of the strain and of the temperature. Superelasticity in Orthodontics was first described for Japanese NiTi, which

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1 ISSN: X CODEN: IJPTFI Available Online through Research Article SUPERELASTICITY OF NiTi Niha Naveed*, Dr. Sri Rengalakshmi Final Year BDS, Department of Orthodontics, Saveetha Dental College, Chennai MDS, Department of Orthodontics, Saveetha Dental College, Chennai Received on: Accepted on: Introduction : The superelastic properties of orthodontic wires and endodontic files are measured as a function of the applied stress, of the strain and of the temperature. Superelasticity in Orthodontics was first described for Japanese NiTi, which delivers an almost constant force over an extended portion of the deactivation range, that is, the stress remains nearly constant despite the strain change within a specific range. The nickel-titanium alloy was invented at the Naval Ordnance Laboratory in the 1960 s and subsequent application to orthodontics became possible during the early 1970 s. [1][2] Evolution of NiTi Alloys : In 1887, Edward Angle used nickel- silver alloys in his orthodontic accessories. Subsequently he replaced them with copper, nickel and silver-free zinc alloys. Eventually, gold alloys became his favourite choice. This was the first generation of orthodontic wires.in 1931, Norris Taylor and George Paffenbarger introduced steel as a substitute for gold claiming it featured greater resilience and was less likely to break under stress. In the 1960s, cobalt-chromium alloys were found their way into Orthodontic practice and were patented as Elgiloy by Rocky Mountain Orthodontics. The first clinical application of Beta titanium alloy in orthodontics occurred in the 1980 s when a different form of titanium called high temperature was introduced. This was the second phase in the evolution. Since then, this titanium gained wide clinical acceptance and popularity. It is commercially available as TMA (titanium molybdenum alloy). Nickel-titanium alloys were developed by the U.S. Naval Laboratory in Silver Spring, Maryland, by researcher William Buehler. He was the first to observe the socalled shape memory effect typical of this material. In 1972, Unitek Corporation produced the NiTi alloy for clinical use under the trade name Nitinol, composed of 55% nickel and 45% of titanium, in an equiatomic structure. In 1985 the clinical and laboratory use of a new superelastic nickel-titanium alloy was reported. It was called IJPT June-2017 Vol. 9 Issue No Page 29499

2 Chinese NiTi. The Japanese NiTi came into use in The third generation of wires were the superelastic NiTi. The fourth phase of evolution was the commercial use of thermodynamic nickel-titanium alloys which began in the 1990s. In addition to the properties of elastic recovery and resilience provided by superelastic wires, thermodynamic nickel titanium wires boast the additional feature of being activated by oral temperature. These were the gradually dynamic NiTi which was the fifth generation of orthodontic wires.in the mid-1990s, nickel-titanium wires with the addition of copper (CuNiTi) first became available on the market. Due to the incorporation of copper these wires feature better de ned thermal properties than NiTi superelastic wires while yielding an outstanding system of forces with increased control over tooth movement. In the mid-2000s different types of aesthetic orthodontic wires have been launched on the market. These were the sixth generation wires and included teflon coated stainless steel wires, stainless steel wires coated with epoxy resin and orthodontic wires comprising a nylon-based matrix reinforced with silicone fibres. The seventh generation of orthodontic wires were made from polymer composite material reinforced with glass fibre. [3][4] Metallurgy of NiTi Alloys : The nickel- titanium wires at room temperature contain approximately equiatomic proportions of nickel and titanium in a stable phase. However some deviations from stoichiometry are possible and the commercially available orthodontic wires are usually titanium-rich (Brantley, 1997). At room temperature the crystal structure of the NiTi alloys consists of a complex bodycentered cubic structure as a single phase or as one component of a two- phase system. The second phase might be either NiTi2 or NiTi3. [5][6] The two major NiTi phases are the austenitic NiTi (or austenite) has an ordered body-centered cubic structure that occurs at high temperatures and low stresses and the martensitic NiTi (martensite), which is formed at low temperatures and high stresses and appears to have a distorted monoclinic, triclinic, or hexagonal structure. The superelastic nickel-titanium wires, including the true shape memory nickel-titanium wires, show exceptional temperature sensitivity. Thus, alterations in mouth temperature could cause a stress fluctuation in nickel- titanium wires during orthodontic treatment. [7][8] Difference Between Shape Memory and Superelasticity: Superelastic and Shape memory effect are the names given at two particular trajectories followed by the Shape Memory Alloys in the stress-strain-temperature space. This alloys exhibit a complex behaviour due the existence of a solid-solid transition between two phases (technically austenite and martensite). This phase transition can be IJPT June-2017 Vol. 9 Issue No Page 29500

3 induced in both directions by applying temperature changes or by mechanical loading. When the material is mechanically loaded at a temperature bellow Mf (Martensite finish temperature) for which the material is in fully martensite state, it remains strained upon unloading (likewise plastically deformed). But original dimensions can be recovered upon heating above a temperature Af (austenite finish). This is the trajectory associated with Shape Memory Effect. Superelastic effect has place when the material in a fully austenite state is mechanically loaded up to a critical stress SA-M, for which the transition to martensite is induced. In this case, the loading direction selects the crystallographic variants of the appearing martensite, and the sample can develop strains up to 10 % in the case of NiTi SMA. Now, when the load is relaxed, the reverse transformation from martensite to austenite has place when the stress drops below a critical vale SM-A. During reverse transformation, the material can recover practically the original dimensions. In this superelastic trajectory, a dissipative hysteresis is described, being possible to utilise this kind of materials as damping devices. [9] Superelasticity of NiTi : The superelasticity of NiTi shape memory alloys is exploited in orthodontics to provide a constant force in the correction of malocclusions, allowing both to fulfil the requirements of the optimal orthodontic force and to considerably shorten the therapy. Though some little drawbacks (superelasticity prevents formability, few choices of the force are available), which can however be overcome, the great benefits offered by superelasticity have dictated the success of NiTi wires in orthodontics. [10] NiTi alloy exhibits unique and ideal mechanical properties for the practice of orthodontic mechanotherapy. The deformation of the superplastic NiTi is associated with the martensitic transformation, which originates in a crystallographically reversible structural change, hence the deformation becomes reversible upon loading and unloading. The most important characteristic in the superelastic behaviour is the generation of a constant force and a large reversible deformation over a long activation span. Besides, the constant force can be adjusted over a wide range by changing manufacturing and metallurgical factors such as thermomechanical treatment, composition, etc. [11] The microscopic superelastic behaviour of a nickel-titanium (NiTi) alloy has been studied by instrumented indentation experiments using both spherical and pyramidal (e.g., Berkovich) diamond indenters in a study conducted by Wangyang Ni and Yang-Tse Cheng David S. Grummon.[12] The indentation load displacement curves for IJPT June-2017 Vol. 9 Issue No Page 29501

4 superelastic NiTi was obtained under a range of indentation conditions. It was found that indentationinduced superelasticity exists under both spherical and pyramidal indenters, which may be exploited for many applications, ranging from microelectromechanical systems to surface engineering. Utilisation of NiTi in Orthodontics: Arch wire material selection can affect the application of optimum orthodontic forces to teeth to produce ideal conditions for safe, comfortable, and fast tooth movement. Storey and Smithdetermined that different teeth require different forces for movement. Much has been done to nickel titanium arch wires to vary the elastic forces within the same nickel titanium arch wire. The nickel-titanium wires have the lightest force delivery and widest elastic range, along with outstanding spring back, particularly for the shape-memory alloys. [13] Innovations began with light forces and heat-activated/thermal NiTi wires. These wires are soft and pliable at room temperature, which permits easier insertion into the bracket slots. As the wire warms to mouth temperature, (98 degrees, +/- a few degrees), the wire becomes more active. This initial wire applies forces to begin tooth movement. Today, nickel titanium wires come in varying applications, ranging from lightest force on anterior teeth, to more active forces on posterior teeth.[14] This unique combination of heat activation and differing, individual force application is why NiTi is the best choice for orthodontic treatment. Nickel titanium arch wire also has added resiliency. However, the wire may be overstressed during insertion into the bracket slot. There is a bending limit to each type of NiTi wire. Forcing the wire past the limit causes unwanted permanent distortion in the wire, from which memory wire cannot recover. The clinician should evaluate the extent to which the wire can be flexed without permanent distortion. To determine whether or not the arch wire is permanently distorted, it is best to partially remove it from the patient s mouth. These distortions could indicate that a smaller diameter NiTi wire should be inserted to permit correction of the tooth malalignment. Later, increased diameter wires can be carefully evaluated to determine if they can be safely inserted, without permanent distortion.[14][15] Nickel titanium wire is also available with Curves of Spee and varying degrees of torque, as well as utility arch forms. There are also NiTi wires with surface coatings, which may reduce wire surface friction by 30%. [16] Rectangular NiTi arch wires may be used early in treatment, to facilitate simultaneous rotation, tipping, levelling and torquing, and thus in some cases, treatment can begin with full-size rectangular wires that nearly fill the bracket slot. The superelastic and shape-memory nickel-titanium wires are particularly useful where large deflections are IJPT June-2017 Vol. 9 Issue No Page 29502

5 necessary for malpositioned teeth, whereas superelastic nickel-titanium coil springs, originally developed for orthodontics, have also found wide applicability. These springs can exert constant light, continuous force over a very wide range, providing an optimal appliance for tooth movement under appropriate clinical conditions. However, it has been found that the force delivery of the superelastic coil springs can be substantially affected by small changes in temperature. [17] On the other hand, since CuNiTi are manufactured for use under three transition temperatures (27 o C, 35 o C and 40 o C) they can be used for different treatment purposes.with the launch of CuNiTi alloys on the market, orthodontic treatment protocols that combine these wires with the use of self-ligating brackets have emerged. These protocols aim to achieve more biologically compatible treatments thanks to the deployment of physiological forces as well as shorter treatment time.[3]currently, the Damon System is the most popular brand of self-ligating brackets on the market. Unlike conventional mechanics, the designers of this system advocate that it is possible to move teeth without bone loss. In such cases, biological dynamics would purportedly enable a physiological adaptation of the alveolar bone in response to the orthodontic treatment. The disadvantages of NiTi wires are that they are expensive, have poor formability, and cannot be soldered or welded. Arch wire-bracket friction is also comparably high when compared to that of beta titanium because of the relatively rough wire surfaces that arise from the high titanium content. [18] However, the arch wire surfaces have now been treated to minimise friction. References : 1. Andreasen GF, Brady PR. A use hypothesis for 55 nitinol wire for orthodontics. Angle Orthod 1972;42: Andreasen GF, Heilman H, Krell D. Stiffness changes in thermodynamic nitinol with increasing temperature. Angle Orthod 1985;55: Ca tia Cardoso Abdo Quinta o, Ione Helena Vieira Portella Brunharo, Orthodontic wires: knowledge ensures clinical optimization, Dental press J Orthodontics, v14, n 6, p , nov-dec Bradley TG, Brantley WA, Culbertson B. Differential scanning calorimetry (DSC) analyses of superelastic and nonsuperelasticity nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop 1996;109: Brantley WA. Orthodontic wires, in: Brantley WA, Eliades T (eds) Orthodontic materials: scientific and clinical aspects. Stuttgard (Germany): Thieme 2001: Burstone CJ, Qin B, Morton JY. Chinese NiTi wire-a new orthodontic alloy. Am J Orthod 1985;87: IJPT June-2017 Vol. 9 Issue No Page 29503

6 7. Cobb III HW, Kula KS, Phillips C, Proffit Wr. Efficiency of multi-strand steel, superelastic Ni-Ti and ionimplanted Ni;Ti archwires for initial alignment. Clin Orthod Res 1998;1: Eliades G, Brantley WA. Instrumental techniques for study of orthodontic materials, in: Brantley WA, Eliades T (eds) Orthodontic materials: scientific and clinical aspects. Thieme;2001: Kamita T., Matsuzaki Y. One-dimensional pseudoelastic theory of shape memory alloys // Smart Mater and Struct. 7 (1998), P ) 10. Eliades T, Eliades G, Athanasiou A, Bradley TG. Surface characterization of retrieved NiTi orthodontic archwires. Eur J Orthod 2000;22: Kapilla S, Sachdeva R. Mechanical properties and clinical applications of orthodontic wires. Am J Orthod Dentofacial Orthop 1989;96: Wangyang Ni and Yang-Tse Cheng David S. Grummon, Applied physics letters, Volume 82, issue Storey E, Smith R: Force in orthodontics and its relation to tooth movement. Aust Dent J 56:11-18, Michael C. Alpern, The Orthoevolution of Orthodontic Archwires, Orthodontic products online, 2015 edition. 15. Alpern M.C.: Gaining Control with Self-Ligation. Seminars in Orthodontics, Vol 14, No 1, March 2008: pp 73-86; Elsevier 16. Thorsten GA, Kusy RP: Effect of archwire size and material on the resistance to sliding of self-ligating brackets with second-order angulation in the dry state. Am J Orthod Dentofacial Orthop122: , Kula K, Phillips C, Gibilaro A, Proffit WR. Effect of ion implantation of TMA archwires on the rate of orthodontic sliding space closure. Am J Orthod Dentofacial Orthop 1998;114: Khier S, Brantley WA, Fournelle RA. Bending properties of superelastic and nonsuperelastic nickel-titanium orthodontic wires. Am J Orthod Dentofacial Orthop1991;99: IJPT June-2017 Vol. 9 Issue No Page 29504