A generational look at adhesives

The take-aways

• Four different types of adhesive systems are available.

• Each category has its unique advantages and disadvantages.

The concept of acid-etching enamel for resin bonding was introduced by Buonocore in 1955.¹ Although the concept was not widely accepted for almost 25 years, we now have more than three decades of widespread clinical experience proving enamel bonding is both predictable and durable.

Dentin is a more difficult and less predictable substrate for resin bonding because of its inherently complex and fluid-filled structure. Over the years, several “generations” of dentin bonding systems have been developed and four such generations can be identified as current options.²

Two major strategies exist for resin bonding to tooth structure: etch-and-rinse (also called total-etch) and self-etch. For each of the two major strategies, specific products use either a somewhat complex or a simplified approach. Thus, four different types of adhesive systems are available.

Three-step etch-and-rinse

The first group of systems has been available for almost 20 years and these materials can be classified as three-step, etch-and-rinse adhesives. These systems use phosphoric acid to etch both enamel and dentin. Acid-etching not only microscopically roughens the enamel, but also removes the smear layer from and demineralizes the superficial dentin.

Etching is followed by priming with special resin monomers carried in a solvent such as ethanol, acetone or water. Priming alters the surface free energy of dentin and initiates resin infiltration of the porous etched surface. The third and final step is application of a fluid bonding agent.

This etch-prime-bond sequence results in the formation of a resin-dentin interdiffusion zone or hybrid layer, a physical combination that links polymerized resin to the dentin.³

Laboratory and clinical studies have reported excellent results with the three-step etch-and-rinse materials.^4-6 Because of their versatility, ability to bond well to both dentin and enamel, and proven clinical performance, these materials are considered by many experts to be the “gold standard” for resin-based adhesive materials.⁷

One-bottle etch-and-rinse

A desire for simplification of the three-step approach resulted in the development of one-bottle, etch-and-rinse adhesives. As etch-and-rinse materials, these products include the same phosphoric acid etchant as the three-step materials. They are described as “one-bottle” systems because they deliver the primer and bonding agent steps in a single solution. This approach is not unique to dentistry, as readers may be familiar with some of the primer and paint combinations currently being sold at home improvement stores. (Although it should be noted, the one-bottle dental adhesives preceded the one-can paints by many years!)

As etch-and-rinse materials, these products provide an excellent bond to enamel. However, the long-term stability of their dentin bond is not as good as that of the three-step systems, and their clinical performance has suffered as a result.^4,5,8 Dentin bonds are particularly susceptible to degradation when they are not protected from the environment by a rim of etched and appropriately bonded enamel.⁹

Because they involve fewer steps and are easier to use than the three-step systems, these materials have been quite popular over the years and have probably performed better in the “real world” than one might expect based on laboratory studies and clinical trials. Anecdotally, the biggest problem with the one-bottle adhesives has been an undesirable incidence of post-operative sensitivity. However, proper technique-including application of a resin-modified glass ionomer liner and maintaining a moist surface after etching-can greatly reduce post-operative sensitivity.¹⁰

It should be noted that many products in this category cannot be used with self-cure or dual-cure composites. Chemical incompatibilities between some adhesives and self-cure/dual-cure composites can result in clinical problems such as debonding of cores.¹¹

Two-step self-etch

The third group of resin-based adhesives is the two-step self-etch materials, or the self-etch primer systems. These products use primers containing special acidic monomers to simultaneously etch and prime the tooth surface.¹² The acidity of most self-etch primers is fairly mild, which is a potential advantage for dentin bonding because it might result in less post-operative sensitivity. Although clinical trials have not been able to prove that self-etch primers actually do result in less post-operative sensitivity,¹³ much anecdotal evidence suggests that this is true.¹⁴

Bonding to dentin is mechanical in nature for all of the resin-based systems, but an additional chemical bond has been demonstrated for one of the self-etch primer systems, Clearfil SE Bond. Because of its mildly acidic nature, some hydroxyapatite remains in the demineralized superficial dentin and the primer’s functional monomer, MDP, forms a chemical bond with ^it.15 This supplemental chemical bond helps to explain the material’s durable dentin bond and excellent clinical performance over years of service.^16,17

Although mild acidity is advantageous for dentin, it is problematic for enamel. Mechanical roughening of the enamel can improve the bond of self-etch primer materials.¹⁸ However, this is not to the same level as that of etch-and-rinse materials. For that reason, some experts are recommending a selective etch of enamel margins before applying the self-etch primer.¹⁹

All-in-one self-etch

The fourth group of adhesives is the self-etch adhesives, or all-in-one adhesives, which deliver the essential etching, priming and bonding functions simultaneously. Some of these products require mixing of components and others do not. These materials have become very popular in recent years.

Despite their widespread popularity, the all-in-one adhesives have the least proven clinical performance. The earliest products had poor enamel or dentin bond strengths or both.²⁰ They are hydrophilic in nature, which could result in more rapid degradation in the oral environment.²¹ In fact, some studies have shown these materials work best when coated with a hydrophobic bonding agent.²² Of course, adding this step means they are no longer acting as all-in-one materials and could be considered as self-etch primers.

Fortunately, these materials do seem to be improving, and laboratory studies are reporting higher bond strengths, but still not comparable to other systems.^23,24 However, given the newness of most products and availability of a few clinical trials,^{25, 26} extensive long-term evidence of clinical performance is lacking.

Which to choose?

Where does the adhesives market stand today? Each category has its unique advantages, but unfortunately, disadvantages as well. For example, the three-step etch-and-rinse systems are considered the gold standard for bonding, but market data indicate that clinicians prefer simpler products. Another example would be the self-etch primer systems, which might be considered optimal for bonding to dentin, but provide a less-than-ideal bond to enamel and is limited to use with direct restorations because of film thickness. Despite the scientific advances that have occurred in bonding and the wide variety of options available to dentists, there remains room for improvement.

New advances

One such improvement might be occurring now, with the introduction of a new self-etch primer system called OptiBond XTR (Kerr). This product includes a new self-etch primer that contains the GPDM adhesive monomer that Kerr has used in its materials for years, other monomers and a water/ethanol/acetone solvent designed to enhance etching and dentin penetration. The primer’s pH has been measured as 1.6. This product also includes a unique filled adhesive that has increased hydrophobic characteristics to maximize bond compatibility with all dual-cured/self-cured composites, resin cements, and core-materials. Its low film thickness (~5 microns) make it suitable for use with indirect procedures. The bonding agent adhesive contains HEMA, Bis-GMA and other monomers, filler particles and ethanol solvent.

In lab testing, OptiBond XTR provides an excellent bond to dentin (Figs. 1A and 1B) and enamel. The primer performs well even on uninstrumented enamel (Fig. 2) compared with enamel conditioned with a self-etch primer (Fig. 3).

In our lab, we have found that bond strengths are equivalent to those of an etch-and-rinse gold standard on enamel and those of a self-etch gold standard on dentin. Although bond strengths measured in the lab 24 hours after specimens are fabricated cannot predict clinical performance with 100% certainty,27 these results suggest that OptiBond XTR will provide excellent clinical results in bonding to both enamel and dentin. Finally, according to the manufacturer, XTR can be used with both direct and indirect restorations. 

About the author

Dr. Ed Swift is Professor and Chair of the Dept. of Operative Dentistry at the University of North Carolina School of Dentistry, Chapel Hill, N.C. He is actively involved in dental materials research, particularly in the area of dentin bonding and other aspects of adhesive and esthetic dentistry. He maintains a part-time intramural practice devoted to restorative and esthetic dentistry. He can be reached at 919-966-2773 or at swifte@dentistry.unc.edu.

Conflict of Interest Disclosure

Dr. Swift has conducted a research study funded by Kerr Corp. and has received an honorarium for writing this article.