Universal Adhesives: New Realm of Possibility
Robert Margeas, DDS
The universal adhesive, the latest generation of adhesive dental systems, provides dentists with the ability to bond numerous materials with predictability to both dentin and enamel substrates with the successful placement of direct and indirect composites among many other materials. They also provide flexibility in the etch protocol. Vital and dynamic, dentin has variations in its structural and chemical composition, while enamel is uniform and more mineralized. Mature enamel is approximately 96% hydroxyapatite by weight with the rest being water and organic material. Generally, dentin is approximately 70% hydroxyapatite by weight, 18% organic, and 12% water.1,2 However, dentin depth, the age of the teeth, and any history of tooth trauma can cause these percentages to vary. This makes consistent and reliable long-term adhesion a challenge for the practitioner. For decades, manufacturers have worked to improve adhesion and bonding products.
Early adhesive systems in the 1970s and early 1980s were hydrophobic and could not penetrate the smear layer, which is the residue on the dentin surface after rotary instrumentation with burs is used, and would bond with it. The smear layer is a thin substance made up of bacteria, inorganic dentin and enamel debris, and degraded collagen.1,3 Total-etch adhesive systems dissolved this smear layer with the use of phosphoric acid, which was washed away during the rinsing step. Self-etching systems used acidic primers to modify or disrupt the smear layer and enable direct adhesive interaction with the dentin substrate.
Acids and acidic primers used in total or self-etching bonding systems also create a thin area of demineralization, exposing collagen fibrils that are either subsequently (total-etch) or concurrently (self-etch) penetrated with primers and resins. Manufacturers sought to penetrate this zone with primers and resins that could be polymerized by light or chemical curing. This resin-infiltrated layer, as described by Nakabayashi and colleagues,4 is called the hybrid layer. Through ionic bonding, monomers chemically interact. The hybrid layer and resin create a resinous surface layer, which serves as a base for chemically compatible restorative materials and resin-based cements.
Regardless of the classification, all modern adhesive systems contain a type of acidic conditioner, dentin primer, and bonding resin. The most recent of these adhesives is the universal. Relatively new to the market, the one-bottle universal adhesives have garnered significant attention. Building on earlier versions eventually reduced the number of etching steps to one; manufacturers have combined the etch, primer, and adhesive in the universal adhesive. It can be used in three modes: total-etch mode, selective-etch mode, or no etch. A number of universal products are on the market: Brush&Bond® (Parkell, parkell.com), Scotchbond™ Universal (3M Oral Care, 3m.com), Futurabond® U (VOCO America, voco.com), CLEARFIL™ Universal Bond (Kuraray, kuraraydental.com), ALL-BOND UNIVERSAL (Bisco, bisco.com), Peak® Universal (Ultradent, ultradent.com), Adhese® Universal (Ivoclar Vivadent, ivoclarvivadent.us) Optibond™ XTR (Kerr, kerrdental.com), and Prime&Bond Elect® (Denstply Sirona, primeandbondelect.com). Manufacturers allow for user discretion as to etching: total, partial, or no etch at all. With the exception of Prime & Bond Elect, all are ethanol-based materials.
The Monomer Within
The holy grail has been the monomer 10-methacryloyloxy-decyl-dihydrogen-phosphate (10-MDP), which has provided the bond strength that dentistry has long sought. One of the first applications of this monomer was Kuraray’s Panavia™ adhesive resin cement. Kuraray held the patent on 10-MDP until 2011, after which time other manufacturers developed and launched their own 10-MDP-based adhesives, known now as the universal adhesives.
One of the reasons that makes it ideal for use in a universal adhesive is that it is an amphiphilic functional monomer with a hydrophobic methacrylate group on one end (bonding to methacrylate-based restoratives and cements) and a hydrophilic phosphate group on the other (bonding to tooth tissues, metals, and zirconia). The bifunctional monomer has a 10-carbon chain. This long carbon chain with a partition coefficient of 4.1 makes it the most hydrophobic of all functional monomers.5 This is key for durability, as water sorption and hydrolytic breakdown of the adhesive juncture has been observed to be a main cause of bond failure.6,7 The stability of hydrophobic 10-MDP provides a long shelf life.
Also, 10-MDP has been shown to bond chemically with the calcium in hydroxyapatite.8,9 During this reaction, stable MDP-calcium salts form and are deposited in self-assembled nano-layers,10,11 which is why this monomer is effective at creating bond durability.10 Nanolayering is the molecular lamination from a chemical interaction of functional groups that exist on the surface of a substrate. The phosphate groups attach to the calcium, and the methacrylate groups attach to themselves.
Not all products are the same, as bond strengths tend to vary, depending on the concentration of 10-MDP.12 Also, pH levels are not consistent between products. A product with a strong acid may have a pH level of less than 1, but a mild acid would have a pH level higher than 2.5.13 The more acidic the adhesive, the less compatible it will be.14 The acid deactivates the aromatic tertiary amines that are the base, which is critical to chemical curing the cement. This may become more problematic when a clinician mixes adhesives and cements from different manufacturers.
Manufacturers claim that universal adhesives can be used for self-cure, light-cure, and dual cure resin-based cements. A truly universal adhesive will contain specific and synergistic monomers, enabling them to react with various substrates and copolymerize with chemically compatible resin-based restoratives and cements. For the majority of my restorations that are zirconia based, I use a resin-modified glass ionomer cement because of its ease of use and durability.
Treating the interior of a crown can vary because the proper protocol is required for predictability. For lithium discilicate, etching the intaglio surface should follow the directions dependent on the percentage of hydrofluoric acid. The etch time may range from 20 to 60 seconds depending on the strength of the hydrofluoric acid.
This makes it crucial for the clinician to understand what kind of material is being used. For zirconia, sandblasting with less than 50 microns of aluminum oxide such as Co-Jet™ (3M Oral Care), followed with a zirconia primer. Products containing 10-MDP can be used inside zirconia crowns. Lithium-discilicate crowns must be treated differently because the material is an etchable, whereas zirconia is not. Ivoclean (Ivoclar Vivadent) might be used to clean the restoration. Then silane should be used on the inside of the crown to increase bond strength.
Tips for Success
The long-term successful outcomes of many current restorative procedures are largely predicated on the dentist’s technique in bonding various materials to tooth tissues. A universal adhesive can be used effectively, particularly if proper technique is used.
If the practitioner is working with enamel, the practitioner should etch enamel and the self-etch mode should not be used. If a scenario involves working on dentin, then the self-etch mode is indicated to help prevent sensitivity. However, much of this depends on good technique. The universal adhesives should be used in a scrubbing or rubbing motion with an applicator—a microbrush—for 20 seconds, with some manufacturers recommending 30 seconds of scrubbing. The days of 4-second application time are long gone with universals.
Complete solvent removal is extremely important for success of the restoration. Incomplete resin polymerization, nanoleakage, and decreased bond strength may result if solvent removal is insufficient.15,16 To ensure complete removal, keep the air syringe approximately 6 inches from the tooth, lightly air thinning as the clinician gets closer to the tooth. Air-thin until no movement of the adhesive can be seen. This is the clinician’s visual indicator that all the solvent, which is the carrier, has been removed. Then, light-cure for 10 seconds. For dual-cure or self-cure composites, some manufacturers require the use of a separate catalyst.
Another key to success is carefully following the respective manufacturer’s instructions. Not all universal adhesives operate the same, with specific placement and handling directions for optimal results. Also, check and adhere to the expiration date on the package because materials can deteriorate, even with refrigeration.
Proper management and technique are essential for a successful outcome for a restoration. Understanding modern materials, the protocols, and the substrates is crucial. More research is needed; however, dentistry has entered a new realm of possibilities with universal adhesives.
Robert Margeas, DDS
Department of Operative Dentistry
University of Iowa
Editor-in-Chief, Inside Dentistry
Des Moines, Iowa
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