Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
JIOS 10.5005/jp-journals-10021-1197 ORIGINAL ARTICLE Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study 1 Ch Sudheer Kumar, 2Shyamala Chandrasekar, 3Sridhar Kodumuru ABSTRACT Aims and objectives: The study was undertaken to evaluate the influence of temperature changes on loading and unloading characteristics of NiTi wires. Materials and methods: NiTi wires included in the study were divided into six groups respectively (group-1 to group-6). Each group consisted of 10 NiTi wires. Three point bending test was done to study the effect of loading and unloading in each group of NiTi wires (Conventional NiTi, HANT and Cu NiTi) using universal testing machine at 37°C and 0°C. Results: Showed that HANT wires exhibited less amount of force during loading and unloading compared to conventional and Cu NiTi at 37°C, whereas, Cu NiTi wires require less force for loading and exhibited gradual recovery during unloading compared to conventional and HANT wires at 0°C. Keywords: NiTi wires, Loading and unloading forces, Universal testing machine. How to cite this article: Kumar CHS, Chandrasekar S, Kodumuru S. Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study. J Ind Orthod Soc 2013;47(4):411-416. INTRODUCTION Orthodontic wire history starts from the use of piano wires, gold wires, stainless steel wires, to the recently introduced transformation thermo NiTi. According to the availability of the arch wire the strategy of the mechanotherapy also keeps on changing. In mechanotherapy the periodic change of wire from the initial phase of treatment to finishing phase is mandatory. In the period prior to seventies when gold and stainless steel were the only available materials, this was affected by altering the cross section of the wire, i.e. from small to large and its geometry, i.e. round to rectangular. Complicated loop designs were required to alter the stiffness characteristics of the wire for a chosen tooth movement. The strategy of wire selection and usage is called variable cross section orthodontics.1 In the mid seventies a host of new arch wire became available, i.e. nitinol and beta-titanium. With the availability 1,3 Reader, 2Professor Department of Orthodontics, Kothiwal Dental College, Moradabad, Uttar Pradesh, India 2 Department of Orthodontics, Saveetha Dental College, Chennai, Tamil Nadu, India 3 Department of Orthodontics, People’s College of Dental Sciences Bhopal, Madhya Pradesh, India 1 Corresponding Author: Ch Sudheer Kumar, Reader, Department of Orthodontics, Kothiwal Dental College, Moradabad, Uttar Pradesh India, Phone: 07500656169, e-mail: drsudheercholleti@ gmail.com Received on: 22/7/13 Accepted after Revision: 1/10/13 of wires with widely varying moduli, it becomes possible for the clinician to select wires with lower moduli for the early stages of treatment and increase the moduli to higher levels toward the finish of the treatment. This approach has been termed by Burstone as variable modulus orthodontics.1 In the nineties, nickel-titanium archwire, that have super elastic and thermodynamic properties were introduced, i.e. Cu NiTi, NeoSentalloy, etc. By taking advantage of the body temperature and setting the alloys transformation temperature (Af) for the martensitic transformation, precise control of the memory phenomenon can be affected. This is called as varying transformation temperature orthodontics. In stress induced phase transformation the austenitic phase of nitinol wires is transformed to martensitic phase during activation. Upon deactivation the reverse occurs. Thermo elastic nitinol is a martensitic active alloy which is influenced by temperature that ultimately exhibit a thermally induced shape memory effect. Transient phase transformation from martensitic to austenite occurs in ambient oral temperature. Experimental prediction of the force exerted by the various wires is necessary to avoid any untoward pressure to the periodontal tissue of the individual patient. Hence, this study was undertaken to evaluate the influence of temperature changes on loading and unloading characteristics of NiTi wires. AIMS AND OBJECTIVES The aims and objectives of this study were: 1. To evaluate the loading and unloading properties of the following different types of round and rectangular nickeltitanium wires, at 37°C. a. 0.016" round conventional NiTi The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416 411 Ch Sudheer Kumar et al b. 0.016" round copper NiTi c. 0.016" round heat activated NiTi d. 16" × 22" rectangular conventional NiTi e. 16" × 22" rectangular copper NiTi f. 16" × 22" rectangular heat activated NiTi. 2. To evaluate the influence of temperature changes on loading (at 0°C) and unloading (at 37°C) properties of the above mentioned group of wires. MATERIALS AND METHODS Wires and their Configurations2 The wires tested included three different commercially available superelastic nickel-titanium orthodontic archwires from different manufacturers most commonly used in the Department of Orthodontics, Saveetha Dental College. The sample wires included in the study were divided in to the following six groups. Groups No. of samples Type of wire Group I 10 Conventional NiTi 0.016" Group II Group III 10 10 Copper NiTi 35°C Heat activated NiTi 0.016" 0.016" Group IV 10 Conventional NiTi 16" × 22" Group V Group VI 10 10 Copper NiTi 35°C 16" × 22" Heat activated NiTi 16" × 22" Fig. 1: Acrylic block Dimensions Manufacturer American orthodontics Ormco Lancer orthodontics American orthodontics Ormco Lancer orthodontics Fig. 2: Hounsfield universal testing machine METHOD Three point bending test was done to study the effect of loading and unloading of 10 specimens from each group of wires at 37°C and 0°C. A total of 120 samples were tested for the same, for the deflection of 1 and 2 mm respectively. Following armamentarium were used to do the three point bending test at 37°C and 0°C. • Specially designed acrylic block (Fig. 1) • Hounsfield universal testing machine (Fig. 2) • A water bath (Fig. 3) • NiTi archwires (Fig. 4) • Twin edgewise brackets (Fig. 5) • Ligature director and other necessary instruments required for bonding (Fig. 6) • Loading of NiTi wire at 37°C (Fig. 7) • Loading of NiTi wire in water bath (Fig. 8) • Operating unit (Fig. 9) • Digital display meter (Fig. 10) • Light cure composite. Fig. 3: Water bath Acrylic Block An acrylic block measuring 60 × 40 × 20 mm was fabricated using self-cure acrylic. A cut was made in the block to a depth 412 Fig. 4: NiTi archwires JIOS Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study Fig. 5: Twin edgewise brackets Fig. 8: Loading of NiTi wire in water bath at 0°C Fig. 6: Armamentarium Fig. 9: Operating unit Fig. 10: Digital display meter Fig. 7: Loading of NiTi wire at 37°C of 10 × 10 mm in the center of the block to allow the deflection of wire samples for the beam test.3 Four 0.018 inch standard medium twin edgewise brackets 3.5 mm wide with 0° torque and angulation were fixed using light cure composite at a distance of 9 and 8 mm.3 From each batch, a 30 mm long piece of wire was cut from the nearly straight, posterior section of the individual archwire blanks. The specimens were tested in orthodontic bending with a hounsfield universal testing machine, fitted with a 500 kg compression load cell calibrated on the 2 kg range with a 2 kg standard weight traceable to the National Bureau of standards. The deflecting rod of the machine was fitted with the common ligature director instrument, to simulate a clinical condition. The testing machine was operated at crosshead speed of 2.0 mm per minute and the deflection was fixed for 2 mm. The jig was placed in the water bath which maintains the water at 37°C and ice cold water at 0°C and testings were conducted for 120 samples to find out loading and unloading at 37°C and 0°C. An industrial thermometer with calibrations from –5°C to 100°C was used to monitor the temperature variations every minute. The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416 413 Ch Sudheer Kumar et al The loading and unloading force values measured in Newtons for all the 120 samples were recorded in the computer and noted down. The readings were tabulated, and subjected to statistical analysis. STATISTICAL ANALYSIS The data values tabulated were subjected to the following statistical analysis: • An analysis of variance. • One-way ANOVA was used to compare the mean values between different study groups. • A multiple range test by Tukey-Honesty significantly different procedure to identify the significant groups at 5% level. Keeping the different wire types constant and varying the temperature, the p-value was calculated. p < 0.05 denoting statistical significance. Mean, standard deviation and test of significance were calculated between different temperatures within each study group. RESULTS Inference Round and rectangular heat activated NiTi wires requires less amount of force (2.36N, 5.68N) to deflect the wire compared to conventional NiTi (2.64, 7.84N) and copper NiTi (2.84, 6.12N) which is statistically significant (Table 1). While unloading, round conventional NiTi (2.00N) and heat activated NiTi (2.00N) has recovered gradually by releasing slow and constant forces compared to copper NiTi (2.08N). During unloading procedure round copper NiTi wire at 37°C return to 0-level suddenly, on the other hand both rectangular conventional NiTi (4.72N) and heat activated NiTi (3.56N) wires reached 0-level instantly, but rectangular copper NiTi (3.28N) wires recovered gradually (Table 2). Round heat activated NiTi wire requires less amount of force (2.52N) to deflect the wire compared to conventional NiTi (3.13N) and copper NiTi (3.36N). Rectangular copper NiTi wire requires less amount of force (7.27N) to deflect the wire compared to conventional NiTi (9.65N) and heat Table 1: Mean, standard deviation and test of significance of mean values between different study groups Temperature Loading at 37°C Unloading at 37°C Groups Mean ± SD p-value Significant groups 5% level I II III IV V VI 2.64 ± 0.21 2.84 ± 0.34 2.36 ± 0.21 7.84 ± 0.77 6.12 ± 0.10 5.68 ± 0.17 0.001 (S) II vs III <0.0001 (S) IV vs V, VI I II III IV V VI 2.00 ± 0.00 2.08 ± 0.10 2.00 ± 0.00 4.72 ± 0.51 3.28 ± 0.62 3.56 ± 0.34 0.007 (S) II vs I, III <0.0001 (S) IV vs V, VI *One-way ANOVA was used to calculate the p-value; Note: Study 1; deflection = 1 mm #Multiple range test by Tukey–HSD procedure was employed to identify the significant groups at 5% level Table 2: Mean, standard deviation and test of significance of mean values between different study groups Temperature Groups Mean ± SD p-value Significant groups 5% level Loading at 37°C I II III IV V VI 3.13 ± 0.57 3.36 ± 0.60 2.52 ± 0.23 9.65 ± 1.96 7.27 ± 1.20 7.45 ± 1.89 <0.0001 (S) II vs III I vs III <0.0001 IV vs V, VI I II III IV V VI 2.19 ± 0.20 2.38 ± 0.33 2.05 ± 0.05 6.32 ± 1.70 4.20 ± 1.14 5.07 ± 1.60 0.0001 (S) II vs I, III 0.0002 IV vs V, VI Unloading at 37°C Note: Study 1; deflection = 2 mm 414 JIOS Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study Table 3: The mean, standard deviation and test of significance of mean values of loading (at 0°) and unloading (at 37°C) of different types of round and rectangular NiTi archwires upto 1 mm deflection Temperature Groups Mean ± SD p-value Loading at 0°C I II III IV V VI I II III IV V VI 2.92 ± 0.29 2.12 ± 0.17 1.20 ± 0.19 5.72 ± 0.94 2.28 ± 0.10 2.96 ± 0.57 2.32 ± 0.29 2.00 ± 0.00 1.00 ± 0.00 4.76 ± 0.08 3.60 ± 0.38 4.84 ± 0.41 <0.0001 (S) I vs II, III II vs III <0.0001 (S) IV vs V, VI <0.0001 (S) I vs II, III II vs III <0.0001 (S) IV vs V VI vs V Unloading at 37°C Significant groups 5% level Note: Study II; deflection = 1 mm activated NiTi (7.45N) wires. While unloading, round heat activated NiTi has recovered gradually by releasing slow and constant forces (2.05N) compared to conventional NiTi (2.19N) and copper NiTi (2.38N). Rectangular copper NiTi recovered gradually (4.20N) compared to conventional NiTi (6.32N) and heat activated NiTi wires (5.07N) (Table 3). That round heat activated NiTi requires less amount of force (1.20N) to deflect the wire compared to conventional NiTi (2.92N) and copper NiTi (2.12N) wires. Rectangular copper NiTi requires less amount of force (2.28N) to deflect the wire compared to conventional NiTi (5.72N) and heat activated NiTi (2.96N) wires. While unloading, round heat activated NiTi (1.00N) wire recovered gradually by releasing slow and constant forces compared to conventional NiTi (2.32N) and copper NiTi (2.00N) wires. Rectangular copper NiTi (3.60N) wire recovered gradually compared to conventional NiTi (4.76N) and heat activated NiTi (4.84N) wires (Table 4). Round heat activated NiTi requires less amount of force (1.40N) to deflect the wire compared to conventional NiTi (3.41N) and copper NiTi (2.30N) wires. Rectangular copper NiTi requires less amount of force (3.06N) to deflect the wire compared to conventional NiTi (7.61N) and heat activated NiTi (3.94N) wires. While unloading, round heat activated NiTi (1.21N) recovered gradually compared to conventional NiTi (2.73N) and copper NiTi (2.17N) wires. Rectangular copper NiTi (4.71N) recovered gradually compared to conventional NiTi (6.48N) and heat activated NiTi (6.29N) wires. DISCUSSION The nickel-titanium wires have the lightest force delivery and widest elastic range, with outstanding spring back, particularly for the shape memory alloys. The physical behavior of the nickel-titanium wires can be interpreted and explained from the metallurgic analysis.5 At the high temperature range the crystal structure of nickeltitanium alloy is an austenitic phase which is body center cubic lattice.4 The martensitic phase which is a closely packed hexagonal lattice is at a low temperature range.6A change in temperature of the wire produces a change in the crystal structure called the martensitic transformation.4-6 Bending stiffness of the nickel-titanium alloy wire is reduced when the wire is subjected to cold temperature which makes it more ductile clinically.8 In this study the bending stiffness was measured after the samples of wires subjected to cold testings at 0°C. It was found that the bending stiffness Table 4: The mean, standard deviation and test of significance of mean values of loading (at 0°C) and unloading (at 37°C) of different types of round and rectangular NiTi archwires upto 2 mm deflection Temperature Groups Mean ± SD p-value Loading at 0°C I II III IV V VI 3.41 ± 0.56 2.30 ± 0.24 1.40 ± 0.25 7.61 ± 2.15 3.06 ± 0.80 3.94 ± 1.14 <0.0001 (S) I vs II, III II vs III <0.0001 (S) IV vs V, VI I II III IV V VI 2.73 ± 0.50 2.17 ± 0.24 1.21 ± 0.22 6.48 ± 1.78 4.71 ± 1.19 6.29 ± 1.52 <0.0001 (S) I vs II, III II vs III 0.0001 (S) IV vs V VI vs V Unloading at 37°C Significant groups 5% level Note: Study II; deflection = 2 mm The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416 415 Ch Sudheer Kumar et al was considerably reduced when compared to the testings at room temperature.7 Elastic recovery of nickel-titanium archwires were superior to that of any other wires. Any permanent distortion of the formed archwire will alter its characteristics and result in lack of tooth movement. Rate of improvement in elastic recovery with permanent deformation becomes very important. The better the elastic recovery of the wire the more difficult it is to deform and it is less ductile. The deterioration in elastic recovery can be circumvented by stress relief annealing, although this eliminates that part of the improvement in elastic properties of formed components which are loaded favorbly.9,10 Julio de A Gurgel11 conducted a study evaluating force deflection behavior of 8 superelastic nickel-titanium orthodontic wires (35°C Cu NiTi, 27°C Cu NiTi, nitinol heat activated, neosent alloy F200, nickel-titanium, rematitan lite, elastinol 35, elastinol 27) under controlled moment and temperature using testing machine. He applied deflections of 0.2 to 2.0 mm at 35°C in the lateral incisor area for copper NiTi 35°C, copper NiTi 27°C, elastinol 35, nickel-titanium, neosentalloy and these wires that exhibited the lowest activation, deactivation forces might be more adaptable clinically in case of mild to moderate crowding. The present study was conducted to evaluate the influence of temperature changes on loading and unloading characteristics of NiTi wires. In the present experiment it was noted that less amount of force was exerted by round and rectangular heat activated NiTi when deflected to 1 mm, but in unloading both conventional and heat activated rectangular NiTi wires recovered faster which could be injurious to periodontal ligaments and painful for the patients. Even though less amount of force was needed for rectangular heat activated NiTi for loading, the behaviour of recovery is not conducive biologically. Though rectangular copper NiTi needed significantly excess force to load but it exhibited a smooth and gradual recovery. A similar observation was reported by Torstein R Meling.7 In the 2 mm activation, in loading, round heat activated NiTi required less force. In unloading, round heat activated NiTi recovered gradually as in 1 mm deflection. Similarly, rectangular copper NiTi recovered gradually. So in the case of minor crowding round and rectangular heat activated NiTi where deflection is minimal and in case of severe crowding where deflection is more round heat activated NiTi and rectangular copper NiTi could be considered. Loading at 0°C for 1 mm of activation round heat activated NiTi and rectangular copper NiTi exerted less amount of force. But in unloading both conventional and heat activated rectangular NiTi wires recovered faster which could induce more force and discomfort for the patients. But rectangular copper NiTi wires while unloading exhibited a smooth and gradual recovery. CONCLUSION The introduction of new alloys into orthodontics offers a new approach in controlling the magnitude of forces used for tooth 416 movement. In this study the basic aim was to explore the effect of temperature variation on bending stiffness of nickeltitanium arch wires without a change in the property of the wires. The result can be summarized as: • Both round and rectangular heat activated NiTi wires required less amount of force for loading. However, though the unloading force was also less, the recovery was faster at 37°C for 1 mm and 2 mm deflections compared to conventional and copper NiTi wires. • At 0°C round and rectangular copper NiTi wires required less amount of force for loading and exhibited gradual recovery during unloading for 1 and 2 mm deflections compared to conventional and heat activated NiTi wires. • Apart from loading and unloading properties, two other factors may be taken into consideration while making choice of wire for initial alignment, namely- cost effectiveness and clinical viability. Copper NiTi is expensive and it needs cooling conditions to 0°C. Taking the above factors into consideration, it is concluded that - Heat activated NiTi wires are preferable in mild to moderate crowding cases, but in periodontally compromised cases copper NiTi wires are preferable taking into account its light and continuous deactivation forces. REFERENCES 1. Burstone CJ. Variable-modulus orthodontics. Am J Orthod 1981;80(7):1-16. 2. Winterbottom JM, Bishara SE, Rim K. Comparison of thermodynamic properties of three nickel-titanium orthodontic arch wires. Angle Orthod 1995;65(2):117-22. 3. Wilkinson PD, Dysart PS, Hood JA, Herbison GP. Load-deflection characteristics of superelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 2002;121(5):483-95. 4. Brantley WA. Structures and properties of orthodontic materials. In: Brantley WA, Eliades T (Eds). Orthodontic materials scientific and clinical aspects. Thieme Stuggart. New York 2001:1-25. 5. Miura F, Mogi M, Okamoto Y. New application of superelastic NiTi rectangular wire. J Clin Orthod 1990;;24;9:544-48. 6. Miura F, Mogi M, Ohura Y, Hamanaka H. Superelastic property of Japanese NiTi. Am J Orthod Dentofacial Orthop 1986;90;1:1-10. 7. Meling TR, Odegaard J. Short-term temperature changes influence the force exerted by superelastic nickel-titanium arch wires activated in orthodontic bending. Am J Orthod Dentofacial Orthop 1998 Nov;114:503-09. 8. Airoldi G, Riva G, Vanelli M, Filippi V, Garattini G. Oral environment temperature changes induced by cold/hot liquid intake. Am J Orthod Dentofacial Orthop 1997;112:58-63. 9. Waters NE, Houston WJ, Stephens CD. The characterization of archwires for the initial alignment of irregular teeth. Am J Orthod 1981;79(4):373-89. 10. Drake SR, Wayne DM, Powers JM, Asgar K. Mechanical properties of orthodontic wires in tension, bending and torsion. Am J Orthod 1982;82;3:206-10. 11. Nakano H, Satoh K, Norris R, Jin T, Kamegai T, Ishikawa F, Katsura H. Force deflection properties of superelastic nickeltitanium arch wires. Am J Orthod Dentofacial Orthop 1999; 115(4):390-95.