10.59987/ads/2024.1.3-8
The purpose of the present study was to evaluate the influence of the two different heat treatments of the mechanical properties of two commercial products: X7 Firewire and X7 Utopia Nickel-titanium rotary instruments (EdgeEndo, Albuquerque, New Mexico). The present article was written following the guidelines of PRILE (Prefered Reporting Items for Laboratory studies in Endodontology). Since both instruments are available with same design, size and taper the influence of the two different heat treatments on various mechanical properties could be properly evaluated. 50 instruments for each group, as determined by power analysis, were selected and tested using methods and devices validated in previous studies. Data were collected and statistically analyzed using a 1-way ANOVA test followed by the post hoc Tukey test with significance set to a 95% confidence level. Results showed that X7 Utopia were found to be more rigid, with a statistically significant difference when compared to X7 Firewire. Similarly, X7 Utopia were found to be significantly more resistant to torsion, while no statistically significant difference was found between the two tested instruments when subjected to a cyclic fatigue test. Cutting efficiency was significantly higher for X7 Utopia instruments, and also instrumentation time was significantly shorter when compared with X7 Firewire. X7 Firewire instruments showed a higher tendency to flues deformations during usage. We can conclude that thermal treatments of nickel-titanium instruments can significantly impact all their mechanical properties in vitro and overall performance in root canal procedures. Therefore, understanding the differences in thermal treatments is crucial for manufacturers to improve instruments and for dental professionals to tailor these instruments to specific clinical requirements.
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Annali di stomatologia
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A Comprehensive Multimethod Analysis of Mechanical Properties of two different heat treatments for endodontic Nickel-titanium instruments
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A Comprehensive Multimethod Analysis of Mechanical Properties of two different heat treatments for endodontic Nickel-titanium instruments
https://www.annalidistomatologia.eu/ads/article/view/281/292
15
A Comprehensive Multimethod Analysis
of Mechanical Properties of two different
heat treatments for endodontic
Nickel-titanium instruments
Nicola Maria Grande1                                                                        Authors
Krithika Datta2                                                                             Nicola Maria Grande -Università Cattolica
Lucila Piasecki3                                                                            del Sacro Cuore, Roma.
Maurilio D’Angelo4
                                                                                            Krithika Datta - Saveetha Dental Institute,
Edit Xhaijanka5                                                                             Chennai, India.
Luca Testarelli4
                                                                                            Lucila Piasecki - New York University at
Dario di Nardo4
                                                                                            Buffalo, USA.
1
  Università Cattolica del Sacro Cuore, Roma                                                Maurilio D’Angelo - Sapienza, Università
                                                                                            di Roma.
2
  Saveetha Dental Institute, Chennai, India
3
  New York University at Buffalo, USA                                                       Edit Xhaijanka - Università di Tirana,
4
  Sapienza, Università di Roma                                                              Albania.
5
  Università di Tirana, Albania                                                             Luca Testarelli - Sapienza, Università di
                                                                                            Roma.

Corresponding author: Maurilio D’Angelo                                                     Dario di Nardo - Sapienza, Università di
                                                                                            Roma.
maurilio.dangelo@uniroma1.it

Abstract
The purpose of the present study was to evaluate the influence of the two different
heat treatments of the mechanical properties of two commercial products: X7 Fire-           License
wire and X7 Utopia Nickel-titanium rotary instruments (EdgeEndo, Albuquerque, New           This work is licensed under a Creative
Mexico). The present article was written following the guidelines of PRILE (Prefered        Commons Attribution-NonCommercial-
Reporting Items for Laboratory studies in Endodontology). Since both instruments            NoDerivatives 4.0 International License.
are available with same design, size and taper the influence of the two different heat
                                                                                            Authors contributing to Annali di
treatments on various mechanical properties could be properly evaluated. 50 instru-         Stomatologia agree to publish
ments for each group, as determined by power analysis, were selected and tested             their articles under the Creative
using methods and devices validated in previous studies. Data were collected and            Commons Attribution-NonCommercial-
statistically analyzed using a 1-way ANOVA test followed by the post hoc Tukey test         NoDerivatives 4.0 International License,
with significance set to a 95% confidence level. Results showed that X7 Utopia were         which allows third parties to copy and
                                                                                            redistribute the material providing
found to be more rigid, with a statistically significant difference when compared to
                                                                                            appropriate credit and a link to the
X7 Firewire. Similarly, X7 Utopia were found to be significantly more resistant to tor-     license but does not allow to use the
sion, while no statistically significant difference was found between the two tested        material for commercial purposes and to
instruments when subjected to a cyclic fatigue test. Cutting efficiency was signifi-        use the material if it has been remixed,
cantly higher for X7 Utopia instruments, and also instrumentation time was signifi-         transformed or built upon.
cantly shorter when compared with X7 Firewire. X7 Firewire instruments showed a
                                                                                            How to Cite
higher tendency to flues deformations during usage. We can conclude that thermal
                                                                                            NM Grande, K Datta, L Piasecki,
treatments of nickel-titanium instruments can significantly impact all their mechan-        M D’Angelo, E Xhaijanka, L Testarelli,
ical properties in vitro and overall performance in root canal procedures. Therefore,       D di Nardo.
understanding the differences in thermal treatments is crucial for manufacturers to         A Comprehensive Multimethod Analysis
improve instruments and for dental professionals to tailor these instruments to spe-        of Mechanical Properties of two different
cific clinical requirements.                                                                heat treatments for endodontic
                                                                                            Nickel-titanium instruments
                                                                                            Annali Di Stomatologia, 15(1), 3-8.
Key words: endodontic instruments, nickel-titanium, heat treatment                          https://doi.org/10.59987/ads/2024.1.3-8

Introduction
Nickel-titanium (NiTi) instruments are widely used in endodontics due to their flexibil-
ity, shape memory, and resistance to cyclic fatigue. In the last decades NITi rotary in-
strumentation technique have been considered as golden standard to achieve proper
canal shaping with a more efficient, rapid and simple clinical approach, even if intra-
canal separation or rotary instruments is still a major concern for the majority of cli-
nicians (1). In the last decades improvements in design, motions and manufacturing
have been proposed to provide clinicians with safer and more efficient NiTi rotary in-
struments. In recent years, however, the majority of manufacturers have focused their
interest in improving heat treatment (HT) procedures before, during or after the grinding
process (1,2) . Several studies have shown the benefits of such treatments; however,


10.59987/ads/2024.1.3-8                                                                                                  3
    A Comprehensive Multimethod Analysis of Mechanical Properties of two different heat treatments for endodontic Nickel-titanium instruments

    they are proprietary and not disclosed by manufactur-                   querque, New Mexico, USA) and have a constant taper,
    ers (3-5).It has been shown that many mechanical prop-                  a parabolic cross-section, a non cutting tip and a 1mm
    erties can be influenced by the heat treatments, even                   maximum flute diameter. Even if design and dimensions
    if overall performance is still a combination of instru-                are same the X7 NiTi rotary instruments are currently
    ments’ design, dimensions, motions and alloy (6-8)                      commercialized with two different names, due to two
    Currently the heat treatment of endodontic NiTi instru-                 different types of heat treatment. “Fire-Wire” X7 instru-
    ments has become a crucial aspect of their manufac-                     ments are made of an Annealed Heat Treated (AHT)
    turing process, playing a pivotal role in enhancing their               nickel-titanium alloy brand named Fire-Wire (more duc-
    mechanical properties and overall performance in root                   tile). According to the manufacturer (11) such alloy im-
    canal procedures. Heat treatment involves subjecting                    proves flexibility, resistance to cyclic fatigue and reduces
    the NiTi alloy to specific temperature and time condi-                  bounce-back effect inside curvatures, and instruments
    tions to modify its microstructure. The primary goals of                closely follow the anatomy of the canal without straight-
    heat treatment for endodontic NiTi instruments are to                   ening out, reducing the risk of ledging, transportation,
    enhance their flexibility, resistance to fracture, and to es-           and perforation. It also allows the NiTi rotary instru-
    t0ablish a balanced combination of hardness and tough-                  ments to be easily straightened with clinicians fingers
    ness (5-8).                                                             (prebendable). The EdgeX7 “Utopia” is a more recently
    The process typically involves a sequence of steps,                     commercialized, NITi rotary instrument that, according
    including solution treatment, quenching, and aging.                     to manufacturer,provides all of the benefits of the origi-
    During solution treatment, the NiTi alloy is heated to a                nal Firewire X7 blade design while taking performance
    temperature where it transitions from a martensitic to                  to a different level with more cutting efficiency, due to a
    an austenitic phase. This phase transition is critical for              different proprietary heat -treatment (11).
    imparting the desired shape memory and superelas-                       Since both instruments are available with same design,
    ticity to the instruments. Quenching follows, where                     sizes and tapers, and are meant to be used with the
    the heated alloy is rapidly cooled to lock in the austen-               same protocol and same motion’s parameters (rotational
    itic phase and achieve the desired mechanical prop-                     speed and torque), the purpose of the present study was
    erties. Subsequently, aging is performed to optimize                    to evaluate the influence of the two different heat treat-
    the balance between hardness and toughness, ensur-                      ments of the mechanical properties of the two commer-
    ing the instrument’s durability during clinical use (1,7).              cial products (X7 Firewire and X7 Utopia instruments).
    The controlled application of heat treatment mainly ad-                 The null hypothesis was that no changes in the in vitro
    dresses some of the challenges associated with NiTi                     mechanical properties were provided by the different
    instruments, such as their susceptibility to cyclic fatigue             heat treatments.
    and potential for breakage. The process enhances the
    instruments’ fatigue resistance by refining the grain struc-            Materials and Methods
    ture and controlling phase transformations, leading to a                The present article was written following the guidelines
    more robust and reliable endodontic tool (9). Manufactur-               of PRILE ( Prefered Reporting Items for Laboratory stud-
    ers continually refine heat treatment processes to tailor               ies in Endodontology (Nagendrabu 2021) , as shown in
    NiTi instruments for specific clinical applications (10).               the flow chart of the study (Fig.1).
    X7 instruments are manufactured by Edge Endo (Albu-




    Figure 1.



4                                                                                                                    10.59987/ads/2024.1.3-8
N.M. Grande et al.

Sample selection                                               length of the fragments (FL) was measured with a digital
                                                               caliber and statistically analyzed to evaluate the correct
A total of 50 new X7 NiTi instruments and 50 new XT            positioning of the instruments inside the artificial canal
Utopia NiTi instruments size 25 and .04 taper (Edg-            and to verify the comparability of the results of the cyclic
eEndo, Albuquerque, New Mexico, USA) were selected             fatigue test. All data were statistically analyzed using a
for the study and divided in 5 groups of ten each (Fig         1-way ANOVA test followed by the post hoc Tukey test
2a). Each group was then subjected to one of the follow-       with significance set to a 95% confidence level. Mean
ing mechanical tests: stiffness, cyclic fatigue, torsional     values, the standard deviations and statistical signifi-
resistance, cutting efficiency, durability. According to       cance of the cyclic fatigue tests are displayed in table 1.
manufacturer all the instruments have same design and
dimensions, while they differ only due to different heat
                                                               Torsional test
treatments, even if such manufacturing processes are
proprietary and not disclosed (11).                            Ten instruments for each product underwent the torsion-
Before starting the laboratory assessment all the 100          al resistance test using a methodology which has been
instruments were examined under dental stereomicro-            validated by studies published in peer-review indexed
scope (Kaps,Asslar,Germany) at x10 magnification to            journals (16) and follows iSO Guidelines 3630-1. Tests
identify major irregularities or defects in the blade de-      were performed with a custom-made torsiometer-like
sign, which could affect properties of the NiTi instru-        device at 300 rpm, because it has been demonstrated
ments and make them not valid for the investigation. No        that rotational speed does not affect the results. The
instrument was discarded and all 100 samples were ac-          device allowed to avoid the bending of the coronal part
cepted for the study.                                          of the instrument and to have a straight angle of inser-
Sample size for each mechanical test was determined            tion, since it has been demonstrated that such coronal
by power analysis and calculated based on preliminar           intereferences and stresses can deeply influence the
data obtained after 6 initial measurements with a power        torsional resistance. The test was performed blocking
of 80% and a 0,05 alpha type of error. For the five previ-     the tip of the instrument with a vise at 3 mm from the tip
ously mentioned tests sample size calculations were 3.         (fig 2d) and rotating it at 300 rpm in the clockwise direc-
4, 3, 4 and 6 respectively and, consequently, a total num-     tion with a dedicated electronic motor (Kavo, Biberach,
ber of 10 instruments per group was considered more            Germany) allowing a real-time (0.1 seconds) recording
than enough for each dependent variable.                       of the torque with a sensitivity of 0.05 Ncm. The torque
                                                               at fracture results were collected on a spreadsheet. The
Stiffness Test                                                 length of the fragments (FL) was measured with a digital
                                                               caliber and statistically analyzed to evaluate the correct
Ten instruments for each product underwent the stiff-          positioning of the instruments’ tip inside the torsiometer
ness test (resistance to bending stress). The stiffness        and to verify the comparability of the results of the tor-
tests were performed using a device, which has been            sional test.
used in previously published peer-review studies (12)          All data were statistically analyzed using a 1-way ANO-
and follows ISO 3630-1 international standard guidelines       VA test followed by the post hoc Tukey test with signifi-
for mechanical tests of endodontic instruments (fig 2b).       cance set to a 95% confidence level. Mean values, the
The device consists of a load cell, an electronic display,     standard deviations and statistical significance of the
and a mobile holder to allow repeatable positioning of         torsional tests are displayed in table 1
the instruments on the load cell. The stiffness tests were
performed by bending each file at a 45° angle at 3 mm          Cutting efficiency test
from its tip and recording the applied force (g). The mea-
surements indicated by the electronic display connected        Ten instruments for each product underwent the cut-
to the load cell were recorded. The higher the values, the     ting efficiency test using a methodology (Fig 2e) which
stiffer (less flexibile) the instrument was. Mean values,the   has been validated by studies published (17,18) in
standard deviations and statistical significance of the cy-    peer-review indexed journals. The device consisted
clic fatigue tests are displayed in table 1.                   of a main frame to which a mobile plastic support for
                                                               the handpiece was connected and a stainless-steel
Cyclic fatigue test                                            block containing the Plexiglas plates (Inplex, Rome,
                                                               Italy), against which the cutting efficiency of the instru-
Ten instruments for each product were subjected to the         ments was tested. A notch 1 mm in depth and width
test . All instruments were rotated in a stainless-steel ar-   had been created on the lateral wall of the Plexiglas
tificial canal of 16 mm characterized by a 90° angle of        plate that measured 1 mm in thickness, to prevent the
curvature and a 2-mm radius of curvature (fig 2c) using        instruments from slipping out the smooth surface of
glycerin as a lubricant to avoid any friction between the      the plastic. The dental handpiece was mounted upon a
files and the artificial canal. The methodology has been       mobile device connected to a fixed weight (150 g), that
validated by many studies published in peer-review in-         for gravity drove the horizontal instrument against the
dexed journals (13-15). Speed ( 300 rpm clockwise) and         Plexiglas block in a precise and reproducible way. The
torque (2N) were selected according to the manufactur-         plastic support for the handpiece allowed for precise
ers’ recommendation and each test was performed by             and simple three-dimensional alignment and position-
the same expert operator. Each instrument was carefully        ing of the instrument, as soon as it came perpendicu-
inserted at the same length (16mm) and rotated inside          larly into contact with the notch created on the wall of
the canal until a visible and/or audible sign of fracture      the Plexiglas specimen without bending. Once every-
was detected. The time to fracture (TtF) was measured          thing was fixed, the motor of the testing device was
using a digital chronometer with a sensitivity of 0.01 sec-    switched on and the instrument removed material and
onds. The test was performed at room temperature. The          penetrated actively The cutting efficiency was tested 6


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    A Comprehensive Multimethod Analysis of Mechanical Properties of two different heat treatments for endodontic Nickel-titanium instruments




    Figure 2.


    mm from the tip of each instrument (max diameter =                      speed and 2,5 N torque). Overall, each instrument was
    0,49 mm) and instruments were rotated at 350 rpm and                    supposed to prepare 20 canals (19) to working length
    2,5 N torque setting for 30 seconds. Each instrument                    without any breakage or deformation of flutes (fig 2g).
    was tested in linear cutting unidirectional lateral mo-                 An initial manual glide-path using a manual k-file n.15
    tion and the maximum penetration depth of the instru-                   was performed to ensure patency and a slight preliminar
    ments was the criterion for cutting Each plastic block                  enlargement to facilitate the .04 25 instruments progres-
    was used to test one instrument from each of the two                    sion to the working length. All canals were prepared by
    groups tested. The precise length of the plastic block                  the same expert clinicians using Mimeraci technique
    cut in 1 min was measured in mm for all groups tested                   in steps: manual insertion, activation and progression
    using a computerized program (Adobe Photoshop CS4)                      in small steps (1-2 mm) , removal of the file from ca-
    with a precision of 0.1 mm. The 1 mm notch was sub-                     nal, cleaning of flutes and irrigation with distilled water.
    tracted to the length obtained. Maximum penetration                     Each step of the technique was repeated till working
    depth was calculated, mean and standard deviations of                   length was reached.The total instrumentation time, the
    each group were calculated and data were statistically                  incidence of instruments ‘ separation or deformation of
    analyzed with a one-way ANOVA test with significance                    flutes ( under microscope inspection at x10 magnifica-
    set at 95 % confidence interval.                                        tion) were recorded. For the instrumentation time mean
                                                                            and standard deviations of each group were calculated
    Durability test                                                         and data were statistically analyzed with a one-way
    Each instrument was used to prepare five artificial 3d                  ANOVA test with significance set at 95 % confidence
    plastic (Fig 2f) molar tooth (Orodeka,Firenze, Italia) with             interval. Data concerning separated or deformed instru-
    the same motor and the same parameters (350 rpm                         ments were only recorded.


    Table 1. Mean (standard deviation) results of different tests for the two instruments
                Test                       Parameters                   x7 firewire              x7 utopia                P-values

     Stiffness                    maximum load                      131,3 +/- 9,2          84, 2 +/- 6,5            <.001
     Cyclic fatigue               time to fracture (s)              19,6 +/- 2,9           20,3 +/- 3,9             .412
     Torsional resistance         Maximum torque (N. cm)            1, 34 +/- 0,28         1.01 +/- 0,19            <.001
     Cutting ability              Penetration (cm)                  13,4 +/- 2,6           9,8 +/- 3,9              <.001
     Instrumentation time         Seconds (s)                       12,9 +/- 7,7           16,1 +/- 9,2             <.001
     Intracanal breakage          number of instruments             0                      0                        NA
     Flute Deformation            number of instruments             4                      0                        NA



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N.M. Grande et al.

Results                                                        Differences in thermal treatments of NiTi endodontic in-
                                                               struments stem from variations in methods and objec-
Results are summarized in table 1. For the stiffness test
                                                               tives (10). A relevant topic is in the pursuit of a balance
X7 Utopia were found to be more rigid, with a statistically
                                                               between flexibility and resistance to cyclic fatigue. Heat
significant difference when compared to X7 Firewire.
                                                               treatment seeks to achieve this equilibrium through care-
X7 Utopia were also found to be more resistant to tor-
                                                               ful control of temperature and time during the thermal
sion, showing significantly higher values for maximum
                                                               processes. The challenge is to prevent excessive hard-
torque at failure when compared to X7 Firewire, while
                                                               ness that could compromise flexibility while ensuring
no statistically significant difference was found between
                                                               sufficient toughness to resist cyclic loading, by creating
the two tested instruments when subjected to a cyclic          gradient structures within the material to optimize both
fatigue test. For both torsional and cyclic fatigue tests no   flexibility and fatigue resistance.
significant differences were noted in the two groups con-      In the present study all the tested instruments had a simi-
cerning fragment lengths, demonstrating a correct test-        lar resistance to a cyclic fatigue test which was performed
ing procedure. Under the conditions of the present test,       in a very challenging complex ,abrupt apical curvature.
cutting efficiency was significantly higher for X7 Utopia      Differences in the thermal treatment protocols contrib-
instruments, and also instrumentation time was signifi-        ute to the development of NiTi instruments with specific
cantly shorter when compared with X7 Firewire. During          characteristics for various clinical applications (1,7). For
durability tests all instruments were able to reach work-      instance, instruments designed for shaping procedures
ing length without any intracanal breakage. On the con-        may undergo thermal treatments that prioritize flexibility
trary four X7 Firewire instruments which exhibited visible     to navigate curved root canals efficiently. In contrast, in-
signs of flute deformation were discarded and were not         struments intended for more simple and rapid techniques
able to prepare all the 20 canals, while no X7 Utopia in-      may prioritize durability and cutting efficiency. Results
strument showed any sign of plastic deformation.               from the present study confirmed the significant impact
                                                               of the different heat treatments. They showed that the
Discussion                                                     X7 Utopia were less flexible, but more resistant to tor-
                                                               sion and efficient in cutting when compared to Firewire
The results of the present study showed that thermal
                                                               X7. This property may be also more helpful in retreat-
treatment of nickel-titanium endodontic instruments in-
                                                               ment cases, making removal of gutta-percha easier and
volveing intricate processes can significantly impact all
                                                               faster. Moreover less plastic deformation of flutes were
their in vitro mechanical properties (flexibility, strength    observed after clinical use, leading to more durability.
and cutting ability) and overall performance in root canal     The question whether such plastic deformations are a
procedures (10, 20). Even if many factors contribute to the    weak point or not is still open. Obviously permanently
success of endodontic therapy, root canal instrumenta-         deformed rotary instruments should be discarded (which
tion has a relevant role, because it create a proper shape     negatively affects durability), but such feature is also
to perform both final irrigation and obturation procedures     considered beneficial, since it is a clinical warning that
correctly (21-25). Understanding the in vitro differences      could prevent sudden, unexpected intracanal failure. It
in thermal treatments is crucial for manufacturers to          is considered a warning because such plastic deforma-
provide instruments with different properties and dental       tions usually occurs immediately prior to breakage.
professionals to tailor these instruments to specific clini-   In summary we may conclude that heat treatment meth-
cal requirements (1,5). In the present study two different     ods provide a well-established positive approach, and
heat treatments from the same manufacturer and applied         new technologies offer more sophisticated and targeted
to the same instrument design were tested and results          modifications, like the differences shown between the two
showed significant differences between the two groups.         tested X7 instruments. Such avenues contribute to the
As dimensions and designs were same these difference           ongoing evolution of NiTi rotary instruments, continually
are only related to the difference in the heat treatments.     improving their performance and expanding their appli-
Heat treatment typically involves processes like aus-          cability in endodontic practice, ultimately benefiting den-
tenitization, quenching, and aging (9). Austenitization        tal practitioners and patients alike by providing tailored
involves heating the NiTi alloy to a specific temperature      instruments for the case and overall by enhancing the
to transform it from a martensitic to an austenitic phase.     efficiency and safety of endodontic shaping procedures.
Subsequent quenching rapidly cools the alloy, fixing
the desired phase and enhancing properties like shape          References
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10.59987/ads/2024.1.3-8                                                                                                               7
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     Contributions       Conceptualization       Methodology       Validation      Data           Writing–         Writing–     Supervision
                                                                                  Curation      Original draft     review
     Author 1
     Author 2
     Author 3
     Author 4
     Author 5
     Author 6
     Author 7

    Funding: This research was partially funded from Sapienza University of Rome “ Progetto Medio di Ateneo 2020 su
    test per strumenti endodontici”
    Conflicts of Interest: The authors declare no conflict of interest.



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