Most bonding agents are light-cured and contain an activator such as camphorquinone and an organic amine. Dual-cured bonding agents include a catalyst to promote self-curing. Although most bonding agents are unfilled, some products contain nanofillers and submicron glasses ranging from 0.5% to 40% by weight. Fillers are described in more detail in Chapter 9. Filled bonding agents may be easier to place on the tooth and may produce higher in vitro bond strengths. Bonding agents may contain fluoride, antimicrobial ingredients, or desensitizers, such as glutaraldehyde. The effectiveness of fluoride and antimicrobial release from a bonding agent has not been demonstrated.

Bond strength tests are, by far, the most popular among in vitro methods. ISO/TS11405 (2003) describes test protocols for both shear and tensile bond strength tests (Figure 13-3). Both tests use relatively large bonding areas (3-6 mm in diameter, 7-28 mm²). Nominal (average) bond strength is calculated by dividing the failure load by the specimen cross-sectional area. The high incidence of cohesive failures of the substrate observed with these tests prompted the development of micro bond strength tests (Figure 13-4), using specimens with much smaller bonding areas (1 mm²). The main limitation of bond strength tests, despite their great popularity, is that results from different studies cannot be directly compared because of the lack of standardization among research groups. Also, because of the heterogeneous stress distribution along the bonded interface, the nominal bond strength value is far from representative of the true stress that initiated debonding.

The quality of the marginal seal obtained with adhesive systems can be estimated by different methods. Microleakage tests use the immersion of a restored tooth in a tracer solution (e.g., methylene blue or silver nitrate). The tooth is sectioned and the extent of dye penetration is evaluated, either qualitatively (using scores) or quantitatively. Interfacial gaps can be measured under a scanning electron microscope (SEM). Because processing of the real specimen for SEM viewing is critical and more gaps can be unintentionally created, replicas of the bonded interface in epoxy resin are preferred. The term nanoleakage applies to a method in which specimens previously immersed in silver nitrate are observed under a transmission electron microscope (TEM). The presence of silver deposits demonstrates the presence of gaps and voids at the bonded interface (Figure 13-5).

Other in vitro methods for evaluating the performance of bonding systems are fracture toughness tests that quantify the critical stress level responsible for initiating debonding, and fatigue testing in which the cyclic fatigue resistance after a predetermined number of loading cycles (usually 10⁵ cycles) is calculated.

Sites of failure for most bonded restorations occur along cervical margins where the bonding is primarily to dentin (Figure 13-6). Studies of bonded composites in class 2 restorations have shown that 95% of all secondary caries associated with the composite restoration is in the interproximal area. These margins are the most difficult to seal during placement of the restoration because they are typically bonded to dentin and cementum rather than enamel, and are hard to access with a light guide for adequate polymerization.

The three-step etch-and-rinse systems remain the “gold standard” for adhesive systems, both in laboratory and clinical evaluations.

Bonding to enamel occurs by micromechanical retention after acid etching is used to preferentially dissolve hydroxyapatite crystals in the enamel outer surface (Figure 13-7). Fluid adhesive constituents penetrate into the newly produced surface irregularities and become locked into place after polymerization of the adhesive.

Gel etchants (typically phosphoric acid) are dispensed from a syringe onto tooth surfaces to be etched. Etching times for enamel vary depending on the type and quality of enamel. Generally, a 15-second etch with 30% to 40% phosphoric acid is sufficient to reach the characteristic clinical endpoint of a frosty enamel appearance. Deciduous unground enamel generally contains some prismless enamel that has not yet worn away and requires longer etching times (20-30 seconds) to create a retentive pattern. Enamel may have been rendered more insoluble as a result of fluorosis. In those cases, extended etching times (15-30 seconds) are required to ensure that sufficient micromechanical bonding can occur. The only caution is that dentin should be protected from exposure to acid because fluorotic dentin is more susceptible to acid than regular dentin.

After the intended etching time, the acid gel is rinsed away and the tooth structure is dried to receive the bonding resin. If a hydrophilic primer or a two-step etch-and-rinse system is used, the surface can be left moist for the next stage of bonding. Then, primer can be flowed onto the surface to penetrate into the available surface irregularities. After curing, primer and adhesive produce resin macrotags by penetrating the space surrounding the enamel prisms. Microtags form where adhesive flows into the etched prisms involving individual hydroxyapatite crystals. Microtags are much more numerous and contribute to most of the micromechanical retention.

Strong self-etch adhesives produce a similar pattern on enamel as that obtained with phosphoric acid. Mild self-etch systems present lower bond strength to enamel compared to etch-and-rinse systems, probably because of a shallower etching pattern.

The infiltration of resin within the collagen scaffold is termed hybridization (Figure 13-8). The result of this diffusion process is called resin-interpenetration zone or resin-interdiffusion zone or simply hybrid layer. Concurrent with hybrid layer formation is the penetration of primer into the fluid-filled dentinal tubules. This generates quite large resin tags. However, these appear to be of little value to overall bonding. This material is generally undercured and behaves as soft flexible tags. If dentin is dehydrated before priming and bonding, these resin tags are more likely to be quite extensive.

Primers contain solvents to displace the water and carry the monomers into the microporosities in the collagen network. During application of the primer, most of the solvent evaporates quickly. Thus several layers usually must be applied to ensure a complete impregnation. The rule of thumb is that one should apply as many layers as are necessary to produce a persisting glistening appearance on dentin.

The thickness of a hybrid layer is not a critical requirement for success. Dentin bond strength is probably proportional to the interlocking between resin and collagen, as well as to the “quality” of the hybrid layer, not to its thickness. Effective etching of dentin does not require long times to produce acceptable dentin bond strengths. Usually, 15 seconds is employed. If etching time is too long and the etched zone is too deep, the decalcified dentin may not be fully impregnated. The etched but not impregnated space may reside as a mechanically weak zone and promote nanoleakage. Although this zone has been detected in laboratory experiments, the clinical results of this process have never been demonstrated to be a problem.

After priming the surface, an adhesive is applied and light cured. Surfaces of the cured bonding agents are initially air inhibited and do not immediately react. However, as composite is placed against the surface, the air is displaced and copolymerization occurs.

Self-etch systems have the great advantages of eliminating the risk of incomplete primer/adhesive penetration into the collagen scaffold and also eliminating the subjectivity when determining the amount of moisture on the dentin surface ideal for primer diffusion. With these systems, the smear layer is dissolved and incorporated into the hybrid layer. The bonding mechanism for strong self-etch adhesives is very similar to that of etch-and-rinse systems. Their bond strength, particularly for all-in-one systems, is relatively low, probably because of their high initial acidity and high water content. Mild self-etch systems demineralize dentin only superficially (a few microns) and leave residual hydroxyapatite attached to collagen fibers. Although the main bonding mechanism is the interlocking between collagen fibers and the polymerized resin, monomers such as 4-META (4-methacryloxyethyl-trimellitic anhydride) and 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate) may bond to this residual hydroxyapatite. Also, the presence of hydroxyapatite may help protect the collagen against degradation, which weakens the bonded interface. Mild self-etch systems may present relatively low bond strength values when applied to sclerotic dentin. Another drawback associated with all-in-one systems is that, due to their high water content, they behave as semipermeable membranes, which increases degradation by hydrolysis.

Low-strength, silica-based ceramics have been successfully bonded to resin cements by etching the restoration’s inner surface with a hydrofluoric acid solution, followed by the application of a silane primer (Figure 13-9). Different acid concentrations are commercially available, from 2.5% to 10%, in liquid or gel forms, and recommended etching times vary from 1 to 4 minutes. Hydrofluoric acid attacks the glass phase of ceramics, to the point where crystals are removed, leaving a microretentive honeycomb-like, high-energy surface. Silane application improves the wettability of the resin cement on the ceramic surface and establishes covalent bonds with both the ceramic surface (via siloxane bonds, -Si-O-Si-) and the resin cement (by carbon double bond polymerization). Hydrolysis of the silane molecule is necessary to convert the methoxy groups (-OCH₃) to silanol (-Si-OH). Silanes are presented in a nonhydrolyzed form (two bottles) or prehydrolyzed (one bottle). In general, prehydrolyzed silanes are less stable, with shorter shelf-life than nonhydrolyzed solutions.