Bonding Through Collaboration

Cementing and bonding both aim to create a strong and durable attachment, but there are fundamental differences between them in terms of the materials used and the mechanisms of adhesion. Cementing is a traditional technique that involves using dental cement to attach restorations to the prepared tooth structure. Dental cement is a luting agent that forms a mechanical bond between the restoration and the tooth surface. It solely relies on the physical properties of the cement, such as its viscosity and compressive strength, to create the desired strong bond. Cementing is more commonly used and is relatively simple and cost-effective.

Bonding, also commonly known as adhesive dentistry, utilizes a more complex and multifaceted mechanism of adhesion. It involves etching the tooth surface with an acid, which creates micro-pores and increases the surface area for bonding. Then, a bonding agent is applied to the tooth surface, forming a chemical bond with the enamel and dentin. Lastly, a composite resin material is layered onto the bonding agent, creating a strong adhesive bond between the restoration and the tooth.

Performed correctly, bonding can allow for a more minimally invasive approach. "In the past, I needed to design my preparations to fit the PFM crown," says Richard A. Sousa, DDS, a private practitioner in Roslyn Heights, New York, who is also a trained dental technician. "Restoration-driven preparations are more aggressive and put the teeth at greater risk for endodontic problems. Now, we can design defect-driven restorations. Approximately 75% of my cases now are partial-coverage, thanks to the ability to bond restorative materials to the tooth structure. It has totally changed the way dentistry is practiced."

Despite those advantages and the fact that so many new restorative materials on the market are bondable—including glass-ceramics, nano-hybrids, 3D printable resins, and yes, even zirconia—many clinicians remain reluctant to bond indirect restorations because it can be time-consuming and challenging. "It is a double-edged sword because doing traditional dentistry really well is better than doing adhesive dentistry poorly, and we see a lot of that," says Dane Avondoglio, DMD, a private practitioner in Cincinnati, Ohio, and faculty member at the University of Cincinnati Advanced Education in General Dentistry residency program. Dental laboratories that position themselves as resources for their dentists can provide valuable support in this area by recommending adhesive products and protocols to maximize the chances that their restorations will succeed. "The laboratory often knows better than many general dentists, so they can teach us," Avondoglio says. Sousa notes that laboratories can suggest how to treat restorations before permanently bonding. "When the dentist tries in a unit, it becomes contaminated with saliva, so the laboratory needs to consider that," Sousa says.

Even with dentists who are more skilled at bonding, laboratories can support by simply communicating what they have or have not done to restorations in preparation. "Distinguishing whether a zirconia restoration has been air particle abraded can be difficult," Avondoglio says. "Something as simple as a note stating that it has been air abraded at 50 µm, for example, can help save the dentist time. Determining whether lithium disilicate has been hydrofluoric acid etched is easier, but still, informing the dentist of the percentage and duration is helpful."

The materials that dentists use to bond traditionally have been classified by generation, based on significant developments in formulations and capabilities.² The newest widely recognized generation, 8th, has been defined as self-etch adhesives containing nano-fillers with an average particle size of 12 nm to increase the penetration of resin monomers and the hybrid layer thickness, which improves the mechanical properties of the bonding systems.²

Within this generation—or in a new generation of its own, according to whom you ask—the category of universal adhesives has become overwhelming popular over the past decade-plus.¹ Universal adhesives have been described as single-bottle, no-mix adhesive systems that can be used in total-etch, self-etch, or selective-etch mode; can be used for the placement of both direct and indirect restorations; and are compatible with self-cure, light-cure, and dual-cure resin-based cements.¹ Universal adhesives also are said to be viable as adhesive primers on substrates such as zirconia, noble and non-precious metals, composites, and various silica-based ceramics.¹ "The use of phosphate monomers such as 10-MDP in single-bottle universal adhesives has been a game-changer," Alex says. "Laboratory and clinical studies generally show these systems perform well, assuming they are used correctly."

Still, no consensus exists regarding whether universal adhesives should be used truly universally. For example, many dentists prefer dedicated metal, zirconia, and porcelain primers.¹ "There are studies that demonstrate some universal adhesives are very effective as zirconia primers," Alex says. "However, there are also studies that found that the bond strength values of some universal adhesives to zirconia decreased significantly after thermocycling or 6 months of water storage. There are also respected chemists and researchers who feel that the most durable bond to zirconia is attained when separate and dedicated primers are employed. It is my personal preference, at least at this time, to still use dedicated silane and zirconia primers." Nathaniel Lawson, DMD, PhD, director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry, says universals typically are less effective for self-etching enamel, citing clinical trials in his laboratory and at the University of Minnesota.³ "Universal adhesives with pH levels closer to 3," Lawson says, "are less ideally effective than self-etching materials with pH levels around 2 for self-etching enamel.³ Lawson adds that, while research has indicated universal adhesives to be effective when bonding porcelain veneers, he "would be nervous" doing so.³

Regardless, the manner in which universal adhesives are used is important. Alex recommends using them only in the selective- or total-etching modes when enamel is present, and in the self-etching mode when bonding full-coverage restorations with little or no enamel remaining. "The new generation of universal adhesives is clinically attractive because they allow dentists to adapt their bonding protocols to specific situations by choosing either etch-and-rinse (total-etch) or self-etch procedures," says renowned researcher Bart Van Meerbeek, DDS, PhD, professor in the Department of Oral Health Sciences at KU Leuven in Leuven, Belgium.⁴

Any adhesive can be technique-sensitive, and universals are no different. Alex says clinicians must be meticulous. "Details such as control and isolation of the working area, cleaning and disinfection of the preparation, proper application of the adhesive system, complete solvent evaporation, and proper light curing of the adhesive when indicated are critical to long-term success," Alex says.

Lawson recommends pairing a universal adhesive with a self-adhesive cement from the same manufacturer. "Once you've performed bond-strength testing on a range of different materials, you understand that mixing and matching just gets complicated."

Laboratories should also be aware that many dentists still achieve successful outcomes with earlier generations of adhesives. Many of the recent developments have centered around ease of use, but older formulations still can produce strong bonds. Avondoglio, for example, uses 4th- and 6th-generation systems, each of which has two bottles—one for the hydrophilic primer and the other for the hydrophobic adhesive."I am not looking to cut corners, and these adhesives work very well for me," he says.

Likely the most popular material to bond is lithium disilicate, with a survival rate that has been measured above 99%.⁵ "Etching for 30 seconds with a hydrofluoric acid gel provides a fantastic etch and tremendous retentive bond to lithium disilicate," Sousa says. "As long as you keep the tooth dry and follow the correct protocol, it is so predictable."

In addition to informing the dentist of any etching that has taken place, the laboratory also can support the dentist by simply understanding the parameters of the materials. "Knowing the minimum thickness requirements is important," Avondoglio says. "While 1 mm might seem to be insufficient thickness, that is really all that is necessary because the bond will strengthen the entire restoration. That should influence the entire design."

The subject of bonding to zirconia, meanwhile, has been controversial, to say the least. Markus Blatz, DMD, PhD, professor of restorative dentistry in the Department of Preventive and Restorative Sciences at the University of Pennsylvania School of Dental Medicine, says it should not be. "In my world of restorative dentistry, prosthodontics, and adhesive dentistry, this is one of the biggest misconceptions that I have ever come across, and I struggle to understand why it exists," says Blatz, who is also Editor-in-Chief of Compendium of Continuing Education in Dentistry. "I started studying bonding to zirconia in the early 2000s, and although we continue to learn how to optimize this bond as new materials come to the market, the phrase, ‘You cannot bond to zirconia' is simply not true."⁶ Literature supports Blatz's stance.^6-12 The 15-year success rate for single-wing resin-bonded zirconia bridges is higher than 90%.⁶

Avondoglio says he does not always bond zirconia, but that he finds it effective in situations when extra retention is needed. "For the single-wing connector on a Maryland bridge or a short clinical crown on a second molar, I prefer to bond," Avondoglio says. Sousa notes that benefits of bonding extend beyond retention of the restoration. "Even if the zirconia crown somehow pops off," Sousa says, "the resin still sticks to the tooth, preventing leakage.

Sousa and Avondoglio both utilize a protocol that Blatz helped developed, known as the APC concept: airborne particle abrasion, priming with a zirconia primer, and composite resin adhesive.⁶

After ultrasonic cleaning, the clinician performs airborne particle abrasion on the intaglio surface and then applies a primer with a phosphate monomer such as MDP that can chemically bond to zirconia, before finally adding the composite resin luting agent. "Applying the primer as soon after air abrasion as possible is best," Blatz says, adding that new self-adhesive resin cements may soon eliminate the need for the primer.⁶

Blatz says the APC concept should be no more technique-sensitive than bonding to silica-based ceramics. Regardless, he says, "Technique sensitivity should not be a deterrent to the performance of proper adhesive dentistry."⁶ Laboratories seeking to become educated on the APC concept in support of their dentists can read more from Blatz in this month's issue of Inside Dentistry at insidedentistry.net.

At least 10 manufacturers have marketed printable materials for permanent crowns, bridges, veneers, and/or inlays/onlays.^13,14 Lawson says testing in his laboratory has indicated that bond strengths comparable to those achieved with lithium disilicate can be created with 3D printing materials, though he cautions that results in the laboratory are not always consistent with what occurs in the oral environment. Lawson recommends that clinicians sandblast, apply a methacrylate-based adhesive (leading brands of universal adhesives qualify), and then utilize resin cement. "This is the most efficient way that we have found to bond a 3D printed crown," Lawson says, adding that silane primer does not appear to be necessary. The surface of the printed crown should not matter, Lawson says, because of the sandblasting. However, laboratories can support clinicians by simply recommending best practices. "Instructions for use are always so important, but many 3D printing manufacturers do not provide guidelines for bonding their materials; they leave it to the cement manufacturers, who are more focused on other materials, so it becomes a no-man's land," Lawson says.

Regardless of the adhesive or restorative material, as always, proper utilization is critical. "We now have many excellent and chemically sound adhesive systems to choose from," Alex says, "but not even good chemistry will overcome poor clinical technique. The biggest variable in adhesive dentistry at this point is not the adhesive systems; it's the dentist. If you want a better adhesive system, then become a better dentist."

While the laboratory's ability to impact the dentist's technique in the chair is limited, it is a factor. Technicians can educate themselves on how best to support their dentists in the bonding process, and Sousa also recommends observing the process up close. "One of the best experiences a technician can have is to visit the dental office and spend a few hours with the dentist as they insert cases, to actually see how it happens," Sousa says.

Providing that level of support can help the laboratory influence the success of its restorations, in terms not only of clinical survival rates but also of better patient satisfaction as more esthetic restorations can be delivered. "The ability to bond is the basis for all of the best cosmetic dentistry today," Sousa says. "It is a total game-changer."