JOURNAL OF ENDODONTICS
`Copyright © 2002 by The American Association of Endodontists
`
`Printed in U.S.A.
`VOL. 28, NO. 8, AUGUST 2002
`
`SCIENTIFIC ARTICLES
`
`Differential Scanning Calorimetric Studies of Nickel
`Titanium Rotary Endodontic Instruments
`
`W. A. Brantley, T. A. Svec, M. Iijima, J. M. Powers, and T. H. Grentzer
`
`Differential scanning calorimetric (DSC) analyses
`were performed between ⴚ130° and 100°C on spec-
`imens prepared from nickel-titanium (NiTi) rotary
`endodontic instruments: ProFile (n ⴝ 5), Lightspeed
`(n ⴝ 4), and Quantec (n ⴝ 3). The ProFile and Light-
`speed instruments were in the as-received condition,
`whereas the Quantec instruments were randomly se-
`lected from a dental clinic and had unknown history.
`The DSC plots showed that the ProFile and Light-
`speed instruments analyzed had the superelastic
`NiTi property, with an austenite-finish (Af) tempera-
`ture of approximately 25°C. Differences in DSC plots
`for the ProFile instruments and the starting wire
`blanks (n ⴝ 2) were attributed to the manufacturing
`process. The phase transformation behavior when
`the specimens were heated and cooled between
`ⴚ130° and 100°C, the temperature ranges for the
`phase transformations, and the resulting enthalpy
`changes were similar to those previously reported
`for nickel-titanium orthodontic wires having super-
`elastic characteristics or shape memory behavior in
`the oral environment. The experiments demon-
`strated that DSC is a powerful tool for materials char-
`acterization of these rotary instruments, providing
`direct information not readily available from other
`analytical techniques about the NiTi phases present,
`which are fundamentally responsible for their clinical
`behavior.
`
`After the pioneering research by Walia et al. (1), which introduced
`nickel-titanium (NiTi) hand files to the endodontic profession, both
`nickel-titanium hand files and particularly rotary instruments have
`achieved widespread popularity. A major reason for their selection
`is the much greater flexibility (i.e. much lower elastic modulus) of
`the nickel-titanium alloy compared with stainless steel (1), which
`offers distinct clinical advantages with curved root canals. Numer-
`ous articles have recently reported the performance of the different
`
`567
`
`brands of nickel-titanium rotary instruments (2– 6) and hand in-
`struments (7–9).
`The manufacture of nickel-titanium instruments for endodontics
`has been discussed in a review article (10). The nickel-titanium
`alloys have the approximate composition of 55% nickel and 45%
`titanium and are based upon the intermetallic compound NiTi (11).
`Originally introduced for orthodontics (12), the nickel-titanium
`archwire alloys are available in nonsuperelastic, superelastic, and
`shape-memory forms (11, 13, 14). The nonsuperelastic nickel-
`titanium alloys have a predominantly heavily cold-worked, stable,
`martensitic NiTi structure, whereas the superelastic and shape
`memory alloys undergo a reversible transformation by twinning on
`the atomic scale (11) from the lower-temperature martensitic NiTi
`structure to the higher-temperature austenitic NiTi structure. The
`superelastic alloys undergo transformation from austenitic NiTi to
`martensitic NiTi with the application of stress. The shape memory
`alloys return to a higher-temperature shape established during
`processing (15), when the temperature is raised above the austen-
`ite-finish (Af) temperature, where the transformation to the auste-
`nitic NiTi structure is completed. The NiTi shape memory alloys
`used in orthodontics have the Af temperature below body temper-
`ature, whereas the superelastic (but not shape memory) orthodontic
`alloys have the Af temperature above that of the oral environment
`(16). The overall phase transformation process can be complex,
`and it has been found that an intermediate R-phase can form during
`the transformation between martensitic and austenitic NiTi (17).
`The structure of the NiTi alloys, which is central to their clinical
`performance, is conveniently studied by differential scanning cal-
`orimetry (DSC), in which the difference in thermal power supplied
`to a test specimen and an inert control specimen heated at the same
`rate is measured very accurately (11, 16). Structural transforma-
`tions in the NiTi alloys are revealed as endothermic peaks on the
`heating DSC curves and as exothermic peaks on the cooling DSC
`curves, and information is obtained about the temperature ranges
`and enthalpy changes for the phase transformations. For NiTi
`alloys, DSC indicates which of the three phases (martensitic NiTi,
`R-phase, or austenitic NiTi) will be present at a given temperature.
`Although X-ray diffraction analysis is a useful complementary
`method to investigate the structure of NiTi orthodontic wires (18),
`this technique only reveals the structure within approximately 50
`␮m of the surface, whereas DSC provides information for the
`overall bulk specimen (19). Electrical resistivity measurements
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`FIG 1. DSC plot for a ProFile specimen. The solid line is the heating curve, and the dashed line is the cooling curve.
`
`may also be used to study the structural transformations in the
`nickel-titanium alloys (11). The purpose of this study was to utilize
`DSC to investigate the phase relationships within three brands of
`popular nickel-titanium rotary endodontic instruments.
`
`MATERIALS AND METHODS
`
`Three brands of rotary nickel-titanium endodontic instruments
`(ProFile 0.04 taper, Dentsply Tulsa Dental, Tulsa, OK; Lightspeed,
`Lightspeed Technology, San Antonio, TX; Quantec, Analytic/
`Sybron Dental Specialties, Orange, CA) were selected for study,
`along with starting 1.0-mm diameter wire blanks for the ProFile
`instruments that were obtained from the manufacturer. Test spec-
`imens were carefully cut with a water-cooled, slow-speed diamond
`saw to minimize mechanical stresses that might change the pro-
`portions of the austenitic and martensitic NiTi phases from those in
`the as-received instruments and wire blanks. Each test specimen
`consisted of 2 to 4 segments, each approximately 4 to 5 mm in
`length, which were placed in an open aluminum pan; no crimped
`pan top was used to avoid mechanical stresses on the specimens.
`An empty aluminum pan served as the inert control specimen for
`the DSC measurements.
`Five specimens were analyzed for the 25-mm long, ISO size 25
`ProFile instruments. Four specimens were randomly selected from
`a package containing ISO sizes 40, 42.5, 45, and 47.5 for the
`25-mm long, Lightspeed instruments. Three specimens were ana-
`lyzed for the Quantec instruments, which were of varying sizes and
`randomly selected from the Endodontic Clinic at the University of
`Texas-Houston Dental Branch. Two replicate specimens of the
`starting wire blanks for the ProFile instruments were analyzed. The
`ProFile and Lightspeed instruments were in the as-received con-
`dition, whereas the Quantec instruments had an unknown clinical
`history.
`The DSC analyses were conducted (model 2910 DSC, TA
`Instruments, Wilmington, DE) over a temperature range from
`⫺130° to 100°C, using the liquid nitrogen cooling accessory (TA
`
`Instruments) to achieve subambient temperatures. For each anal-
`ysis, the specimen was first cooled from room temperature to
`⫺130°, then heated from ⫺130° to 100°C to obtain the heating
`DSC curve, and subsequently cooled from 100°C back to ⫺130°C
`to obtain the cooling DSC curve. The linear heating or cooling rate
`was a standard 10°C per minute (16), and during each analysis, the
`DSC cell was purged with dry nitrogen at a rate of 50 cm3/min.
`Temperature calibration of the DSC apparatus was performed with
`n-pentane, deionized water, and indium. The DSC plots were
`analyzed by computer software (TA Instruments) to obtain the
`peak temperatures for the phase transformations, along with the
`enthalpy changes associated with these processes. The interpreta-
`tions of the plots were based upon previous DSC studies of NiTi
`alloys in orthodontics (11, 16, 20).
`
`RESULTS
`
`Figures 1 and 2 show the two types of DSC plots that were
`obtained for the ProFile specimens. In all of the DSC plots, the
`heating curve is shown as a solid line at the bottom of the figure,
`and the cooling curve is shown as a dashed line at the top of the
`figure. The lines obtained by extrapolating the baselines adjacent
`to the peaks show the temperature ranges for the transformations.
`The other construction lines are used by the computer software to
`calculate values of enthalpy change (⌬H) and determine onset
`temperatures for the transformations.
`A pair of endothermic peaks can be seen on the heating curve in
`Figure 1, whereas an unresolved smaller endothermic peak exists
`on the left shoulder of the main peak at approximately 10°C. These
`peaks correspond to the initial transformation of martensitic NiTi
`to R-phase at lower temperatures, followed by transformation at
`higher temperatures of R-phase to austenitic NiTi, which is com-
`pleted at approximately 25°C. The area under both peaks repre-
`sents a total enthalpy change (⌬H) of approximately 3.3 J/g for the
`overall transformation from martensitic NiTi to austenitic NiTi. On
`the cooling curve, there is a broad exothermic peak at approxi-
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`DSC Analysis of NiTi Files
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`569
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`FIG 2. DSC plot for a second ProFile specimen. The solid line is the heating curve, and the dashed line is the cooling curve.
`
`mately 5°C; and the overall enthalpy change of approximately 3.4
`J/g also includes a contribution from the unresolved smaller exo-
`thermic peak on the left shoulder, i.e. at lower temperatures than
`the main peak. Note that the vertical scale has been expanded for
`the heating curve, compared with the cooling curve. The main peak
`on the cooling curve corresponds to the initial transformation of
`austenitic NiTi to R-phase, which then transforms at lower tem-
`peratures to martensitic NiTi with a much lower enthalpy change
`than that for the transformation from austenitic NiTi to R-phase.
`In Figure 2, for another ProFile specimen, the smaller endother-
`mic peak at approximately ⫺18°C on the heating curve, corre-
`sponding to the initial transformation from martensitic NiTi to
`R-phase, is now resolved. The second endothermic peak at ap-
`proximately 1°C corresponds to the subsequent transformation
`from R-phase to austenitic NiTi. The enthalpy value of 4.4 J/g is
`applicable to the overall transformation from martensitic NiTi to
`austenitic NiTi when the sample is heated. For this specimen, the
`transformation to austenitic NiTi on heating (Af temperature) is
`again completed at approximately 25°C. The interpretation of the
`cooling curve follows that for Figure 1, with the main exothermic
`peak at approximately ⫺4°C, corresponding to the transformation
`from austenitic NiTi to R-phase. The transformation at lower
`temperatures from R-phase to martensitic NiTi again appears as an
`unresolved exothermic peak on the left shoulder of the main peak
`and has a lower enthalpy change than the transformation from
`austenitic NiTi to R-phase. The enthalpy change of 2.8 J/g corre-
`sponds to the overall transformation from austenitic NiTi to mar-
`tensitic NiTi when the sample is cooled. Note that both the heating
`and cooling curves have the same vertical scale in Figure 2.
`The other three ProFile specimens had DSC plots that were very
`similar to Figures 1 and 2. For the five specimens, the enthalpy
`change for the overall transformation from martensitic NiTi to
`austenitic NiTi on the heating curve ranged from approximately
`3.3 to 4.4 J/g, and the enthalpy change for overall transformation
`from austenitic NiTi to martensitic NiTi on the cooling curve
`ranged from approximately 2.8 to 3.6 J/g. The transformation to
`the austenitic NiTi structure on heating was largely completed for
`all five specimens at approximately 25°C.
`
`Figure 3 shows a DSC plot for a specimen prepared from the
`starting wire blanks used to manufacture the ProFile instruments;
`nearly the same results were obtained for the two specimens that
`were analyzed. The heating curve contains a clearly resolved first
`endothermic peak at approximately ⫺25°C for the transformation
`from martensitic NiTi to R-phase, followed by a second endother-
`mic peak at approximately 2°C for the transformation from R-
`phase to austenitic NiTi, which is completed at approximately
`25°C. The enthalpy change of 4.8 J/g corresponds to the overall
`transformation from martensitic NiTi to austenitic NiTi. The cool-
`ing curve contains a single resolved exothermic peak at approxi-
`mately ⫺6°C, corresponding to the transformation from austenitic
`NiTi to R-phase. As in Figures 1 and 2, the peak for the subsequent
`transformation from R-phase to martensitic NiTi is an unresolved
`left shoulder on the main exothermic peak and has a much lower
`enthalpy change than the initial transformation from austenitic
`NiTi to R-phase. The enthalpy change of 3.4 J/g corresponds to the
`overall transformation from austenitic NiTi to martensitic NiTi.
`(Note that the vertical scale for the heating curve has been sub-
`stantially expanded, compared with the scale for the cooling
`curve.)
`Figure 4 shows a DSC plot for a Lightspeed test specimen; very
`similar results were obtained for the other three samples. The
`heating curve contains two endothermic peaks at approximately
`⫺16°C and 2°C that correspond to the transformation from mar-
`tensitic NiTi to R-phase, followed by the transformation from
`R-phase to austenitic NiTi, respectively. The enthalpy change for
`the overall transformation from martensitic NiTi to austenitic NiTi
`is 8.2 J/g. As in the ProFile specimens in Figures 1 and 2, the
`cooling curve in Figure 4 contains a single exothermic peak at
`approximately ⫺2°C, but the extended left shoulder suggests the
`presence of an unresolved second exothermic peak with a smaller
`enthalpy change. Accordingly, the cooling curve is assumed to
`indicate the transformation of austenitic NiTi to R-phase, followed
`by the transformation at lower temperature from R-phase to mar-
`tensitic NiTi. The enthalpy change of 2.6 J/g arises from the overall
`transformation from austenitic NiTi to martensitic NiTi. For the
`four Lightspeed specimens, the overall enthalpy change on the
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`FIG 3. DSC plot for a specimen prepared from the NiTi wire blanks used to manufacturer the ProFile rotary instruments. The solid line is the
`heating curve, and the dashed line is the cooling curve.
`
`FIG 4. DSC plot for a Lightspeed specimen. The solid line is the heating curve, and the dashed line is the cooling curve.
`
`heating curves ranged from 7.4 to 8.4 J/g, and the overall enthalpy
`change on the cooling curves ranged from 1.8 to 2.6 J/g. The
`transformation to the austenitic NiTi structure on heating was
`completed at 25°C for all four specimens.
`The three Quantec specimens, which had an unknown clinical
`history, had more variable DSC plots than the ProFile and
`Lightspeed specimens, which were prepared from known as-
`received instruments. Figure 5 shows the DSC plot for one
`Quantec specimen; the general appearance of the plots was
`similar for the other two specimens, although there was sub-
`stantial variation in the peak temperatures. The heating curve in
`
`Figure 5 contains two clearly resolved endothermic peaks at
`approximately ⫺24° and 3°C that correspond to the transfor-
`mation from martensitic NiTi
`to R-phase, followed by the
`transformation from R-phase to austenitic NiTi. The enthalpy
`change of 4.3 J/g is for the overall transformation from mar-
`tensitic NiTi to austenitic NiTi. The cooling curve contains a
`single, broad, exothermic peak at approximately 1°C, which is
`assumed to include both the transformation from austenitic NiTi
`to R-phase at higher temperatures, followed by the transforma-
`tion from R-phase to martensitic NiTi because of the long
`shoulder on the left side of this broad peak. The enthalpy change
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`Vol. 28, No. 8, August 2002
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`DSC Analysis of NiTi Files
`
`571
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`FIG 5. DSC plot for a Quantec specimen. The solid line is the heating curve, and the dashed line is the cooling curve.
`
`TABLE 1. Properties determined from the differential scanning calorimetric (DSC) plots for the three brands of
`endodontic instruments
`
`Property
`⌬H (martensitic NiTi to austenitic NiTi) on heating (J/g)
`⌬H (austenitic NiTi to martensitic NiTi) on cooling (J/g)
`Peak temperature, martensitic NiTi to R-phase on heating (°C)
`Peak temperature, R-phase to austenitic NiTi on heating (°C)
`Peak temperature, austenitic NiTi to R-phase on cooling (°C)
`Approximate completion temperature for transformation to
`austenitic NiTi (°C)
`
`ProFile*
`
`3.3 – 4.9
`2.8 – 3.6
`⫺25 – 14‡
`1 – 10
`⫺6 –⫹5
`25
`
`Lightspeed
`
`7.4 – 8.4
`1.8 – 2.6
`⫺18 – ⫺16
`2 – 3
`⫺3 – 0
`25
`
`Quantec†
`
`4.3 – 7.7
`1.6 – 2.7
`⫺25 – ⫺9
`3 – 24
`1 – 26
`25 – 50
`
`* Includes data for instruments and blanks.
`† Used instruments.
`‡ Not clearly defined for samples from three instruments, i.e. based on samples from two instruments and two wire blanks.
`
`of 2.7 J/g is likewise assumed to correspond to the overall
`transformation from austenitic NiTi to martensitic NiTi.
`The second Quantec specimen had endothermic peak tempera-
`tures of ⫺12° and 19°C on the heating curve, with an overall
`enthalpy change of 5.8 J/g. On the cooling curve, the exothermic
`peak temperature was 16°C with an overall enthalpy change of 1.6
`J/g. The third Quantec specimen had endothermic peak tempera-
`tures of ⫺9° and 24°C on the heating curve, with an overall
`enthalpy change of 7.7 J/g. On the cooling curve, the exothermic
`peak temperature was 26°C with an overall enthalpy change of 2.6
`J/g. Whereas the transformation to austenitic NiTi on heating was
`nearly completed at 25°C for the Quantec specimen in Figure 5, the
`temperatures were approximately 40° and 50°C for the other two
`specimens.
`Table 1 summarizes the properties determined from the DSC
`measurements on the three brands of NiTi rotary endodontic
`instruments. Ranges of the enthalpy changes are provided for
`the overall forward and reverse transformations between mar-
`tensitic NiTi and austenitic NiTi on heating and cooling, re-
`spectively. Ranges in peak temperatures are listed for the trans-
`formations from martensitic NiTi to R-phase and from R-phase
`to austenitic NiTi on heating and for the transformation from
`austenitic NiTi to R-phase on cooling. (There were no clearly
`
`defined peaks on the cooling DSC plots for the transformation
`from R-phase to martensitic NiTi.) Lastly,
`the approximate
`completion temperature (Af) is given for the transformation to
`austenitic NiTi on heating.
`
`DISCUSSION
`
`The DSC results in Figures 1, 2, and 4 show that the as-received
`ProFile and Lightspeed rotary endodontic instruments analyzed
`would be essentially in the completely austenitic NiTi condition at
`room temperature. These instruments would undergo superelastic
`behavior during clinical usage, where the imposition of stress
`causes transformation to martensitic NiTi and the removal of stress
`(withdrawal of the instrument from the root canal) results in
`reversible transformation to the original austenitic NiTi structure,
`unless more than approximately 10% tensile strain takes place
`(13). By analogy with the DSC results for shape memory NiTi
`orthodontic wires (11, 16), the NiTi alloy for the ProFile and
`Lightspeed rotary instruments would be capable of true shape
`memory under clinical conditions, if the manufacturers performed
`the necessary processing procedures (15). However, the shape
`memory property would not be necessary for the clinical use of
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`these instruments, provided that any tensile strain generated did not
`exceed approximately 10%.
`Differences in the transformation temperatures and enthalpies of
`the NiTi alloys for the ProFile and Lightspeed instruments (Figs.
`1, 2, and 4) are indicative of some variations in processing, but it
`is evident that the manufacturers have emphasized the importance
`of the Af temperature being near 25°C to permit superelastic
`behavior during clinical use. Differences in Figures 1–3 are attrib-
`uted to the manufacturing processing for converting the starting
`wire blanks to the ProFile instruments, but it is noteworthy that for
`both the wire blanks and the instruments the Af temperature is
`close to 25°C.
`Because of their unknown clinical history, it is not possible to
`comment about the sources of substantial variability in the DSC
`plots for specimens from the three Quantec instruments. This
`variability may have been due to processing differences for the
`as-received instruments or to their subsequent clinical use. A study
`is currently in progress to examine the effect of controlled simu-
`lated clinical usage on the DSC plots for the ProFile and Light-
`speed instruments.
`The enthalpy changes observed for the overall martensitic NiTi
`to austenitic NiTi transformations in the specimens ranged from
`2.8 to 4.4 J/g for the as-received ProFile instruments and from 1.8
`to 8.4 J/g for the as-received Lightspeed instruments. These values
`lie within the range of 1.7 to 19.2 J/g (0.4 – 4.6 cal/g), which has
`been reported (11) for some nickel-titanium orthodontic wires. It is
`tempting to speculate that the higher enthalpy changes of 7.4 to 8.4
`J/g for the overall transformation from martensitic NiTi to auste-
`nitic NiTi in the Lightspeed instruments, compared with values
`ranging from 3.3 to 4.4 J/g for the ProFile instruments, is due to
`lower amounts of stable, work-hardened martensitic NiTi (which
`does not undergo transformation to austenitic NiTi) in the former
`instruments after manufacturing. Although this might suggest that
`the ProFile instruments have a less advantageous microstructure,
`future research is required to provide unambiguous proof of this
`hypothesis.
`
`The authors thank Dr. John C. Mitchell (Department of Geological Sci-
`ences, The Ohio State University) for providing the laboratory facilities to
`prepare the test specimens.
`
`Dr. Brantley is professor, Section of Restorative Dentistry, Prosthodontics
`and Endodontics, and director of the graduate program in Dental Materials
`Science, College of Dentistry, The Ohio State University, Columbus, Ohio. Dr.
`Svec is associate professor, Division of Endodontics, Department of Stoma-
`tology; Dr. Powers is professor and vice chair, Department of Restorative
`Dentistry and Biomaterials, and director, Houston Biomaterials Research
`Center and the graduate program in Dental Materials Science, University of
`Texas-Houston Health Science Center, Dental Branch, Houston, Texas. Dr.
`Iijima is visiting scholar, College of Dentistry, The Ohio State University,
`Columbus, Ohio. His permanent affiliation is instructor, Department of Orth-
`odontics, School of Dentistry, Health Sciences University of Hokkaido, Ish-
`ikari-tobetsu, Japan. Mr. Grentzer is a staff member in Analytical Services &
`
`Technology, Research & Development Department, Ashland Specialty
`Chemical Company, DublinUAddress requests for reprints to Dr. Timothy A.
`Svec, Division of Endodontics, University of Texas-Houston Dental Branch,
`Houston, TX 77030-3402.
`
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