Orthodontic archwires: the alloy is chosen for the phase, not for the brand

The archwire is the engine of fixed appliance therapy, and the choice of alloy determines how much force reaches the tooth and for how long. It is a decision taken at every visit, and one that tends to be made from habit.

The parameter that matters is the relationship between deflection and force delivered. An ideal wire during levelling deflects a great deal and returns a low, constant force as it recovers its original shape.

Superelastic nickel-titanium does exactly this. It has a force plateau: across a wide range of deformation, the force it releases stays almost constant, regardless of how far it was bent to engage it.

That property makes it indispensable at the outset, where teeth are irregular and deflections are large. Steel in the same situation would deliver very high forces initially and none after a few weeks.

Excessive forces do not accelerate movement: they arrest it. Pressure above the capillary pressure of the periodontal ligament interrupts the blood supply, and the area becomes hyalinised and acellular.

In that zone movement stops until macrophages remove the tissue. The result is slower movement, not faster, and a greater risk of root resorption.

Thermoactive nickel-titanium adds a useful characteristic: it is soft at room temperature, hence easy to engage even in severe crowding, and develops its force on reaching oral temperature.

A patient drinking cold liquids temporarily reduces the applied force, which is an acceptable and sometimes welcome side effect in the days after activation.

Stainless steel enters in the later phases, once alignment is achieved and control and stability are required. It is highly rigid, bends accurately and holds its shape: it is the wire on which space-closure mechanics are performed.

TMA, or beta-titanium, sits between them: roughly a third of the stiffness of steel, yet formable. It suits situations needing custom bends with moderate forces, typically during finishing.

A sensible sequence therefore follows increasing dimension and increasing stiffness. One starts with small round nickel-titanium, moves to rectangular sections, and reaches steel when torque must be controlled.

Skipping steps to shorten treatment is the commonest cause of unwanted effects. A rigid rectangular wire engaged before alignment is complete expresses torque on teeth that are not yet in the right position.

In summary: nickel-titanium to align, steel to control and close, TMA to finish. And the sequence is not shortened, because force that does not serve movement ends up on the roots.