Etching the dental tissues with an acidic solution is a standard clinical procedure that results in the demineralization of the superficial layer of enamel and dentin. Different acids have been proposed over the years: phosphoric acid (from 10% to 50%), fluoridated phosphoric acid, pyruvic acid, citric acid, maleic acid, oxalic acid, tannic acid, EDTA, trichloracetic acid, and polyacrylic acid, among others.^1-6

Recent studies have shown that the pH and the pKa of the acid solution are important parameters that influence the aggressiveness of the acids and their ability to demineralize the surface.^7,8 The physical status of the solution (either gel or liquid) is an important parameter. The gel type etching agent is easier to apply on the enamel surface than the liquid form, and it is reported to produce a wider and deeper enamel penetration.⁸ As to methods of application, a continuous brushing technique may be useful when applying an acid-etching gel to obtain a more defined etched pattern for an improved marginal adaptation of the composite resin restoration.⁹

Etching of the enamel results in 3 types of patterns.¹⁰ A preferential removal of prism core material, leaving the periphery intact, constitutes the Type I etching pattern (Figure 1B). Type II is defined as a preferential removal of periphery core material, leaving the prism core relatively unaffected (Figure 1A). The Type III pattern represents a more random etching morphology in which adjacent areas of the tooth surface correspond to Types I and II, mixed with regions in which the pattern could not be related to prism morphology (Figure 2).

The consequences of etching dentin are different from etching enamel. Acids remove the superficial mineral dentin to expose the organic matrix, in particular, the collagen dentin network. The characteristic collagen banding is formed by a longitudinal overlapping of the molecules that involve approximately one quarter of the length of the fibril monomer, leaving a space between the extremity of one collagen triple helix and the beginning of the next.¹¹ This “hole” has been considered a site for accumulation of hydroxyapatite crystals inside the collagen molecule.³³ Collagen banding has been also reported in the peritubular region.¹² The lack of collagen banding in some areas of etched dentin may be caused by the aggressiveness of the etchant, since dentin collagen, after demineralization, is susceptible to proteolytic degradation.¹³

In spite of assumptions that etching dentin results in collagen denaturation and precludes the establishment of durable bonding, it has never been validated, nor has it been proven that changes in collagen ultrastructure are detrimental for the performance of dentin adhesives. The results from a recent immunocytochemical study provided additional evidence that application of phosphoric acid for 15 seconds results in a mineral dissolution of the crystals, enveloping the superficial collagen fibers without damaging the collagen ultrastructure.¹⁴  

Other molecules of the dentin matrix that might have an important role in adhesion are the proteoglycans (PG). The complex molecular conformation of the collagen fibrils network that is exposed by the etching agent involves the presence of lateral chains of proteoglycans that have a fundamental role in stabilizing the fibrillar arrangement.¹⁵ Proteoglycans are important components of the extracellular matrix of all human connective tissues; keratan sulfate (KS) and chondroitin sulfate (C4-S and C6-S) are present in predentin, dentin, and cement.^15,16 Due to the ability of PG to bind water molecules,¹⁷ PG regulate the biophysical properties of the collagen fibrils and are able to imbibe a great amount of water. This peculiarity is transferred to the nonmineralized connective tissues as a high resistance to a load force.¹⁸ On the other hand, the presence of the collagen fiber in the connective dentin matrix gives the typical high resistance to traction forces. Collagen fibers and PG interact closely, creating a complex network of fibrillar structures that act as the scaffold of the dentin matrix and react with the adhesive agent during infiltration.¹⁹ The role of proteoglycans is crucial, as water is fundamental in preventing collagen collapse after etching.

DENTAL ADHESIVES: CURRENT CONCEPTS

A group of simplified dentin adhesive systems has been recently developed and received considerable attention from clinicians, due to a simplified clinical application. In these one-bottle dental adhesive systems, enamel and dentin are etched with phosphoric acid simultaneously, followed by the application of a single solution that combines primer and adhesive resin from the early generation. These simplified systems contain hydrophilic (ie, HEMA, PENTA, BPDM, or PMDM) and hydrophobic resins (ie, BisGMA, UDMA), dissolved in high vapor pressure solvents, such as acetone and ethanol. The solvents may be able to displace water from the dentin surface and from the moist collagen network and allow the monomers to intermingle with the filigree of collagen fibers to form a “hybrid layer” (Figure 3).^20,21 However, the idea that organic solvents lead the monomers into direct contact with the moist collagen fibers and prevent their collapse has never been substantiated. In fact, some organic solvents may actually cause shrinkage of moist demineralized dentin and interfere with the penetration of resin monomers into dentin.²²

The demineralized dentin matrix collapses when dentin is air dried following etching.²³ When the “wet-bonding” technique²⁴ was first considered as a standard procedure associated with total-etch, the moist dentin surface was deemed as the ideal bonding substrate, and most manufacturers recommend it.²⁵ The concept behind it is to obtain high bond strengths in vitro, supporting the idea that moist dentin provides a porous collagen network that allows infiltration of adhesives into the nanospaces between adjoining collagen fibers(Figures 4 and 5).^26-28 The immediate high-bond strengths obtained, decrease when the specimens are stored in aqueous environments.^29,30

The demineralized dentin matrix collapses easily in vitro when it is air-dried after being rinsed with water.^23,31-33 After dissolution of the hydroxyapatite crystals by the acid, it is essential to keep dentin moist, so that air-drying and consequent collapse of the fibrillar structure of the collagen scaffold is prevented, in vitro.^27,28,30-33  

Air-drying of etched preparations was taught in dental schools as a method to check for etched aspect of enamel. Some clinicians still dry the preparation after rinsing the etching gel. As a result of air-drying enamel, dentin is also dried, which causes dentin collagen to collapse resulting in the closing of the pores in intertubular collagen in vitro. Re-wetting dried dentin with water raises the collapsed collagen to a level compared to a “wet bonding” technique and restores the bond strengths when dentin is re-wet for twice as long as the time spent with drying.^27,34,35