10.59987/ads/2012.2.59-63
Aims. The aim of this study is to investigate cyclic fatigue resistance of four nickel – titanium rotary (NTR) instruments produced by a new method or traditional grinding processes.
Methods. Four NTR instruments from different brands were selected: group 1. Twisted File produced by a new thermal treatment of nickel – titanium alloy; group 2. Revo S SU; group 3. Mtwo and group 4. BioRaCe BR3 produced by traditional grinding processes. A total of 80 instruments (20 for each group) were tested for cyclic fatigue resistance inside a curved artificial canal with a 60 degree angle of curvature and 5 mm radius of curvature. Time to fracture (TtF) from the start of the test until the moment of file breakage and the length of the fractured tip was recorded for each instrument. Means and standard deviations (SD) of TtF and fragment length were calculated. Data were subjected to one-way analysis of variance (ANOVA).
Results. Group 1 (Twisted File) showed the highest value of TtF means. Cyclic fatigue resistance of Twisted File and Mtwo was significantly higher than group 2 (Revo S SU) and 4 (BioRace BR3), while no significant differences were found between group 1 (Twisted File) and 3 (Mtwo) or group 2 (Revo S SU) and 4 (BioRaCe BR3).
Conclusions. The cyclic fatigue resistance of Twisted File was significantly frigher than instruments produced with traditional grinding process except of Mtwo files.
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Annali di stomatologia
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Cyclic fatigue resistance of four nickel-titanium rotary instruments: a comparative study
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Cyclic fatigue resistance of four nickel-titanium rotary instruments: a comparative study
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                                                                                                Original article


Cyclic fatigue resistance of four nickel-titanium rotary
instruments: a comparative study




                                                                                        li
Eugenio Pedullà, DDS, PhD1                                Introduction
Gianluca Plotino, DDS, PhD2




                                                                                     na
Nicola Maria Grande, DDS, PhD2                            All nickel-titanium rotary (NTR) instrument systems cur-
Alfio Pappalardo, DDS1                                    rently on the market are constructed from “Nitinol” an
Ernesto Rapisarda, DMD,DDS1			                            alloy that was applied in endodontics since 1988 (1).
                                                          Endodontic treatment has benefited from the introduc-




                                                                                  io
1
  Department of Surgery,                                  tion of NTR root canal instruments (2). This is due to a
University of Catania, Italy                              combination of unique mechanical properties of the al-
2
  Department of Endodontics,                              loy, innovative file design, greater tapers and a crown




                                                                               az
‘Sapienza’ University of Rome, Italy                      down instrumentation procedure (3). As a result, there
                                                          are now many systems available commercially, that use
                                                          NTR instruments of different designs and dimensions, to
Corresponding author:                                     produce a desirable tapered root canal form, with a low




                                                                  n
Eugenio Pedullà, MD                                       risk of transporting the original canal foramen and fa-
Via Cervignano, 29                                        cilitate cleaning and shaping procedures (4-7). However,




                                                               er
95129, Catania, Italy                                     clinicians remain concerned about their breakage in use
Phone: +39 339 2613264                                    although the actual prevalence of such breakages has
E-mail: eugeniopedulla@gmail.com                          been indicated to be low (about 5%) (8). Several studies
                                                          have attempted to examine the reasons for fracture of


Summary
                                                  nt      NTR instruments after clinical or simulated use and have
                                                          shown that fractures can occur as a result of tensional
                                                          or cyclic fatigue (9-13). Torsional fatigue is the twisting
                                                iI
                                                          of a metal about its longitudinal axis at one end while
Aims. The aim of this study is to investigate cyclic      the other end is in a fixed position. Cyclic fatigue oc-
fatigue resistance of four nickel – titanium rotary       curs when a metal is subjected to repeated cycles of ten-
                                              on

(NTR) instruments produced by a new method or             sion and compression that causes its structure to break
traditional grinding processes.                           down, ultimately leading to fracture (14).
Methods. Four NTR instruments from different              Cyclic fatigue failure was implicated in more than one-
brands were selected: group 1. Twisted File pro-          third of those instruments fractured clinically and occurs
                                      i


duced by a new thermal treatment of nickel – tita-        unexpectedly without any sign of previous permanent
                                   iz


nium alloy; group 2. Revo S SU; group 3. Mtwo             deformation (9, 15, 16).
and group 4. BioRaCe BR3 produced by traditional          The fracture was due to the alternating tension/compres-
grinding processes. A total of 80 instruments (20 for     sion cycles which instruments are subjected to, when
                   Ed




each group) were tested for cyclic fatigue resistance
                                                          flexed in the region of maximum curvature of the canal
inside a curved artificial canal with a 60 degree angle
                                                          (17).
of curvature and 5 mm radius of curvature. Time to
                                                          Since the introduction of nickel-titanium alloy to end-
fracture (TtF) from the start of the test until the mo-
                                                          odontics, there have been many changes in instrument
ment of file breakage and the length of the fractured
                                                          design, but no significant improvements in the raw ma-
tip was recorded for each instrument. Means and
 IC




standard deviations (SD) of TtF and fragment length       terial properties, or enhancements in the manufacturing
were calculated. Data were subjected to one-way           processes. Recently, a new manufacturing process was
analysis of variance (ANOVA).                             developed by SybronEndo (Orange, CA), to create a
Results. Group 1 (Twisted File) showed the high-          NTR instrument called Twisted File (TF). It is produced
C




est value of TtF means. Cyclic fatigue resistance of      by twisting the alloy after heat treatment in order to im-
Twisted File and Mtwo was significantly higher than       prove super-elasticity and increase cyclic fatigue resis-
group 2 (Revo S SU) and 4 (BioRace BR3), while no         tance (18).
significant differences were found between group 1        The purpose of the present in vitro study was to compare
©




(Twisted File) and 3 (Mtwo) or group 2 (Revo S SU)        the cyclic fatigue resistance of a nickel – titanium rotary
and 4 (BioRaCe BR3).                                      instrument produced by a new method (Twisted File) to
Conclusions. The cyclic fatigue resistance of Twist-      three instruments produced by traditional grinding pro-
ed File was significantly frigher than instruments        cesses.
produced with traditional grinding process except
of Mtwo files.

Key words: cyclic fatigue, grinding process, nickel-
titanium, rotary instruments, twisted file.

Annali di Stomatologia 2012; III (2): 59-63                                                                       59
E. Pedulla et al.



Materials and methods                                                Means and standard deviations (SD) of TtF and frag-
                                                                     ment length were calculated for each system. Data were
Four different NTR instruments from different brands,                subjected to one-way analysis of variance (ANOVA) and
of identical sizes (.06 taper and 0.25 tip diameter) were            Dunn’s multiple comparison post-hoc test to determine
selected and evaluated: group 1. Twisted File (Sybro-                significant differences between groups. Significance was




                                                                                                     li
nEndo, Orange, CA, USA) produced by a new ther-                      set at the 95% confidence level.
mal treatment of nickel – titanium alloy; group 2. Revo




                                                                                                  na
S SU (MicroMega, Besancon, France); group 3. Mtwo
(Sweden-Martina, Padova, Italy) and group 4. BioRaCe                 Results
BR3, (FKG, La Chaux de Fonds, Switzerland) produced
by traditional grinding processes. Twenty NTR files from             Mean values and standard deviation of Time to Fracture




                                                                                               io
each manufacturer were tested for cyclic fatigue resis-              (TtF) and fragment lengths are displayed in Table 1. A
tance, resulting in 80 instruments. All instruments had              higher TtF is due to a higher resistance to cyclic fatigue
been previously inspected using a measuring micro-                   of the tested instruments. Group 1 (Twisted File) showed




                                                                                            az
scope for dimensional analysis, and for any signs of vis-            the highest value of TtF means. Cyclic fatigue resistance
ible deformation. None was discarded. Size 06-25 was                 of Twisted File (Group 1) and Mtwo (Group 3) was signif-
selected, being the master apical NTR file in many op-               icantly higher than group 2 (Revo S SU) and 4 (BioRace
erative sequences. Cyclic fatigue tests were performed               BR3) (P<0.05). No statistically significant difference was




                                                                             n
by a device as described in previous studies (19, 20).               noted between groups 1 (Twisted File) and 3 (Mtwo) or
The device consists of a main frame to which a mobile                groups 2 (Revo S SU) and 4 (BioRaCe BR3) (P>0.05)




                                                                          er
plastic support for the hand-piece is connected, and a               (Fig. 2).
stainless-steel block containing the artificial canals. The          Mean length of the fractured segment was also recorded
hand-piece was mounted upon a mobile device to allow                 to evaluate the correct positioning of the tested instru-
precise and reproducible placement of each instrument                ment inside the canal curvature, and whether similar

                                                 nt
inside an artificial canal. This ensured three-dimension-
al alignment, and positioning of the instruments to the
same depth.
                                                                     stresses were being induced. No statistically significant
                                                                     difference (P>0.05) was reported in the mean length of
                                                                     the fractured fragments for all of the instruments tested
                                               iI
The artificial canal was manufactured reproducing in-                (Fig. 3).
strument size 25 and taper .06. It provided the instru-
ment with a suitable simulated root canal with a 60 de-
                                             on

gree angle of curvature and 5 mm radius of curvature
(Fig. 1). Radius was measured to the central axis of the
curvature. The center of the curvature was 6 mm from
the tip of the instrument and the curved segment of the
                                     i


canal was approximately 5 mm in length. Instruments
                                  iz


were rotated at a constant speed of 300 rpm using a 6:1
reduction hand-piece (Sirona Dental Systems GmbH,
Bensheim, Germany), powered by a torque-controlled
                    Ed




motor (VDW Silver, VDW GmbH – Dentsply International
Inc., Munich, Germany). Torque was set at 2N/cm. All
instruments were rotated until fracture occurred. Time to
fracture (TtF) from the start of the test until the moment
of file breakage was recorded with a chronometer to an
                                                                     Figure 2 - Column bar plot of Time to Fracture (TtF) for instru-
 IC




accuracy of 0.1 second. The length of the fractured tip              ments.
was also recorded for each instrument.
C
©




Figure 1 - Mtwo instrument inserted in the stainless-steel artifi-   Figure 3 - Column bar plot of fragment length for instruments.
cial canal.

60                                                                                            Annali di Stomatologia 2012; III (2): 59-63
Cyclic fatigue resistance of four nickel-titanium rotary instruments

   Table 1 - Time to Fracture and fragment length (mean and SD) of size 25, taper.06 instruments.

       Instruments                      Time to Fracture (TtF)                              Fragment length
                                        Mean               SD                            Mean              SD
                                       (seconds)                                      (millimeters)
       Twisted File                      148.5                   17.48                    5.25                     0.58




                                                                                                     li
        Revo S SU                         43.5                   5.66                     5.05                     0.49
          Mtwo                            112                    21.49                    5.9                      0.87




                                                                                                  na
       Biorace BR3                        42.9                   10.97                    5.8                      0.95

Table 1 - Time to Fracture and fragment length (mean and SD) of size 25, taper.06 instruments.




                                                                                               io
Discussion                                                             creased phase transformation temperatures and its su-




                                                                                            az
                                                                       perior flexibility (26).
File fracture is a major concern during endodontic treat-              Results of this study showed that size 25.06 of Twisted
ment. Although multiple factors are responsible for in-                File was more resistant to cyclic fatigue than the other
strument separation in use, cyclic fatigue has been                    tested size 25.06 NTR instruments in absolute value.




                                                                               n
shown to be an important cause because the rotary                      It has been shown that the fatigue life of NTR instru-
instrument might be used in curved root canals (9, 13,                 ments made from conventional NiTi wire and that are the




                                                                            er
16). The shorter the radius of curvature, the greater is               same size, was not significantly affected by differences
the chance of fatigue breakage (13, 21). Cyclic fatigue                in cross-sectional shape and/or design of the cutting
occurs when a metal is subjected to repeated cycles                    flutes (27). Another recent study showed no obvious dif-
of tension and compression that cause its structure to                 ference on the fatigue life among ProFile, TYP (triangu-

                                                  nt
break down (as a result of concentration of stress at the
propagating crack front) and ultimately fracture (21, 22).
Among the four instruments tested, Twisted Files were
                                                                       lar configuration), and DS NEYY (square configuration)
                                                                       files, which were made from conventional NiTi wire (28).
                                                                       Moreover, Twisted file and Bio Race BR3 both have an
                                                iI
produced by processes of heating and twisting while                    equilateral triangular cross-section, but significant differ-
the other three instruments were produced by traditional               ences were found between TtF of Twisted Files and Bio
grinding methods.                                                      Race BR3 or Revo S SU in our study (P<0.001) (29).
                                              on

No scientific method was developed to date, which eval-                So the improvement in cyclic fatigue resistance for size
uated the resistance to fatigue of NTR instruments (18).               25.06 Twisted File would be also related to the manufac-
Device or methods for fatigue testing didn’t incorporate               turing processes. In fact, heating treatment of Twisted
into international standards for endodontic instruments.               Files transform the austenitic crystalline structure of NiTi
                                      i


The cyclic fatigue testing device used in this study has               alloy into the rhombohedral (R-) phase (30). R-phase
                                   iz


been utilized in previous studies of cyclic fatigue resis-             shows good super-elasticity and shape memory effect
tance and has generated the maximum stress at the                      (30); its Young’s modulus typically is lower than that of
center of simulated curve (about 6 mm from the tip) (23).              austenite. Thus, an instrument made up of the R-phase
                   Ed




No difference (P > 0.05) between fragment lengths of all               would be more flexible, allowing a greater amount of de-
NiTi files confirmed the correct positioning of the tested             formation than austenitic NiTi. (29, 31).
instrument inside the canal curvature, and whether simi-               On the other hand, cross-sectional configuration and
lar stresses were being induced.                                       manufacturing processes may influence flexibility of
Although the simulated canal does not duplicate the in                 NTR instruments and have a substantial impact on their
 IC




vivo situation, it allows the comparative testing of differ-           fatigue lifetime (32, 33).
ent instruments in a standardized environment (24).                    Our study showed no statistically significant difference
Among the four instruments tested, Twisted Files were                  of TtF between Twisted File and Mtwo. Indeed, although
produced by processes of heating and twisting, while the               produced by the traditional grinding processes, Mtwo
C




other three instruments (Revo S SU, Bio Race BR3 and                   instruments showed resistance to cyclic fatigue only
Mtwo) were produced by traditional grinding methods.                   slightly lower in absolute value than of new instruments
Kuhn & Jordan proposed some suggestions to improve                     manufactured by thermal methods, such as Twisted
the longevity of endodontic files, which include the fol-              File. This is probably due to the lower flexural rigidity for
©




lowing: (1) thermal treatments before machining to de-                 cross-section of Mtwo rather that one of Twisted File.
crease the work-hardening of the alloy; (2) choosing ma-               Moreover a significant difference was found between
chining conditions adapted to the NiTi alloy; and (3) elec-            Mtwo and Bio Race BR3 or Revo S SU despite being
tropolishing to reduce the machining damage on the sur-                both made by traditional grinding processes. Instead
face. The production method for manufacturing Twisted                  the comparison of TtF of Bio Race BR3 and Revo S SU
File instruments would incorporate the first two of these              showed no significant difference (P>0.05).
suggestions (25). Hou suggested that new method of                     The lower resistance to cyclic fatigue of instruments pro-
manufacturing NiTi instruments by twisting coupled with                duced by traditional grinding process can be explained
heat treatment of Twisted File might contribute to the in-             by the fact that it has many drawbacks. More precisely,

Annali di Stomatologia 2012; III (2): 59-63                                                                                      61
E. Pedulla et al.



cutting across the grain of the crystalline structure of                 nium root canal instruments with different cross-sections. J
the nickel-titanium wire limits the overall strength of the              Endod. 2006; 32: 372-375.
instruments, due to the formation of micro-cracks and                 8. Cheung GS, Darvell BW. Fatigue testing of a NiTi rotary in-
                                                                         strument. Part 1: Strain-life relationship. Int Endod J. 2007;
defects along the surface of the instruments and with-
                                                                         40: 612-618.
in the internal structure (29). Kuhn et al. described the             9. Sattapan B, Nervo GJ, Palamara JEA, Messer HH. Defects




                                                                                                     li
crack nucleation stage being facilitated by a high density               in rotary nickel–titanium files after clinical use. J Endod.
of surface defects, and then fatigue failure is largely a                2000; 26: 161–5.




                                                                                                  na
crack propagation process (34).                                      10. Spanaki-Voreadi AP, Kerezoudis NP, Zinelis. Failure
For the cyclic fatigue resistance, residual stresses have                mechanism of ProTaper nickel–titanium rotary instruments
been considered as an important factor not only after                    during clinical use: fractographic analysis. Int Endod J
(simulated) clinical uses but also for a brand-new instru-               2006;39:171–178.




                                                                                               io
                                                                     11. Zelada G, Varela P, Martın B, Bahıllo JG, Magan F, Ahn
ment (35, 36). When an instrument is machined (ie, be-
                                                                         S. The effect of rotational speed and the curvature of root
ing ground), plastic deformation occurs at the surface of                canals on the breakage of rotary endodontic instru- ments.
the metal, resulting in residual stresses that remain at                 J of Endod 2002; 28: 540–542.




                                                                                            az
the surface (37). Although the exact nature and extent               12. Yared G. In vitro study of the torsional properties of new
of such residual stress after manufacture are unknown,                   and used ProFile nickel titanium rotary files. J Endod 2004;
any internal stresses might act as a negative factor to                  30: 410–2.
the martensite substructures (38).                                   13. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing




                                                                             n
It is important for clinicians to realize that preexisting               of nickel-titanium endodontic instruments. J Endod 1997;
                                                                         23: 77–85.
conditions associated with the manufacturing process
                                                                     14. HibbelerRC.Mechanicsofmaterials.3rded.UpperSaddleRiv-




                                                                          er
might contribute to the propagation of instrument frac-                  er, NJ: PrenticeHall; 1997.
tures during use (39, 40).                                           15. Peng B, Shen Y, Cheung GSP, Xia TJ. Defects in ProTaper
                                                                         S1 instruments after clinical use: longitudinal examination.
                                                                         Int Endod J 2005; 38: 550–557.
Conclusions
                                                nt
Twisted Files 25.06, produced by thermal and twisted
                                                                     16. Shen Y, Cheung GS, Bian Z, Peng B. Comparison of de-
                                                                         fects in ProFile and ProTaper systems after clinical use. J
                                                                         Endod. 2006; 32: 61–65.
                                              iI
                                                                     17. Gambarini G. Cyclic fatigue of nickel-titanium rotary instru-
processes, showed the best cyclic fatigue resistance that
                                                                         ments after clinical use with low-and high-torque endodon-
was significantly better than the other NTR instruments                  tic motors. J Endod. 2001; 27: 772-774.
tested produced with the traditional grinding process, ex-           18. Testarelli L, Grande NM, Plotino G, Lendini M, Pongione G,
                                            on

cept Mtwo files. Additional studies are also necessary to                Paolis GD, Rizzo F, Milana V, Gambarini G. Cyclic fatigue
improve correlations between in vitro and in vivo fatigue                of different nickel-titanium rotary instruments: a compara-
resistance of NTR instruments.                                           tive study. Open Dent J. 2009; 16: 55-58.
                                                                     19. Plotino G, Grande NM, Sorci E, Malagnino VA, Somma F. A
                                   i


                                                                         comparison of cyclic fatigue between used and new Mtwo
                                iz


                                                                         Ni-Ti rotary instruments. Int Endod J 2006; 39: 716-723.
Acknowledgement
                                                                     20. Grande NM, Plotino G, Pecci R, Bedini R, Malagnino VA,
“The authors deny any conflicts of interest”                             Somma F. Cyclic fatigue resistance and three-dimensional
                    Ed




                                                                         analysis of instruments from two nickel-titanium rotary sys-
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                                                                            er
    2001; 27: 516–20.




                                                  nt
                                                iI
                                      i       on
                                   iz
                   Ed
 IC
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©




Annali di Stomatologia 2012; III (2): 59-63                                                                                         63
