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).
“However, the length of time you etch is material-specific, and also depends on the concentration of the hydrofluoric acid,” Fasbinder adds. “For example, lithium disilicate (ie, IPS e.max) is only etched with a 4.9% hydrofluoric acid solution for 20 seconds, but with the same solution leucite-reinforced glass ceramic (ie, IPS Empress) is etched for 1 minute. There are other solutions with a concentration up to 9%, which would then change the length of etching time, so it’s important to understand the material differences because if you overetch, you’ll have a friable surface that won’t bond well.”
“The laboratory protocol is normally to sandblast the internal aspect to create a rougher surface, followed by an etch with hydrofluoric acid,” Pizzi explains. “Because lithium disilicate and zirconia are much denser and harder surfaces, some studies are showing that the protocol of etching with hydrofluoric acid is doing something, and some studies are showing it’s not accomplishing anything.”
Recently, Blatz and his colleagues applied decades of research on how to achieve high and long-term, durable bond strengths to high-strength ceramics to the problems associated with bonding zirconia restorations.1 The “APC” zirconia bonding concept synthesizes this research into a simplified protocol necessary for proper cementation. The three-step protocol requires: (A) air-particle abrasion; (P) zirconia primer; and (C) adhesive composite resin, Blatz explains.
Cohesive or Adhesive: What’s Involved?
Cementing a restoration is less technique sensitive because the protocol does not require the use of etchants or a separate primer, Margeas explains. For cementation of indirect restorations, a variety of different cements are available, he says. Likewise, various adhesives are also available, ranging from all-in-one adhesives, which are universal cements that do not require separate etchants, primers, or bonding agents. When universal cements are used, Margeas emphasizes the need to ensure good retention and resistance form in the preparation, because sometimes the cement may not be as strong as an etch-and-rinse type of adhesive cementation system.
“Self-adhesive resin cements are unique in that they will bond to both tooth structure and most restorative substrates without an additional primer,” Powers says. “Adhesive resin cements require a primer for tooth structure but will bond to most substrates without a primer. However, use of a primer with self-adhesive and adhesive resin cements will increase the bond strength. Esthetic resin cements require both tooth structure and restorative substrate primers.”
The choice between adhesive or cohesive is really case specific. If the preparation provides good retention, is long, and there is good resistance form, then a universal adhesive may be the go-to choice, Margeas suggests. In cases involving a short preparation, the choice may be a resin cement requiring either selectively etched enamel and a separate primer, or a separate resin cement and adhesive.
Additionally, if the adhesive cannot be cured prior to cementation, then a chemical-cure component for cementation in light-curing obscured regions is necessary (ie, dual-cured material), notes Sorensen. Another important consideration is the film thickness of the dentin adhesive and whether pre-polymerization is possible prior to cementation.
“We can get into the habit of always using similar materials over and over again, on both sides—the clinical and the laboratory side,” Pizzi admits. “However, there are times when patients have issues with their enamel, for example, that can affect bondability, and we would rather cement, and there are other times when cementing doesn’t seem to be an option. The goal is to choose what material will work best in that environment for that patient.”
Implications of Improper Bonding
Among the potential issues that could result from improper bonding are compromised periodontal health and the need for retreatment as a result of failed restorations. How frequently does that occur?
“Discussing bonding failure rates is a catch-22 because you’re not going into the process to do it poorly, so if a failure occurs, the evaluation is retrospective to determine where it failed and whether it was a material failure, a preparation failure, or a bonding failure,” Fasbinder explains. “So, when the question is whether the restoration failure rate partially results due to improper bonding, I can almost 100% guarantee that it does. But again, nobody tries to do it poorly to see what the outcome is.”
Bond failures can result in increased microleakage and recurrent decay, as well as loss of retention, requiring re-cementation, Powers cautions. Bond failure resulting in additional decay might require a more extensive tooth preparation or even loss of the tooth, requiring an implant or bridge.
Other consequences of improper or incomplete bonding include interfacial staining, shade changings, caries, immediate sensitivity, chipping, and fractures.
“A characteristic of bonding—especially with posterior ceramic restorations—that isn’t always discussed is the importance of what the bond does to the ceramic besides hold it in place,” Fasbinder elaborates. “Another advantage of bonding is actually enhancing the strength properties of the ceramic—such as glass ceramics—by sealing the internal surface.”
McLaren adds that what is not commonly known is that a benefit of adhesive bonding—combined with the restorative material—is stress distribution. The higher the bond strength, the better the stress distribution into the tooth and the restoration. Further, the more even the stress distribution, the less likely a crack is to form. Improper bonding techniques jeopardizes this otherwise beneficial property.
But improper bonding also correlates to periodontal issues, McLaren says. If there are leakage and contour problems with restorations due to bonding or poor finishing, there will be visible gingival tissue inflammation.
“The better you have sealed your teeth—which is a bonding procedure, and the better you have chosen a material that is polishable, the healthier your soft tissue will be,” McLaren observes. “This is another reason for adhesion today, and for using materials that have polishability and smoothness that are similar to natural tooth structure.”
Realistically Managing Patient Expectations
The other “bonding”—direct restorations—aren’t without their challenges. It’s often patient expectations of esthetic results that weigh heavily on whether directly bonded composite restorations are successful. Fortunately, companies have introduced many different shades and formulations of direct composites that demonstrate translucencies, opacities, and dentin and enamel characteristics that better mimic natural tooth structure, Margeas observes.
“For me, I’m determining the material by each particular case,” he says. “There are times when I’m able to only use one shade or a body shade. Otherwise, I layer my composites using multiple shades of translucency, opacity, and fluorescence to create a restoration that’s much more esthetic.”
But managing patient expectations extends beyond satisfying immediate esthetic demands to fulfilling long-term functional requirements. A younger patient with spaces between their teeth who wants porcelain veneers needs to be educated about the fact that no restoration is permanent, including porcelain veneers; they may need to have them redone two or three times in their lifetime.
I would much rather do conservative restorations in composite resin—where it’s much more cost-effective and conservative, yet creates a beautiful result,” Margeas says.
Conclusion
In essence, bonding enables dentists to be less invasive and retain more of their teeth—and their natural tooth structure—longer. That may explain why dentists strive to bond restorations despite its cumbersome protocol, which requires multiple steps and very delicate treatment execution, Blatz says.
“If it doesn’t work, then you’re looking at failure,” Blatz cautions. “But if you do it properly and with the proper materials, we have the ability to be less invasive and help patients keep their teeth longer, so it’s obviously the way to go.”
When it comes to placing restorations, success ultimately depends upon material choice per case, our experts suggest. Therefore, it’s important to look at each case individually and choose what restorative materials are best, and then coordinate the adhesive or cement accordingly.
Reference
1. Blatz MB, Alvarez M, Sawyer K, Brindis M. How to bond zirconia: the APC concept. Compend Cont Educ Dent. 2016;37(9):611-617.
Ceramic Thickness Requirements When Supported by Dentin
In a master’s thesis completed this summer by graduate prosthodontics resident Dr. Carlota Suarez, the occlusal thicknesses of 0.3 mm, 0.5 mm, 0.8 mm, and 1.5 mm of bonded lithium disilicate onlays supported by the clinically equivalent situation of all of the enamel abraded away from a mandibular molar and therefore supported almost completely by dentin were evaluated.1 The researchers ran the specimens through 1.2 million cycles of dynamic loading in a thermal-cycled aqueous environment. Dr. Suarez demonstrated that when a dentin substrate supports the ceramic, a greater thickness of ceramic (ie, at least 0.8 mm) is required. Comparatively, when enamel supports the onlay, the ceramic thickness can be in the range of 0.3 mm to 0.5 mm.1,2
References
1. Suarez C. Fracture Resistance of Minimal Thickness Ceramic Partial Coverage Restorations Under Dynamic Loading. MSD Thesis, University of Washington, 2016.
2. Suarez C, Beuer F, Sorensen JA. Fracture resistance of minimal thickness ceramic partial coverage restorations under dynamic loading. J Prosthet Dent. Manuscript in preparation.
Shear Bond Strength of Adhesives to Dentin with Varying Moisture Conditions
The results of new research have demonstrated how a new class of adhesives is highly tolerant of dentin moisture conditions and can ultimately significantly reduce technique sensitivity.1,2 Last year, 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, and his team conducted in vitro studies measuring shear bond strength to dentin and enamel controls. They set up their tests with four different dentin moisture surface conditions after treatment with phosphoric acid: Group 1 had a layer of water; Group 2 was “moist dentin” typically considered to be the ideal dentin bonding condition; Group 3 was air-dried for 3 seconds; and Group 4 was air-dried for 10 seconds. Manufacturers’ directions were then followed for application of dentin adhesives, and then direct filling materials were placed and light-cured.
“We were very impressed with the bond strengths observed with several of the adhesives,” Sorensen says. “The adhesives tested demonstrated significant improvements in tolerance of dentin surface moisture conditions.”
The results of these bond strength tests with the same four dentin moisture surface conditions combined with cements showed that several adhesives/cements were highly tolerant of varying dentin moisture conditions. The two best systems were Adhese Universal/Variolink Esthetic (Ivoclar Vivadent; www.ivoclarvivadent.com), with dentin bond strength values of dentin moisture groups 1, 3, and 4 ranging from 82% to 99% of the ideal moist dentin; and Scotchbond™ Universal/RelyX™ Ultimate (3M; www.3M.com) ranging from 77% to 93% of moist dentin.1,2
“We also evaluated the self-etching bond strengths and found that of the four adhesives/cements evaluated, the self-etch enamel bond strengths ranged from 64.3% to 86.3% for the phosphoric-acid–etched enamel,” Sorensen says. “The self-etch dentin bond strengths ranged from 64% to 87% for the phosphoric-acid–treated moist dentin.”
This means that the new generation of adhesive/cement systems bring, first, a reduction in technique sensitivity with a considerable improvement in the tolerance of dentin moisture surface conditions, Sorensen explains. Secondly, with the improvements in the self-etching qualities of these adhesives, dentists can have a second line of defense by knowing the dentin self-etching qualities of the adhesives can correct potential procedural mistakes made during the adhesive preparation process.1,2
References
1. Sorensen JA, Chen Y-W. Shear bond strength of adhesives to dentin with varying moisture conditions. J Dent Res. 2016: Spec. Iss. Ab# 1342.
2. Sorensen JA, Chen Y-W. Shear bond strength of cements to dentin with varying moisture conditions. J Prosthet Dent. Manuscript in preparation.