Dentin Adhesives: Essential Components to Successful Conservative Dentistry

The process of bonding restorative materials to tooth structure has undergone many changes and improvements since the introduction of tooth bonding by Bounocore in 1955.¹ In the beginning, only tooth enamel was bonded. Dentin bonding was avoided because it was thought that putting acid on dentin would devitalize the tooth. The problem with adhesion to dentin was directly related to the chemistry of the bonding agent; adhesives were chemically hydrophobic, which created an incompatibility with moisture-containing vital dentin. The early adhesives provided good bond strengths to enamel but much poorer bond strengths to dentin.

In the 1980s, Fusayama introduced the idea of etching dentin combined with hydrophilic monomers, which resulted in much higher bond strengths to dentin.² However, this generated a major concern about the effects etching might have on pulp. Subsequent research led to the successful use of the dentin etch-and-rinse technique for many years; however, postoperative sensitivity was a constant concern with this highly technique-sensitive procedure, especially as the practice of posterior composite resin placement increased.

To mediate these concerns, manufacturers developed self-etch materials, which did not require the pretreatment of an acid but used acidic monomers. Issues with this series of products had to do with the relatively lower strength of the acid and its ability to adequately etch enamel. This has led to the development of "universal" bonding systems, which can be used with or without prior application of an acid, usually around 32% ortho-phosphoric acid (H₃PO₄).

The balance of this article will review these three dentin bonding systems, but instead of classifying them by their somewhat confusing bonding "generations," the author will classify them as etch-and-rinse, self-etch, and universal dentin bonding systems. All bonding systems consist of three components: an acid for etching the enamel and dentin, which removes the smear layer from the dentin and creates a roughened surface on the enamel to allow for micromechanical retention; a primer, which is a bifunctional molecule that allows a hydrophobic material (ie, resin) to attach to a water-rich, hydrophilic surface (ie, dentin); and adhesive, which is light-cured and creates an interface to which the composite resin restorative material can attach. These three component parts can be used separately or be combined into one component.

During tooth preparation, a smear layer is created on the surface of the dentin and enamel. The smear layer, which is poorly attached to the tooth, acts as a type of bandage for the dentin and contains tooth remnants, bacteria, and possibly even miniscule parts of the bur used in the tooth preparation procedure. This layer has been the topic of a great deal of research with regard to what should be done with it. One option is to remove it. It cannot be effectively removed by rinsing or scrubbing, but it can be removed with an acid. Historically, phosphoric acid has been the most commonly used method to effectively remove the hydroxyapatite-collagen layer. One concern has been that over-etching the dentin may result in postoperative sensitivity. Another concern is the over-drying of the dentin, which can cause a collapse of the collagen fibers. This can lead to lower bond strengths because there is not enough surface area in the collagen fiber network to create a good micromechanical bond. Depending on the solvent there may be a need to moisturize the dentin surface before adhesive application. Used correctly, etch-and-rinse adhesives provide adhesion to dentin through a hybrid zone and to enamel through micromechanical retention.

These etch-and-rinse systems started out requiring three separate steps, which, though successful, made for time-consuming bonding. These systems have remained the standard to which newer systems are compared. Due to the tedious and technique-sensitive nature of using these systems, the different parts were combined in various ways. The etch was combined with the primer, or the primer was combined with the adhesive leaving a separate etching step.

Because of the complexity of the etching, rinsing, and bonding steps, manufacturers began to develop self-etch systems. In these systems the smear layer is incorporated into the bonding layer. A hybrid layer of dentin is formed when primers and bonding agents penetrate through the smear layer into the dentin, forming a resin-dentin zone similar to what is seen with etch-and-rinse adhesives. Self-etch bonding systems create a much thinner hybrid layer; however, the completeness of this layer is more important than its thickness. The self-etch systems combined all the bonding steps into one or two bottles. Some brands combine the acid and primer into one bottle with a separate adhesive bottle, while others package all three parts in one bottle. These systems are very popular in today's dental practices because of the reduced number of steps, which makes them less technique-sensitive; shortened step application times; and potentially less postoperative sensitivity issues.³

Self-etching adhesives have been classified according to their acidity, as strong (pH ≤ 1), intermediate (pH = 1.5), and mild (pH ≥ 2).⁴ Weak and mild self-etching systems demineralize dentin, superficially resulting is a thin hybrid layer, while strong self-etching systems demineralize dentin similarly to etch-and-rinse systems. Most single-step dentin adhesives are very hydrophilic so that they may interact with the underlying dentin. However, a water-permeable adhesive layer may be formed and thus compromise bonding performance.⁵ Some systems recommend the application be performed by scrubbing into the dentin,⁶ and some work more effectively with two applications. All require the evaporation of the solvent with a light application of air prior to light-curing.

When using self-etching systems, pre-etching enamel may enhance the bond strength and longevityof composite restorations to the level of that found with etch-and-rinse adhesive systems, and this may improve their overall performance in clinical use.⁷ However, if the etching includes the dentin, the bond strength may potentially be reduced.

The dictionary defines universal as "applicable or common to all purposes, conditions, or situations." In recent years, dental manufacturers have developed bonding systems that are called universal. The question is, are they truly universal? If one were to limit the definition of a universal bonding system to a single-bottle, no-mix adhesive system that can be used in total-etch, self-etch, or selective-etch mode depending on the specific clinical situation and personal preference of the operator⁸ then certainly progress is being made. However, if one were to factor in variables for direct and indirect restorations, manner of curing (eg, light-cure, dual-cure, or self-cure), and restorative material being bonded (eg, zirconia, porcelain, or metal), then some of these systems may not truly meet the definition of universal.

The addition of 10-MDP (methacryloyloxy-decyl-dihydrogen phosphate) to these newer systems makes them effective, as they bond both micromechanically and chemically to tooth structure. However, the more acidic the bonding system, the more problems there may be with certain resin cements. An additional bottle of bonding material may sometimes be needed to optimize effectiveness.

Problems can be associated with each method of dentin bonding. In self-etch systems, the acid may not be strong enough to effectively etch enamel properly. The etch-and-rinse mode is effective on enamel, but its use may lead to over-etching the dentin, incomplete removal of the acid, or incomplete infiltration of the dentin with adhesive. A universal system used in a selective-etch mode results in an effective etching of the enamel; however, it is extremely difficult to keep the etchant off the dentin, and this can lead to inadvertent over-etching of the dentin. Moreover, when etchant is rinsed from enamel, it may accidentally get on the dentin. It is critical not to over-dry the dentin and to evaporate all the solvent before light-curing the bonding agent.

Though dentin bonding systems are becoming increasingly simplified with fewer steps, dentists still must follow manufacturers' directions, adhere to the correct sequence of steps, observe the proper application times of each step, and, if necessary, use an extra bottle of bonding material to make the adhesive truly universal. Though fewer bottles are needed with newer systems, clinicians should understand the intricacies of each material that is being bonded. Not all tooth structure is the same, and not all material, be it composite, porcelain, zirconia, or metal, can be treated similarly.

James F. Simon, DDS, MEd
Professor and Director, Division of Esthetic Dentistry, University of Tennessee Health Science Center College of Dentistry, Memphis, Tennessee

James C. Ragain, Jr., DDS, MS, PhD
Associate Professor and Dean, University of Tennessee Health Science Center College of Dentistry, Memphis, Tennessee