Brief history:

The first attempt at tooth bonding was made by Hagger who developed an adhesive system in 1951 based on glycerophosphoric acid dimethacrylate (GPDM) which was able to penetrate the dentin surface and he used it to seal the margin and stick to the cavity walls. It is associated with a self-curing acrylic resin for crown restorations.

But the modern era of adhesive dentistry was initiated in 1955 by Buonocore when he suggested that tooth enamel should be treated with phosphoric acid (at an initial concentration of 85%) in order to improve its bonding with the restorative resins. 
Acid etching became really useful a few years later owing to its high efficiency to increase the bond strength with the composite resin, although it has been subject to some changes in principles or techniques, such as decreasing the phosphoric acid concentration from 85% to 30-40% and the etching time from 40-60 to 15-20 seconds, as well as the use of etching products in the form of gel, etc.

Enamel adhesives

Enamel bonding is, clinically speaking, the most significant. Therefore, the central rule in cavity preparation for adhesive restoration is to have as large enamel surfaces as possible for adhesion. Enamel is not isotropic. Thus, the optimum utilization of enamel walls is possible when they are cut transversely during the beveling of cavosurface margins. The transverse sections provide significantly higher bond strength than the longitudinal sections. With enamel prisms being cut longitudinally, the adhesive may only penetrate into the laterally loosened enamel parts. The necessity of enamel bevels is of great importance, especially in posterior teeth.

Several studies have shown that the acid-etching step is particularly critical to the formation of the mechanical bond between the resin and the enamel surface through what are so-called "resin tags". Resin tags guarantee micromechanical interlocking. Another way in which micromechanical interlocking is guaranteed is by intercrystallite retention.

The most common acid used in dental restoration is phosphoric acid. Phosphoric acid products are normally provided as a gel in concentrations of 35-40% applied for 15-30 s followed by rinsing with air/water spray to remove both acid and precipitates formed during acid etching. The ideal mix is 37% and 30 seconds.

Phosphoric acid irreversibly removes about 10 μm enamel with a roughness of 50 μm underneath, the so‐called etch pattern. The retentive etching pattern provides high surface energy allowing good wettability of etched enamel. To guarantee successful wetting of the etched enamel surface, appropriate isolation has to be obtained during treatment to avoid contamination with blood, saliva, sulcus fluid, or oil. Any contamination impedes penetration of low‐viscosity adhesives into the retentive surface and corroborates retention.

Acid etching increases the surface energy of the enamel, which brings about a decrease in the contact angle between the liquid adhesive and the enamel surface from 22-23 degrees to 5-6 degrees, namely an increase in wettability. As compared to dentin, the prevailing mineral structure of the enamel determines its lower permeability to liquids. The structure of hydroxyapatite favors the chemical bonding of composite restorations to the etched enamel through a liquid resin with high wettability. 

Following the preferential and partial dissolution of prism hydroxyapatite crystals, the resultant etch pits will be filled with adhesive fluid components. Resin tags up to 25 μm in length and 6 μm in diameter are formed into the microporosities of the conditioned enamel. After adequate polymerization, they provide a long-lasting bond by mechanical interlocking. In this way, due to enhancing the contact surface with the adhesive, the formed resin tags significantly improve bond strength.   Prismless enamel is detectable only in very outer and not abraded layers. This layer is typically found on the cervical surfaces and within pits and fissures. This aprismatic enamel layer (averaging 30 microns) may be found at the surface of newly erupted permanent teeth and is always found at the surface of newly erupted primary teeth. It is more mineralized (absence of organic prism boundaries) and will not yield good bond strength unless slightly ground with a diamond bur or etched more heavily. Etching time has to be prolonged to 60 s in order to dissolve prismless enamel areas.  Shorter etching times are possible on prepared enamel. Even though aprismatic enamel can be viewed as a natural-acid-resistant barrier to caries, its removal is recommended for improving bonding.

Another important tip is to avoid rubbing the etched enamel with microbrushes during the application of the adhesive resin or even while priming the dentin to avoid any destruction to the fragile etch pattern.

The etch-and-rinse adhesive techniques on enamel require two main steps; the first step is etching the surface with acid and the second step is to apply the bonding agent and use a cure light to polymerize the resin, in situ, directly inside the treated surface.

To be continued...