The Restorative Interface: Practical Approaches for Better Outcomes

With the advancement of adhesion science, newer approaches allow the possibility to customize the adhesion process. By evaluating the presenting tooth structure, combined with material properties and their application, clinicians are able to determine what adhesion methodology works best. However, with any procedure, variables exist and can be introduced throughout the bonding sequence, creating potential restoration failure. Today, when performing adhesion dentistry it is imperative that clinicians incorporate specific guiding principles into their decision-making process to achieve proper restorative results that yield a strong and durable adhesion interface.

Adhesive dentistry as we know it today has evolved considerably since its introduction in the 1950s. Dr. Michael Buonocore discovered that acrylic-based materials could be better retained by treating enamel first with phosphoric acid.² A decade later, resin tags that filled the defects from etching were identified as “micromechanical retention.”³ The term “hybrid layer” was portrayed. Although it hadn’t yet been introduced as such, the hybrid layer was described as neither the enamel nor resin interlocking, but rather a hybridization of the two.⁴ By the 1980s, the concept of bonding to dentin had emerged and gained widespread acceptance with etch-and-rinse adhesives.⁵

The mechanisms by which adhesives work are multifold. Initially the tooth surface, whether enamel or dentin, is demineralized by an acid, making the surface more receptive for bonding. Next, hydrophilic monomers and solvents wet or “prime” the tooth structure, followed by the bonding agent monomers that penetrate the surfaces to form resin microtags. Finally, light-curing completes the polymerization of the resin microtags to achieve a mechanical bond, sealing the tooth anatomy to form the restorative interface.^6,7

Modern materials utilize this science to offer adhesive chemistries that affect enamel, dentin, and the smear layer in distinct application methods that are directly related to the tooth anatomy. Adhesion strategies have emerged that support either an etch-and-rinse or self-etch methodology.⁷

It is well documented that when preparing enamel, acid-etching facilitates a durable bond.² Clinically, if a large amount of enamel is present, the clinician may opt for an etch-and-rinse adhesive choice. Selective etching of the enamel is one such example whereby only the enamel is carefully etched and rinsed, and the bonding agent applied.

Conversely, dentin is a more delicate structure because of its organic composition, and it requires a more prudent treatment approach. Thus, self-etching adhesive systems may be indicated. These systems are simpler to use and do not require a separate etching step. Rather, they work by simultaneously demineralizing and penetrating the tooth surface, relative to their pH level. Self-etch materials have improved significantly and provide a more predictable bond strength to dentin than previously.⁷

Combining etch, primer, and adhesive into one bottle, universal adhesives are the latest generation of adhesives to be introduced. These systems afford versatility in etching protocol and also offer compatibility with various substrates to which they can bond besides dentin and enamel. What sets this class of adhesives apart from the field is the incorporation of monomer 10-MDP (methacryloyloxy-decyl-dihydrogen-phosphate), which provides an extremely hydrophobic feature, bond durability, and chemical bond to calcium.^8-10

Although the trend is to employ fewer clinical application steps and bottles, adhesives often remain confusing, technique-sensitive, and misunderstood by clinicians as to their best and most predictable use. Central to the adhesive system decision-making process is that success of the restorative interface is only as good as the clinician’s understanding and application of the material.

Commonplace during the adhesive process are extrinsic variables that need to be identified and addressed. These factors can greatly influence the success of the restorative interface, thereby impacting the strength and durability of the bond. If these variables are taken cavalierly, the restoration may be subjected to leakage, marginal staining, postoperative sensitivity, and/or secondary caries.

Isolation is a key factor during the adhesive agent application. Although many bonding agents claim to be hydrophilic, they are not immune to blood, saliva, airborne debris, and other contaminants.¹¹ Rubber dams continue to be the gold standard; however, other quadrant isolation and evacuation devices are becoming more popular.

Bond degradation of the adhesive interface over time has warranted much discussion, especially when considering the variations of available bonding systems. Many systems possess acidic and hydrophilic properties, which tend to attract water and create “water trees” that lead to overall decreased bond strengths. In addition, matrix metalloproteinases (MMPs) are slow-acting enzymes that break down the collagen matrices within the hybrid layer regardless of whether self-etch or etch-and-rinse systems are used.^12,13 To counter this degradation process, a 2% chlorhexidine conditioner has been shown to inhibit the activity of MMPs, thus increasing bond longevity.¹⁴

The use of desensitizers as rewetting agents has become prevalent to reduce postoperative sensitivity. These rewetting agents are applied when dentin is dried after acid-etching and rinsing. Studies have shown bonding to dentin is improved by maintaining a hydrated dentinal surface following the acid-etching process.¹⁵

When using phosphoric acid-etch, the author recommends a high-viscosity, low-flow gel. This will allow for greater control and more precise placement, especially if performing the selective-etch method. The simple process of timing the etch placement is also recommended: 15 to 30 seconds for freshly cut enamel, and 10 to 15 seconds for dentin. The gel must be copiously rinsed with a water stream, not a combination of syringed air and water, for 5 to 10 seconds, allowing the tooth structures to remain moist.¹⁶

As mentioned previously, the chemistries of dental adhesives vary. Regardless of its ethanol- or acetone-based formulation, the adhesive should be freshly dispensed due to evaporation, which alters its efficacy. Care should be given to shake the bottle before dispensing, and refrigeration of the adhesive may be advised by the manufacturer. The manner in which the adhesive is to be applied is critical. Most adhesives require a scrubbing application to properly agitate and infiltrate the surfaces. Again, following the manufacturers’ recommendations as to the proper amount of “scrubbing application time” is essential.

Many adhesives require air-thinning before light-curing. Not only does this factor bring the adhesive’s film thickness into consideration, but also the technique of syringing air. Thicker adhesives can pool in the corners of the preparations and may cause misdiagnosis of secondary caries, a void, or a gap. Thicker adhesives may also cause a clinical dilemma if not properly thinned, especially when utilized in an indirect adhesive treatment process.

Clinicians should use a stream of oil-free syringe air held approximately one-half inch from the height of the preparation and blow gentle air for 3 to 5 seconds in a zigzag movement and continue moving closer to the preparation, blowing air for another 1 to 2 seconds. The goal is to properly thin and evaporate the adhesive, not just displace it.¹⁷ Care must be taken to clear the syringe tip from any premature moisture before proceeding over the preparation to thin the adhesive. Newer disposable air-water syringe tips are a practical solution to deliver dry air with no water contamination or biofilm build-up.

Thorough light-curing is yet another important factor. Many curing lights lose their curing energy as the light extends to the depth of the preparation, are not measured for their output, and may perform in a limited wavelength. Light-curing tips need to be clear and free of debris, held at right angles, and as close to the preparation as possible. Once again, curing the adhesive must be properly timed and not slighted.¹⁸

Finally, dentists and their assistants must read the manufacturers’ instructions and be cognizant of expiration dates of the adhesives. Traditionally, adhesive systems and any other required components such as a primer or activator are recommended to be used as a family; therefore, introducing and mixing various products from other manufacturers is never indicated.¹⁹

Successful direct adhesive procedures are complex and rely on attention to detail, and each adhesive individually possesses unique chemical properties. Although the trend is the simplification of these systems and their application techniques, none are identical. Adhesion protocols are a personal preference, and clinicians must evaluate what is clinically required and combine that decision with the capability of the adhesive system in order to properly restore the natural dentition. Finally, by considering how specific extrinsic variables directly affect the protocol, clinicians can better ensure positive, long-term outcomes at the restorative interface.

Lori Trost, DMD
Private Practice
Columbia, Illinois