Basic Research—Technology
`
`Bending Properties of a New Nickel-Titanium Alloy
`with a Lower Percent by Weight of Nickel
`Luca Testarelli, DDS, PhD,* Gianluca Plotino, DDS, PhD,* Dina Al-Sudani, DDS,†
`Valentina Vincenzi, DDS,* Alessio Giansiracusa, DDS,* Nicola M. Grande, DDS, PhD,*
`and Gianluca Gambarini, MD, DDS*
`
`Abstract
`Introduction: The aim of the present study was to
`evaluate the bending properties of Hyflex instruments,
`which exhibit a lower percent in weight of nickel (52
`Ni %wt) and compare them with other commercially
`available nickel-titanium (NiTi)
`rotary instruments.
`Methods: Ten instruments with tip size 25, 0.06 taper
`of each of the following NiTi rotary instrumentation
`techniques were selected for the study: Hyflex, EndoSe-
`quence, ProFile, Hero, and Flexmasters. All instruments
`from each group were tested for stiffness by comparing
`their bending moment when they attained a 45-degree
`bend. Experimental procedures strictly followed testing
`methodology described in ISO 3630-1. All data were
`recorded and subjected to statistical evaluation by using
`analysis of variance test. Statistical significance was set
`at P < .05). Results: Statistical analysis of the data re-
`vealed that Hyflex files were found to be the most flex-
`ible instruments, with a significant difference (P < .05) in
`comparison with the other instruments. Among the
`other files, a significant difference has been reported
`for EndoSequence instruments compared with ProFile,
`Hero, and FlexMaster (P < .05), whereas no significant
`differences have been reported among those 3 files
`(P > .05). Conclusions: Results of the present study
`have illustrated an increased flexibility of the new NiTi
`alloy over conventional NiTi alloy, and they highlight
`the potential of
`the new manufacturing process.
`(J Endod 2011;37:1293–1295)
`
`Key Words
`Endodontic
`instruments, flexibility, nickel-titanium,
`thermal treatments
`
`From the *Department of Endodontics, ‘‘Sapienza’’ Univer-
`sity of Rome, Rome, Italy; and †Department of Endodontics,
`KSU University, Riyadh, Saudi Arabia.
`Address requests for reprints to Dr Gianluca Plotino, Via
`Tommaso Salvini, 57, 00197 Rome,
`Italy. E-mail address:
`endo@gianlucaplotino.com
`0099-2399/$ - see front matter
`Copyright ª 2011 American Association of Endodontists.
`doi:10.1016/j.joen.2011.05.023
`
`One of
`
`Testing and Materials (ASTM) F 2082. Typical tolerances for Active A(f) are 5
`
`the major innovations in endodontics has been the introduction of
`nickel-titanium (NiTi) alloy to manufacture root canal instruments. This is mainly
`because of the superelasticity of the NiTi alloy, which provides increased flexibility and
`allows the instruments to effectively follow the original path of the root canal (1). Nitinol
`is a simple binary mixture of nickel and titanium at about 50 atomic percent each (about
`55% by weight of nickel). However, subtle adjustments in the ratio of the 2 elements
`make a large difference in the properties, particularly the transformation temperatures,
`ie, the temperatures at which the crystal structure changes from austenite to martensite
`or vice versa. The sensitivity of the transformation temperature to composition is so
`great that chemistry is not used to specify the alloy. Instead, transformation temperature
`is the most accurate means to specify the alloy. The temperature most frequently spec-
`ified for the finished product is the active austenite finish temperature, Active A(f). This
`is determined by differential scanning calorimetry (DSC) by using American Society for
`
`C.
`For shape memory NiTi alloys, the Active A(f) determines the completion of the
`shape recovery transformation on heating. For superelastic NiTi alloys, the Active
`A(f) must be below the product use temperature. A superelastic material will remain
`
`superelastic up to a temperature from the Active A(f) to a temperature about 50
`C above
`
`Active A(f). Therefore, a material with an Active A(f) of about 15
`C will exhibit good
`
`C. However, the greatest ability to recover occurs close
`superelasticity up to about 65
`to A(f), as shown by Duerig et al (2).
`The transformation temperatures change because of mechanical processing and
`annealing; therefore, the Active A(f) of NiTi rotary instruments will be different than the
`transformation temperature of the original ingot. In a recent article, Hou et al (3)
`showed that a different manufacturing process, including a proprietary heat treatment,
`produced instruments (Twisted File; SybronEndo, Orange, CA) with significantly
`different Active A(f) and, consequently, significantly different bending properties. In
`previous studies, it has been shown that files produced with this new manufacturing
`technique resulted in instruments having enhanced mechanical properties and resis-
`tance to cyclic fatigue, compared with instruments manufactured with a traditional
`grinding process (4–6).
`Possible strategies to increase efficiency and safety of NiTi rotary files include an
`improvement in the manufacturing process or the use of new alloys that provide supe-
`rior mechanical properties. Heat treatments are performed during the production of the
`NiTi ingot but can also be performed by endodontic manufacturers before, during, and
`after the manufacturing of the NiTi rotary instruments. Unfortunately, all these heat treat-
`ments are usually proprietary, and they are not disclosed by manufacturers. According
`to Zinelis et al (7), this explains, contrary to other dental and biomedical alloys, why the
`mechanical properties and transformation temperatures of NiTi alloys are usually domi-
`nated by the hidden thermomechanical history rather than the elemental composition.
`One of the very few exceptions, however, can be Hyflex instruments (Coltene-Whale-
`dent, Allstetten, Switzerland), which exhibit a lower percent in weight of nickel (52
`Ni %wt) than the common 54.5–57 Ni %wt shown by the great majority of commercially
`available NiTi rotary instruments (7). Hyflex is a new NiTi rotary file for root canal prep-
`aration that has been commercialized since 2011. Hyflex NiTi Files have been produced
`by an innovative methodology (patent pending) that uses a unique process that controls
`
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`Basic Research—Technology
`
`the material’s memory (a complex heating and cooling treatment).
`According to many authors, such compositional deviations between
`54 and 57 Ni %wt, probably attributed to raw material variations during
`manufacturing, did not seem to affect the mechanical properties of NiTi
`instruments (8–11). However, none of these studies evaluated Hyflex
`instruments, the only one with a 52 Ni %wt. Therefore, the aim of the
`present study was to evaluate bending properties of
`the Hyflex
`instruments and compare them with other commercially available
`NiTi rotary instruments.
`
`Materials and Methods
`Ten instruments with tip size 25 and 0.06 taper of each of the
`following NiTi rotary instrumentation techniques were selected for
`the study: (1) Hyflex (Coltene-Whaledent, Allstetten, Switzerland),
`(2) EndoSequence (Brasseler, Savannah, GA), (3) ProFile (Dentsply-
`Maillefer, Baillagues, Switzerland), (4) Hero (MicroMega, Besancon,
`France), and (5) Flexmaster (VDW, Munchen, Germany).
`All
`instruments from each group were tested for stiffness by
`comparing their bending moment when they attained a 45-degree
`bend. Experimental procedures strictly followed testing methodology
`described in ISO 3630-1 (12) by using a computerized device
`(Fig. 1).
`Testing device (MAGTROL FTS-100, Buffalo, NY) and test proce-
`dures were in accordance with specifications described for root canal
`files. Three millimeters of the tip of each instrument was clamped in
`a chuck connected to a digital torque meter memocouple and to a strip
`chart for data recording. The amplifier was set at an angular deflection
`of 45 degrees, at which point the test stopped automatically. The
`bending moment was then measured and recorded by the memocouple,
`and the value was read directly on the strip chart.
`All instruments had been previously examined under a measuring
`microscope (MS2 Walter Uhl, Asslar, Germany) at D3 (3 mm from the
`file tip) and D16 (16 mm from the file tip) to ensure uniformity of
`dimensions (according to the tolerance indicated by ISO 3630-1)
`and under a stereomicroscope (Karl Kaps GmbH & Co KG, Asslar,
`
`Germany) at 20 magnification to ensure uniformity of cutting flutes
`
`and defect-free surfaces. All defective instruments were eliminated
`from the study and substituted with new ones.
`All data were recorded and subjected to statistical evaluation with
`analysis of variance test. Statistical significance was set at P < .05).
`
`Results
`Mean values and standard deviation for each group of instruments
`are shown in Table 1. The higher the value, the more rigid were the
`instruments. Statistical analysis of the data revealed that Hyflex files
`were found to be the most flexible instruments, with a significant
`difference (P < .05) in comparison with the other instruments. Among
`the other files, a significant difference was found for EndoSequence
`instruments compared with ProFile, Hero, and FlexMaster (P < .05),
`whereas no significant differences have been reported among the latter
`3 files (P > .05).
`
`Discussion
`Thermal treatment of NiTi alloys for endodontic use is a new field
`of research, and little information is currently available. Thermal treat-
`ment of the alloy is known to produce a better arrangement of the crystal
`structure, thus leading to improved flexibility (superelastic behavior),
`and also changes in the percentage of phases (a different grain struc-
`ture) of
`the alloy,
`thus leading to improved resistance or plastic
`behavior. Patented, proprietary processes are highly influenced by
`temperature and time intervals, and each small change makes every
`single manufacturing process unique.
`Energy-dispersive x-ray analysis performed by Zinelis et al (7)
`showed that Hyflex instruments had a lower percent in weight of nickel
`(52.1 Ni %wt) than the other tested NiTi rotary instruments. In fact, the
`values for FlexMaster, EndoSequence, Hero, and ProFile were 56.2,
`56.0, 55.4, and 54.6 Ni %wt, respectively. These values were close to
`those (56 wt% Ni, 44 wt% Ti) reported previously (1, 13–17). These
`values are within the nominal composition range specified in the
`ASTM standards for wrought NiTi alloys used in medical devices and
`surgical implants (F2063-05 2005).
`Results from the present study showed that Hyflex instruments
`were the most flexible instruments, with a significant improvement in
`flexibility over the other tested commercially available instruments
`(Table 1). Despite the fact that the flexibility is influenced by instru-
`ment’s design, such a great improvement is probably related to the
`proprietary manufacturing process (which is not disclosed by the manu-
`facturer) and/or the different percent in weight of nickel of the Hyflex
`instruments, because they have a cross-sectional design very similar
`to EndoSequence. Because the Hyflex instruments are new to the market
`and there is no older similar files that were manufactured with
`
`Figure 1. The testing device used in the present study for the stiffness test.
`
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`TABLE 1. Mean Values  Standard Deviation of the Values Obtained in the
`
`Stiffness Test
`
`Instrument
`Hyflex
`EndoSequence
`ProFile
`Hero
`FlexMaster
`
`g/cm
`14.46  2.7a
`46.01  3.9b
`64.18  4.2c
`67.11  5.7c
`68.39  4.9c
`
`The lower the value, the more flexible the instrument is. Different superscript letters indicate statis-
`tically significant differences.
`
`a traditional grinding process without any thermal treatment or with
`a similar manufacturing process (with proprietary heating and cooling
`processes) but with a 55% Ni wt, it is not easy to determine whether the
`improvement in the flexibility is due to the unique composition, the
`property thermal proportion, or both. Several studies have shown that
`phase transformation behavior has an impact on the mechanical prop-
`erties of NiTi instruments including flexibility (18–22), and the former
`is easily influenced by factors including chemical composition,
`heat treatment, and manufacturing processes (1, 23). Furthermore,
`Zinelis et al (7) reported that Hyflex compositional deviations did not
`seem to affect the mechanical properties of the NiTi instruments, and
`that the thermomechanical history has a much more crucial effect on
`the final mechanical strength (8–11). It can be postulated that the
`proprietary thermal processing plays an important role in increasing
`flexibility of Hyflex instruments when compared with the other tested
`instruments, which have a traditional manufacturing process with
`no property
`thermal
`treatments
`and,
`consequently, different
`thermomechanical history. The potential role of different percentage
`in weight of nickel of the Hyflex instruments still remains uncertain.
`However, because this compositional deviation was attributed to raw
`material variations during manufacturing and thermal processing (7),
`we might consider it as a result of a unique thermomechanical history
`of the alloy. Therefore, it is difficult to determine whether there is
`a crucial effect of a single factor or, more likely, a combination of
`different correlated factors in determining the final mechanical proper-
`ties of Hyflex instruments.
`On the contrary, the improvement in flexibility shown by EndoSe-
`quence compared with Hero, FlexMaster, and Profile, all manufactured
`with the same grinding process and with similar energy-dispersive x-ray
`patterns, is mainly related to the unique shape of the EndoSequence
`instruments, with a smaller inner core and alternate flute design.
`During the last decade several changes in cross-sectional and flute
`design have been introduced to increase flexibility of greater tapered
`NiTi rotary instruments, which are intended to be used in apical portion
`of curved root canals (ie, size 25/0.06), but only small improvements
`were achieved. Therefore, a significant improvement in the flexibility of
`the alloy should be highly beneficial, providing NiTi instruments of
`greater taper with a superior ability to negotiate curved canals, to reduce
`the tendency of iatrogenic errors, and to allow dimensionally adequate
`apical preparations of curved canals while maintaining the original
`path.
`
`Flexibility is one of the most important properties of NiTi rotary
`instruments, but it cannot be the only characteristic on which a choice
`among different instruments is based. Moreover, when new root canal
`instruments are produced with innovative manufacturing process or alloy
`properties that differ markedly from conventional files, several character-
`istics (ie, torsional and flexural resistance, cutting ability, wear, etc) need
`to be investigated and tested to allow an efficient and safe clinical usage.
`Therefore, more tests are needed to fully evaluate advantages and, even-
`tually, disadvantages of instruments produced with an alloy that exhibits
`
`Basic Research—Technology
`
`a lower percent in weight of nickel (52 Ni %wt) than the common 54.
`5–57 Ni %wt shown by the great majority of commercially available
`NiTi rotary instruments.
`Results of the present study have illustrated an increased flexibility
`of the new NiTi alloy over conventional NiTi alloy, and they highlight the
`potential of the new manufacturing process.
`
`Acknowledgments
`The authors deny any conflicts of interest related to this study.
`
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`JOE — Volume 37, Number 9, September 2011
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`Bending Properties of New Alloy
`
`1295
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`PGR2015-00019 - Ex. 1017
`US ENDODONTICS, LLC., Petitioner