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Evaluating the importance of adhesive procedures among the myriad of techniques, materials, and technologies in modern dentistry can be challenging for most dentists. However, when one imagines daily practice without adhesion, its importance becomes clear. Without adhesion, how would a class IV cavity be restored? How about cementing a porcelain veneer or making a fissure sealant with good durability? How would diastemata be closed or orthodontic brackets be attached? What about minimally invasive preparations? Without adhesion, would the minimally invasive concept exist in dentistry? Would imperceptible esthetic restorations be possible? Would the technologic development of the dental profession have occurred in the same way without the adhesive revolution?1
It is difficult, not to say impossible, to think about modern dentistry without Buonocore's acid-etching technique,2 the development of composite resins,3 or the consequent popularization of various bonding techniques. However, high-quality adhesive procedures are not easily performed. A dependable adhesive interface is obtained only when quality materials are used under rigorous control and with a well-defined, understood, and practiced protocol.
Other factors can affect adhesion success, such as a lack of humidity control in the operatory field or proper isolation in the region to be restored. Rubber dam use is recommended when using adhesive systems.4 In cases when a rubber dam can not be placed, such as esthetic restorations that require visualizing periodontal tissues, the operatory field can be isolated with retraction cords and cotton rolls, as long as they do not affect the quality of the working-field isolation.
A high-quality tooth–restoration interface, especially the dentin–composite interface, must be obtained to prevent degradation over time. Degradation may be related to operator error with a bonding product, as well as to an inadequate chemical composition of the adhesive system. This article will discuss these problems and present some solutions.
Current Adhesive Systems: Classification and Composition
For many years, adhesive systems were classified based on generations (first through seventh generation).5 However, this classification system was confusing, impractical, and hard to remember because it had no relationship with the clinical steps of bonding. Thus, a new classification for adhesive systems based on adhesive strategy and number of clinical steps was created, and is commonly used in the literature. Contemporary adhesive systems are divided according to their bonding strategy: etch-and-rinse or self-etch. A second differentiation is based on the number of clinical steps for the adhesive procedure (number of different products that need to be applied): three steps, two steps, or one step (Table 1).
With both bonding strategies, etch-and-rinse or self-etch, adhesive systems are available with the primer and the bonding resin in different bottles as well as with the primer and bonding resin mixed in a single container (Table 2). This difference appears to modify the composition of the bonding agent and the product's ability to withstand degradation over time.7-10 Modern bonding systems contain an acid, a primer, and a bonding resin. However, the components responsible for those functions, as well as their distribution among the bottles, vary.11
Systems that mix the primer and bonding resin in a single bottle become highly hydrophilic, which appears to have no affect on their immediate bond strength but compromises long-term durabilty. Durability is associated with adhesive systems that have a separate bonding resin composed of hydrophobic monomers.7-10
Adhesion Durability
Dental manufacturers regularly release new adhesive systems with fewer clinical steps and shorter application times, in attempts to ease the clinician's job in bonding dental substrate to composite. When some of these products are introduced, the manufacturers release studies demonstrating their very high bond strength values and low marginal leakage scores; however, most of the tests were performed immediately after bonding. It is important to remember that, over time, adhesive restorations are exposed to mechanical (occlusal forces), thermal (temperature variations), and chemical (oral environment exposure, hydrolytic and enzymatic degradation) challenges.7 When analyzed in long-term studies, many of these simplified bonding agents present disappointing results.
The majority of the problems related to bonding durability are found in the hybrid layer the interdiffusion zone between dentin and resin that is composed primarily of demineralized dentin (collagen) infiltrated by fluid resins (bonding agent).12 Two main types of degradation occur in the hybrid layer: (1) bonding resin; and (2) exposed collagen fibers.
Degradation of Bonding Resin
Degradation of the resinous component of the hybrid layer occurs by a chemical phenomenon, called hydrolysis, that breaks molecules by adding water. Hydrolysis happens when the polymer (bonding agent) absorbs water; therefore, each product is affected by its polymer's water sorption degree. Water sorption degree is directly correlated to the hydrophilicity degree.4,8,9,13 Simplified bonding systems that combine hydrophilic primer and bond resin in a single bottle do not produce a membrane on the dental substrate that is capable of interrupting the water flux, and are considered semipermeable.8,14,15 These adhesive systems have high water sorption degrees and, consequently, are more prone to hydrolysis.4,8,9,13 Further, the monomer–polymer conversion degree is reduced significantly in highly hydrophilic adhesive systems, which increases their permeability and degradation rates.16,17
Conversely, adhesive systems that require a separate coat of a hydrophobic material allow less water sorption, are less permeable, have higher monomer–polymer conversion degrees and, therefore, are less prone to degradation over time.4,8,9,13,16,17
Degradation of Exposed Collagen Fibers
In an ideal hybrid layer, collagen fibers always should be covered by bonding resin. However, adhesive monomers are not always capable of infiltrating completely the acid-demineralized dentin. In these cases, a zone of collagen fibers becomes exposed, which commonly occurs when the etch-and-rinse strategy is used.18,19 Even when acid-demineralized dentin is infiltrated completely by the adhesive monomers, collagen fibers can become exposed because of the hydrolytic degradation of the bonding resins used to cover them. This degradation occurs primarily with simplified, highly hydrophilic adhesive systems.7,13
Collagen fibers not covered by resin are prone to hydrolytic degradation and to the action of enzymes (ie, matrix metalloproteinases) that contribute to the deterioration of the collagen matrix (Figure 1, Figure 2 and Figure 3).7,8,13
Consequences of Hybrid Layer Degradation
Deterioration of the main components of the hybrid layer seriously compromises the adhesion between dentin and composite. Sealing of dentin tubules is jeopardized, the restoration's retention is compromised, and marginal leakage occurs. These problems may lead to negative outcomes of restorative procedures.
Laboratorial studies have identified that simplified adhesive systems (with priming and bonding functions combined into a single bottle) lose most of their capacity to adhere dentin to composite after 1 year in contact with water.20,21 Longitudinal evaluations of different adhesive systems have demonstrated adhesive interface degradation in the clinical setting. These evaluations, usually performed in Class 5 restorations, have shown that long-term adhesion occurs with the use of bonding agents with a separate hydrophobic component (etch-and-rinse, three steps; self-etch, two steps).8,10
However, studies have shown that in restorations with enamel margins the deterioration of the hybrid layer is slow, which can delay problems caused by the degradation of the dentin–composite interface.22,23 Preservation of enamel margins is, therefore, always recommended when clinically possible.
Improving the Performance of Current Adhesive Systems
Formation of a durable and dependable adhesive interface is an indispensable procedure for the achievement of restorative success. Thus, the available adhesive systems must be used in an ideal manner, even if it means additional steps or increased application time. This section of the article highlights aspects of the bonding protocol that may contribute to the attainment of a high-quality adhesive interface. A general protocol for each category of adhesive systems is presented, but small variations may be required depending on the specific brand of adhesive used.
Etch-and-Rinse, Three Steps
A total etch should be performed with 32% to 40% phosphoric acid, beginning with the enamel and extending to the dentin where the acid is allowed to act for 15 seconds. After copious rinsing of the acid with an air–water spray, the humidity excess should be removed. Overwet dentin will not be infiltrated adequately by the bonding agent because of the collapse of the collagen fibers. In excessively humid dentin, primers will not be able to evaporate the water with their solvents, and the bonding agent components will be "diluted, compromising the adhesive interface.24
Errors in primer application are common. Primer requires time to evaporate the water into the collagen-fiber matrix and to infiltrate its monomers into the full extent of the demineralized dentin. The primer should be allowed to act for 20 to 30 seconds, followed by a mild air stream.25 Then, the bonding agent should be applied uniformly and light-activated for approximately 20 seconds at a light intensity of 600 mW/cm2.16,17
Etch-and-Rinse, Two Steps
With these adhesive systems, total etching, rinsing, and humidity removal procedures are the same as those described in the three-step protocol. The bonding agent (primer and bonding resin in a single bottle) should be applied in coats. Each coat should take approximately 10 seconds and be applied by vigorously brushing the adhesive system against the cavity walls,26 followed by a mild air stream to enhance solvent evaporation. Most manufacturers recommend two coats of the bonding agent. However, evidence suggests that increased water evaporation and more complete adhesive resin infiltration into demineralized dentin occurs when four layers of the bonding agent are applied.27 After four coats of the bonding agent are applied, light-activation is performed. Because this category of adhesive systems does not create a hydrophobic layer, polymerization is jeopardized, and better results are obtained only when the light exposure time is increased. Thus, the recommended activation time is 40 seconds at 600 mW/cm2.16,17
Self-Etch, Two Steps
With self-etching systems, product instructions note that no initial phosphoric acid-etching step is required, which reduces the adhesive systems' clinical application times. However, the best results for enamel bonding with self-etch, two-step systems are associated with initial acid etching28-30 or by doubling the self-etching primer's application time.31
With the initial acid-etching technique, the enamel only (dentin should not be etched with the phosphoric acid) should be etched with 32% to 40% phosphoric acid for 15 seconds, followed by rinsing and air drying. With the self-etching-primer technique, the self-etching primer should be applied to the complete preparation, both enamel and dentin, and actively brushed for 20 to 30 seconds, then followed by a mild air stream. With both techniques, the next step is to apply uniformly the hydrophobic bonding resin, followed by light-activation for 20 seconds at 600 mW/cm2.16,17
One self-etch, two-step adhesive system has a mild pH (≈ 2) and contains the functional monomer 10-methacryloxydecyl dihydrogen phosphate. In comparative testing, this composition appears to create a chemical bond to hydroxyapatite, with low water solubility, which may enhance bonding results.32
Self-Etch, One Step
Self-etch, one-step systems, even those with a low pH, can benefit from an initial etching step with phosphoric acid on enamel.31 This step should be performed as described in the two-step protocol. The adhesive should be applied for approximately 15 seconds, followed by an air stream. Then, this procedure should be repeated, and light-activation performed for 40 seconds at 600 mW/cm2, if the adhesive is highly hydrophilic.16,17 Applying a hydrophobic bonding resin onto the treated surface also can improve the bonding results, essentially transforming self-etch, one-step systems into two-step systems.30,33 Table 3 summarizes the suggested clinical protocols for the currently available adhesive systems that show the best laboratorial and/
or clinical results.
Conclusions
Studies on the durability of the bond, especially the clinical evaluations, are important to adhesion research. These studies have shown that simplified adhesive systems, ones that mix priming and bonding agents in a single bottle, do not provide the long-term adhesion noted in systems with a separate hydrophobic resin layer.7-10
Modified application protocols have been presented in the literature, aiming to increase the quality of the bond with simplified adhesives. However, most of these modifications increase application time, eliminating the greatest advantage of the simplified systems.
Some recent reviews of the current status of adhesion systems affirm that the etch-and-rinse, three-step adhesive systems are still the best products to promote bonding between dental substrate and composite.7-10
Self-etch, two-step systems best approximate the results of the etch-and-rinse, three-step adhesives. However, the self-etch, two-step systems lack the ability to etch enamel as well as phosphoric acid does,7,9,10 and require a separate enamel-etching step,28-30 increasing application time.
Etch-and-rinse, two-step systems offer quality immediate results and regular medium-term results, but seem to deteriorate over time because of their high hydrophilicity,7-10 especially when restorations margins are on dentin.22,23,34
One-step, self-etching systems do not offer acceptable results in promoting minimally dependable adhesion between dental substrate and composite.7-10
Because of the importance of the daily adhesive procedures in the dental practice, it seems prudent to use nonsimplified adhesive systems that can promote a dependable and durable adhesive interface. Further, to ensure that the highest quality bond is created, ideal application protocols need to be rigorously followed, even if they increase application time. The time reduction promoted by simplified adhesives is quite insignificant in the context of a complete restorative procedure and does not seem to be an acceptable reason to reduce the quality of the adhesive interface.
Acknowledgements
The authors would like to thank the Brazilian National Council for Scientific and Technological Development (CNPq) and the Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES) for grants Nos. 142966/2006-1 and BEX2759/07-1, respectively; the German Academic Exchange Service (DAAD), and the Graduate Program in Dentistry of the Federal University of Santa Catarina for the research opportunities.
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About the Authors
Leandro Augusto Hilgert, DDS, MSc
PhD Candidate, Department of Operative Dentistry
Universidade Federal de Santa Catarina
Florianópolis, Brazil
Visiting Researcher, Department of Prosthodontics
Ludwig-Maximilians-Universität
Munich, Germany
Guilherme Carpena Lopes, DDS, MSc, PhD
Associate Professor, Department of Operative Dentistry
Universidade Federal de Santa Catarina
Florianópolis, Brazil
Élito Araújo, DDS, MSc, PhD
Full Professor, Department of Dental Clinics
Universidade Federal de Santa Catarina
Florianópolis, Brazil
Luiz Narciso Baratieri, DDS, MSc, PhD
Full Professor, Department of Operative Dentistry
Universidade Federal de Santa Catarina
Florianópolis, Brazil