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After studying this chapter, the student will be able to do the following:
1. Describe an “adhesive.”
2. Explain the difference between micromechanical bonding and macromechanical bonding and provide an example of each type.
3. Recall three benefits the patient receives from restorations that are bonded to tooth structure.
4. Compare the differences between the microanatomy of enamel and dentin regarding etching and bonding. The comparison should include the following terms:
- Orthophosphoric acid
- Enamel tags
- Smear layer
- Hybrid layer
5. Discuss two of the earlier fallacies about dentinal bonding and how research has changed current practice.
6. Summarize the main differences between glass ionomer cements and dentinal bonding.
enamel bonding resin
glass ionomer cements
Adhesion, or bonding, is the joining together of two objects, using a glue or cement. It is common in everyday life; it is used in manufacturing, repairs, and dentistry. It is also important when a protective layer is applied to an object, such as when a metal surface is painted to prevent rust or when a pit and fissure sealant is applied to prevent decay.
The definition of adhesion or bonding in dentistry is not concise. A material that can stick to a flat surface or bond two flat surfaces together is typically called an “adhesive.” Most dental materials that are adhesive involve micromechanical adhesion or bonding. Remember that all dental materials must function in a wet, hostile environment for an extended period of time to be useful. Therefore, the oral environment limits the types of adhesives used in dentistry.
True adhesion involves chemical bonds between the materials being joined, but not all bonding to tooth structures is truly adhesive. In this text, the terms “adhesion” and “bonding” will be used interchangeably, but neither will signify chemical bonding (unless specifically stated).
Micromechanical bonding of dental materials to tooth structure is common. Micromechanical bonding also occurs in everyday life, when materials such as superglue are used. We will define micromechanical bonding as bonding using surface irregularities smaller than can be seen with the naked eye or felt with a dental explorer. The result of micromechanical bonding can be difficult to distinguish from true adhesion.
Macromechanical bonding is also common in everyday life and in dentistry. With this type of bonding, surface roughness can be seen and/or felt. Macromechanical bonding is the mechanism by which most glues join two pieces of wood, repair broken toys, and do many other things.
The mechanisms for micromechanical and macromechanical bonding are much the same. The difference is that they occur at a different scale or physical size. The glue or cement flows into surface irregularities and fills them. The glue then sets or hardens and is locked into the surface irregularities of the objects being joined. If the glue is strong, the objects are now joined together. The main advantage of micromechanical bonding is that a greater number of small surface irregularities are used compared to macromechanical bonding. In addition, force is more evenly distributed on the joint with micromechanical bonding, making it stronger than macromechanical bonding. Screws, nails, nuts, bolts, and other fasteners are examples of macromechanical joining of objects at an even larger scale. With this type of joining, stress is greatly concentrated in the vicinity of the fastener.
In dentistry, macromechanical bonding is used for cementing or luting crowns and bridges to teeth with “nonadhesive” cements. Dental cements fill in the roughness on the surface of the tooth and on the inside of the crown. The crown is luted or glued in place in the same manner as two pieces of wood are glued together. A crown is shown in Figure 1.4.
I. Adhesive Materials in Dentistry
A. Use of Adhesion/Bonding in Dentistry
1. Retention of Restorations
Adhesion is commonly used to keep restorations in place. Undercuts (as illustrated in Fig. 1.3) and other mechanical locks are not necessary when adhesive materials are used. Sometimes, adhesion is used to bond a weak esthetic restorative material, such as a porcelain veneer, to the stronger remaining tooth structure so that the tooth supports the weak restorative material. Adhesion is also used to attach orthodontic brackets and other appliances to teeth.
2. Reduction of Microleakage
Adhesion reduces or eliminates microleakage of restorations (Fig. 4.1). It also reduces postoperative sensitivity. Microleakage is the seeping and leaking of fluids and bacteria between the tooth/restoration junction or interface. Microleakage increases the likelihood of recurrent caries and postoperative sensitivity. Postoperative sensitivity is caused by fluids and bacteria moving in and out of the interface between the restoration and the tooth. If the pulp is irritated by fluid movement or bacterial metabolic wastes (acids), pain occurs.
FIGURE 4.1. Illustration of the effects of temperature changes and microleakage. When the coefficient of thermal expansion of a restorative material does not match that of the tooth structure, uneven expansion and contraction occur. In turn, gaps, leakage and percolation occur at the interface of the restoration and the tooth.
When teeth are heated and cooled by the ingestion of hot and cold foods, expansion and contraction occur. If the coefficient of thermal expansion for a restorative material does not match that of the tooth, they expand and contract at different rates. Repeated expansion and contraction of teeth and restorations at different rates result in fluids being sucked in and pushed out at the margins of a restoration. This phenomenon is called percolation and is illustrated in Figure 4.1.
Adhesion also reduces staining of the margins of esthetic materials. Margins are the junction of the tooth and the restoration. Margins that leak frequently become dark, stained, and unesthetic (see Fig. 4.2). Sealing the margins of restorations reduces or eliminates microleakage and reduces postoperative sensitivity and staining.
FIGURE 4.2. Photograph of several anterior composite restorations. Note the significant staining of the margin of tooth #10.
3. Reduction of Recurrent Caries
The most important reason to reduce microleakage is to minimize the likelihood of recurrent caries (secondary decay). Recurrent caries is decay that occurs at the margin of a restoration. If no space exists between the restoration and the tooth, bacteria do not have a well-protected niche in which to colonize and proliferate. Smooth surfaces of teeth and sealed margins are much more resistant to decay than are pits, fissures, and gaps at the margins of restorations.
B. Development of Dental Adhesives
1. Historical Perspective
Acid etching was initially conceived by Dr. Michael Buonocore in the 1950s to seal pits and fissures. An acid is applied to enamel to etch the surface. The etched surface is rough, allowing a low-viscosity adhesive (resin system) to flow into the irregularities and then cure (or set). Acid etching is a micromechanical bonding technique that was first used to retain pit and fissure sealants. Later, when dental composite restorations were developed in the 1960s, acid-etching techniques were used during placement. This reduced leakage and staining of margins.
Many other uses for acid etching and composite materials were developed in the 1970s and 1980s. With composite materials and acid-etching techniques, orthodontic brackets could be bonded to the labial surface of teeth rather than needing to be welded onto a metal band for every tooth. Soon, researchers learned that the enamel of the tooth and the metal of the fixed bridge could be both etched and then bonded together. Plastic, composite, and porcelain veneers were developed that could be bonded to the labial surface of anterior teeth to hide discolored enamel, to close spaces, and to change the shape of teeth. Dentists used etched composites to bond together mobile, periodontally involved teeth; to stabilize replanted, avulsed teeth; and to stabilize segments of fractured jaws. The acid-etched composite is the “gold standard” of adhesion in dentistry, against which all other materials and techniques are compared to judge strength of bond, utility of use, and longevity.
2. Chemical Adhesion in Dentistry
Acid etching solved the problem of bonding dental materials to enamel, but bonding dental materials to dentin was more difficult. In the 1970s, Dennis Smith developed the first chemically adhesive dental cement, called polycarboxylate cement. Polycarboxylate cements use polyacrylic acid and zinc oxide. Later, Wilson, Crisp, and McLean developed glass ionomer cement. Glass ionomer cements also use polyacrylic acid, but they include glass powder instead of zinc oxide. Both materials are based on polyacrylic acid, and both chemically bond to dentin and enamel. They are discussed later in this chapter.
A number of glass ionomer materials were developed for various uses, with luting and restorative materials being the most popular. However, glass ionomer materials lack the esthetic appearance and mechanical toughness of dental composites.
3. Dentinal Bonding Agents
In the 1970s and 1980s, products were developed that supposedly bonded composite materials to dentin. By the 1990s, dentinal bonding of composites had become a clinically proven reality. Because dentinal bonding incorporates acid etching, it should be thought of as an extension of the acid-etching process rather than as a replacement for it. Dentinal bonding systems continue to be developed and are now used to bond amalgam and ceramic and cast metallic restorations to dentin and enamel. In fact, nearly every restorative material can now be bonded to dentin or enamel with the use of some product and technique. However, the longevity and efficacy of some of these bonding techniques continue to be evaluated by clinical research.
C. Surface Factors
When applying an adhesive to an object, the surface must be clean. Otherwise, the adhesive will bond to the dirt and debris on the surface rather than to the surface itself. This would be like putting a Band-Aid on Pig-Pen of the “Peanuts” comic strip. (Pig-Pen is the character who is so dirty that a dust cloud follows him wherever he goes.) The Band-Aid would bond to the dirt rather than to Pig-Pen. Adhesives will not bond to any surface irregularities that are filled or covered by debris. If the surface and the adhesive are not somewhat chemically compatible, the adhesive will not wet the surface adequately, the adhesive will not flow into the irregularities, and the bonding will be poor. Whether the adhesive bonding is macromechanical, micromechanical, or truly adhesive (chemical), the surface must be clean to allow intimate association of the adhesive (bonding material) and the adherend (the surface).
In the oral cavity, it can be difficult to keep surfaces clean. A clean surface is one that is uncontaminated by oral fluids, such as saliva, blood, or crevicular fluid. Once a surface is contaminated by any oral fluid, it immediately becomes covered by a layer of biofilm. A biofilm is a coating that derives from organisms, both large and small. Biofilms in the mouth start as molecular coatings (the enamel pellicle) and grow into a community of microorganisms (plaque). For bonding purposes, the surface is no longer amalgam, enamel, or composite; the surface the adhesive “sees” or “feels” is biofilm. Biofilms reduce (or even prevent) bonding of many dental adhesives. To remedy this, use of a dental rubber dam is recommended when working with adhesive materials. The biofilm from saliva helps to lubricate the food bolus for swallowing, so it should not be surprising that biofilms are readily soluble in stomach acid and, therefore, do not inhibit the digestion of food. Luckily, the enamel pellicle is easily removed when acids are used to etch enamel and dentin.
D. Testing Adhesion: Optional
Much work has been done to measure the bond strength of various materials that are bonded to dentin and enamel. Usually, a small portion of material is bonded to a tooth and then pushed or pulled in an attempt to remove it. The force necessary to push or pull the bonded material off the tooth is measured in megapascals (mPa). One megapascal is equal to 145 pounds per square inch (psi). The resulting numbers are used to compare the effectiveness of the adhesive. A bond strength of 20 to 25 mPa (2,900–3,400 psi) is necessary for clinical success in high-stress areas of the mouth. Such numbers are useful only for general comparisons, however. In addition, one must know how the material broke off the tooth (where the fracture occurred). If the adhesive came off cleanly, then the break occurred at the interface. This type of break is called an adhesive failure. This is a test of bond strength. If the failure occurred inside the bonding material, the break is called a cohesive failure. This is a measure of the strength of the bonding material, not of the bond itself. If, during the testing procedure, the adhesive breaks the tooth, this is also a cohesive failure, and it signals that the strength of the bond is greater than the strength of the teeth. A bond that is stronger than tooth structure provides no advantage because the teeth, rather than the restoration, will break during failure.
II. Acid Etching
Acid etching was the first successful technique for bonding dental materials to tooth structure (Fig. 4.3). Acid creates a microscopically rough enamel surface, as shown in Figures 4.3 and 4.4A. This roughened surface has sometimes been termed “enamel tags” or “micropores.” A low-viscosity liquid polymer system is applied to the roughened surface. This liquid must wet the surface adequately so that it will flow into the micropores created by the etchant. The polymer system reacts chemically (polymerizes), changing from a liquid into a solid. The new solid is now bonded to the micromechanically roughened enamel surface. A variety of acids and polymer systems are possible, but because of time restrictions and oral conditions, only a few are suitable for dental use.
FIGURE 4.3. Schematic representation of the acid-etching process for enamel. A. Vertical bars represent a clean surface composed of enamel rods. B. Etching dissolves some of the enamel rods, creating a rough surface. C. Adhesive flows into the irregularities between and within the rods. The adhesive then sets and covers the surface with a layer of resin. The adhesive is micromechanically locked into the spaces between the enamel rods. D. The composite restorative material is applied and bonds to the underlying resin.
FIGURE 4.4. A. Scanning electron micrograph of etched enamel. (Reproduced from Hormati AA, Fuller JL, Denehy GE. Effects of contamination and mechanical disturbance on the quality of acid-etched enamel. J Am Dent Assoc. 1980;100(1):34–38, with permission) B. Photograph of etched enamel on the second molar (taken in a mirror). Note the chalky or frosty appearance of the surface, and compare this with the glossy surface of the unetched first molar. (Courtesy of Dr. Ronald House, Bethesda, MD.)
A. The Acid-Etching Process
First, the enamel surface is cleaned with pumice or a similar abrasive. The debris and pumice are then rinsed away with water, and the area is dried with compressed air (Fig. 4.3A ). The acid or etchant, which is typically 37% orthophosphoric acid, is applied for 15 to 30 seconds to permanent teeth. The acid is rinsed away with water, and the surface is completely dried again with suction and compressed air (Fig. 4.3B). Next, the liquid bonding resin (polymer system) is applied. The polymer system chemically reacts or “cures” (Fig. 4.3C ). Finally, layers of restorative materials are chemically bonded to this initial layer of bonding resin (Fig. 4.3D).
The term “etchant” is preferred in front of a patient rather than any word or words that use the term “acid.” Sometimes, the etchant is called a “conditioner.” However, that term can be confusing because other different dental materials are also called conditioners.
1. The acid-etching technique is used to bond materials to enamel, but not to dentin. The technique is simple and micromechanical, and it has not changed appreciably over the years.
2. It is more effective to bond the polymer resin to the ends of enamel rods than to the long axis of the rods.
3. The acid-etching technique has a “built-in” quality control check. If the enamel is properly etched and dried, it appears chalky or frosty white, as shown in Figure 4.4B.
4. Years of clinical data demonstrate the advantages of using acid-etching techniques for bonding to enamel. Pit and fissure sealants prevent caries, and the margins of composite restorations stain less frequently. Composites can be bonded to teeth to correct fractures, rotations, or other defects.