Bonding and Bonding Agents
Acid-etch technique—The process of cleaning and roughening a solid surface by exposing it to an acid and thoroughly rinsing the residue to promote micromechanical bonding of an adhesive to the surface.
Adhesion—A molecular or atomic attraction between two contacting surfaces promoted by the interfacial force of attraction between the molecules or atoms of two different species; adhesion may occur as chemical adhesion (formation of covalent bonds, hydrogen bonds, or polar bonds via van der Waals attraction), mechanical adhesion (structural interlocking), or a combination of both types.
Adhesive bonding—Process of joining two materials by means of an adhesive agent that solidifies during the bonding process.
Bonding—A means of binding two adjoining materials, for example, dental hard tissues, metal, composite, or ceramic, and providing resistance to their separation.
Cement/Cementing—Substance that hardens from a viscous state to a solid union between two surfaces. For dental applications, cements act as a base, liner, restorative filling material, or adhesive to bond devices and prostheses to tooth structure or to each other. See also, “Luting agent.”
Contact angle—Angle of intersection between a liquid and a surface of a solid that is measured from the solid surface through the liquid to the liquid–vapor tangent line originating at the terminus of the liquid–solid interface; used as a measure of wettability, whereby absolutely no wetting occurs at a contact angle of 180 degrees and complete wetting occurs at an angle of 0 degrees.
Dentin bonding agent—A thin layer of resin between conditioned dentin and the resin matrix of a resin composite restorative material.
Dentin conditioner—An acidic agent that dissolves the inorganic structure in dentin, resulting in a collagen mesh that allows infiltration of an adhesive resin.
Filler/Reinforcing filler—A distribution of solid particles that are dispersed in a resin matrix to increase rigidity, strength, and wear resistance and to decrease thermal expansion, polymerization viscosity, shrinkage, and swelling in water and other solvents.
Hybrid layer—An intermediate layer of resin, collagen, and dentin that is produced by acid etching of dentin and infiltration of resin into the conditioned dentin.
Luting agent—A viscous cement-like material that also fills a gap between the bonded materials. See also, “Cement.”
Microleakage—The flow of oral fluid and bacteria into the microscopic gap between a prepared tooth surface and a restorative material.
Micromechanical bonding—The mechanical interlocking that is associated with bonding of an adhesive to a roughened adherend surface.
Obturate/Obturation—To form an obstruction, or to obstruct. In endodontic dentistry, filling of a root canal completely and densely with a nonirritating hermetic sealing agent to prevent leakage.
Preventive-resin restoration (PRR)—A conservative, sealed, resin-based composite restoration, usually placed in a minimally prepared occlusal fissure area, with the sealant extending into contiguous uncut fissures.
Primer—A hydrophilic, low-viscosity resin that promotes bonding to an adherend substrate, such as dentin.
Sandwich technique—The process of restoring a prepared tooth by initially placing a layer of type II glass ionomer cement for fluoride release followed by an overlayer of resin-based composite.
Smear layer—Poorly adherent layer of ground dentin produced by cutting a dentin surface; also, a tenacious deposit of microscopic debris that covers enamel and dentin surfaces that have been prepared for a restoration.
Surface energy—The excess energy of molecules at the surfaces of materials above that of molecules found in the interior of a material.
Surface tension—The tension at the surfaces of liquids that results from the physical driving force to minimize the total energy of a system. This tension causes a liquid to minimize its surface by forming a spherical drop or a droplet with a contact angle against a solid surface. See also, “Contact angle.”
Wetting—Relative interfacial tension between a liquid and a solid substrate that results in a contact angle of less than 90 degrees.
Bonding and adhesion comprise a complex set of physical, chemical and mechanical mechanisms that allow the attachment and binding of one substance to another. A dental bonding system performs three essential functions: (1) provides resistance to separation of an adherend substrate (i.e., enamel, dentin, metal, composite, ceramic) from a restorative or cementing material; (2) distributes stress along bonded interfaces; and (3) seals the interface via adhesive bonding between dentin and/or enamel and the bonded material, thus increasing resistance to microleakage and decreasing the risk for postoperative sensitivity, marginal staining, and secondary caries.
Prior to the middle of the twentieth century, dental bonding consisted of various methods of mechanical retention, such as forming undercuts in cavity preparations for amalgam restorations (see Chapter 15). Luting, using zinc phosphate and other nonadhesive dental cements, also falls into this category of bonding (see Chapter 14). In the late 1940s, Oskar Hagger, at the De Trey division of Amalgamated Dental, developed the first bonding agent, Sevriton Cavity Seal. This system was based on glycerophosphoric acid dimethacrylate (see Figure 12-5 on page 263) as a self-adhesive or self-etching component for both enamel and dentin bonding. However, this product had very limited clinical durability because of the large interfacial stresses that developed because of the high polymerization shrinkage and high thermal expansion of the unfilled methacrylate-based resins used at that time. Shortly after, Michael Buonocore investigated stronger acids and discovered that phosphoric acid provides superior enamel etching, and it is still in use today.
As shown in Figure 12-1, these discoveries of acid etching led to our ability to produce clean, high-energy, roughened enamel surfaces capable of establishing a durable micromechanical retentive interface with resin-based cementing and restorative materials launched the current era of adhesive dentistry. These events soon led to a burgeoning growth in the development of adhesive materials and bonding techniques, into the beginning of the twenty-first century. This progress is summarized in Figure 12-1 and is discussed in detail in several of the later sections of this chapter. At the present time, adhesive bonding can only be relied on for long-term retention in very select circumstances using highly specialized materials and clinical techniques, many of which are discussed in detail below. Nevertheless, dentistry is now well into the era of adhesive bonding and its associated field of esthetic dentistry.
Applications for Bonding
Today, acid etching is one of the most effective ways to promote restoration retention and to ensure a sealed interfacial joint at restoration margins. This procedure has markedly expanded the use of resin-based restorative materials because it provides a strong, durable bond between resin and tooth structure and has formed the basis from many innovative dental procedures as diverse as orthodontic bracket bonding and porcelain laminate veneer bonding. Additional applications include pit and fissure sealants; amalgam bonding; both enamel and dentin bonding; adhesive cements, including glass-ionomer restorative materials; and endodontic sealers. Many of these applications are discussed in this chapter; however, other bonding applications are also discussed in greater detail throughout the book but with particular emphasis in Chapters 2, 13, 14, 15, and 18.
Mechanisms of Adhesion
True adhesion has been the “holy grail” of dental restorative materials for many decades. If true adhesion of restorative materials to tooth structure is to be achieved, three conditions must be satisfied:
1. Sound tooth structure must be conserved.
2. Optimal retention must be achieved.
The science of adhesion has been covered in Chapter 2. In this chapter, only those principles of adhesion needed for an understanding of dental bonding will be discussed.
Oral hard tissues and their environment are complex. However, the fundamental mechanism of adhesion to tooth structure can be regarded simply as an exchange by which inorganic tooth material (hydroxyapatite) is replaced by synthetic resins. This process involves two parts: (1) removing hydroxapatite to create micropores and (2) infiltration of resin monomers into the micropores and subsequent polymerization. As a result, resin tags are formed that micromechanically interlock or interpenetrate with the hard tissue. There may also be chemical interactions with the tooth substrate if monomers having acidic or chelating functional groups are present. In general, the following factors can play major or minor roles in achieving adhesive bonds:
2. Interpenetration (formation of a hybrid zone)
3. Micromechanical interlocking
Wetting is the essential first step for the success of all adhesion mechanisms. An adhesive cannot form micromechanical interlocks, chemical bonds, or interpenetrating networks with a surface unless it can form intimate contact with the surface, spread over it and penetrate by capillary attraction into any microscopic and submicroscopic irregularities. These conditions are, by definition, achieved if the adhesive wets the surface.
As explained in Chapter 2, wettability of a liquid on a solid can be characterized by the contact angle that forms between a liquid and solid, as measured within the liquid. Categories of wettability include “mostly nonwetting” (>90 degrees), “absolutely no wetting” (180 degrees), “mostly wetting” (<90 degrees), and absolute wetting (0 degrees). See Figure 2-15 for a schematic illustration of wetting situations.
Generally, wettability can be enhanced by increasing the surface energy of the substrates (e.g., dentin, enamel, and synthetic materials). Since a clean, microroughened tooth surface has higher surface energy than unprepared tooth surfaces, organic adhesives are inherently able to wet and spread over such a surface unless a low surface tension material contaminates it before the adhesive can be applied. The acid-etch technique (see below), by which contaminants are removed and microporosities are created, is widely used to generate high-energy tooth surfaces and promote wetting by adhesive monomers.
Although wetting is an essential requirement for intraoral adhesion, it is not sufficient to ensure durable bonding. As explained below, to achieve strong bonding through the micromechanical interlocking mechanism, wetting monomers must intimately adapt to enamel and fill enamel surface irregularities and/or infiltrate into a demineralized collagen network in dentin. Some acid monomers with a phosphate (e.g., phenyl-P) or carboxyl group (e.g., 4-MET) have the additional potential of forming chemical bonds with calcium in the residual tooth tissue. The chemical structures of these acidic monomers are shown later in Figure 12-4 and Figure 12-6.
Another requirement for achieving lasting intraoral bonds is hydrolytic stability (resistance to chemical degradation by water). Enamel and dentin are hydrated, hydrophilic, and permeable to water. Even if an enamel or dentin surface is initially dried before applying an adhesive, inadvertent contamination and diffusion can easily result in water becoming strongly bound to both the hard tissue and the adhesive. Thus, for an adhesive monomer to wet hard tooth tissue as well as form a durable bond in the moist environment of the mouth, it must be both hydrophilic for water compatibility and hydrolytically stable to ensure longevity.
Whenever both enamel and dentin tissues are mechanically cut, especially with a rotary instrument, a layer of adherent grinding debris and organic film known as a smear layer is left on their surfaces and prevents strong bonding. Different quantities and qualities of smear layer are produced by the various cutting and instrumentation techniques, as occurs, for example, during cavity or root canal preparation. In dentin, the smear layer becomes burnished into the underlying dentinal tubules and lowers dentin permeability, which is a protective effect. However, it is also a very weak cohesive material and interferes with strong bonding. Therefore, various cleaning or treatment agents and procedures are employed to either remove the smear layer or enhance its cohesive strength and other properties. As explained in greater detail below, application of acid is used to remove the smear layer from both enamel and dentin. Alternatively, in dentin the smear layer can be left partially in place and modified such that adhesive resins penetrate through it and bond to the intact dentin structures below.
The first meaningful demonstration of intraoral adhesion was reported by Michael Buonocore (1955). Buonocore etched enamel surfaces with various acids, placed an acrylic restorative material on the micromechanically roughened surfaces, and found a great increase in the resin–enamel bond strength (~20 megapascals [MPa]). One of the surface conditioning agents he used, phosphoric acid, is still the most widely used etchant today for bonding to both enamel and dentin. Depending on the concentration, phosphoric acid removes the smear layer and about 10 microns of enamel to expose prisms of enamel rods to create a honeycomb-like, high energy retentive surface (Figure 12-2). The higher surface energy ensures that resin monomers will readily wet the surface, infiltrate into the micropores, and polymerize to form resin tags. The pattern of etching enamel may vary from selective dissolution of either the enamel rod centers (type I etching) as shown in Figure 12-2, or the peripheral areas (type II etching) as indicated by the resin tags in Figure 12-3. In either case, the resin tags are approximately 6 µm in diameter and 10 to 20 µm in length and lead to micromechanical interlocking.
Prior to the introduction of enamel acid etching and the use of enamel bonding agents, restorative materials were placed directly on the smear layer of the prepared tooth. It is evident that the apparent bond strength is the cohesive strength (5–10 MPa) of the smear layer, which is not sufficient to withstand the daily mechanical forces experienced in the mouth. As a result, debonding and leakage of oral fluids within the microscopic space between prepared teeth and restorative materials was an ongoing problem. Unlike other types of dental restorative products, resin-based composites have no mechanism to counteract the effects of marginal leakage (e.g., the corrosion of amalgam over time produces a deposit such as tin oxide and/or tin oxychloride along the tooth-restoration interface to form a relatively leakproof seal). Resin-based composites also cause problems because of the shrinkage stress (~15 MPa) generated during polymerization, exacerbating fracture of the tooth-restorative interface. Stronger and longer lasting bonds result if the smear layer is removed, because resins can then directly bond to the intact hard tissue.
As illustrated in Figure 12-1, dentin etching did not gain wide acceptance until Fusayama introduced the total-etch concept in 1979. For this method, both dentin and enamel are etched simultaneously, typically using 37% phosphoric acid. His study demonstrated that not only was restoration retention substantially increased but also pulp damage did not occur as had been generally assumed. A subsequent study by Nakabayashi et al. (1984) revealed that hydrophilic resins can infiltrate the surface layer of acid-demineralized collagen fibers that is produced in etched dentin and it can form a layer of resin-infiltrated dentin with high cohesive strength. Such a hybrid layer structure forms very strong resin bonds through the development of an interpenetrating network of polymer and dentinal collagen, together with numerous micromechanical interlocks at the resin–hybrid layer interface. By the early 1990s, dentin etching had gained worldwide acceptance. Since the total-etch technique usually involves etching with an acid followed by rinsing to remove the acid, this technique is also known as the etch-and-rinse technique.
Dentin etching is more technique sensitive than enamel etching because of the complexity of the dentin structure. Unlike enamel, dentin is a living tissue, consisting of 50 vol% (volume percentage) of calcium phosphate mineral (hydroxyapatite), 30 vol% of organic material (mainly type I collagen), and 20 vol% fluid. Acid etching removes hydroxyapatite almost completely from several microns of sound dentin, exposing a microporous network of collagen suspended in water. Whereas etched enamel must be completely dry to form a strong bond with hydrophobic adhesive resins, etched dentin must be moist to form a hybrid layer. The amount of water left in etched dentin is critical. If insufficient water is present, the collagen network will collapse and produce a relatively impermeable layer that prevents resin infiltration and subsequent hybridization. If too much water remains, resin infiltration cannot fully replace the water in the collagen network and, consequently, sets the condition for later leakage into those locations. Therefore, a priming step is required to maintain a hydrated collagen network while removing excess water (see details in the following sections).
Process and Procedural Factors
Several aspects of the total-etch, etch-and-rinse, and related techniques are critical to the success or failure of bonding systems, as discussed below.
The optimal application time for the etchant may vary somewhat, depending on previous exposure of the tooth surface to fluoride and other factors. For example, a permanent tooth with a high fluoride content may require a somewhat longer etching time, as do primary teeth. In the latter, increased surface conditioning time is needed to enhance the etching pattern on primary tooth enamel that is more aprismatic than permanent tooth enamel. Currently, the etching time for most etching gels is approximately 15 seconds. The advantage of such short etching times is that they yield acceptable bond strength in most instances, while conserving enamel and reducing treatment time.
Rinsing and Drying Stage
Once the tooth is etched, the acid should be rinsed away thoroughly with a stream of water for about 20 seconds, and the rinsed water must be removed. When enamel alone is etched and is to be bonded with a hydrophobic resin (e.g., bisphenol A glycidyl methacrylate [bis-GMA]–based resin; see Chapter 13), it must be dried completely with warm air until it takes on a white, frosted appearance. Dentin, in contrast, cannot withstand such aggressive drying, which would cause bond failure because of the formation of impermeable, collapsed collagen fibers. In the total-etch technique, a dentin bonding agent and primer must be used that are compatible with both moist dentin and moist enamel.
Cleanness of the Bonding Surfaces
The etched surfaces must be kept clean (free of contaminants) and sufficiently dry until the resin is placed to form a sound mechanical bond. Although etching raises the surface energy, contamination can readily reduce the energy level of the etched surface. Reducing the surface energy, in turn, makes it more difficult to wet the surface with a bonding resin that may have too high a surface tension to wet the contaminated surface. (Surface energy and wetting are described in detail in Chapter 2.) Thus, even momentary contact with saliva or blood can prevent effective resin tag formation and severely reduce the bond strength. Another potential contaminant is oil that is released from the air compressor and transported along the air lines to the air–water syringe. If contamination occurs, the contaminant should be removed, and the surface should be etched again for 10 seconds.
The acid-etch technique was not widely used in the years immediately following its introduction (see Figure 12-1). The principal reason was the inferior properties of the acrylic filling materials used at that time. With those resins, curing (>6 vol% shrinkage) and thermal dimensional changes (coefficients of thermal expansion in excess of 100 parts per million per degree Celsius [ppm/°C]) generated interfacial stresses sufficient to rupture the bond to etched enamel. After highly filled resin-based composites were marketed beginning in the mid-1960s, the acid-etch technique was “rediscovered.” Acid etching is a very effective way to improve bonding and durability as well as to ensure a sealed interface. It has markedly expanded the use of resin-based restorative materials because it provides a strong bond between resin and teeth, forming the basis for many innovative dental procedures.
Dentin Bonding Agents
Dental bonding agents are designed to provide a sufficiently strong interface between restorative composites and tooth structure to withstand mechanical forces and shrinkage stress. The success of adhesives is dependent on two types of bonding:
1. Micromechanical interlocking, chemical bonding with enamel and dentin, or both
2. Copolymerization with the resin matrix of composite materials
Before the total-etch technique was adopted, enamel bonding agents were used only to enhance the wetting and adaptation of resin to conditioned enamel surfaces. Generally, enamel bonding agents are made by combining different dimethacrylates from resins of composite materials (e.g., bis-GMA) with diluting monomers (e.g., triethylene glycol dimethacrylate [TEGDMA], Figure 12-4) to control viscosity and to enhance wetting. These agents have no potential for adhesion, but they improve micromechanical bonding by optimal formation of resins tags within the enamel. Because enamel can be kept fairly dry, these rather hydrophobic resins work well as long as they are restricted to enamel.
During the past few years, these bonding agents have been replaced by the same systems that are used on dentin. This transition occurred because of the benefit of simultaneously bonding resin to both enamel and dentin, not because of any substantial improvement in bond strength.
A wide variety of chemistries have been explored and marketed in the search for species that can produce strong, permanent bonds to dentin. As discussed in the earlier section on adhesion mechanisms, a successful dentin bonding system must meet several requirements:
1. Adequate removal or dissolution of the smear layer from enamel and dentin
2. Maintenance or reconstitution of the dentin collagen matrix
4. Efficient monomer diffusion and penetration
5. Polymerization within tooth structure
Irrespective of the number of bottles or components (see Figure 12-7 and Table 12-1), a typical dentin bonding system includes etchants, resin monomers, solvents, initiators and inhibitors, fillers, and sometimes other functional ingredients such as antimicrobial agents.
Classification of Dental Bonding Systems and Representative Examples of Commercial Products Currently Available for Clinical Use