The term bonding is a little bit misleading from the standpoint that many things used in dentistry are bonded, that is they are actually fastened, “bonded” to the tooth. Brackets used in orthodontics are bonded to the enamel surface. Veneers, which will be talked about later, are bonded to the tooth. Some crowns can be bonded while some are cemented. Sealants are actually bonded to the enamel of the tooth. Generally speaking, when someone refers to having had their teeth bonded, they are referring to having had tooth colored material placed, bonded, to their tooth to close a gap or change tooth color. An advantage over veneers is cost, but the material may not be as color stable as a veneer and may have small imperfections, such as pits, develop over time as the material wears and exposes a bubble that was trapped. It is basically the same thing as placing a tooth colored filling after removing the decay from a tooth, just on the surface of the tooth, rather than in a hole in the tooth from the cavity.
For bonding to be successful, whatever is being bonded, there has to be some type of surface irregularity, usually microscopic, that a bonding agent can flow into and then harden, locking it into place. Tooth enamel, the outer hard part of a tooth can be etched, creating a frosty appearance similar to etched glass. Dentin, the inner hard part of a tooth can be primed, giving a microscopic roughened surface. Porcelain can be etched using a different etchant. Metals can be sandblasted with a very small particle size powder. Orthodontic brackets come with a roughened surface. When bonding a veneer or bracket, once the surfaces are ready to go, a thin layer of bonding agent is placed and cured and then a thicker (viscosity), tooth colored material is sandwiched between and cured (hardened). Some materials cure with a high intensity light, others are self-curing, some are both. When bonding to close a gap or replace a fractured corner of tooth, the tooth colored material (composite) can be added in layers, even using different shades to mimic natural tooth which is usually lighter at the incisal (biting) edge. Once placed and cured, composite can be ground on to finalize its shape and then polished to appear smooth and shiny just like enamel.