Adhesives: 60 Years of Clinical and Chemical Improvement by Kyle Patton, Associate Editor, Dentaltown Magazine

Dentaltown Magazine interviewed Drs. Byoung I. Suh and Paul L. Child Jr., both experts on dental adhesion.

Suh founded Bisco in 1981, and in 2013, wrote "Principles in Adhesion Dentistry." He has also published more than 150 articles in various dental journals and magazines. Child is the former executive vice president of Bisco, and an international lecturer. He maintains a private practice in the greater Chicago area.

The doctors addressed the ins and outs of adhesion.

What are the biggest advancements in adhesive chemistry during the past 60 years?
Suh: Ever since Buonocore introduced the concept of acid-etching enamel for resin bonding in 1955,1 dental adhesives have evolved from first- to seventh-generation adhesive systems, from total-etch to self-etch, and from complex, multistep, multicomponent to simplified, single-step, single-component adhesives.

However, each category has both advantages and disadvantages. For example, the "gold standard" three-step, etch-and-rinse systems (fourth generation) are less convenient, but the most convenient single-step self-etch systems (seventh generation) have some potential issues such as incompatibility with self-cure materials, permeability, and faster bond degradation.

The most recent class of "universal" adhesives such as All-Bond Universal (BISCO) and Scotchbond Universal (3M ESPE) are the biggest advancement in adhesive technology. Universal adhesives can have both the advantage of a single-step system, and the performance of a gold-standard adhesive if the formulation optimizes the acidity (pH), bond strength, hydrophobicity and solvent system.

Why do you use a universal adhesive for all procedures? Even veneers?
Child: Universal adhesives are designed to work with all procedures—even veneers! However, some cured universal adhesives have a higher film thickness, which may interfere with seating of a veneer, especially if allowed to pool on a margin. Universal adhesives not only give me flexibility in the etching mode I select, but the chemistry has improved significantly to be compatible with anything from basic operative procedures to complex cementation protocols. Some researchers mistakenly refer to universal adhesives as simply "multi-modal" regarding the etching mode. But they are more than that. They have taken the best of the previous generations and combined it into one bottle.

How does a clinician read between the lines with all the confusing marketing and KOL information?
Child: A clinician needs to consider several factors when choosing an adhesive to use in her or his practice. Evidence-based dentistry has been defined by the American Dental Association as: "an approach to oral health care that requires the judicious integration of systematic assessments of clinically relevant scientific evidence, relating to the patient's oral and medical condition and history, with the dentist's clinical expertise and the patient's treatment needs and preferences."2

This definition dictates that a clinician should use his or her own clinical expertise, the patient's needs, and the most current scientific evidence when deciding upon what materials to place in a patient's teeth.

However, many dentists simply look to a key opinion leader in a lecture or a distributor to tell them what to use. If this method is to be continued, the clinician should ask for evidence that supports the promotion of the adhesive or product in question. Further, clinicians should rely upon long-term observation and data to support their decisions.

Hydrophilic versus hydrophobic adhesives. What does this mean and why is it important in the development of adhesives?
Suh: Hydrophilic means having a strong affinity to water, while hydrophobic means having little or no affinity to water. Dentin is hydrophilic and contains water. A dental adhesive should be hydrophilic before polymerization in order to penetrate into the dentin structure to have great wetting ability and maximum mechanical bonding.

However, water is the enemy of dental adhesives after restoration since water can degrade dental restorations. A hydrophilic adhesive would attract more water into the adhesive layer and would have faster degradation.

Therefore, the ideal adhesive should be hydrophilic before polymerization and hydrophobic after polymerization. Adhesive functional monomers in general are acidic, but some are more hydrophilic than others, which can increase the permeability of the adhesive layer, even after polymerization. For example, MDP and BPDM are the most hydrophobic monomers, while GPDM and Penta-P are more acidic and hydrophilic.

Does the water content matter in an adhesive, and if so, why?
Suh: Unlike acetone or ethanol, water in dental adhesive is difficult to remove during application. Residual water left in the adhesive has many potential problems, such as increased permeability of the cured adhesive, and faster hydrolytic degradation.

Too much water in the adhesive is very difficult to completely evaporate. Some adhesives utilize an azeotropic amount of ethanol-and-water solvent system to ensure easy and complete evaporation of residual water. In addition, high water content results in higher dissociation of the acidic monomer, which leads to higher acidity (lower pH) and may reduce the shelf life of a single-bottle adhesive.

Is it possible to reduce the number of steps while maintaining the same high bond strength?
Suh: Some new universal adhesives are single-step systems, but still have bond strengths as high as multiple-step adhesive systems for dentin bonding. However, for enamel bonding, a separate phosphoric-acid-etching step is recommended to optimize bond strength.

How important is bond strength? What other characteristics are equally important?
Child: A recent survey I conducted to determine the most desirable characteristics of an adhesive found that bond strength was the most important characteristic to clinicians. Truthfully, I find this a little humorous, as I believe the whole bond-strength war among various manufacturers to be tiring and outdated. Clinicians can't see bond strength, nor do they know what happens to it over time.

As such, I believe that bond strength maintained over time is highly superior to a simple lab bench study of immediate bond strength. Other characteristics that are equally important: it's fully compatible with dual- and self-cure resins, bonds to all substrates, can be used for both direct and indirect procedures, offers ease of use, and of course, is cost effective.

Are one-layer adhesives good enough?
Suh: Traditional one-layer adhesives are permeable, which may cause sensitivity, faster degradation, and incompatibility with slow-curing resin materials. The two-layer adhesive system, utilizing a hydrophobic second layer to eliminate these issues, is considered the gold standard. Today, some one-layer universal adhesives are designed to be as hydrophobic as a two-layer system. Only these hydrophobic one-layer systems are as good as the two-layer gold standards.

Can one manufacturer's adhesive be used with another manufacturer's composite, cement, etc.? Or is it best to stay with all the same manufacturer's products?
Child: Most adhesives are manufactured to be compatible with other manufacturers' composites and cements (with the exception of Filtek Silorane 3M ESPE).

However, the reverse is not always true. Some dual-cure cements require use of their adhesive for best results (or at least, that is what the manufacturer claims). Staying within a system designed to maximize results can be beneficial, especially in terms of ease of use.

The system and steps become familiar for the clinician and generally result in exceptional outcomes. Kits have become popular among manufacturers (e.g., eCEMENT from BISCO, Variolink Esthetic from Ivoclar, or RelyX Ultimate from 3M EPSE). These kits are designed to use all the components needed for the treatment of both teeth and the restoration. However, with the advent of universal adhesives and the concept of "one bottle for everything," I would recommend using an adhesive that is compatible with other manufacturers' products as well.

What was the tipping point that made you switch from fourth-, fifth-, or sixth-generation adhesives to a universal adhesive?
Child: I have to admit, I was a little slow to adopt universal adhesives after their introduction in 2011. I was using a fourth- generation and a sixth-generation adhesive with great success. While working at CR with Gordon Christensen, I had evaluated and tried at least 40 different adhesives, but had settled on two.

It wasn't until I started to read the scientific literature intently and learn about the in-depth characteristics of adhesives that I decided to use universal adhesives. With their hydrophobic, one-bottle, multi-etch, dual-cure compatible characteristics, I simply couldn't refuse to give them a try. Would I turn back now? Heavens, no!

How do you differentiate seventh-generation adhesives from universal adhesives?
Suh: Seventh-generation adhesives can only be used in self-etch mode and are hydrophilic, while universal adhesives are more hydrophobic and can be used in both self-etch and etch-and-rinse modes.

Why do some universal adhesives require a separate self- or dual-cure activator?
Suh: Most universal adhesives contain an excessive amount of water and remain acidic, even after solvent evaporation, and are thus too acidic, or hydrophilic, and permeable. They are not compatible with self-cure or dual-cure resin materials without a separate self-cure activator, due to this acidity.

What effect does the pH of an adhesive have on enamel, dentin, or restorative materials?
Suh: Contemporary self-etch adhesives can be classified into different categories: aggressive (pH ? 1), intermediate (pH ≈ 1.5), mild (pH ≈ 2) or ultra-mild (pH ? 2.5), according to their pH.2

The pH of most universal adhesives ranges from 2.3-3.2.

For bonding to enamel, the pH of any self-etch adhesive is not acidic enough (phosphoric acid etchant has a pH of 0.1, which is 1,000 times more acidic than an adhesive with a pH of 3) to dissolve and etch enamel. In order to have a maximum mechanical bond, selective etching of the enamel with phosphoric acid is required.

For bonding to dentin, more acidic self-etch adhesives will dissolve more of the smear layer, which may result in lower bond strength and higher sensitivity rates.3

Adhesives with low pH cause incompatibility between the acidic adhesive and the self-cure mechanism of a dual-cure resin composites/cements.4-5

The major reason for the incompatibility is because the amine-initiator of the composite/cement is deactivated by the ‘acid-base' reaction between the acid (H+) from the oxygen- inhibited layer of the cured adhesive and the amine-initiator of the composite/cement. The consequence is no polymerization at the adhesive and composite/cement interface, resulting in no bonding due to the incompatibility.

Do you believe that self-etch on dentin is as good and durable as total-etch on dentin?
Suh: Yes, depending on the adhesive formulation, self-etch on dentin can be as good and durable as, or even better than, total-etching dentin. Several of the new universal adhesives utilize MDP as its acidic functional monomer, which forms a strong and durable chemical bond with the tooth.

Do you believe a single-bottle adhesive should be used as an all-substrate primer as well? E.g., silane contained in an adhesive.
Child: Not presently. There are universal adhesives that incorporate silane into their formulas but the resins in the adhesive interfere with the chemical process of silanization. Current research has demonstrated that a separate pure silane performs better than a mixed-adhesive resin that contains silane.

A total-etch, three-step process has been considered the gold standard. Do you still consider it so? If not, why?
Suh: Some of the new universal adhesives form strong and durable chemical bonds with the tooth, especially if the enamel is selectively etched with phosphoric acid. They are single-step systems, but also provide great performance equivalent to the three-step gold standard. It is a benefit for dentists to try a new universal adhesive system instead of a three-step system due to the simplification of the application process.

What's next? What improvements can be made to universal adhesives?
Child: Are universal adhesives the end of improvements? Certainly not! This questions whether some universal adhesives are truly universal. Improvements can be made to prolong the bond beyond what it is capable of achieving now. Antibacterial features that inhibit biofilm growth indefinitely can be provided.

Resistance to adhesive breakdown by MMPs has yet to be incorporated. And we are still learning about the complex nature of dentin and how bonding can be enhanced. Adhesives will continue to evolve in the next decade until perhaps we no longer need them! Until then, new universal adhesives are the best and a culmination of more than 60 years of improvement.

  1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J. Dent. Res. 1955; 34(6):849-853.
  2. American Dental Association Center for Evidenced Based Dentistry (2015, July 20). What is EBD? Retrieved from
  3. Van Meerbeek B, Peumans M, Poitevin A, et al. Relationship between bond-strength tests and clinical outcomes. Dent Mater. 2010; 26:e100-121.
  4. Schittly E, Bouter D, Le Goff S, et al. Compatibility of five self-etching adhesive systems with two resin luting cements. JAdhes Dent 2010;12:137-142.
  5. Suh BI, Feng L, Pashley DH, et al. Factors contributing to the incompatibility between simplified-step adhesives and self-cured or dual-cured composites. Part III. Effect of acidic resin monomers. J Adhes Dent 2003;5:267-282.

Dr. Byoung Suh received his master's degree in chemistry from California State University in San Francisco, and his PhD in dental medicine from Tsurumi University in Yokohama, Japan. Suh founded BISCO, Inc. in 1981. In addition to his dental materials research, from which he has published more than 150 articles, Dr. Suh has become a well-known and sought-after lecturer throughout the world. Suh has given more than 200 lectures at various dental associations and research conventions in the United States, Canada, and in more than 40 countries around the world. His book, Principles in Adhesion Dentistry, was published in 2013.

Dr. Paul L. Child Jr. is a prosthodontist and certified dental technician. He maintains a private practice in the greater Chicago area where he enjoys providing all aspects of dentistry, but primarily focuses on prosthodontics, esthetics, implants, complex restorative, CAD/CAM, and all aspects of surgery. Child is the former CEO of CR Foundation (formerly CRA), where he lectured and conducted extensive research with Drs. Gordon and Rella Christensen. He is also the former executive vice president of BISCO Dental Products. He lectures nationally and internationally, maintains membership in many professional associations and academies, and is on the editorial boards of several journals.

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