This study evaluated the cyclic fatigue resistance of three rotary instrument systems - Sequence, HyFlex EDM, and ProTaper Next (PTN) - after immersion in 5.25% sodium hypochlorite (NaOCl) solution for different time periods. The instruments immersed in NaOCl showed lower cyclic fatigue resistance than the non-immersed controls. HyFlex EDM instruments demonstrated the highest resistance to fatigue, while Sequence instruments had the lowest. Immersion in NaOCl negatively impacted the fatigue life of the instruments, with longer immersion times further reducing resistance. The study found that NaOCl immersion can decrease the fatigue performance of rotary instruments.

1. EFFECT OF IMMERSION IN SODIUM
HYPOCHLORITE ON THE CYCLIC FATIGUE
RESISTANCE OF THREE ROTARY
INSTRUMENTS
Nogueira D, Silveria Bueno CE, Kato AS, De Martin AS, Pelegrine RA, Silva
Limoeiro AG, et al J Conserv. Dent 2020;23:554-7
Presented by
Dr Nadeem Aashiq
MDS 1ST YEAR

2. INTRODUCTION
 The introduction of nickel titanium (NiTi) alloy in the manufacture of
endodontic instruments represented a breakthrough in endodontics.
 Its favorable mechanical properties allowed the fabrication of
instruments with new designs, greater taper, alternative systems,
and the introduction of a continuous rotation movement for the
cleaning and shaping the root canals.
 Several strategies have been used by manufacturers to improve
fatigue strength, including different designs in cross-sections, alloys
with different mechanical properties, or optimized manufacturing
processes.
 The new NiTi alloys, such as M-wire, Phase-R, and CM-wire,
exhibit properties, such as superior flexibility and higher fracture
resistance, advantageous characteristics for the instrumentation of
more complex root canals.

3.  Torsional fatigue and cyclic fatigue are the most common fracture
causes in NiTi rotary instruments.
 Among the factors that may influence the resistance of the
endodontic instrument are the anatomical conditions, such as radii
and angles of curvature of the root canal, the number of uses of the
instrument, the torque used, and the ability of the operator.
 The speed of rotation employed, the number of sterilization cycles
and the type of irrigation substance can also influence the cyclic
fatigue resistance of the NiTi instruments.
 Of the irrigating solutions currently used in endodontic treatment,
sodium hypochlorite (NaOCl) satisfies most of the requirements and,
due to its effective antimicrobial action, and mainly due to its
capacity for tissue dissolution, has become the most used by
endodontic professionals.
 NiTi instruments with CM-wire offer better performance and safety.
The surface treatment makes the files more flexible, more resistant
to fatigue, and efficient in cutting.

4. AIM & OBJECTIVES
 The objective of this work was to evaluate and compare the cyclic
fatigue strength of three systems with different alloys and heat
treatments, after immersion in 5.25% NaOCl.
 The null hypotheses tested were as follows:
1. That there would be no difference between the cyclic fatigue
resistance of sequence rotary file, HyFlex EDM,and ProTaper Next
(PTN)
2. The immersion of the instruments into the NaOCl would not affect
the cyclic fatigue resistance.

5. METHODS
 90 new files of three different instruments were
selected for test
Sequence rotary file (SEF)
HyFlex EDM (EDM)
Pro Taper Next (PTN)
 The thirty files each group were separated into
three sub groups
 Group 1 ( SEQ1, EDM1, PTN1) not immersed in
5.25% of Naocl
 Group 2 ( SEQ2, EDM2, PTN2) immersed in
5.25% of Naocl for 1 min
 Group 3 (SEQ3, EDM3, PTN3) immersed in
5.25% of Naocl for 5 min

6.  All the instruments were previously analyzed using the optical
microscope Opto (Opto, São Carlos, Brazil) with a ×10, to discard
those with deformities.
 The endodontic instruments were fixed to the X-Smart electric motor
(Dentsply Maillefer, Baillagues, Switzerland) and operated at a
constant speed according to the manufacturers’ recommendations
(Sequence Rotary File-400 RPM and 2.0 Ncm, Hyflex EDM-500 RPM
and 2.5 Ncm, and PTN -300 RPM and 2.0 Ncm).
 The instruments were inserted 19 mm from its tip, into a recipient that
was filled with the amount of 5.25% NaOCl. After immersion, the
instruments were washed in distilled water to neutralize any effect of
the solution.

7.  All instruments were subjected to the cyclic fatigue test in a device
with a simulated artificial stainless steel canal and the ability to fix the
handpiece of the X-Smart electric motor (Dentsply Maillefer,
Baillagues, Switzerland)
 The instruments were positioned and operated at speed
recommended by the manufacturer, and the time elapsed between
the beginning of the experiment, and the time of the fracture was
registered (Vollo VL-501 Stopwatch).
 During the test, to reduce friction between the instruments and the
canal walls, dental material lubricant was used (Pana Spray Plus-
NSK GmbH Germany).

8.  For each instrument, the number of cycles up to the fracture was
calculated by multiplying the time in seconds until the fracture time by
the number of revolutions per second.
 The results were analyzed using the GraphPad Prism program, version
6.01.The fracture surfaces of the files of each group were observed
under a scanning electron microscope (Zeiss EVO 50) at ×200,×500,
×1.000 and ×3.000, to determine fracture patterns after immersion in
5.25% NaOCl.

9. RESULTS
The number of cycles until fracture of the instruments, measured in the
different groups, is shown in Table 1.
The control group showed a greater number of cycles until the fracture occurs
than the same instruments submitted to NaOCl (P < 0.001); these, in turn, were
also statistically different from each other (P < 0.001).

10.  Sequence instruments showed the lowest values of cycles until a
fracture occurs (P < 0.001). There was no significant difference between
treatments (P = 0.998).
 PTN instruments, showed intermediate values of fracture resistance
when compared to the HyFlex and Sequence instruments (P < 0.001),
but without intragroup differences.
 Two-way ANOVA for two independent factors, type of instrument and
treatment, revealed that in the control group, and the fragments of the
instruments presented different sizes among each other (P < 0.001). In
NaOCl treated groups, HyFlex and Sequence instruments presented
similar fragment sizes (P > 0.05), whereas PTN instruments had
significantly smaller fragment sizes (P < 0.001).

11.  The fracture of the sequence system occurred before the point of
curvature, the Hyflex EDM system had the fracture located at the
maximum point of curvature, and the PTN fractured after the point of
curvature.
 The fracture pattern of the species observed using the scanning
electron microscopy corresponds to the fracture characteristics due to
cyclic fatigue

12. DISCUSSION
 This study was aimed to evaluate and compare the cyclic fatigue
strength of three systems with different alloys and heat treatments
(sequence rotary file, HyFlex EDM, and PTN) after immersion in
5.25% NaOCl in two different times, 1 and 5 min, compared to no
immersion. Up to this moment, no study has evaluated the cyclic
fatigue resistance of these three instruments after immersion in
NaOCl.
 Corroborating with previous studies, the Hyflex EDM instruments
presented greater resistance to cyclic fatigue than PTN, and this
can be justified by their new thermal treatment. Thus, the first
hypothesis tested should be rejected.

13.  Gündogar and Özyürek investigated the cyclic fatigue resistance of the
OneShape, HyFlex EDM, WaveOne Gold, and Reciproc Blue systems,
where Hyflex EDM was superior to the other systems tested.
 Hyflex EDM showed an increase in fatigue strength of up to 700%
when compared to the Hyflex CM system.
 The unusual structural properties (a new shape adjustment and file
surface hardness) may be the reason why the number of cycles of this
system has been considerably larger when compared to the other two
systems tested in the present study.
 Cyclic fatigue resistance of instruments after immersion in NaOCl with
different protocols presented conflicting results in previous studies

14.  In this study, the fatigue resistance was negatively affected when the
instruments were immersed in NaOCl compared to those in which
were not immersed.
 The D-RaCe instruments had the highest cyclic fatigue resistance
than ProTaper and Mtwo after immersion in NaOCl.
 The endodontic instruments K3, K3XF, and Vortex, presented lower
resistance to cyclic fatigue when exposed to 5.25% NaOCl, with
temperature variation at 22°C, 37°C, and 60°C.
 Moreover, the NaOCl caused deteriorations and irregularities in the
surface of wave one and reciproc files, predisposing the instruments
to possible unexpected fractures

15.  Erik and Özyürek evaluated the effect of irrigating solutions on the
cyclic fatigue resistance of the HyFlex EDM, WaveOne Gold, and
Reciproc Blue instruments, using the association of 6% NaOCl
solution with 18% etidronic acid.
 Hyflex EDM was significantly more resistant than the other groups
under all conditions tested. In the intragroup comparison, all groups
showed a significant reduction in resistance to cyclic fatigue when
in contact with the combination of NaOCl and 18 % etidronic acid.
 The use of this solution caused corrosion in the systems tested and
may have been the factor causing the reduction in the resistance to
cyclic fatigue of the files.
 NaOCl at different concentrations and temperatures influenced the
cyclic fatigue resistance of the ProTaper Gold system, agreeing
with the results of PTN and Sequence presented in this study.
NaOCl did not have any effect on the cyclic fatigue resistance of
the WaveOne and WaveOne Gold, Twisted File and Hyflex CM
instruments, corroborating the results found in the Hyflex EDM
system.

16.  Regarding immersion in NaOCl solution, the studies present
controversial results, in which some concluded that the NaOCl
reduces the resistance to cyclic fatigue of the tested
instruments, and others indicated that the solution did not
change the resistance of the systems tested. This discrepancy
in the results observed in the literature can be explained by
the different methodologies and immersion protocols used,
making further studies necessary to standardize the methods
and devices used.

17. CONCLUSIONS
 Hyflex EDM instruments were the most resistant to cyclic fatigue
after immersion in 5.25% NaOCl. The PTN instruments had the
intermediate values and the sequence instruments had the
lowest values of resistance to cyclic fatigue.

18. REFERENCES
 Cyclic Fatigue Resistance of Heat-treated Nickel-titanium
Instruments after Immersion in Sodium Hypochlorite and/or
Sterilization
Pedullà E, Benites A, La Rosa GM, Plotino G, Grande NM, Rapisarda E, Generali L J Endod. 2018
Apr;44(4):648-653.
 Aim :- The purpose of this study was to assess the effects of sodium
hypochlorite (NaOCl) immersion and sterilization on the cyclic fatigue
resistance of heat-treated nickel-titanium (NiTi) rotary instruments.

19.  Methods: Two hundred ten new 25/.06 Twisted Files (TFs; SybronEndo, Orange,
CA) and Hyflex CM (Coltene Whaledent, Cuyahoga Falls, OH) files were divided into
7 groups (n = 15) for each brand. Group 1 (control group) included new instruments
that were not immersed in NaOCl or subjected to autoclave sterilization. Groups 2
and 3 were composed of instruments dynamically immersed for 3 minutes in 5%
NaOCl solution 1 and 3 times, respectively. Groups 4 and 5 consisted of instruments
only autoclaved 1 and 3 times, respectively. Groups 6 and 7 recruited instruments
that received a cycle of both immersion in NaOCl and sterilization 1 and 3 times,
respectively. Instruments were subsequently subjected to a fatigue test. The surface
morphology of fractured instruments was studied by field-emission scanning electron
microscopy and x-ray energy-dispersive spectrometric (EDS) analyses. The means
and standard deviations of the number of cycles to failure (NCF) were calculated and
statistically analyzed using 2-way analysis of variance (P < .05).
 Results: Comparison among groups indicated no significant difference of NCF (P >
.05) except for the groups of TFs sterilized 3 times without and with immersion in
NaOCl (P < .05). HyFlex CM files exhibited higher cyclic fatigue resistance than TFs
when files were sterilized 3 times, independently from immersion in NaOCl (P < .05).
EDS analysis showed the presence of an oxide-rich layer on the Hyflex CM files'
external surface. No morphologic or chemical differences were found between files of
the same brand subjected to different treatments.

20.  Conclusions: Repeated cycles of sterilization did not influence the
cyclic fatigue of NiTi files except for TFs, which showed a significant
decrease of flexural resistance after 3 cycles of sterilization.
Immersion in NaOCl did not reduce significantly the cyclic fatigue
resistance of all heat-treated NiTi files tested.

21.  Effects of Sodium Hypochlorite Concentration and Temperature on the
Cyclic Fatigue Resistance of Heat-treated Nickel-titanium Rotary
Instruments
Alfawaz H, Alqedairi A, Alsharekh H, Almuzaini E, Alzahrani S, Jamleh A. J Endod. 2018
Oct;44(10):1563-1566.
 Introduction: We tested the cyclic fatigue resistance of heat-treated instruments
immersed in sodium hypochlorite solution under different concentrations and
temperature conditions.
 Methods: Based on the irrigating solution's concentration and temperature, 135
ProTaper Gold (PTG; Dentsply Sirona, York, PA) F2 instruments were divided
equally into 9 groups of 15. Cyclic fatigue testing was performed by using a block
with artificial canals with a curvature angle of 60°, a curvature radius of 5 mm, and a
curvature center 5 mm from the instrument tip. The block was fixed inside a water
bath of distilled water, 2.5% sodium hypochlorite (NaOCl), or 5.25% NaOCl. The
temperature was preset at 25°C, 37°C, or 60°C. The instrument was rotated at 300
rpm until fracturing occurred. The number of cycles to fracture was calculated, and
the fragment length was measured. Fractured surfaces were examined via scanning
electron microscopy. NCF data were analyzed statistically via Kruskal-Wallis and
Mann-Whitney tests. All statistical analyses were performed using SPSS software
Version 22 (IBM Corp, Armonk, NY) at a 5% significance level.

22.  Results: The number of cycles to fracture of the PTG F2 was
highest in distilled water at 25°C and lowest in 5.25% NaOCl at 60°C.
Changing the irrigating solution from distilled water to NaOCl and
increasing the surrounding temperature reduced the fatigue
resistance.
 Conclusions: NaOCl irrigating solution at different concentrations
and temperatures influenced the cyclic fatigue resistance of PTG
instruments. Future NiTi instrument failure studies should be
conducted under simulated body temperature conditions in
commonly used irrigating solutions.

23. THANK YOU