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. While enamel is composed of 96% hydroxyapatite (mineral) by weight, dentin contains a significant amount of water and organic material, mainly type-I collagen. Dentin is more humid and more organic than enamel. This humid and organic nature of dentin makes bonding to this hard tissue extremely difficult.

When the tooth structure is cut with a bur or other instrument, the residual components form a ‘‘smear layer’’ of debris on the surface. This debris forms a uniform coating on enamel and dentin and plugs the entrance of the dentinal tubules, reducing the permeability of dentin. As the smear layer constitutes a true physical barrier, it must be dissolved or made permeable so the monomers in the adhesives can contact the dentin surface directly. In spite of different classifications of adhesive systems, the current adhesion strategies depend exclusively on how dental adhesives interact with this smear layer. 

  • One strategy involves etch-and-rinse adhesives, which remove the smear layer and superficial hydroxyapatite through etching with a separate acid gel. 

  • The second strategy involves self-etch adhesives, which make the smear layer permeable without removing it completely.

Etch-and-rinse bonding strategy

This technique involves the application of the acid etchant (35–40% phosphoric acid gel) as an initial separate step followed by substrate rinsing and water removal. The acid penetrates along the dentinal tubules that have been opened by its effect. This results in the complete removal of the smear layer and demineralization of the most superficial hydroxyapatite crystals. The demineralization depth is about 5 μm. The next steps consist of applying the primer, which is the diffusion-promoting agent indispensable to achieving adhesion, and finally, the adhesive resin, which is the bonding agent. 

The duration of phosphoric acid etching leads to different demineralization depths being additionally dependent on acid agitation during application. In general, an application time of 15–20 seconds is recommended. Prolonged etching times lead to deeper demineralization and possibly suboptimal infiltrated areas.
This technique was previously called “total etching”  because the entire cavity was etched by acid. However, more accurate is the term “etch and rinse” because self‐etch adhesives also etch totally, emphasizing that phosphoric acid is removed by rinsing. The invariable and characteristic step is the compulsory and separate acid etching followed by rinsing. 

Thus, the etch-and-rinse strategy involves three separate steps (3 E&R), or two (2 E&R) in the case of the simplified version when the primer and the bonding agent are mixed.

Enamel bonding with E&R adhesives:

The approach which basically requires enamel etching by acid is still the most effective for achieving solid and stable bonding to enamel. Acid etching of hydroxyapatite crystals produces microporosities on enamel, where the resin absorbed by capillary attraction is polymerized in situ and envelopes the remaining exposed crystals. The adhesive infiltrated within the prismatic structures of the enamel, interlocks with the etch pits in two ways, forming macro-and micro resin tags. The macro tags fill the space surrounding the enamel prisms, while the more numerous micro tags, which result from resin infiltration and polymerization within the small etch pits at the core of the etched enamel prisms, play a major contributory role in the micromechanical bonding between adhesive and enamel. This intimate micromechanical entanglement of resin monomers with etched enamel results in a better marginal fit.

Dentin bonding with E&R adhesives:

The phosphoric-acid treatment exposes the collagen network that is nearly totally “depleted” of hydroxyapatite crystals, thus displaying numerous microporosities within which nearly all calcium phosphates have been dissolved and removed. Consequently, the primer is applied to the etched dentin. Following this chemical etching, a mixture of resin monomers (primer/adhesive) dissolved in an organic solvent is applied to infiltrate the etched dentin. Consequently, the primary bonding mechanism of etch-and-rinse adhesives to dentin is first and foremost diffusion-based and depends on the infiltration of resin within the exposed collagen fibril scaffold.  The resin monomers permeate the water-filled spaces between adjacent dentin collagen fibers that used to be occupied by hydroxyapatite crystals. This infiltration results in a hybrid layer composed of collagen, resin, residual hydroxyapatite, and traces of water. This intimate micromechanical entanglement of resin monomers with etched dentin may result in decreased postoperative sensitivity, may make for a better marginal fit, and may even act as an elastic buffer that compensates for the polymerization shrinkage stress during contraction of the restorative composite.

This technique involves delicate technical issues and there is a risk of errors during application and manipulation of the substances, which could lead to either deterioration of the hybrid layer, resulting in loss of marginal seal integrity (opening the way for nanoleakage), or postoperative pain, because of the dentinal fluid movement within dentinal tubules.

The main drawback of this technique is the significant delay between the moment when the microporosities are formed by acid etching and the moment when the adhesive resin infiltrates these gaps. In this time period, the conditions for filling the etch pits by diffusion must be created and maintained. 

Self-etch bonding strategy

The latest development in dental adhesion is based on simplification and reduced application time. The self-etch (non-rinsing) adhesives do not require a separate acid-etch step as they condition and prime enamel and dentin simultaneously by infiltrating and partially dissolving the smear layer and hydroxyapatite to generate a hybrid zone that incorporates minerals and the smear layer. Clinically, the self-etch bonding strategy is the most promising regarding user-friendliness and the reduction of technique sensitivity. In such an adhesive system, etchants are mixed with the primer. As no separate acid etching step followed by rinsing and drying is required, the clinical application time is reduced, and the risk of errors during application and manipulation is lowered. The self-etch approach involves two steps or a single step in the case of the simplified version when the primer and the bonding agent are mixed together.  

The first self-etch adhesives were composed of two solutions, an acidic primer, and a bonding resin. Recently, many clinicians have shifted to one-step self-etch systems (all-in-one adhesives) in which manufacturers have attempted to incorporate all the primary components of an adhesive system (etchant, primer, and bonding resin) into a single solution bottle. Both the acidic primer of two-step self-etch adhesives and the all-in-one solution are composed of aqueous mixtures of acidic functional monomers, generally phosphoric-acid or carboxylic-acid esters, with a pH higher than that of phosphoric acid gels. Water is an essential component of self-etch adhesives as it participates in the ionization of the acidic moieties.

All-in-one adhesives are user-friendly in that fewer steps are required for the bonding protocol. What is peculiar to this technique is the fact that the mandatory separate etching stage followed by rinsing is no longer present. The elimination of separate etching and rinsing steps simplified the bonding technique, making these systems more popular in daily practice.

As the infiltration of resins occurs simultaneously with the etching process in this bonding strategy, the problems related to the delay between the emergence of etch pits following acid etching and monomeric resin infiltration within the gaps are largely diminished (if not completely removed). On the other hand, there is still the question of the long-term effects of incorporating dissolved hydroxyapatite crystals and residual smear-layer remnants within the bond. At the same time, it is important to know how much of the self-etching primer/adhesive solvent is retained within the interfacial structure. A solvent surplus will directly weaken the bond integrity (diminishing the bond strength at the substrate) and provide channels for nanoleakage, or it may inhibit the polymerization of the infiltrated monomers. Another significant aspect is related to the fact that the resultant interfacial structure is much more hydrophilic and, consequently, more prone to hydrolytic degradation. Because self-etch adhesives are not as aggressive as the phosphoric acid gel in etch-and-rinse adhesives, most do not remove the smear layer. 

Enamel bonding with SE adhesives:

The enamel bond strengths of the earliest self-etch adhesives were lower than those associated with adhesives that rely on a separate etching step. Because of their higher pH, two-step self-etch adhesives (2 SE) result in shallower enamel demineralization compared with that of the phosphoric acid. Nevertheless, roughening of enamel to remove prismless enamel improves the enamel-bonding ability of self-etch adhesives. A separate phosphoric-acid enamel-etching step (selective etching) also enhances the efficacy of self-etch adhesives. 

Two-step self-etch adhesives bond at an acceptable level to normal dentin and to ground enamel in vitro. Conversely, two-step self-etch materials may not bond as well to intact enamel and sclerotic dentin.

All-in-one self-etch systems (1 SE) are not as acidic as the phosphoric acid used with the etch-and-rinse adhesives. This characteristic has raised concerns about the performance of all-in-one self-etch systems on intact enamel. Several studies have reported low resin-enamel bond strength of all-in-one self-etch materials. Grinding the enamel during a bevel or cavity preparation, for instance, makes the substrate more receptive to bonding with all-in-one self-etch systems. Despite the increased popularity of self-etch adhesives, etching with phosphoric acid is still considered the gold standard and the benchmark against which new materials are tested. Self-etch adhesives resulted in severe enamel microleakage following thermal stresses. 

Dentin bonding with SE adhesives:

In spite of their user-friendliness and low technique sensitivity, all-in-one adhesives have resulted in low bonding effectiveness in vitro while their clinical reliability has often been questioned. Another drawback associated with all-in-one adhesives is their behavior as semipermeable membranes because of the high hydrophilic nature of the acidic monomers and the high water concentration required for ionization of the acidic monomers. These materials allow the movement of water across the bonded interface, which potentially leads to hydrolytic degradation.

An inadequate resin penetration into tooth substrates may result in accelerated degradation of the structure of the bonding interface. As polymerization shrinkage stresses the bonding interface, dentin adhesives that do not resist these stresses result in low bond strengths, marginal gaps, recurrent caries, and pulpal irritation.

For some all-in-one adhesives, performance may depend on the application method, as the number of coats recommended by the manufacturer may not suffice. Application of the all-in-one adhesive in multiple layers may result in higher bond strengths and better infiltration into the hybrid layer. One manufacturer recommends rubbing the adhesive continuously for 15 seconds, followed by the application of a second coat after gentle air-drying and curing the first coat. This second coat prevents the formation of dry spots on the dentin surface and may result in better impregnation of the monomers into the hybrid layer.

Low enamel and dentin bond strengths have been reported when acetone-based all-in-one adhesives are applied as per the manufacturer’s directions. The magnitude of dentin bond strengths depends on the degree of infiltration of the resin monomers into the collagen pretreated with an acidic conditioner or with phosphoric acid. It is known that hybrid layers are particularly susceptible to degradation when the cavosurface margins are not in enamel. The degradation of the dentin bonding interface is caused by the availability of exposed collagen fibrils at the base of the hybrid layer or by hydrolytic degradation of resin components in the hybrid layer. Water can also infiltrate and plasticize the resin matrix, which decreases the mechanical properties of the polymer.

Porosities (or blisters) that occur at the enamel and dentin bonding interfaces may play a role in the weak bonding performance of acetone-containing all-in-one adhesives because most all-in-one adhesives behave as semipermeable membranes. These porosities may be a result of water accumulation either caused by an osmotic gradient or by monomer-solvent phase separation upon evaporation of the acetone. The number and size of these blisters may also depend on the intensity of the air-drying step. High hydrophilicity, and consequent higher water sorption, have been reported associated with another acetone-based all-in-one adhesive containing.

To be continued...