Adhesive dentistry gained considerable importance within the field of modern restorative dentistry and it is meanwhile established as a daily routine. Adhesion is the attraction between two surfaces - the adhesive and the adherend - of bonding capacity in very close contact due to intermolecular forces that act at a very small distance. The material able to bond two surfaces is called adhesive, and the material on which the adhesive is applied is called the adherent or substrate. Tight contact is the main factor for adhesion. A liquid-phase adhesive must fill the remaining spaces between surfaces as a guarantee for increased wettability. Ideally, the adhesion substrate provides high surface energy and the adhesive low viscosity (i.e. surface tension). Additionally, surface roughness is another important factor for adhesion resulting in increased surface area.

The ultimate goal of a bonded restoration is to attain an intimate adaptation of the restorative material with the dental substrate. This task is difficult to achieve as the bonding process for enamel is different from that for dentin. 

Enamel Vs. Dentin

Dentin is more humid and more organic than enamel. While enamel is composed of 98% hydroxyapatite (minerals) by weight, dentin contains a significant amount of water and organic material, mainly type-I collagen. In contrast to enamel, dentin has a different response to adhesion due to its structure and chemical composition, so bonding is much more difficult to achieve. While enamel is predominantly mineral, dentin is a vital tissue. The permeability of dentin depends on the number and diameter of the dentin tubules. The continuous flow of dentinal fluid leads to a permanent modification of the surface and dilution of the applied substances.

When tooth structure is cut with a bur or other instrument, the residual components form a microscopic layer of remnant debris, known as the smear layer, which is deposited on the dentin. It is a thin layer consisting of microcrystalline particles enveloped in a modified organic matrix, which results from the friction, vibration, and heat produced during preparation. The basic composition of the smear layer is hydroxyapatite and altered collagen with an external surface formed by gel-like denatured collagen. 

The smear layer looks like a porous, amorphous, and relatively flat structure with a thickness of between 0.5 and 5 μm. The morphology and thickness of the smear layer are determined to a large extent by the type of cutting instrument that creates it and by the site of dentin where it is formed. Diamond burs produce a rougher surface than carbide burs, and dry cutting produces greater smearing and roughness than wet cutting. 

The smear layer occludes the dentinal tubules and acts as a diffusion barrier that decreases dentinal permeability; it can be considered as an obstruction that plugs the entrance of the dentinal tubules and prevents resin from reaching the underlying substrate. 

Many adhesive techniques can now be applied to many clinical situations and achieve results that have only recently been made possible. Good cement adhesion to dental structures can decrease the risk of secondary caries and marginal leakage. The majority of studies in the field of dentistry assume that the resin‐enamel and resin‐dentin bonds are of a micromechanical nature mainly because chemical interactions have seldom been reported. Mechanical interlocking also enables the adhesive to penetrate into the microscopic or submicroscopic irregularities of the adhered surface of a substrate. This type of bonding is known as micromechanical adhesion. Distinguishing between purely chemical adhesion, a quality intrinsic to glass-ionomer cement, for example, and micromechanical adhesion, a characteristic of resin materials associated with an enamel-dentin bonding system, is important. 

Micromechanical adhesion exploits microscopic retention areas created on the dental surface and subjected to a surface treatment. If micromechanical adhesive cementation using a resin-based system is chosen, the clinician should create a dentin-resin hybrid layer after dental preparation to ensure dentinal isolation. Micromechanical adhesion represents a preservation method against pulpal irritations. References are going to be collected and mentioned... All the related topics are coming one after the other... Stay tuned