The effect of temperature on the mechanical behavior of nickel-titanium orthodontic initial archwires
To investigate and compare the characteristics of commonly used types of traditional and heat-activated initial archwires at different temperatures by plotting their load/deflection graphs and quantifying three parameters describing the discharge plateau phase. Forty-eight archwires of cross-sectional diameters ranging from 0.010 inches to 0.016 inches were obtained from seven different manufacturers. A modified three-point wire-bending test was performed on three analogous samples of each type of archwire at 55°C and 5°C, simulating an inserted archwire that is subjected to cold or hot drinks during a meal. For each resulting load/deflection curve the plateau section was isolated and the mean value of each parameter for each type of wire was obtained. Permanent strain was exhibited by all wires tested at 55°C. Statistically significant differences were found between almost all wires for the three considered parameters when tested at 55°C and 5°C. Loads were greater at 55°C than at 5°C. Differences were also found between traditional and heat-activated archwires, the latter of which generated longer plateaus at 55°C, shorter plateaus at 5°C, and lighter mean forces at both temperatures. The increase in average force seen with increasing diameter tended to be rather stable at both temperatures. All nickel-titanium wires tested showed a significant change related to temperature in terms of behavior and force for both traditional and heat-activated wires. Stress under high temperatures can induce permanent strain, whereas the residual strain detected at low temperatures can be recovered from as temperature increases.ABSTRACT
Objectives:
Materials and Methods:
Results:
Conclusions:
INTRODUCTION
Nickel-titanium (NiTi) archwires have become increasingly popular in recent years because of their ability to release constant, light forces, which are considered to improve the efficiency and efficacy of treatment, especially during initial alignment and during the leveling phase. The second mechanical characteristic of these alloys is the shape memory property, the materials' ability to show complete recovery even when deformed, providing the clinician with the distinct advantage of being able to activate a NiTi archwire over a long time span without risk of permanently deforming the appliance.1
The shape memory and superelastic properties of Nitinol wires are attributed to their transformation between a high-temperature austenite phase and a low-temperature martensite phase. This transformation is the result of changes in the crystal lattice of the archwire material, and it can occur either by lowering the temperature or, within a defined temperature range, by applying stress.1 At lower temperatures, the alloy is completely present in the martensitic phase until the increase in temperature causes the progressive transformation into austenite.2 Each NiTi alloy has a specific temperature range in which this phase transition takes place, called the “TTR.” Shape memory property is the capability of NiTi wires to be plastically deformed in their martensite phase. If heated above a certain temperature range they will return to an austenite phase, recovering their initial form.1 Superelasticity is the transformation from the austenitic to the martensitic phase that occurs by stress application within a temperature range and is reflected in a load/deflection graph characterized by a flattish slope, known as the plateau, which indicates that the force exerted is relatively constant during this change.3,4 The martensite so formed is called stress-induced martensite (SIM). Within a temperature range martensite can be made stable with the application of stress, but it becomes unstable again when the stress is removed.1
Since oral temperature is not constant, but rather varies with every intake of air or hot and cold drink,5 Tonner and Waters6 have demonstrated that the forces applied by these wires will vary over a wide range and in some cases will become zero if the temperature falls below some critical value. Half of the wires tested by Meling and Odegaard7 were markedly influenced by the ambient temperature changes. Similar results related to superelastic wire behavior as it is affected by temperature were obtained by Laino et al.,8 who proposed a model to predict the elastic modulus of superelastic wires. Most of the research, though, was conducted on rectangular wires of significant size.7,9
The aim of the present study, therefore, was to investigate the effect of temperature (55°C and 5°C) on the mechanical behavior of round NiTi wires, typically employed during the first stages of orthodontic treatment, during the discharge phase, evaluating the entity of displacement at which the average force is approximately constant, and to determine the effective degree of constancy of the plateau phase.
MATERIALS AND METHODS
Forty-eight types of NiTi archwires were tested and grouped into two macrocategories: 22 were classed as traditional NiTi wires and 26 as heat-activated wires; all were circular in cross section and had a diameter measuring between 0.010 inches and 0.016 inches. The archwires were provided by G&H (Franklin, Ind), Ortho Technology (Tampa, Fla), 3M Unitek (St Paul, Minn), Ormco (Orange, Calif), American Orthodontics (Sheboygan, Wis), Dentaurum (Ispringen, Germany), and Forestadent (St Louis, Mo), as summarized in Table 1. Samples of each archwire were obtained by cutting 5.5 cm of the straightest distal portion of an archwire.
Means of Deflection
Tests were performed on three samples. We tested them in a three-point bending experiment.6,10,11 To evaluate the samples under conditions similar to the final operating one, they were mounted in four passive self-ligating brackets (slot, 0.022 inch; Damon 3Mx, Ormco).12 These brackets were glued to an acrylic resin base13 in such a way as to create a 14-mm span between the internal sides of two adjacent brackets. The resin base was, in turn, placed in a Plexiglas bath filled with water kept at a constant temperature of 55°C and 5°C: the 55°C was maintained with the aid of a heating pump (Julabo, Julabo Labortechnik Gmbh, Seelbach, Germany; Figure 1), and the tests at 5°C were performed with the aid of a refrigerator (TECO, Ravenna, Italy; Figure 2), both placed in a separate water bath connected to the first by a hydraulic circuit. Each sample was immersed in the water for a minimum of 60 seconds before being subjected to the testing procedure to reach thermal equilibrium. Water temperature was controlled by means of a thermocouple (Tekkal T8303, Tekkal, Milan, Italy) submerged in the test bath and was monitored continuously by the same operator responsible for performing the mechanical tests. The force applied was regulated by means of an Instron 4467 dynamometer (Instron, Norwood, Mass) connected to a 100-N load cell. A metal blade, with a curvature range of 1 mm at its extremity, was fixed to the load cell to deflect the archwires (Figure 3). Each wire was deflected 4 mm, at a deflection speed of 1 mm/min, and then was returned to its horizontal starting point at the same speed. This degree of deflection was chosen because of its possible occurrence under clinical conditions, although some authors14 maintain that far smaller deflections are more authentic.
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Measurements
Data were gathered by means of a personal computer connected to the measuring device and were processed using Labview 8.5 (National Instruments Corporation, Austin, Tex). Data thereby collected were presented in spreadsheet form using Microsoft Excel (Microsoft Corporation, Redmond, Wash) and were then used to plot a graph for each test, showing deflection of the test strip on the x-axis and the force exerted on the y-axis. Each curve thereby obtained represented the initial loading phase, of no particular clinical relevance, and the discharge phase, which indicates the entity of the force exerted on the teeth during orthodontic treatment.
We then characterized the behavior of the archwires, at different temperatures, in the discharge plateau phase by measuring three parameters already introduced for this kind of test15: average plateau force, plateau length, and plateau slope (Figure 4). A load/deflection curve was obtained for each of the samples of each type of wire tested. A sole operator subjectively identified and isolated on each graph the discharge plateau and calculated the values yielded by the samples for each of the parameters considered at the two considered temperatures.
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Statistical Analysis
Statistical analysis of the data was performed with analysis of variance (ANOVA), with P values of <.05 considered significant. For each of the parameters and each temperature, an internal comparison of each group of wires of the same type (traditional and heat-activated wires) and diameter was made. ANOVA was performed only on 0.012-, 0.014-, and 0.016-inch wires because data pertaining to 0.010-inch heat-activated and 0.013-inch traditional wires were insufficient for statistical purposes.
RESULTS
The analysis was focused on differences in the parameters examined between traditional and heat-activated NiTi archwires.
Tests at 55°C
Most of the graphs plotted displayed readily identifiable plateau regions in which the force was relatively high (Figures 5 and 6). The difference in values for each of the three parameters considering traditional and heat-activated wires of the same diameter and manufacturer revealed for the heat-activated wires an increase in plateau length and slope, except in the case of the 3M and American Orthodontics wires, which showed a decrease in slope. A decrease was also registered in the average plateau force between the heat-activated wires of all of the manufacturers tested (Table 2). These data showed statistically significant differences between manufacturers' archwires of the same type and diameter, except for the slope values yielded for 0.014-inch traditional wires, the plateau length values yielded for 0.016-inch heat-activated wires, and the values yielded for 0.012- and 0.013-inch heat-activated wires. The percentage of average increase in plateau force observed in traditional wires with increasing diameter was as follows: 98% from 0.012 inch to 0.014 inch; 228% from 0.012 inch to 0.016 inch; and 70% from 0.014 inch to 0.016 inch. The percentage of average increase associated with the heat-activated wires was as follows: 51% from 0.012 inch to 0.014 inch; 158% from 0.012 inch to 0.016 inch; and 61% from 0.014 inch to 0.016 inch.
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
The force expressed by traditional archwires shows a slightly greater tendency to increase with increasing diameter with respect to heat-activated wires, amounting to roughly 80% and 220% with a 0.002-inch and a 0.004-inch increase in diameter, respectively.
Tests at 5°C
The graphs plotted displayed identifiable plateau regions in which the force was low if compared to that registered for the tests at 55°C (Table 3; Figures 7 and 8). ANOVA revealed statistically significant differences between archwires of the same type and diameter, except for the plateau length value yielded for 0.013-inch heat-activated wires. The difference in values for each of the three parameters in a comparison of traditional and heat-activated wires of the same diameter and manufacturer showed for the heat-activated wires an increase in plateau slope and a decrease in plateau length and force. The percentage average increase in plateau force observed in traditional wires with increasing diameter was as follows: 52% from 0.012 inch to 0.014 inch; 166% from 0.012 inch to 0.016 inch; and 72% from 0.014 inch to 0.016 inch. The percentage average increase for the heat-activated wires was as follows: 42% from 0.012 inch to 0.014 inch; 139% from 0.012 inch to 0.016 inch; and 55% from 0.014 inch to 0.016 inch.
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
Citation: The Angle Orthodontist 83, 2; 10.2319/040612-287.1
The force expressed by traditional archwires shows, as at 55°C, a slightly greater tendency to increase with increasing diameter with respect to heat-activated wires, amounting to roughly 60% and 160% with a 0.002-inch and a 0.004-inch increase in diameter, respectively.
These results were compared to those published by Lombardo et al.,15 who observed the mechanical behavior of the same wires but tested at the temperature of 37°C. A significant reduction in slope and plateau length and a concomitant increase in force were noted between wires tested at 37°C and 55°C. Moreover, at 55°C wires showed permanent strain. A residual deflection (and, consequently, permanent strain) was recorded by our experiments at the temperature of 55°C (Table 4). The residual deflection values were calculated using Microsoft Excel and choosing, from each wire test, the deflection value correspondent with the force value closest to zero. An average residual deflection for each wire was obtained, and from these the permanent strain (εper) was calculated using the linear elasticity theory, as follows:
in which d is the wire's diameter, fres is the wire's residual deflection, and l is the wire's span. A reduction in force and a variable behavior in slope were detected between wires tested at 37°C and those tested at 5°C. In addition, the length plateau parameter was noted to get low above the group of the heat-activated wires and tends to be variable for traditional wires. A residual, but not permanent, deflection was also noted at the temperature of 5°C; however, a further increase in temperature allowed the wires to recover.
DISCUSSION
Today “light continuous forces” are thought of as physiologically suitable and efficacious, but in this case, the term is used somewhat arbitrarily. In fact, no consensus whatsoever has been reached among the scientific community about what constitutes a “light” force and what does not, and clinicians must judge for themselves the most suitable force for each particular clinical situation. In this context, NiTi archwires have become increasingly popular in recent years because of their ability to release constant, light forces, which are considered to improve the efficiency and efficacy of treatment, especially during initial alignment and leveling phases. It is well known nowadays that heat-activated archwires tend to impart less force than traditional NiTi archwires.16 In this context, the aim of our study was to evaluate and compare the behavior of NiTi archwires with regard to the force exerted by each at different temperatures, bearing in mind both the practical impossibility of defining this value in vivo17 and the fact that force values obtained using self-ligating brackets are lower than those associated with conventional archwires.18 The method chosen for this comparison not only considered the force expressed by each wire at specific points of deflection but it also aimed to characterize the behavior of each type of archwire in all of the discharge phases at different temperatures, so each could be described from a clinical perspective. In fact, the greater the capacity of an archwire to exert more constant forces with increasing displacement, the better its performance in terms of dental movement. During our study particular attention was paid to wires of round cross section and small diameter to investigate the behavior of so-called light wires.
The temperatures chosen for our tests are in agreement with those used in the study of Moore et al.,19 who reported during the 24-hour period a temperature range from 5.6°C to 58.5°C at the incisor site and from 7.9°C to 54°C at the premolar site. If the force expressed by NiTi wires increases with the increase in temperature, the force will decrease with the drop in temperature, as observed in our study; in particular, we reported a decrease, on average, of 82% for the heat-activated wires and a decrease of 77% for the traditional wires. This confirms the behavior described by Tonner and Waters,6 who demonstrated that at high temperatures, when the NiTi wires exist solely in the austenitic phase, a greater stress is required to produce strain than when other phases coexist, such as the martensite and transition phases at lower temperatures. Moreover, our tests showed, both at 55°C and at 5°C, an average increase in plateau force with increasing diameter of archwires. The force expressed by traditional archwires shows a slightly greater tendency to increase with increasing diameter with respect to heat-activated wires. The NiTi wires, both traditional and heat-activated, showed a permanent strain at the temperature of 55°C. According to Miyazaki et al.,20 this can occur when the stress required to induce SIM is greater than that required to cause plastic strain. Residual deflection, however, is very important to the clinician. A permanently deflected archwire below a certain value of deflection (specific for each NiTi wire) doesn't work anymore and has to be substituted by a new one in order to complete the alignment. Instead, the residual deflection noted at 5°C was recoverable, increasing the temperature and for this reason does not have a clinical importance.
CONCLUSIONS
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All NiTi wires tested showed a significant change related to temperature as well as changes in mechanical behavior and force for both traditional and heat-activated wires.
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Stress under high temperatures can induce permanent strain, whereas residual strain detected at low temperatures can be recovered from as temperature increases.
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Comparing our results to those published by Lombardo et al.,15 we detected a reduction in plateau length and an increase in force between wires tested at 37°C and those tested at 55°C; when tested at 37°C and 5°C, the force decreased and the plateau length showed a reduction for heat-activated wires and variable behavior for the traditional wires.
Heating pump.
Refrigerator.
Deflection, with 1-mm blade, of mounted archwire, kept in a water bath.
Isolation of discharge plateau and three parameters considered: length, mean force, and slope.
Load/deflection curve of three samples at 55°C of 0.014-inch NiTi Classic (3M) (a), 0.014-inch NiTi Super Elastic (3M) (b), 0.014-inch NiTi Titanium memory wire (American Orthodontics) (c), 0.014-inch NiTi Rematitan Lite (Dentaurum) (d), 0.014-inch Titanol-Superelastic (Forestadent) (e), 0.014-inch Orthoforce G4 (G&H) (f), 0.014-inch TruFlex (Ortho Technology) (g), and 0.014-inch CuNiTi (Ormco) (h).
Load/deflection curve of three samples at 55°C of 0.014-inch Tensic (Dentaurum) (i), 0.014-inch Biostarter (Forestadent) (l), 0.014-inch Orthoforce M5 (G&H) (m), 0.014-inch TruFlex Thermal (Ortho Technology) (n), 0.014-inch Therma-Ti (American Orthodontics) (o), 0.014-inch Therma-Ti D (American Orthodontics) (p), and 0.014-inch Therma-Ti Lite (American Orthodontics) (q).
Load/deflection curve of three samples at 5°C of 0.014-inch NiTi Classic (3M) (a), 0.014-inch NiTi Super Elastic (3M) (b), 0.014-inch NiTi Titanium memory wire (American Orthodontics) (c), 0.014-inch NiTi Rematitan Lite (Dentaurum) (d), 0.014-inch Titanol-Superelastic (Forestadent) (e), 0.014-inch Orthoforce G4 (G&H) (f), 0.014-inch TruFlex (Ortho Technology) (g), and 0.014-inch CuNiTi (Ormco) (h).
Load/deflection curve of three samples at 5°C of 0.014-inch Tensic (Dentaurum) (i), 0.014-inch Biostarter (Forestadent) (l), 0.014-inch Orthoforce M5 (G&H) (m), 0.014-inch TruFlex Thermal (Ortho Technology) (n), 0.014-inch Therma-Ti (American Orthodontics) (o), 0.014-inch Therma-Ti D (American Orthodontics) (p), and 0.014-inch Therma-Ti Lite (American Orthodontics) (q).
Contributor Notes