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Introduction
Dr. Michael Buonocore established the basis for modern restorative dentistry in 1955 when he described a technique for bonding acrylic materials to enamel using phosphoric acid.2 However, it took many years for his techniques to become universally accepted. It was originally thought that the application of phosphoric acid to enamel was acceptable but that application to dentin could cause serious pulpal damage. In 1982, Nakabayashi3 reported good adhesion by etching both the dentin and enamel, followed by infiltrating the etched surface with hydrophobic and hydrophilic monomers. At about the same time, Fusayama was doing his own research, and in 1993 he published his book that shed new light on the application of phosphoric acid to dentin, which has led to widespread use of this technique.4
Dentin bonding involves adhesion. There are three types of adhesion: chemical, van der Waals interaction, and mechanical (either micromechanical or macromechanical). Adhesive systems can use the types of adhesion either individually or together, depending upon the manufacturer. Chemical adhesion involves bonding at the atomic or molecular level. There is an ionic bond to the mineralized component of the dentin and a covalent bond to the organic constituents of the dentin. This type of bonding occurs with glass ionomer materials. The van der Waals interaction is based on an electrostatic or dipole interaction between the bonding agent and the substrate. The strength of the dipole interaction is directly measured by the contact angle. The contact angle is the angle formed when a liquid is placed in contact with a solid. The contact angle is a good indicator of the wettability of solid by a particular liquid5 and whether a solid is hydrophilic or hydrophobic (Figure 1 and Figure 2). The highest bond strengths are obtained when van der Waals forces are optimized. This is the primary bonding of resin to dentin and enamel. Micromechanical bonding is based on retention by the blocking of movement of the restoration (also known as “luting”). Adhesion is an attempt to minimize the space between the tooth structure and the restorative material.
Obstacles to Adhesion
1. Dentin contains an organic component (collagen 27% by volume percent [vol%]) and hydroxyapatite (47 % by vol%).6
2. Dentin has a low surface energy and is a protein. It is this protein component that is responsible for the low surface energy.
3. Dentin is hydrophilic by nature (dentin is 21% water by vol%).6
4. Constant surface contamination from oral fluids.
5. Morphological as well as chemical changes that take place due to aging and pathology.
6. Composite resin is hydrophobic.
Enamel and dentin are two histologically different materials that lead to different responses to acid etching and resin penetration. Enamel is primarily composed of hydroxyapatite (95% by weight percent [wt%]) with the remainder being noncollagenous proteins, lipids, ions (1% by wt%) and water (3% by wt%).6 The acid works on enamel by removing the contaminated outer surface (saliva, protein, fluoride) to create a surface that is easily wetted by the resin (Figure 1). The contact angle for resin on enamel changes drastically by etching the enamel. It was this observation by Buonocore that led him to pursue enamel bonding.2 This bond has been reported to be 20-25 MPa and is universally accepted to be strong enough to resist the forces of composite polymerization shrinkage and create an excellent seal if done properly. The acid of choice seems to be orthophosphoric acid at a 30% to 38% concentration.
Dentin is a completely different substrate to attempt to bond. Several factors contribute to dentin being a more complex material for bonding. Compared to enamel, dentin is comprised of more organic content and less inorganic content. Dentin is comprised of hydroxyapatite (70% by wt%), collagen (18% by wt%), and water (10% by wt%).6 In addition, the physical composition and microstructure of dentin can change both physiologically and pathologically from patient to patient, tooth to tooth, and depth into the tooth. Dentin’s response to decay with the formation of reparative dentin and sclerotic dentin all lead to changes in the structure of dentin that may affect the ability of materials to bond to it.7 Bonding to a multicomponent or multiphasic material such as dentin is more difficult because the different phases have different surface energies. After Buonocore established the bonding to enamel, his technique was tried on dentin with disastrous results. For years, bonding to the inorganic phase of dentin was tried with poor results (glass ionomer bond to calcium in dentin). Also, the Buonocore technique required that the substrate be dry, which is very difficult to achieve for dentin. By etching the dentin vigorously, such that the entire inorganic phase is removed and where bonding takes place only to collagen, then successful bonding was achieved. The discovery that collagen needed to be wet to be preserved made dentin bonding a reliable and clinically dependable method. The very low contact angle of the bonding agent to etched dentin confirms that dipole interaction is a major contributor to the bond strength.
When the tooth surface is altered by rotary and manual instrumentation during cavity preparation, cutting debris is smeared over the enamel and dentin surfaces, forming what is termed the smear layer.7 This layer of debris has a great influence on the bond to dentin because it is loosely attached and limits the bond strength. This layer of debris can not be washed off but can be removed by an acid. Two methods have been proposed to overcome this potential weak link in the attachment of composite to dentin: (1) removal of the smear layer prior to bonding with an acid (the etch-and-rinse method) and (2) using the bonding agent to penetrate the smear layer and bind it to the underlying dentin (the self-etch method).
All current composite resin formulations undergo some degree of polymerization shrinkage during their setting. Most of the present composites shrink 2% to 3% by volume upon curing; however, some new composites have been introduced for posterior restorations that have shrinkage values less than 2%.8 This shrinkage causes stresses to be placed on the bonding adhesive, which acts to pull the bonding agent from the bonding surface. It has been determined that this shrinkage stress is in the range of 18 MPa.9 Also, differences in coefficient of thermal expansion between the composite resin and the dentin surface will further stress the adhesive interface during any thermal changes experienced in the oral environment. Therefore, for a composite restoration to be successful, the bond strength of the bonding agent used must be greater than the forces generated by polymerization shrinkage and thermal expansion/contraction of the restoration.
Before discussing the different generations in dentin bonding agents, it is important to look at the processes involved in dentin bonding. Three separate processes are required to achieve dentin bonding; etching, priming, and bonding.10 Depending on the bonding system used, these processes may be accomplished by individual steps or in combination. For example, one manufacturer’s system may be three steps, first using an etchant, followed by the primer, and finally the adhesive while another manufacturer’s system may accomplish all processes by incorporating all components in one bottle and one application step.
Total Etching
Preparation of the dentin surface for bonding starts with etching. The total-etch technique uses phosphoric acid as the etchant. Etching the dentin removes the smear layer and creates a demineralized zone of dentin, exposing collagen fibrils. This creates a porous surface allowing for primer penetration. Application of the etching material for too long a period of time on the dentin can lead to a demineralized zone that can not be filled with resin and is a potential cause of postoperative sensitivity. The pH of self-etching and self-priming adhesives systems is low enough to etch through the smear layers and underlying dentin without a separate etching step.10
Priming
Primer solutions ensure efficient wetting and displacement of moisture (water) from the collagen fibril network for attachment of the adhesive component.11 Primers consist of hydrophilic monomers carried in one of three solvents; acetone, ethanol, and/or water. When applying primers, consideration must be given to the properties of the solvent being used to achieve the best dentin bond. In addition, the low concentration of the primer in solution frequently requires multiple applications to achieve adequate dentin penetration.10 Once etched, the dentin surface is impregnated with the primer solution. The addition of the primer leads to the formation of the hybrid layer, a process of resin interlocking in the demineralized dentin surface, thereby providing micromechanical retention and microtags, which are critical to dentin bonding.7,10 The dentin bond strength is related more to the number of microtags formed than the length of the microtags. During the formation of the hybrid layer, the primer also penetrates into open dentinal tubules forming macrotags. Though larger than microtags, the macrotags do not seem to contribute much to the dentin bond strength.10
Bonding
Bonding agents are hydrophobic, dimethacrylate oligomers that are compatible with the monomers used in the primer and the composite.10After placement of the primer, the bonding agent is applied to the treated dentin surface and light cured. This leaves an air-inhibited layer on the surface of the treated dentin. Once the composite is placed, air is displaced and with light curing, copolymerization takes place: adhering the composite to the dentin surface.10
Bonding Systems
To simplify the classification of bonding systems, other classification systems have been introduced. Many bonding systems today are classified by their procedural steps. This has led to the classification of bonding systems such as “total etch” (etch-and-rinse) and “self-etch” (no rinse). Previously, the systems were classified by “generations.”
First-Generation Systems
The first-generation dentin bonding agents combined BisGMA (bisphenyl-A glycidyl methacrylate) unfilled resin and polymerizable phosphates. These products were phosphonated esters of BisGMA. The addition of the phosphonated ester groups thinned the BisGMA, allowing it to penetrate the etched enamel easier. The dentinal smear layer was used with these systems to act as a barrier against penetration of the composite resin to the pulp and it also helped to keep the water component of the dentinal tubules from competing for adhesive bonding sites of the collagen and calcium. Thus, an entangled gel was formed as the unfilled resin intertwined into the smear layer, forming a mechanical interlock along with chemical bonding involving the calcium and phosphonated ester groups of the BisGMA. Bond strengths from one quarter to one third those found in enamel bonding were established. The bond strength of etched enamel is 20–25 MPa. Products in this category included: Bondlite (Kerr Corporation, Orange, CA); Clearfil (Kuraray Co. Ltd., New York, NY); and Scotchbond (3M Corporation, St Paul, MN).
Second-Generation Systems
Second-generation dentin bonding agents were modifications of the first-generation phosphonated esters. The difference was that they were light cured. These adhesives involved an ionic bond to calcium by chlorophosphate groups. The bond strength was slightly better, but was not greater than the stress created by the polymerization shrinkage of the composite materials. Though second-generation adhesives produced weak bonds and revealed poor clinical results, enhanced surface wetting capabilities were noted. Bondlite (Kerr Corporation) is a representative example.
Third-Generation Systems
Third-generation systems used conditioners and/or primers that altered or removed the smear layer. Conditioners were applied to the dentin and washed off, where primers were usually applied and dried onto the dentin surface. Typical examples of conditioners included weak organic acids and included nitric acid, ethylenediaminetetraacetic acid (EDTA), citric acid, maleic acid, and phosphoric acid. Primers are hydrophilic wetting agents that are compatible with both dentin and bonding resins. Primers aid in adhesion because they reduce the contact angle, which improves contact between the hydrophilic dentin and the hydrophobic BisGMA. Typical examples of primers include hydroxyethyl methacrylate (HEMA), pyromellitic diethylmethacrylate (PMDM; usually in acetone), and solvents that act as carriers and include glycerophosphate dimethacrylate (GPDM), phosphonated penta-acrylate ester (PENTA), and glutaraldehyde, which acts to stabilize the organic matrix in the altered dentin. Some systems contained both conditioners and primers, however many used just primers. Once the surface is primed, adhesives are added that are filled or unfilled bonding agents. Several systems incorporate HEMA with either BisGMA or urethane dimethacrylate. The HEMA makes the BisGMA more hydrophilic because it helps thin it. Products included in this group are: Tenure, Scotchbond 2, Scotchbond Multi-Purpose Dental Adhesive, Prisma Bond, XR Bond, Amalgambond, and Gluma. These materials have shown increased in-vitro bond strengths of from 8–20 MPA, and decreased susceptibility to microleakage. However, these systems are more complex, containing several steps that can be somewhat difficult for the clinician to emulate. The manufacturer's instructions must be carefully adhered to for optimum results. These products were found to be more successful if the dentin was left moist after rinsing off the acid.
Fourth- and Fifth-Generation Systems
This generation of dentin bonding agents acts through the formation of a "hybrid layer" that is formed by the treatment of the dentin surface with a mild organic acid conditioner (usually 30% to 35% phosphoric acid), allowing removal of the smear layer and selective removal of hydroxyapatite in the dentin, but which leaves the collagen fibrils intact provided it is not left in contact with the dentin for too long. A hydrophilic primer, like HEMA, combined with a water chaser such as acetone or alcohol, is applied to the moist, conditioned dentin surface. The HEMA intertwines around the collagen and penetrates the interstitial spaces of the intertubular dentin, being carried with the acetone or alcohol as it seeks water interstitially down the open dentin tubules. After polymerization of the overlying composite resin is completed, shear bond strengths of 18–35 MPa have been reported. These bond strengths surpass those of enamel bond strengths. The primary problem with systems is their technique sensitivity. It is easy to over-etch or over-dry the dentin prior to the placement of the primer, which can lead to postoperative sensitivity. Some examples of these systems are All Bond 2 (Bisco, Schaumburg, IL), OptiBond FL and OptiBond Solo Plus (Kerr, Orange, CA), Prime&Bond NT (Caulk/Dentsply, Milford, DE), and Excite (Ivoclar, Amherst, NY).
Sixth- and Seventh-Generation (Self-Etch) Systems
Self-etch adhesives were first developed by raising the amount of acidic monomers in HEMA/water-based adhesives. They do not require a separate etch-and-rinse phase as they contain acidic monomers that simultaneously condition and prime enamel and dentin. As a result, the dissolved smear layer and demineralization products are not rinsed away, but incorporated in the adhesive resin. They are categorized by the number of steps and their acidity. The primers have a hydrophilic end (HEMA) and a hydrophobic end (BisGMA) placed in a solvent such as ethanol or acetone with high concentrations of a water component and initiators for light curing. These products are marketed as if they were only one step, without separate etching of the enamel or dentin. It has been found for some of the products that the results were poor on uninstrumented enamel and so etching the enamel was recommended making it a two-step process. However, etching the dentin with these systems reduces their bond strength; therefore, one must be careful to keep the acid off the dentin if one is going to use this technique. Bond strengths seem to have decreased slightly from the fourth generation to the seventh generation, but there was also a reported decrease in postoperative sensitivity because of their ease of use. The newer products seem to be just as effective in bonding to dentin and many practitioners have started using these single-step products. Some problems remain with these products. Most can not be used to bond in the indirect resins or porcelain inlays/onlays as they have no chemical cure component, so practitioners need at least two different systems. The air-drying step subsequent to their application is particularly crucial to reduce the amount of solvent and water in the adhesive layer as much as possible. Some of the present one-step systems are Xeno V (Caulk), All-Bond SE (Bisco), Adper Scotchbond SE (3M ESPE, St. Paul, MN), and OptiBond All-In-One (Kerr).
Total Etch vs Self-Etch
The adhesive systems used today can be categorized as either “total etch” or “self etch” systems. (“Rinse” or “no rinse” is another way to categorize today’s adhesive systems.)
Total-etch techniques require at least two steps, the etching of the dentin and enamel with phosphoric acid and then the application of the primer/adhesive. Total-etch systems are the most reliable and versatile bonding agents. They have a proven track record and are compatible with any type of composite. However, a major drawback is postoperative sensitivity. This may be attributed to the fact that the application of total-etch systems can be very technique sensitive. The enamel and dentin require etching for different lengths of time, which could cause one to etch the respective surfaces for too short or too long a period. It is recommended that the etchant be placed on the enamel surfaces first, followed by the placement of the etchant on the dentin. Etching the dentin surface for too long may lead to the surface being etched deeper than the primer can penetrate. Another problem is that most modern bonding agents work best when applied to moist dentin surfaces. Due to preparation design, it is difficult in a clinical setting to have uniform wetting of the dentin surfaces. Too dry a dentinal surface will lead to collapse of the collagen fibrils and too wet a surface will dilute the primer and interfere with the resin penetrating the etched dentin surface.10
The increased popularity of self-etch systems has been due to their low incidence of postoperative sensitivity. Because the acid is contained within the primer, the dentin surface is etched no further than the primer can infiltrate.12 In addition, since no phosphoric acid etchant is used, there is no need to rinse off the etchant. This leaves the demineralized smear layer to be incorporated into the hybrid layer. Another advantage of self-etch systems is that the acidified primers are placed onto a dry dentin surface. Obtaining the proper “dry surface” is easier to accomplish than a proper “moist surface.”13 However, because these adhesives contain water along with the monomer, it is critical to ensure that the drying of the adhesive after application is effectively completed, and this inclusion of water makes some of the product’s shelf stability less certain than the etch-and-rinse systems.14
A problem with self-etch primers is that the acidic monomer is not strong enough to properly etch enamel. The resultant low enamel bond may lead to microleakage and early marginal leakage. Many users pre-etch the enamel surface with phosphoric acid before placement of the acidic primer to improve the enamel bond. Another problem with a light-cured self-etch system is its incompatibility with self-cure composites. The low pH of the acidic monomer used in light-cured systems can deactivate the catalyst (tertiary amine) of self-cure composites preventing the polymerization of the composite.10 Manufacturers have introduced dual-cured versions of the self-etch systems for use with self-cured composites.
Summary
Due to the inorganic nature of enamel, bonding to this surface has been successful and predictable. With a larger organic makeup, successful bonding to dentin has been more problematic. Bonding to dentin requires three processes; etching, priming, and bonding. Over the past two decades, seven generations of dentin bonding systems have been introduced and new ones seem to be introduced monthly. The trend towards simplifying bonding systems has led to a simpler classification system such as etch and rinse, or self-etch, or no rinse. Generations four through seven are still being used today. Each system has its advantages and disadvantages. Because of the variations in use of each system, one must be aware of the nature of the material being bonded, the function of the components in the systems, and be sure to read and follow the manufacturer’s instructions. Deviations from the recommended steps can lead to unwanted results.
References
1. Leinfelder KF. Understanding dentinal adhesives. Dentaltown Magazine. 2007:60-66.
2. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces, J Dent Res. 1955;34(6):849-853.
3. Nakabayashi N, Kojima K, Masuhara E. The promotion of adhesion by the infiltration of monomers into tooth substrates. J Biomed Mater Res. 1982;16(3):265-273.
4. Fusayama T. In: A Simple Pain-Free Adhesive Restorative System by Minimal Reduction and Total Etching. St. Louis, Mo: Ishiyaku EuroAmerica,Inc; 1993.
5. van Noort R. Principles of adhesion. In: Introduction to Dental Materials. 2nd ed. St. Louis, Mo: Mosby; 2002:68-78.
6. LeGeros RZ. Calcium phosphates in oral biology and medicine. Monogr Oral Sci. 1991;15:1-201.
7. Summit JB, Robbins JW, Hilton TJ, Schwartz RS. Bonding to Enamel and Dentin. In: Summit JB, Robbins JW, Hilton TJ, Schwartz RS, eds. Fundamental of Operative Dentistry. 3rd ed. Hanover Park, Il: Quintessence; 2006:183-260.
8. Simon JF, deRijk WG. Low shrink composites. Inside Dentistry. 2009;5(3):56-60.
9. O’Brien,WJ. Polymeric restorative materials: composites and sealants. In: O’Brien WJ, ed; Dental Materials and Their Selection. 3rd ed. Carol Stream, Il: Quintessence; 2002:113-131.
10. Powers JM, Sakaguchi RL. Bonding to Dental Substrates. In: Craig’s Restorative Dental Materials. 12th ed. St. Louis, Mo: Mosby Elsevier; 2006:213-234.
11. Owens B; King K. The science of dental adhesives 2006: state of the art, state of the profession. US Dentistry. 2007. Available at: www.touchbriefings.com.
12. Miller M. The Ratings: Bonding Agents: Total Etch. Reality Publishing. Available at: www.realityesthetics.com/portal/index.php?option=com_jreviews&task=listcategory§ion=3&cat=260&dir=1&Itemid=17. Accessed: February 5, 2009.
13. Pashley DH. The evolution of dentin bonding from no-etch to total-etch to self-etch. Adhesive Technology Solutions. Kurary, New York, NY. Available at: www.kuraraydental.com/newsletters/ats_premier.pdf. Accessed: February 6, 2009.
14. Latta M. Clinical perspectives on current dental adhesives. PennWell. Available at: www.ineedce.com. Accessed: February 6, 2009.
About the Authors
Kenneth King, DDS, Associate Professor, University of Tennessee, College of Dentistry,Memphis, TN
James F. Simon, DDS, Professor, Director, Division of Esthetic Dentistry, Director, Division of Clinical Research, University of Tennessee, College of Dentistry, Memphis, TN
Waldemar de Rijk, DDS, MS, PhD, Associate Professor, Director, Division of Biomaterials, University of Tennessee, College of Dentistry,Memphis, TN
