Archwires

Three metal alloys that are clinically relevant as archwire material are nickel–titanium, titanium–molybdenum alloy (TMA or beta-titanium), and stainless steel. Again, there is currently no standardized methodology for establishing the elastic properties of these materials, and manufacturers’ claims and subsequent research on this topic are therefore difficult to compare. The American Dental Association (ADA) specification no. 32 advocates the unilaterally supported beam approach, in which a straight wire with a set length (1 inch according to ADA specification no. 32) is deflected up to 90°, and either the required force or the resulting bending moment is measured. This methodology may not be ideal, particularly in relation to the mechanical properties of wires made of nitinol (a substance named after the alloy components nickel and titanium, NiTi, and the institution that developed it, the Naval Ordinance Laboratory in White Oak, Maryland, USA). This is why the bilaterally supported beam approach is preferred in engineering ( Fig. 2.30 ). But a number of variations exist even for this, particularly with a view to standardizing the exact fixation of the wires in the testing equipment. A typical force deflection diagram is shown in Fig. 2.31 .

Flexible wires made of nickel–titanium alloys can have both superelastic and thermoelastic properties. The terminology applying to these properties has not been standardized in the literature, and manufacturers’ information and research studies can therefore be difficult for the practicing clinician to interpret. Generally, thermo-elasticity refers to the property of NiTi to change phases based on temperature: the martensite phase exists at lower temperatures and the austenite phase at higher temperatures. An archwire that has been deformed, within limits, in the martensitic phase will resume its original shape in the austenitic phase. This is known as the “memory effect” of NiTi, and while it is theoretically quite useful, it is difficult to implement in practice in an orthodontic treatment protocol, as the transition to the austenite phase occurs immediately after the wire enters the warmer oral cavity.

Superelasticity usually refers to the property the alloy exhibits in the transition from the austenite phase to the martensite phase when tension is applied to the wire, creating what is known as “tension-induced martensite” (TIM). This state is not stable and will immediately convert back to the austenite phase, and the original shape associated with that phase, when the tension is released. The orthodontically interesting phenomenon occurring here is not only that the wire resumes its original shape, but that it does so at lower force levels than were required to deform the wire (hysterisis) ( Fig. 2.30b ).

In clinical practice, however, we often observe that superelastic archwires do undergo plastic deformation, depending on the quality and composition of the NiTi alloy ( Fig. 2.32 ).

The friction of an archwire in the bracket slot is also determined by its surface characteristics. Microscopic investigations have demonstrated the variations seen in the surface structure of different archwires ( Fig. 2.33 ).

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