“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.