Adhesive Dentistry: Just Bond It!

Scientific documentation of successful bonding strategies to tooth structures dates back to the late 1940s.^1,2 However, despite constant improvements and simplification attempts, the complexity of the dentin substrate continues to challenge researchers and manufacturers in the development of the ideal dental adhesive system.¹ Phosphoric acid-etching, used in the total-etch technique, has long been considered the "gold standard" for enamel bonding, as it forms microporosities on the crystalline enamel surface that allow for strong interlocking of resin tags. In contrast, dentin is a more organic substrate with varying compositions based on factors like location, depth, pulpal fluid flow, sclerotic status, and level of possible carious infection, requiring a substantially different approach.¹

Total-Etch or Self-Etch Dental Adhesives

The classification of dental adhesives by generations (first through eighth) has been popular for distinguishing new developments but has become confusing as it provides little information about the adhesive strategy employed. Also, a new generation of adhesive may imply improvement of clinical success, which is not necessarily so. It makes more sense to categorize adhesive systems by the applied adhesive strategy: etch-and-rinse (total-etch) and self-etch (etch-and-dry) systems.³ Etch-and-rinse (ER) systems comprise two or three steps and involve phosphoric acid pretreatment of the enamel as well as the dentin with subsequent infiltration of the demineralized dentinal collagen. Past debate about whether to leave the dentin surface wet or moist after etching was mainly based on laboratory studies, while recent clinical studies could not confirm a significant effect of dentin moisture levels on restoration success. The application technique, especially active rubbing of the adhesive into the etched dentin and adequate solvent drying to remove residual water from inside the resin-adhesive interface, seems to have a much greater and positive effect.¹

Self-etch (SE) systems are one- or two-step solutions of different pH levels that interact with the tooth structures via functional monomers. Unlike ER systems, they do not dissolve and remove the smear layer but incorporate it into the adhesive interface. The formation and quality of this hybrid layer is key for proper resin-dentin adhesion.⁴ SE adhesives, particularly two-step systems, have shown excellent performance to dentin through implementation of functional monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP), which provide some chemical adhesion to the hydroxyapatite crystals in the dentin. Without the use of phosphoric acid, however, the bond to enamel, especially when left uncut, is compromised. A simple measure is the selective enamel etching (SEE) technique where phosphoric acid is applied to the enamel only. Clinical trials indicate better marginal integrity and retention rate of composite restorations when SE adhesives are combined with SEE.

Because they provide better bond strengths to dentin, SE adhesives are preferred for direct restorations and cavities predominantly in dentin. Other factors such as film thickness and light-cure mode add to that determination. Several manufacturers now offer dual-cure activators for their SE systems to expand the range of clinical indications toward indirect restorations. Still, ER systems are widely considered the materials of choice for indirect restorations and cavities that are mostly confined to enamel.

Universal Adhesives

Universal adhesives represent a generation of one-bottle adhesives, indicated to bond not only to tooth structures but also to various dental materials, such as composites, silica-based ceramics, and high-strength ceramics (ie, zirconia). Based on the indication, these adhesives may be used with ER, SE, or SEE bonding strategies, making them highly versatile. They typically contain acidic functional monomers, such as MDP, while their actual concentration and combination with other components seem to greatly influence their susceptibility to hydrolytic degradation.

Recent clinical studies recommend phosphoric acid-etching (ER or SEE) when using universal adhesives. It was also shown that the typical recommended solvent evaporation time of 5 seconds may be too short and gentle air-drying of at least 15 seconds could improve bond strength durability.¹ Further, several studies have questioned the effectiveness of the incorporated silanes when bonding to glass-based ceramics as they may not be stable in the low-pH adhesive solutions. Application of a separate silane coupling agent may still be beneficial.¹

Self-Adhesive and Bioactive Materials

Most prominent among self-adhesive materials are resin cements for indirect restorations. Self-adhesive resin cements provide high bond strengths to tooth substrates and various dental materials, including composites, ceramics, and metal alloys. They combine the physical and optical advantages of composites with the simple use of conventional cements, requiring no additional bonding steps or priming agents. Self-adhesive resin cements also seem to have biological advantages over conventional cements with the ability to reduce postoperative sensitivity.⁵ Glass-ionomer cements and resin-modified glass-ionomer cements belong in this group as well. While bond strengths are lower, they have self-adhesive properties and are still considered the preferred materials for noncarious cervical lesions.

Some self-adhesive flowable composites were developed for direct restorations and fissure sealing, but clinical data has not been favorable so far.

So-called "bioactive" self-adhesive restorative materials and cements have also gained popularity. The term "bioactive," however, has various interpretations and is often confused with biocompatible, antibacterial, bacteriostatic, and other terms. These self-adhesive materials are typically a hybrid of glass-ionomer and calcium aluminate. Clinical performance and substantiation of the bioactive claims still lack validation through long-term clinical trials.

Any bonding surface contamination from saliva, blood, or sulcus or other fluids significantly diminishes resin bonds.⁶ Therefore, isolation of the operating field is essential for adhesive dentistry.

Composite Resins

Offering esthetic tooth-colored restorations, composite resin restorative materials have been steadily evolving. For anterior composite restorations, loss of retention is no longer a main reason for failure, provided dependable adhesive systems are used correctly. Instead, marginal deterioration and discoloration have become primary reasons for replacement. Posterior composite restorations are subject to secondary caries. The individual caries risk of a patient plays a significant role in long-term restoration survival.

Current trends suggest simplification of placement technique with low-shrinkage-stress bulk-fill composite resins. These materials have varying properties and are often applied as flowable base materials veneered with more viscous hybrid composite resins or inserted in thick increments and cured in one step to eliminate time-consuming layering techniques. Adhesion between two composite resin layers is achieved in the presence of an oxygen-inhibited layer of the unpolymerized resin. Successful bonding depends on a surface with a high number of unreacted vinyl groups (C=C) that can then be cross-polymerized to the resin in the bonding composite. Because already-polymerized composites contain fewer free radicals on their surfaces, the preferred method for indirect composite restorations implies air-particle abrasion with aluminum-oxide particles and application of a silane coupling agent.

Ceramics

All-ceramic restorations, popular for their esthetics and durability, are classified into silica-based ceramics (ie, feldspathic porcelain, leucite-reinforced, and lithium-silicate ceramics) and non-silica-based high-strength ceramics (ie, zirconia). Hydrofluoric acid (HF)-etching followed by application of a silane coupling agent is recommended for silica-based glassy matrix ceramics.⁷ HF selectively dissolves the glass or weak crystalline components of the ceramic and produces a porous, irregular surface of increased wettability. Application of a silane coupling agent on the etched surface increases the chemical adhesion between resin materials and the ceramic through siloxane bonds.

The high strength of zirconium dioxide (zirconia) allows for cementation of retentive restorations such as crowns and bridges with conventional cements. When retention is insufficient or resin-bonded treatment options such as resin-bonded fixed dental prostheses are applied, the preferred surface treatment method for strong and long-term durable resin bonds to zirconia is air-particle abrasion with aluminum oxide followed by application of a special ceramic primer containing an acidic adhesive monomer such as MDP. This is also known as the APC (air abrasion, primer, composite) zirconia bonding technique.⁸ Alternatively, application of a silica layer by means of air-particle abrasion or firing followed by a silane coupling agent has also shown promising results. Numerous laboratory and clinical trials have indicated excellent long-term success rates of resin-bonded zirconia restorations,⁹ dispelling the belief that "zirconia cannot be bonded."

Metal Alloys

Multifunctional adhesives for both noble and base metal alloys typically contain monomers with functional groups, such as sulfur, amino, and carboxyl, and have demonstrated high and durable bond strengths.

Adhesive dentistry facilitates minimally invasive, esthetic, and tooth-preserving treatments, offering excellent long-term success rates. Specific and somewhat technique-sensitive bonding protocols for various tooth structures and dental materials, however, are required for clinical success. Adhesive techniques, technologies, and clinical concepts are constantly being updated and enhanced, shaping the future of oral healthcare for the benefit of patients. Providing minimally invasive dentistry is not just another treatment option for clinicians, it is an ethical obligation.

Markus B. Blatz, DMD, PhD

Professor of Restorative Dentistry, Chairman, Department of Preventive and Restorative Sciences, Assistant Dean for Digital Innovation and ProfessionalDevelopment, University of Pennsylvania School of DentalMedicine, Philadelphia, Pennsylvania; Editor-in-Chief,Compendium of Continuing Education in Dentistry