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VOLUME 9 - NUMBER 1 - 2018
Effect of heat treatment on the cyclic fatigue resistance of NiTi endodontic openers



 Di Nardo D., Morese A.., Ferri V., Obino F.V., Seracchiani M., Testarelli L. 
Original Article, 24-26
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Aim: Aim of the present study is to evaluate if a thermal treatment adopted for endodontic NiTi instruments could increase their cyclic fatigue lifespan. Methods: 25 thermally treated One Flare (MM, Besancon, France) and 25 non-thermally treated Endo Flare (MM, Besancon, France) were mechanically rotated in a precurved artificial canal until fracture. Differences between Time to Fracture (TTF) and Fragment Length (FL) were statistically analyzed (Student’s t test, (p&lt;0.05). Results: One Flare’s time to fracture (47.24 seconds; SD±4.24) was significantly longer when compared to Endo Flare’s (21.2 seconds; SD±3.08) at the same operating conditions (p&lt;0.005). Differences between the length of fractured segments showed no statistical significance differences (p&gt;0.005). Conclusions: Heat treated NiTi alloy showed an improved time to fracture when compared to a non-thermally treated one at the same operating conditions. Thermal treatments could enhance the characteristics of NiTi improving the lifespan of rotary endodontic instruments.




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Annali di stomatologia
endodontic instruments
Effect of heat treatment on the cyclic fatigue resistance of NiTi endodontic openers
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Original Article
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Ann Stomatol (Roma)
L. Testarelli 
2018
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Effect of heat treatment on the cyclic fatigue resistance of NiTi endodontic openers
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                                                                                                   Original article



    Effect of heat treatment on the cyclic fatigue
    resistance of NiTi endodontic openers




                                                                                               li
                                                                                             na
    Dario Di Nardo DDS, PhD                                Introduction
    Antonio Morese DDS
    Valerio Ferri DDS                                      The introduction of nickel-titanium (NiTi) alloy for rotary




                                                                                           io
    Federico Valenti Obino DDS                             endodontic instruments improved the clinical skills in
    Marco Seracchiani DDS                                  root canal treatment, making it safer and more pre-
    Luca Testarelli DDS, PhD                               dictable (1-3). NiTi alloy allowed the manufacturers to




                                                                     az
                                                           produce instruments with a greater mass and taper
                                                           than the stainless-steel instruments: this resulted in a
    Department of Oral and Maxillofacial Sciences,         more predictable clearness for the root canal system
    “Sapienza” University of Rome, Rome, Italy             (4, 5). The continuous motion guaranteed a higher cut-




                                                                   rn
                                                           ting ability than the balanced force motion used with
                                                           the stainless-steel instruments (6-8).
                                                           Despite the improved performance of rotary instru-
    Corresponding author:                                  ments, their relatively higher speed of rotation may




                                                       te
    Antonio Morese                                         lead to an intracanal separation of the endodontic in-
    Department of Oral and Maxillofacial Sciences          strument (9-11). Quality of the alloy, manufacturing
    Via Caserta 6                                          defects, and cyclic stress accumulation are involved
    00161 Rome, Italy
                                                     In    in intracanal leakage of endodontic instruments (12-
    Tel.: +39 366 1905474                                  14). Intraoperational strain affect the integrity of the
    E-mail: moreseantonio@gmail.com                        alloy produces microcracks which grow at every ro-
                                                           tating cycle of the instrument. Microcracks may lead
                                                           to intracanal separation after certain cycles and this
                            ni

    Summary                                                is an unavoidable aspect after prolonged use (15-18).
                                                           Aim of the present study is to evaluate if a propri-
    Aim: Aim of the present study is to evaluate if a      etary thermal treatment could increase the resistance
                          io


    thermal treatment adopted for endodontic NiTi in-      to cyclic fatigue of a high tapered endodontic instru-
    struments could increase their cyclic fatigue lifes-   ment.
    pan.
                        iz



    Methods: 25 thermally treated One Flare (MM, Be-
    sancon, France) and 25 non-thermally treated En-       Materials and methods
    do Flare (MM, Besancon, France) were mechani-
          Ed




    cally rotated in a precurved artificial canal until    25 Endo Flare (MicroMega, Besancon, France), tip
    fracture. Differences between Time to Fracture         diameter #25, taper 12%, length 15 mm, and 25 One
    (TTF) and Fragment Length (FL) were statistically      Flare (MicroMega, Besancon, France), tip diameter
    analyzed (Student’s t test, (p<0.05).                  #25, taper 9%, length 17mm were tested (n=50).
    Results: One Flare’s time to fracture (47.24 sec-      A validated cyclic fatigue testing device was adopted
    onds; SD±4.24) was significantly longer when           to evaluate time to fracture at known conditions (19-
 IC




    compared to Endo Flare’s (21.2 seconds;                20). The electric handpiece was mounted on a mo-
    SD±3.08) at the same operating conditions              bile device to allow reproducible placement of each
    (p<0.005). Differences between the length of frac-     instrument inside the artificial canal to the same
    tured segments showed no statistical signifi-          depth. The artificial canal had a 60° angle of curva-
C




    cance differences (p>0.005).                           ture and 5 mm radius of curvature. All the instru-
    Conclusions: Heat treated NiTi alloy showed an         ments were inserted at the same length and then ro-
    improved time to fracture when compared to a           tated at 350 rpm with maximum torque until fracture
    non-thermally treated one at the same operating        occurred. For each instrument, the time to fracture
©




    conditions. Thermal treatments could enhance           was recorded with a 1/100 sec chronometer. All frag-
    the characteristics of NiTi improving the lifespan     ments were collected and measured. For each
    of rotary endodontic instruments.                      group, mean and standard deviation were calculated.
                                                           Differences among groups were statistically exam-
    Key words: heat treatment, Endo Flare, One Flare,      ined using the Student’s t test (significance level was
    nickel-titanium, endodontic instruments.               set at p<0.05). Data was statistically analyzed using



    24                                                                           Annali di Stomatologia 2018;IX(1):24-26
    Effect of heat treatment on the cyclic fatigue resistance of NiTi endodontic openers


    the SPSS 13.0 software (SPSS Incorporated, Chica-                Discussion
    go, IL, USA).
                                                                     The thermal treatment applied to the NiTi rotary in-
                                                                     struments allowed changes in the metallurgical
    Results                                                          phase at environmental temperature. At higher tem-




                                                                                                        li
                                                                     perature the NiTi alloy is in austenitic phase, at lower
    Results from the cyclic fatigue tests are shown in               temperature it is in martensitic phase and at average




                                                                                                      na
    Table 1 and Figure 1. Mean values of time to fracture            temperature the alloy is in R phase. Thermal treat-
    for Endo Flare instruments were 21.2 seconds                     ments allow the achievement of improved qualities
    (SD±3.08) and were 47.24 seconds (SD±4.24) for                   for the rotary instruments, such as superelasticity
    One Flare. Different time to fracture between groups             and shape memory (21). The vacuum casting of an
    showed statistical significance (p<0.005).                       ingot, hot molding, rolling and cold drawing followed




                                                                                                    io
    Mean values for fragment length for Endo Flare in-               by heat treatment are typical applied processes for
    struments were 6 mm (SD±0.8) and for One Flare                   the NiTi wires. The thermal treatment applied to the
    were 7 mm (SD±0.8). No statistically significant dif-            rotary instrument is usually under patent. Usually, in-




                                                                                az
    ferences were found between fragment lengths                     struments are brought to high temperature, between
    (p>0.005).                                                       450° and 550° degrees, in furnaces that can work in
                                                                     controlled atmosphere or air. This kind of thermal
                                                                     treatment allows an increase in the instrument per-




                                                                              rn
                                                                     formances, reaching the superelastic or the shape
                                                                     memory characteristics (22). Thermal treatments dra-
    Table 1. Time to Fracture (TtF) in seconds (s) and length        matically affect the in vitro instruments’ performance,
                                                                     resulting in an increased cyclic fatigue resistance.




                                                             te
    (mm) of fractured fragments (FL).
                                                                     Geometrical design, rotary motion and thermal treat-
                Endo Flare            One Flare                      ment of the alloy could improve the characteristics of
                                                                     the endodontic instrument (19-24). Superelastic and
    Group       Mean         SD       Mean          SD
                                                           In        shape memory characteristics could be reached
    TtF (s)     21.2         3.08     47.24         4.24             treating the alloy at certain temperatures at con-
                                                                     trolled atmosphere (15).
    FL (mm)     6            0.8      7             0.8              The orifice opener is a high tapered instrument used
                                                                     for the enlargement of the coronal third of the en-
                               ni

                                                                     dodontic canal (flaring) due to facilitate irrigation and
                                                                     the penetration of the subsequent instrument (25).
                                                                     Flexibility of One Flare is higher therefore, it may also
                             io


                                                                     allow a safer and more predictable clinical usage.
                                                                     The One Flare seems to be mostly indicated for ob-
                                                                     taining straight access to constricted orifice and\or
                           iz



                                                                     bad positioned orifice. Due to its flexibility and duc-
                                                                     tility, One Flare presents a lower risk of canal perfo-
                                                                     ration and/or stripping. Endo Flare presents a 12%
              Ed




                                                                     taper, this makes the instrument harder and less
                                                                     flexible when compared to One Flare. In addition, no
                                                                     thermal treatments were applied to its alloy and this
                                                                     could represent a detriment for its cyclic fatigue lifes-
                                                                     pan (26).
                                                                     High tapered instruments should be thermally treated
 IC




                                                                     to improve their resistance to fracture, contrasting
                                                                     the minor flexibility and ductility given by the in-
                                                                     creased mass of alloy. Data obtained in this study
                                                                     are in accordance with previous studies which en-
C




                                                                     lightened the importance of heat treatments in im-
                                                                     proving NTRIs’ resistance to breakage (12, 15, 18,
                                                                     24, 26).
©




                                                                     Conclusions

                                                                     MM proprietary thermal treatment applied at One
                                                                     Flare improves the resistance to cyclic fatigue, allow-
                                                                     ing a safer use by the clinician compared to the non-
    Figure 1. One Flare vs Endo Flare time to fracture box plot.
                                                                     thermally treated Endo Flare. Apply a thermal treat-

    Annali di Stomatologia 2018;IX(1):24-26                                                                                25
    D. Di Nardo et al.


    ment in manufacturing NiTi endodontic rotary instru-                     12. Capar ID, Ertas H, Arslan H. Comparison of cyclic fatigue
    ments may improve mechanical properties and lifes-                           resistance of novel nickel-titanium rotary instruments. Aust
                                                                                 Endod J. 2015;41:24-28.
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                                                                             13. Gambarini G, Plotino G, Sannino GP, Grande NM, Gian-
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                                                                                 endodontic glide path. Odontology. 2015;103:56-60.




                                                                                                                      li
                                                                             14. Plotino G, Costanzo A, Grande NM, Petrovic R, Testarelli L,
    Declaration of conflicting interests                                         Gambarini G. Experimental evaluation on the influence of au-




                                                                                                                    na
                                                                                 toclave sterilization on the cyclic fatigue of new nickel-tita-
    The Authors deny any conflict of interest.                                   nium rotary instruments. J Endod. 2012;38:222-225.
                                                                             15. Zinelis S, Darabara M, Takase T, Ogane K, Papadimitriou
                                                                                 GD. The effect of thermal treatment on the resistance of nick-
                                                                                 el-titanium rotary files in cyclic fatigue. Oral Surg Oral Med
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