A permanent or fixed retainer is a thin wire cemented directly to the lingual (tongue-side) surface of the teeth, typically along the lower front six teeth. This orthodontic device maintains the alignment achieved after active treatment with braces or aligners. Because the appliance is bonded, it ensures continuous retention and prevents teeth from shifting back to their original positions.
The Primary Metal Alloy Used
The most common material used for bonded retainers is medical-grade stainless steel wire. This alloy is often multi-stranded or braided, which increases its flexibility compared to a solid wire.
The composition includes a high percentage of iron, along with chromium (17–22%) and nickel (8–12%). Chromium creates a passive oxide layer on the surface, providing excellent corrosion resistance in the oral environment. Nickel enhances the alloy’s strength and flexibility, allowing the wire to bend without permanent deformation.
Nickel-titanium (NiTi) alloys are also used for fixed retainers, known for their superelastic qualities. They can be significantly bent and still return to their original shape. However, stainless steel remains the standard choice due to its high strength and established use in orthodontics.
Material Properties Essential for Retention
The selection of stainless steel alloys is based on properties that allow the retainer to function effectively over many years. Biocompatibility is primary, meaning the metal must be inert and safely tolerated by the body and soft tissues of the mouth. The chromium oxide layer significantly contributes to this by preventing the metal from interacting with saliva and tissue.
The material must possess high corrosion resistance to withstand the acidic and enzymatic environment created by food and oral bacteria. This resistance prevents metal ion release and device degradation. The wire also needs a high elastic modulus (stiffness) to effectively resist the forces that cause teeth to shift.
The wire must also have flexibility to allow for the minor movements teeth naturally undergo during chewing and speaking. The braided design provides stability and shape retention capacity while offering springiness. This ensures the retainer maintains corrected tooth positions without causing discomfort.
Alternatives for Patients with Metal Allergies
While stainless steel is widely used, its nickel content can concern patients with metal sensitivities. Nickel is a frequent allergen, and exposure from a fixed retainer can lead to localized oral irritation. Orthodontists offer several alternative options for patients with a known nickel allergy or who develop sensitivity after placement.
A primary alternative is a titanium-based wire. Titanium is highly biocompatible and nickel-free, making it a safe choice for allergic individuals. However, titanium wires may be softer or exhibit different mechanical properties than stainless steel, potentially making them more susceptible to deformation.
Other options include nickel-free alloys, such as cobalt-chromium based wires, or a fiber-reinforced composite (FRC) retainer. FRC retainers are non-metallic fibers embedded in a dental resin, offering a completely metal-free alternative. Less common options include custom retainers using materials like zirconium ceramics, which offer excellent mechanical strength.