Introduction
Adhesive systems (bonding agents) join resin composite to enamel and dentin, mainly by micromechanical interlocking of resin in the etched tooth surface, in some systems helped by a chemical bond. They allow minimally invasive cavities without retentive undercuts and seal the margins against microleakage, secondary caries and postoperative sensitivity.
Enamel, dentin and the smear layer
Enamel is about 96 % mineral by weight, dry and homogeneous, and bonds reliably. Dentin is only about 70 % mineral, with about 20 % organic matter (mainly type I collagen) and 10 % water; its tubules carry outward fluid and become wider and denser towards the pulp. Dentin is therefore wet and variable, and much harder to bond to.
The smear layer is a 1–5 µm layer of cutting debris (hydroxyapatite, denatured collagen, bacteria) left on every prepared surface; it also plugs the tubule openings as smear plugs. It lowers dentin permeability but is loosely attached, so a bond made on top of it is weak. Every adhesive therefore either removes it (etch-and-rinse) or dissolves and incorporates it (self-etch).