Universal Adhesives: The Evolution of Adhesive Solutions Continues
To most dentists, the ideal adhesive would offer one-layer, one-component (ie, one bottle) convenience, would be usable as a total-etch (TE) and a self-etch (SE) adhesive, and could be used in indirect procedures without the need for an additional activator. Also, the adhesive would bond to all indirect substrates, including zirconia- and silica-containing ceramics.
In an effort to satisfy all of these conditions, new “universal” adhesives have been introduced. Designed to be used in total-etch and self-etch applications, the hope is that these products can replace all previous generations of adhesives. In general, the new universal adhesives are more hydrophobic than previous self-etch products to address permeability issues. The bond strengths to dentin in both TE and SE modes are very good. Bond strengths to enamel are reasonable, but to ensure long-term durability the selective-etch technique (etching of enamel only) is recommended for universal adhesives. Methacryloyloxydecyl dihydrogen phosphate (MDP) can chemically bond to Ca++ ions and form stable MDP-Ca salts, according to the “adhesion-decalcification” concept; these salts deposit at the adhesive interface, forming “self-assembled nanolayers,” which may be responsible for the good long-term performance of MDP-containing adhesives on dentin, both in vivo and in vitro (Yoshida Y, et al. J Dent Res. 2012;91:376-381). Thus, the selective-etch technique is especially recommended for MDP-containing universal adhesives.
Adhesives with a pH of less than 3 require a self-cure (SC) or dual-cure (DC) activator to be used when cementing indirect restorations adhesively with a DC luting resin based on a peroxide-amine redox system, unless an amine-free luting resin is used. However, use of an adhesive with a pH above 3, such as BISCO’s All-Bond Universal®, with a pH of 3.2, allows the clinician to choose any DC luting resin, as it will be compatible with most common SC/DC luting resins.
Some universal adhesives are formulated with a silane monomer (porcelain primer), with the strategy being that the silane will be stable in the acidic adhesive (which contains an acidic monomer, such as MDP) and will bond chemically to silica-containing ceramics. In theory, a universal adhesive with silane would be very convenient for users. However, water contact angle (CA) and shear bond strength (SBS) studies have demonstrated that a silane primer does not function when it is mixed with other resin monomers (Chen L, et al. J Prosth Dent. 2013;110:402-407). The reason silane is not effective when combined into a universal adhesive is most likely due to the MDP and bis-GMA resins in the adhesive. A silane monomer in an acidic condition (MDP + water) may be unstable due to the self-condensation reaction of the silanol groups in silane. Additionally, the bis-GMA resin may interfere with the silane-coupling condensation reaction between the silane and the hydroxyl (-OH) group of a silica-containing ceramic surface (Chen L, et al. J Prosth Dent. 2013;110:402-407; Lung CY, Matinlinna JP. Dent Mater. 2012;28:467-477). Consequently, a pure silane primer should be used for a durable chemical bond to porcelain.
The presence of an MDP (or other organophosphate) monomer makes a universal adhesive bondable to zirconia (Zr) or other metallic substrates. Chen et al proved that MDP chemically bonds to Zr (Chen L, et al. Am J Dent. 2012;25:103-108). However, MDP-containing universal adhesives must be light-cured to be used as a Zr primer due to the presence of other resins, which may not cure when placed under a self-cured resin cement. Light-curing the universal adhesive on the restoration, however, may cause film thickness issues. To avoid light-curing the adhesive on Zr/metal, a pure Zr primer, such as BISCO’s Z-Prime™ Plus, should be utilized.
All-Bond Universal is a single-bottle adhesive that does not require a SC/DC activator; like all universal adhesives, it requires a separate silane primer for chemically bonding to silica-containing ceramics. All universal adhesives, with or without silane incorporated into the adhesive formula, are not as effective as pure silane primers. Therefore, the author recommends using a separate silane primer on silica-containing ceramics (ie, porcelain and lithium disilicate) when using any universal adhesive.
While not quite the “ideal” adhesive, universal adhesives are superior to any previous generation and are here to stay.
Disclaimer: This article was provided by Dr. Suh.
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Byoung I. Suh, MS, PhD is President, BISCO, Inc., Schaumburg, Illinois. Dr. Suh is a Member of the American Chemical Society, the American & International Academies of Dental Research, and the Korean Scientists & Engineers Association, and also serves on the UCLA School of Dentistry Board of Counselors. A well-known lecturer and teacher, he has published more than 30 articles and 80 abstracts, and is author of “Principles of Adhesion Dentistry.”