Highlights
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The results of the study indicate that the Bend and Free Recovery (BFR) method is suitable as a standard test method to determine the transformation temperatures of heat-activated Nickel-Titanium (Ni-Ti) orthodontic archwires.
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In this study, transformation temperature values, A s and A f , are comparable between the Differential Scanning Calorimetry (DSC) and BFR test methods.
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Since BFR enables testing of unmodified, as-received wires that can be strained to clinically appropriate levels, it is a more clinically relevant test method than DSC, and it is more economical.
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In this study, the tested Ni-Ti archwires fully transitioned to austenite well before the manufacturer-listed temperatures.
Abstract
Objectives
The purpose of this study was to investigate the suitability of the Bend and Free Recovery (BFR) method as a standard test method to determine the transformation temperatures of heat-activated Ni-Ti orthodontic archwires. This was done by determining the transformation temperatures of two brands of heat-activated Ni-Ti orthodontic archwires using the both the BFR method and the standard method of Differential Scanning Calorimetry (DSC). The values obtained from the two methods were compared with each other and to the manufacturer-listed values.
Methods
Forty heat-activated Ni-Ti archwires from both Rocky Mountain Orthodontics (RMO) and Opal Orthodontics (Opal) were tested using BFR and DSC. Round (0.016 inches) and rectangular (0.019 × 0.025 inches) archwires from each manufacturer were tested. The austenite start temperatures (A s ) and austenite finish temperatures (A f ) were recorded.
Results
For four of the eight test groups, the BFR method resulted in lower standard deviations than the DSC method, and, overall, the average standard deviation for BFR testing was slightly lower than for DSC testing. Statistically significant differences were seen between the transformation temperatures obtained from the BFR and DSC test methods. However, the A f temperatures obtained from the two methods were remarkably similar with the mean differences ranging from 0.0 to 2.1 °C: A f Opal round (BFR 26.7 °C, DSC 27.6 °C) and rectangular (BFR 27.6 °C, DSC 28.6 °C); A f RMO round (BFR 25.5 °C, DSC 25.5 °C) and rectangular (BFR 28.0 °C, DSC 25.9 °C). Significant differences were observed between the manufacturer-listed transformation temperatures and those obtained with BFR and DSC testing for both manufacturers.
Significance
The results of this study suggest that the Bend and Free Recovery method is suitable as a standard method to evaluate the transformation temperatures of heat-activated Ni-Ti orthodontic archwires.
1
Introduction
Within orthodontics, standards for the manufacturing of products provide distinct guidelines and clarity to manufacturers and consumers mutually . Set standards that provide requirements for measurement and labeling of wire size, along with requirements for testing and presenting of physical and mechanical properties of orthodontic wires, have made the comparison between products easier for clinicians. However, many U.S. manufacturers do not provide packaging and labeling information required by ANSI/ADA and ISO standards for orthodontic wires. In particular, both ANSI/ADA Standard No. 32 “Orthodontic Wires” and ISO 15841 “Dentistry–Wires for use in Orthodontics” require that, when applicable, the austenite finish temperature (A f ) of nickel-titanium (NiTi) wires be provided with the packaging and labeling information . Yet, information on the austenite finish temperature is often not found on the labels of orthodontic wires claiming to be “heat-activated.”
Nickel-titanium alloys have the ability to exhibit a shape memory effect. The ASTM Committee F04 on Medical and Surgical Materials and Devices defines a “shape memory alloy” to be an alloy that, after it is plastically deformed in the martensitic phase, “undergoes a thermoelastic change in crystal structure when heated through its transformation temperature range resulting in a recovery of the deformation .” It is this shape memory effect exhibited by NiTi alloys that is used by the Bend and Free Recovery method to determine transformation temperature values, as described below. The high temperature phase for NiTi shape memory alloys (SMAs) is referred to as the austenitic phase, and the lower temperature phase is the martensitic phase . When in the austenitic phase, NiTi has a body-centered cubic crystal structure, making it difficult to displace; however, when it is in the martensitic phase, it has a close-packed hexagonal crystal structure, which allows the molecules to slide across one another more easily . The martensitic phase has a lower modulus of elasticity (∼50 GPa) than the austenitic phase (∼120 GPa), which essentially means the martensitic phase is more flexible .
The temperature range at which NiTi changes between its two solid phases (martensite and austenite) is called the Transformation Temperature Range (TTR) . Both phases exist within this range in a dynamic equilibrium . The austenite start temperature ( A s ) is the temperature at which the martensitic phase starts to transform to the austenitic phase when the alloy is heated . Once the temperature is equal to or greater than the austenite finish temperature ( A f ), the wire is entirely in the austenitic phase. Above A f , the archwires have the ability to exhibit superelastic behavior. The archwires must be above A f for the “nonlinear recoverable deformation behavior” characteristic of superelasticity to take place . This is because the behavior comes from the “stress-induced formation of martensite on loading and the spontaneous reversion of this crystal structure to austenite upon unloading” . As stated above, when the temperature is below A s and the wire is in the martensitic phase, it is more flexible . Thus, since the archwire will exhibit different behaviors whether it is below A s or above A f , the transformation temperature range is one of the most important features of a thermoelastic (heat-activated) wire. Moreover, these heat-activated wires are significantly more expensive than many other types of NiTi archwires available for purchase, so it important to clinicians that these wires actually transition at the claimed clinically relevant temperature.
The majority of published orthodontic studies use Differential Scanning Calorimetry (DSC) to test the transformation temperatures of orthodontic wires. Also, standards for orthodontic wires, specifically ANSI/ADA Standard No. 32 and ISO 1584, specify DSC as the method for determination of the austenite finish temperature ( A f ) for orthodontic archwires . However, for some manufacturers within the medical device industry, DSC is not the preferred test method for determination of the A f of NiTi devices. The Bend and Free Recovery (BFR) method, as described in ASTM F 2082 “Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery”, is also used to test and verify the A f temperature of medical products such as nitinol stents . Both of these methods (DSC and BFR) are straightforward to perform, able to test small specimens, and are reproducible . However, since the BFR method has the ability to test a finished medical product without sectioning, the results obtained from this method can be more clinically relevant. Furthermore, it is the only method that utilizes the shape memory effect of NiTi wires during testing, as noted by ASTM F 2082 : “measurement of the specimen motion closely parallels many shape memory applications and provides a result that is applicable to the function of the material.” Also, when NiTi wire is bent around a mandrel of a suitable radius of curvature to induce “an outer fiber strain level of 2−2.5%”, ruggedness testing has shown that the effect of applied strain is not significant . However, BFR allows higher strain levels to be applied if the product being tested is subjected to higher strain levels during clinical use and the researchers would like to simulate the higher levels during testing. Since increasing strain has been shown to shift transformation temperatures to higher levels, simulating clinical strain levels is important . Additionally, the apparatus used for BFR testing is much more economical in comparison to the price of DSC equipment.
Given this information, the absence of the BFR method for the testing of heat-activated NiTi archwires within the orthodontic literature is surprising. Therefore, the purpose of this study was to investigate the suitability of the Bend and Free Recovery method as a standard test method to determine the transformation temperatures of heat-activated NiTi orthodontic archwires. This was done by determining the transformation temperatures of two brands of heat-activated NiTi orthodontic archwires using both the Bend and Free Recovery method and the standard method of Differential Scanning Calorimetry. The values obtained from the two methods were compared with each other and to the manufacturer-listed values.
2
Materials and methods
The experimental groups consisted of commercially available thermoelastic NiTi orthodontic archwires from two different manufacturers, Opal Orthodontics (Opal; South Jordan, UT, USA) and Rocky Mountain Orthodontics (RMO; Denver, CO, USA). Round archwires with a diameter of 0.016 inch (0.41 mm) were chosen, since these are commonly used in the initial leveling and aligning phase of orthodontic treatment. Rectangular archwires with dimensions of 0.019 inch × 0.025 inch (0.48 × 0.635 mm) were also tested, since many practitioners use such wires early in treatment. Manufacturers were asked to provide wires from two different lots: Opal round – 258999 and 245990; Opal rectangular – 261376 and 258671; RMO round − F1111747 and F1202886; and RMO rectangular − F1204539 and F1209259. There were a total of eight groups, each comprised of 10 specimens from two different lots, which is double the sample size used for the precision and bias statements of ASTM F 2082-06 and ASTM F 2004-05. All specimens were stored at room temperature prior to testing.
2.1
Bend and Free Recovery (BFR) test method
The Bend and Free Recovery test method was performed using the Recovery Temperature Testing Apparatus (RTTA). This apparatus was built at the American Dental Association (Chicago, IL, USA) using the apparatus requirements set forth in ASTM F 2082-06 . Since a closed BFR testing system was not used, testing of specimens was randomized to account for the potential environmental differences within the laboratory at different test times. An outside participant numbered specimens 1 through 20 for each group. These numbers were then randomized using the randomization feature in Microsoft® Excel (Redmond, WA, USA) to determine the order of testing. To avoid cutting and grinding, which can cause cold working of the material that affects the transformation temperature , and to use actual orthodontic archwire products with material volumes relevant to their clinical function, the wires were tested as received without being cut. The wires were tested at a consistent location along their straight portions, 15 mm from the end of each archwire.
In brief, to determine the transformation temperature of a specimen by BFR, ASTM standard F 2082 states that the specimen must be cooled “to its nominally fully martensitic phase,” deformed, and heated back to its fully austenitic phase . During the heating process, the specimen movement is monitored; therefore, specimen displacement can be plotted versus specimen temperature. From the temperature−displacement graph, the A s and A f of the specimen can be determined ( Fig. 1 ).
Before testing, each specimen was marked with a permanent marker 20 mm from one end. To begin a test, an individual wire was mounted on the test recovery fixture of the RTTA, with the wire clamped in position on the forming mandrel such that the 20 mm mark and the recovery fixture clamp were aligned, as shown in Fig. 2 a . A bath was then filled with a water−glycerin solution that was cooled down to a minimum of -20 °C. Next, the test recovery fixture, with the test wire mounted on it, was placed in the water−glycerin bath, and a T-type thermocouple, with a resolution of 0.1 °C, was positioned as close as possible to the test wire (the thermocouple was calibrated by comparison with a NIST traceable, mercury reference thermometer with a resolution of 0.05 °C using a method similar to one described in ASTM E 220-02 ). In order to allow the wire and RTTA parts to equilibrate to the bath temperature, the test wire remained in the water−glycerin solution for a minimum of 3 min prior to testing.
After 3 min, the wire-forming lever was moved over the test wire, bending it against the forming mandrel ( Fig. 2 b). This wire deforming step resulted in the round wires being subjected to an outer surface strain of 2.5%, and the rectangular wires being subjected to a slightly higher outer surface strain of 2.95%. After the wire deformation step, the core of a linear variable displacement transducer (LVDT, Model DC 750-250-10, MacroSensors, Pennsauken, NJ, USA) was lowered onto the test wire 15 mm from the end. The LVDT specifications are the following: range ±6.3 mm, full-scale output 0 to ±10 V DC, and linearity error < ± 0.25% of full range output (note that the linearity was verified to be within specification using a procedure similar to the one outlined in ASTM F 2537) . The weight of the LVDT core was counterbalanced such that the weight on the test wire was no more than 3 g. Fig. 3 shows an illustration of the Recovery Temperature Testing Apparatus with the LVDT core lowered on to the test wire.
After the LVDT core was positioned, a polyimide film insulated heater (Kapton® flexible heater, 10 W/in 2 , Omega Engineering Inc., Stamford, CT, USA) was turned on to heat the water glycerin bath, and a stirrer was turned on to circulate the solution. The heating rate was limited to 1.4−1.6 °C/min. At the same time the heater was turned on, a data acquisition system (CompactDAQ, National Instruments Corp., Austin, TX, USA) was initiated to acquire the signals from the thermocouple and LVDT. From the acquired signals, temperature and displacement were monitored using a custom written program (LabVIEW software, National Instruments Corp.). For wires from both manufacturers, the tests were stopped at 50 °C, since this temperature was at least 10 °C above the A f of both wire groups as determined by pilot testing.
The data from the data acquisition program were saved as text files and imported into a spreadsheet (Microsoft® Excel) for plotting. For each test, a temperature versus time graph was created to determine the heating rate for the individual test. Also, for each test, a temperature versus displacement graph was created to determine A s and A f . This was done by using the spreadsheet tools to draw lines tangent to the different linear portions of an individual curve, in accordance with the procedure set forth in ASTM F 2082 . Fig. 1 shows a sample curve with the tangent lines drawn, and A s and A f determined by the intersection of the tangent lines.
2.2
Differential Scanning Calorimetry (DSC) test method
The DSC testing was performed using a Mettler Differential Scanning Calorimeter (Model 822e Mettler-Toledo Inc., Columbus, OH, USA). Specimen preparation included sectioning 5 mm segments from the straight portion end of each archwire using a low-speed, water-cooled diamond saw (Isomet, Buehler Ltd, Lake Bluff, IL, USA). For an individual test, a 5 mm segment was placed in an aluminum crucible and sealed (note that it was not necessary to bend the straight, 5 mm segment to fit it in the crucible). The test crucible and an empty aluminum crucible were placed in the differential scanning calorimeter at room temperature, and the temperature was scanned from -100 to 100 °C and back to -100 °C at a rate of 10 °C per minute. Liquid nitrogen was used as the coolant and nitrogen gas for purging.
The DSC plots were analyzed using the DSC manufacturer’s software. A s and A f values were determined by the intersection of the baseline of the heating curve with tangents to the heating peak, as specified and illustrated in ANSI/ADA Standard No. 32 and ISO 15841 . Cooling peaks were also analyzed but were not included for comparison because the BFR did not record analogous values. Fig. 4 shows a representative DSC curve.
2
Materials and methods
The experimental groups consisted of commercially available thermoelastic NiTi orthodontic archwires from two different manufacturers, Opal Orthodontics (Opal; South Jordan, UT, USA) and Rocky Mountain Orthodontics (RMO; Denver, CO, USA). Round archwires with a diameter of 0.016 inch (0.41 mm) were chosen, since these are commonly used in the initial leveling and aligning phase of orthodontic treatment. Rectangular archwires with dimensions of 0.019 inch × 0.025 inch (0.48 × 0.635 mm) were also tested, since many practitioners use such wires early in treatment. Manufacturers were asked to provide wires from two different lots: Opal round – 258999 and 245990; Opal rectangular – 261376 and 258671; RMO round − F1111747 and F1202886; and RMO rectangular − F1204539 and F1209259. There were a total of eight groups, each comprised of 10 specimens from two different lots, which is double the sample size used for the precision and bias statements of ASTM F 2082-06 and ASTM F 2004-05. All specimens were stored at room temperature prior to testing.
2.1
Bend and Free Recovery (BFR) test method
The Bend and Free Recovery test method was performed using the Recovery Temperature Testing Apparatus (RTTA). This apparatus was built at the American Dental Association (Chicago, IL, USA) using the apparatus requirements set forth in ASTM F 2082-06 . Since a closed BFR testing system was not used, testing of specimens was randomized to account for the potential environmental differences within the laboratory at different test times. An outside participant numbered specimens 1 through 20 for each group. These numbers were then randomized using the randomization feature in Microsoft® Excel (Redmond, WA, USA) to determine the order of testing. To avoid cutting and grinding, which can cause cold working of the material that affects the transformation temperature , and to use actual orthodontic archwire products with material volumes relevant to their clinical function, the wires were tested as received without being cut. The wires were tested at a consistent location along their straight portions, 15 mm from the end of each archwire.
In brief, to determine the transformation temperature of a specimen by BFR, ASTM standard F 2082 states that the specimen must be cooled “to its nominally fully martensitic phase,” deformed, and heated back to its fully austenitic phase . During the heating process, the specimen movement is monitored; therefore, specimen displacement can be plotted versus specimen temperature. From the temperature−displacement graph, the A s and A f of the specimen can be determined ( Fig. 1 ).
Before testing, each specimen was marked with a permanent marker 20 mm from one end. To begin a test, an individual wire was mounted on the test recovery fixture of the RTTA, with the wire clamped in position on the forming mandrel such that the 20 mm mark and the recovery fixture clamp were aligned, as shown in Fig. 2 a . A bath was then filled with a water−glycerin solution that was cooled down to a minimum of -20 °C. Next, the test recovery fixture, with the test wire mounted on it, was placed in the water−glycerin bath, and a T-type thermocouple, with a resolution of 0.1 °C, was positioned as close as possible to the test wire (the thermocouple was calibrated by comparison with a NIST traceable, mercury reference thermometer with a resolution of 0.05 °C using a method similar to one described in ASTM E 220-02 ). In order to allow the wire and RTTA parts to equilibrate to the bath temperature, the test wire remained in the water−glycerin solution for a minimum of 3 min prior to testing.
After 3 min, the wire-forming lever was moved over the test wire, bending it against the forming mandrel ( Fig. 2 b). This wire deforming step resulted in the round wires being subjected to an outer surface strain of 2.5%, and the rectangular wires being subjected to a slightly higher outer surface strain of 2.95%. After the wire deformation step, the core of a linear variable displacement transducer (LVDT, Model DC 750-250-10, MacroSensors, Pennsauken, NJ, USA) was lowered onto the test wire 15 mm from the end. The LVDT specifications are the following: range ±6.3 mm, full-scale output 0 to ±10 V DC, and linearity error < ± 0.25% of full range output (note that the linearity was verified to be within specification using a procedure similar to the one outlined in ASTM F 2537) . The weight of the LVDT core was counterbalanced such that the weight on the test wire was no more than 3 g. Fig. 3 shows an illustration of the Recovery Temperature Testing Apparatus with the LVDT core lowered on to the test wire.
After the LVDT core was positioned, a polyimide film insulated heater (Kapton® flexible heater, 10 W/in 2 , Omega Engineering Inc., Stamford, CT, USA) was turned on to heat the water glycerin bath, and a stirrer was turned on to circulate the solution. The heating rate was limited to 1.4−1.6 °C/min. At the same time the heater was turned on, a data acquisition system (CompactDAQ, National Instruments Corp., Austin, TX, USA) was initiated to acquire the signals from the thermocouple and LVDT. From the acquired signals, temperature and displacement were monitored using a custom written program (LabVIEW software, National Instruments Corp.). For wires from both manufacturers, the tests were stopped at 50 °C, since this temperature was at least 10 °C above the A f of both wire groups as determined by pilot testing.
The data from the data acquisition program were saved as text files and imported into a spreadsheet (Microsoft® Excel) for plotting. For each test, a temperature versus time graph was created to determine the heating rate for the individual test. Also, for each test, a temperature versus displacement graph was created to determine A s and A f . This was done by using the spreadsheet tools to draw lines tangent to the different linear portions of an individual curve, in accordance with the procedure set forth in ASTM F 2082 . Fig. 1 shows a sample curve with the tangent lines drawn, and A s and A f determined by the intersection of the tangent lines.
2.2
Differential Scanning Calorimetry (DSC) test method
The DSC testing was performed using a Mettler Differential Scanning Calorimeter (Model 822e Mettler-Toledo Inc., Columbus, OH, USA). Specimen preparation included sectioning 5 mm segments from the straight portion end of each archwire using a low-speed, water-cooled diamond saw (Isomet, Buehler Ltd, Lake Bluff, IL, USA). For an individual test, a 5 mm segment was placed in an aluminum crucible and sealed (note that it was not necessary to bend the straight, 5 mm segment to fit it in the crucible). The test crucible and an empty aluminum crucible were placed in the differential scanning calorimeter at room temperature, and the temperature was scanned from -100 to 100 °C and back to -100 °C at a rate of 10 °C per minute. Liquid nitrogen was used as the coolant and nitrogen gas for purging.
The DSC plots were analyzed using the DSC manufacturer’s software. A s and A f values were determined by the intersection of the baseline of the heating curve with tangents to the heating peak, as specified and illustrated in ANSI/ADA Standard No. 32 and ISO 15841 . Cooling peaks were also analyzed but were not included for comparison because the BFR did not record analogous values. Fig. 4 shows a representative DSC curve.
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