Hani F Ounsi et al 10.5005/jp-journals-10024-2181 REVIEW ARTICLE Evolution of Nickel–titanium Alloys in Endodontics 1Hani F Ounsi, 2Wadih Nassif, 3Simone Grandini, 4Ziad Salameh, 5Prasanna Neelakantan, 6Sukumaran Anil ABSTRACT and alloys, they have an obligation to deal more intimately with engineering consideration to not only take advantage of their To improve clinical use of nickel–titanium (NiTi) endodontic possibilities but also acknowledge their limitations. rotary instruments by better understanding the alloys that compose them. A large number of engine-driven NiTi shaping Keywords: Cyclic fatigue, Endodontics, Martensitic alloys, instruments already exists on the market and newer generations M-wire, Nickel–titanium, Surface treatment, Torsional stress. are being introduced regularly. While emphasis is being put on How to cite this article: Ounsi HF, Nassif W, Grandini S, Salameh Z, design and technique, manufacturers are more discreet about Neelakantan P, Anil S. Evolution of Nickel–titanium Alloys in alloy characteristics that dictate instrument behavior. Along Endodontics. J Contemp Dent Pract 2017;18(11):1090-1096. with design and technique, alloy characteristics of endodontic instruments is one of the main variables affecting clinical per- Source of support: Nil formance. Modification in NiTi alloys is numerous and may yield Conflict of interest: None improvements, but also drawbacks. Martensitic instruments seem to display better cyclic fatigue properties at the expense of surface hardness, prompting the need for surface treatments. INTRODUCTION On the contrary, such surface treatments may improve cutting efficiency but are detrimental to the gain in cyclic fatigue resis- The endodontic treatment requires proper cleaning and tance. Although the design of the instrument is vital, it should shaping of the root canal space, i.e., removing tissues in no way cloud the importance of the properties of the alloy whether vital or necrotic and reducing the bacterial load and how they influence the clinical behavior of NiTi instruments. in the case of infection, which is done through chemo- Clinical significance: Dentists are mostly clinicians rather mechanical detersion protocols. To reach the apical part, than engineers. With the advances in instrumentation design practitioners have to improve the rheology of the root canal by giving it a continuously tapered shape. Until 1Department of Endodontics, Faculty of Dental Medicine the early 1990s, this was conventionally done using Lebanese University, Beirut, Lebanon; Department of stainless steel instruments that have a natural tendency Endodontics and Restorative Dentistry, Siena University, Siena to straighten curved canals when used in sizes 20/100 Italy and above due to the inherent stiffness of the alloy and 2 Department of Prosthodontics, Faculty of Dental Medicine could not follow curvatures even in moderately curved Lebanese University, Beirut, Lebanon canals. They had thus to be precurved to reach length, 3 Department of Endodontics and Restorative Dentistry, Siena which in turn forced operators to use them exclusively in University, Siena, Italy filing motion. This resulted in a high incidence of proce- 4 Department of Research, Faculty of Dental Medicine, Lebanese dural errors, such as ledges, elbows, zipping, strippings, University, Beirut, Lebanon and perforations. Nickel–titanium alloys in dentistry1 5 Department of Endodontology, Faculty of Dentistry, The allowed for endodontic instruments that have reduced University of Hong Kong, Hong Kong stiffness and increased elasticity, specifically engine- 6 Department of Dental Health, Dental Biomaterials Research driven NiTi instruments that proved to be a valuable Chair, College of Applied Medical Sciences, King Saud University Riyadh, Kingdom of Saudi Arabia addition to the endodontic armamentarium. Since their introduction, these files have seen numerous improve- Corresponding Author: Hani F Ounsi, Department of Endodontics, Faculty of Dental Medicine, Lebanese University ments in not only file design and clinical sequences but Beirut, Lebanon; Department of Endodontics and Restorative also metallurgy. If modified clinical sequences and file Dentistry, Siena University, Siena, Italy, e-mail: ounsih@ design are relatively easy for practitioners to follow, gmail.com modification in the alloys may prove more challenging 1090 JCDP Evolution of Nickel–titanium Alloys in Endodontics and alloys designed to overcome a specific weakness twinned martensite structure can hence, untwin on stress, may impact on another property of the alloy. This article but the load has to be nearly constant or slightly increas- presents an overview of the NiTi alloys currently used ing. Furthermore, this untwinning is not an elastic strain: in endodontics. Atoms move from one energy minimum to another. This atom shifting phenomenon allows to absorb up to 8% of About NiTi Smart Alloys the strain as compared with the 0.1 to 0.2% strain limit of many other alloys.3 Nickel–titanium alloys (also known as NiTi or NITINOL for Naval Ordinance Laboratory) are known, when in Conventional NiTi Alloys used in Endodontics nearly equiatomic proportions, to display an array of interesting properties, such as shape memory, super- The main advantage of using NiTi alloys in root canal elasticity, and damping characteristics that arise from shaping instruments is the alloy’s high flexibility.8 reversible crystallographic changes. It is noteworthy Martensitic transformation can be stress induced from that a 0.1% change in composition will result in a 10°C the austenitic phase over a narrow range of tempera- change in the transformation temperature of the alloy tures. Superelasticity occurs when a large reversible and subsequently in its mechanical characteristics.2 The deformation occurs while increasing, stress appears to be NiTi alloys form Ni3Ti, NiTi, and NiTi2 intermetallic constant (plateau). It happens as follows: Conventional compounds. The nearly equiatomic NiTi alloy has broad NiTi alloys are in the austenite phase at body/room compositional limits in the eutectoid phase field above temperatures9-11 Activation of austenitic NiTi produces 630°C, whereas Ni3Ti and NiTi2 are sharply defined an elastic deformation that follows a linear stress/strain compounds.3 Below this temperature, a two-phase field function (the slope of the curve representing the elastic (Ni3Ti, NiTi2) stretches between around 25 to 66% Ti. It is modulus). If deformation (stress) increases, the superelas- possible to preserve the metastable phase by cooling NiTi tic deformation appears, whereas strain remains constant. beneath this temperature.4 When heated, NiTi displays a This superelastic behavior is a direct consequence of the body-centered cubic structure, i.e., known as austenite. martensitic transformation which occurs at the crystal- On cooling, a classic linear thermal contraction is visible lographic level. The strain will remain constant until the until a certain limit [martensite start (Ms)] beyond which entirety of the NiTi mass has shifted to the martensitic, the contraction accelerates. This is caused by a progres- which in turn will sign the end of the superelastic domain. sive shear transformation to a monoclinic structure Continuing the activation beyond that point will reveal called martensite.5 On further cooling, the contraction conventional martensitic deformation with a classic linear rate becomes linear again at a certain point [martensite stress/strain relationship as the crystallographic deforma- finish (Mf)], pointing to the fact that the proportion of tion’s potential to absorb strain is exhausted.8 Thus, if the martensite phase in the alloy has reached 100%. Reheating load is relieved before reaching the plastic deformation this martensite will eventually reverse the process, yield- limit, the deformation will be reversible, both ordinary ing an austenite phase similarly an austenite start (As) austenitic elasticity and the pseudoelastic deformation and an austenite finish (Af) points. Generally, the As and due to phase change. Af temperatures are about 20°K above the Mf and Ms Again, as for thermal modification, the hysteresis temperatures, signing the presence of a hysteresis phe- phenomenon is present and the loading and unloading nomenon above the temperature transformation range. curves will not match. It is noteworthy that although the A third rhombohedral phase or R-phase can also be mechanism of action is similar, the aspect of stress–strain described.6,7 In general, it appears only on cooling before curves will vary significantly depending on the diameter the martensitic transformation is complete. The face- of the wire, temperature, and annealing properties.12 centered cubic planar organization may be described as Having tested instruments from several manufacturers, A, B, C, A, B, C…. It reflects the atoms’ position in suc- Ounsi et al13 established that the earlier generations cessive planes as compared with those in a randomly of instruments were all manufactured from a unique chosen reference plane. This organization involves a 55%Ni–45%Ti. This becomes obvious when one consid- regular displacement of each plane in a specific direction. ers that transformation temperatures of such alloys are If the stacking sequence is reversed A, B, C, B, A, C… for highly sensitive to the composition and even 1% devia- instance, the crystal is said to be twinned. This happens tion in these percentages would almost yield a 100°C in some alloys when compressive