Chapter 8 Adhesion
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.
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.
Box 8.1 Generations of Dental Adhesives
The first-generation adhesives in the late 1970s were really rather ineffective. Although their bonding strength to enamel was high (generally, adhesives of all generations except for the sixth bond well to the microcrystalline structure of enamel), the strength of their bond to the semi-organic dentin was the major problem facing dentists. Their adhesion to dentin was pitifully low, typically no higher than 2 MPa. Bonding was achieved through the chelation of the bonding agent to the calcium component of the dentin. Although tubular penetration did occur, it contributed little to restoration retention. It was common to see debonding at the dentinal interface within several months. These bonding agents were recommended primarily for small, retentive class III and class V cavities. Post-operative sensitivity was common when these bonding agents were used for posterior occlusal restorations.
In the early 1980s a distinct second generation of adhesives was developed. The enamel was etched and rinsed prior to bonding, keeping the acid away from the dentin. This generation targeted the smear layer on the dentin (a post cavity preparation layer composed of dentinal debris, dead and live bacteria) as a bonding substrate. The smear layer is typically bonded to the underlying dentin at a negligible level of 2 to 3 MPa. The weak bonding strengths of these second-generation agents—2 to 8 MPa to dentin—meant that a mechanical retention form in cavity preparations was still required. Restorations with margins in dentin saw extensive microleakage, and posterior occlusal restorations were likely to exhibit significant post-operative sensitivity. The long-term stability of second-generation adhesives was problematic. For direct restorations, 1-year retention rates were as low as 70%.
In the late 1980s, two-component primer-adhesive systems were introduced. The marked improvement that these bonding agents represented warranted their classification as third-generation adhesives. Significant increases in bonding strength to dentin, 8 to 15 MPa, decreased the need for a retention form in the cavity preparation. Erosion, abrasion, and abfraction lesions were treatable with minimal tooth preparation, hence the introduction of ultraconservative dentistry. A noticeable decrease in post-operative sensitivity observed with posterior occlusal restorations was very welcome. Enamel etching was routine but it was feared that etching the dentin would cause pulpal necrosis and necessitate endodontic treatment. The third-generation adhesives were the first agents that bonded not only to tooth structure but also to dental metals and ceramics. The downside to these bonding agents was their limited longevity. Various studies demonstrated that adhesive retention with these materials started to diminish after 3 years intraorally. However, patient demands for tooth-colored restorations convinced some dentists to begin routinely providing posterior composite fillings.
In the early 1990s fourth-generation bonding agents transformed dentistry. Their high bonding strength to dentin, 17 to 25 MPa, and decreased post-operative sensitivity in posterior occlusal restorations encouraged many dentists to begin the tectonic switch from amalgam to direct posterior composite restorations. Both the enamel and the dentin are etched and then rinsed simultaneously with the “total etch” technique. This generation is characterized by the hybridization process at the dentin-composite interface.
Hybridization is the replacement of the hydroxyapatite and water in the surface dentin by resin material. This resin, cured with the remaining collagen fibers, constitutes the hybrid layer. Hybridization involves both the dentinal tubules and the intratubular dentin, dramatically improving bonding strength to dentin. Total etching (both enamel and dentin) and moist dentin bonding, concepts developed by Fusayama and Nakabayashi in Japan in the 1980s and introduced in North America by Dr. Raymond Bertollotti, are innovative hallmarks of the fourth-generation adhesives.
The fourth generation adhesives are distinguished by their components; there are two or more ingredients that must be mixed, preferably in precise ratios, or utilized in a specific sequence. This is easy enough to accomplish at the research laboratory but much more complicated, and perhaps impossible, chairside. The number of mixing steps involved and the requirement for precise component measurements tend to confuse the process; imprecise procedures and inaccurate ratios reduce or eliminate the bonding strengths to dentin.
This led to the development and the great popularity of the fifth-generation dental adhesives. These materials adhere well to enamel, dentin, ceramics, and metal, but, most important, they are characterized by a single component in a single bottle (in addition to the etching gel). There is no mixing involved in the adhesion process, and hence less opportunity for error. Bonding strengths to dentin are in the 20- to 25-MPa range, suitable for all dental procedures (except self-curing resin cements and self-curing composites).
Dental procedures tend to be both technique sensitive and stressful. Where some of this stress can be eliminated, dentists, staff, and patients all benefit. There is little technique sensitivity in a material that can be applied directly from the bottle to the prepared tooth surface. Post-operative sensitivity was reduced appreciably. The fifth-generation bonding agents, easy to use and predictable, were the most popular adhesives of their day.
Over the years, dentists and researchers have sought to eliminate the etching step or to include the surface conditioning process chemically as part of another component. The sixth-generation adhesives require no separate etching step, at least at the dentinal surface. Since 2000, a number of dental adhesives specifically designed to eliminate the etching step have been introduced. These products have a dentin-conditioning liquid as part of one of their components; the acid treatment of the dentin is self-limiting, and the etch byproducts are permanently incorporated into the dental-restorative interface.
Significant questions were raised by researchers concerning the quality of the bond after aging in the mouth, typically at the 3-year milestone. Interestingly, the bond to the dentin (17 to 22 MPa) remains strong; it is the bond to the unetched, unprepared enamel that tends to fail in 30% or more of cases. In addition, the multiple components and multiple steps required for the various sixth-generation agents can be confusing and lead to clinical error. In practice, it is possible to eliminate bond failure at the enamel interface simply by etching or roughening the enamel before placing the adhesive. This, however, introduces a separate etching step to a supposedly non-etching generation.
A novel, simplified seventh-generation adhesive system overcomes all previous objections. Just as the fifth-generation bonding agents made the leap from multi-component systems to a rational and easy-to-use single bottle adhesive the seventh-generation simplifies the multi-component, multi-step sixth-generation materials into a single-component, single-bottle system. The seventh-generation adhesives are typically more acidic (1.0 pH lower) than their sixth-generation counterparts. This adequately etches all exposed enamel for effective bonding strengths. The etch is self-limiting immediately after its application to the tooth surface, and the byproducts are ultimately incorporated into the adhesive interface during the light-curing step. Because the conditioning materials are never rinsed off the tooth surface before polymerization, there is far less likelihood that any vital dentinal tubules are left open, and thus made more prone to after-treatment discomfort. Thus, postoperative sensitivity is virtually never observed after the deployment of seventh-generation adhesives.
Both the sixth- and seventh-generation adhesives are available for self-etching, self-priming adhesion for improved procedures with minimal technique sensitivity and little or no post-operative sensitivity. Seventh generation adhesives are easier to use, less technique sensitive, and do not require a “moist” surface.
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).
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.
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.
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.
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).
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.
There are no contraindications to adhesion in clinical dentistry. There have been no reports of allergy to the materials. Restorations that failed have done so only when the adhesion was improperly implemented, or early in the adhesive era when bond strengths were simply too weak. It is very important that adhesion be accomplished rigorously, following the instructions and the requirements of the materials involved.
There are many different adhesive procedures. Specific composite materials may require specific adhesive components. Some composite materials are not compatible with all adhesive components. Thus it is essential that the adhesives used be appropriate to the procedure and the restorative material and that the entire system be used according to the manufacturer’s instructions. Certain restorative materials, bases, cements, and ionomers do not require separate adhesives because the bonding chemistry is incorporated into the restorative material itself. In these situations, the adhesive may be superfluous. It may even compromise the restorative to tooth bond strength and the overall success of the bonding procedure.
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).
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.
(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.
The best choice for restorative dentistry today is a seventh-generation adhesive. With a single-component, single-step process, the bonding process is straightforward and predictable, and no clinical mistakes can be made. The practitioner simply takes the bonding agent from the bottle or the individual dispenser compule and applies it to the tooth surfaces. There the adhesive is agitated or left to infiltrate the tooth structure, according to the instructions. The excess adhesive is air dried until no droplets remain on the surface. Then it is light cured. There is virtually no post-operative sensitivity with seventh-generation adhesives. (The sixth-generation adhesives also cause little sensitivity, but fourth- and fifth-generation bonding agents are associated with significant post-operative sensitivity. These earlier adhesives can be useful but are more problematic clinically.)
Clinically, the most efficient procedures use the fifth- and seventh-generation adhesives; there are fewer components and fewer steps. The time required for bonding may not seem relevant for the practitioner at first glance; 30 seconds per restoration for a fifth or seventh generation compares very favorably to the far longer application times of fourth-generation adhesives at up to 180 seconds. A difference of 1 to 2 minutes in chairside time bonding time has a significant impact on productivity when it is considered that the average practice day comprises more than 20 adhesive procedures. Patients who must keep their mouths wide open for the entire procedure, might be uncomfortable with extended procedures, and will certainly appreciate a more streamlined treatment.
Often the fourth generation is referred to as the gold standard. Although this adhesive category is definitely the most researched over the years, fourth-generation adhesives are also the most difficult, most technique sensitive, and often the most time-consuming to use. They are known to cause more post-operative sensitivity in posterior teeth than adhesives of the other generations, even when used exactly according to instructions. This group of bonding agents requires the development of a hard-to-define moist surface prior to the application of the adhesive.
Sixth-generation products are easier to use than fourth-generation products, but despite having eliminated the acid etching step, sixth-generation procedures still involve numerous components. The most significant problem with sixth-generation agents, however, is their unpredictable adhesion to unprepared, unetched enamel. Hence the need to separately etch or roughen the enamel surfaces to increase adhesion predictability.
The remaining options are the fifth- and seventh-generation adhesives. Both are fairly rapid and easy to use, but fifth-generation agents involve a separate acid etching step, more components, and more steps. The seventh-generation bonding agents are better, faster, and easier, as well as more predictable. They offer the patient-pleasing bonus of eliminating virtually all post-operative sensitivity.
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.
Although the currently available dentin bonding agents effectively adhere composites to the dentinal surface, there is still room for improvement. Assuming application as prescribed by the instructions under carefully controlled conditions, the clinical longevity of the bonded resin is comparable to that of any other material currently used by the restorative dentist. Unfortunately, some bonding systems are more technique sensitive than originally presumed. In a study that examined fourth-generation dental adhesives (and with findings that may apply to fifth-generation products as well), Hashimoto demonstrated that gradual debonding at the dentinal surface can occur over time. The bond strength of posterior composite resin restorations adhered with fourth-generation materials decreased by nearly 75% over a 3-year period. In addition, scanning electron microscopy demonstrated that some of the collagen fibers beneath the hybrid zone had undergone various levels of degradation. Although this study was conducted on primary posterior teeth, the same conclusion could be extrapolated to restored permanent teeth. This rationale is based on the fact that the mechanism of bonding to collagen and the formation of the hybrid zone are similar for both deciduous and permanent dentition.
Although the specific reasons for these findings have not been determined, the most probable cause can be attributed to the manipulative procedures associated with the bonding process itself. Specifically, it is probable that once the decalcification (acid etching) process is completed, the bonding agent primer fails to penetrate completely into all of the evacuated dentinal tubules and spaces among the peritubular collagen fibers. Without the protection of either the naturally occurring hydroxyapatite or the resin component of the dentin bonding agent, the exposed collagen fibers simply undergo a process of biologic degradation.
This problem can be related in part to the manner in which either the fourth- or fifth-generation bonding agents are used. In both procedures, the acid etching agent is first used to demineralize the dentin. Once this step has been completed and the etchant rinsed, the clinician applies the dentin bonding agent to the prepared surface to effectively reverse the etching process; all the open dentinal tubules and intracollagenous spaces created by the demineralization must be completely filled with resin adhesive. Unless the dentist is very careful to follow instructions precisely, apply the correct number of primer layers, and allow the time required for the complete diffusion of the adhesive into the denatured dentin, adequate resin penetration may not be achieved. Other factors may influence the level of penetration as well; overdrying the preparation and thus failing to leave some water on the surface (for the mandated moist bonding) may prevent the primer from penetrating the dentin. Excess water on the surface may also prevent the infusion of the bonding agent. Premature vaporization of the alcohol or acetone solvent of the bonding agent (a problem that occurs when the adhesive is dispensed too early and the solvent allowed to evaporate in the well) may also cause inadequate diffusion and bond failure.
The recent introduction of self-etching dentin bonding agents (sixth and seventh generations) has been met with great enthusiasm. The most important reason for this is the relative ease of use of these products. Many practitioners view self-etching adhesives as materials that can etch both dentin and enamel in a single step. They also perceive these bonding agents as systems that can simultaneously apply the primer and/or adhesive in the same step. A second reason for the rapid acceptance of these materials is related to the virtual absence of post-operative sensitivity associated with their use. Together, these two factors have convinced many practitioners to abandon earlier adhesive systems for a process that they perceive to offer better, faster, easier, and more predictable bonding to tooth structure.
The inherent advantage of the self-etching dentin bonding agents is that they etch and deposit the primer simultaneously. This procedural sequence makes it far less likely that underfilling (incomplete replacement by resin of the etched minerals in the dentinal tubules and intracollagenous areas) will occur. Consequently, the possibility of both long-term bond strength degradation and short-term post-operative sensitivity is significantly diminished. Furthermore, the number of steps needed for bonding composites to the dentin surface is reduced, minimizing technique sensitivity. The latest-generation adhesives make bonded dental procedures easier, better, and more predictable.
The technologic advances that have played a role in the process of dental adhesion are very important. The earliest bonding agents were supplied in one or more bottles. The components were dispensed and mixed as required and then were picked up by a brush or synthetic foam stick to be applied to the tooth structure. If the applicator was inserted into the bottle, the remaining adhesive was immediately contaminated.
It is important that the adhesive be at its optimal chemistry when it is applied to the tooth surface. Because the most common dental solvents—acetone and alcohol—are highly volatile at room temperatures, the practice of anticipating a procedure by dispensing the material into a well 2 to 3 minutes before application allows excessive solvent evaporation, an altered chemistry, and premature bond failure.
One of the first major innovations was the development of individual dispenser containers, or disposable wells wherein the components were quickly premixed, eliminating the possibility of cross-contamination. Each cap or individual dispenser was designated for a single patient use, for one or more teeth; they certainly could not be shared among patients.
The issue of chemical contamination of the adhesive by the plastic bottle material was solved, initially by using glass containers and then by adjusting the chemistries of either the adhesive or the bottle plastic, or both.
Many of the earlier applicators could not transport sufficient volumes of adhesive to the tooth surface effectively or quickly. Manufacturers began to provide innovative reservoir mechanisms to overcome this problem. Tooth applicators went from simple brushes to foam carriers and then advanced to foam-brush carriers that were capable of incorporating significant amounts of adhesive to be brought to the tooth in a single carry. With current adhesives, it is imperative that the adhesive be carried to the tooth as quickly as possible in sufficient quantity, applied to the tooth, agitated on the surface, and then, once completely air dried, light cured.
Another technologic innovation has been the factory-level mixing of components. In a clinical situation when two materials must be mixed freehand, one material may be used in a greater proportion than the other. This is often demonstrated by dentists finishing one component of a two-bottle system well before the other, indicating that too much of one component (or too little of the other) was used. The resulting decrease in the adhesive’s chemical properties can damage the functionality and decrease the longevity of the restoration.