The archwire in braces is the active component that applies the controlled force necessary to move teeth into their correct positions. The wire is bent and engaged into the brackets, exerting continuous, gentle pressure. Since a patient’s needs change throughout treatment, orthodontists use different metal alloys, each tailored to a specific phase of alignment. Selecting the correct wire is paramount for efficient tooth movement and minimizing discomfort.
Primary Wire Materials
Orthodontic archwires are constructed from three primary metal alloys, each offering a distinct combination of strength and flexibility. Nickel-Titanium (NiTi) wires, also called Nitinol, are composed of nickel and titanium in roughly equal proportions. These wires are the most flexible and are reserved for the initial stages of treatment.
Stainless steel wires have been a staple in orthodontics since the 1920s. They are primarily composed of iron, chromium, and nickel. The common austenitic type contains approximately 18% chromium and 8% nickel, providing excellent strength, corrosion resistance, and stability. These wires are the stiffest and are used when precise control is needed for final tooth positioning.
Beta-Titanium, also known as Titanium Molybdenum Alloy (TMA), balances the properties of the other two types. This alloy is composed mainly of titanium and molybdenum and notably does not contain nickel. Beta-Titanium is intermediate in stiffness, delivering a force softer than stainless steel but greater than NiTi wires.
Unique Mechanical Properties
The effectiveness of each archwire material stems from its distinct mechanical behavior under stress. Nickel-Titanium wires are known for shape memory and superelasticity. Superelasticity allows the wire to be significantly deformed when seated into misaligned brackets, yet it returns to its original shape. It exerts a constant, light force over a wide range of deflection.
Stainless steel wires provide greater stiffness and withstand higher forces without deforming. This makes them effective for final detailing and stabilization, where small, precise adjustments are needed. However, stainless steel lacks the superelastic range of NiTi, meaning small deformation results in a much greater force.
Beta-Titanium wires are prized for their excellent formability, allowing the orthodontist to easily place specific bends and loops into the wire. This formability, coupled with greater springback than stainless steel, allows for complex wire designs that deliver moderate, controlled forces. Customizing the wire shape is useful for individual tooth movements and torque control during the intermediate phase of treatment.
Stages of Treatment and Wire Selection
The archwire material selected directly corresponds to the stage of orthodontic treatment. Treatment begins with initial leveling and alignment, addressing crowding and rotation. For this phase, highly flexible, superelastic Nickel-Titanium wires are chosen because they deliver the gentlest continuous forces required to initiate tooth movement.
Next is the intermediate or working phase, addressing larger movements like space closure or root movement. This stage often utilizes stiffer Nickel-Titanium or Beta-Titanium wires. Beta-Titanium is useful here because it allows the clinician to introduce specific adjustments and bends to control the roots of the teeth, offering moderate force and greater control than NiTi.
The final stage, known as finishing, focuses on precise alignment, proper bite relationship, and minor detailing. This phase requires the stiffest wires, primarily stainless steel. The high stiffness and stability of stainless steel allow the orthodontist to place exact bends that hold the teeth firmly in their final positions.
Safety and Material Considerations
Orthodontic materials are rigorously tested for safety, but metal sensitivity is a factor in wire selection. Nickel is a common component in both Nickel-Titanium (up to 50%) and stainless steel (up to 9%) wires, and it frequently causes contact dermatitis. While the nickel in stainless steel is bound tightly within the alloy, NiTi wires have a higher potential for nickel ion release into the oral environment.
Patients with a diagnosed nickel allergy avoid high-nickel NiTi wires. Nickel-free alternatives, such as Beta-Titanium, offer a safe option for intermediate treatment phases. Other alternatives include pure titanium or gold-plated wires, or NiTi wires with a resin coating to reduce nickel leaching. All materials used in the mouth are biocompatible, meaning they are designed to be safe and well-tolerated by the human body.