10.59987/ads/2017.2.53-58
Aim. Aim of the study was to evaluate effectiveness of different heat treatments in improving Ni-Ti endodontic rotary instruments’ resistance to fracture.
Methods. 24 new NiTi instruments similar in length and shape: 12 M3 instruments, tip size 25 and .06 taper (United Dental, Shanghai, China), and 12 M3 Pro Gold instruments tip size 25 and .06 taper (United Dental, Shanghai, China), were tested in a 60° curved artificial root canal. Each group received a different heat treatment. Cycles to fracture were calculated for each instrument. Differences among groups were evaluated with an analysis of variance test (significance level was set at P&lt;0.05.).
Results. Statistical analysis found significant differences (p&lt;0.0213) between groups. The M3 Pro Gold instruments were significantly more resistant to fatigue (mean values = 1012, SD +/- 77) than M3 instruments (mean values = 748, SD +/-62). No statistically significant differences were found between fragments’ lengths (p&gt;0,05).
Conclusions. An increased flexibility and the reduction of internal defects produced by heat treatments during or after manufacturing processes, may be responsible for improving resistance to cyclic fatigue and flexural stresses.
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Annali di stomatologia
cyclic fatigue
In vitro resistance to fracture of two nickel-titanium rotary instruments made with different thermal treatments
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cyclic fatigue
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In vitro resistance to fracture of two nickel-titanium rotary instruments made with different thermal treatments
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                                                                                                Original article


    In vitro resistance to fracture of two nickel-titanium
    rotary instruments made with different thermal
    treatments




                                                                                             li
                                                                                           na
    Gabriele Miccoli DDS                                   Key words: endodontics, endodontic instruments,
    Gianfranco Gaimari DDS, PhD                            nickel-titanium, cyclic fatigue.
    Marco Seracchiani DDS




                                                                                         io
    Antonio Morese DDS
    Tatyana Khrenova DDS                                   Introduction

                                                           In the last decades the introduction of nickel-titanium
    Dario Di Nardo DDS




                                                                     az
                                                           (NiTi) alloy in the manufacturing of endodontic instru-
                                                           ments resulted in a significant improvement for root
    Department of Oral and Maxillo-Facial Sciences,        canal preparations which resulted easier in shaping
    “Sapienza” University of Rome, Rome, Italy             procedures, faster, and more predictable (1-3). The
                                                           superior mechanical properties of the NiTi alloy al-




                                                                   rn
                                                           lowed the clinical use of NiTi rotary instruments with
                                                           greater tapers (4, 5). This increase in dimensions and
                                                           continuous rotation movements significantly improved




                                                        te
    Dario Di Nardo                                         effectiveness and rapidity of the cutting and simplified
    Corresponding author:

    Department of Oral and Maxillo-Facial Sciences,        the achievement of a successful root canal treatment
    "Sapienza" University of Rome                          (6-8). Unfortunately, these features could also lead to
    Via Caserta 6                                          an increased risk of intracanal separation of the in-
                                                      In
    00161 Rome, Italy                                      struments (9-11).
    E-mail: dario.dinardo@uniroma1.it                      Several clinical and experimental studies demonstrat-
                                                           ed that multiple factors contribute to instrument’s sep-
                                                           aration: cyclic fatigue has been proven to be one of
                                ni

                                                           the leading causes (12-14). Fatigue failure usually
                                                           begins with the formation of microcracks that arise
                                                           from the surface’s irregularities of the instruments.
    Summary

                                                           During each loading cycle, microcracks develop and
                              io


                                                           deepening until complete separation of the file (15-
    Aim. Aim of the study was to evaluate effective-

                                                           17). All NiTi rotary instruments show some irregulari-
    ness of different heat treatments in improving Ni-

                                                           ties and inner defects on the surface as a conse-
                            iz



    Ti endodontic rotary instruments’ resistance to

                                                           quence of the manufacturing processes and the dis-
    fracture.

                                                           tribution of these defects influences their strength
    Methods. 24 new NiTi instruments similar in

                                                           (18).
            Ed




    length and shape: 12 M3 instruments, tip size 25

                                                           In recent years, manufacturers tried to find different
    and .06 taper (United Dental, Shanghai, China),

                                                           solutions to develop instruments with enhanced resis-
    and 12 M3 Pro Gold instruments tip size 25 and

                                                           tance to flexural and torsional stresses, aiming at re-
    .06 taper (United Dental, Shanghai, China), were

                                                           ducing the incidence of intracanal breakage. The
    tested in a 60° curved artificial root canal. Each

                                                           three basic ways to achieve these improvements
    group received a different heat treatment. Cycles

                                                           were mainly related to changes in design, heat treat-
    to fracture were calculated for each instrument.
 IC




                                                           ments of the alloy and the use of reciprocating mo-
    Differences among groups were evaluated with an

                                                           tions (19-23). Different and proprietary heat treat-
    analysis of variance test (significance level was

                                                           ments have been developed and commercialized in
    set at P<0.05.).

                                                           the last decade, aiming at improving both flexibility
    Results. Statistical analysis found significant dif-
C




                                                           and resistance to breakage, but they are not dis-
    ferences (p<0.0213) between groups. The M3 Pro

                                                           closed by manufacturers in detail (24). Changing the
    Gold instruments were significantly more resis-

                                                           thermal history of the alloy, could produce an alloy
    tant to fatigue (mean values = 1012, SD +/- 77)

                                                           with different characteristics for the endodontic use.
    than M3 instruments (mean values = 748, SD +/-
©




                                                           By changing the thermal treatments, manufacturer
    62). No statistically significant differences were

                                                           can quickly and significantly modify clinical perfor-
    found between fragments’ lengths (p>0,05).

                                                           mance of NiTi instruments, with no need of modifica-
    Conclusions. An increased flexibility and the re-

                                                           tions to the quality of the raw material or grinding ma-
    duction of internal defects produced by heat

                                                           chines. M3 Rotary and M3 Pro Gold are an example
    treatments during or after manufacturing pro-

                                                           of similar instruments, produced by same manufac-
    cesses, may be responsible for improving resis-
    tance to cyclic fatigue and flexural stresses.

    Annali di Stomatologia 2017;VIII (2):53-58                                                                  53
    G. Miccoli et al.


    turer (United Dental, Shanghai, China), but with dif-         on cyclic fatigue resistance (25, 26). The device con-
    ferent performance due to different heat treatments.          sists of a mainframe to which is connected the elec-
    The aim of the present study was to evaluate the hy-          tric handpiece and a stainless-steel block containing
    pothesis that different heat treatments can significant-      the artificial canal. The electric handpiece was
    ly affect the in vitro resistance to cyclic fatigue of NiTi   mounted on a mobile device to allow precise and re-
    rotary instruments. The null hypothesis was that no           producible placement of each instrument inside the
    difference would be found between similar instru-             artificial canal to the same depth (18 mm) (Fig. 1). A




                                                                                                      li
    ments with different heat treatments.                         simulated root canal with a 60° angle of curvature




                                                                                                    na
                                                                  and 5 mm radius of curvature was used for all the
                                                                  tested instruments. All instruments were inserted at
                                                                  the same length (16 mm) and then rotated at 350 rpm
                                                                  with maximum torque until fracture occurred. For
    Material and methods

    A total of 24 new NiTi instruments 25 mm in length            each instrument, the time to fracture was visually as-




                                                                                                  io
    was used in the present study: 12 M3 instruments, tip         sessed and recorded with a 1/100 sec chronometer.
    size 25 and .06 taper (United Dental, Shanghai, Chi-          Number of cycles to fracture was calculated for each
    na) and 12 M3 Pro Gold instruments tip size 25 and            instrument (NCF). Fragments were collected, mea-




                                                                            az
    .06 taper (United Dental, Shanghai, China). All instru-       sured and underwent to fractographic analysis per-
    ments were the same in size and design, but they re-          formed by a scanning electron microscope (SEM) to
    ceived a different heat treatment. All of them had            determine fracture mode.
    been previously inspected using an optical stereomi-          All data were recorded. For each group mean and




                                                                          rn
    croscope at x20 magnification for morphological anal-         standard deviations were calculated. Differences
    ysis and checked for any signs of visible deformation.        among groups were statistically evaluated with an
    If defective instruments were found, they were dis-           analysis of variance test (significance level was set at
    carded.                                                       P<0.05.). Data was statistically analyzed using the




                                                           te
    The cyclic fatigue testing device used in the present         SPSS 17.0 software (SPSS Incorporated, Chicago,
    study has been used for previously performed studies          IL, USA).
                                                         In
                              ni
                            io
                          iz
           Ed
 IC
C
©




                                                                           Figure 1. The testing device for cyclic fatigue.

    54                                                                                Annali di Stomatologia 2017;VIII (2):53-58
    In vitro resistance to fracture of two nickel-titanium rotary instruments made with different thermal treatments


    Table 1. Results of cyclic fatigue tests (number of cycles to failure).

    Group                    NCF Mean Values                SD                Mean Fragment Lenght                 SD
    M3                       748                            (+/- 62)          5,4                                  (+/- 0,4)
    M3 ProGold               1012                           (+/- 77)          5,5                                  (+/- 0,5)




                                                                                                         li
                                                                                                       na
    Table 2. Time to fracture in seconds (s).                          Discussion
    Group            Seconds to fracture            SD                 NiTi endodontic alloys can exhibit three phases (27):
    M3               128,3                          (+/- 11,6)         the high-temperature B2 austenitic phase, the low-




                                                                                                     io
                                                                       temperature B19’ martensitic phase (monocyclic
    M3 ProGold       173,5                          (+/- 14,7)
                                                                       structure) and intermediate temperature R-phase
                                                                       (rhombohedral structure). Transformations of these




                                                                                 az
                                                                       phases are fundamental, because they determine the
                                                                       superelastic and shape memory characteristics of
                                                                       these instruments and their mechanical and function-
    Results

    Results from the cyclic fatigue tests are shown in Ta-             al properties and performance. These transforma-
                                                                       tions can proceed by various ways, depending on the




                                                                               rn
    bles 1 and 2. Mean values for time to fracture for M3
    Pro Gold instruments were 173,5 seconds (SD +/-                    thermal history of the alloy during the manufacturing
    14,7) and for M3 instruments were 128,3 seconds                    processes (18).
    (SD +/- 11,6). Statistical analysis found significant dif-         The heat treatment’s parameters which are chosen to




                                                            te
    ferences (p < 0.0213) between the two groups. The                  set the properties of the NiTi instruments are critical
    M3 Pro Gold instruments were significantly more re-                (23). In general, temperatures as low as 400°C and
    sistant to fatigue (NCF mean values = 1012, SD +/-                 times as short as 1-2 minutes can set the shape, but
                                                          In
    77) than M3 instruments (NCF mean values = 748,                    generally, endodontic instrument’s manufacturers
    SD +/- 62). Mean value of the fragments’ length for                prefer temperatures closer to 500°C for over 5 min-
    M3 Pro Gold instruments was 5,5 mm (SD +/- 0,5)                    utes. A rapid cooling of the instruments is preferred
    and for M3 instruments was 5,4 mm (SD +/- 0,4). No                 via water quench or rapid air cool. Longer heat treat-
    statistically significant differences were found be-               ment times and higher temperatures will increase the
                                 ni

    tween fragment lengths (p > 0,05), showing proper in-              actuation temperature and often give a sharper ther-
    sertion of the instruments inside artificial canals and            mal response (in the case of shape memory ele-
    consequently the same mechanical stresses applied                  ments). However, there is usually a concurrent drop
                               io


    on the same portions of the instruments (Fig. 2).                  either in peak force (for shape memory alloys) or in
                             iz
            Ed
 IC
C
©




    Figure 2. Differences between instruments' time to fracture, number of cycles to fracture (NCF) and lenght of the fractured
    segment


    Annali di Stomatologia 2017;VIII (2):53-58                                                                                 55
    G. Miccoli et al.


    plateau stresses (for superelastic alloys). There is al-    ments are induced (11). Moreover, also room temper-
    so an accompanying decrease in the ability of the in-       ature can dramatically influence fatigue resistance of
    struments to resist permanent deformation (28).             the NTRIs, as shown in recent studies (29, 30).
    Clinically, such changes result in different mechanical     Therefore a correct comparison can only be made if
    properties and behaviour. Even if the raw material          the NTRIs are tested with the same device and artifi-
    and the machining processes are the same, the dif-          cial canal in the same conditions.
    ferent and proprietary heat treatment exhibits more         Fractographic analysis using SEM showed similar




                                                                                                    li
    evident superelastic characteristics. On the contrary,      fracture patterns in both instruments. Metal fatigue
    M3 Pro Gold instruments are more ductile and softer,        commonly leads to ductile fracture with some plastic




                                                                                                  na
    exhibiting more evident shape memory characteris-           deformations and a typical dull dimpled surface (10).
    tics. Differently than M3, M3 Pro Gold can be easily        The dimpling involve the whole fracture surface, with
    precurved for easier insertion in curved canal and to       some microvoids present as black dots (Figs. 3, 4). In
    reduce restoring forces. In the present study, the last     ductile fracture, microvoids are produced inside the




                                                                                                io
    characteristic allowed M3 Pro Gold instruments to           metal and their cohalescence weaken the structure
    better withstand the bending stresses. The null hy-         and results in fracture.
    pothesis was rejected: M3 Pro Gold instruments were         NiTi instruments have constantly gained popularity




                                                                          az
    found to be significantly more resistant to fatigue         during last decades because they offer more distinct
    when compared to M3. The originality of the present         clinical advantages with curved root canals than
    study was in the possibility of testing instruments pro-    stainless-steel instruments, due to their higher flexi-
    duced by the same manufacturer with the same NiTi           bility, by virtue of their superelasticity. The superelas-




                                                                        rn
    alloy, but with different heat treatments. In the majori-   ticity has made it possible to carry out conservative
    ty of previous studies about heat treated NTRIs, dif-       and better centered shapes, with less canal trans-
    ferent instruments were tested from different manu-         portation and with more respect of the original anato-
    facturers. In many cases, when manufacturers pro-           my (22). NiTi shape memory alloys undergo transfor-




                                                         te
    duce a new version of endodontic instruments with a         mation from cubic austenitic to monocyclic martensite
    different heat treatment, they also change instru-          when the applied stress in the austenitic phase is
    ments’ design and this could make more difficult to         enough to promote the stress-induced transformation
                                                       In
    evaluate the real effect of the heat treatment. The da-     (27). This stress-induced martensitic transformation
    ta are in accordance with previous studies which en-        reverses spontaneously upon release of the stress
    lightened the importance of heat treatments in im-          and the material returns to its original shape and size
    proving NiTi rotary instruments’ resistance to break-       (23). Furthermore, the rhombohedral R-phase forma-
    age (12, 15, 18).                                           tion, thermoelastic phase, often precedes the marten-
                             ni

    However, it is always difficult to compare different        sitic transformation under certain conditions and is
    cyclic fatigue studies, because many factors can in-        considered to be the main reason for increased flexi-
    fluence the final results. The main factor is the shape     bility of NiTi instruments over traditional stainless
                           io


    and dimension of the artificial canal (16). Each varia-     steel instruments. Such an increased flexibility and
    tion of canal curvature and diameter may affect the         the reduction of internal defects produced by heat
    way that mechanical loads are applied and conse-            treatments during or after manufacturing processes,
                         iz



    quently different mechanical stresses on the instru-        are responsible for greater resistance to flexural
           Ed
 IC
C
©




                                                                                 Figure 3. SEM microscope image of a
                                                                                 fractured M3 Pro Gold instrument.

    56                                                                              Annali di Stomatologia 2017;VIII (2):53-58
    In vitro resistance to fracture of two nickel-titanium rotary instruments made with different thermal treatments


                                                                                              Figure 4. SEM microscope image of a
                                                                                              fractured M3 Pro Gold instrument.




                                                                                                                  li
                                                                                                                na
                                                                                                              io
                                                                                      az
                                                                                    rn
    stresses and consequently to cyclic fatigue. Further                        dodontic instruments: effects on bending properties and
    studies, however, are needed to evaluate how much                           shaping abilities. Int Endod J. 2011 Sep;44(9):843-849.




                                                                  te
    different heat treatments affect torsional resistance                 5.    Testarelli L, Plotino G, Al-Sudani D, Vincenzi V, Giansira-
                                                                                cusa Rubini A, Grande NM, Gambarini G. Bending prop-
    and hardness of the alloy in NiTi rotary instruments.
                                                                                erties of a new nickel-titanium alloy with a lower percent
                                                                In
                                                                                by weight of nickel. Journal of Endodontics. 2011;37
                                                                                (9):1293-1295.
    Conclusions                                                           6.    Plotino G, Giansiracusa Rubini A, Grande N.M, Testarelli
                                                                                L, Gambarini G. Cutting efficiency of reciproc and
    Changing the thermal treatments, allows manufactur-                         waveone reciprocating instruments. Journal of Endodon-
                                 ni

    ers to quickly and significantly improve clinical perfor-                   tics. 2014;40(8):1228-1230.
    mance of NTRIs, with no need of changing quality of                   7.    Gambarini G, Pongione G, Rizzo F, Testarelli L, Cavalleri
    the raw material or modifying the grinding machines.                        G, Gerosa R. Bending properties of nickel-titanium instru-
                                                                                ments: a comparative study. Minerva stomatologica.
    M3 Rotary and M3 Pro Gold are an example of NTRIs
                               io


                                                                                2008;57(9):393-398.
    with same design and alloy, produced by the same                      8.    Di Fiore PM, Genov KA, Komaroff E, Li Y, Lin L. Nickel-ti-
    manufacturer, which show different resistance to                            tanium rotary instrument fracture: a clinical practice as-
    cyclic fatigue due to different heat treatments.
                             iz



                                                                                sessment. Int Endod J. 2006 Sep;39(9):700-708.
                                                                          9.    Gambarini G, Giansiracusa Rubini A, Sannino G, Di Gior-
                                                                                gio G, Piasecki L, Al-Sudani D, Plotino G, Testarelli L.
            Ed




                                                                                Cutting efficiency of nickel-titanium rotary and reciprocat-
                                                                                ing instruments after prolonged use. Odontology. 2016
    Declaration of conflicting interests

    The Authors declare that there is no conflict of interest.                  Jan;104(1):77-81.
                                                                          10.   Parashos P, Messer H. Rotary NiTi instrument fracture
                                                                                and its consequences. J Endod. 2006;32:1031-1043.
                                                                          11.   Plotino G, Grande N.M, Cordaro M, Testarelli L, Gambari-
                                                                                ni G. Measurement of the trajectory of different NiTi rotary
 IC




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            Ed
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    58                                                                                         Annali di Stomatologia 2017;VIII (2):53-58
