What are the different types of orthodontics arch wire materials?
Since its introduction to orthodontics in the 1930’s, stainless steel (SS) arch wires have remained a popular choice in orthodontics; as they offer many advantages such as: formability, biocompatibility, environmental stability, stiffness, resilience, and low cost [1, 3, 4]. One advantage of stainless steel is the ability to alter composition and ultimately physical properties through manipulations of formulations in the manufacturing process. This is beneficial as it allows for the percentage weight of each substituent in stainless steel to be adjusted, allowing for differential properties. Originally, stainless steel arch wires typically contained 17-25% chromium and 8-25% nickel, with the remaining being iron . Whereas, more recent formulations contain 18% chromium and 8% nickel, and are thus commonly regarded to as 18-8 stainless steel . A resulting important advantage of stainless steel composition, especially considering its use in the oral cavity, is its rust resistance properties. This is due to chromium content within the alloy; it has been shown that when the percentage of chromium is greater than 10%, chromium forms a surface film of chromium oxide over the stainless steel to render it resistant to corrosion . Further, the physical properties of stainless steel are generally regarded to have a high strength value, boosting a stiffness of 93-100% stronger than conventional carbon steels . Moreover, not only is it possible to alter stainless steels’ composition through specific formulations, as discuss above, but this can also be compounded with variations in the manufacturing protocols, allowing for adaptation of its properties. This is usually done by varying the amount of cold working and annealing during manufacturing . A general principle that is found in this process is the indirect relationship between strength and formability; thus, as one increases strength, generally, the degree of formability decreases, and vice versa . Through the process of cold working and annealing, two main grades of stainless steel can be classified, regular and super grade. Regular grade has a high degree of formability and low fracture rate; whereas, the super grade has higher yield strength, with lower deformation properties . It is through these properties, as discussed above, that stainless steel arch wire continues to offer a high degree of versatility and durability, remaining as one of the most widely used arch wires in orthodontics.
It was not until the 1960’s that cobalt-chromium alloys were introduced . The composition of cobalt chromium is cobalt (40%), chromium (20%), iron (16%), and nickel (15%) by percentage mass . Cobalt-Chromium and stainless steel have similar physical properties as both have a high stiffness value; however, a main advantage of CoCr is its availability in four different tempers, made possible through the process of heat treatment . This is an important advantage as it gives the practitioner a variable amount of formability to work with, allowing for the ability to bend loops, place V-bends, and place various other offsets into the arch wire [3, 5]. Thus, with these properties CoCr arch wire proves useful under certain clinical circumstances in orthodontics; as its ability to be hardened by heat treatment after being shaped and its different formable states may be particular advantageous in certain cases .
Nickel-Titanium (NiTi) Alloys
The advent of Nickel-Titanium alloys, which were originally intended for the space program, were found to have a place in arch wire orthodontics; after being introduced in 1970 through the University of Iowa [2, 5]. Shortly after, Nickel-Titanium (commonly regarded as Nitinol), was introduced it quickly became highly regarded, as its physical properties allowed for a high degree of springiness . This was found to be very advantageous as it allowed for the ability of NiTi arch wires to exhibit a shape memory effect, in which they exhibited the potential to return to their original form after deformation . Further, the physical properties of NiTi, which is generally 50% nickel and 50% titanium, allowed for light continuous forces to be offered through the shape memory affect; rather than abrupt heavy forces [3, 5]. In some cases, it is possible to exploit NiTi’s physical properties and allow for a superelastic form, allowing for so much springback, it can approach pure plasticity . On the other hand, the early NiTi alloys did have some limitations. One such limitation of conventional NiTi’s was its lack of formability, becoming apparent as wires broke under thresholds of deformation . Although, this lack of formability still remains today in conventional NiTi arch wires, the initial brittleness which plagued earlier models has been minimized . This is evident in the newer martensitic alloys such as Titanal or Orthonol, which have similar strength and springiness to Nitinol but better formability .
In addition to the conventional NiTi alloys, two other generic nitinol-type alloys are available, an austenitic active alloy and a martensitic active alloy . Martensitic and austenitic are both important as their physical properties allows for shape memory rebound after deformation .
Martensite is generally represented by the low stiffness phase; whereas, austenite respresents the higher stiffness phase . The ability to shift through these phases accounts for the properties of certain NiTi alloys. For example, the austenitic active alloy is able exert three times the force per activation compared to conventional martensitic nitinol alloys; however, this higher force is short lived and a plateau phase is soon reached . Furthermore, it was found that the austenitic arch wire could exert about the same force whether it was deflected a relatively small or large distance, a highly desirable characteristic . This can explained through phase shift of austenite to martensite phases, as discussed above . A key limitation of the austenitic alloy is that wire bending is not practical clinically, as very high forces are needed for the material to undergo plastic deformation. Finally, some currently-marketed NiTi’s have been able to enhance on these properties, allowing for the creation of an arch wire which is dead soft at room temperature, but in the environment of higher temperatures, such as the oral cavity, it becomes elastic . This becomes particularly advantageous for patients after an arch wire adjustment, as sipping cold water changes the properties of the elastic strain, ultimately resulting in decreased applied force. It is clear, current NiTi systems are beneficial in certain orthodontic circumstances, as their ability to supply a light and constant force is more predictable and efficient compared to stainless steel and cobalt chromium.
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