Failure to Adhere

One- and two-bottle adhesive systems, total-etch and selective-etch techniques, and newer universal adhesives are among the material and technique options that dentists can choose from depending on the individual case, restorative material being used, and tooth substrate (ie, enamel or dentin) available for bonding. Considering these multiple variables, it's not surprising that the same product may yield very different results, depending on the technique employed by the dentist. This unpredictability raises the question: Are the products themselves lacking or are the accompanying techniques/training lacking?

“Although there may be adhesive-specific factors that can lead to the failure of bonded restorations, the most critical factors that result in failure are applicable to most, if not all, adhesive categories,” says André Ritter, DDS, MS, MBA, chair, Department of Operative Dentistry, UNC School of Dentistry. “Assuming that a material with good provenance is used, adhesion can be challenged by the substrate and the application technique. Relative to the substrate, we know that adhesives tend to perform better with enamel and “normal” dentin substrates. By “normal” dentin, I mean dentin that has not been altered significantly by caries or other physiologic, pathologic, or restorative-related processes.”

Adhesive system options are more confusing than ever for the average clinician, notes Jeff Brucia, DDS, an assistant professor of dental practice at the University of the Pacific School of Dentistry and co-director of the FACE Institute. Direct adhesive restorations provide many options from which to select, as well as systemsdesigned for different modes of use.

“However, when compared with direct restorations, indirect restorations are a very different story,” Brucia says. “There are concerns related to compatibility and complete seating of the restoration that must continue to be evaluated with independent in vitro and in vivo studies.”

Available adhesive systems include one- and two-bottle systems as well as universal adhesives. One-bottle adhesives are self-etch adhesives that essentially combine the primer (hydrophilic) and bonding agent (hydrophobic) chemistry. Their acidity is insufficient for etching; therefore, they do not demonstrate good micromechanical retention with enamel, explains Marcos Vargas, BDB, DDS, MS, a professor in the Department of Family Dentistry at the University of Iowa College of Dentistry. Enamel should always be etched with 35% phosphoric acid for at least 15 seconds (or longer for uncut enamel).

Two-bottle adhesives can be self-etch, two-step systems or etch-and-rinse, two-step systems. “Universal adhesive” is the term used for newer adhesives that manufacturers claim can be bonded to various substrates, including ceramics, composites, enamel, and dentin. The term is also applied to adhesives that can be used with or without an etching step, Vargas adds.

“Self-etch, two-step adhesives have shown to be more reliable and produce better long-term results than the self-etch, one-step adhesives,” Vargas says. “Etch-and-rinse, two-step adhesives are also inferior to the etch-and-rinse, three-step systems.”

According to Jorge Perdigão, DMD, MS, PhD, aprofessor in the Department of Restorative Sciences at the University of Minnesota School of Dentistry, the bonding provided by one-bottle adhesives is not as stable or durable when compared with that of two-bottle, self-etch adhesives. For example, a two-step, self-etch adhesive (Clearfil™ SE Bond, Kuraray Dental) resulted in excellent retention of composite restorations after 13 years, with or without selective enamel etch, notes Perdigão.¹

“Another clinical study compared the 18-month failure rate of cervical restorations placed with a one-bottle adhesive both with or without an extra layer of hydrophobic bonding resin, basically comparing the one-step version with the two-step version of the same adhesive,” Perdigão continues. “The two-step version of this commercially available adhesive resulted in significantly better clinical performance than the one-bottle version.²”

Universal adhesives, however, raise an interesting question: Are universal adhesives truly universal? The answer depends on how “universal” is defined, Ritter explains.

“If by universal you mean an adhesive that can be used in any tooth or restorative substrate without any application technique variation, then the answer is no,” says Ritter, adding that he believes the term universal has been used more as a marketing label than as an accurate technical description of an adhesive's ability to be used universally in all clinical situations. “If by universal you mean an adhesive that can be used on both enamel and dentin, either with or without acid etching, then the answer may be yes.”

Perdigão explains that, similar to older one-step, self-etch adhesives, universal adhesives are one-bottle adhesives with a pH between 1.6 and 3.2. However, unlike traditional one-step, self-etch adhesives, most universal adhesives contain 10-MDP, a molecule that bonds chemically to calcium in hydroxyapatite.³ Therefore, he says universal adhesives are not truly universal for several reasons.

First, etching dentin with phosphoric acid removes calcium, which results in dentin nanoleakage and precludes any potential chemical bonding to dentin. On the other hand, enamel must be etched with phosphoric acid prior to the application of a universal adhesive, because these materials do not bond effectively to unetched enamel, Perdigão says.

“Consequently, universal adhesives should only be used following a selective-etch approach,” Perdigão emphasizes. “This restriction makes universal adhesives about as universal as older self-etch adhesives.”

Second, with a few exceptions, universal adhesives must be mixed with their respective self-cure activators to make them compatible with dual- and self-cure composite resins, including build-up composite materials and resin cements, Perdigão continues. In fact, most dual-cure resin cements are based on a peroxide-amine redox system, making them incompatible with simplified adhesives with a low pH. He says at least two recently released resin cements are amine-free, making them compatible with all simplified adhesives, including universal adhesives, without the need to add a dual-cure activator.⁴

Finally, although some universal adhesives contain a silane coupling agent in their composition to simplify the bonding protocol to glass-matrix ceramics, some studies have concluded that a separate silane solution is still needed, notes Perdigão.⁵ The silane in the adhesive solution seems to be neutralized by the resin monomers, he adds.

“Most multiple-bottle, total-etch adhesive systems are compatible with all modes of resin cement curing; however, well-documented science exists that demonstrates a concern with the compatibility of one-bottle systems when used with light-, dual-, or self-cure materials where delayed, minimal, or no light energy can reach the cement-adhesive interface,” Brucia says. “This incompatibility between the adhesive and resin system could lead to adhesive failure and possible clinical failure, as demonstrated by recurrent decay, restoration fracture, or complete loss of the restoration.”

Relative to application techniques, adhesion can be challenged by multiple factors, including moisture contamination, incorrect application sequence of the adhesive components, inappropriate manipulation of the adhesive components (eg, failure to dry in between layers when indicated), inadequate light curing of the adhesive, and poor restorative technique in general, says Ritter.

“We can't always control the substrate—meaning, enamel or dentin—but we should be able to control the application technique,” Ritter emphasizes.

Enamel and dentin are very different tissues. Enamel is highly mineralized, possessing low water and organic content, whereas dentin is less mineralized and contains more water and organic content, Vargas explains. Therefore, dentin should be etched briefly when using an etch-and-rinse technique with one- or two-step adhesives. It does not need to be etched when using self-etch adhesives or newer, so-called “universal” adhesives, he adds.

“The dentin requires a more hydrophilic material to allow interaction with its moist and complex surface,” Brucia elaborates. “The hydrophobic bond material then allows the primed surface to interface with the restorative material to be placed.”

The enamel surface does not require a hydrophilic material to establish an excellent and durable bond, Brucia notes. Therefore, when working in the total-etch mode with a mostly all-enamel preparation, a simple etched and dried enamel surface can be coated with the bond bottle only (which is a mostly hydrophobic adhesive material), cured, and then layered with the restorative material.This technique option is limited to multiple-bottle systems, in which the primer and bond materials are provided in separate bottles, he adds.

“When dentists use the total-etch or etch-and-rinse technique, the acid is rinsed off after etching the dentin. The complete replacement of the rinsing water by monomers in the adhesive is unfeasible, resulting in hybrid layers that contain residual water and/or voids that can lead to the degradation of the dentin-adhesive interface,” Perdigão elaborates.^6-8 “When using self-etch adhesives, acid etching the dentin has been shown to compromise the integrity of the dentin-adhesive interface. Selective enamel etching should always be the first choice.”

In addition, Perdigão cautions that the solvent evaporation time currently recommended for one-step adhesives (including universal adhesives) is too short, potentially allowing residual water from the adhesive to cause hydrolytic degradation of the resin and dentin collagen. Increasing solvent evaporation time by up to 15 seconds is recommended, he says.⁹

“All self-etch and etch-and-rinse primers, as well as etch-and-rinse, two-step adhesives, contain solvents that must be fully evaporated with moisture-free air,” Vargas emphasizes. “The best indication is a shiny surface that does not move with the air stream after a few seconds. Larger preparations require longer evaporation/drying times.”

Additionally, dentin adhesives that employ a hydrophobic bonding resin as the last bonding step (ie, three-step, etch-and-rinse adhesives and two-step, self-etch adhesives) are more stable than their simplified counterparts (ie, two-step, etch-and-rinse adhesives and one-step, self-etch adhesives), Perdigão says.² For simplified adhesives (including universal adhesives), vigorous scrubbing of the adhesive may improve the durability of the bonded dentin interface.

Although not all indirect restorations require adhesive bonding for placement, the type of indirect restoration should determine whether or not adhesive bonding is indicated and what type of adhesive should be used. For example, thin porcelain veneers can be bonded with the same light-cure adhesive used for direct restorations because they allow for good light penetration through the restorative material, particularly if the veneers are lighter in color and not too opaque, explains Ritter.

For most indirect esthetic materials, such as ceramics and laboratory-processed composites, thickness and translucency (characterized by translucency parameter and contrast ratio) determine what type of adhesive should be used. If the curing light will not reach the adhesive through the restoration, a dual-cure or self-cure adhesive should be used, Ritter advises.

However, if dentists are directing the curing light so that it is shaded by the tooth, the composite resin is less cured. It is important to realize that this will reduce the intensity of the curing light by as much as 40%, cautions John Burgess, DDS, MS, adjunct professor at the UAB School of Dentistry.

Other indirect materials (eg, metal alloys, PFM crowns) do not allow the curing light to reach the intaglio surface where the adhesive is applied. Therefore, if adhesive is used in those cases, it must meet those requirements, Ritter says.

“When used for indirect procedures, some simplified adhesives must be mixed with a dual-cure activator in order for them to co-polymerize with specific dual-cure resin cements,” notes Perdigão.

Additionally, in the case of indirect restorations, pre-curing the adhesive results in higher bond strengths when several resin cement/adhesive combinations are used, says Vargas. Unfortunately, when adhesives are cured for indirect restorations, there is a risk of the restorations not seating properly.

Most one-bottle adhesive system strongly recommend completely curing the adhesive prior to placing the restoration, but many multiple-bottle systems do not require this step, Brucia explains. If the adhesive must be cured prior to restoration placement, then it is critical to air thin the material to a thickness that will not interfere with completely seating the restoration, but not too thin to prevent a curable layer in the presence of oxygen.

“This air thinning may only be possible with an unfilled or slightly filled material, and more complex preparation designs can make materials difficult-to-impossible to thin sufficiently,” Brucia adds. “Many studies demonstrate improved performance of one-bottle systems when applied with a multiple layer and separate cure technique.”

However, this wouldn't be possible with the delayed sealing technique most commonly performed for the cementation of indirect restorations, Brucia says. One technique that may reduce these concerns involves placing a quality adhesive at the time of the preparation, prior to impressing. This technique is known as immediate dentin sealing.

The impact of curing on adhesive bonding is very significant, regardless of the type of restoration. It has been widely documented that light curing is the most effective when the material is cured directly (ie, without having to direct the light through a substrate or restoration). Therefore, when possible, it is always best to use a light-cure adhesive and not a dual-cure adhesive, Ritter advises.

“Proper cure is the foundation for all light-cured adhesive restorations, and success cannot be achieved without it,” Ritter says. “This point can't be emphasized enough.”

Unless the adhesive is too viscous, curing the adhesive separately (even for a dual-cure adhesive) is indicated and does not compromise the clinical outcome, Perdigão says.

“The adhesive should always be light-polymerized before the application of direct restorations, which results in higher bond strengths. To prevent polymerization shrinkage, the adhesive needs to be set and the bond formed before composite resin placement,” Vargas explains. “With indirect restorations, the adhesive is not fully polymerized by the resin cement in areas where light penetration is shallow.”

However, a number of factors influence light polymerization and subsequently, adhesively placed direct and indirect restorations. Among them are the curing lights themselves (ie, power, intensity, wavelength emitted), adhesive and composite material components, exposure time, light direction and distance, and the preparation required (ie, material depth).

“I remember when we had curing lights with outputs of 300 mW/cm². Now the average output is probably greater than 1000 mW/cm², so light-curing technology certainly has advanced,” observes Burgess. “Additionally, the divergence of the light produced has also decreased, so we're seeing a more uniform light emission.”

Burgess explains that previously, curing light tips had hot and cold spots related to how the light was transmitted through them. Much of that problem has been eliminated, so newer curing lights deliver a more consistent and higher output. When combined with composite materials incorporating new photoinitiators that respond to different light wavelengths (eg, bulk-fill direct restoratives), such improvements help to speed up curing times and enhance depth of cure, Burgess says.

“I think that in the future, you're going to see shorter and shorter curing times. That means the attention to detail for light-curing procedures must be even more precise,” Burgess explains. “If you have a 3-second curing time, and you're moving the curing light the entire time, you'll produce a very poor depth of cure because you're not properly irradiating that restoration.”

Therefore, clinicians must pay attention to the quality and quantity of the light emitted and the procedure they use (ie, adequate exposure time, direction, distance, etc.) in order to achieve the optimal cure for a material, advises Jack Ferracane, PhD, professor and chair in the Department of Restorative Dentistry at the OHSU School of Dentistry.

“I think with current systems, we have gone beyond what is necessary and possibly prudent in terms of overall light power in order to theoretically shorten the curing time,” says Ferracane. “Because the relationship between power and time is limited, the bigger danger is undercuring, even with a high-powered light.”

To increase light penetration and consequently depth of cure, some bulk-fill composites and flowables incorporate modified and/or additional monomers, fillers, and photoinitiators in their formulations, which increase polymerization conversion and decrease stress during and after polymerization, explains Gary Alex, DMD. He notes that some of these photoinitiators (eg, Lucirin TPO) are outside the spectral range of many curing lights.

“The degree of polymerization conversion has a direct effect on shrinkage stress. All other things being equal, the higher the conversion of any given composite, the greater the shrinkage stress,” Alex elaborates. “In fact, some studies suggest that the reason why ‘delayed polymerization' or ‘soft start' polymerization may reduce composite shrinkage stress is not because they prolong the gel point of the composite (as some have stated), but because the final polymerization conversion is less when compared with continuous curing, which results in lower stress.”

Material, technique, and equipment advances have done wonders to increase efficiency and ensure better and more predictable outcomes with adhesively placed restorations. However, the use of more efficient materials with enhanced capabilities doesn't negate the need for precision and adherence to appropriate protocols. Adhesive bonding success (as well as clinical and functional success) ultimately depends on the material choice, making it imperative to coordinate treatments, restorative materials, and adhesives based on an evaluation of each individual case and its requirements.

“As with all patient care, a risk assessment is critical to the success of clinical treatment,” Brucia emphasizes. “High caries risk patients and those with significant functional forces require different restorative approaches and will experience more rapid restorative failure if preventative techniques and materials are not considered, discussed, and introduced.”

According to our experts, adhesive failure is predominantly operator dependent. When adhesion fails, a number of potential issues could result. These include—but are not limited to—increased microleakage, recurrent decay, interfacial staining, immediate sensitivity, loss of retention, chipping and fractures, and compromised periodontal health. The following table provides recommendations to help avoid adhesive failures: