Bending characteristics of nitinol wire

Abstract

U ntil recently only a limited group of alloys have been used for orthodontic wires, namely, gold-caper, austenitic stainless steel, and chromium-cobalt-nickle (CrCo-Ni) based materials. In general, the last two groups have similar mechanical properties although processing, stress-relieving, heat-treating, and minor alloying adjustments allow for small product differences. On the other hand, the gold-based wires have a modulus of elasticity approximately one half that of the other two groups. Unfortunately, the yield strength of gold alloys is less by approximately the same factor, which gives them a ratio of yield strength to modulus of elasticity that is nearly the same as that of stainless steel and Cr-Co-Ni alloys, Since springback or resilience is proportional to this ratio, all three categories of alloys are approximately the same in regard to this clinical characteristic. When used in orthodontic appliances, wires with low moduli of elasticity in combination with high resilience aid in delivering clinically desirable low continuous forces and increased working time.’ It is these benefits, coupled with technologic advances in metallurgy and wire processing, which have promulgated the introduction of several lowstiffness, high-springback orthodontic wires. Braided wires use traditional stainless steel but achieve their improved properties through unique design of the wire’s cross section. A second approach to obtaining “high-deflection” wires is through the use of novel alloys, the most notable being nitinol, a nickel-titanium alloy.2 Nitinol was developed by William F. Buehle? in the early 1960’s. The original alloy contained 55 percent nickel and 45 percent titanium, which resulted in a one-to-one stoichrometric ratio of these elements. The most unique feature of this NiTi intermetallic compound is the “memory” phenomenon, which is a result of temperature-induced crystallographic transformations.” Andreasen’ suggested that these shape changes might be used by the orthodontist to apply forces. This memory principle is not used clinically, although it would appear plausible. The commercially available nitinol orthodontic wire contains 1.6 percent cobalt to modify the transition temperature and mechanical properties. Even without the “memory” effect, the unusually low modulus of elasticity of 4.8 x lo6 p.s.i. and high resilience offer desirable features to the orthodontist. Using a clinical model, Andreasen and Barrett” demonstrated that nitinol had a lower stiffness than stainless steel and could be deflected further without permanent deformation when tied into malaligned brackets. Andreasen and Morrow’ have evaluated the bending characteristics and spring rate of