Critical Elements of Bonding Success
Bonded resin composites are the most frequently placed direct tooth-colored restoratives and, released from the constraints of classical preparations required for amalgam restorations, clinicians are now able to remove the minimal amount of tooth structure required for caries removal with no requirement for a retentive preparation form.
As such, resin composite restorations can play a positive role in the drive towards minimally invasive dentistry. A number of elements are critical for the success and durability of composite restorations. Selecting a composite with suitable properties including esthetics, high compressive and tensile strength, and minimal polymerization shrinkage is critical. Proper use of a resin adhesive, isolation such that a dry field is obtained for composite placement, careful attention to placement and final form, and marginal integrity are essential. Resin composite restoration success rates can be outstanding, and clinical studies have demonstrated that long-term success with bonded composite restorations is possible. Success, however, requires that both a suitable bonding agent and an appropriate technique are used.1,2,3 In one 8-year prospective study an annual failure rate of 2% was observed,4 while a review of multiple studies of at least five years’ duration has shown an annual failure rate of 1% to 3%, with secondary caries being the most common cause of repair/replacement.5 From the patient’s perspective, the definition of success would be a restoration that is comfortable (pain-free), functional, and esthetic.
Adhesive Systems
It was only following the introduction of adhesive technology that composite resins became a reality in dentistry and since their original introduction, adhesive systems have advanced dramatically. The first adhesive systems were based on etching of only enamel while, later, removal of the dentin smear layer by etching was also performed along with use of a hydrophilic monomer to obtain some adhesion to dentin. At the current time, a plethora of adhesive systems and resin composites is available to choose from—however, not all are created equal nor are they all universally compatible. Adhesive systems are arguably misunderstood yet they are the most important factor in successful short- and long-term outcomes for bonded resin composite restorations. Use of a compatible adhesive system in the manner intended is essential and adhesive systems for resin composites have been categorized in several ways. The generational classification is sequential and based on when the adhesive system was introduced. The sequence does not however correlate to the technique used (Table 1). A simpler method is to base categories on two simple measures—whether they are etch-and-rinse (total etch) or self-etch adhesives. Within this simple classification, the subsets are then based on the number of containers required for the system. Thus, etch-and-rinse adhesive systems can be broken down into 3-step, 3-bottle systems
that have separate etchant, primer and bonding agent or 2-step, 2-bottle systems that have etchant and a separate combined primer/bonding agent. The phosphoric acid etchant component of etch-and-rinse adhesives is typically 32%-37% with a pH range of 0.1-0.4. Self-etch adhesives consist of either a 2-step (2-bottle) or a 1-step (2-bottle or 1-bottle) system (Table 2). These contain acidic monomers in a water-based adhesive and do not require separate etching or rinsing steps. After placement, the solvent must be thinned with the air syringe to avoid having a layer that is too thick and that would negatively impact bonding. More recently, self-etch adhesives have also been stratified by the pH of the acidic monomer incorporated for etching—for example, a “strong” self-etch adhesive has a lower pH than a ‘mild’ self-etch adhesive (around pH<1 versus 2).6 Cross-linking agents may also be used to reinforce dentin bonding.7 The objective of lower-pH selfetch adhesives is to improve enamel etching and bonding.
Mechanisms of Action
Etch-and-rinse adhesives involve the use of phosphoric acid etchant to demineralize enamel and dentin. In the case of dentin, this exposes the microscopic collagen fibers and completely removes the smear layer. The separate primer and bonding agents or single primer/bonding agent are designed to penetrate into the demineralized
areas, creating micromechanical locking (once the resin tags are cured) and a minimal amount of hybridization. With self-etch adhesives, the etchant is milder which results in the loss of less enamel structure during demineralization. Retention to enamel occurs through the same micromechanical locking of resin tags observed with etch-and-rinse adhesives. In dentin, the intent is both micromechanical locking and chemical bonding. Unlike with etch-and-rinse adhesives, the smear layer is not fully removed and is instead incorporated into the tooth-adhesive interface through hybridization. Since the etching monomers result in less demineralization, the collagen fibrils are not fully exposed and still have some mineral attached to them. To avoid the presence of free matrix metalloproteinases following demineralization, and the associated risk of long-term bond degradation, the adhesive monomer must be able to fully penetrate, encase and seal the dentin and dentinal tubules.
Key Drivers for Success
Etch-and-rinse adhesion is still considered by many researchers to be the gold standard. However, differences in bonding results vary with the operator performing the procedures regardless of which adhesive system is used. For Class V restorations bonded using etch-and-rinse or selfetch adhesives were equally reliable in a separate study. 8A contemporary study of Class III, IV and V restorations, published in 2013, found both self-etch and etch-and-rinse adhesive systems to be effective.9Marginal integrity relies on correct use of the adhesive system for bond strength and durability in the presence of an intraoral environment that results in stress during mastication, periodically low pHs and dramatic changes in temperature. A bond is likely to fail sooner than the restorative material under adverse conditions, and if a bond either does not develop—or fails—microleakage occurs. Etiologically, microleakage is the main reason for marginal staining, recurrent/secondary caries and sensitivity due to the loss of marginal integrity. Bond strength is therefore an important consideration when choosing an adhesive system, and the higher the bond strength the better. Other factors that must be considered include polymerization shrinkage, shrinkage stress and poor enamel bonding which are also associated with loss of marginal integrity and microleakage. The strongest evidence for bonded composites is based on in vivo trials.
Adhesive Compatibility
As stated earlier, the adhesive system selected must be compatible with the material being used. Considerations include whether a resin is light-cured, self-cured or dual cured or which indirect restorative material is being used. If a manufacturer indicates that a specific adhesive system should be used only for certain composite resins or materials, it is wise to follow this direction to avoid the risk of failure. Oftentimes, incompatibility may only be realized when catastrophic failure/debonding occurs. Incompatibility between an adhesive system and composite results in bonding failure, which is associated with lack/loss of marginal integrity and restoration failure.
Universal Adhesives
By definition, a universal adhesive is one that can be universally applied—in other words, it can be used for all tooth substrates and for all direct and indirect restorative materials and resin-based cement. Thus, it is suitable for enamel and dentin bonding to direct and indirect resin composites, as well as indirect ceramics and metals. Truly universal adhesives are also suitable for light-cured, dualcured and self-cured resin composites. The definition universal does not imply the number of bottles or containers that must be used nor whether an etch-and-rinse or self-etch technique is to be used. As clarification, a 2-step or 1-step self-etch adhesive that can be used for all direct composite and indirect restorative materials and tooth substrates is a universal adhesive—as is an etch-and-rinse adhesive that can be similarly utilized. Key attributes for a universal adhesive include ease-of-use, a strong and stable bond in vivo, and preferably the ability to use a simplified protocol. Using an adhesive system that is compatible with as many types of restorative materials as possible keeps things simple, avoids confusion and saves on inventory. Adhesive systems now exist that are “universal” and can be used with all direct and indirect restorative materials. A two-step etch-and-rinse universal adhesive (Optibond Solo Plus), as well as a 2-step self-etch universal adhesive (Optibond XTR). This universal adhesive can
be utilized for adhesion of direct composite restorations, composite core build-ups and indirect composite restorations, as well as indirect ceramic, metal and metal-ceramic restorations. Just as with more traditional adhesives, the intaglio surface of indirect metal and ceramic restorations must be pretreated, however the clinical process is greatly simplified by the use of a universal adhesive. Optibond XTR contains a monomer designed to enhance bonding through cross-linking and by providing for both chemical and micromechanical adhesion, while the solvent system aids penetration of the self-etchant and primer into the dentin which results in longer resin tags and micromechanical locking, and the filler increases bond strength. It has been recommended that enamel margins be prepared with a diamond or tungsten carbide bur to roughen the surface prior to self-etching with self-etch adhesives,10 with the objective of increasing etching and therefore bond strength, and—due to reliance on enamel bonding—self-etch adhesives for Class IV restorations were discouraged. The OptiBond XTR universal adhesive chemistry has resulted in bond strengths that are comparable to or higher than gold standard etch-and-rinse agents, durable, and therefore sufficient for all Classes of preparations without prior treatment of the enamel margins. Superior bond strengths for universal self-etch adhesive have been found for both cut and uncut enamel, and clinically the adhesive performance has been superior.11 A further advantage as self-etch adhesives is the reduced number of steps and no risk of dessicating dentin which could result in post-operative sensitivity.
Summary
Universal adhesives are now available that offer high bond strengths at least as high as conventional etch-andrinse gold standard adhesives. Using a self-etch universal adhesive further reduces complexity as no separate etching and rinsing step is required. Using a universal adhesive that is suitable for all resin composites as well as all indirect restorative materials simplifies treatment, reduces inventory and saves time.
References
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