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Esthetic Bonding: How to Avoid Catastrophe Before It Starts
By Allison M. DiMatteo, BA, MPS
With all of the new bonding materials on the market, dentists are quick to pull the trigger and sometimes end up using them without gaining the proper training first. The patient may be none the wiser until they run into a future problem with a broken restoration, tooth sensitivity, or a periodontal issue and the need to seek additional treatment. With proper training, practice, the correct materials and techniques, and following instructions for use, clinicians can position themselves for success from the outset.
Adhesive dentistry and resin bonding have presented the profession and patients with a whole new range of treatment options beyond direct restorations. The applications and indications of many indirect restorations are now possible as a result of the newer materials available today.
“We’re talking about silica-based ceramics, zirconia, hybrid ceramics, and new indirect composites that we can apply much more broadly and less invasively by bonding them to the tooth structure,” explains Markus B. Blatz, DMD, PhD, professor of restorative dentistry and chairman of the Department of Preventive and Restorative Sciences at the University of Pennsylvania School of Dental Medicine. “The variety of indications for these materials would not be possible without resin bonding, which really has changed how we do dentistry.”
Placing an indirect restoration requires bonding the tooth structure (ie, enamel and dentin) to a restorative substrate (ie, silica-based ceramic, zirconia ceramic, resin ceramic, gold alloy, base metal alloy), explains John M. Powers, PhD, publisher of The Dental Advisor and clinical professor of oral biomaterials at the University of Texas School of Dentistry at Houston. Bonding to tooth structure could be accomplished by a total-etch (ie, phosphoric acid) technique or a self-etch technique. Universal bonding agents are compatible with both techniques, he says.
“The bond to etched enamel is the gold standard in terms of multi-decade durability, resistance to microleakage, and the highly important enamel property of being a high elastic modulus (ie, high stiffness) support for bonded ceramics. This is a significant factor contributing to the unprecedented clinical success of bonded ceramics,” says John A. Sorensen, DMD, PhD, FACP, professor of restorative dentistry, director of the Biomimetics Biomaterials Biophotonics Biomechanics & Technology Laboratory, and director of the Graduate Prosthodontics Program at the University of Washington School of Dentistry. “Very different from bonding to enamel is the challenging task of bonding to a dentin tooth substrate. Its organic, living, moist nature creates an adversarial environment for achieving a reliable, long-term bond for cementation.”
Ed McLaren, DDS, MDC, director of the UCLA Center for Esthetic Dentistry and professor in the Department of Biomaterials and Advanced Prosthodontics, notes that the total-etch technique using phosphoric acid with fourth-generation bonding agents have shown excellent long-term success and have the most “evidence,” and when used for etching should have an etch pH of approximately 0.7 or 0.8, depending on the dentist’s comfort zone and whether or not they’ll etch the dentin. There are several agents that have been shown to reduce the main concern from etching—ie, sensitivity—the best being a 30-second application of material containing 5% glutaraldehyde and 35% HEMA. These materials have also shown inhibitory effects of the class of enzymes called MMPs, which are thought to contribute to long-term bond breakdown by collagen dissolution and subsequent hydrolosis. If dentists choose not to etch the dentin, then short-term data suggests that universal adhesives may work, he says, but there is almost universal agreement that enamel still should be etched.
“I do think we should be sealing the dentin, and one of the big benefits is increased adhesion, especially in a non-retentive, high-stress situation,” McLaren emphasizes. “One of the best benefits of dentin adhesion is stress distribution over minimized crack formations.”
For decades, dentistry has considered “moist dentin” to be the ideal bonding substrate, Sorensen elaborates. The rationale, as described by David Pashley, was that if the phosphoric acid-conditioned dentin was over-dried, the collagen fibers would collapse, reducing the capacity to impregnate the dentin surface with the adhesive.
“The question that clinicians always ask is, ‘How wet is moist dentin?’” Sorensen says, adding that the elastic modulus of dentin is significantly lower than enamel, producing poorer support of the ceramic and increased tensile forces in occlusion (See “Ceramic Thickness Requirements When Supported by Dentin,” sidebar). “Achieving this ideal tooth condition has been elusive and made many adhesive systems overly technique-sensitive.”
According to Robert C. Margeas, DDS, a private practitioner in Des Moines, Iowa, and editor-in-chief of Inside Dentistry, bonding requires very good isolation in order to be successful. When preparation margins are subgingival, it can be difficult to bond the restoration due to isolation requirements (eg, packing cord, placing a rubber dam) to ensure a dry field.
“Poor isolation leads to a contaminated surface, which automatically by default gives you a poor bonding surface and a poor bonding outcome, so you’ve got to be able to control the moisture, bleeding, saliva, and soft tissue in the oral environment so you’re working on a dependable, predictable surface,” explains Dennis J. Fasbinder, DDS, a clinical professor at the University of Michigan School of Dentistry. “The other consideration is the type of surface you’re bonding to: enamel or dentin. We would prefer to bond to enamel over dentin, although we are getting better and better at dentin bonding (See “Shear Bond Strength of Adhesives to Dentin with Varying Moisture Conditions,” sidebar).”
The Connection Between Restorative Materials and Bonding Options
Understanding the restorative material being placed is equally important, Fasbinder says.
“When you’re talking about indirect restorations, you’re talking about zirconia, lithium disilicate, or regular porcelain,” Margeas explains. “Lithium disilicate and regular porcelain really need to be bonded for good retention, whereas zirconia can be cemented, and there’s a difference between cementing versus bonding.”
Therefore, to achieve successful bonding, selection of the restorative material and cement, as well as preparation design, are important factors, Powers asserts. For example, a zirconia restoration can be either cemented or bonded to a retentive preparation. Cementation of a zirconia crown with a glass-ionomer cement might be desirable for a patient with a high caries index.
“However, bonding a zirconia crown with a strong esthetic resin cement might be the best choice for a preparation with poor retention,” Powers notes.
According to Peter Pizzi, CDT, MDT, owner of Pizzi Dental Studio, Inc, the best form of bonding is to enamel, and when it comes to material choices, not every material demonstrates the same bonding abilities. Lithium disilicate, for example, can be adhesively bonded or cohesively cemented.
“I think there are questions about how well harder, newer structures actually work in the bonded world,” Pizzi believes. “On the clinical side, it’s really about having enamel. On the laboratory side, it’s more about understanding which material you’re working with that can or cannot be bonded.”
Zirconia requires an acidic phosphate monomer (eg, phosphoric-acid methacrylate) primer, and various other material substrates also require appropriate primers, explains Powers. Silica-based ceramics (eg, lithium disilicate) are typically etched with hydrofluoric gel and then treated with silane primer, while gold and base metal alloys require a sulfide monomer (eg, sulfide methacrylate).