In the fourteenth century, Europe emerged from the Dark Ages via the Renaissance. This emergence encompassed not only art, science, and literature, but more profoundly, thought itself. What had been considered impossible was now possible. That, which had been unthinkable, was now a lucid thought with limitless possibilities. In a very real sense, we are in the midst of a Renaissance in adhesive dentistry. The concepts of total etch,1,2 wet bonding,3 hybrid layer formation,4 and effective single bottle adhesive systems, were incomprehensible just a short time ago. It is through the dedication and insight of those who have developed and researched these concepts that we find ourselves emerging from the dark ages of adhesion, and into a new era.
In 1955, Buonocore introduced a revolutionary technique for bonding acrylic resins to enamel surfaces.5 He found that if he treated the enamel surface with phosphoric acid, the restorative resins subsequently placed adhered more durably to the tooth structure (Figure 1). The elucidation of this interaction by Gwinnett, Buonocore, and Matsui6,7 was the first step in the modern adhesive story. Subsequent research, clinical observation, and anecdotal evidence have established the reliability of the long-term bond to acid etched enamel. Several significant changes have taken place regarding acid etching of enamel surfaces over the last 20 years. These include the development of gel and semi gel etching agents, a reduction in the acid application time,8,9 and attempts at using acidic conditioners other than phosphoric acid (i.e., nitric, citric, oxalic, maleic, 10-MDP, Phenyl-P) to etch enamel surfaces. While many of these acids perform well in terms of preparing dentin surfaces for bonding, there are concerns regarding their ability to etch enamel surfaces as capably as phosphoric acid10,11,12,13,14 in the recommended concentrations and application times. This may be significant when using “self-etching primers” such as Liner Bond 2 (Kuraray/J. Morita) and Prompt L-Pop (ESPE). These products do not recommend a separate etching step on cut enamel with phosphoric acid prior to primer application. It is assumed the acidic primers will adequately etch the enamel while simultaneously conditioning and priming the dentin. While there is evidence to support this contention15,16,17 some studies demonstrate higher bond strength and less microleakage when enamel surfaces are treated with phosphoric acid prior to the application of self-etching primers.18,19,20 One of the advantages of self-etching primers is their clinical effectiveness is not dependent on the hydration state of the dentin. This eliminates ambiguities associated with the “wet bonding” technique (how wet is wet?). Despite this advantage, their erratic performance in terms of enamel etching capabilities is a concern.46 Based on their own testing, one widely read dental publication recommends etching both enamel and dentin with phosphoric acid for 15 seconds prior to the application of Liner Bond 2.21 After discussions with knowledgeable colleagues, reviewing the literature, and personal testing, I believe we should still be etching enamel with phosphoric acid (30-40% for 15 seconds) when using current “self-etching” primers. Of course this somewhat defeats the purpose of a self-etching primer, but it does alleviate concerns regarding their enamel etching capabilities. It also provides for a cleansing of the cavity preparation, which may be significant in terms of the removal of debris and potential contaminants prior to primer application. Until there is compelling evidence to the contrary, traditional phosphoric acid, in recommended concentrations and application times, still appears to be the most reliable and predictable way to etch enamel surfaces.
While bonding to enamel has proven to be a reliable and predictable treatment modality, bonding to dentin has proven to be a much more difficult clinical endeavor. This is due to morphologic, histologic, and compositional differences between these two substrates.
Enamel is a non-vital tissue with a mineral content (by weight) of approximately 96% hydroxyapatite.9 Dentin on the other hand is an intrinsically wet vital tissue comprised of approximately 70% hydroxyapatite (by weight), and 30% water and organic material (mainly type I collagen22). These percentages are not consistent and can vary significantly depending on a number of factors including dentin depth, the age of the teeth, and history of tooth trauma or pathology. Superficial, middle and deep dentin surfaces can be quite different in their structural and chemical composition.23 In addition, dentin is permeated with a fluid filled tubular network containing odontoblastic processes, which communicate directly with the pulp. All this variability, coupled with the relatively high water content of dentin, present a challenge for consistent and reliable long term bonding.
The ideal dentin-enamel bonding system should provide gap-free restorations, achieve rapidly developing high bond strengths, be biocompatible, function in the presence of moisture, and treat dentin and enamel simultaneously.24 Producing gap-free restorations is especially important because it helps minimize microleakage. Patients’ demands for more esthetic restorations have caused an increase in the use of tooth colored restorative materials such as composite. All current polymeric composite restorative resins shrink (2%-6% by volume) during polymerization. This results in the development of tensile stress, shear stress, or both at the tooth restoration interface. This, coupled with different thermal coefficients of expansion and contraction between tooth structure and composite, water sorption, and occlusal load, can lead to gaps, cracks in enamel,25,26 and microleakage. While gap-free composite restorations have been demonstrated both in-vitro and in-vivo,27 microleakage has not been totally eliminated.19,28-30 This may not be microleakage in the classic sense (between restorative material and tooth structure) but caused by nanoleakage within porosities in acid demineralized dentin that has not been fully penetrated by resin.31.32
Total etch, that is the simultaneous etching of both enamel and dentin surfaces, has gained in popularity over the last decade. There is good reason for this as both dentin and enamel surfaces can be conditioned in one step. This saves time and reduces the potential for operator error. Acid removes the dentin smear layer (Fig. 2) created by instrumentation, exposes and opens dentin tubules, raises surface energy, and demineralizes the dentin substrate so that it can be infiltrated by subsequently placed primers (resins). The depth of demineralization is contingent on the type of acid, its concentration, and duration of exposure.33 With few exceptions (such as self-etching primers) virtually all current adhesive systems are “total etch” systems using varying concentrations and formulations of phosphoric acid to treat both dentin and enamel surfaces prior to primer application. Up until recently, the intentional placement of acids on dentin, especially phosphoric acid, was considered taboo.34,35 It is now widely accepted that the acidic treatment of dentin is in fact not damaging to the pulp providing the pulp is healthy to begin with, and the dentin is well sealed after etching.36,37 In any case, total etch procedures have been used for a number of years with excellent clinical success.38 One of the reasons for this success is the ability of hydrophilic resin primers in current adhesive systems to infiltrate acid demineralized dentin, polymerize, and form a “hybrid” layer.4 Readers are referred to studies by Tay et al. and Van Meerbeek et al.39-40 for excellent descriptions of the physical and chemical nature of hybrid layer formation. Of utmost importance is the diffusion of primer (resin) through the entire thickness of demineralized dentin and its interaction with the unaltered substrate below.9 Failure to completely diffuse through the dem
ineralized zone could result in lower bond strength, nanoleakage, and sensitivity.
Kanca found that the presence of moisture on the dentin substrate, prior to primer application, significantly enhanced the bonding capabilities of some adhesive systems.3,24 This is now called “wet bonding” and its understanding and application is crucial for clinical success with most current adhesive systems. As previously mentioned dentin has a high collagen content. Dentin collagen fibers are surrounded and encapsulated by a mineral component (hydroxyapatite). When the dentin surface is conditioned with acid, the mineral component is removed, leaving behind a friable collagenous network. It is this collagen network (demineralized zone) that must be infiltrated by subsequently placed primers in order to achieve good bonding. Even a few seconds of air drying can cause this collagen layer to collapse into an amorphic gel-like mass that is difficult for primers to penetrate. By maintaining the collagen in a moist state the collagen fibers remain open and the substrate is more permeable to primer penetration (Figures 3-6). Adhesives that employ acetone as their primary solvent are especially sensitive to the hydration state of dentin prior to primer application (i.e., One-Step/BISCO, Prime and Bond NT/Caulk, Gluma One Bond/ Kultzer). These are effective systems when placed on a moist dentin substrate but far less effective when placed on air-dried dentin.41,42 Even systems which employ ethanol as their primary solvent (i.e., Single Bond/3-M, Optibond Solo Plus/Kerr, Permaquik/Ultradent) generally demonstrate higher in-vitro bond strength values when applied to moist dentin.12,43 On moist dentin, acetone and ethanol based primers dynamically displace water in the demineralized zone. Upon drying, the solvents and water evaporate, leaving behind a resinous layer which can then be polymerized. Subsequently placed composite materials will chemically bond to this layer which, in essence, is micro-mechanically locked into the dentin substrate. In order to prevent the collagen from collapsing it is important that the dentin is not dried excessively after etching. One effective way to accomplish this clinically is to not dry at all after rinsing off the phosphoric acid. In my own practice after washing off the acid, and prior to primer placement, I simply blot the preparation with a cotton pellet or sponge. The idea is to remove any puddles or pools of liquid but leave behind a surface that has a definitive shine to it. Bonding to wet enamel (no frosted appearance) is not a problem as long as it also is treated with primer (bond strength values can actually be enhanced in some cases).44,45
RE-WETTING AFTER DRYING
While I believe the best approach to “wet bonding” is not to dry at all and use the blot technique described above, there are times when the dentin is purposely or inadvertently dried leading to collagen collapse. It is often recommended that dried dentin be rewet with various agents (i.e., Tubulicid Red/Global Dental, AquaPrep-F/ BISCO, Gluma Desensitizer/ Kulzer, Concepsis/Ultradent, Microprime/Danville Engineering, etc.) to open the collapsed collagen network prior to primer application. Many of these products contain an anti-microbial in their formulation. There is speculation that the use of disinfecting solutions to re-wet dentin may be beneficial in terms of decreasing post operative sensitivity.47 While this seems to make sense, much of this supposed benefit is anecdotal in nature and there are concerns that some of these solutions could adversely effect the bond strength values of some adhesive systems.48,49 Other studies have shown no adverse effect on bond strength when using specific adhesives and specific re-wetting/disinfecting solutions.50 Kanca and Alex demonstrated that rehydration of dentin may be a time dependant phenomena.41 We found that several rewetting solutions could be employed without adversely effecting bond strength values when testing One-Step (BISCO) as long as the solutions were allowed to dwell for a sufficient time prior to primer application. We found that long drying periods require long hydration periods. While further research is indicated, we currently recommend if the dentist elects to air dry the dentin, it should be dried for only a few seconds and then re-hydrated for 30 seconds prior to primer application.
ADHESIVE DENTISTRY IN THE FUTURE
Much of the research in adhesive dentistry today is geared toward the simplification of clinical protocol. The emergence of reliable single component systems (One-Step/ BISCO, Optibond Solo/Kerr, Prime & Bond NT/Caulk, Single Bond/3M, etc.) has greatly expedited many adhesive procedures. There is continuing development of self-etching primers that will eliminate the ambiguities associated with the wet bonding technique. If these products can demonstrate consistent, long term, bonding to enamel as well as to dentin, they may become the adhesives of the future. Perhaps the most significant change will not be in the adhesives, but in the materials they are designed to bond in place. The “Holy Grail” of composite research is the development of non-shrinking polymers, with good physical and esthetic properties that will eliminate problems associated with polymerization shrinkage stress.
Although the renaissance in adhesive dentistry is upon us, much research has yet to be done. Improvements and refinements will continue in the future. Used in a knowledgeable fashion, with attention to technique and materials, adhesive technology offers expanded treatment options that can lead the clinician confidently into restorative techniques previously considered impossible.
Acknowledgement: This paper is dedicated to a friend, colleague, teacher, and mentor, the late Dr. John Gwinnett.
Gary Alex, DMD, is an accredited member of the American Academy of Cosmetic Dentistry and past president of the AACD New York chapter. He maintains a practice geared toward comprehensive prosthetic and cosmetic dentistry.
Oral Health welcomes this original article.
References available from the managing editor of Oral Health.