Bonding Protocols for Improved Long-Term Clinical Success

Both the etch-and-rinse (total-etch) and self-etch (etch-and-dry) adhesive concepts have advantages and disadvantages in different clinical situations. The phosphoric acid used with etch-and-rinse adhesives not only removes the layer of debris from tooth preparation (ie, the smear layer) but also opens the dentinal tubules and exposes the underlying collagen mesh. While exposed dentinal tubules are sealed by the adhesive resin, neither acetone nor ethanol—vehicles in etch-and-rinse systems—provide complete infiltration of the demineralized dentin. The exposed collagen fibrils may consequently suffer hydrolytic degradation by matrix metalloproteinase (MMPs). Meanwhile, self-etch adhesives, particularly two-step systems, have shown excellent bonding performance to dentin (and lower susceptibility to MMP degradation) through implementation of functional monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP), but without the use of phosphoric acid, the bond to enamel —especially to uncut enamel—may be compromised.¹ In light of these considerations, self-etch adhesives are recommended for cavities predominantly in dentin, while etch-and-rinse systems are preferred for indirect restorations and cavities that are mostly in enamel.¹

In the search for the “ideal” dental adhesive system, newer “universal” systems may improve ease of use, as they can be used in both etch-and-rinse and self-etch modes. Regardless of the bonding concept used, an adhesive should: minimize phosphoric acid pretreatment of dentin and only require selective etching of enamel; provide a mild self-etch with a universal adhesive monomer such as MDP; be solvent free; and have antibacterial properties.

Current trends in the use of composite resin for direct restorations suggest simplification of the placement technique with low-shrinkage-stress bulk-fill composite resins. These new materials have varying properties and are often applied as flowable base materials veneered with more viscous hybrid composite resins or inserted in 4-mm to 5-mm thick increments and cured in one step to eliminate time-consuming layering techniques. However, the recommended placement technique continues to be small increments to allow for flow of the composite material away from free space and toward a bonded substrate.² This technique ensures an optimal conversion rate upon photopolymerization and a restoration with superior physical properties.

Composites

Adhesion between two composite resin layers is achieved in the presence of an oxygen-inhibited layer of the unpolymerized resin. Successful bonding depends on establishing 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.³ Because already polymerized composites contain fewer free radicals on their surfaces, several methods have been suggested to improve the composite-to-composite adhesion. Surface roughening with airborne particle abrasion, etchants such as acidulated phosphate fluoride, hydrofluoric acid, or phosphoric acid with the use of intermediate adhesive resins (IARs) either in a silane and/or an adhesive system have been recommended. The preferred method is a combination of air abrasion, application of a silane coupling agent, and an IAR.⁴

Ceramics

The two major categories of all-ceramic materials are silica-based (ie, feldspathic, leucite-reinforced, and lithium disilicate) and non-silica–based (ie, zirconia or yttria stabilized zirconia, alumina) high-strength ceramics. The clinical success of either resin-bonded or repaired ceramic restorations depends on the quality and durability of the bond between the composite resin and ceramic. This bond typically depends on the surface topography of the substrate, surface energy, and chemical interaction with the resin.⁵

Silica-Based Ceramics—Hydrofluoric acid (HF) etching followed by application of a silane coupling agent is recommended for use with glassy matrix ceramics.^5,6 HF selectively dissolves the glass or weak crystalline components of the ceramic and produces a porous surface of increased wettability. Application of a silane coupling agent on the etched ceramic surface increases the chemical adhesion between the ceramic and resin materials by coupling the silica (silicon oxides) in glassy matrix ceramics to the organic matrix of resin materials by means of siloxane bonds. Because silica-based ceramics are brittle, blunt surface-roughening methods such as air-particle abrasion or grinding should be avoided.

High-Strength Ceramics—The high strength of alumina- (Al₂O₃) and zirconia-based (ZrO₂) ceramics allows for cementation with conventional cements. If adhesive bonding is selected for final insertion, however, some unique properties have to be considered. The bio-inert high-crystalline and low-glass structure makes high-strength ceramics corrosion- and acid-resistant, rendering adhesion protocols applied for silica-based ceramics ineffective.⁷ The preferred surface treatment method is air-particle abrasion with aluminum oxide, which removes loose contaminated layers, and the roughened surface provides some degree of mechanical interlocking with the adhesive material. Application of a special ceramic primer containing an acidic adhesive monomer such as MDP provides superior bond strengths to air-abraded high-strength ceramic surfaces.⁷ Alternatively, silica coating followed by silanization or chemical activation seems similarly successful.

The selective infiltration-etching technique by heat treatment has been recently proposed to improve zirconia bonding. The surface is coated with a glass-containing conditioning agent and heated above its glass-transition temperature. After cooling, the glass is dissolved in an acidic bath, creating a porous surface and achieving promising bond strengths.⁸

Prefabricated fiber-reinforced polymer (FRP) posts, a popular choice for retaining coronal restorations in endodontically treated teeth, are usually luted with resin cements to increase retention and mechanical performance of the restored teeth while reducing the risk of root fracture. FRP posts are made of carbon or silica fibers surrounded by a matrix of polymer resin, usually epoxy resin. Because fiber posts are passively retained in the root canal, the effectiveness of the adhesive cement and luting procedure plays an important role. Ideally, the intracoronal dentin is treated with etch-and-rinse adhesives and ethylenediaminetetraacetic acid (EDTA). A silane coupling agent is typically applied to the post surface to enhance adhesion.

Recently developed resin-based self-adhesive cements eliminate pretreatment steps and have become popular for cementation of fiber posts. Self-adhesive resin cements contain multifunctional hydrophilic monomers with phosphoric acid groups, which can react with hydroxyapatite and also infiltrate and modify the smear layer. They can offer bond strengths comparable to etch-and-rinse systems.

The development of techniques for adhesion of composite resins to metallic substructures has greatly expanded restorative treatment options. Current development of adhesion to noble dental alloys has focused on the use of functional monomers, especially those containing sulfur. 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.⁹ Silica coating through air-particle abrasion followed by application of a silane coupling agent has demonstrated excellent bond strengths of metal alloy surfaces.

The most common clinical problems with bonded indirect posterior restorations include hard-tissue conservation, impression-taking, and adhesive cementation, as well as provisional restorations. An original treatment protocol¹⁰ to overcome these problems includes four main concepts: First, dual bonding pertains to the substrate treatment, which seals the dentin with a dentin-bonding agent to prevent further tissue dehydration and contamination and protect the tooth against sensitivity while improving bond strength and stability of the adhesive interface. The second concept, cavity design optimization (CDO), limits removal of sound tooth structure during preparation by applying a flowable composite liner to fill all undercuts and create an ideal cavity geometry. The third concept, cervical margin relocation (CMR), is used for deep proximal preparations (intrasulcular). The final concept, controlled adhesive cementation (CAC), has advantages in complex cavity designs. Combined with the CMR technique, visual margin examination and proper cement removal are simplified. A highly filled fine/microhybrid composite is recommended for cementation, and its viscosity is reduced during restoration placement with a special ultrasonic or sonic cementation tip. The reduced restoration thickness (CDO concept) supports proper light transmission.

Offering patients minimally invasive treatment, adhesive techniques and technologies are constantly being updated and improved. As such, they continue to shape the future of dentistry.

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry
Chairman of the Department of Preventive and Restorative Sciences
University of Pennsylvania School of Dental Medicine
Philadelphia, Pennsylvania