10.59987/ads/2013.1.149-151
Aim.The aim of the present study was to compare the resistance to cyclic fatigue of Hyflex® (Coltene-Whaledent, Allstatten, Switzerland) size 40 taper.04 nickel titanium instruments used in continuous rotation versus the use with a reciprocating motion. The null hypothesis was that different file motions had no influence on the lifetime of instruments subjected to a cyclic fatigue test.
Methodology.24 Hyflex® size 40 taper.04 nickel titanium instruments were randomly divided in two groups (n=12each), and submitted to a cyclic fatigue test. The firstgroup (CR group) were used with a continuous rotation,while the second one (RCP group) with a reciprocating motion. The cyclic fatigue tests were performed by using a stainless steel block containing an artificial canal shaped with a 135° angle. All instruments were rotated or reciprocated until fracture occurred. The time to fracture was recorded visually with a 1/100 second chronometer. Data were recorded and statistically analysed.
Results. Results indicated that instruments used with are ciprocating motion showed a significant increase in the meantime to failure when compared to those used in continuous rotation.
Conclusions. The null hypothesis was rejected. Results ofthe present study showed that reciprocating motion extended resistance to cyclic fatigue of the tested nickel titanium instruments, when compared to continuous rotation.
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Annali di stomatologia
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Influence of file motion on cyclic fatigue of new nickel titanium instruments
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Original Article
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G. Gambarini 
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Influence of file motion on cyclic fatigue of new nickel titanium instruments
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                                                                                                           Original article


Influence of file motion on cyclic fatigue
of new nickel titanium instruments




                                                                                                  li
                                                                                               na
Alessio Giansiracusa Rubini, DDS1                                Introduction
Gianpaolo Sannino, MD1




                                                                                            io
Giancarlo Pongione, DDS1                                         The introduction of Nickel-Titanium (NiTi) alloy in en-
Luca Testarelli, DDS, PhD1                                       dodontics was a significant improvement, allowing excel-
Dina Al Sudani, DDS2                                             lent result in terms of cleaning and shaping of root canals,
                                                                 while reducing operative time and minimizing iatrogenic




                                                                                         az
Jeeraphat Jantarat, DDS3
Massimo De Luca, MD, PhD1                                        errors. Thanks to the superior mechanical properties of
Gianluca Gambarini, MD, PhD1                                     the NiTi alloy (1) it was possible to use endodontic instru-
                                                                 ments in continuous rotation, increasing the effective-
                                                                 ness and rapidity of the cutting. However, several studies




                                                                          n
1
    Department of Oral and Maxillo Facial Sciences,              reported an increased tendency to intracanal separation
    “Sapienza” University of Rome, Italy                         of NiTi Rotary instruments when compared to stainless




                                                                       er
2
    King Saud University, Riyadh City, Saudi Arabia              steel manual instruments (2-4).
3
    Mahidol University, Bangkok, Thailand                        This increased risk of intracanal separation is due to the
                                                                 continuous rotation movement inside a curved canal,
Corresponding author:
Alessio Giansiracusa Rubini, DDS
                                             nt
Department of Oral and Maxillo Facial Sciences,
                                                                 which submits NiTi instruments to greater flexural stress
                                                                 and consequently to increased metal fatigue, when com-
                                                                 pared to stainless steel file used manually (5).
                                                                 Despite the improvements in instrument design over the
                                           iI
“Sapienza” University of Rome
                                                                 last 20 years, intracanal separation of NiTi rotary instru-
Via Giuseppe Rosso, 5
                                                                 ments remains a quite common (from 2% to 9% of cases)
00136 Rome, Italy
                                                                 procedural error, which can dramatically affect the out-
                                         on

Phone: +39 06 3231452
                                                                 come of the endodontic treatment (6).
E-mail: alessiogiansiracusa@gmail.com
                                                                 Instrument fracture can occur because due to excessive
                                                                 torsion or fatigue, or, more often, due to a combination of
Summary                                                          the two factors. Torsional fracture occurs when the tor-
                                i


Aim. The aim of the present study was to compare the re-         sional torque applied to the instrument exceed torsional
                             iz


sistance to cyclic fatigue of Hyflex® (Coltene-Whale-            elastic limit of instrument itself. Clinically it could be hap-
dent, Allstatten, Switzerland) size 40 taper.04 nickel tita-     pen when an instrument is locked into the canal while the
nium instruments used in continuous rotation versus the          motor continues to apply a moment of force.
              Ed




use with a reciprocating motion. The null hypothesis             Fatigue fracture occurs when instrument undergoes to a
was that different file motions had no influence on the          cyclic stress. A material submitted to cycle of stress lower
lifetime of instruments subjected to a cyclic fatigue test.      than fracture force limit get a fracture without any previ-
Methodology. 24 Hyflex® size 40 taper.04 nickel titanium         ous sign of plastic deformation.
instruments were randomly divided in two groups (n=12            Intracanal failure is not clinically predictable; because it is
each), and submitted to a cyclic fatigue test. The first         not possible to evaluate how much fatigue an instrument
 IC




group (CR group) were used with a continuous rotation,           accumulates during its work inside the canal. Research
while the second one (RCP group) with a reciprocating            has shown that intracanal cyclic fatigue which an instru-
motion. The cyclic fatigue tests were performed by us-           ment accumulates during canal shaping of curved canals
ing a stainless steel block containing an artificial canal       depends on anatomy (7), rotation speed (8). Instrument
C




shaped with a 135° angle. All instruments were rotated or        design and manufacturing (9, 10).
reciprocated until fracture occurred. The time to fracture
                                                                 Recently, a new manufacturing technique has been devel-
was recorded visually with a 1/100 second chronometer.
                                                                 oped and commercialized: the Hyflex® nickel titanium in-
Data were recorded and statistically analysed.
                                                                 struments (Coltene-Whaledent, Allstatten, Switzerland).
©




Results. Results indicated that instruments used with a
                                                                 These NiTi files have been manufactured utilizing a
reciprocating motion showed a significant increase in
the meantime to failure when compared to those used in           process that controls the material’s memory, making the
continuous rotation.                                             files extremely flexible, while significantly reducing the
Conclusions. The null hypothesis was rejected. Results of        restoring force. This gives the file the ability to precisely
the present study showed that reciprocating motion ex-           follow and respect the original root canals trajectories, re-
tended resistance to cyclic fatigue of the tested nickel tita-   ducing the risk of ledging and transportation or perfora-
nium instruments, when compared to continuous rotation.          tion. Moreover the thermally treated alloy (CM wire tech-
                                                                 nology) used for the manufacturing of Hyflex instruments
Key words: cyclic fatigue, nickel titanium, instruments.         has been shown to improve cyclic fatigue resistance (11).

Annali di Stomatologia 2013; IV (1): 149-151                                                                               149
A. Giansiracusa Rubini et al.


In order to minimize these risks, Yared proposed the use
of nickel titanium instruments of increased tapers in a re-
ciprocating motion (12). In the proposed technique, canal
is negotiated with a stainless steel size 08 hand file and
than an F2ProTaper NiTi rotary instrument is used for the




                                                                                                  li
canal preparation in a clockwise (CW) and counter clock
wise (CCW) movement. The CW and the CCW rotations




                                                                                               na
used by Yared were four-tenth and two-tenth of a circle re-
spectively and the rotational speed was 400 rpm (12). The
concept of using a single NiTi instrument to prepare the
entire root canal is interesting, and it is possible due to the




                                                                                            io
fact that reciprocating motion reduced instrumentation
stress. However, such a drastic change in the movement
kinematics needs to be assessed in terms of in vitro test-
ing of facture resistance. Recent literature seems to show




                                                                                         az
that reciprocating movements can provide mechanical ad-
                                                                  Figure 1. The testing apparatus.
vantages compared to continuous rotation (13, 14). These
preliminary positive results need further studies, also be-
cause many different reciprocating movement and many              compared to continuous rotation. In continuous rotation




                                                                          n
different instrument designs can be used in clinical prac-        (CR group) instruments mean time to failure was 8,75 sec
tice, thus affecting the overall results.                         (SD 1,34) while for the reciprocating motion (RCP group)




                                                                       er
The aim of the present study is to valuate if reciprocating       mean time to failure was 14,55 sec (SD 1,56).
motion can increase Hyflex instrument lifetime in a cyclic
fatigue test compared to continuous rotation. The null hy-

related to the different operative mode.
                                             nt
pothesis is that there is no difference in fatigue resistance     Discussion

                                                                  The results of the present study showed that reciprocat-
                                                                  ing motion could extend cyclic fatigue life of Hyflex NiTi in-
                                           iI
Materials and methods
                                                                  struments compared with continuous rotation. The null hy-
                                                                  pothesis was rejected, since the different movements
24 Hyflex® size 40 taper.04 nickel titanium instruments
                                                                  affected the lifespan of the NiTi instruments. These results
                                         on

were randomly divided in two groups (n=12 each). All in-
                                                                  are in accordance with another recent research article,
struments had been previously inspected by using an op-
                                                                  which demonstrated that movement kinematics, had a
tical stereomicroscope with 20 magnification for morpho-
                                                                  significant influence on cyclic fatigue life (13).
logic analysis and for any signs of visible deformation. If
                                                                  Fatigue failure usually occurs by the formation of micro
defective instruments were found, they were discarded.
                                i


                                                                  crack at the surface of the file that starts from surface ir-
                             iz


All instruments were then submitted to a cyclic fatigue
                                                                  regularities. During each loading cycle micro cracks de-
test. The first group (CR group) were used with a contin-
                                                                  velop, getting deeper in material, until complete separa-
uous rotation, while the second one (RCP group) with a
                                                                  tion of the file (15). All endodontic file shows some
              Ed




reciprocating motion. A specific endodontic motor (Acteon
                                                                  irregularities on the surface, and inner defect, as a con-
dual endoSatelec, France), that allows both movements,
                                                                  sequence of the manufacturing process, and distribution
was used. The selected reciprocation motion was the
                                                                  of these defects influence fracture strength of the en-
following one: 150 degree of counter clock wise (CCW) ro-
                                                                  dodontic instruments (16, 17). Consequently instrument
tation followed by a 30 degree of clock wise (CW) rotation.
                                                                  fatigue life can be regarded as a function of the load, ir-
In both movements speed was set at 300 rpm.
                                                                  regularities and size of crack on the surface (15).
 IC




The cyclic fatigue tests were performed by using a stain-
                                                                  Endodontic rotary file are subjected to a bending stress
less steel block containing an artificial canal shaped to
                                                                  during their clinical use in a curved canal. A bending
form a 135° angle (Fig. 1). Instruments were placed into
                                                                  force applied to an instrument result in a tensile stress,
the artificial canal at the same depth, and rotated or re-
                                                                  that tends to open micro cracks, on the convex side, and
C




ciprocated until fracture occurred. The time to fracture was
                                                                  in a compression stress, that tend to close micro cracks,
recorded visually with a 1/100 second chronometer. The
                                                                  on the concave side. When an instrument is in continuous
same operator performed all tests.
                                                                  rotation in a curve, tensile stresses follow a sinusoid
Means and standard deviations of time to fracture were
                                                                  trend. In fact there is an alternation of tensile and com-
©




calculated, and statistical analysis were performed by
                                                                  pression stresses when a certain portion of the instrument
using univariate analysis or post-hoc analysis in soft-
                                                                  translate from convex side to concave side.
ware (SPSS for Windows 11.0) and the level of signifi-
                                                                  Each 360° rotation identify one loading cycle in which
cance was set to p<0.05.
                                                                  each point of instrument surface go through maximum
                                                                  tensile stress and maximum compressive stress.
Results                                                           In other words during one cycle the crack open and close
                                                                  once. On the opposite, the Reciprocating motion used in
Results indicated that reciprocating motion showed a              the present study consisted in 150° of rotation CCW and
significant increase (p<0,05) in the time to failure when         30° of CW rotation.

150                                                                                  Annali di Stomatologia 2013; IV (1): 149-151
Influence of file motion on cyclic fatigue of new nickel titanium instruments


Therefore, the instrument approximately turned 5/12 of the                 struments: a large cohort clinical evaluation. J Endod 2006;
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                                                                     4.    Iqbal MK, Kohli MR, Kim JS. A retrospective clinical study
rotation. Therefore, in the same amount of time (overall ro-
                                                                           of incidence of root canal instrument separation in an en-




                                                                                                         li
tational speed, which is calculated by number of rotations                 dodontics graduate program: a PennEndo database study.
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                                                                                                      na
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                                                                                                   io
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                                                                                                az
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                                                                               n
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                                                                            er
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facturing technique (CM wire technology) used in the                       Endod 2006; 32:372-5.
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                                               nt
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                                                                     11.
                                                                           tanium instruments produced with a new manufacturing
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                                           on

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                                 i


We may conclude that reciprocating motion extended re-               14.   You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifes-
                              iz


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              Ed




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©




Annali di Stomatologia 2013; IV (1): 149-151                                                                                           151
