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Bonding with the Latest Generation
Thomas E. Dudney, DMD
The most recent category of adhesives, often referred to as "universal" or "eighth generation," has continued to grow in popularity since its introduction in late 2011. The level of success enjoyed by these adhesives can be attributed to the considerable research and development behind them. While there are some differences between manufacturers’ products in this category, they all incorporate a phosphate ester as their primary adhesive functional monomer (the most common of which is 10-MDP) and are compatible with total-etch, self-etch, and selective-etch modes. Most products in this category come in one bottle (not to be confused with one-bottle, self-etch seventh generation adhesives), which simplifies the number of steps and application; however, two manufacturers have products that are marketed as "universal," but are actually two component systems, and another manufacturer’s product comes in a blister pack that must be mixed before use. Some universal adhesives require a dual-cure activator in order to be compatible with many dualcure and all self-cure resins and cements, whereas some do not, as long as the adhesive layer is light-cured first.
The great thing about universal adhesives is that, regardless of the requirements of the clinical situation, only one type or "generation" of adhesive is needed. For example, when bonding to all or mostly all enamel, a total-etch fourth or fifth generation system would be recommended for optimal bond strengths, but when bonding to mostly dentin, a two-bottle, sixth generation self-etch system would be preferred—requiring the dentist to have and maintain more products. Because universal adhesives can be used and are compatible with all etching modes, including selective-etch if desired, in any clinical situation, the dentist is able to simplify procedures, streamline product inventory, and because most universal adhesives have a two-year shelf life, reduce waste.
It is well documented that bonding to enamel utilizing phosphoric acid with an etch and rinse technique results in predictable and durable bonds, but long-term bonding to dentin is less predictable and more subject to hydrolysis and degradation over time. The challenge in bonding to dentin is that ideally you would want the adhesive to be hydrophilic when applying it to the tooth, but to be hydrophobic when seating or placing a restoration. In other words, hydrophilic enough to be able to properly wet and infiltrate the dentin (which is desirable initially), but hydrophobic enough to bond to restorative materials and be durable and resist hydrolysis over time. In order to address these issues, the manufacturers of universal adhesives incorporate a delicate balance of hydrophilic and hydrophobic monomers that work together synergistically, resulting in an adhesive that is hydrophilic when applied to dentin, but hydrophobic after light curing. That is why it is important to light-cure the adhesive layer before placing the restoration. A lot of the success with universal adhesives can be attributed to the incorporation of an acidic phosphate functional monomer such as 10 MDP. This monomer is able to bond chemically to calcium in hydroxyapatite by forming stable MDPCa salts that are deposited in self-assembled nano-layers. Additionally, MDP is the most hydrophobic functional monomer, possessing a long carbon chain that helps prevent water absorption and hydrolytic breakdown, thus contributing to long-term bond durability. Universal adhesives that incorporate the MDP monomer are also capable of bonding to substrates such as zirconia, precious and non-precious metals, composites, and glass ceramics—further enhancing their desirability. However, in order to insure the highest bond strengths, most manufacturers recommend a dedicated ceramic primer for zirconia and metals and silane for all glass ceramics.Dr. John Burgess, a well-respected lecturer, researcher, and product evaluator at the University of Alabama Birmingham School Of Dentistry, included universal adhesives in his article, "Materials You Cannot Work Without." And Dr. Gary Alex, also a well-respected lecturer and author, expresses his belief that universal adhesives may soon dominate the market place in his article, "Universal Adhesives: The Next Evolution in Adhesive Dentistry?" Therefore, if you have not yet tried a universal adhesive, it might be time to consider giving one a try. However, it is important to remember that products and their handling can vary, and it is critical to carefully read and follow the directions provided by the manufacturer. Factors such as number of coats applied, duration, agitation method of adhesive, evaporation of solvent, length of curing time, and the necessity of a dual-cure activator are all critical to success and may change from one manufacturers product to another.
About the Author
Thomas E. Dudney, DMD, maintains a private practice in Alabaster, Alabama.