Relevance to Esthetic Dentistry

Over the past two decades the evolution of adhesive techniques has transformed the scope of dental practice. Today, most direct and indirect restorations are bonded to natural tooth structures rather than cemented or mechanically retained. Extensive research and product development have consistently improved the adhesive armamentarium available to the dentist, broadening its applications and range. Patient interests and demands reflect a new-found interest in oral appearance and health, most commonly associated with adhesive procedures.

The widespread demand for, and use of, dental adhesives has fueled an intensive development of better and easier-to-use dental adhesives in rapid succession; dentists have literally been inundated with successive “generations” of adhesive materials. Although the term generation has no scientific basis in the realm of dental adhesives and is to a great extent arbitrary, it has served a useful purpose in organizing the myriad materials into more comprehensible categories.

Brief History of Clinical Development and Evolution of the Procedure

The “generational” definitions help to identify the chemistries involved, the strengths of the dentinal bond, and the ease of use for the practitioner (Box 8-1). Ultimately this type of classification benefits the dentist and patient by simplifying the clinician’s chairside choices.

Relating Function and Esthetics

Conservative dentistry, a treatment process whereby a minimum of healthy tooth structure is removed during the restorative process, is inherently a desirable dental objective. Natural enamel and natural dentin are still the best dental materials in existence, and thus, minimally invasive procedures that conserve more of the original, healthy tooth structure are preferable.

Minimally invasive dental procedures are beneficial from a patient’s point of view as well. There is less discomfort, less need for local anesthesia, and a real prospect that the repaired natural tooth will last a lifetime. The replacement of existing amalgam restorations with newer amalgam involves ever larger restorations that have shorter life spans than their predecessors. The replacement procedures may nick or otherwise damage adjacent healthy teeth.

In many parts of the world, restorative dentistry has been described and taught as “conservative dentistry.” It has hardly been conservative of tooth structures, however; traditional methods and materials have been aggressive and highly invasive, requiring the removal of otherwise healthy enamel and dentin for various reasons, including extending the cavity for the retention of the final restoration and extending a preparation for the prevention of recurrent decay. Thus, healthy tooth structures were condemned to removal by the demands of non-adhesive restorative materials (Figure 8-1).

FIGURE 8-1 A, Small interproximal decay. B, Nonadhesive amalgam extended for retention and prevention.

Fortunately, the current era of dentistry has witnessed the development of new materials, new techniques, and new instruments that make conservative dentistry practical and ultraconservative dentistry a reality. Adhesive restorations eliminate the need for more extensive retentive preparations (Figure 8-2). Enamel-mimicking composites (both hybrid and flowable) offer long-lasting tooth structure replacement with minimum requirements for restorative bulk. Little or no healthy tooth material must be removed simply to allow for adequate thickness of the filling material. Innovative materials, particularly when combined with early detection and conservative treatment make the development of esthetics possible within every dental practice.

FIGURE 8-2 Composite resin restoration without extension for retention and prevention.

Clinical Considerations

Early Detection and Treatment

Routine diagnosis and treatment of large, visible dental decay is relatively easy. Its presence and location are readily accessible. Over the past several decades, however, there have been major changes in the pattern of dental decay. Owing largely to the advances in the dental education of the public, there is a greatly increased dental awareness among many population groups. Combined with more frequent and more thorough preventive care by dentists, this has led to fewer and smaller cavities, particularly among the younger age groups.

Although this represents great progress for the dental profession (as well as the general population), this trend toward fewer and smaller cavities has raised some new questions:

• How does one effectively diagnose these much smaller lesions in the teeth?

• Should these smaller lesions be left to grow larger for easier diagnosis and access or should they be intercepted while they are still small?

The accurate diagnosis of minute lesions may be quite difficult with traditionally accepted techniques. The shape of pit and fissure lesions tends to mask the size and extent of the defect when the dentist is using an explorer. Forty-two percent of these fissures have a narrow occlusal opening and vary in shape as they progress inward in the tooth. Caries is initiated in the lateral walls of the fissure and progresses downward toward the dentino-enamel junction (DEJ). The narrow occlusal opening tends to prevent the entry of the explorer into the larger chambers of the lesion. Often only a stickiness of the instrument in the tooth surface is reported. In fact, histologic cross-section has confirmed a ratio of only 25% accuracy in diagnosing decay underlying the occlusal surface using the traditional explorer method (Figure 8-3). This is hardly an impressive rate of success.

FIGURE 8-3 Explorer used on tooth to diagnosis decay.

Radiographic diagnosis is an important tool for the practicing dentist. Radiographs can detect caries when none are observed clinically. However, negative radiographic results can be misleading. All too often the tooth has caries that the radiographic process will not reveal. This is known as the phenomenon of hidden caries, a condition in which the tooth appears caries free clinically and/or radiographically but is found to be carious by other diagnostic means (Figure 8-4, A). Subsequent cross-sectioning of the tooth clearly reveals caries that has originated at the base of a fissure and is now spreading along the DEJ (Figure 8-4, B).

FIGURE 8-4 A, Radiograph showing an apparently healthy tooth. B, Cross-section of tooth showing hidden caries spreading along the dentino-enamel junction.

The dilemma of clinically diagnosing small caries at an early stage is a very real problem that cannot easily be solved by existing diagnostic techniques. Explorers and radiographs are simply not adequate tools for this common type of dental lesion. A further complication is the aggressive use of fluoride on a regular basis in fluoridated communities. A study in the Netherlands determined that the entire Dutch population (among others) may be overdosing on fluoride. This may have resulted in an undiagnosed hidden caries level of approximately 15% in the younger population. The surface-hardening effect of fluoride on the enamel makes the tooth surface more impenetrable to exploration, while at the same time masking the carious activity occurring just below the tooth surface and along the DEJ.

The clinical dentist is faced with the option of (1) watching and waiting until the early caries, which may be far more active just under the enamel surface, becomes larger and destroys more healthy tooth structure, or (2) aggressively eliminating these early lesions and restoring the cavities with ultraconservative restorations. The older tradition of “watching” incipient decay is no longer tenable. Extensive recent research has clearly indicated that incipient surface decay may, particularly in fluoridated communities, mask much greater subsurface carious activity within the tooth. Incipient decay must therefore be intercepted at the earliest possible opportunity to prevent the spread and growth of caries and to permit the most conservative restoration possible.

The practice of sealing pits and fissures has enjoyed widespread acceptance. There is continued concern, however, about the placement of sealants over undiagnosed caries. Because it is often difficult to determine the status of caries activity in fissures, an exploratory technique, or excisional biopsy, offers the best access and the best diagnostic and conservative technique for the maximum retention of healthy tooth structures combined with the assured removal of all decay. The excisional Fissurotomy bur (SS White Burs, Inc., Lakewood, New Jersey) remodels the anatomy of the fissure, facilitating the access, the acid etching, and the bonding of composite resin into the cavity preparation. lf this can be accomplished with minimal patient discomfort, preferably without any anesthetic, patient acceptance will be high, and the dentist’s conservationist goals can be attained (Figure 8-5).

FIGURE 8-5 A, Fissurotomy bur is used to excise early decay to the depth of the dentino-enamel junction or just beyond without need for local anesthetic. B, Acid etching of the prepared cavities. C, Fifth-generation adhesive applied to etched surfaces. D, Flowable composite inserted into the small occlusal preparations. E, Completed ultraconservative restorations.

Material Options

The dental practitioner has many options in the area of adhesives. As described earlier, there are seven distinct generations of adhesives. Each generation has its own advantages and disadvantages; some of the earlier generations are currently not in widespread use because better methods have superseded them quite effectively. Only generations four through seven are commonly used at this time (Table 8-1).

The first-generation adhesives bonded quite well to enamel through resin-infiltration of the enamel microcrystals but did not bond to dentin. Etching was not yet well established as a technique; in fact, there were numerous controversies about whether etching should precede bonding. Later on, etching, performed either directly or as part of one of the adhesive components, became an essential part of the bonding process.

First-generation materials did not include dentin conditioners, and it is questionable whether they were capable of removing the smear layer at the dentin surface. It can be assumed that the etching step would have eliminated the smear layer of the dentin; however, if no etching was done, the smear layer was essentially left intact. Therefore no hybrid layer could be created and moist bonding was not required. Typically, the number of bottles in the first-generation kit was one or two, and two or three steps were needed to complete the procedure. Because the dentinal tubules were not opened by acid etching, there was little if any postoperative sensitivity. The enamel bond was significantly strong, typically 20 to 30 MPa, but the dentin bond at 1 to 3 MPa was essentially nonexistent. This made bonding to exposed dentinal surfaces such as Class V abfractions impossible.

The second-generation adhesives were also effective in bonding to enamel. With respect to the dentin, they bonded to the smear layer, the organic debris that is found on the surface of the prepared tooth. These adhesive bonds at the dentin interface were weak, ionic bonds that were generated by the hydrolysis of calcium. Etching was a routine part of the bonding procedure, but dentin conditioning had not yet been introduced. The second-generation adhesives bonded to the smear layer but did not remove it and did not develop a hybrid layer. Moist bonding was not a requirement. Typically, second-generation techniques involved two bottles and three steps. There were few reports of postoperative hypersensitivity unless the dentin was over-etched. Dentinal bonding reached levels of 2 to 8 MPa, whereas enamel bonding remained in the 20- to 30-MPa range.

The third-generation adhesives were the first ones specifically designed to remove and/or modify the smear layer. Adhesion to enamel was as with earlier generations, but there was a major focus on the dentinal surfaces. When this interface was examined under the electron microscope, the polymerized intratubular resin had the appearance of spaghetti-like projections that acted as anchors inside the tubules and provided increased dentinal adhesion. Both the enamel and the dentin required etching, and for the first time a conditioner was used on the prepared dentinal surface, which removed the smear layer to allow the adhesive to enter into the dentinal tubules. No hybrid layer was created, and moist bonding was not yet a requirement. Most third generation kits had two to four components and three or four distinct clinical steps. The smear plugs were partially or completely removed by the etching and conditioning, unblocking the dentinal tubules. Because not all of the vital dentinal tubules were effectively resealed by the adhesive, there was some postoperative sensitivity observed with third-generation bonding agents. Dentinal bonding of 8 to 15 MPa was achieved, along with the expected 20 to 30 MPa for enamel surfaces.

The fourth-generation adhesives were the first to require a total etch of all prepared tooth surfaces. Dentin bonding to moist dentin was 17 to 25 MPa, at least theoretically, whereas enamel bonding remained fairly constant at 20 to 30 MPa. The fourth-generation bonding protocol involved etching both enamel and dentin and conditioning the dentin. The elimination of the smear layer was a key part of the procedure. A hybrid layer was created at the adhesive-tooth interface through the interaction of the adhesive material with the moistened tooth surface. Moist bonding became a clinical requirement.

This generation was the first wherein the bonding strength to dentin was greater than the polymerization shrinkage of the composite. As a result, the composite did not shrink away from the tooth-resin interface during polymerization, thereby leaving a gap that could develop into a collector for oral fluids and bacteria. The drawback of fourth-generation systems was the need for multiple components. Kits had three to five components and required three to seven distinct steps in their protocol, a very time-consuming and technique-sensitive exercise. Certain components had to be mixed equally, and in a specific order, chairside. This is rather difficult to accomplish predictably on a regular basis, as evidenced by the common situation in which one of the “equal mix” components was always used up more quickly than the other. The unequal amounts of the components often compromised the adhesive strength of the bonding agent. Although the final bonding strength to dentin was theoretically 25 MPa, in actual fact it was often less than 17 MPa, the minimum adhesion needed to avoid marginal gaps caused by polymerization shrinkage of the composite. Clinically, the greater the number of steps, the greater the likelihood of inadvertent procedural error (Figure 8-6).

FIGURE 8-6 Clinically, the greater the number of steps, the greater the decrease in bond strength from theoretical to actual.

Postoperative sensitivity was observed in 10% to 30% of posterior restorations (far less with anterior restorations). This may have been a result of the prescribed technique that effectively eliminated the entire smear layer as well as the smear plugs in all the exposed dentinal tubules. In order for post-treatment sensitivity to be prevented, the bonding agent had to infiltrate and seal every single one of the opened dentinal tubules to plug them. Any open dentinal tubules allowed the outflow of intra-tubular moisture, creating hypersensitivity that could cause the patient pain for days, weeks, or even longer. The sensitivity was much more acute and more likely to occur in posterior teeth, possibly because of the higher C factors in these preparations. Postoperative sensitivity was often observed even when technically precise procedures were performed with great clinical care.

Fourth-generation adhesives are dual cure; this means that after mixing they can be polymerized within seconds with a curing light. In the absence of a photo-catalyst, these adhesives will cure within 60 to 90 seconds after mixing. Thus they polymerize both in the presence and the absence of a curing light.

The fifth-generation adhesives combined all the necessary bonding components into a single bottle, not including the etchant. They had somewhat lower bonding strength to dentin than the fourth-generation products, at least in theory. However, as there were no components to mix chairside, the formulation having been completed under controlled factory conditions, the adhesive was more likely to always be at its optimal chemistry. The single-component fifth-generation adhesives bond to both enamel and dentin; they require total etching however (a 15- to 25-second process).

Etching is very predictable and is easily accomplished by practitioners. The most common fifth-generation problems relate to the moist bonding surface requirement. Although it generally has been accepted that a “moist” bonding surface is a must, the definition of “wet” or “moist” surfaces can be rather controversial and confusing. Some academics and practitioners assume that a very slight moistness is enough; others maintain that a liquid sheen must be visible on the surface to be bonded. Because there has been little research to quantify or specify the correct level of moisture for optimal adhesion, there is little consensus within the profession on this topic.

The fifth-generation adhesives are specifically designed to remove the smear layer and to generate a hybrid layer created through moist bonding. There is one etchant bottle and one adhesive bottle. The number of indicated steps is two: a relatively simple and rapid etching step, and a second, adhesive step that varies in complexity and technique sensitivity from product to product. There is less post-operative sensitivity observed with fifth-generation bonding agents—typically 5% or less in posterior teeth and quite rare in anteriors. This benefit may be a result of the more consistent manufacturer premix of the adhesive components and the reduction of the recommended etching time from the earlier 60 to the more sensible 15 seconds. The dentin bond is a more predictable 20 to 25 MPa (the premix eliminates mixing errors and variability), and the enamel bond range is 20 to 30 MPa.

The chemistry of fifth (as well as sixth and seventh) generation adhesives is not compatible with dual cure restorative materials such as cements and core buildups. Thus, an accessory dual-cure additive was introduced by many manufacturers to make the strictly light-cured fifth-generation adhesives compatible with dual-cure restorative materials. Theoretically, the dual-cure fifth-generation adhesives were light initiated but would polymerize within 5 minutes in the absence of light, as well. According to a number of published reports based on clinical testing, only Pulpdent’s DenTASTIC UNO-DUO system (Figure 8-7, A) and Bisco’s One-Step Plus system (Figure 8-7, B) were shown to truly have dual-cure properties. Many practitioners who attempted the use of fifth-generation dual-cure products other than the two noted above were very disappointed with their cementation results.

FIGURE 8-7 A, DenTASTIC UNO-DUO system. B, One-Step Plus system.

(A courtesy Pulpdent Corporation, Watertown, Massachusetts. B courtesy Bisco, Schaumburg, Illinois.)

The sixth-generation adhesives were designed to eliminate the separate etching step. (In fact, there is no separate etching step with sixth- or seventh-generation products; the surface conditioning is accomplished by the chemistry of the bonding agent.) The sixth-generation adhesive materials bond to both enamel and dentin. The dentin bond is 17 to 22 MPa, which is relatively acceptable. The bond to enamel, however, particularly at the enamel interface, all too often fails within the first 3 years. Thus most sixth-generation adhesives cannot be indicated for enamel bonding without an additional enamel etching or enamel roughening step.

The sixth-generation adhesives are placed on the tooth surfaces at a very low initial pH, immediately etching all the surfaces that they contact until the adhesive-tooth complex is titrated (approximately 1 to 2 seconds). The etching of the tooth surfaces is accomplished as part of the overall adhesive process, not as a separate step. The sixth-generation adhesives were the first to incorporate the etching chemistry into the adhesive components, but unfortunately reverted to the multibottle, multistep application. This reintroduced the drawbacks of unpredictable chairside mixing, incorrect component ratios, and possibly an inappropriate application sequence. Some sixth-generation adhesives such as ExciTE F (Ivoclar Vivadent, Schaan, Liechtenstein) use innovative chemistries to overcome the problem of inadequate enamel etching

The clinical concern is that the acidity of the pH and the tooth application time of most sixth-generation adhesives are simply inadequate to etch the enamel sufficiently. Therefore, enamel interfaces have a tendency to break down. Whereas the dentinal interfaces are likely to stay intact in the long term, enamel interfaces may not. There are two very simple solutions to this problem: the practitioner must chemically etch or mechanically roughen the enamel surfaces of the preparation before sixth-generation adhesion. This additional step, however, creates a paradox: it makes the sixth-generation a non-etching adhesive that still requires etching materials to be used on the enamel—not a truly practical solution.

Most sixth-generation adhesives are supplied in two or three components and require two or three clinical steps. This generation uses dentin conditioners to chemically alter but not to remove the smear layer. The adhesion is to the etched enamel and conditioned dentin surfaces. The chemistry of the conditioned dentin and the hybridized layer raises the question of whether a moist surface is required. The jury is still out on this point.

The seventh-generation adhesives are furnished in a single bottle or ampule that includes all the required components for self-etching, surface conditioning, hybridization, desensitization, adhesion, and often fluoride release, as well. All the necessary adhesive steps are accomplished chemically by the simple application of the adhesive to the tooth surface. No additional clinical steps are required, no mixing is needed, and the bond to the enamel, unlike with sixth-generation adhesives, is very acceptable. The seventh-generation bond to dentin is the highest among all the adhesive groups, in the range of 23 to 30 MPa and higher. The enamel bond developed with seventh-generation agents is successful simply because of the lowered application pH (1.0 pH lower or 10 times more acidic than sixth-generation adhesives at the enamel surface on application). The enamel surface is etched quickly and effectively.

The most important innovation of seventh-generation adhesives is the added advantage of moisture independence. The acid-base reaction of the seventh-generation adhesive on the dentin surface is that of an acid acting on an organic base. This chemical reaction generates organic salt and water. Thus, seventh-generation adhesive procedures effectively create their own moisture. They can be applied to a “moist surface” (however that may be defined) or a dried surface (much more easily described). Because seventh-generation systems are supplied in a single pre-mixed container and require a single application step, and no moist or wet surface, few mistakes are possible and there is no technique sensitivity. The process is simple: apply the adhesive to the tooth surface, agitate or scrub if required, and then, after a brief wait, simply air dry the surface. Once the bonded surface is dry (no longer moist with bonding), it is light cured. There are no separate etching or conditioning steps, and the smear layer is not removed. There has been virtually no post-operative sensitivity reported with seventh-generation bonding agents. The hybrid layer is created by the chemistry of the adhesive. The number of bottles is one; the number of steps is one. There is no post-operative sensitivity, and dentin bonds of 23 to 30 MPa and higher have been reported.

Of the seven bonding agent generations available to the dentist, the first-, second-, and third-generation agents are rarely used. The main reason for this is that their adhesion to dentin is less than the minimum force required to resist the forces of polymerization contraction of composite (17 MPa). When adhesives of these generations are used to bond composite restorations, a marginal gap often develops at the interface between the composite and the tooth surfaces. This gap attracts plaque and bacteria, resulting in acid formation that causes soft and hard tissue breakdown. Current dental practice essentially focuses on fourth-, fifth-, sixth-, and seventh-generation adhesives. Although all of these adhesives can be used successfully (it is crucial to follow instructions and to proceed in a logical step-by-step in sequence), the newer-generation products are clinically easier and far more predictable than the earlier ones.

Other Considerations

One of the most important clinical considerations for the selection of adhesive products is the bonding strength required at the adhesive interface. This question first arose with the development of adhesive materials in the 1950s, and it is still a somewhat controversial topic. Certain basic principles have been conclusively established and are well accepted. In the 1980s and 1990s, a number of studies, including Munksgaard in 1985 and Retief in 1994, found that a minimum of 17 MPa of adhesion to tooth structure was required for successful adhesion. The 17 MPa represents the force of the polymerization contraction of the composite resin restorative material. If there is less than 17 MPa of adhesion to either the enamel or the dentin, the polymerization force of the composite resin is greater than the force adhering the material to the enamel, dentin, or both. The forces of polymerization cause the resin to contract toward the center of the composite, pulling the restorative material away from the walls of the cavity (Figure 8-8). A small gap is created, which then allows micro-infiltration of bacteria and plaque that eventually causes marginal breakdown. The fluid inflow and outflow at the restorative interface carries bacteria and sugars deep into the tooth-restorative interface and eventually causes decay. In time, a dark line appears at the margin of the restoration.

FIGURE 8-8 Less than 17 MPa of adhesion results in the polymerization forces, causing the resin to contract toward the center of the composite, which pulls the restorative material away from the walls of the cavity.

(Courtesy Dr Ray Bertolotti.)

Where the bonding agent’s adhesive strength to the dentin and the enamel exceed the 17 MPa of polymerization contraction, the shrinkage of the composite is toward the walls of the cavity (Figure 8-9). The laws of entropy dictate that the polymerization contraction process always tends to go in the direction of least resistance (or higher attraction). The composite is thus more attracted to the dentin and to enamel surfaces than it is to itself. Because the shrinkage is toward the walls and away from the center, no marginal gap develops. This makes marginal infiltration of bacteria and oral fluids far less likely, and thus prevents decay and eventual breakdown. The meniscus developed in the center of the restorative material is simply filled in by the next layer of composite resin. This is why an adhesive must have bonding strengths both to enamel and to dentin of more than 17 MPa to be clinically acceptable. Ideally, the bond strengths to enamel and dentin should be relatively equal. If, for example, the adhesion to the enamel is far greater than the bond to the dentin, the stronger force at the enamel interface will tend to pull the composite away from the dentinal margin during the polymerization process, weakening the dentin interface.

FIGURE 8-9 When there is more than 17 MPa of adhesion, the polymerization contraction causes shrinkage of the composite toward the walls of the cavity.

(Courtesy Dr Ray Bertolotti.)

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