Adhesive Interface: Key Component of Contemporary Bonded Restorations

Adhesive systems have progressed from the largely ineffective systems of the 1970s and early 1980s to the relatively successful total-etch (etch and rinse) and self-etch (etch and no rinse) systems of today. While long-term bonding to phosphoric acid-etched enamel surfaces has proven to be highly reliable and predictable, long-term bonding to dentin has been considerably more problematic. This is due mainly to morphologic, histologic, and compositional differences between the two substrates.^2,3 Enamel is a hard crystalline and highly mineralized non-vital tissue that generally is uniform throughout, while dentin is a vital, dynamic, and highly variable substrate. Superficial, middle, and deep dentin can vary significantly in structural, physical, and chemical composition. This, coupled with the relatively high water and collagen content of dentin, presents a significant challenge to achieving consistent and reliable long-term adhesion to this substrate.⁴ The relative success of current adhesive systems can be attributed not just to improved chemistries, but to a better understanding of dentin dynamics, improved clinical techniques, and the selective use of adjunct materials (such as antimicrobials) to optimize the physical/chemical interaction and durability of adhesives to dentin.

To understand adhesive dentistry one has to start with the smear layer.^5,6 The smear layer is the residue left on the surface of dentin and enamel after rotary instrumentation is performed with diamond or carbide burs. It is a thin amorphous layer largely composed of degraded collagen, bacteria, and various inorganic dentin and enamel debris. Early adhesive systems were extremely limited and generally ineffective, in part because they bonded directly to the smear layer and were, thus, limited by its low intrinsic cohesive strength.⁷ At some point, researchers recognized that the smear layer needed to be removed and/or modified and bypassed so that adhesive primers and resins could interact directly with the dentin. In the case of total-etch adhesive systems, the smear layer is essentially dissolved with phosphoric acid and subsequently washed away during the rinsing step. With self-etch systems, various acidic primers and/or conditioners are used to modify, disrupt, and/or solubilize the smear layer to permit direct adhesive interaction with the dentin substrate, even though the remnants are not washed away as with total-etch systems.

The acids and/or acidic primers and conditioners used with either total- or self-etch bonding systems do not just remove and/or disrupt the smear layer but create a thin zone of demineralization, in which a network of collagen fibrils are exposed that are either subsequently (total-etch) or concurrently (self-etch) infiltrated with various functional and cross-linking primers and resins. The degree and depth of demineralization is dependent on the type of acid used, its concentration, and how long it is applied.

One of the goals in developing a successful adhesive interface is the complete infiltration of and penetration through this acid-demineralized collagen-rich zone (whatever its thickness) with various primers and/or resins that can be subsequently polymerized by light and/or chemical curing mechanisms. This thin layer of resin-infiltrated dentin, which was first described in a classic 1982 paper by Nakabayashi and colleagues, is called the hybrid layer (a zone that is neither dentin nor resin but a mixture, or hybrid, of the two).⁸ Although micromechanical resin infiltration and entanglement with the tooth tissues appears to be the primary attachment mechanism to both enamel and dentin, strong evidence suggests that certain monomers (such as 10-methacryloyloxydecyl dihydrogen phosphate [10-MDP]) chemically interact, via ionic bonding, to calcium in hydroxyapatite as well.^9-11 The hybrid layer and associated resin tags form a thin polymerized micromechanically, and in some cases chemically, attached resinous surface layer that acts as the foundation for subsequently placed and chemically compatible restorative materials and resin-based cements.

Placing a bonded restoration is one thing; having it be durable over time is something else entirely. There are numerous reasons why, and places where, a bonded interface might fail. In the case of dentin this includes, but is not limited to, microleakage, nanoleakage, hydrolysis of the interface, incomplete penetration of primers/resins through acid-demineralized dentin, occlusion, operator error and/or poor clinical technique, poor product performance, inadequate polymerization, poor oral hygiene, and enzymatic breakdown of the adhesive interface.^12-16 The late Dr. John Gwinnett, a highly respected academician, researcher, dentist, mentor, and teacher, taught his students to think of a bonded restoration as a chain, that is, a series of links that taken together form a bonded assembly.¹⁷ Much like a chain, a bonded restoration is only as strong and durable as its weakest link. In this regard, anything that can be done to strengthen the weakest link in the chain has the potential to improve the long-term clinical expectations of bonded restorations.

A key area in adhesive research today focuses on the use of various chemical agents that inhibit proteolytic enzymes such as matrix metalloproteinases (MMPs) and cysteine cathepsins.^18,19 These enzymes, which are inherent in dentin and play a vital role during tooth morph-ogenesis, become dormant after the tooth is formed. However, they can become active again after acidic primers and conditioners are used to demineralize dentin (and etch enamel) during the bonding protocol. Once activated, MMPs can break down the supportive collagen (a protein) scaffolding of the hybrid layer, thus weakening this link in the bonding chain. Antimicrobial solutions, such as chlorhexidine, gluteraldehyde, and benzalkonium chloride, have been shown to not only bebactericidal and act as re-wetting agents, but inhibit MMP activity as well. Because of this, many practitioners now routinely use these products at various points in the bonding protocol. In addition, some manufacturers are incorporating antimicrobials and MMP inhibitors directly into their adhesive formulations. Indeed, much research is being conducted on materials and chemistries that will improve hybrid layer and adhesive interface durability by inhibiting the enzymes responsible for proteolysis as well as materials that, once polymerized, are bacteriostatic with active ion exchange mechanisms that can potentially remineralize and strengthen tooth tissues.^20,21

The trend in recent years has been toward not just the simplification of adhesive systems, but making them "universal" as well. Manufacturers typically state that universal adhesives can be used for the placement of both direct and indirect restorations and are compatible with self-cure, light-cure, and dual-cure resin-based cements. They also typically claim that universal adhesives can be used not only to bond to dentin and enamel, but as primers on substrates such as zirconia, noble and nonprecious metals, composites, and various silica-based ceramics. In principle, this would enable bonding to these surfaces without the need for dedicated and separately placed primers such as silane and various products marketed as metal and zirconia primers.

The question is whether universal adhesives can really do all of the things manufacturers claim they can. Clearly, there must be significant practical and chemical challenges in developing such a versatile product, placing all the chemistry required into one or even two bottles, have it perform as claimed, and have it remain stable for a reasonable period of time. Among other things, the use of phosphate esters such as 10-MDP has made universal adhesives possible. The 10-MDP monomer (used in most universal adhesives) has many positive attributes. It is a versatile amphiphilic functional monomer with a hydrophobic methacrylate group on one end (capable of chemical bonding to methacrylate-based restoratives and cements) and a hydrophilic polar phosphate group on the other (capable of chemical bonding to tooth tissues, metals, and zirconia). The question isn't whether universal adhesives are capable of bonding to substrates such as zirconia, noble and nonprecious metals, composites, and silica-based ceramics (they are), but are they as effective, both initially and, more importantly, over time, as separately placed dedicated primers? There is some controversy surrounding this question. Opinions differ from manufacturer to manufacturer, and, in the author's view, additional objective independent research is needed before making definitive recommendations. In any case universal adhesives, while not perfect, represent a simplified and viable choice for adhesive dental procedures. (The author has previously discussed the chemical, technical, and clinical aspects of universal adhesives in great detail.^4,22)

Effective management of the adhesive interface is critical for the predictable placement of adhesively bonded restorations. Thus, the clinician must understand the material being placed, the substrate being bonded to, and the clinical protocol being utilized. Each adhesive system has idiosyncrasies, strengths, and weaknesses. Indeed, the operator, ie, the dentist, may be the biggest variable of all in adhesive dentistry. While there are many excellent and chemically sound adhesive systems, even good chemistry will not overcome poor clinical technique. To maximize the performance of an adhesive system, dentists must pay meticulous attention to details, such as control and isolation of the working area, proper conditioning and priming, and proper solvent evaporation followed by adequate light polymerization. The more dentists know about the adhesive agents they are using on tooth tissues and various restorative materials, the better results they will likely achieve. Although it is not necessary to be a chemist, those dentists who understand the fundamental concepts and principles of adhesive chemistry stand a better chance of achieving consistent and predictable results.

Gary Alex, DMD
Private Practice, Huntington, New York; Accredited Member, American Academy of Cosmetic Dentistry; Member, International Association for Dental Research