Advancements in Adhesion and Bonding

Q&A with Gary Alex, DMD

Inside Dentistry interviews Editorial Advisory Board member Gary Alex, DMD, owner of a private practice specializing in prosthetic and cosmetic dentistry in Huntington, New York, a member of the International Association of Dental Research, and an accredited member of the American Academy of Cosmetic Dentistry

Inside Dentistry (ID):What have been the most significant developments in adhesives and bonding over the years, and why were they significant?

Gary Alex, DMD (GA): Dentin adhesive systems have progressed from the largely ineffective systems of the 1970s and early 1980s to the relatively successful total-etching, self-etching, and universal systems of today. Regarding bonding to dentin, early generation adhesive systems were extremely limited and generally ineffective, in part because they bonded directly to the smear layer. The smear layer is the residue that is left on the surface of the dentin after rotary instrumentation with diamond and carbide burs. At some point, it was recognized that the smear layer needed to be removed (total-etching systems) or modified or bypassed in some fashion (self-etching systems) so that the adhesive primers and resins could interact directly with the dentin. Acidic removal or modification of the smear layer, combined with improved chemical formulations, led to increased bond strengths and greater predictability with adhesive systems. As adhesive systems continued to evolve, they generally became simpler to use and offered greater versatility in terms of the potential to bond to a variety of organic and inorganic substrates.

ID: What are the most important considerations involving adhesion and bonding when treatment planning?

GA:Optimal adhesion requires an understanding of the materials being used and the substrates being bonded to, control and isolation of the working area, and a correct and precise clinical protocol. In addition, it is important to have a good working knowledge of the histology and morphology of both dentin and enamel and the ability to mentally visualize what is happening to these tissues as various adhesive and conditioning agents are applied. On the restorative side, clinicians need to know the optimal way to treat the intaglio surface of whatever type of restoration they are placing because surface treatment and adhesive techniques can vary significantly depending on the material being bonded to.

ID: What are some of the most popular new material types, techniques, and/or applications in adhesion and bonding?

GA: The latest players in the adhesive marketplace are the so-called "universal adhesives." In principle, these systems have the potential to significantly simplify and expedite adhesive protocols and represent the latest evolution in adhesive dentistry. Space constraints prevent me from discussing these materials at length, but I have previously written a very comprehensive narrative on universal adhesives that includes an in-depth discussion of their chemistry, interactions, and limitations.¹ I highly recommend that interested clinicians read this article to gain a better understanding of universal adhesives and adhesion in general.

ID:Are there any specific concepts in adhesion that are particularly controversial right now?

GA:There are always controversies when it comes to adhesive dentistry! Currently, two controversies related to universal adhesives involve their ability to bond to enamel and ceramics: (1.) Universal adhesives are inherently acidic, but do they etch enamel as effectively as phosphoric acid? (2.) Several universal adhesives incorporate silane into their formulations, but are these silane-containing universal adhesives as effective as stand-alone, pure silane primers when used to optimize adhesion to etchable ceramics? Based on the results of numerous published studies, my personal opinion and answer to both of these questions is no. I routinely use a universal adhesive, but always etch any enamel present with phosphoric acid, using either a total- or selective-etch technique. I also use a separate, dedicated silane primer on etchable ceramics.

ID: Are there any areas where you feel there is a lack of adequate knowledge and a need for more extensive education?

GA:I think some dentists lack a complete understanding of exactly what is going on when they use an adhesive system. In other words, too many dentists are placing adhesive agents on tooth tissues and various restorative materials without really understanding what it is that they just placed and what it is actually doing. Knowledge is power. Although it is not necessary to be a chemist, those dentists who understand at least the fundamental concepts and principles of adhesive chemistry stand a much better chance of achieving consistent and predictable results. It is incumbent on every dentist to learn at least the basics regarding adhesive systems, including how they work, their idiosyncrasies, their strengths and weaknesses, and how to optimize their performance.

ID:What are some of the remaining challenges to adhesion and bonding that have not yet been solved by new advancements from manufacturers?

GA: Long-term bond strength to dentin is still a significant issue in adhesive dentistry. In general, most current adhesive systems bond very well to both enamel and dentin-at least in the short-term. Studies have demonstrated that, over time, bond strength to enamel remains relatively unchanged while bond strength to dentin typically decreases (sometimes quite dramatically). This is partially explained by the significant morphologic, histologic, and compositional differences between the two substrates. All adhesive systems use acid conditioners and/or primers that demineralize the dentin to some extent. This demineralized, collagen-rich dentin is subsequently (total-etch) or concurrently (self-etch) infiltrated with various bifunctional primers and resins. This resin-infiltrated zone of dentin is referred to as the "hybrid layer." Over time, the hybrid layer can be subjected to hydrolysis, enzymatic breakdown, water infiltration, and other insults that may contribute to the eventual failure of the adhesive interface. Indeed, much research is being conducted with materials that directly and/or indirectly improve the long-term stability of the hybrid layer.

ID: What do you envision on the horizon in the next 5, 10, or 20 years with respect to transformational advancements in this realm?

GA: Today, one of the most interesting and potentially important areas in adhesive research focuses on the use of various chemical agents that inhibit proteolytic enzymes such as matrix metalloproteinases and cysteine cathepsins. Both of these enzymes, which are inherent in dentin and activated when acidic primers and conditioners are used during the bonding protocol, can break down the supportive collagen scaffolding of the hybrid layer and thus weaken the adhesive interface. Many of these agents are also antimicrobial in nature. In the future, it is likely that antimicrobials and matrix metalloproteinase inhibitors will be incorporated, either directly or indirectly, into adhesive, conditioner, and restorative formulations and protocols. There will also be continued development of self-adhesive restorative materials that don't require the use of a separately placed adhesive. These and other exciting new developments on the horizon should make adhesive dentistry even more durable and predicable.

Reference

1. Alex, G. Universal adhesives: the next evolution in adhesive dentistry? Compendium. 2015;36(1):15-26.