The Evolution of Adhesive Dentistry
Bonding to enamel and dentin is more predictable than ever
Marc Geissberger, DDS, MA, BS, CPT
Enamel evolved in teeth as a covering to protect the underlying structures of dentin and pulp. Because of the acidic nature of most diets, enamel is relatively difficult to demineralize, so to do so requires prolonged exposure to acids or exposure to acids in higher concentrations than are normally found in a balanced diet. The first attempts to develop bonding agents in dentistry were primarily focused on enamel adhesion. For well over half a century, the profession of dentistry has refined the practice of etching enamel to achieve a robust and sustainable bond. Since Buonocore demonstrated that enamel adhesion improved with acrylics when the surface of enamel was modified, decades of scientific research have led to a deeper understanding of appropriate etching time, etchant viscosity, and the types of etchant used.1 Thanks to this research, we can now predictably and routinely bond well to enamel.
Dentin Etching and Bonding
Initially, dentin bonding proved to be relatively problematic and unpredictable. Dentin, unlike enamel, has several inherent characteristics that make it a less-than-desirable bonding substrate. Although enamel possesses very little protein and organic material, dentin has a considerable amount of organic material present. Furthermore, enamel is histologically similar regardless of where a cross section is made, whereas dentin exhibits considerable histologic differences from the dentino-enamel junction to the pulpal interface, adding to the complexity of establishing an adequate bond. Additionally, when enamel is desiccated, little effect is seen on overall bond strengths. By comparison, when dentin is desiccated, bond strengths can vary substantially. These factors made initial attempts to bond to dentin quite challenging. Early research and reports indicated that bonding to dentin would provide less than favorable results and outlined the challenges that laid ahead.2
Dentin Bonding with Etch and Rinse Systems
At the inception of dentin bonding, what was not well understood was the fact that etching dentin transformed the relatively hard, mineral rich surface into one that was depleted of minerals and rich in dentin collagen. This collagen network could easily be desiccated, leading to a collapse of these fibers that further compromises the dentin bond by blocking access to underlying dentin tubules.3 To overcome many of the initial shortcomings in bonding, the first systems utilized a 3-step process that incorporated separate etching, priming (rehydrating), and resin infiltration steps.
Over the next several decades, dental researchers explored dentin bonding with great vigor. While many practicing dentists believed that dentin bonding was highly unpredictable and virtually impossible to do successfully, the scientific community began to improve dentin bonding with several significant discoveries and advances. In the early 1980s, Nakabayashi and others first described the “hybrid layer” as a layer established after dentin was treated with an acid and rinsed with water.4 This layer was relatively hydrophobic, resistant to acids, and rich in collagen. Once the surface was impregnated with resin adhesives, this layer was truly a hybrid of both dentin and resin.
Etch and Rinse Systems that Utilize “Wet Bonding” Techniques
The next major advance in our understanding of dentin bonding occurred with the introduction of “wet bonding,” which was proposed by Kanca and others in the early 1990s.5 This process described the profound effect achieved by drying dentin after acid etching. The drying of dentin caused a significant change in the collagen network by decreasing its volume and creating a relatively impermeable surface for dentin adhesives. To overcome the effects of rinsing and drying, bonding systems of this generation employed the use of a priming agent to aid in the rehydration of the dentin surface, which ultimately increased overall bond strengths substantially.
The Dawn of the 2-Step Self-Etching Dentin Bonding Systems
The problems associated with total-etch wet bonding lead many researchers to search for alternative approaches to overcome the challenges of effective dentin bonding. If one could increase the acid levels in dentin primers to a level effective enough to etch dentin, one could eliminate the separate etching step in dentin bonding. This is exactly what has occurred with self-etching dentin adhesive systems. Essentially, the acid levels in self-etching primers were raised to 30% to 40% by weight and combined with hydroxyethyl methacrylate (HEMA). This effectively created a system that was both self-etching and self-priming in one step. These products were lumped into the self-etching category of dentin bonding agents and saw rapid growth throughout the 1990s and early 2000s. To be effective, these systems generally required the tooth surface to be scrubbed to effectively modify the dentin smear layer and ensure adequate bond strengths. Following this, the dentin surface was lightly dried. To complete the bonding process, a relatively hydrophobic adhesive layer was applied and light cured.
All-In-One Dentin Bonding Systems
Following the introduction of two-step self-etching dentin bonding materials, manufacturers focused their research on creating a bonding agent that would simultaneously etch, prime, and create a bonding surface with dentin adhesive components. The initial formulations of all-in-one dentin adhesives had two fundamental characteristics. First of all, they were all more acidic and hydrophilic than their two-step self-etching counterparts, creating bonding surfaces that were quite thin (5 to 10 microns). This extremely thin bonding layer increased the likelihood of incomplete curing, even after the use of light curing, as oxygen could easily diffuse into the surface and inhibit the reaction. Secondly, due to their relatively acidic and hydrophilic properties, these systems tended to draw in water, creating so called “water trees,” which lead to decreased overall bond strengths when compared to their two-step self-etching counterparts.6
Universal Bonding Agents— The Next Step Forward in Dentin Bonding
Because dentin and enamel were so dramatically different, to optimally bond to them required employing two different systems. When bonding to enamel, a total-etching system lead to superior results and sustained bonds. When bonding to dentin, a self-etching system created the strongest, most predictable bond. What the market place was demanding was the creation of a bonding system that could be employed in a total-etch, self-etch, or selective-etch mode. Recently, several dental manufacturers have launched “universal” bonding systems. These systems have been designed to be used with or without a separate etching step, making them highly adaptable to all clinical situations in which bonding to tooth structure is required.
Conclusion
In the short time that bonding agents have been available, these systems have undergone huge transformations and revolutionized the practice of dentistry. The introduction of self-etching and universal bonding agents has made bonding to dentin more predictable than ever. Dentists should look beyond what they have traditionally done in practice, embrace the emerging science of dentin bonding, and move their protocols into the 21st century.
References
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. Brudevold F, Buonocore MG, Wileman W. A report on a resin composition capable of bonding to human dentin surfaces. J Dent Res. 1956;35(6):846-851.
3. Pashley DH, Ciucchi B, Sano H. Dentin as a bonding substrate. Dtsch Zahn Z. 1994;49:760-763.
4. Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth substances. J Biomed Mater Res. 1982;16(3):265-273.
5. Kanca J. Improving bond strength through acid etching of dentin and bonding to wet dentin surfaces. J Am Dent Assoc. 1992;123(9):35-43.
6. Tay FR, Pashley DH. Water treeing—a potential mechanism for degradation of dentin adhesives. Am J Dent. 2003;16(1):6-12.
About the Author
Marc Geissberger DDS, MA, BS, CPT
Professor, Department of Preventive and Restorative Dentistry
University of the Pacific
San Francisco, California
Private Practice
Greenbrae, California
