Adhesive Classification

The first step to improving clinical outcomes is knowing which bonding agent is being used. But to know that, one must understand that essentially all bonding agents are composed of 3 major components in some form, including an acid-etchant, primer, and adhesive. These components are separated or combined in various manners to create the adhesives available today. These combinations create the classification systems that help quickly distinguish one adhesive from another. Perhaps the easiest classification is by inclusion or separation of the acid-etch component. Bonding agents with a separate phosphoric acid etch step are termed “etch-and-rinse,” whereas those with the acid-etch included in the adhesive are termed “self-etch.” Within these categories themselves, the number of application steps varies. The manufacturer’s instructions included with the adhesive will state the required application protocol, providing the practitioner with the information needed to assign a classification. A summary of the classification breakdown is seen in Table 1 .

Classification Characteristics

Now that it has been determined which adhesive system is being used, one can begin to understand the pros and cons of each. For simplicity, each bonding agent will be referred to by its generation number going forward. At this juncture, it is also important to understand that the fifth and seventh generation bonding agents will tend to have similar characteristics, whereas the fourth and sixth generation bonding agents will have their own similar characteristics. The reason for this distinction is the presence of a neutral adhesive layer, which is provided by a separate step involving the adhesive alone. This means that the acidic priming resin, characteristic of the fifth and seventh generation group, will be separated from the composite placed on top. This separation is key, allowing for the creation of a hydrophobic adhesive layer that is also more resistant to hydrolysis over time. This also allows fourth and sixth generation adhesives to be compatible with dual-cure and self-cure composites. A summary of the characteristics of these bonding agent groups is listed in Table 2 .

Given the benefits of the fourth and sixth generation adhesives, we would expect this to translate to clinical success. Interestingly, a systematic review by Peumans and colleagues evaluated the effectiveness of contemporary adhesives placed in noncarious cervical lesion clinical trials between 1950 and 2013. It was found that the glass ionomers and two-step self-etch mild adhesives (2SEa-6th generation) displayed the most favorable and durable clinical bonding performance. The 3-step etch-and-rinse adhesives (3E&Ra-4th generation) also performed favorably in this study, providing further evidence to support the use of these adhesives.

This study also highlighted an interesting finding in clinical effectiveness within the self-etch adhesives. If the pH of the adhesive was less than 1.5, it was termed strong, whereas a pH of greater than 1.5 was termed mild. When a strong pH adhesive was used, it was associated with a higher annual failure rate for self-etch adhesives. This conclusion is important, as it has implications in the discussion of the next group of adhesives, the universal bonding agents.

Universal Bonding Agents

Universal adhesives have gained in popularity due to their simplicity of use, versatility, and clinical effectiveness. The definition of a universal adhesive, however, is still vague, as the term “universal” seems to imply it can be used in all different ways at all times (self-etch, total-etch, self-cure, dual-cure, light-cure, bonds to tooth, bonds to composites, metals, ceramics, and zirconia), all while coming in one bottle. If that were the definition, it could be argued that no such adhesive currently exists. Nonetheless, it appears that these universal adhesives are here to stay and are deserving of a classification all their own.

One of the key benefits of universal adhesives is their ability to be used for both indirect and direct restorations. This means they can be used with direct composites, as well as bonding a veneer or crown. This versatility reduces the number of materials needed in the practice without compromising the quality of the restoration. What makes this versatility possible in many of the universal adhesives is the inclusion of the adhesive monomer 10 methacryloyloxydecyl-dihydrogen-phosphate (10-MDP for short). This monomer is the same one that has been used for many years in Kuraray’s Panavia adhesive resin cement (Osaka, Japan). With its addition, the adhesive can bond to methacrylate-based restoratives and cements, as well as to tooth, metal and zirconia. 10-MDP is also a highly hydrophobic monomer, making it less prone to water sorption and hydrolytic breakdown. In addition, it can bond chemically to the tooth via its interaction with calcium in the hydroxyapatite of the tooth, forming stable MDP-Ca salts, which, along with nano-layering, explain the high stability of this bond and resistance to degradation.

As stated previously, the pH of the adhesive seems to play an important role in clinical effectiveness, with a milder pH being beneficial. Although a mild pH self-etch adhesive is desired on dentin, it is a concern on enamel, as the ability to adequately etch the substrate is in question. To improve this bond, the popular selective-etch technique is recommended. ^, This is where the enamel margin is etched with phosphoric acid, rinsed, dried, and then followed by placement of the universal adhesive. When used in this manner, a positive effect on the bond durability is noted.

Another concern with pH of the adhesives is the effect of its compatibility with dual-cure and self-cure composites and resin cements. In general, a stronger acidity/lower pH means less compatibility. Without proper polymerization at the adhesive interface, we have no bond. To overcome this, the use of dual-cure activators or amine-free cements must be used. Thus, it is critical to read the manufacturer’s instructions regarding the adhesive being used, as different adhesives have differing formulations. It should also be noted here that it is advised to not mix and match adhesives and resin cements when possible. With different chemistries present, adequate polymerization may not be occurring, thus negatively impacting the bond and clinical outcome.

Tooth Preparation

Before discussing the application technique of bonding agents, we need to go back a step further. Although seemingly insignificant, the bur used in the tooth and cavity preparation process is important. With the enamel being etched by phosphoric acid, we focus on the dentin. Although both carbide and diamond burs are used in the cavity preparation process, carbide burs are recommended on the dentin due to the thinner smear layer and higher bond strengths they create when using a self-etching adhesive. ^, When using an etch-and-rinse technique on dentin, no difference in bond strength was noted with respect to bur type used, although the bond strengths were all lower than the self-etch adhesive.

Another factor to consider is the cavity preparation depth. Although enamel is relatively homogeneous throughout, dentin properties and composition vary considerably depending on the distance from the pulp. It has been shown that deeper dentin (closer to the pulp) exhibits lower bond strengths than superficial dentin (closer to the dentinoenamel junction), regardless of the adhesive system used. This finding demonstrates the importance of intertubular dentin, which increases in total area toward the enamel. As the intertubular dentin increases, the dentinal tubule volume decreases, promoting greater development of the hybrid layer as coined by Nakabayashi and colleagues, which appears to be more important for bond strength than resin tag development. Furthermore, the deeper the dentin, the more permeable the substrate due to greater tubule diameter and number. This increase in permeability can negatively influence the bond strength achieved at the adhesive interface, especially for the etch-and-rinse adhesives in which the phosphoric acid removes the peritubular dentin and completely opens the tubules. Thus, one proposed method of counteracting this fluid movement is through the use of local anesthetics with vasoconstrictors along with mild self-etching adhesives, which function to both decrease the pulpal pressure in the tooth and leave the smear plugs in the tubules largely intact, respectively.

Adhesive Application

Regardless of the adhesive used, following the manufacturer’s instructions is essential. Each will have slightly different protocols and recommended application times that are tailored to the specific chemistry of that adhesive. Nonetheless, a few key principles apply to all adhesives during the application process.

Acid-Etch Application

As mentioned previously, there is a general concern regarding the ability of self-etching adhesives being strong enough to adequately condition the enamel to the level of that achieved with 35% to 37% phosphoric acid, especially with uncut enamel. Thus, etching the enamel through the selective-etch technique is often advocated with self-etch adhesives, as higher bond strengths have been reported when compared with enamel treated with the self-etch adhesives alone. An application time of 15 seconds appears necessary to adequately condition the enamel surface, with longer etch times increasing surface roughness but producing no significant increase in bond strength.

In the dentin, 15 seconds also appears adequate to achieving adequate bond strength, as longer etching times have been shown to reduce bond strength. However, as is often the case in the clinical setting, we often run into nonideal dentin substrate. One such variation is that of sclerotic and older dentin, which has been shown to be more acid-resistant and require longer etching times in the range of 20 to 30 seconds. ^, Longer etching times however negatively affects any normal surrounding dentin, making subsequent adhesive infiltration more challenging.

Moist Bonding

Contrary to enamel, in which we often look for the “frosty” appearance after rinsing and drying of the phosphoric acid, dentin is a very different substrate that behaves differently. After etching and rinsing, effort should be made to avoid overdrying the dentin. This is because of the collapse of the collagen matrix and decreased infiltration by the adhesive that results, producing decreased bond strengths, leakage, and sensitivity. Dentin should thus remain moist during the application process. This is the reason for the often stated shiny or hydrated appearance to the surface that should be present after etching and rinsing.

Various methods can be used to achieve this appearance, including gentle air-drying, blot drying, or suction tip. ^, With the use of self-etch adhesives, the guesswork is largely removed, as no rinsing is involved. Interestingly, a study by Unlu and colleagues compared self-etch and etch-and-rinse adhesives amongst dental providers with varied experience levels. They found that bond strengths achieved were higher with the self-etch method across all levels and that operator experience does influence the values obtained.

Solvent Removal

Proper evaporation of the solvent is perhaps one of the most overlooked steps in adhesive application. The solvent is water, ethanol, acetone, or some combination in the bonding agent. Water/ethanol-based solvents have been demonstrated to perform better than acetone-based solvents, largely due to the thinner adhesive layer generated. Nonetheless, failure to evaporate the solvent has been shown to lead to poor bond strengths and nanoleakage as a result of dilution, incomplete polymerization, and phase separation of the adhesive. Consequently, longer air-drying times may be beneficial. In addition, active agitation of the adhesive can help aid removal of the solvent through enhanced movement of monomer inward and solvent outward, thus improving the bond strength achieved and clinical performance. ^, To aid this step, the use of rigid microbrushes is recommended over flexible or long fiber-based ones in which adequate application pressure cannot be achieved.

It is important that this principal of air thinning with solvents not be directly applied to the adhesive portion of the bonding agent. For instance, in fourth and sixth generation adhesives, in which the adhesive is applied in a separate step, caution should be exercised with excessive or prolonged air thinning, as this may significantly reduce the dentin bond strength. As a result, thinning the adhesive with a microbrush may be a superior alternative to air thinning.

Bond Durability

It is known that the resin-dentin bonds decrease over time, and that this phenomenon is due to the degradation of the resin and collagen fibrils responsible for bond formation. Matrix metalloproteinases (MMPs) have largely been implicated as the primary enzymes contributing to this degradation, which become activated during dentin bonding procedures. Thus, to extend the life of this bond and improve its durability, several techniques are advocated. One such technique is to simply apply multiple coats of the adhesive. Although there does not appear to be uniform agreement on the number of coats needed, doing so has been demonstrated to decrease the degradation rate with self-etch adhesives and improve the overall performance.

Another technique with much interest is the use of an MMP inhibitor, with the most widely known agent being Chlorhexidine. Common agents using chlorhexidine include Cavity Cleanser (BISCO, Inc, Schaumberg, IL) and Consepsis (Ultradent, Inc, South Jordan, UT). Even at low concentrations, chlorhexidine has been shown to be effective in preventing degradation at the adhesive interface after acid-etching. An application time of 15 to 30 seconds appears to be all that is necessary for effectiveness. Use of chlorhexidine with self-etch adhesives on the other hand is not as conclusive. Although its placement on the dentin substrate before self-etch placement has shown some promise, further research is necessary to validate this method.

Benzalkonium chloride is another agent that has demonstrated MMP inhibition properties. This agent is included in the acid etchants of BISCO Inc and is included in the cavity disinfectant Tubulicid Red (Dental Therapeutics, Saltsjo-Boo, Sweden), and has been shown to strongly bind to demineralized dentin even after rinsing. When compared with untreated teeth showing decreases in bond strength after 6 and 12 months, the use of benzalkonium chloride demonstrated stable bond strengths over the same period. In addition, glutaraldehyde containing desensitizers such as Gluma (Kulzer, Hanau, Germany) have been implicated as MMP inhibitors in matrix-bound dentin, contributing to prevention of collagen degradation. However, it’s impact on bond strength of overlying resin remains unclear, with conflicting evidence available. ^, It should also be noted that this agent is marketed primarily for desensitizing reasons, and has cytotoxicity concerns. ^, Thus, its use on acid-etched dentin remains unclear at present and warrants additional investigation.

Light Curing

Once the adhesive is placed, light curing is generally required among most bonding agents to maximize the bond strength achieved. This critical step can easily be overlooked, as it often receives less attention than most other topics. Nevertheless, proper knowledge and technique with curing lights is of paramount importance in the dental practice, as it allows the provider to provide much of today’s dental procedures, from sealants and composites to bonded indirect restorations. It is no surprise then that its correct use is critical to the successful execution of adhesive dentistry.

It should be kept in mind, however, that high-power light-emitting diode curing lights available today are able to generate significant increases in temperature. This is especially important when curing the bonding agent in deep preparations where little dentin remains over the pulp, as pulpal damage can occur. Although increasing the distance from the curing light to the dentin may decrease the temperature, compromised polymerization may result, especially in areas difficulty for the light to reach. One method that shows promise to reduce the temperature rise with prolonged light curing is directing a stream of air at the tooth during light exposure. In addition, undercuts may be present in the cavity preparation, producing shadows and areas of poor light exposure. This may require the operator to move the curing light around over a longer period of time to ensure polymerization of all areas.

In any case, it is important that this critical step be carried out correctly to ensure adequate polymerization of the light cured adhesive and resins. In addition to technique, simple steps that can be taken to optimize the output from the curing light include periodic monitoring with a radiometer and routine examination of the light tip for damage and debris. Clear barrier sleeves and wraps can also be used as a method of protection for the light tip during treatment to minimize contamination without significantly compromising the curing of resin or the light spectrum emitted.