The adhesive used to secure orthodontic braces to teeth is not a simple household glue but a specialized dental material engineered for a unique biological environment. This substance is designed to create a strong, temporary bond with tooth enamel, resisting the constant forces of chewing and orthodontic pressure over a period of months or years. Unlike permanent dental fillings, the adhesive must also be safely removable at the end of treatment without causing lasting harm to the tooth surface. The material must possess specific properties, including high bond strength, biocompatibility, and the ability to set rapidly under clinical conditions.
The Composition of Orthodontic Adhesives
The materials used for bonding orthodontic brackets are broadly categorized as resin-based composites or glass ionomer cements. Resin-based composites are the most widely used materials, chosen for their superior strength and ability to be quickly set using light. These composites are a mixture of an organic resin matrix and inorganic filler particles. The resin matrix typically contains monomers like Bisphenol A-Glycidyl Methacrylate (Bis-GMA) and Triethylene Glycol Dimethacrylate (TEGDMA).
The inorganic fillers, which can include silica, barium silicate, or strontium silicate, are finely ground glass or ceramic particles suspended within the resin. These fillers are what give the adhesive its strength and durability, preventing it from fracturing under the constant stress of orthodontic mechanics. The ratio and size of these filler particles are controlled to optimize the material’s handling and mechanical performance.
An alternative is the use of glass ionomer cements (GICs), which are created from a reaction between polyacrylic acid and fluoroaluminosilicate glass powder. Resin-modified GICs are often used to combine the chemical bonding and fluoride-releasing properties of GICs with the enhanced strength and faster setting time of a resin component. All materials are formulated to be biocompatible, though concerns exist regarding the potential for trace amounts of unpolymerized monomers to leach out, which is mitigated by thorough light-curing.
Achieving a Secure Bond
Securing a bracket to the tooth is a precise, multi-stage process. The first critical stage is the preparation of the tooth enamel, which involves the use of a mild acid, typically 30% to 40% phosphoric acid. This acid is applied for a short period to selectively dissolve mineral content from the enamel surface. This process, known as etching, creates a microscopically porous surface that resembles a frosted pattern without damaging the tooth.
These microscopic pores are the foundation of the mechanical retention, as they allow the subsequent liquid materials to penetrate and form tiny resin tags. Following the acid etch, the tooth surface is thoroughly rinsed and dried with oil-free air to prevent contamination from saliva or moisture. A liquid primer, or sealant, is then applied to the conditioned enamel, acting as a bridge between the etched tooth surface and the main adhesive material.
The adhesive paste is placed onto the back of the bracket and the bracket is positioned accurately on the tooth. The final step is polymerization, or curing, which is typically achieved by exposing the adhesive to a high-intensity visible light source, often a blue light. This light activates a photoinitiator chemical within the resin, causing the monomers to link together and harden rapidly, locking the bracket securely in place through the mechanical interlock created by the resin tags in the etched enamel.
The Safe Removal of Adhesives
The removal of orthodontic appliances requires a careful procedure to protect the underlying enamel. The bracket itself is typically removed using specialized debonding pliers that apply controlled pressure to the bracket base. This pressure fractures the adhesive bond, allowing the bracket to be lifted away from the tooth surface. The goal is for the bond to break primarily at the interface between the bracket and the adhesive, leaving the majority of the cured resin on the tooth.
Once the bracket is off, the cured adhesive residue must be completely removed without compromising the enamel surface. This is generally accomplished using a low-speed dental handpiece fitted with a specialized rotary instrument, such as a tungsten carbide bur. Tungsten carbide burs are preferred because they efficiently shave away the hard resin material while causing the least amount of roughness to the enamel.
The final phase of the process involves polishing the tooth surface to restore its original smooth texture. Fine-grit polishing discs, rubber cups, or pumice slurry are used to remove any remaining trace of adhesive and eliminate the microscopic scratches created by the bur. This finishing process ensures the tooth is left smooth, reducing the likelihood of plaque accumulation and restoring the natural appearance of the enamel after orthodontic treatment.