Riadh F.doc J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry159 In-Vitro evaluation of load-deflection characteristics and force levels of nickel titanium orthodontic archwires Riyadh Abdu Al-Hamza Ruwiaee, B.D.S. (1) Akram Faisal Al-Huwaizi, B.D.S., M.Sc., Ph.D. (2) ABSTRACT Background: Nickel-titanium (NiTi) archwires have become increasingly popular because of their ability to release constant light forces, which are especially useful during initial alignment and leveling phase. The aim of the present study was to investigate and compare the load–deflection characteristics of four commercially available NiTi archwires. Materials and methods: 200 NiTi 0.014, 0.016, 0.018, 0.016x0.022 and 0.019x0.025-inch nickel–titanium archwires from four different manufacturers (3M, Ortho Technology, Jiscop and Astar) were tested. The load-deflection properties of these archwires were evaluated by a full arch bending test in both palatal and gingival directions at 37°C temperature using a universal material testing machine. Forces generated at maximum loading of 2mm and at unloading of 1.5mm were measured. Results: All the tested NiTi wires showed an increase in loading and unloading force with increased wire dimension. Generally, 3M gave the most flexible round wires and relatively stiff rectangular wires; with linear load deflection curves. Ortho Technology wires were flexible. Jiscop gave the stiffest round wires and the most flexible rectangular wires. Astar wires were stiff which gave the highest force levels during unloading. Conclusion: Force levels vary greatly from brand to brand and so NiTi wire brands must be selected with consideration to their load-deflection characteristics and mechanical properties. Key words: Load-deflection; force level; nickel titanium archwires. (J Bagh Coll Dentistry 2015; 27(3):159-164). INTRODUCTION Dental arch alignment and leveling is the initial stage of orthodontic treatment. Satisfactorycompletion of this first stage is essential if esthetic; function and stability are to be achieved (1). A well-planned orthodontic treatment starts with very flexibleand superelastic wires fully engaged into the bracket on each arch. Usually, the ideal archwire for that initial first stag generates a light and continuous force over a long period of time (2). Super-elastic nickel-titanium (Ni-Ti) alloy wires with low stiffness and high superelasticity aregenerally used in the leveling and alignmentstages of orthodontic treatment for efficient toothmovement and a desirable biological response (3). These austenitic-active Ni- Ti alloys are predominantly in theaustenitic phase at room temperature. Nickel-titanium (NiTi) alloys have been widely usedin orthodontics because of their favorable mechanicalproperties, a remarkable feature of which is their super-elasticity (4). Super- elasticity is the transformation from austenitic to martensitic that occurs by stress application within a temperature range and is manifested by a flat or nearly flat plateau in a force-deflection curve (5). (1)Master student. Department of Orthodontics. College of Dentistry, University of Baghdad. (2)Professor. Department of Orthodontics. College of Dentistry, University of Baghdad. The transition between the two phases is termedmartensitic transformation, and it is responsible for thememory effect. This transformation is the result of changes in the crystal lattice of the material. Shape-memory property is the plastic deformationof NiTi wires from the martensite phase to an austenitecrystal structure (6). Most of the information about the behavior of these wires is based on mechanical laboratory testing without simulating the many variables encountered in clinical situations (7). The most appropriate wire tests those that reproduce conditions encountered clinically, with the wire constrained as part of a fixed appliance (8). Variations in model design have been shown to affect unloading deflection plots (9). Recent studies reveal all commercial wires do not necessarily behave in the same manner. Minor differences in the production process contribute to the variation in the behavior of these wires (10). This investigation details a comparison of forces achieved in different commercial NiTi superelastic wires in a deflection test of activation and deactivation that attempts to approximate clinical conditions (11). Full arch (palatal and gingival deflection) tests for four different brands of Ni-Ti alloy wires are made under the same testing conditions to clarify their load-deflection properties. MATERIAL AND METHODS Five gauges of NiTi wires (0.014, 0.016, 0.018, 0.016x0.022 and 0.019x0.025 inch) were J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry160 tested to compare their mechanical properties. The sample comprised of wires from four brands 3M Unitek (Monrovia, USA), Ortho Technology (Tampa, Florida, USA), Jiscop (Dangieahg-Dong, gunpo-si, Kyeanggi-do, Korea) and Astar (Shanghai, China). Preformed archwire were tested with phantom head jaw (Shanghai, China) in palatal and gingival deflectionswith greater stability and positional accuracy.The teeth of a plastic phantom head jaw were fitted with Roth prescription 0.022×0.028 inch slot passive self-ligating brackets and buccal tubes(Ortho Technology, Tampa, Florida, USA).Secure attachment was achieved forboth by bonding the base of them to the crown. Accurate slot alignment was achieved by using a plain 0.021x0.025 stainless steel arch wire as a former while the bonding was light cured The load site simulated a misaligned upper right canine with 15mm between the midpoints of the brackets. This interbracket distance was derived from typical tooth dimensions (8).The bending test was carried out with Universal Material Tester by deflecting the wire at the midpoint.Each bending test was done 10 times, with a new piece of wire for each repetition.All tests were carried out in a water bath at temperature 37°C ±0.5°C with digital thermometer control (Fig.1). Load at maximum deflection of 2mm was registered as a measure of flexibility.Load during unloading phase at 1.5mm deflection was registered as a measure of elasticity (Fig.2). (12). Figure 1: A test in progress on the phantom head jaw in a. Palatal b. Gingival deflection test. Figure 2. Typical X-Y plot of load deflection curve for NiTi wire at 2 mm load-deflection test Whereas UDP, unloading deflection point at 1.5mm. RESULTS Most, but not all, load-deflection graphs of both palatal and gingival tested NiTi wires confirmed features of superelasticity, with plateau regions varying in gradient and load value depending on the testing direction, wire dimension and wire brands (Fig.3). After reach the maximum force at 2mm deflection, the unloading plot for all bending tests typically dropped very rapidly followed by a plateau region during which a relatively constant force was produced. In this superelastic range, the load curves for loading and unloading were consistent with the definition of hysteresis (13). J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry161 The results of the ANOVA and LSD show that the forces generated by the four brands of the five NiTi wire gauges at loading and unloading showed highly significant difference at the p<0.001 level. Figures 4 and 5 showed the force at maximum loading of 2mm and unloading at 1.5 mm deflection using both palatal and gingival deflection tests for the five NiTi wires gauges from four brands. From these figures the following can be noted: 1. All the tested NiTi wires showed an increase in loading and unloading force levels with increase of wire dimension. The differences of force level were small in round cross section wires, but were noticeably large in rectangular cross section wires 2. In general, for all round (0.014, 0.016 and 0.018 inch) wires, both Astar and Jiscop displayed high loading and unloading forces while 3M gave the lowest forces. Whereas for both rectangular (0.016x0.22 and 0.019x0.025-inch) wires, Astar and 3M displayed high loading and unloading forces while Jiscop gave the lowest forces. Ortho Technology wire’s force levels were intermediate mostly in both tests. DISCUSSION The factors that determine the mechanical properties of Ni-Ti alloy wires include composition, heat treatment, and degree of working. Concerning the composition ratio of nickel and titanium, most manufacturers are cautious about releasing such information, as it is regarded as a trade secret (14). This study agreed with Nakano et al. (15) who observed great variations in force values with different NiTi wires of the same diameter, indicating that the wires are intrinsically different and therefore should be differentiated according to their characteristics. Loading curve represents the force required to insert the wire in the bracket on the crowded teeth, therefore, the force is usually measured at the last deflection of loading curve (maximum force level). The wires with highest maximum force were stiffer, while the wires with lowest force were flexible (16). The differences of forces may be due to that the martensitic transformation (SIM) occurred earlier for the lowest force wires than for the highest force wires (14). For round wires, 3M Ni-Ti wires exerted the least maximum loading force which agrees with the findings of Gatto et al. (10) who also found their load-deflection curves to be narrow and steep at 2mm deflection but were wider with larger plateau at 4mm deflection. This means that at 2mm these wires did not express their superelasticity as greater deformation generate the martensitic transformation induced by this stress (SIM). On the other hand, 3M 0.019x0.025 inch wires showed the highest maximum loading force which may be due to that some austenitic NiTi wires exhibit stiffness higher than that of TMA wires, if the deformation does not reach that of the proportional limit (17). The unloading curve represents the force delivered to teeth during treatment and usually is measured in several deflection points. However, the different brands of Ni-Ti alloy wires tested varied widely in the force levels they exerted. The level of susceptibility of the periodontium is one of the essential factors for determining the effective and safe value of the force which should not be exceeded when applied to a single tooth (18). An ideal archwire should be able to deliver differential forces to the arch segments. The force should range from about 70g to 80g in the incisor area and gradually increase toward the posterior segments, up to 300g. (19) An optimal performance of austenitic NiTi wires will be obtained in cases of severe dental crowding, when an accentuated deflection due to the irregular interbracket span will generate SIM in a localized area of the arch, usually the lower incisor area. Mild crowding does not necessarily require the use of superelastic wires, and a small diameter alloy such as 3M wire will generally perform as well (20). Our study agreed with the study of Sarul et al. (18) during testing the mechanical properties of the NiTi wires of various diameters, they found that some round section wires release forces which fall within the range of optimal forces. That makes them more clinically useful. Some rectangular wires as with 0.019x0.025 inch Jiscop wires, the loading force were relatively high but, after 1.5mm unloading the force were the lowest in range of 884g to 643g for both tests. This could be explained by Garrec and Jordan (21) who stated that the value of stiffness appears to vary with wire size but depends on the ratio of volume of martensitic transformation i.e. a large-size rectangular wire does not produce necessarily high forces during unloading. So, in this study, the archwires can be classified according to their flexibility (from highest to lowest) into 3M, Ortho Technology, A- star and Jiscop wires for both round and rectangular wires. J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry162 As conclusions; 1. All the tested NiTi wires showed an increase in loading and unloading force with increase of wire dimension. 2. In general, for round wires, Astar and Jiscop displayed high loading and unloading forces while 3M gave the lowest forces. Whereas for rectangular wires, Astar and 3M displayed high loading and unloading forces while Jiscop gave the lowest forces. Ortho Technology wire’s force levels were intermediate. 3. Wires can be classified (from highest to lowest) according to their flexibility as 3M, Ortho Technology, Astar and Jiscop. Palatal deflection tests Gingival deflection tests 0. 01 4 in ch 0. 01 6 in ch 0. 01 8 in ch 0. 01 6x 0. 02 2 in ch J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry163 0. 01 9x 0. 02 5 in ch Figure 3: Load deflection curves for the 0.014, 0.016, 0.018, 0.016x0.022 and 0.019x0.025 inch wires from four brands using bothpalatal and gingival deflections. Figure 4: Maximumloading Forces at 2mm deflection for the five NiTi wires gauges from four brands using both palatal and gingival deflection tests. Figure 5: Unloading forces at 1.5mm deflection for the five NiTi wires gauges from four brands using both palatal and gingival deflection tests. REFERENCES 1. Khier SE, Brantley WA, Fournelle RA. Bending properties of superelastic and nonsuperelastic nickel- titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1991; 99: 310- 8. 2. Miura F, Mogi M, Ohura Y, Hamanaka H. The superelastic property of the Japanese NiTi alloy wire for use in orthodontics. Am J Orthod Dentofacial Orthop 1986; 90: 1-10. 3. Andreasen GF, Hileman TB. An evaluation of 55- cobalt substituted wire for orthodontics. J Am Dent Assoc 1971; 82: 1373-5. 4. Brantley WA. Orthodontic wires. In: Brantley WA, Eliades T (eds.) Orthodontic materials: scientific and clinical aspects. Stuttgard: Thieme; 2001. p. 91-9. 5. Segner D, Ibe D. Properties of superelastic wires and their relevance to orthodontic treatment. Eur J Orthod 1995; 17: 395-402. 6. Meling TR, Ødegaard J. The effect of short term temperature changes on superelastic nickel-titanium arch wires activated in orthodontic bending. Am J Orthod Dentofacial Orthop 2001; 119(3): 263-73. 7. Mullins WS, Bagby MD, Norman TL. Mechanical behavior of theromoresponsive orthodontic arch wires. Dent Mater 1996; 12: 308-14. 8. Mallory DC, English JD, Powers JM, Brantley WA, Bussa HI. Force-deflection comparison of superelastic nickel-titanium archwires. Am J Orthod Dentofacial Orthop 2004; 126(1): 110-2 9. Elayyan F, Silikas N, Bearn D. Mechanical properties of coated superelastic archwires in conventional and self-ligating orthodontic brackets. Am J Orthod Dentofacial Orthop 2010; 137: 213-7. Palatal Gingival Palatal Gingival J Bagh College Dentistry Vol. 27(3), September 2015 In-Vitro evaluation Pedodontics, Orthodontics and Preventive Dentistry164 10. Gatto E, Mateares G, Di bella G, Nucera R, Cordasco G. Load deflection characteristics of super and thermal Ni-Ti wires. Eur J Orthod 2011; 10: 1-9 11. Lombardo L, Toni G; Stefanoni F, Mollica F, Guarnerie M, Siciliani G. The effect of temperature on the mechanical behavior of nickel-titanium orthodontic initial archwires. Angle Orthod 2012; 83(2): 298-305 12. Wilkinson PD, Dysart PS, Hood JAA, Herbison G. Load-deflection characteristics of superelastic nickel- titanium orthodontic wires. Am J Orthod Dentofacial Orthop 2002; 121: 483-95. 13. Bednar JR, Grueneman GW, Sandrik JL. A comparative study of frictional forces between orthodontic brackets and arch wires. Am J Orthod Dentofacial Orthop 1991; 100: 513-22 14. Liaw YC, Su YY, Lai YL, Lee SY. Stiffness and frictional resistance of a superelastic nickel-titanium orthodontic wire with low-stress hysteresis. Am J Orthod Dentofacial Orthop 2007; 131(5): 578.e12-8. 15. Nakano H, Satoh K, Norris R, Jin T, Kamegai T, Ishikawa F, Katsura H. Mechanical properties of several nickel-titanium alloy wires in three-point bending tests. Am J Orthod Dentofacial Orthop 1999; 115(4): 390-8. 16. Schemann-Miguel F, Cotrim-Ferreira F, Streva AM, Chaves AVOA. Comparative analysis of load/deflection ratios of conventional and heat- activated rectangular NiTi wires. Dental Press J Orthod 2012; 17(3): 23.e1-6. 17. Proffit WR, Fields HM, Sarver DM. Contemporary orthodontics. 5th ed. St. Louis: CV Mosby 2013. 18. Sarul M, Kawala B, Antoszewska J.Comparison of Elastic Properties of Nickel-Titanium Orthodontic Archwires. Adv Clin Exp Med 2013; 22(2): 253-60 19. Tonner RI, Waters NE. The characteristics of super- elastic Ni-Ti wires in 3-point bending. 1. The effect of temperature. Eur J Orthod 1994; 16(5): 409-19 20. Santoro M, Olivier FN, Thomas JC. Pseudoelasticity and thermoelasticity of nickel titanium alloys: a clinically oriented review. Part I: Temperature transitional range. Am J Orthod Dentofacial Orthop 2001; 119(6): 587-93 21. Garrec P, Jordan L. Stiffness in bending of a superelastic Ni-Ti orthodontic wire as a function of cross sectional dimension. Angle Orthod 2004; 74(5): 691-6.