Short-term temperature changes influence the force exerted by superelastic nickel-titanium archwires activated in orthodontic bending

ORIGINAL ARTICLE Short-term temperature changes influence the force exerted by superelastic nickel-titanium archwires activated in orthodontic bending Torstein R. Meling, MD, DrPhilos,a and Jan Ødegaard, BDS, MS, DrOdont Oslo and Stavanger, Norway

Background: Alterations in mouth temperature may lead to changes in the force exerted by an activated superelastic wire. It has been assumed that variations in archwire stiffness associated with short-term cooling or heating are transient. This investigation studied the effect of short-term cooling or heating on the bending force exerted by nickel-titanium archwire. Material and methods: Six rectangular superelastic wires and one conventional nickel-titanium wire were tested in bending at 37oC. The test specimens were deflected 0.5 mm, and the bending force was measured continually. The activated specimens were subjected to cold (10oC) or hot (80oC) water under constant deflection, simulating an inserted archwire that is subjected to cold or hot drinks or food during a meal. Results: The conventional nickel-titanium wire was marginally affected by brief cooling or heating. In contrast, some of the superelastic wires were strongly affected by short-time application of cold or hot water. Whereas the effect of brief heating disappeared quickly, some wires continued to exert sub-baseline bending forces (up to 32% less) after short-time application of cold water and showed little or no tendencies toward increase even after 30 minutes of postexposure restitution (up to 43% less). Conclusions: Short-term exposures to hot liquid increased the bending force exerted for a given deflection transiently. The effect of short-term exposures to cold liquid was not always transient; the bending force remained sub-baseline for a number of the thermosensitive wires tested for a prolonged time. (Am J Orthod

Dentofacial Orthop 1998;114:503-9)

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uperelastic nickel-titanium (NiTi) archwires may show exceptional temperature-sensitivity of some key characteristics.1-3 Transient mouth temperature changes associated with ingestion of cold or hot food can lead to changes in the force exerted by an activated thermosensitive wire.3 The responsiveness toward thermal stimuli of a superelastic wire can be visualized in a stress versus temperature plot.4 A steep slope implies a high degree of thermosensitivity. Thus, slight elevations in the deformation temperature can significantly increase the archwire stiffness, thereby increasing the load exerted on periodontal structures and lead to patient discomfort if the pain threshold is exceeded.4 Conversely, transient reductions in wire stiffness in response to a lower deformation temperature can decrease the load on periodontal structures, This study was supported by a research grant from Research Forum, Ullevaal Hospital, FUS. aDepartment of Maxillofacial Surgery, Ullevaal Hospital, Oslo, Norway. bIn private practice, Stavanger, Norway. Reprint requests to: Dr Torstein R. Meling, Erika Nissensvei 3A, 4022 Stavanger, Norway Copyright © 1998 by the American Association of Orthodontists. 0889-5406/98/$5.00 + 0 8/1/87602

thereby leading to a transiently increased perfusion of these tissues.5 It has been hypothesized that because this may allow replenishment of cells and nutrients to the cellular elements involved in the repair process in areas where blood stasis has occurred due to capillary strangulation,4,5 tooth movement may be accelerated.4 In a previous investigation, we studied the effect of short-term temperature variations on the torsional stiffness of eight rectangular superelastic wires.3 Test-specimens were activated to 20° in longitudinal torsion at body temperature and subjected to cold or hot water while the deflection was kept constant, simulating an inserted archwire that is subjected to cold or hot drinks or food during a meal. The torsional stiffness of some wires was strongly affected by brief cooling or heating. Whereas the effect of heating was transient, the amount of torque exerted by certain superelastic archwires remained well below baseline (up to 85% less than baseline) after being briefly exposed to cold water. The most thermosensitive archwires showed incremental reductions in torsional stiffness when cold water was repeatedly applied. Furthermore, the torsional stiffness remained low (up to 50% less than baseline) and 503

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Fig 1. Schematic drawing of test apparatus. Archwire test piece is held in position by three orthodontic brackets. Loading of the test piece occurs through the center bracket as carried by a center sliding bar. The linear pulling force required to keep the test specimen constantly deflected at .5 mm is recorded by a load cell. The deflection is measured by an extension meter.

showed no tendency toward increase even after 2 hours of postexposure restitution. To our knowledge, the effect of temperature on the bending stiffness of superelastic wires has not been investigated in depth. The purpose of this investigation was to evaluate the effect of transient alterations in temperature on the elastic responses of superelastic nickel-titanium-based archwires to orthodontic bending when tested in a situation simulating application of an extrusive load to a single tooth. MATERIAL AND METHODS

Four manufacturers supplied a total of six different .017 × .025 inch nickel-titanium (NiTi) or copper-nickel-titanium (CuNiTi) wire batches; all claimed to exhibit superelastic properties. The wires were Rematitan Super Elastic (Dentaurum Pforzheim, Germany), NeoSentalloy F200 (GAC Central, Islip, N.Y.), CuNiTi 27oC, CuNiTi 35oC, and CuNiTi 40oC (Ormco, Glendora, Calif.), and Nitinol SE (3M/Unitek, Monrovia, Calif.). In addition, one conventional, work-hardened NiTi wire was included for comparison (Nitinol, 3M/Unitek). The wires were supplied in arch forms and tested in as-received conditions. From each batch, a 25-mm long piece was cut from the nearly straight, posterior section of three individual archwire blanks. The specimens were tested in orthodontic bending in a specially designed apparatus6 (Fig 1). The archwire test-piece was held in position by three .018 inch standard medium twin edgewise brackets 3.5 mm wide with 0° torque and angulation (Ormco). The center bracket (A) was fixed to a center sliding bar (B) on ball bearings (C), while the two supporting brackets (D) were stationary. When mounted, the brackets were oriented such that an orthodontic wire passing through the bracket slots will have its cen-

tral axis perpendicular to the central axis of the sliding bar. The interbracket distance was set at 5 mm. A Minibeam load cell (E) type MB-10 (Interface Inc., Scottsdale, Ariz.) was indirectly coupled to the sliding bar. The load cell was attached to a moveable stage (F) and connected via a wire to the end of the sliding bar (B). The stage can be moved in both directions through a spindle (G). By moving the stage away from the sliding bar, the load cell will pull on the connecting wire and hence the bar. Thus, loading of the wire test specimen occurs through the center bracket as carried by the center sliding bar. The linear pulling force was recorded by the load cell (E). The deflection was measured by a digital extension meter (H) (Model 543-126B, Mitutoyo, Tokyo, Japan) to the nearest .01 mm. The experiments were carried out in an electrically heated room with dimensions of 5 × 8 feet. The temperature was regulated by a thermostat capable of controlling the temperature with an accuracy of ± .5°C (Micromatic, Vejle, Denmark). The temperature was monitored continually with an electronic thermometer accurate to 0.1oC and capable of recording minimum, maximum, and actual temperature. A pilot study demonstrated that the preset temperature could be maintained within ± .5oC (unpublished results). To study the effect of brief temperature alterations, three test specimens from each wire type were tested at 37oC ambient temperature and subjected to a sequence of short-term exposure to cold (10oC) or hot (80oC) water. The water temperatures were chosen after multiple temperature measurements of drinks with cold soda and hot coffee with a calibrated thermometer accurate to ± 1oC (unpublished results). The test-pieces were ligated into the three holding brackets with Alastic A-Modules (3M/Unitek) and subsequently activated by pulling the center sliding bar .5

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mm. The deflection magnitude was read off the extension meter. The load cell was coupled to a PC using an A/D converter card (Strawberry Tree, Sunnyvale, Calif.) and programmed to record the bending load once per second and make each recording as a mean of 10 subrecordings. The program was developed by Mr. Robert Sandvik. As we found a slight tendency of wire creep during the initial phase of activation, the wire was kept activated for 2 minutes before further testing. After this initial period, the wire was subjected to 5 ml of cold water for 10 seconds and kept activated for 5 minutes. The wire was then deactivated for 60 seconds before subsequent testing in order to examine whether permanent changes in wire properties had occurred during cold exposure. The wire was reactivated, and after a restitution phase of 1 minute, it was subjected to 5 ml of hot water for 10 seconds and kept activated for 5 minutes. The wire was then deactivated for 60 seconds before subsequent testing to examine whether permanent changes in wire properties had occurred during heat exposure. Subsequently, the wire was reactivated and underwent another restitution phase of 1 minute. Then the wire was subjected to 5 ml of cold water for 10 seconds, kept activated for 5 minutes, subjected to 5 ml of hot water for 10 seconds, and kept activated for another 5 minutes. Finally, to control for any permanent wire damage or creep, the wire was deactivated for 60 seconds, reactivated 0.5 mm, and kept constant for a final 2-minute period. To study the effect of repetitive archwire cooling, one test specimen from each of the seven test wires was selected. The test specimen was activated 0.5 mm at body temperature and kept activated for 2 minutes. Subsequently it was subjected to 10 cycles of cold water exposures. Each cycle consisted of a cold water exposure period of 10 seconds with a 50-second postexposure restitution phase. Thereafter, the test specimen was kept activated undisturbed for 30 minutes to study the speed and extent of recovery. RESULTS Effect of short-term temperature changes

The effects of short-term temperature changes on archwire stiffness are shown in bending load-versustime diagrams for .5 mm of constant deflection (Fig. 2). The superelastic wires were generally more affected by exposure to cold than to hot water. The effect of heating disappeared quickly, and the baseline bending load level was completely restored. Deactivations followed by reactivations restored the initial bending load level regardless of prior heat or cold exposure, indicating that no permanent damage was inflicted on the test specimens by the exposure to hot or cold water. After

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short-term cooling, the initial bending load level was restored after a short-term exposure to hot water. The Dentaurum Rematitan Super Elastic wire (Fig. 2A) showed minor sensitivity toward cooling, and the load fell by 28% from baseline. The bending load was minimally influenced by hot water. After being subjected to cold or hot water, the bending load rather quickly approached the baseline level. The GAC NeoSentalloy F200 wire (Fig. 2B) showed a marked drop (45% below baseline) after application of cold water and had an incomplete ability to regain its original bending load level. Five minutes postexposure, the load level was 14% below baseline. The effect of heating was notable, with a maximum load of 26% above baseline. The load level quickly fell down to baseline postexposure. The bending force exerted by the Ormco CuNiTi wires (Fig. 2C) dropped markedly when the activated test specimens were subjected to cold water (52% to 54% of baseline). These wires tended to stabilize at a subbaseline load level (68% to 80% below baseline). However, the baseline bending load level was restored when the test specimens were activated after a brief (1 minute) deactivation period. Application of hot water led to a slight increase in the load (up to 12% above baseline), but the effect was transient and the load level quickly approached baseline. The Unitek Nitinol SE wire was more affected by heating than cooling. However, the load level after short-term cooling did not completely reach baseline level after 5 minutes postexposure restitution (90% of baseline). Brief heating increased the load exerted by the archwire up to 29% above baseline. However, the effect was transient. The conventional, work-hardened Unitek Nitinol wire was marginally affected by temperature provocations. Exposure to cold water decreased the bending load to 8% below baseline. Conversely, short-term heating increased the load exerted by the activated wire to 19% above baseline. In general, this wire returned to the original bending load level quickly, regardless of temperature stimulus. Effect of repeated cold exposure

The effects of repeated cold exposures are shown in Fig. 3. The general trend was that the wires most affected by a single short-term cooling were strongly influenced by repeated cooling and showed a tendency to stabilize at a very low load level. For some of the wires tested, the original load level was not regained even after a 30-minute postexposure restitution phase. The Dentaurum Rematitan Super Elastic wire (Fig. 3A) exerted less force when repeatedly being subjected

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Fig 2. Effect of short-term temperature changes on the bending stiffness of superelastic Ni-Ti .017 × .025 inch wires. Ambient temperature constant at 37oC. Temperature stimulus: cold water (10oC) or hot water (80oC). A, Typical plot of a superelastic wire with low thermosensitivity; Dentaurum Rematitan SE. B, Typical plot of a superelastic wire with intermediate thermosensitivity; GAC NeoSentalloy F200. C, Typical plot of a superelastic wire with high thermosensitivity; Ormco CopperNi-Ti 35oC.

to cold water, up to 36% less than baseline. Repetitious cooling did not produce a striking additive effect. The effect of cooling was transient, and the load level was similar to the original within 30 minutes after the last exposure to cold water. The force exerted by an activated GAC NeoSentalloy F200 wire (Fig. 3B) decreased markedly when exposed to cold water (69% below baseline). Furthermore, repetitive cooling produced a slight additive effect. As we observed after a single exposure to cold water, the bending force returned toward the original level. However, baseline bending load level was not completely restored within the 30-minute postexposure restitution phase (23% below baseline). The three Ormco CuNiTi wires were dramatically affected by exposure to cold water, and the bending force dropped to 82% to 90% below the baseline level after 10 cycles of cold water exposures. Furthermore,

the effect of repetitious archwire cooling seemed to be additive. Although the wires regained some of their bending stiffness after the last cooling, the CuNiTi wires exerted significantly lower bending forces after being subjected to cold water. The CuNiTi 27oC wire exerted about 29% less than baseline, the 35oC wire (Fig. 3C) about 33%, and the 40oC wire about 43% a half an hour after the last cycle. The bending load level observed 10 minutes after a single cold exposure was similar to that recorded 30 minutes after repeated archwire cooling. The bending force exerted by the Unitek Nitinol SE wire was 35% below baseline after being exposed to cold water for the 10th time. The effect of repetitious archwire cooling was somewhat additive. The bending load increased toward the original level but remained 18% below baseline after a 30-minutes postexposure restitution period.

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Fig 3. Effect of repeated short-term exposures to cold water (10oC) on the torsional stiffness of temperature-sensitive superelastic Ni-Ti .017 × .025 inch wires. Ambient temperature constant at 37oC. A, Typical plot of a superelastic wire with low thermosensitivity; Dentaurum Rematitan SE. B, Typical plot of a superelastic wire with intermediate thermosensitivity; GAC NeoSentalloy F200. C, Typical plot of a superelastic wire with high thermosensitivity; Ormco Copper-Ni-Ti 35oC.

Repeated exposure to cold water lead to a slight decrease in the bending force exerted by the Unitek Nitinol wire. The bending load was 13% below baseline during the last cycle. However, this wire returned to its original bending load level quickly. DISCUSSION

Thermosensitive superelastic nickel-titanium wires exert lower forces for a given deflection at low temperatures.2,7,9 It has been suggested that brief archwire cooling encountered during ingestion of cold food or drinks may reduce the load imparted on the dental structures from an activated wire.4,7 This effect has been believed to be transient.4,7 We have studied the effect of short-term temperature changes on the bending stiffnesses of nickel-titanium-alloy archwires under clinically relevant test conditions. A sample of rectangular so-called superelastic

archwires were subjected to controlled bench-testing at body temperature. The test specimens were subjected to orthodontic bending and the deflection was kept constant while the wire specimens were exposed to a sequence of transient archwire cooling or heating, simulating temperature exposures as would occur during ingestion of cold or hot drinks. The bending force exerted by the activated archwire was measured continually during this sequence. This study shows that brief heating of an activated archwire lead to an increase in the bending force exerted for a given deflection (ie, wire stiffness). For the majority of the wires tested, the effect was small. Furthermore, the baseline bending load level was quickly restored after temperature provocation was ended. As expected, the conventional 3M/Unitek Nitinol archwire was rather temperature-insensitive. In contrast, a single exposure to cold water was sufficient to produce a

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marked reduction (up to 54% below baseline) of the force exerted by the most thermosensitive archwires (Fig. 2C). Furthermore, the baseline bending force level was not completely restored for these wires within the 5-minute postexposure restitution phase. Repetitive cold exposures to the most temperature-sensitive wires lead to a 69% to 90% reduction in exerted force immediately after the last cold exposure (Fig. 3). The wires rapidly regained some of their stiffnesses but exerted from 23% to 43% less than the baseline load level even after 30 minutes of postexposure restitution (Fig. 3C). Brief heating subsequent to archwire cooling restored the baseline bending force level (Fig. 2). Furthermore, when the wire was deactivated and then reactivated, baseline bending load level was also restored (Fig. 2). This indicates that the observed reduction in exerted bending force was not caused by permanent damages inflicted on the test specimens by the transient cooling. Although the partial restitution of wire stiffness after cooling may occur faster intraorally, it may not necessarily occur to a greater extent. A 30-minute postexposure restitution phase at 37oC ambient temperature should presumably be sufficient to reheat the test specimens up to body temperature. Furthermore, a previous study on torque demonstrated that restitution was only partial even after 2 hours for some of the wires tested.3 In a previous investigation, we demonstrated that short-term application of cold water lead to a marked and sustained reduction in the amount of torque exerted by the most temperature-sensitive wires.3 The current study shows that the archwires behave likewise in bending. Furthermore, the general agreement between the findings of this study as compared with the results from testing superelastic wires in longitudinal torsion was very good. Although the test sample was small, the percentage reductions in torque and bending force levels as a result of archwire cooling, as well as the percentage gain in torque and bending force after 30minutes postexposure restitution, were almost identical for the majority of the wires. Thus, the current study serves to confirm our initial rather surprising findings. In patients with compromised periodontal conditions, a thermosensitive superelastic archwire may be exploited as a way of applying intermittent forces. An archwire that is very flexible and exerts low forces at body temperature will transiently become stiffer and may exert orthodontically active forces only when heated during ingestion of hot meals or drinks. However, this study demonstrates that increases in archwire stiffness as a result of heating by hot water exposure is very transient. Whether such very short-acting forces are sufficient to produce efficient tooth movement is at present unknown.

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It has been hypothesized that intermittent reductions in the force exerted by an activated archwire may produce a positive effect on tooth movement by allowing reperfusion and thereby replenishment of cells and nutrients to the periodontal tissues.4 If this hypothesis should be correct, the superelastic wires that are moderately temperature sensitive and rebound completely after transient cooling may be beneficial. The clinical effect of the most temperature-sensitive wires seems more unpredictable. Although it has been assumed that temperature variations brought about by food or drink are transient in nature,4,7 we have demonstrated that short-term application of cold water lead to marked and sustained reductions in the torque3 and bending force exerted by the most temperature-sensitive wires. On the other hand, we have also demonstrated that these wires will rebound to baseline torque3 or bending force levels after ingestion of hot drinks or food. Whether these two opposing effects cancel out during a meal remains uncertain. Furthermore, the baseline torque 3 or bending force level can be restored through a deactivation-reactivation. The effect of mastication and the accompanying archwire movement may have a similar effect and serve to modify the effect of temperature-related changes in archwire stiffness. Thus, the final result is even more unpredictable. CONCLUSIONS

From the foregoing, the following conclusions seem valid: 1. The bending stiffness of the superelastic nickeltitanium wires tested were markedly influenced by a single short-term application of cold (10oC) or hot (80oC) water. The effect of hot water was transient. However, after cooling, the majority of the wires exerted subbaseline bending forces during the 5-minute postexposure restitution phase. In contrast, the conventional nonsuperelastic nickel-titanium wire was marginally affected by cooling and heating and regained its baseline bending force quickly. 2. The most thermosensitive wires also showed a tendency toward incremental reductions in bending stiffnesses when cold water was repeatedly applied. Furthermore, the bending force exerted remained low for a prolonged period of time and showed little or no signs to increase after 30 minutes of postexposure restitution. We acknowledge the material put at our disposal from the various manufacturers. The help of Mr. Robert Sandvik is greatly appreciated.

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REFERENCES 1. Duerig TW, Zadno R. An engineer´s perspective of pseudoelasticity. In: Duerig TW, editor. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990:369-93. 2. Meling T, Ødegaard J. The effect of temperature on the elastic responses to longitudinal torsion of rectangular nickel-titanium archwires. Angle Orthod (In press). 3. Meling T, Ødegaard J. The effect of short-term temperature changes on the mechanical properties of rectangular nickel-titanium archwires tested in torsion. Angle Orthod (In press). 4. Sachdeva RCL, Miyazaki S. Superelastic Ni-Ti alloys in orthodontics. In: Duerig TW, editor. Engineering aspects of shape memory alloys. London: Butterworth-Heinemann, 1990:452-69.

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5. Reitan K. Some factors determining the evaluation of forces in orthodontics. Am J Orthod 1957;43:32-45. 6. Ødegaard J, Meling T, Holte K, Meling E, Segner D. An evaluation of the formulas for bending with respect to their use in estimating force levels in orthodontic appliances. Kieferorthopädische Mitteilungen 1995;9:73-88. 7. Tonner RI, Waters NE. The characteristics of super-elastic Ni-Ti wires in three-point bending. Part I: The effect of temperature. Eur J Orthod 1994;16:409-19. 8. Yoneyama T, Doi H, Hamanaka H, Okamoto Y, Mogi M, Miura F. Super-elasticity and thermal behavior of Ni-Ti alloy orthodontic arch wires. Dent Mater J 1992;11:1-10. 9. Andreasen GF, Heilmann H, Krell D. Stiffness changes in thermodynamic nitinol with increasing temperature. Angle Orthod 1985;55:120-6.

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