The Chemistry of Universal Adhesives
Understanding the differences in functional monomers
Gregg A. Helvey, DDS, MAGD, CDT
Adhesive dentistry began in the 1960s with the introduction of acid etching. Initially, acid etching was confined to only the enamel with the goal of improving the bond strength between it and the resin being used. This etching technique continued through the 1970s, but with better chemistry, which resulted in improved bond strength.
The concept of etching both the enamel and dentin was introduced in the 1980s. This technique was referred to as total-etch (ie, etch-and-rinse.) The technique had three separate steps, which consisted of etching and rinsing, applying a primer, and applying the resin adhesive. In the 1990s, the total-etch concept was still being used; however, many manufacturers had combined the primer with the adhesive, which eliminated a step.
Then in the 2000s, manufacturers started introducing resin adhesives that combined the etchant, the primer, and the adhesive into one bottle. The application of resin adhesive could now be accomplished in one step. These one-step bonding agents received mixed reviews, many citing the lack of deep enamel etching patterns. In time, modifications to the chemistry of these one-step bonding agents produced what became known as “universal” adhesives. They were marketed as bonding agents that could be used with or without acid etching. If etching was used, it was left up to the clinician to decide whether to employ a total-etch or selective-etch technique.
More recently, universal adhesive bonding agents have become very popular, which is likely due to their heavy promotion by manufacturers. Many of these manufacturers claim that their universal adhesive can be used effectively in all three modes (ie, non-etch, total-etch, selective-etch) and achieve virtually the same results. Although they are all classified as “universal,” there are functional differences in the chemistry of these materials that make them very different from one another.
The backbone of any bonding agent is its functional monomer, but not all bonding agents share the same chemistry. Some rely primarily on chemical retention, whereas others achieve adhesion through micro-mechanical retention. When selecting a material to adhesively bond a restoration to tooth substrate, the chemical differences in the functional monomers of the various available products should be considered.
A 68-year-old female patient presented to the office with a failing crown restoration on her maxillary right lateral incisor. The existing 15-year-old crown was made from a laboratory-processed resin composite material (Figure 1). Because the original material was no longer available, options were presented that included a chairside-fabricated CAD/CAM ceramic crown. During a discussion of the risks and benefits, the patient was informed that the restoration would be made of a different material, ceramic (IPS e.max CAD, Ivoclar Vivadent),which would have different optical qualities than lab-processed resin composite. The patient was assured that every effort would be made to create as close a match as possible.
Before the existing restoration was removed, a custom shade guide (made from leftover material from previously milled and crystalized IPS e.max CAD blocks) was used to select the shade (Figure 2). The existing restoration was removed, and the tooth was prepared and scanned. After designing and milling, the pre-sintered crown was tried in and finished in one step with a Touchdown Disc (Dental Ventures of America), which replaces the use of 3-step diamond impregnated wheels (Figure 3). Unlike some other finishing wheels, this finishing/polishing disk does not generate heat and make the restoration uncomfortable to hold. To delineate the areas where the ceramic stains would be applied, a wax margin pencil was used to draw lines that prevent the different stains from bleeding into each other (Figure 4). The specific stains were then applied to the facial surface of the crown (Figure 5), and it was crystallized. The wax lines burn off during the crystallization of the ceramic.
Once the crystallization cycle was complete, the crown was allowed to bench cool. The intaglio surface was lightly air abraded to remove any debris and then etched with 9.5% hydrofluoric acid for 20 seconds. After thorough rinsing and drying, a ceramic primer was applied (EA-Z-Y Primer™, Parkell). The tooth was then etched for 15 seconds (Figure 6), rinsed, and dried. One drop of bonding adhesive (Brush&Bond® Universal, Parkell) was placed in a mixing well then stirred for 2 seconds with a Brush&Bond® Standard Activator Brush. These brushes contain chemical activators that are necessary for the dual-cure aspect of the polymerization process. The adhesive was applied to the entire tooth and kept moist for 20 seconds. Unlike other bonding adhesives for which active application (ie, agitation of the microbrush on the tooth surface) is advocated, passive application is recommended with this material (Figure 7). This mode of application can help prevent postoperative sensitivity.
After the 20 seconds had elapsed, an air-water syringe was used to evaporate the solvents and remove the dissolved smear layer by blowing it with a gentle air stream into a high-volume evacuator tip for 10 seconds (Figure 8). The adhesive resin was then light-cured. Following this, a dual-cure resin cement (SEcure® Resin Cement, Parkell) was mixed and placed into the restoration, and it was seated. After a 2- to 3-second exposure to the curing light on both the lingual and labial surfaces, the excess cement was removed. Floss was gently passed through the contact area towards the gingiva and removed by letting go of the lingual end of the floss and pulling it out in the direction of the labial. Before any occlusal adjustment was made, the restoration was once again light-cured for 30 seconds per side. Despite the slight differences in optical properties of the adjacent restorations, the final result was acceptable to the patient. In cases with juxtaposed dissimilar restorations, the goal of “blending in” the two materials can achieve restorative success (Figure 9 and Figure 10).
In order to maximize the particular strengths, it is important for dentists to understand the unique chemistry of any bonding adhesive they employ. There is not a “universal” way to use universal adhesives—the instructions are specific to each manufacturer's product, and they should be carefully read before the material is used.