Orthodontic Wire Types UK — NiTi, Stainless Steel and TMA Explained

Why different wires are used at different treatment stages

Fixed brace treatment broadly follows three phases: alignment and levelling, active correction, and finishing and detailing. Each phase requires different mechanical properties from the archwire. In the alignment phase, teeth may be significantly displaced and the wire needs to be flexible enough to engage all brackets without generating damaging forces — high stiffness would cause discomfort and potential root resorption. In the active correction phase, controlled, consistent forces are needed to move teeth bodily or correct the bite. In the finishing phase, fine adjustments require a wire with good formability and spring-back. Wire material selection is a clinical decision made by the orthodontist based on tooth position, treatment stage, and individual patient factors.

Nickel-titanium (NiTi) wires — properties and early treatment use

Nickel-titanium wires are characterised by superelasticity and shape memory. They can be deflected significantly from their ideal arch form to engage a malpositioned tooth, and they then exert a gentle, continuous force as they return to their original shape — a property known as the plateau effect. This makes them ideal for the early alignment and levelling phase, when teeth may be severely crowded or displaced. NiTi wires generate relatively light forces over a long range of activation, which is both more comfortable for the patient and biologically appropriate for tooth movement. Heat-activated NiTi wires, which become more active at mouth temperature, are a variant that allows very light initial forces in severely crowded arches. Patients often experience the most soreness when NiTi wires are first fitted, as the wire is working hardest to engage displaced teeth.

Stainless steel wires — properties and active treatment use

Stainless steel is the workhorse wire of the active correction phase. It is stiffer and less flexible than NiTi, which means it can deliver more controlled, predictable forces — particularly important when torquing roots, applying precise bracket prescriptions, or using auxiliaries such as coil springs and elastics. Steel wires are also easily bent and adjusted by the orthodontist, allowing bends to be placed to correct individual tooth positions or compensate for residual discrepancies. The progression from thin, round NiTi wires to larger, rectangular stainless steel wires is a hallmark of conventional fixed brace treatment. Rectangular wires engage the bracket slot fully and allow three-dimensional tooth control — including root torque — that round wires cannot provide.

TMA (titanium-molybdenum alloy) wires — properties and finishing use

TMA, also known as beta-titanium, occupies a mechanical middle ground between NiTi and stainless steel. It has approximately half the stiffness of steel, making it more flexible while still being formable — unlike NiTi, it can be bent and will hold its shape. This combination makes TMA well suited to the finishing stage of treatment, where fine adjustments to tooth position and torque are needed without the stiffness of full steel. TMA is also biocompatible and nickel-free, making it an alternative for patients with confirmed nickel sensitivity who still require a more active wire than stainless steel alone can provide. TMA wires are more expensive than steel and are typically reserved for cases where their specific properties are clinically advantageous.

Wire sizes and what they mean

Orthodontic wires are described by their cross-sectional dimensions in thousandths of an inch or in millimetres. Round wires are described by a single diameter — for example, 0.014-inch NiTi is a thin, flexible wire commonly used at the start of treatment. Rectangular wires are described by two dimensions — width and height — such as 0.019 × 0.025-inch stainless steel, which fills most of a standard bracket slot and provides maximum control. As treatment progresses, orthodontists typically increase wire dimensions in a planned sequence — this is sometimes called the wire sequence. Larger wires engage the bracket slot more fully and allow finer control of tooth position in all three planes of space. The wire sequence is planned to match the biological response of the supporting bone and tissues.

Aesthetic wires — tooth-coloured and clear coated options

For patients with ceramic or clear brackets who want a more discreet appearance throughout the wire, tooth-coloured and white-coated archwires are available. These are typically NiTi or stainless steel wires with a rhodium or tooth-coloured epoxy coating. They improve aesthetics significantly, particularly in photographs and at conversational distances. However, coated wires have some limitations: the coating can chip or discolour over time with food and drink, particularly in the finishing stage when wires are under sustained stress. Some orthodontists reserve coated wires for the alignment phase and use uncoated steel in the active correction stage where the aesthetic benefit is lower and mechanical demands are higher. Patients interested in aesthetic wire options should discuss the trade-offs with their orthodontist.

Useful related pages

This guide is for general information only. It is not a diagnosis, treatment plan, or substitute for advice from a registered dentist or orthodontist.