
Aim. A new manufacturing method aiming at to producing more flexible and resistant NiTi endodontic instruments has been recently developed (Hyflex, produced with CM wire). The purpose of the study was to determine whether this new manufacturing method produces NiTi instruments (Hyflex) of superior flexibility and/or superior resistance to cyclic fatigue, when compared with instruments produced by a traditional manufacturing process or thermally treated NiTi alloy (M-wire).
Materials and methods. Twelve .06 size 25 Hyflex instruments (Coltene, Allstatten, Switzerland), and twelve 06.25 Vortex instruments (Dentsply-Tulsa, OK, USA) and twelve 06.25 Endosequence instruments (Brasseler, Savannah, GA, USA) were initially evaluated for stiffness on bending, followed by a cyclic fatigue test. For the stiffness test test procedures strictly followed ISO 3630-1, and bending moment was measured when the instrument attained a 45 degrees bend. The cyclic fatigue test was performed in a customized artificial stainless steel canal (60° degree curvature with 5 mm radius). Instruments were rotated at 300 rpm until fracture. All data obtained were recorded and statistically analyzed using an ANOVA test.
Results. Statistical analysis of data showed that bending moments were significantly greater (P &lt; .05) for Vortex and EndoSequence instruments (mean values 59.06 g/cm and 48,98 g/cm respectively), compared to the Hyflex instruments (mean value 35.60 g/cm). For the cyclic fatigue test Hyflex and Vortex were significantly more resistant than EndoSequence instruments (P &lt; .05).
Conclusions. Results of the present study demonstrate the ability of the new CM-wire manufacturing process to produce NiTi rotary instruments more flexible and more resistant to cyclic fatigue than instruments produced by a traditional manufacturing process or a thermally treated NiTi alloy (M-wire).
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Annali di stomatologia
resistance of endodontic instruments
Flexibility and resistance to cyclic fatigue of endodontic instruments made with different nickel-titanium alloys: a comparative test
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Flexibility and resistance to cyclic fatigue of endodontic instruments made with different nickel-titanium alloys: a comparative test
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                                                                                                            Original article


Flexibility and resistance to cyclic fatigue
of endodontic instruments made with different
nickel-titanium alloys: a comparative test




                                                                                                  li
                                                                                               na
Giancarlo Pongione, DDS                                          resistant to cyclic fatigue than instruments produced by
Giorgio Pompa, MD                                                a traditional manufacturing process or a thermally
                                                                 treated NiTi alloy (M-wire).




                                                                                            io
Valerio Milana, PhD
Stefano Di Carlo, MD
Alessio Giansiracusa, DDS                                        Key words: cyclic fatigue test, NiTi alloy, rotary instru-
Emanuele Nicolini, PhD                                           ments, resistance of endodontic instruments.




                                                                                         az
Francesca De Angelis, PhD

                                                                 Introduction
Department of Oral and Maxillofacial Sciences, Sapienza




                                                                          n
University of Rome, Italy
                                                                 It is well known that during root canal preparation, en-
                                                                 dodontic instruments are subjected to different forces, in-




                                                                       er
Corresponding author:                                            cluding flexion, torsion, traction and apical pressure. Thus,
Giorgio Pompa, MD                                                they should have properties capable of preventing me-
Department of Oral and Maxillofacial Sciences, Sapienza          chanical failure and also minimizing the possibility of unde-
University of Rome, Italy
Via Caserta 6
00161 Rome, Italy
Phone: +39 06 49976614
                                              nt                 sirable alterations in canal anatomy. Loss of flexibility may
                                                                 result in ledges, transportations and perforations. This is why
                                                                 nickel-titanium (NiTi), a very unique alloy with specific prop-
                                                                 erties very suitable for the endodontic use, has become pop-
                                            iI
E-mail: giorgio.pompa@uniroma1.it                                ular in the last decades. The super-elastic property of the al-
                                                                 loy can be described briefly. The stress value remains fairly
                                                                 constant up to a certain point of instrument deformation. At
                                          on

Summary
                                                                 the same time, when the deformation rebounds, the stress
Aim. A new manufacturing method aiming at to produc-
                                                                 value remains fairly constant. These characteristics are
ing more flexible and resistant NiTi endodontic instru-
ments has been recently developed (Hyflex, produced              very useful in clinical practice, allowing an increased elas-
with CM wire). The purpose of the study was to determine         tic deformation and, consequently, increasing flexibility of the
                                 i


whether this new manufacturing method produces NiTi              endodontic instruments. The process of manufacturing of
                              iz


instruments (Hyflex) of superior flexibility and/or supe-        endodontic instruments from nickel-titanium was investi-
rior resistance to cyclic fatigue, when compared with in-        gated initially by Walia et al.(1), who demonstrated that
struments produced by a traditional manufacturing                NiTi instruments have a greater degree of elastic flexibility
              Ed




process or thermally treated NiTi alloy (M-wire).                in bending and torsion, as well as superior resistance to tor-
Materials and methods. Twelve .06 size 25 Hyflex instru-         sional fracture, when compared with stainless-steel instru-
ments (Coltene, Allstatten, Switzerland), and twelve 06.25       ments manufactured with the same process. The mechan-
Vortex instruments (Dentsply-Tulsa, OK, USA) and twelve          ical properties of endodontic instruments, like flexibility and
06.25 Endosequence instruments (Brasseler, Savannah,             resistance to fracture, are affected by several factors, such
GA, USA) were initially evaluated for stiffness on bend-         as dimensions, tip size and conicity, design, chemical com-
 IC




ing, followed by a cyclic fatigue test. For the stiffness test   position of the metallic alloy and thermo-mechanical
test procedures strictly followed ISO 3630-1, and bend-          processes applied during manufacturing (2-4). To date, de-
ing moment was measured when the instrument attained             spite the fact that NiTi alloys demonstrate improved elastic-
a 45 degrees bend. The cyclic fatigue test was performed         ity, which significantly reduces the restoring force, and re-
C




in a customized artificial stainless steel canal (60° degree
                                                                 duces the amount of transportation that occurs during canal
curvature with 5 mm radius). Instruments were rotated at
                                                                 preparation, this elasticity is limited by the size and taper of
300 rpm until fracture. All data obtained were recorded
                                                                 instrument used (5,6). Therefore the fabrication of instru-
and statistically analyzed using an ANOVA test.
                                                                 ments of greater taper resulted in a significant increase of
©




Results. Statistical analysis of data showed that bending
moments were significantly greater (P < .05) for Vortex          the stiffness of rotary instruments, and increased risk of fa-
and EndoSequence instruments (mean values 59.06 g/cm             tigue failure (7). This is not an ideal characteristic for an in-
and 48,98 g/cm respectively), compared to the Hyflex in-         strument which is going to reach full working length in a
struments (mean value 35.60 g/cm). For the cyclic fa-            curved canal, resulting in an increased risk of canal trans-
tigue test Hyflex and Vortex were significantly more re-         portation in the portion of the canal after the start of the cur-
sistant than EndoSequence instruments (P < .05).                 vature, and in an increased risk of intracanal failure.
Conclusions. Results of the present study demonstrate            An ideal solution to increase instruments’ performance is
the ability of the new CM-wire manufacturing process to          to use an improved nickel-titanium alloy with superior flex-
produce NiTi rotary instruments more flexible and more           ibility and resistance to fracture. Recently, new manufac-

Annali di Stomatologia 2012; III (3/4): 119-122                                                                              119
G. Pongione et al.


turing processes (which are patented and not disclosed by         VDW motor (Munich, Germany) until fracture occurred.
manufacturers) involving heat treatment of the alloy have         For each instrument, the time in seconds was recorded by
been developed in order to improve mechanical properties          the same operator with a chronometer to an accuracy of
of the alloy for the endodontic use. The M-wire technology        0.1 s. After positioning the instrument in the canal and as
developed by Tulsa Dental was one of the first thermally          soon as rotation started, timing was initiated. Timing




                                                                                                 li
treated NiTi alloy used for the endodontic use. A new             stopped when instrument breakage was observed. Num-
manufacturing method aiming at to producing more flexi-           ber of cycles at failure (NCF) was calculated for each in-




                                                                                              na
ble and resistant NiTi endodontic instruments has been re-        strument, by multiplying time to failure (in seconds) by 5
cently developed by Coltene (CM wire). On these basis,            (which is 5 rotation x second = 300 rpm). For both tests
the aim of the present study was to evaluate the bending          all data were recorded and subjected to statistical evalu-
properties and the cyclic fatigue resistance of NiTi rotary       ation (p<.05) using ANOVA test.




                                                                                           io
instruments produced with thermally treated alloys and
compare them with those of commercially available NiTi in-
struments manufactured with traditional methods.                  Results




                                                                                        az
                                                                  For the bending test mean values and standard deviation
Materials and methods                                             for each group of instruments are shown in Table 1. The
                                                                  higher the value, the more rigid the instruments. Table 2
Twelve instruments of each following NiTi rotary instru-          shows a statistical comparison of results: Hyflex were




                                                                          n
mentation technique were selected for the study:                  found to be the most flexible instruments, showing a sig-
                                                                  nificant difference (P < .05) in comparison with the other




                                                                       er
a) EndoSequence 0625 (Brasseler, Savannah, GA, USA);              instruments. Vortex were not found to be significantly
b) Hyflex 0625 (Coltene, Allstatten,Switzerland);                 more flexible than EndoSequence instruments).
c) Vortex 0625 (Denstply-Tulsa, Tulsa, OK, USA).                  For the cyclic fatigue test mean values and standard de-


                                             nt
The NiTi rotary instruments were selected based on a
similar cross-sectional design and according to three dif-
ferent manufacturing processes: traditional grinding of
                                                                  viation for each group of instruments are shown in Table
                                                                  3. The higher the value, the more resistant the instru-
                                                                  ments. Table 4 shows a statistical comparison of results:
                                                                  Hyflex were found to be the most resistant instruments,
                                           iI
NiTi (EndoSequence), M-wire (Vortex) and CM wire                  showing a significant difference (P <.05) in comparison
(Hyflex) technologies.                                            with EndoSequence and Vortex instruments.
Before the tests, all instruments were examined under a
                                         on

measuring microscope at D3 and D16 to ensure uniformity
of dimensions (according to the tolerance indicated by            Discussion
ISO 3630-1), and under a stereo microscope (magnifica-
tion x20) to ensure uniformity of cutting flutes and defect       In the last years many new endodontic instruments, alloys
                                i


free surfaces. All defective instruments were eliminated          and manufacturing processes have been commercial-
                             iz


from the study, and substituted by other new instruments          ized in an attempt to improve performance and safety of
from the same manufacturer. Bending moment was meas-              root canal instrumentation. When new root canal instru-
ured when the instrument attained a 45° bend. Experimen-          ments with innovative manufacturing process which dif-
              Ed




tal procedures strictly followed testing methodology de-          fers markedly from conventional ones are produced, sev-
scribed in ISO 3630-1 (8), using a computerized device.           eral characteristics need to be investigated and tested to
After the stiffness test the same instruments were sub-           allow an efficient and safe clinical usage. On these basis,
jected to a cyclic fatigue test. The cyclic fatigue testing de-   the aim of the present study was to determine whether a
vice used in the present study has been used in many              new manufacturing method (CM wire) produces NiTi in-
previous studies on cyclic fatigue resistance performed by        struments of superior flexibility and fatigue resistance,
 IC




the authors (9-14). The device consists of a mainframe to         when compared with instruments produced by a tradi-
which a mobile plastic support for the electric hand-piece        tional or a thermally treated NiTi alloy (M-wire).
is connected and a stainless steel block containing the ar-       Results from the present study showed that Hyflex were
tificial canals. The electric hand-piece was mounted on a         the most flexibile instruments, with a significant improve-
C




mobile device to allow the precise and reproducible place-        ment (P <.05) in flexibility ranging over the other tested
ment of each instrument inside the artificial canal. This         commercially available instruments produced with the M-
placement ensured three-dimensional alignment and the             wire or the traditional grinding process (Tabs. 1-2). De-
positioning of the instruments to the same depth. The ar-
©




tificial canal was manufactured by reproducing an instru-                              Mean values       Standard deviation
ment’s size and taper, thus providing the instrument with
a suitable trajectory that respects the parameters of the         Hyflex 0625              19,46               (SD 2,7)
curvature chosen. A simulated root canal with a 60° an-
gle of curvature and 5-mm radius of curvature was con-            Vortex 0625              67,58               (SD 4,2)
structed for instrument type. The center of the curvature
was 5 mm from the tip of the instrument, and the curved           EndoSequence 0625        62,11               (SD 5,7)
segment of the canal was approximately 5 mm in length.            Table 1. Bending test: mean value and standard deviation for
All of the instruments were rotated at 300 rpm using a            group of instruments.

120                                                                               Annali di Stomatologia 2012; III (3/4): 119-122
Flexibility and resistance to cyclic fatigue of endodontic instruments made with different nickel-titanium alloys…


                                                                    These improvements in the manufacturing process shown
Hyflex vs Vortex                   Significant difference           by the CM wire technology can very useful in clinical prac-
                                   (P<.05).                         tice. Nickel-titanium possess an unique flexibility that can
                                                                    usually withstand the rapid, repeated distortions of rotation
Hyflex vs EndoSequence             Significant difference           in curved canals (10), but, unfortunately, flexibility is lim-
                                   (P<.05).




                                                                                                       li
                                                                    ited by the size and taper of instrument used. Therefore,
                                                                    a tip 25/06 taper NiTi rotary instrument is usually more rigid
Vortex vs EndoSequence             No significant difference




                                                                                                    na
                                   (P>.05).                         than small stainless steel K-files, i.e. size 15 or 20. Sev-
                                                                    eral changes in cross-sectional and flute design have
Table 2. Statistical comparison of results: Hyflex were found the   been introduced in the last years in order to increase flex-
most flexible in comparison with the other instruments.             ibility of greater tapered instruments which are intended to




                                                                                                 io
                                                                    be used in apical portion of curved root canals, but no clin-
spite the fact that flexibility is influenced by instruments        ically significant improvement was achieved (2). Therefore
design, such a great improvement is mainly related to the           a significant improvement in the flexibility of the alloy
new manufacturing process (CM wire), which seems to                 should be highly beneficial, providing NiTi instruments of




                                                                                              az
play a major role in increasing flexibility of the Hyflex in-       greater taper with a superior ability to negotiate curved
strument. No significant difference were noted between              canals, to reduce the tendency of iatrogenic errors and to
Vortex and EndoSequence. Even if some articles report               allow dimensionally adequate apical preparations of
an increased flexibility of CM-wire vs traditional NiTi, data       curved canals while maintaining the original path (21-23).




                                                                             n
from the present study does not support those findings.             Moreover, endodontic files that show an increased flexi-
Since the tested instruments were similar but not identi-           bility are also perceived to be more resistant to cyclic fa-




                                                                          er
cal in cross-sectional design, it can be stated that the the-       tigue. A possible explanation is that when the instrument
oretical increase of flexibility of m-wire is not enough to         is rotated inside a curved canal and is subjected to ten-
overcome a slight increase of stiffness due to a different          sile and compressive stress, a more flexible instruments
cross-sectional design.

                                              nt
In the present test M-wire technology did not produced
a significant improvement in the cyclic fatigue testing
over the traditional grinding method, while a statistically
                                                                    could accommodate this stress in a better way, thus in-
                                                                    creasing fatigue resistance. In the present study such cor-
                                                                    relation between flexibility and fatigue resistance was
                                                                    found to be valid for the all the tested instruments.
                                            iI
significant difference was noted with CM wire (Tabs. 3-
4). Such differences may be due to the different alloys
but also due to different instrument design. In fact, while         References
                                          on

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122                                                                                          Annali di Stomatologia 2012; III (3/4): 119-122
