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Force-deflection comparison of superelastic nickel-titanium archwires
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Force-deflection comparison of superelastic nickel-titanium archwires Daniel C. Mallory, DDS,a Jeryl D. English, DDS, MS,b John M. Powers, PhD,c William A. Brantley, PhD,d and Harry I. Bussa, DDS, MSe Houston, Tex, and Columbus, Ohio This in vitro study compared the force-deflection behavior of 6 superelastic nickel-titanium orthodontic archwires (0.016 ⫻ 0.022 in) under controlled moment and temperature. To simulate leveling, maxillary canine brackets and first molar tubes were bonded in such a manner as to remove the tip and angulation from the system. The wires (n ⫽ 10) were passively self-ligated into stainless steel brackets attached to an acrylic jig to simulate the maxillary arch. A testing machine recorded deactivations of 3 distances (5, 4, and 3 mm) at 37°C in the canine position. Force-deflection measurements were recorded from the deactivations only. Forces produced during deactivation, at deflections of 2.5, 2.0, and 1.5 mm, were compared by analysis of variance. Significant differences (P ⬍ 0.0001) in forces were observed among the wires at the various deflections. All wires exhibited superelastic behavior, and rankings were derived according to statistically significant differences for each deflection distance. (Am J Orthod Dentofacial Orthop 2004;126:110-2)
O
rthodontic archwires, through their engagement with the bracket, generate biomechanical forces necessary to move the teeth. Orthodontists are constantly searching for the most effective archwire. An ideal archwire should move teeth with light continuous forces, which would reduce the risks of patient discomfort, periodontal ligament necrosis, and undermining resorption.1 It would be a clinical advantage if an archwire could produce this constant delivery of force over a period of weeks or months. Four archwire alloys with desirable properties are currently used to accomplish this objective: stainless steel, cobalt-chromium, nickel-titanium (Ni-Ti), and beta-titanium.2 Superelastic Ni-Ti archwires have been widely accepted for initial alignment of malocclusions mainly because of their unique properties of superelasticity and shape memory.3 An impressive characteristic of supera
Graduate, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. b Professor and chairman, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. c Professor, Department of Restorative Dentistry and Biomaterials; Director, Houston Biomaterials Research Center, University of Texas Health Science Center at Houston Dental Branch. d Professor, Section of Restorative and Prosthetic Dentistry; Director, Graduate Program in Dental Materials, Ohio State University, Columbus. e Associate professor, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. Reprint requests to: Professor Jeryl D. English, UT-Houston Dental Branch, 6516 M. D. Anderson Blvd., Houston, TX 77030-3402; e-mail, Jeryl.D. [email protected]. Submitted, December 2003; revised and accepted, March 2004. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2004.03.012
110
elastic Ni-Ti is its ability to produce light, continuous forces over long ranges of activation.4,5 When this wire begins to recover its original shape, it provides a light, continuous force to the dentition and supporting periodontium without root resorption or necrosis.6,7 Numerous studies have discussed the properties of superelastic Ni-Ti alloy.8-22 The purpose of this study was to investigate the force-deflection characteristics of the martensitic-active or heat-activated superelastic Ni-Ti wires, during large deflections, by using a modified bending test. This study details the comparison of forces achieved during the deactivation of a deflection test that attempts to imitate a clinical situation. MATERIAL AND METHODS
The deflection test used a device fabricated to simulate the movement of a maxillary canine. The jig included an acrylic plate cut to resemble a typical maxillary arch, in which the portion holding the left canine bracket was free to move with the vertical application of forces. To simulate an intraoral clinical situation, maxillary 0.022 ⫻ 0.028-in brackets and first molar tubes (Damon, Ormco, Glendora, Calif) were bonded to the lateral surface of the acrylic plate. This was accomplished while the brackets were engaged to a full-size archwire to remove any angulations or torque from the system. The attachments were bonded with an interbracket distance of 5 mm for each segment. This distance was chosen as an average clinical interbracket distance. Individual 0.016 ⫻ 0.022-in archwire specimens were inserted into the system and engaged by using the passive cover provided by the
American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 1
Mallory et al 111
Table. Codes and manufacturers of 0.016 ⫻ 0.022 in superelastic wires tested Code
Wire
Manufacturer
C27 C35 C40 NH NM NL
27°C copper Ni-Ti 35°C copper Ni-Ti 40°C copper Ni-Ti Neosentalloy (240 g) Neosentalloy (160 g) Neosentalloy (80 g)
Ormco, Glendora, Calif Ormco, Glendora, Calif Ormco, Glendora, Calif GAC, Islip, NY GAC, Islip, NY GAC, Islip, NY
appliance. The 6 wires tested were C27, C35, C 40 (Ormco), NH, NM, and NL (GAC, Islip, NY). After each archwire specimen was inserted into the system, the canine region of the specimen was chilled for 3 seconds with a frozen cotton tip applicator and activated (engaged) to the designated distance (5, 4, or 3 mm). Water heated to 37°C was then added to the system until the specimen was entirely submerged. The deactivations were measured and recorded by the testing machine (Instron, Canton, Mass) with a 5-kg load cell and a crosshead speed of 1.0 mm/min. The deactivation from the 3 different distances for each of the 6 heat-activated superelastic Ni-Ti archwires (Table) was repeated 10 times. Means and standard deviations (n ⫽ 10) of the forces generated during deactivation at deflections of 2.5, 2.0, and 1.5 mm were calculated. The data were analyzed by analysis of variance (ANOVA) (StatView, SAS Institute, Cary, NC). Means were compared by using Fisher’s protected least-significant difference interval, calculated at the 0.05 level of significance. RESULTS
Means and standard deviations of the deactivation forces at deflections of 2.5, 2.0, and 1.5 mm were recorded. These deactivation deflections were selected to statistically compare the activations of 5, 4, and 3 mm of each archwire. The ANOVA of the deactivation forces showed significant differences among the wires (Fig). Wires C40 and NL were similar in that they had the lowest deactivation forces. The differences in activation distances were observed to be as expected: 5 mm ⬍ 4 mm ⬍ 3 mm in most instances. NM varied from the expected reading, with the 4-mm activation significantly less than the 5-mm activation at a distance of 1.5 mm. DISCUSSION
Due to their unique engagement capability, high resiliency, and production of continuous forces, the use of heat-activated superelastic Ni-Ti archwires has become
Fig. Force-deflection graph for all wires at 5-mm activation.
widely accepted in orthodontics. Selection of these archwires should be based on their ability to produce a constant force over differing amounts of deflection. Some of these wires have been designed to reach different amounts of force during their superelastic plateau. For this reason, the clinician should determine the optimum force for the type of tooth movement desired. To quantify clinically relevant data, the test design should approximate as closely as possible a clinical situation. Variations in forces have been reported for these types of wires depending on the test model or the system design. The testing conditions should be identical to compare data from different studies. Differences such as ligation techniques and dry or wet heat sources are common variables.14,15,19 This study used a passive bracket system to limit the influence of friction on the deflection study.21 This allowed the testing machine to more accurately record the forces produced by the archwires. Water, controlled with a thermostat rather than an oven, was used to heat the specimens to 37°C to more closely approximate the clinical environment.15,23,24 Despite the best efforts to replicate a clinical situation, each study can present unique circumstances. Therefore, it has been suggested that more emphasis should be placed on the rank order of the forces produced by the wires rather than on the actual value in grams shown in the data tables.19,25 An interesting phenomenon occurred during the deactivation of the 5-mm deflections for all wires tested (Fig). During the first 0.5 mm of deactivation, there was a sharp decline in force production. C40 actually produced a negative value or hindered movement of the canine from 4.5 to 4.0 mm. The combination of a severe deflection and the lack of enough heat to transform the wire back to a completely austenitic phase could explain why this wire did not produce a
112 Mallory et al
positive force. After 1.0 mm of deactivation, all wires began to “recover” and produce increasing amounts of force until the 1.5-mm deflection point was reached. This explains how the forces for NM during the 5-mm activation became significantly higher than the 4-mm activation at 1.5-mm deflection. Filleul and Jordan23 reported not only that forces generated depended on the severity of the deformation, but also that the crystalline structure and the temperature transition range (TTR) were equally important. Another study found that the TTR could be shifted to a higher temperature in an alloy that forms stress-induced martensite during its activation. This would require a higher temperature than expected to transform the stress-induced martensite back to its austenite form. This also means that the TTR values given by the manufacturers might not be accurate in certain clinical situations.6 It has been suggested that the stress-related TTR should be evaluated for the wires so that a more accurate clinical selection can be made.6,26 Ideally, the crystalline structure of the alloys and the amounts of phase transformation should be confirmed by either radiographic diffraction or differential scanning calorimetry.8,9,19 CONCLUSIONS
This laboratory study compared the force-deflection performance of 6 commercially available 0.016 ⫻ 0.022-in heat-activated superelastic Ni-Ti archwires. The wires were tested with a device that simulated the oral environment: a maxillary arch with a bonded edgewise appliance. The results indicated that the wires produce various amounts of force at different activation distances. These were rank ordered by significant differences. C40 and NL produced the least amount of force in all 3 activation categories. The wire rankings showed that there was a significant decrease in force for subsequent increases in activation distances. REFERENCES 1. Proffit WR. Contemporary orthodontics. 3rd ed. St. Louis: Mosby; 2000, p. 304. 2. Kusy RP. A review of contemporary archwires: their properties and characteristics. Angle Orthod 1997;67:197-208. 3. Miura F, Mogi M, Ohura Y, Hamanaka H. The super-elastic property of the Japanese NiTi alloy wire for the use in orthodontics. Am J Orthod Dentofacial Orthop 1986;90:1-10. 4. Chen R, Zhi YF, Arvystas MG. Advanced Chinese NiTi alloy wire and clinical observations. Angle Orthod 1992;62:59-66. 5. Miura F, Mogi M, Ohura Y, Karibe M. The super-elastic Japenese NiTi alloy wire for use in orthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J Orthod Dentofacial Orthop 1988;94:89-96. 6. Santoro M, Beshers DN. Nickel-titanium alloys: stress-related temperature transitional range. Am J Orthod Dentofacial Orthop 2000;118:685-92.
American Journal of Orthodontics and Dentofacial Orthopedics July 2004
7. Kusy RP. The future of orthodontic materials: the long-term view. Am J Orthod Dentofacial Orthop 1998;113:91-5. 8. Brantley WA, Eliades T. Orthodontic materials. Stuttgart, New York: Thieme; 2001, p. 78 –103. 9. Bradley TG, Brantley WA, Culbertson BM. Differential scanning calorimetry (DSC) analyses of superelastic and nonsuperelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1996;109:589-97. 10. Hurst CL, Duncanson MG Jr, Nanda RS, Angolkar PV. An evaluation of the shape-memory phenomenon of nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1990;98: 72-6. 11. Barwart O. The effect of temperature change on the load value of Japanese NiTi coil springs in the superelastic range. Am J Orthod Dentofacial Orthop 1996;110:553-8. 12. Meling TR, Odegaard J. The effect of short-term temperature changes on superelastic nickel-titanium archwires activated in orthodontic bending. Am J Orthod Dentofacial Orthop 2001;119: 263-73. 13. Burstone CJ, Qin B, Morton JY. Chinese NiTi wire- a new orthodontic alloy. Am J Orthod Dentofacial Orthop 1985;87: 445-52. 14. Gurgel JA, Kerr S, Powers JM, LeCrone V. Force-deflection properties of superelastic nickel-titanium archwires. Am J Orthod Dentofacial Orthop 2001;120:378-82. 15. Nakano H, Satoh K, Norris R, Jin T, Kaemgai T, Ishikawa F, et al. Mechanical properties of several nickel-titanium alloy wires in three-point bending tests. Am J Orthod Dentofacial Orthop 1999;115:390-5. 16. Oltjen JM, Duncanson MG, Ghosh J, Nanda RS, Currier GF. Stiffness-deflection behavior of selected orthodontic wires. Angle Orthod 1997;67:209-18. 17. Tonner RIM, Waters NE. The characteristics of super-elastic NiTi wires in the three-point bending. Part 1: the effect of temperature. Eur J Orthod 1994;16:409-19. 18. Tonner RIM, Waters NE. The characteristics of super-elastic NiTi wires in the three-point bending. Part 2: Intra-batch variation. Eur J Orthod 1994;16:421-5. 19. Wilkinson PD, Dysart PS, Hood JA, Herbison GP. Loaddeflection characteristics of superelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 2002;121:483-95. 20. Kapila S, Angolkar PV, Duncanson MG, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofacial Orthop 1990;9811726. 21. Pizzoni L, Ravnholt G, Melson B. Frictional forces related to self-ligating brackets. Eur J Orthod 1998;20:238-91. 22. Meling TR, Odegaard J. The effect of temperature on the elastic responses to longitudinal torsion of rectangular nickel titanium archwires. Angle Orthod 1998;68:357-68. 23. Filleul MP, Jordan L. Torsional properties of Ni-Ti and copper Ni-Ti wires: the effect of temperature on physical properties. Eur J Orthod 1997;10:637-46. 24. Mullins WS, Bagby MD, Norman TL. Mechanical behavior of thermo-responsive orthodontic archwires. Dent Mat 1996;12: 308-14. 25. Dowling PA, Jones WB, Lagerstrom L, Sandham JA. An investigation into the behavioural characteristics of orthodontic elastomeric modules. Br J Orthod 1998;25:197-202. 26. Santoro M, Nicolay OF, Cangialosi TJ. Pseudoelasticity and thermoelasticity of nickel-titanium alloys: a clinically orientated review. Part I: temperature transitional ranges. Am J Orthod Dentofacial Orthop 2001;119:587-93.
Force-deflection comparison of superelastic nickel-titanium archwires Daniel C. Mallory, DDS,a Jeryl D. English, DDS, MS,b John M. Powers, PhD,c William A. Brantley, PhD,d and Harry I. Bussa, DDS, MSe Houston, Tex, and Columbus, Ohio This in vitro study compared the force-deflection behavior of 6 superelastic nickel-titanium orthodontic archwires (0.016 ⫻ 0.022 in) under controlled moment and temperature. To simulate leveling, maxillary canine brackets and first molar tubes were bonded in such a manner as to remove the tip and angulation from the system. The wires (n ⫽ 10) were passively self-ligated into stainless steel brackets attached to an acrylic jig to simulate the maxillary arch. A testing machine recorded deactivations of 3 distances (5, 4, and 3 mm) at 37°C in the canine position. Force-deflection measurements were recorded from the deactivations only. Forces produced during deactivation, at deflections of 2.5, 2.0, and 1.5 mm, were compared by analysis of variance. Significant differences (P ⬍ 0.0001) in forces were observed among the wires at the various deflections. All wires exhibited superelastic behavior, and rankings were derived according to statistically significant differences for each deflection distance. (Am J Orthod Dentofacial Orthop 2004;126:110-2)
O
rthodontic archwires, through their engagement with the bracket, generate biomechanical forces necessary to move the teeth. Orthodontists are constantly searching for the most effective archwire. An ideal archwire should move teeth with light continuous forces, which would reduce the risks of patient discomfort, periodontal ligament necrosis, and undermining resorption.1 It would be a clinical advantage if an archwire could produce this constant delivery of force over a period of weeks or months. Four archwire alloys with desirable properties are currently used to accomplish this objective: stainless steel, cobalt-chromium, nickel-titanium (Ni-Ti), and beta-titanium.2 Superelastic Ni-Ti archwires have been widely accepted for initial alignment of malocclusions mainly because of their unique properties of superelasticity and shape memory.3 An impressive characteristic of supera
Graduate, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. b Professor and chairman, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. c Professor, Department of Restorative Dentistry and Biomaterials; Director, Houston Biomaterials Research Center, University of Texas Health Science Center at Houston Dental Branch. d Professor, Section of Restorative and Prosthetic Dentistry; Director, Graduate Program in Dental Materials, Ohio State University, Columbus. e Associate professor, Department of Orthodontics, University of Texas Health Science Center at Houston Dental Branch. Reprint requests to: Professor Jeryl D. English, UT-Houston Dental Branch, 6516 M. D. Anderson Blvd., Houston, TX 77030-3402; e-mail, Jeryl.D. [email protected]. Submitted, December 2003; revised and accepted, March 2004. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2004.03.012
110
elastic Ni-Ti is its ability to produce light, continuous forces over long ranges of activation.4,5 When this wire begins to recover its original shape, it provides a light, continuous force to the dentition and supporting periodontium without root resorption or necrosis.6,7 Numerous studies have discussed the properties of superelastic Ni-Ti alloy.8-22 The purpose of this study was to investigate the force-deflection characteristics of the martensitic-active or heat-activated superelastic Ni-Ti wires, during large deflections, by using a modified bending test. This study details the comparison of forces achieved during the deactivation of a deflection test that attempts to imitate a clinical situation. MATERIAL AND METHODS
The deflection test used a device fabricated to simulate the movement of a maxillary canine. The jig included an acrylic plate cut to resemble a typical maxillary arch, in which the portion holding the left canine bracket was free to move with the vertical application of forces. To simulate an intraoral clinical situation, maxillary 0.022 ⫻ 0.028-in brackets and first molar tubes (Damon, Ormco, Glendora, Calif) were bonded to the lateral surface of the acrylic plate. This was accomplished while the brackets were engaged to a full-size archwire to remove any angulations or torque from the system. The attachments were bonded with an interbracket distance of 5 mm for each segment. This distance was chosen as an average clinical interbracket distance. Individual 0.016 ⫻ 0.022-in archwire specimens were inserted into the system and engaged by using the passive cover provided by the
American Journal of Orthodontics and Dentofacial Orthopedics Volume 126, Number 1
Mallory et al 111
Table. Codes and manufacturers of 0.016 ⫻ 0.022 in superelastic wires tested Code
Wire
Manufacturer
C27 C35 C40 NH NM NL
27°C copper Ni-Ti 35°C copper Ni-Ti 40°C copper Ni-Ti Neosentalloy (240 g) Neosentalloy (160 g) Neosentalloy (80 g)
Ormco, Glendora, Calif Ormco, Glendora, Calif Ormco, Glendora, Calif GAC, Islip, NY GAC, Islip, NY GAC, Islip, NY
appliance. The 6 wires tested were C27, C35, C 40 (Ormco), NH, NM, and NL (GAC, Islip, NY). After each archwire specimen was inserted into the system, the canine region of the specimen was chilled for 3 seconds with a frozen cotton tip applicator and activated (engaged) to the designated distance (5, 4, or 3 mm). Water heated to 37°C was then added to the system until the specimen was entirely submerged. The deactivations were measured and recorded by the testing machine (Instron, Canton, Mass) with a 5-kg load cell and a crosshead speed of 1.0 mm/min. The deactivation from the 3 different distances for each of the 6 heat-activated superelastic Ni-Ti archwires (Table) was repeated 10 times. Means and standard deviations (n ⫽ 10) of the forces generated during deactivation at deflections of 2.5, 2.0, and 1.5 mm were calculated. The data were analyzed by analysis of variance (ANOVA) (StatView, SAS Institute, Cary, NC). Means were compared by using Fisher’s protected least-significant difference interval, calculated at the 0.05 level of significance. RESULTS
Means and standard deviations of the deactivation forces at deflections of 2.5, 2.0, and 1.5 mm were recorded. These deactivation deflections were selected to statistically compare the activations of 5, 4, and 3 mm of each archwire. The ANOVA of the deactivation forces showed significant differences among the wires (Fig). Wires C40 and NL were similar in that they had the lowest deactivation forces. The differences in activation distances were observed to be as expected: 5 mm ⬍ 4 mm ⬍ 3 mm in most instances. NM varied from the expected reading, with the 4-mm activation significantly less than the 5-mm activation at a distance of 1.5 mm. DISCUSSION
Due to their unique engagement capability, high resiliency, and production of continuous forces, the use of heat-activated superelastic Ni-Ti archwires has become
Fig. Force-deflection graph for all wires at 5-mm activation.
widely accepted in orthodontics. Selection of these archwires should be based on their ability to produce a constant force over differing amounts of deflection. Some of these wires have been designed to reach different amounts of force during their superelastic plateau. For this reason, the clinician should determine the optimum force for the type of tooth movement desired. To quantify clinically relevant data, the test design should approximate as closely as possible a clinical situation. Variations in forces have been reported for these types of wires depending on the test model or the system design. The testing conditions should be identical to compare data from different studies. Differences such as ligation techniques and dry or wet heat sources are common variables.14,15,19 This study used a passive bracket system to limit the influence of friction on the deflection study.21 This allowed the testing machine to more accurately record the forces produced by the archwires. Water, controlled with a thermostat rather than an oven, was used to heat the specimens to 37°C to more closely approximate the clinical environment.15,23,24 Despite the best efforts to replicate a clinical situation, each study can present unique circumstances. Therefore, it has been suggested that more emphasis should be placed on the rank order of the forces produced by the wires rather than on the actual value in grams shown in the data tables.19,25 An interesting phenomenon occurred during the deactivation of the 5-mm deflections for all wires tested (Fig). During the first 0.5 mm of deactivation, there was a sharp decline in force production. C40 actually produced a negative value or hindered movement of the canine from 4.5 to 4.0 mm. The combination of a severe deflection and the lack of enough heat to transform the wire back to a completely austenitic phase could explain why this wire did not produce a
112 Mallory et al
positive force. After 1.0 mm of deactivation, all wires began to “recover” and produce increasing amounts of force until the 1.5-mm deflection point was reached. This explains how the forces for NM during the 5-mm activation became significantly higher than the 4-mm activation at 1.5-mm deflection. Filleul and Jordan23 reported not only that forces generated depended on the severity of the deformation, but also that the crystalline structure and the temperature transition range (TTR) were equally important. Another study found that the TTR could be shifted to a higher temperature in an alloy that forms stress-induced martensite during its activation. This would require a higher temperature than expected to transform the stress-induced martensite back to its austenite form. This also means that the TTR values given by the manufacturers might not be accurate in certain clinical situations.6 It has been suggested that the stress-related TTR should be evaluated for the wires so that a more accurate clinical selection can be made.6,26 Ideally, the crystalline structure of the alloys and the amounts of phase transformation should be confirmed by either radiographic diffraction or differential scanning calorimetry.8,9,19 CONCLUSIONS
This laboratory study compared the force-deflection performance of 6 commercially available 0.016 ⫻ 0.022-in heat-activated superelastic Ni-Ti archwires. The wires were tested with a device that simulated the oral environment: a maxillary arch with a bonded edgewise appliance. The results indicated that the wires produce various amounts of force at different activation distances. These were rank ordered by significant differences. C40 and NL produced the least amount of force in all 3 activation categories. The wire rankings showed that there was a significant decrease in force for subsequent increases in activation distances. REFERENCES 1. Proffit WR. Contemporary orthodontics. 3rd ed. St. Louis: Mosby; 2000, p. 304. 2. Kusy RP. A review of contemporary archwires: their properties and characteristics. Angle Orthod 1997;67:197-208. 3. Miura F, Mogi M, Ohura Y, Hamanaka H. The super-elastic property of the Japanese NiTi alloy wire for the use in orthodontics. Am J Orthod Dentofacial Orthop 1986;90:1-10. 4. Chen R, Zhi YF, Arvystas MG. Advanced Chinese NiTi alloy wire and clinical observations. Angle Orthod 1992;62:59-66. 5. Miura F, Mogi M, Ohura Y, Karibe M. The super-elastic Japenese NiTi alloy wire for use in orthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J Orthod Dentofacial Orthop 1988;94:89-96. 6. Santoro M, Beshers DN. Nickel-titanium alloys: stress-related temperature transitional range. Am J Orthod Dentofacial Orthop 2000;118:685-92.
American Journal of Orthodontics and Dentofacial Orthopedics July 2004
7. Kusy RP. The future of orthodontic materials: the long-term view. Am J Orthod Dentofacial Orthop 1998;113:91-5. 8. Brantley WA, Eliades T. Orthodontic materials. Stuttgart, New York: Thieme; 2001, p. 78 –103. 9. Bradley TG, Brantley WA, Culbertson BM. Differential scanning calorimetry (DSC) analyses of superelastic and nonsuperelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1996;109:589-97. 10. Hurst CL, Duncanson MG Jr, Nanda RS, Angolkar PV. An evaluation of the shape-memory phenomenon of nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1990;98: 72-6. 11. Barwart O. The effect of temperature change on the load value of Japanese NiTi coil springs in the superelastic range. Am J Orthod Dentofacial Orthop 1996;110:553-8. 12. Meling TR, Odegaard J. The effect of short-term temperature changes on superelastic nickel-titanium archwires activated in orthodontic bending. Am J Orthod Dentofacial Orthop 2001;119: 263-73. 13. Burstone CJ, Qin B, Morton JY. Chinese NiTi wire- a new orthodontic alloy. Am J Orthod Dentofacial Orthop 1985;87: 445-52. 14. Gurgel JA, Kerr S, Powers JM, LeCrone V. Force-deflection properties of superelastic nickel-titanium archwires. Am J Orthod Dentofacial Orthop 2001;120:378-82. 15. Nakano H, Satoh K, Norris R, Jin T, Kaemgai T, Ishikawa F, et al. Mechanical properties of several nickel-titanium alloy wires in three-point bending tests. Am J Orthod Dentofacial Orthop 1999;115:390-5. 16. Oltjen JM, Duncanson MG, Ghosh J, Nanda RS, Currier GF. Stiffness-deflection behavior of selected orthodontic wires. Angle Orthod 1997;67:209-18. 17. Tonner RIM, Waters NE. The characteristics of super-elastic NiTi wires in the three-point bending. Part 1: the effect of temperature. Eur J Orthod 1994;16:409-19. 18. Tonner RIM, Waters NE. The characteristics of super-elastic NiTi wires in the three-point bending. Part 2: Intra-batch variation. Eur J Orthod 1994;16:421-5. 19. Wilkinson PD, Dysart PS, Hood JA, Herbison GP. Loaddeflection characteristics of superelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 2002;121:483-95. 20. Kapila S, Angolkar PV, Duncanson MG, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofacial Orthop 1990;9811726. 21. Pizzoni L, Ravnholt G, Melson B. Frictional forces related to self-ligating brackets. Eur J Orthod 1998;20:238-91. 22. Meling TR, Odegaard J. The effect of temperature on the elastic responses to longitudinal torsion of rectangular nickel titanium archwires. Angle Orthod 1998;68:357-68. 23. Filleul MP, Jordan L. Torsional properties of Ni-Ti and copper Ni-Ti wires: the effect of temperature on physical properties. Eur J Orthod 1997;10:637-46. 24. Mullins WS, Bagby MD, Norman TL. Mechanical behavior of thermo-responsive orthodontic archwires. Dent Mat 1996;12: 308-14. 25. Dowling PA, Jones WB, Lagerstrom L, Sandham JA. An investigation into the behavioural characteristics of orthodontic elastomeric modules. Br J Orthod 1998;25:197-202. 26. Santoro M, Nicolay OF, Cangialosi TJ. Pseudoelasticity and thermoelasticity of nickel-titanium alloys: a clinically orientated review. Part I: temperature transitional ranges. Am J Orthod Dentofacial Orthop 2001;119:587-93.