An Uncommon Bond

As the base of this knowledge, the technician must be familiar with the difference between adhesive bonding and conventional cementation. The latter refers to the adhesion of a material to the tooth structure through mechanical retention. Conversely, adhesive bonding binds the restoration to the tooth structure through chemical adhesion, requiring additional steps which may include applying bonding liquid, etching, and setting with a curing light.

The choice between these methods is not always easy or clear-cut. It can be based primarily on the material chosen for the restoration, but shade, available restorative space, difficulty of isolating the tooth, and many other factors can play a part.

"For basically everything that's zirconia-based, alumina-based, or metal-based, I use conventional cementation, relying on mechanical surface area and retention for that restoration to stay in place. I use bonding when it's a more traditional ceramic—feldspathic, leucite-reinforced, lithium disilicate—that can be etched and have a silane primer applied," says M. Reed Cone, DMD, CDT, owner of Nuance Dental Specialists in Portland, Maine.

Cementation and bonding both have advantages and disadvantages that must be carefully weighed before a final decision is made. Conventional cementation has the benefit of familiarity; the first zinc phosphate cement was introduced back in 1879, and it is still the baseline against which modern cements are measured. Newer categories of cement include zinc polycarboxylate, glass ionomers, and the highly popular resin-modified glass ionomers.¹ In general, cementation is less time consuming, has fewer steps, and may be more advantageous to use in areas of the mouth that are not easily isolated from blood and saliva during the procedure. However, depending on the restoration, it may not be the best choice for esthetic considerations.

"If you're using conventional cementation, especially with all-ceramic, lithium disilicate, or zirconia restorations, the coloration of the cements may be a disadvantage," says IDT Editor-in-Chief Peter Pizzi, MDT, CDT, owner of Pizzi Dental Studio in Staten Island, New York. "It can actually affect the coloration of the final restoration. That's an important thing to be aware of as technicians. Obviously, thickness is a factor also; with 1.5 mm or 2 mm of restorative space, the cement will not play a major role in the coloration, but in today's minimally invasive world, we often work with much less space. In that situation, the cement can affect the optical ability of the restoration, especially when working with lithium disilicate or translucent zirconia."

Bonding has the advantage of exceptionally strong retention, but restorations are more time- and labor-intensive to place. The procedure is also not uniform; each bonding agent and system has specific steps that must be followed for optimal results, according to the manufacturer's recommendations. Inadequate technique can lead to less-than-stellar results for the patient.

"The chemistries employed in bonding systems can be sensitive to a multitude of factors," says Gary Alex, DMD, a private practitioner in Huntington, New York, "including the hydration state of the dentin and enamel prior to application (eg, dry, wet, moist), the application time, how the material is manipulated during application, and how long the bonding agent is air-dried to evaporate the solvents after placement, as well as factors related to light polymerization, such as light curing unit wavelength, intensity, collimation, technique, and duration. In addition, many bonding systems are adversely affected when the working area is contaminated with blood or saliva. In short, durable cohesive dentistry is an exact science, and attention to the smallest details can be the difference between clinical success or failure."

Furthermore, the dentist's preferences and level of comfort with the process may be a factor.

"Most dentists do not really love to bond if they don't need to, so the majority are cementing when possible," Pizzi says.

That approach can be detrimental, however.

"Half of the strength of a lithium disilicate restoration comes from bonding it in place; why would you forgo that strength?" Cone asks. "You could make an argument that conventional cementation is easier to clean, has fewer steps, and is less technique sensitive, but in that kind of work, performing the extra steps to get the desired result shouldn't be a hardship, and attention to detail and technique sensitivity are already required if you're doing high-end anterior esthetic restorations."

The decision becomes even more complex when multiple materials are in use. The qualities of both materials must then be considered.

"When you are mixing materials together, there are many cohesive options. Most lithium disilicates and nanohybrids, all lithium silicates and silicate nanohybrid materials, and even PEEK materials all have great bondability. Because of that, the only possible negative in the interface is the material to which it will be bonded. When bonding to natural tooth structure, you have great bondability from material to nature, but when bonding to another material such as titanium, metal ceramics, or metal itself, the bondability may not be as good as it would be on natural tooth structure. However, it has really improved over the years because of improved bonding agents," Pizzi says.

With so many options and possible combinations, no rule is absolute.

"It really boils down to analyzing the situation and knowing the materials," Cone says. "There are very few things that I would commit to ‘always' doing. I always bond veneers, but there's probably been one or two times in every other situation where there was a reason to use glass ionomer or resin cement instead of whatever I would have done 99% of the time."

As zirconia has risen in popularity, an accompanying debate has grown up around it: Can zirconia be bonded, or is cementation the only option? Opinions vary.

"I cement zirconia restorations," Cone says. "I ensure there is adequate retention and resistance form, that the line angles are all correctly positioned, that everything is straight and parallel—and that I don't need to rely on a chemical bond because I don't think that is realistic with zirconia. It is so slippery, which is partially why it's great; nothing wants to stick to it, and it's very biocompatible."

Alex disagrees: "A misconception is that ‘you cannot bond to zirconia.' The truth, however, is that you can bond very predictably and durably to zirconia surfaces using a combination of particle abrasion (ie, sandblasting), a phosphate ester primer (eg, 10-MDP), and an appropriate resin-based cement. Proper management of both the zirconia substrate and tooth tissues is crucial to achieving predictable and durable clinical outcomes." ^2,3

Many just aren't sure currently. More research and experimentation is necessary.

"Cohesive dentistry is our future and is the best version of dentistry that we can do, but we also must understand that not every material accepts cohesion," Pizzi says. "Studies show that if zirconia is properly etched, better bonds can be achieved—but the bond really decreases the value or the strength of the material, so is it worth it? I'm not sure of that answer today."

The best course for laboratories is to keep pace with the most current research literature so they can offer up-to-date knowledge if a dentist asks for advice on the subject.

"Bonding zirconia requires a special procedure. We have bonding guides and other resources on our website that dentists can consult to find out which materials and bonding agents to use for a particular situation," says Kurtis Helm, CDT, owner of Helm Dental Laboratory in Wylie, Texas.

We have come a long way from the single-option zinc phosphate days of the 19th century, but it's clear that the evolution of cements and bonding agents isn't done yet. The next wave of development is focused on increasing bond strength and broadening the range of indications that can be addressed with a single product.

Within the last decade, all-in-one or universal adhesives have come to prominence.⁴ While they offer great efficiency and convenience, there are still questions about whether they are truly superior to the more specialized adhesives.

"In order to develop a truly universal adhesive, very specific and synergistic functional and cross-linking monomers that are multifunctional in nature are required. They must be capable of reacting with a number of different substrates, be able to copolymerize with chemically compatible resin-based restoratives and cements, and have some hydrophilic character, yet at the same time, be as hydrophobic as possible once polymerized to discourage hydrolysis and water sorption over time," Alex says.

Laboratories should be aware of the drawbacks of all-in-one cements but also of the benefits.

"When you try to design a cement to do everything at once, they may not work as well overall. However, the quality of the result with a specialized cement also depends upon the dentist performing each step correctly along the way. In some cases, all-in-one adhesives are not a bad option to provide flexibility without putting all of the risk on the dentist," Pizzi says.

While all-in-one adhesives still need refining, there is potential for greater things in the future.

"Although the current universal adhesives are not perfect, they offer a viable choice if they are correctly used. In theory, these systems have the potential to significantly simplify and expedite adhesive protocols and may indeed represent the next evolution in adhesive dentistry," Alex says.

Another area of development is increased bond strength. Some of the newest bonding products can actually grab onto dentin and enamel better than dentin and enamel bond to each other. This is another area where the future is unclear: Is this extreme bond strength necessary? Does it provide a net benefit, or can it be a detriment in the long run?

"From personal experience, a bond that strong—where it has fused to the underlying tooth—can cause catastrophic damage if the tooth cracks or breaks. Probably once a week, a patient presents with a zirconia bridge on one abutment that has loosened, and that we can't remove without too much damage to the underlying structure because of the bond strength. When they do sustain trauma, that tooth snaps at the gingiva because there is no give. The underlying natural tooth structure pays the price for that super strong bond," Cone says, adding that he rarely sees bonds fail on older restorations. "A ‘stickier' cement is no substitute for proper preparation design."

Increased bond strength capacity offers obvious benefits, of course.

"The fact that we can form a stronger bond between the material and the natural tooth structure than tooth structure can form to itself is amazing," Pizzi says. "I understand that can be challenging if a restoration has to be removed or if there is a very small preparation that is fracturable, but I believe the best way to deal with those situations is for the restorative team to shift their thinking and approach the restoration in a different manner. Adhesion today is almost miraculous compared to what it once was, and I see that as a good thing."

In order to successfully complete a case, collaboration between the dentist and the laboratory is essential. Each side has something to offer when it comes to material selection, and the choice of adhesive or cohesive protocol thereafter.

"There are some things that the dentist should do themselves, and others for which only the laboratory has the necessary expertise," Cone says. "Sometimes there can be a communication disconnect, but as both a dentist and a technician, I try to speak for both sides and act as liaison. Materials selection is a gray area where I believe dentists and technicians have the greatest opportunity to form a common bond. I routinely ask my laboratory to select the material that they think will work best in their hands unless I see a clinical contraindication on my side, and I encourage discussion of the best protocols to use for the case long before I've ever laid bur to tooth."

As the process of adhesion/cohesion has become more complex, dentists have come to rely on laboratories for advice on the latest products and processes.

"It's really important for the laboratory as a dental partner to understand cementation, whether it's conventional or bonding, because often dentists are looking to you for advice in the restorative process. I can't tell you how many laboratories are getting calls from dentists every day asking for recommendations on how to cement lithium disilicate, zirconia, etc, and I think it makes the laboratory stronger to have that information and to understand what to use and when to use it," Pizzi says.

There are a number of ways that laboratories can be a resource for dentists. One of the most essential ways is to provide educational opportunities.

"It's good for laboratories to design their own guide to reflect their material usage, ultimate shade goals, and optimal strategies for different situations," Helm says. "It's really just a matter of understanding the materials, asking a lot of questions to a lot of manufacturers, researching, and then passing that knowledge on to dentists."

Helm has also developed a strategy to help overcome hesitance to use bonding on the clinical side.

"To alleviate some of the problems, we started carrying bonding kits that we loan to dentists," he says. "Dentists don't want to purchase a kit for just one restoration, so it's a good service to provide."

In the end, teamwork, collaboration, and the sharing of knowledge are the most essential parts of successful cementation and bonding protocols.

"The more knowledge that we have of what to use and when to use it, the better it is for all of us," Pizzi says. "Not only for the laboratory, but also for the final esthetic and functional results for the patient."