Introduction
Adhesive systems bond composite materials to the tooth. Adhesive dentistry began in 1955, when M. Buonocore showed that resin bonds strongly to enamel treated with phosphoric acid. Bonding to enamel and to dentin works differently.
Bonding to enamel
Enamel is about 90% inorganic, so it bonds reliably.
- Etch: apply 20 to 38% phosphoric acid, usually 37%, for 30 seconds. This cleans the surface, removes the smear layer and creates micropores about 20 µm long.
- Rinse and dry: rinse for 30 seconds and dry for 15 seconds, until the enamel looks frosted.
- Apply enamel bond: this is a hydrophobic resin, such as Bis-GMA with TEGDMA. It flows into the micropores and polymerizes.
The surface of the adhesive stays uncured because oxygen inhibits it. This oxygen-inhibited layer has free double bonds that join covalently to the composite.
Bonding to dentin
Dentin is much harder to bond to. It is less than 50% inorganic, wet, and variable, and its smear layer is thicker and infected. In 1982, Nakabayashi showed that resin soaking into etched dentin forms a hybrid layer, in which polymerized resin surrounds collagen fibers.
This needs a primer, which has two ends: a hydrophilic part that wets the moist dentin, and a hydrophobic part that joins the adhesive. A simple primer contains:
- HEMA, 30 to 60%: a small hydrophilic monomer
- Methacrylic acid, about 18%
- Maleic acid, about 5%, to remove the smear layer
- Water or alcohol as a solvent