Dental adhesives explained

A dental resin filling materials syringe, etching enamel gel, and adhesive bonding system for restorative dentistry. NENAD/ STOCK.ADOBE.com

When it comes time to choosing a dental adhesive, you need to balance ease of use, technique, strength and efficacy. But with so many dental adhesive options on the market today, it can be hard to keep them all straight. Which ones boast the strongest bonds? Which ones are the easiest to use? How many bottles will you need? And how many steps is it going to take to complete a restoration?

It all depends on which generation of dental adhesive you’re using. Whether it be the total-etch and no-etch systems of the fourth and fifth generations, or the self-etch advancements offered by the sixth generation and beyond, it’s important to understand what each generation brings to the table—and what it has to offer in your practice.

First generation

First-generation materials debuted in the 1950s, when Dr. Michael Buonocore identified a lack of adhesion to tooth substrates as a major culprit in restoration failure. In 1955, he proposed the use of acid etching for bonding, demonstrating that resin containing glycerophosphoric acid dimethacrylate (NPG-GMA) could bond to acid-etched dentin, greatly improving the adhesion of filling materials to enamel.1

While this discovery was groundbreaking, initial bond strength was underwhelming (ranking only in the 1-3 MPa range), reduced even further by exposure to the moist environment of the mouth. So, while these initial attempts were largely unsuccessful, they must be lauded for laying the groundwork for future innovation and the dental adhesives we know and use today.

Second generation

The 1970s ushered in an effort to improve on the first generation of adhesive materials. To accomplish this, polymerizable phosphates were added to bis-GMA resins to increase bond strength through the creation of an ionic bond to calcium in the tooth.2 

But much was still unknown about dental bonding. The smear layer had not yet been discovered, resulting in unreliable bond strength. While small improvements had been made (second-generation adhesives had a slightly increased bond strength of 4-6 MPa), the bonds would still degrade quickly when exposed to moisture (which again, the mouth is full of).

Until the smear layer was addressed, improvements in bonding could only be minimal. Luckily, in the late 1970s, Dr. David Eick identified the smear layer and its detrimental effects on bonds, ushering in the third generation of materials and rendering second-generation adhesives essentially obsolete.

Third generation

In the late ’70s and 1980s, removing the smear layer became the critical goal of bonding systems. Thanks to Eick’s discovery of the smear layer, acid etching the dentin was introduced in an attempt to remove (or at least disrupt) this coating of organic and inorganic material created during cavity preparation.

The introduction of acid etching was not well received by all dentists and became controversial, as many clinicians believed that dentin shouldn’t be etched. Researchers were on the right path, however: Acid etching served to open the dentin tubules allowing smear layer particles to be more efficiently removed. After the acid was rinsed, a primer was applied, and unfilled resin was placed on the dentin and enamel. Unfortunately, despite a raised bond strength of 12-15 MPa, many of the frustrating bonding issues of prior generations remained unsolved, as the smear layer resisted penetration by most unfilled resins.3

Fourth generation

Enter the fourth generation of materials, that changed the bonding game permanently. It took four generations to get there, but in the 1980s researchers finally discovered a way to effectively remove the smear layer.

But this success didn’t come easily. While these adhesives produced unprecedented bond strengths in the low- to mid-20s MPa when implemented correctly, correct implementation wasn’t simple.2 The numerous application steps and the use of multiple bottles in these complex adhesive systems left abundant opportunities for error and contamination, which could disrupt bond strength.

However, the efficacy of fourth-generation materials has made them long-lasting standbys for many clinicians. Familiar names like OptiBond (Kerr), Clearfil (Kuraray America) and All-Bond (BISCO) emerged—the predecessors of products commonly used today, thanks to their reliability and track record of excellent clinical results.

“The adhesive materials with the longest evidence of clinical success are definitely the two-bottle systems—either the fourth-generation etch-and-rinse materials or the sixth-generation self-etch materials,” says Dr. Nathaniel Lawson, director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry.

Since these fourth-generation systems can be implemented in a variety of bonding protocols for indirect and direct restorations (including self-cure, dual-cure and light-cure procedures) they are some of the most versatile bonding systems available on the market. According to Lawson, when implemented correctly, these products become the gold standard against which newer generations should be judged.

“The gold standard are those which contain separate primer and adhesive (i.e., fourth- and sixth-generation products),” Lawson says. “A new adhesive should compare favorably to one of these.”

Fifth generation

With smear layers conquered and bond strength improved by the fourth generation, the goal for fifth-generation bonding systems became simplification, in an effort to reduce both procedural time and post-operative sensitivity (both of which became concerns with fourth-generation systems).

So, with the birth of the fifth generation came the introduction of one-bottle systems. These products combined the primer and adhesive into one bottle, making it possible to apply to the enamel and dentin simultaneously. This eliminated the inconvenient waiting periods and cumbersome multiple application steps of the prior generation, and meant that the tooth was only exposed to acid for 15-20 seconds. The adhesives also didn’t sacrifice bond strength, which ranged around 25 MPa, comparable to fourth-generation materials.

But, with simplicity came drawbacks. Not all fifth-generation adhesives could be used with self- or dual-cure materials. The one-bottle systems also contained hydrophilic primers, which made the adhesives much more susceptible to moisture degradation. Despite the drawbacks, however, the one-bottle system was well received by dentists, since it reduced chair time and didn’t require them to stock multiple types of adhesives and primers.

Sixth generation

As mentioned by Lawson, sixth-generation systems have been a resounding success and have set a high bar for future generations of adhesives. While this generation of self-etching primers returned to the multiple-bottle system (and requires a separate adhesive and etch-primer), they have eliminated the etching step entirely, and no longer require the dentin preparation surfaces to be etched.

While the addition of the second bottle meant more products to juggle, manufacturers in the 1990s and early 2000s worked their hardest to counteract the inconvenience. Cartridges that combine the unmixed adhesive and self-etch primer in exact ratios were introduced, simplifying the dispersal process and ensuring the proper mixture was achieved. Since these bonding agents can simultaneously diffuse primer and etch dentin and enamel, the sensitivity issues of the fourth and fifth generations have been greatly reduced.

One concern with sixth-generation materials however is that margins are often discolored, believed to be caused by the fact that this generation of products are not as effective as phosphoric acid for etching the enamel surface. As a result, the bond strength to enamel can be up to 25 percent weaker than the fourth and fifth generations.4 To counteract this, some manufacturers now recommend etching the enamel with phosphoric acid prior to the self-etching adhesive application, which adds an extra step.

Overall, however, the sixth generation of materials is an adequate addition to any dentist’s arsenal. While bond strength to enamel is decreased, sixth-generation bonding agents boast impressive bond strength to dentin (up to 41 MPa after 24 hours). Though sixth-generation products may not be as popular as other generations, they are certainly an improvement on the earliest iterations and are used successfully by many clinicians.

Seventh generation

Not to be outdone by the development of one-bottle and self-etch systems, the seventh generation went one step further by combining them. Developed in the early 2000s, these bonding agents include etch, primer and adhesive in one bottle.

This unprecedented simplicity has presented incredible benefits for clinicians. Because the three key ingredients for bonding are already combined, it eliminates the mixing steps—thus minimizing opportunities for errors that can compromise bond strength. And you can’t afford to compromise bond strength with seventh-generation materials: While bond strength is incredibly consistent, it is also lower (in both initial and long-term studies) than any other system. 

These adhesive systems’ ingredients present another challenge, as they are acidic and prone to hydrolysis since they contain high concentrations of water. This makes them susceptible to chemical breakdown, which results in solution instability. Essentially, the seventh-generation adhesives become unstable quicker than past generations, and are more hydrophilic and disposed to moisture absorption than the sixth-generation, two-step systems.

While seventh-generation systems laid the groundwork for ultimate stability, they ultimately are not as popular as earlier generations. It would take one more generation to find the right mix of simplicity and effectiveness.

Eighth generation/Universal adhesives

If you’re looking for ease of use, universal adhesives are the way to go. Like seventh-generation systems, the universal one-bottle systems combine etching, priming and bonding into one solution, eliminating the need for mixing and reducing both products and steps.

“Universals really make things simpler, and are inarguably less technique sensitive,” says Dr. Jeff Lineberry, a general dentist practicing in Mooresville, North Carolina. “But the question is longevity and durability, and we see promising results with these adhesives released three or four years ago, but we don’t yet have a long history to evaluate.

Lawson agrees.

“One-bottle universal adhesives are probably the most popular bonding agents,” Lawson says. “And, while we don’t have long-term statistics yet, there are medium length studies showing good clinical performance.”

And they are popular for a good reason. The versatile universal materials can be used with a range of protocols (from self-etch to total-etch) on both dentin or enamel, and are much more effective for enamel etching than seventh-generation materials. Universals are also compatible with light-, self- or dual-cure composite, and bond well to a wide range of materials, including zirconia, composites and glass ceramics. To top it all off, they have an impressive bond strength (to unprecedented rates of over 30 MPa). 5

These benefits are largely due to the addition of the 10-MDP (methacryloyloxy-decyl-dihydrogen-phosphate) monomer. This hydrophobic material eliminates many of the moisture-related issues in previous generations, particularly the incredibly hydrophilic nature of seventh-generation materials. Because of 10-MDP, universal adhesives have much broader applications than prior generations.

“I think there’s a place for universals, because there are situations where you might not want to use other generations with different techniques such as self-etch,” says Lineberry. “In short, the universals can save you a lot of time while working with many different materials.”

They also come in handy for situations where isolation is difficult in indirect restorations.

“I have less time with good isolation when bonding indirect and that I don't want too thick of an adhesive layer when bonding indirect,” Lawson says. “I generally turn to a single-bottle universal adhesive or one-bottle self-etch primer when bonding indirect restorations.”

Conclusion

Dental adhesives have come a long way, and there’s still room for multiple generations in any practice. Different indications call for different options, so dentists shouldn’t marry themselves to one generation exclusively. Despite new developments, traditional fourth- and fifth-generation materials still remain incredibly popular. Sixth and seventh generations have opened new doors in bonding. And universals, well, the possibilities are endless. Whichever generation you prefer, keep an eye out for the latest developments in bond strength, simplicity and efficacy. We’re sure to see even more options in the years to come.

References

1. Buonocore, M. G. (1955). A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces. Journal of Dental Research, 34(6), 849–853.

2 Sofan, E., Sofan, A., Palaia, G., Tenore, G., Romeo, U., & Migliau, G. (2017). Classification review of dental adhesive systems: from the IV generation to the universal type. Annali di stomatologia, 8(1), 1–17.

3. Tao L, Pashely DH, Boyd L. (1988). Effect of different types of smear layers on dentin and enamel shear bond strengths. Dent Mater, 4(4):208-16.

4. De Munck J, Van Landuyt K, Peumans M. (2005). A critical review of the durability of adhesion to tooth tissue: methods and results. J Dent Res 84:118-132

5. Kamble, S. S., Kandasamy, B., Thillaigovindan, R., Goyal, N. K., Talukdar, P., Seal, M. (2015). In vitro Comparative Evaluation of Tensile Bond Strength of 6(th), 7(th) and 8(th) Generation Dentin Bonding Agents. Journal of international oral health: JIOH, 7(5), 41–43.