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One key factor that determines the clinical success of a composite restoration is the quality of the bond at the tooth-restoration interface. The composite placement requires the use of a meticulous technique, not only for esthetic form and function, but also to achieve effective bonding via dental adhesives. Since Buonocore’s reports on bonding to enamel in 19551 and to dentin in 1956,2 dental adhesives have been improved in many aspects. Enamel bonding has been predictable, but bonding to dentin remains more of a challenge due to the composition of dentin itself and its fluid-filled dentinal tubules. From the 1960s to early 1990s, researchers and manufacturers have attempted to create adhesive systems that provide reliable bond strength to both substrates. The outcome has been the fourth-generation adhesives, currently known as multistep total-etch or etch-and-rinse adhesives, which could bond equally well to dentin and enamel.3 Research findings showed etching enamel with 30% to 40% phosphoric acid followed by sufficient rinsing (etch-and-rinse) resulted in surface microporosities for reliable mechanical retention.4 More research outcomes indicated the simultaneous application of the acid on enamel and dentin (total-etch) combined with hydrophilic adhesives resulted in a hybrid layer as a bonding mechanism.5 Some products from this group are still in use, such as OptiBondTM FL (Kerr, kerrdental.com) and AdperTM ScotchbondTM Multi-Purpose Plus (3M ESPE, 3mespe.com); OptiBond is considered a gold standard.6
Simplification has been the key word for the millennium. Minimizing steps to simplify the bonding procedure not only saves clinical time, but also potentially decreases discrepancies, thus improving efficacy. Manufacturers have offered simplified adhesives that correspond with their multistep precursors by combining hydrophilic primer and hydrophobic resin into one bottle, eg, Adper Single Bond (3M ESPE), One-Step® (Bisco, bisco.com), and OptiBond Solo (Kerr).7 Separate etching and rinsing steps are required for this group. Therefore, the application time is not substantially reduced.
Another group of simplified adhesives introduced in the late 1990s are the self-etch systems, which do not require separate etching and rinsing steps. The difference is that the original self-etch primers were used only for dentin and separate selective enamel etching and rinsing steps were necessary. The current self-etch adhesives simultaneously condition (etch) and prime both enamel and dentin. They are subdivided, according to their bonding procedures, into two-step systems that still require a separate adhesive resin to be applied after the self-etch primer, and one-step or all-in-one systems that combine the conditioning, priming, and adhesive application together.8 Acidic monomers in the mild self-etch adhesives, such as methacryloyloxi-decyl-dihydrogen-phosphate (10-MDP), 4-methacryloyloxyethy trimellitate anhydride (4-META), and 2-(methacryloyloxyethyl)phenyl hydrogenphosphate (phenyl-P), promote chemical bonds to hydroxyapatite in addition to the micromechanical retention of the hybrid layer.9
The latest trend in dental adhesives is universal bonding. Some examples are Scotchbond Universal (3M ESPE), Prime&Bond Elect® (Dentsply Sirona, dentsply.com), ClearfilTM Universal Bond (Kuraray), and All-Bond Universal® (Bisco). Universal means that these adhesives perform well in the self-etch mode, selective enamel etching mode, and etch-and-rinse mode.10 Universal also indicates that the functional monomers provide chemical bonding to other substrates such as silica-based glass ceramics or zirconia.11 Universal adhesives are dependent on similar acidic monomers as those of mild self-etch adhesives. The monomer 10-MDP provides the most stable bond in an aqueous environment.9,12
Clinicians desire the best treatment for their patients and want to select ideal products. However, with so many adhesives available, the choice is not easy. Numerous laboratory and clinical studies have been performed to evaluate their performances. Research data, manufacturer information, and publications help clinicians in the selection of adhesives suitable for certain clinical conditions. However, just as important or even more important than the adhesive itself is the application. This point is best illustrated by a study from the University of California at San Francisco, in which shear bond strengths to dentin of one product varied from 8 MPa to 32 MPa depending on operator experience and (most likely) the ability to precisely follow the manufacturer’s instructions.13 Essentially this indicates a clinician can have the best adhesive on the market but will not necessarily obtain a good outcome if the procedure is not performed correctly. Therefore, an ideal adhesive gives a reliable bond despite being misused. In other words, dental adhesives that are less technique sensitive, or more forgiving, routinely perform better.
In this article, we will discuss current dental adhesives based on technique sensitivity, common pitfalls in bonding procedures, and ways to avoid those issues.
Pitfalls in Bonding Procedures
Carious or Sclerotic Dentin
No matter the adhesive material used, bonding to caries-affected and sclerotic dentin is a challenge. Caries-affected dentin has areas with high mineral loss, a disorganized collagen matrix, and dentinal tubules occluded with minerals. Sclerotic dentin also has crystalline deposits that obliterate the tubules with thick hypermineralized surface layers that are not easily etched. In general, research has shown etch-and-rinse adhesives were more effective than self-etch adhesives for both caries-affected and sclerotic dentin.14-16 Thirty-percent phosphoric acid performed better than 10% phosphoric acid for etching sclerotic dentin. Certain components of the adhesives, particularly polyalkenoic acid in Scotchbond products and 10-MDP in self-etch and universal adhesives, are beneficial because they are able to bond chemically with the mineral phase in sclerotic dentin.16 Extending etching time or using more of the phosphoric-acid etch-and-rinse increased bond strengths in one study;17 however, this may lead to over-etching of sound dentin.
Contamination
Contamination with saliva or blood reduces the bond strength. The reported research results are not consistent due to differences in experimental design, timing of contamination, and tested materials. Some studies showed a universal adhesive (Scotchbond Universal) and total-etch one-bottle adhesive (Adper Single Bond [3M ESPE] and Prime&Bond NT [Dentsply Sirona]) were not affected by saliva contamination.18-20 When saliva contamination occurred, rinsing off the saliva and drying before repeating the adhesive procedures recovered the bond strength of self-etch and universal adhesives.19,21 For the total-etch adhesive, one study reported that blotting the surface and applying the primer (without re-etching) recovered the bond strength after saliva contamination.22
Over-etching Dentin/Under-etching Enamel
Currently, both enamel and dentin have to be etched or acid conditioned to achieve reliable bonding. Finding a balance between under-etching enamel and over-etching dentin is important. For enamel, using phosphoric acid etch-and-rinse resulted in better marginal integrity and less discoloration than employing self-etch primer/adhesives.23 However, phosphoric acid is prone to over-etching dentin. Partial dentin demineralization for resin infiltration into the collagen network to form a hybrid layer is crucial. Aggressive etching leaves a zone of weak dentin under the hybrid layer, increasing susceptibility to long-term degradation.5 The use of self-etch adhesives solved the issue of over-etching dentin by simultaneously demineralizing and infiltrating the exposed collagen scaffolds.8 On the other hand, self-etch adhesives may under-etch enamel. A few approaches may overcome the over-etch/under-etch dilemma:
A) For the etch-and-rinse total-etch systems (eg, ScotchBond Multipurpose, SingleBond Plus, Optibond FL, Optibond Solo Plus), manufacturers usually instruct clinicians to etch enamel and dentin for 15 seconds. Faculty in the University of Tennessee Health Science Center’s Restorative Dentistry Department teach predoctoral students to apply the phosphoric acid etchant precisely (by using a syringe-type etching gel) along the enamel margins for 15 seconds, apply the etching gel on dentin for 10 seconds, rinse thoroughly for 15 seconds, and blot dry. This prevents over-etching of dentin while the enamel has a longer etching time.
B) Most all-in-one or one-step self-etch adhesives are not good choices because they are too hydrophilic and have not performed well clinically, even though they are more acidic for effective enamel etching.24 However, newer products may change this assessment when results from long-term clinical studies become available.12
C) Mild two-step self-etch adhesives are considered the gold standard for dentin adhesion.25 For the enamel margin, however, slight discoloration and small marginal defects were observed in a 5-year clinical study.26 Selective enamel etching with phosphoric acid improves marginal integrity, but spoils the simplified bonding concept by restoring the etch-and-rinse step in the self-etch procedure. More importantly, dentin bond strength was decreased when dentin was (inadvertently) etched during selective enamel etching.27 In short, self-etch adhesives are not forgiving in terms of bonding to enamel, especially to uncut enamel. Perhaps the prudent way to increase enamel bonding of self-etch adhesives is to prepare the enamel margin.28 Finishing the cavity walls with an extra-fine diamond bur results in thinner smear layers, which is preferable for the self-etch adhesives.9
D) Universal adhesives have been developed to function in either self-etch, selective etch, or etch-and-rinse modes. Findings from laboratory and 18-month clinical studies showed comparable bonding results to dentin with or without phosphoric acid pre-etching for most commercially available universal adhesives.10,11 However, marginal discrepancies at the enamel margins (which could be repolished) were found more frequently in the self-etch mode.10 Even though they are less technique sensitive, universal adhesives are highly hydrophilic. High water content in these adhesives may increase permeability of resin-dentin interfaces and affect long-term stability of the bond. In addition, when using the etch-and-rinse technique, the etched dentin surface may be depleted of apatite crystals to provide stable bonds to acidic monomers as expected in the self-etch mode. Therefore, clinicians may wish to wait for the results of long-term clinical studies to become available before switching to new materials.
Overwet/Overdry
The demineralized collagen scaffold easily collapses when etched dentin is dried after the etch-and-rinse procedure.29 This phenomenon suggests that etch-and-rinse dentin adhesives are sensitive to overdrying. It is well known that etched dentin should be moist before the application of the adhesive. The definition of moist, however, is typically open for interpretation. Hydration conditions can be described as follows: overwet is water slightly pooled on the surface, moist is a shiny appearance on the dentin surface after excess water is removed by blotting, and overdry is when the dentin surface is gently air dried for 15 seconds with the air source approximately 2 cm away from the tooth.13 An alternative description of a hydrated condition is: “…the surface should be gently dried until the etched enamel presents its white-frosted appearance and dentin loses its shine and turns dull.”9 Adhesives with hydrophilic primer (eg, Scotchbond Multipurpose, Optibond FL) can rehydrate a gently air-dried collapsed collagen network and, thus, are less sensitive to overdrying.30 Clinicians should also be aware that overwetting the dentin surface could interfere with sealing due to the formation of blister-like voids. Acetone-based etch-and-rinse adhesives are more sensitive to the overwet state, such as water puddles in the proximity of preparation line angles.31 Self-etch adhesives are not sensitive to overdrying/overwetting because they no longer require the etching, rinsing, and drying steps.
Adhesive Layer Being Too Thin
The resin layer on top of the hybrid layer acts as a shock absorber that accommodates deformations and thus helps relieve stresses at the interface. Adhesives belonging to the one-bottle group, which combine hydrophilic primer/adhesive, are prone to having bonding layers that are too thin, thus becoming water permeable. They should be applied copiously or in multiple layers to saturate the exposed collagen network and create a thick resin layer, which appears as a glossy film. Adhesives with high acetone content (such as All-Bond) are more vulnerable to having a thin bonding layer, and, hence, are more technique sensitive than water- and ethanol-based adhesives.9 Highly hydrophilic adhesives, such as the universal and all-in-one, exhibit the possibility for being too thin as well. Nanofillers added to the adhesives may help the formation of a thicker resin film.8 Self-etch adhesives also benefit from multiple coats by increasing etching ability with an additional supply of fresh acidic monomers.32 Adding a hydrophobic resin layer on top has been suggested to improve clinical performance.10,26 The only drawback is that this defeats the purpose of simplification or time savings.
Insufficient Solvent Evaporation
Dental adhesives contain water and organic solvents (ethanol or acetone) to facilitate monomer infiltration into collagen networks of etched dentin. When applying an adhesive, complete evaporation of the solvent and water optimizes the polymerization. Solvent evaporation is usually accomplished by agitating the adhesive on the dentin/enamel surfaces and by using compressed air. Extended solvent-evaporation time improved the bonding of adhesives across the board. Agitation by vigorous rubbing and extended evaporation time also allow better resin infiltration into the collagen network.32 Universal adhesives in etch-and-rinse mode had higher bond strengths and less nanoleakage when the evaporation times were increased from 5 seconds or 10 seconds to 25 seconds.33 Acetone-based universal adhesive required a longer evaporation time than that recommended by the manufacturer (15 seconds vs 5 seconds) and took longer to evaporate than water- and alcohol-based adhesives, even though acetone has a higher vapor pressure.33 Sufficient solvent evaporation is even more important when the bonding layer is thick. Manufacturer’s instructions usually recommend gentle air blowing to achieve a uniform and even layer of adhesive because aggressive use of compressed air can cause excessive thinning. Conversely, a new approach suggests the use of strong air drying or a doubling of the normal pressure improves bonding effectiveness by increasing resin infiltration and facilitating removal of solvent and water.34,35
Light Curing of Adhesive
Regardless of adhesive type, the bonding layer has to be satisfactorily light cured prior to the placement of restorative composite to achieve reliable bonding.36-38
Shelf Life and Evaporation of Solvent
Dental adhesives are a mixture of monomers and other chemical components. These compositions can gradually change, and, hence, adhesives have a period in which they retain optimum bonding efficacy, known as the shelf life. One-step self-etch adhesives tend to have limited shelf life because methacrylate monomers mixed with water under acidic conditions are prone to hydrolysis.39 In addition, shelf life is limited by the evaporation of the volatile organic solvent during use. Acetone-based adhesives are more vulnerable to evaporation. Lower dentin bond strength was reported when bottles were repeatedly opened.40 An ethanol-based universal adhesive had impaired bond strength when the evaporation is more than 50%.41 Evaporation from repeatedly opening the bottle is no longer an issue in unidose packaged adhesive. Whether bonding effectiveness is reduced when a unidose package is left open during a treatment visit has been addressed in the accompanying research study conducted by the authors of this continuing education article (dentalaegis.com/go/cced1006).
Beyond Current Adhesives
New developments are on the horizon, although an in-depth discussion is not within the scope of this paper. From the beginning, researchers and manufacturers have focused on the morphology of tooth substrates and the physical and chemical properties of adhesives. Contemporary concepts include biologic factors in the longevity of bonded interfaces. Endogenous enzymes, such as matrix metalloproteinases and cysteine cathepsins, can degrade exposed collagen under or within the hybrid layer. Research is underway to incorporate enzyme inhibitors into dental adhesives. The use of chlorhexidine has been shown to eliminate or delay collagen degradation when applied before the adhesive. More information can be found in an excellent review by Tjäderhane.42 Other innovative approaches in the research phase are, for example, biomimetic remineralization of dentin to improve bond stability43 and incorporation of bioactive components into dental adhesives.44 However, even if a silver bullet of the highest-quality long-lasting adhesive were to emerge from these developments, the effectiveness would still be dependent upon technique sensitivity and, thus, proper use.
Summary
Dental adhesives that are forgiving under less-than-ideal clinical conditions contribute to reliable bonding. Among the many details reviewed in this article, a number of common features were noted. Etch-and-rinse adhesives provide better enamel bonding than self-etch adhesives, but their use has the potential to lead to over-etching dentin and are sensitive to moisture levels. Moisture-free compressed air used to evaporate solvent and water from the adhesive layer is critical, especially for the acetone-based adhesives. Adhesives with separate bonding steps (ie, multistep etch-and-rinse, two-step self-etch) provide consistent bonding layers, whereas the combination of a primer in one-bottle or one-step/universal self-etch adhesives is at risk for creating permeable bonding layers. Each group of dental adhesive has (slightly) different handling characteristics. Understanding the pitfalls of each adhesive and employing meticulous bonding techniques are crucial for the success of composite restorations.
Acknowledgments
Supported by the University of Tennessee Health Science Center College of Dentistry Alumni Endowment Fund and the Tennessee Dental Association Foundation.
About the Authors
James A. Lane, DDS
Assistant Professor
Department of General Practice Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Samantha J. Hughey, DDS
Former dental student
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Private Practice
Hendersonville, Tennessee
Paul N. Gregory, DDS, MHA
Assistant Professor
Department of General Practice Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Daranee Versluis-Tantbirojn, DDS, MS, PhD
Professor
Department of Restorative Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
James F. Simon, DDS, MSEd
Professor and Director
Division of Esthetic Dentistry;
Professor
Department of Restorative Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Janet Harrison, DDS
Professor and Former Chair
Department of Restorative Dentistry
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Antheunis Versluis, PhD
Professor and Director
Biomaterials Research
Department of Bioscience Research
College of Dentistry
University of Tennessee Health Science Center
Memphis, Tennessee
Queries to the authors regarding this course may be submitted to authorqueries@aegiscomm.com.
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