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Orthodontics

Authors: Dr. Shah Aakash M, Dr. Shah Purvesh M, Dr. Shah Romil B

Abstract

Archwires are the basic of orthodontics. They contain the force to move the teeth. A review of archwires is necessary for orthodontists attempting to maximize treatment outcomes. Therefore a brief review of archwires is discussed in this article.

Keywords: Archwires, malocclusion

INTRODUCTION

The properties of an ideal archwire for efficient orthodontic movement can be defined based off the biomechanical principle that light, continuous forces produce the most biologically efficient tooth movement. These properties include: 1. high strength, 2. high range, 3. low stiffness, 4. high formability, and 5. affordability. In addition, the archwires should be solderable or weldable so that stops or hooks can be attached to the wire if needed. Not just one single archwire material, however, meets all of the aforementioned requirements. As a result, a series of different archwires made out of various materials are used as treatment progresses to accomplish the multiple goals of the individual phases of orthodontic treatment including leveling and aligning, space closure and A/P correction, detailing and finishing, and retention.1

In the early 1900s, during the beginning of the “archwire era of orthodontics”, noble metal alloys, such as gold, were primarily used for orthodontic wires. This is because nothing else would withstand intra-oral conditions. However, in the 1930s, with the introduction of different alloys with lower cost and better mechanical properties, the initial noble metal alloy archwires became obsolete in modern day orthodontics. Today, the three main categories of orthodontic archwires that have replaced the noble metal alloys include: stainless steel, beta titanium, and nickel-titanium.1

Stainless steel archwires became broadly accepted within orthodontics by the 1960s. Stainless steel has greater strength and springiness than the pioneer wires in orthodontics; the noble metal alloys. Additionally, it also has the beneficial property of corrosion resistance, which is due to its high chromium content. The chromium forms a thin oxide layer that prevents the diffusion of oxygen. Stainless steel is commonly used in the later stages of orthodontic treatment for leveling, detailing, and finishing due to its properties of low friction, good formability, stiffness, and less spring-back compared to some of the other categories of archwires.2

Beta-titanium wires are titanium molybdenum alloys that were introduced for orthodontic use in 1979 by Goldberg and Burstone. Beta-titanium archwires, commonly referred to as TMA archwires after the first commercially available version, have advantages such as low elastic modulus, excellent formability, weldability, good spring- back, and a low potential for hypersensitivity. However, the use of beta-titanium wires has disadvantages such as high surface roughness, which increases friction at the wire-bracket interface during the wire sliding process, and susceptibility to fracture during bending. With properties in between nickel-titanium and stainless steel, these wires are used commonly as an intermediate wire between the two. As a result, many clinicians use this wire as their main “working wire” during orthodontic treatment.1

Nickel-titanium is the final category of wires used in modern orthodontics. Nickel-titanium, commonly referred to as “NiTi,” was first discovered in the late 1950s by the U. S. Naval Ordnance Laboratory (NOL). It was developed by the space program, and was known as “Nitinol.” Although not initially developed for orthodontics, its clinical advantages in leveling and aligning teeth were soon noticed. Nickel-titanium archwires are known in orthodontics for their exceptional springiness and strength as well as their poor formability. As a result, nickel-titanium is extremely useful during initial orthodontic alignment of the teeth due to its ability to apply a light force over a large range of activation.2

NICKEL-TITANIUM PROPERTIES

Nickel-titanium alloy’s clinical advantages in orthodontics are based upon the fact that these alloys can exist in two different crystal structures: martensite and austenite. At low temperatures and higher stress, the martensitic form is more stable, and at higher temperatures and lower stress, the austenitic form is more stable. The transition between these two phases/structures is fully reversible and occurs at low temperatures. The two different phases of nickel-titanium are responsible for two clinically significant properties of nickel-titanium: shape memory and superelasticity.3, 4

Shape memory is the capability of the wire to return to its original shape after being plastically deformed. If the wire is cooled below the transition temperature, it will transform into the martensite form, and it can plastically deform. Once it is heated back above the transition temperature, the wire will return back to the austenite phase and return to its original form. Shape memory refers to the temperature-induced change in crystal structure, and it is also known as thermoelasticity.5

Superelasticity refers to the large, reversible strains that the nickeltitanium wires can withstand due to the martensite-austenite transition. This property, also referred to as pseudoelasticity, is possible because the transition temperature between the two crystal phases is very close to room temperature. This property is evident in the almost flat section of the loaddeflection curve. Clinically, this is a useful property in that the initial archwire can exert the same force, whether it was deflected a small or large distance.5

COPPER-NICKEL-TITANIUM ARCHWIRES

Since the introduction of nickel-titanium archwires into orthodontics, a couple of different chemical elements have been added to the nickel-titanium in order to derive clinical advantages. One of the chemical elements that have been added to nickel-titanium is copper.

In 1991, Ormco applied for a patent for copper-nickel-titanium archwires, stating that the reason for the production of this wire was that there were some limitations associated with the existing nickel-titanium archwires commonly used in orthodontics. The first limitation stated by Ormco was that the amount of force applied by the NiTi orthodontic archwire to the orthodontic bracket is relatively low thus requiring longer treatment time. The second problem stated that the initial force necessary to engage the wire with the orthodontic bracket is quite high, thus making it difficult for the orthodontist to apply the archwire to the bracket, especially if the tooth is severely mal- aligned. The final problem listed by Ormco was that the

substantially constant load is effective for only a relatively short distance and at a relatively low level of force. Ormco stated that they discovered that by controlling the composition of the shape memory alloy, with the addition of copper, the previously mentioned disadvantages of shape memory archwires could be eliminated or minimized. Therefore, Ormco claims that with the addition of copper to the nickel-titanium archwires, the archwires would have a lower loading stress while still providing relatively high unloading stress for more effective orthodontic tooth movement of teeth. Thus, the wires would deliver more force per tooth movement, and maintain a substantially constant force as the teeth move closer to their intended position.6

Since Ormco’s patent on copper-nickel-titanium archwires expired, most of the major orthodontic companies have begun producing their own version of the copper- nickel-titanium archwire. Copper-nickel-titanium archwires are commonly marketed by these companies to orthodontists by stating that they exhibit a more constant force/deformation relationship, thereby providing superior consistency from archwire to archwire. Companies claim that these archwires demonstrate consistent transformation temperatures, ensuring consistency of force from batch to batch. Furthermore, they claim that this property allows the orthodontist to customize treatment to various patients by choosing between specific different amounts of force correlated to transition temperatures, i.e. 27˚C, 35˚C, or 40˚C.

It is easy for a clinician to assume that all copper-nickel-titanium wires of the same type from the same manufacturer have the same mechanical properties. Research has been conducted to determine if this is in fact true. Pompei-Reynolds and Kanavakis7 sought to test the potential variability in mechanical and thermal properties among copper-nickel-titanium wires with the same advertised characteristics from their company. When this study was conducted, copper-nickel-titanium archwires were only commercially available from two companies: Rocky Mountain Orthodontics and Ormco. The results of the 3-point bend test showed statistically significant differences between manufacturers in interlot force delivery variations on unloading. When interlot force variations are large, it is questionable if the orthodontist can rely on the same clinical properties. The study confirmed that interlot variations exist between copper-nickel- titanium archwires of the same type from the same manufacturer.7

CONCLUSION

Wires of the same materials, dimensions, and transition temperature, but from different manufacturers do not always have the same mechanical properties. There are significant differences in activation and deactivation forces among the different manufacturers of archwires. Improvements should be made in the standardization of the manufacturing process of archwires in order to provide orthodontists with archwires that have consistent mechanical properties despite the manufacturing brand that produces them.

References
  1. Kusy RP. A review of contemporary archwires: their properties and characteristics. The Angle Orthodontist 1997:67;197-208.
  2. Proffit WR, Fields HW, Sarver DM, Ackerman JL. Journal of Contemporary Orthodontics. 5th ed. St. Louis, MO: Mosby, 2013.
  3. Burstone C, Qin B, Morton J. Chinese NiTi wire-a new orthodontic alloy. American Journal of Orthodontics 1985:87:445-452.
  4. Miura F, Mogi M, Ohura Y, Hamanaka H. The superelastic property of the Japanese NiTi alloy wire for use in orthodontics. American Journal of Orthodontics and Dentofacial Orthopedics 1986:90:1-10.
  5. Fernandes DJ, Peres RV, Mendes AM, Elias CN. Understanding the Shape-Memory Alloys Used in Orthodontics. International Scholarly Research Notices Dentistry 2011:132408.
  6. Sachdeva RCL, Miyazaki S, Farzin-Nia F. Orthodontic archwire and method of moving teeth. U.S. Patent 5,044,947, September 3, 1991.
  7. Pompei-Reynolds E, Kanavakis G. Interlot variations of transition temperature range and force delivery in copper-nickel-titanium orthodontic wires. American Journal of Orthodontics 2014:146 (2); 215–226