Laura Dorr is the former executive editor of DPR's Modern Dental Network and a regular contributing author.

Laura Dorr

Change is scary. Switching jobs, moving to a different city—heck, even getting a new haircut can be majorly anxiety inducing. But what happens when it’s time to make a change in a clinical situation where even the smallest misstep can lead to a huge problem?

Many dentists shudder when they think about changing procedures or products and (perhaps rightfully) have a “go with what you know” or “if it ain’t broken, why fix it” mentality. But sometimes, change is necessary.

Whether you’re making a shift because of issues with your current products, or you want to try a new generation of a tried-and-true favorite, the process of switching materials can be understandably intimidating. But with so many fresh, innovative generations of products, including dental adhesives, hitting the market seemingly every day, the temptation to switch can outweigh trepidations about adopting something new. And, when implemented correctly, a new adhesive system could end up being the best choice in the long run.

There are many reasons to adopt a new adhesive. Maybe you’re looking for better longevity in your restorations, or you find your current product is causing an unacceptable amount of postop sensitivity. Perhaps you’re looking for a material that’s easier to work with and cuts down on procedure time. Possibly you want a product that is more versatile, and is compatible with different composite and cement materials. Conceivably, your colleague raved about a new product and you just want to take it for a spin.

Whatever your reason, it’s important to identify what you’re looking for and why you want to make a switch.

“Generally, if I’m switching something, it’s because I’m having a problem and am looking to solve that,” says Dr. Jennifer Sanders, a general dentist practicing in Frenchtown, Montana. “If there’s something I don’t like about the previous system, I’m looking to make sure the new one doesn’t have that issue. It takes some legwork, but finding the appropriate adhesive for your situation is an important step for successful adoption of the product.”

Once you pinpoint what you’re trying to gain with a new adhesive, you can narrow down which adhesive meet those needs. Then you’re ready to try them out.

It may seem obvious, but introducing a new product to your repertoire requires education. Each product comes with its own steps, benefits and quirks, so understanding how—and why—it works can be the difference between success and failure. Essentially, you’ll need to know the adhesive system inside and out, and understand the specific instructions you’ll have to follow.

“When dental materials experts get together to discuss the most important advice to give clinicians when using a new adhesive, the advice is simple: Read the directions,” says Dr. Nathanial Lawson, director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry. “Every adhesive has its unique instructions and the reality is most of us who practice, believe that all adhesive handle the same. This simply isn’t true.”

Other practitioners agree that Lawson’s recommendation can’t be stressed enough.

“Read the instructions,” concurs Dr. John Flucke, Dental Products Report’s technology editor who practices in Lee’s Summit, Missouri. “Read them before using the product and make sure you fully understand them. This very well might be the smartest things to do.”

Even if you’ve worked with similar adhesives—or even previous generations or other comparable products from the same manufacturer—it’s not safe to assume that the new adhesive will behave like the past one. Does this product require etching? Is it a one- or two-bottle system? What about curing time? Manufacturers often update products with features to help simplify procedures or eliminate steps, but if you aren’t aware of these time-saving developments, it can actually cost you valuable procedure time and increase the risk of errors.

“Making sure you understand the steps to the adhesive is critical, and it’s also really important to ensure that it will work with your current materials,” Sanders explains.

Once you’ve learned the steps and have identified any you think might be problem areas, it’s important to trial the product so you know how it handles and can perfect your technique.

“I always bench test something before I ‘go live’ with it,” Flucke says. “Understanding how the product feels is important. Is it runny, hard to mix, hard to get on a brush or instrument, or hard to spread evenly? These are all things you can discover by tinkering with it before you throw all the patient variables of saliva, movement, limited opening, etc. into the equation.”

Running these bench tests and taking a product for a test drive can make a big difference in the success of an actual procedure. Flucke emphasizes the importance of understanding how a product works, because uncertainty can affect procedure time. Since contamination is such a persistent problem with bonding, it’s critical that clinicians can implement a product efficiently and smoothly to minimize opportunities for contaminates to enter the field.

“If you approach a clinical situation and have to work slowly because you don’t understand how to use the product efficiently, you are going to run the risk of fighting contamination throughout the procedure,” he explains. “Ultimately, you’re going to end up with a less-than-stellar results.”

While problems can arise from numerous issues, uncertainty about a new product can add another element to the mix. Identifying where the problems arose—whether it be from a product or a procedural error—can help prevent complications in future cases.

“Sometimes we see the results at the next recare appointment, forget we struggled and blame the product,” Flucke says. “If you can move efficiently and keep contamination to a minimum, when you see the restoration at recare, you can then know for sure it wasn’t your product that caused the less-than-optimal result.”

Flucke further recommends making sure your other equipment is functioning properly, so you can focus on success with the new adhesive.

“I advise checking your curing lights on a regular basis,” he says. “If your lights do not have sufficient output, you cannot adequately cure materials. You want to remove as much blame from the product as possible—so that if there is an actual problem with the adhesive, it’s easy to identify.”

Ensuring you document the procedure will also give you a reference point to help narrow down the contributing factors if there are issues down the road.

“I make sure my procedure notes are accurate to reflect the system I’m using,” says Sanders. “That way if a problem arises, I can see what system I used and troubleshoot from there.”

In the end, the combination of education, attention to detail and practice make perfect. While adopting a new adhesive, or any other material or product, is a learning curve, it can make a big difference in how you practice—and for the end success of your restorations.

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Implementing a new adhesive system can be daunting, but there are several steps that can minimize the struggle.

Best practices for adopting a new dental adhesive

Let’s face it: mistakes happen. Even the best dentist is going to run into problems with adhesives every now and then. And that’s okay, as long as you know how to fix them.

But where can it all go wrong? There are several areas that are particularly problematic and can lead to big problems.

“Aside from not following the directions, the two biggest mistakes that I suspect dentists make while bonding are not achieving sufficient isolation and improper light curing,” says Dr. Nathanial Lawson, director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry. “If you’re making errors on these steps, you’re going to incite major problems.”

Dr. Jennifer Sanders, a general dentist practicing in Frenchtown, Montana, agrees. “In my experience, the biggest cause of adhesive errors are poor isolation and the age-old mistake of not following the directions for the system.”

So, when things do go wrong, how can you fix them? Start by reading the manufacturer’s instructions—and then review our top tips below for solutions to some of the other most-common issues.

It goes without saying that isolation is one of the most important things to get right—and also where many procedures go wrong. Impeccable isolation and contamination prevention can be tantamount to success. While there are several ways to ensure good isolation, some methods make it easier than others.

“During a procedure, the best thing you can do is get adequate isolation,” says Dr. John Flucke, who practices in Lee’s Summit, Missouri. “There are products such as Isolite, Isodry, DryShield, and Zirc’s Mr. Thirsty that will help isolate and provide amazing vacuum. This greatly simplifies isolation and contamination. There is also the rubber dam or copious amounts of cotton.”

While often used in dental school, rubber dams can feel cumbersome and time-consuming for practicing dentists, so it’s good to be up to date on the aforementioned (and other) options available on the market.

“As rubber dams may not seem practical for some practicing dentists, a combined retraction/suction system, such as Isodry, may provide a higher level of isolations than cotton rolls alone,” Dr. Lawson says. “If a dentist does contaminate their bonding surface with saliva, the ideal treatment would be to clean the surface with a bur or sandblasting.”

If those critical steps are not performed, there are a few different solutions that can be taken, depending on which stage the error occurred in (though if it gets to that point, Dr. Lawson says, outcomes may be compromised). If contamination occurs after etching, Dr. Lawson recommends rinsing off the saliva and blotting dry, but not re-etching.

If it occurs after applying the bonding agent but before curing, the saliva should be rinsed and dried, and another layer of bonding agent should be applied. And, if the contamination happens after curing the bonding agent, the saliva should be rinsed and dried, and then the composite can be placed.

“If I etch a prep and then, during the rinse, the tissue begins to bleed, I’ll stop, use a hemostatic agent and start the etch process over,” Dr. Flucke says. “If the field is contaminated, it just tends to lead to other problems later.”

And, there’s no shame in starting over, a lesson Dr. Flucke says he learned from his grandfather.

“If there is an error during bonding, it is almost always easier to go back and start over,” he says. “My grandfather used to say, ‘Son, there’s never time to do it right, but there’s always time to do it over. So just do it over in the first place.’ He was a smart man.”

When it comes to light-cured composites, curing can make or break everything. And while it may seem as simple as pointing the light at a restoration and hitting the magic button, it’s a little more involved than that. Selecting the right curing light, keeping it properly maintained, and following curing instructions are necessary for success. A high-powered curing light can also speed up procedure time, minimizing the risk of isolation failure and contamination.

Even with a top-notch curing light that is expertly maintained, technique blunders can also be an issue. Mistakes such as not curing for the recommended time, not matching the wavelength to the adhesive requirements, having the light too far away, or curing at an angle can lead to undercuring and subsequent side effects such as debonding, discoloration, or fracture.

“A technique error that is often made is that the curing light is placed in such a way that the bonding surface is not being cured,” Dr. Lawson says. “This often happens in the bottom of Class 2 boxes when the tooth mesial to the restored tooth blocks the curing light due to improper positioning of the light.”

At the basic level, curing errors can be avoided by ensuring that the curing light is directly on the restoration and is actually curing the area as intended, and by adhering to curing-time instructions. Other recommendations include:

Make sure portions of the area to be cured aren’t shadowed. The light needs to reach the entire area for a successful cure. If there are areas that cannot be reached, consider switching to a dual-cure or hybrid adhesive.

Ensure that the wavelength of the curing light matches the adhesive so that a strong, fast cure can be achieved.

Test your curing light. To ensure maximum efficacy, curing lights should be tested to make sure the output is consistent. Output can change due to battery charge or light burnout (particularly in lower-end curing lights, which are susceptible to maintenance issues like these), so it’s important to test regularly.1

Employ heat management techniques. Strategies such as taking breaks in between curing cycles to allow heat dissipation can prevent overheating, which can cause damage to the soft tissue or dry out the restoration.

Sometimes there are hiccups in procedures that may seem inconsequential in the moment, but which could cause problems in the long run. Rather than crossing your fingers that it works out alright, it’s important to document the potential issue so it can be easily and effectively addressed in the future.

“If something doesn’t go ideally during the procedure then, I note it,” Dr. Sanders says. “For instance, if the crown is cemented on, it won’t do the tooth any good to mess with it unless there is a problem, so I’ll make a note of it. That way if someone returns with an adhesive failure, I can just read back on my notes, which makes it easier to troubleshoot what went wrong. Then I can identify if I need to take extra care or try a different adhesive.”

When mistakes do happen (and inevitably, they will happen to everyone) Dr. Sanders recommends being transparent and opening a dialogue with the patient about any concerns. This way the patients knows in advance of any potential issues that could arise and expectations can be managed.

“If I have concerns about the bond, I would let the patient know,” Dr. Sanders says. “And, I would check on it more often than I otherwise would.”

Dr. Flucke concurs that building solid patient relationships can go a long way when it comes to remedying adhesive failures or errors.

“As far as failures a month or two down the road, we all have them,” he says. “It’s nothing to be ashamed of. Doing a new restoration is the right thing to do and I feel that doing the occasional piece of ‘warranty work’ can show patients that their oral health is more important to us than profits.”

1. Che, F, Felix, C. (2019). Endurance Testing of Dental Light Curing Units. J Dent Res, Vol 98 A, Abstract #1105

Identifying the biggest adhesive mistakes and how to rectify them.

How to avoid adhesive technique mistakes

A dream adhesive is one that is effortless to use, creates a strong bond with pristine esthetics, and leaves the patient with zero post-operative sensitivity. Where does one find this perfect adhesive? The bad news is, it doesn’t exist.

So, if you can only have a few of these ideal characteristics, which ones are the most important? Should practitioners be stressing over strong bonds, or focusing on streamlining procedures through ease of use? Are esthetics 

 important if it means keeping the patient more comfortable?

With each clinical situation being unique, there may not be a definitive answer to these questions. However, some properties stand out above others in terms of importance.

When considering indirect restorations, the ultimate goal is to create a strong, long-lasting bond between the substrate and the dental adhesive, as well as between the adhesive and the restoration. In the end, forming these bonds can mean the difference between restoration success and failure.

“There are many things to take into account, but when I am considering an adhesive, the first thing that I would want to see would be a good five-year clinical trial,” explains Dr. Nathaniel Lawson, director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry. “And in a clinical trial, I would be considering retention as the primary consideration. Secondary to that, I’d look at staining around the margins”

“In the laboratory, the most critical property we measure on adhesives is bond strength,” Lawson says. “Ideally, bond strength should be measured after at least three months of water storage.”

But with most modern bonding systems meeting—or surpassing—the 30 MPa level, does bond strength become that important of a factor? While bond strength is still a critical consideration, some practitioners feel comfortable assuming that bond strength is a veritable guarantee, since most modern adhesives have the ability to keep a restoration placed for years to come, when implemented and applied properly.

However, the key there is that adhesives must be properly applied. Any issue with isolation, application, light curing or contamination can compromise the strength of a bond.

“If each step is not performed properly, there are some solutions which can be taken afterwards to correct them and form a strong bond—but you’re looking at compromised outcomes,” Lawson explains.

An adhesive with bad handling properties can eat up a lot of procedural time, costing practitioners time and money and increasing the risk of contamination or complications that could ultimately lead to restoration failure. This means ease of use is paramount to restoration success. In fact, almost all doctors surveyed recently by Dental Products Report reported ease of use as a major consideration for adhesives, particularly when trying to save time and maximize productivity.

“As far as adhesives go, the most important thing for me is ease of use,” one doctor states. “Sensitivity reduction is nice, but basically because I work in a public-health office based on seeing as many people as possible (or at least that was the goal pre-COVID), for me it's about saving time.”

Simple handling, consistent viscosity and fewer steps are all important factors that fall under the “ease of use” umbrella and can simplify procedures and streamline workflows. Extended working time and decreased setting time also help speed up procedures, while reducing the risk of contamination. Easy mixing and cleanup are also critical considerations that can make restorations less frustrating for practitioners and patients alike.

While ensuring a bond stays intact is obviously a critical consideration, the esthetics of a restoration are also essential. With an end goal of trying to ensure a restoration is indistinguishable from the surrounding dentition, it’s extremely important to color match to ensure natural-looking results. This can require a little more finesse—and potentially, extra steps.

For example, light-cured cements are the most color stable, but are not an option in all situations. On the other hand, traditional dual-cure resin cements often discolor and darken over time, or even change shade during curing.

As such, it’s important to consider the procedural steps of an adhesive when thinking about esthetics, as well as the range of shades and viscosities available. To improve esthetics, practitioners should consider adhesives that are simple to dispense, apply and clean up, with viscosity levels that limit drifting after a restoration is seated, and lasting color stability. While this may mean the procedure will take a little longer, it can reinforce long-term esthetic success.

No doctor wants to get a call from a patient complaining of post-operative sensitivity a few days (or weeks, or months) after a procedure. Limiting patient discomfort should be a priority for all clinicians. To achieve this, clinicians should do their research on adhesives and know their properties as well as the necessary application steps.

“When evaluating an adhesive in a clinical trial, the third thing I’d look at is post-operative sensitivity,” Lawson says. “That can be a big problem for patients.”

Technique matters when reducing post-op sensitivity. For example, total-etch systems used on dentin require acid etching to open the dentin tubules, subsequently allowing the adhesive to penetrate into the collagen fiber network. If the collagen layer is hydrated, all goes according to plan. If the collagen layer is dry, however, the collagen fibers can collapse, preventing solid adhesion between the substrate and the adhesive. These collapsed collagen fibers contribute greatly to post-op sensitivity.

However, less technique-sensitive self-etch adhesives can reduce this risk of sensitivity. This is because self-etch systems prevent collagen fibers from collapsing, since the material etches and penetrates simultaneously. (This also counteracts the need for moist bonding, eliminating a concern and simplifying the procedure.)

Composite restorations also face the danger of microleakage at the restoration margins due to polymerization shrinkage, which results in the debonding of the restoration and potential enamel cracking. Thus, proper technique to prevent microleakage is critical to reducing post-op sensitivity.

When evaluating adhesive properties, it’s important to keep in mind that different generations of bonding agents come with different strengths. This means you may have to weigh the cruciality of bond strength against the importance of ease of use or esthetics.

For example, adhesives with the best reputation of clinical success are two-bottle systems, such as fourth- and sixth-generation materials. However, fourth-generation materials remain complex and time consuming to use, requiring multiple steps. The use of multiple bottles and the resulting numerous application steps leave increased room for error.

Sixth-generation systems, while also requiring multiple bottles, introduced self-etching technology, which eliminated the etching step found in fourth- and fifth-generation materials. One downside is that the bond strength of sixth-generation products has been found to decrease significantly over time. If you’re looking for ultimate ease of use, universal adhesives should be a go-to, though since they are newer to the market, their long-term bond strength and efficacy are not as well measured.

Although there’s no one-size-fits all adhesive that’s ideal for all situations, product developers have been getting closer and closer to that end goal. Until then, practitioners should consider which properties matter most on a case-by-case basis and choose an adhesive that meets those goals and works well for them.

1. Yu H, Zheng M, Chen R, Cheng H. Proper selection of contemporary dental cements. Oral Health and Dental Management. 2014; 13: 1: 54-49

Exploring the importance of balancing considerations for dental adhesives.

 What property is most important when choosing a dental adhesive? 

When it comes time to choosing a dental adhesive, you need to balance ease of use, technique, strength and efficacy. But with so many dental adhesive options on the market today, it can be hard to keep them all straight. Which ones boast the strongest bonds? Which ones are the easiest to use? How many bottles will you need? And how many steps is it going to take to complete a restoration?

It all depends on which generation of dental adhesive you’re using. Whether it be the total-etch and no-etch systems of the fourth and fifth generations, or the self-etch advancements offered by the sixth generation and beyond, it’s important to understand what each generation brings to the table—and what it has to offer in your practice.

First-generation materials debuted in the 1950s, when Dr. Michael Buonocore identified a lack of adhesion to tooth substrates as a major culprit in restoration failure. In 1955, he proposed the use of acid etching for bonding, demonstrating that resin containing glycerophosphoric acid dimethacrylate (NPG-GMA) could bond to acid-etched dentin, greatly improving the adhesion of filling materials to enamel.1

While this discovery was groundbreaking, initial bond strength was underwhelming (ranking only in the 1-3 MPa range), reduced even further by exposure to the moist environment of the mouth. So, while these initial attempts were largely unsuccessful, they must be lauded for laying the groundwork for future innovation and the dental adhesives we know and use today.

The 1970s ushered in an effort to improve on the first generation of adhesive materials. To accomplish this, polymerizable phosphates were added to bis-GMA resins to increase bond strength through the creation of an ionic bond to calcium in the tooth.2 

But much was still unknown about dental bonding. The smear layer had not yet been discovered, resulting in unreliable bond strength. While small improvements had been made (second-generation adhesives had a slightly increased bond strength of 4-6 MPa), the bonds would still degrade quickly when exposed to moisture (which again, the mouth is full of).

Until the smear layer was addressed, improvements in bonding could only be minimal. Luckily, in the late 1970s, Dr. David Eick identified the smear layer and its detrimental effects on bonds, ushering in the third generation of materials and rendering second-generation adhesives essentially obsolete.

In the late ’70s and 1980s, removing the smear layer became the critical goal of bonding systems. Thanks to Eick’s discovery of the smear layer, acid etching the dentin was introduced in an attempt to remove (or at least disrupt) this coating of organic and inorganic material created during cavity preparation.

The introduction of acid etching was not well received by all dentists and became controversial, as many clinicians believed that dentin shouldn’t be etched. Researchers were on the right path, however: Acid etching served to open the dentin tubules allowing smear layer particles to be more efficiently removed. After the acid was rinsed, a primer was applied, and unfilled resin was placed on the dentin and enamel. Unfortunately, despite a raised bond strength of 12-15 MPa, many of the frustrating bonding issues of prior generations remained unsolved, as the smear layer resisted penetration by most unfilled resins.3

Enter the fourth generation of materials, that changed the bonding game permanently. It took four generations to get there, but in the 1980s researchers finally discovered a way to effectively remove the smear layer.

But this success didn’t come easily. While these adhesives produced unprecedented bond strengths in the low- to mid-20s MPa when implemented correctly, correct implementation wasn’t simple.2 The numerous application steps and the use of multiple bottles in these complex adhesive systems left abundant opportunities for error and contamination, which could disrupt bond strength.

However, the efficacy of fourth-generation materials has made them long-lasting standbys for many clinicians. Familiar names like OptiBond (Kerr), Clearfil (Kuraray America) and All-Bond (BISCO) emerged—the predecessors of products commonly used today, thanks to their reliability and track record of excellent clinical results.

“The adhesive materials with the longest evidence of clinical success are definitely the two-bottle systems—either the fourth-generation etch-and-rinse materials or the sixth-generation self-etch materials,” says Dr. Nathaniel Lawson, director of the division of biomaterials at the University of Alabama at Birmingham School of Dentistry.

Since these fourth-generation systems can be implemented in a variety of bonding protocols for indirect and direct restorations (including self-cure, dual-cure and light-cure procedures) they are some of the most versatile bonding systems available on the market. According to Lawson, when implemented correctly, these products become the gold standard against which newer generations should be judged.

“The gold standard are those which contain separate primer and adhesive (i.e., fourth- and sixth-generation products),” Lawson says. “A new adhesive should compare favorably to one of these.”

With smear layers conquered and bond strength improved by the fourth generation, the goal for fifth-generation bonding systems became simplification, in an effort to reduce both procedural time and post-operative sensitivity (both of which became concerns with fourth-generation systems).

So, with the birth of the fifth generation came the introduction of one-bottle systems. These products combined the primer and adhesive into one bottle, making it possible to apply to the enamel and dentin simultaneously. This eliminated the inconvenient waiting periods and cumbersome multiple application steps of the prior generation, and meant that the tooth was only exposed to acid for 15-20 seconds. The adhesives also didn’t sacrifice bond strength, which ranged around 25 MPa, comparable to fourth-generation materials.

But, with simplicity came drawbacks. Not all fifth-generation adhesives could be used with self- or dual-cure materials. The one-bottle systems also contained hydrophilic primers, which made the adhesives much more susceptible to moisture degradation. Despite the drawbacks, however, the one-bottle system was well received by dentists, since it reduced chair time and didn’t require them to stock multiple types of adhesives and primers.

As mentioned by Lawson, sixth-generation systems have been a resounding success and have set a high bar for future generations of adhesives. While this generation of self-etching primers returned to the multiple-bottle system (and requires a separate adhesive and etch-primer), they have eliminated the etching step entirely, and no longer require the dentin preparation surfaces to be etched.

While the addition of the second bottle meant more products to juggle, manufacturers in the 1990s and early 2000s worked their hardest to counteract the inconvenience. Cartridges that combine the unmixed adhesive and self-etch primer in exact ratios were introduced, simplifying the dispersal process and ensuring the proper mixture was achieved. Since these bonding agents can simultaneously diffuse primer and etch dentin and enamel, the sensitivity issues of the fourth and fifth generations have been greatly reduced.

One concern with sixth-generation materials however is that margins are often discolored, believed to be caused by the fact that this generation of products are not as effective as phosphoric acid for etching the enamel surface. As a result, the bond strength to enamel can be up to 25 percent weaker than the fourth and fifth generations.4 To counteract this, some manufacturers now recommend etching the enamel with phosphoric acid prior to the self-etching adhesive application, which adds an extra step.

Overall, however, the sixth generation of materials is an adequate addition to any dentist’s arsenal. While bond strength to enamel is decreased, sixth-generation bonding agents boast impressive bond strength to dentin (up to 41 MPa after 24 hours). Though sixth-generation products may not be as popular as other generations, they are certainly an improvement on the earliest iterations and are used successfully by many clinicians.

Not to be outdone by the development of one-bottle and self-etch systems, the seventh generation went one step further by combining them. Developed in the early 2000s, these bonding agents include etch, primer and adhesive in one bottle.

This unprecedented simplicity has presented incredible benefits for clinicians. Because the three key ingredients for bonding are already combined, it eliminates the mixing steps—thus minimizing opportunities for errors that can compromise bond strength. And you can’t afford to compromise bond strength with seventh-generation materials: While bond strength is incredibly consistent, it is also lower (in both initial and long-term studies) than any other system. 

These adhesive systems’ ingredients present another challenge, as they are acidic and prone to hydrolysis since they contain high concentrations of water. This makes them susceptible to chemical breakdown, which results in solution instability. Essentially, the seventh-generation adhesives become unstable quicker than past generations, and are more hydrophilic and disposed to moisture absorption than the sixth-generation, two-step systems.

While seventh-generation systems laid the groundwork for ultimate stability, they ultimately are not as popular as earlier generations. It would take one more generation to find the right mix of simplicity and effectiveness.

If you’re looking for ease of use, universal adhesives are the way to go. Like seventh-generation systems, the universal one-bottle systems combine etching, priming and bonding into one solution, eliminating the need for mixing and reducing both products and steps.

“Universals really make things simpler, and are inarguably less technique sensitive,” says Dr. Jeff Lineberry, a general dentist practicing in Mooresville, North Carolina. “But the question is longevity and durability, and we see promising results with these adhesives released three or four years ago, but we don’t yet have a long history to evaluate.

“One-bottle universal adhesives are probably the most popular bonding agents,” Lawson says. “And, while we don’t have long-term statistics yet, there are medium length studies showing good clinical performance.”

And they are popular for a good reason. The versatile universal materials can be used with a range of protocols (from self-etch to total-etch) on both dentin or enamel, and are much more effective for enamel etching than seventh-generation materials. Universals are also compatible with light-, self- or dual-cure composite, and bond well to a wide range of materials, including zirconia, composites and glass ceramics. To top it all off, they have an impressive bond strength (to unprecedented rates of over 30 MPa). 5

These benefits are largely due to the addition of the 10-MDP (methacryloyloxy-decyl-dihydrogen-phosphate) monomer. This hydrophobic material eliminates many of the moisture-related issues in previous generations, particularly the incredibly hydrophilic nature of seventh-generation materials. Because of 10-MDP, universal adhesives have much broader applications than prior generations.

“I think there’s a place for universals, because there are situations where you might not want to use other generations with different techniques such as self-etch,” says Lineberry. “In short, the universals can save you a lot of time while working with many different materials.”

They also come in handy for situations where isolation is difficult in indirect restorations.

“I have less time with good isolation when bonding indirect and that I don't want too thick of an adhesive layer when bonding indirect,” Lawson says. “I generally turn to a single-bottle universal adhesive or one-bottle self-etch primer when bonding indirect restorations.”

Dental adhesives have come a long way, and there’s still room for multiple generations in any practice. Different indications call for different options, so dentists shouldn’t marry themselves to one generation exclusively. Despite new developments, traditional fourth- and fifth-generation materials still remain incredibly popular. Sixth and seventh generations have opened new doors in bonding. And universals, well, the possibilities are endless. Whichever generation you prefer, keep an eye out for the latest developments in bond strength, simplicity and efficacy. We’re sure to see even more options in the years to come.

1. Buonocore, M. G. (1955). A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces. 

2 Sofan, E., Sofan, A., Palaia, G., Tenore, G., Romeo, U., & Migliau, G. (2017). Classification review of dental adhesive systems: from the IV generation to the universal type. 

3. Tao L, Pashely DH, Boyd L. (1988). Effect of different types of smear layers on dentin and enamel shear bond strengths. 

4. De Munck J, Van Landuyt K, Peumans M. (2005). A critical review of the durability of adhesion to tooth tissue: methods and results. 

5. Kamble, S. S., Kandasamy, B., Thillaigovindan, R., Goyal, N. K., Talukdar, P., Seal, M. (2015). In vitro Comparative Evaluation of Tensile Bond Strength of 6(th), 7(th) and 8(th) Generation Dentin Bonding Agents. 

Journal of international oral health: JIOH

Unpacking the different iterations of adhesives and what they bring to the table, from first-generation materials to universals.

Dental adhesives explained

A “one-stop-shop, magic bullet for all your dental adhesive needs,” was the promise heralded with the introduction of universal adhesives. Although these versatile adhesives can bond to a wide variety of materials-including glass ceramics, zirconia, lithium disilicate, and composites—are they really the right choice for every situation? Are they, in essence, truly universal?

They certainly seem to be at first glance. Since universal adhesives will bond to so many different materials, it gives them broader applications than previous generations of bonding agents. Generally defined as one-bottle systems that don’t require mixing, they are an etchant, primer, and bonding agent all in one. They can be used with self-etch to total-etch protocols on dentin or enamel, and are compatible with light-, self-, or dual-cure composites. In short, they are a comprehensive product that offers convenient solutions that simplify procedures while saving time and money-all without compromising on bond strength.

“The newer universals are definitely less technique sensitive and they really set the stage for making procedures simpler,” says Dr. Jeff Lineberry, a general dentist practicing in Mooresville, North Carolina. “But you should always consider the clinical indications before making a choice.”

"For me, I’ve always said I believe that the best approach is a tailored approach to each clinical situation,” says Dr. Jason Goodchild, director of clinical affairs at Premier Dental. “For instance, if there’s enamel, I want to etch it. If it is dentin, I usually don’t want to etch it. So, having a product that can do everything is great. But I also [believe] a lot of dentists think ‘I’ve got to have multiple bonding agents,’ so you see dentists with a drawer full of bonding agents. And it’s not necessarily a bad thing to have different options for different indications.”

While previous generations of bonding agents also combined etching, priming, and bonding, universals were developed with a focus on increased bond strength. This is bolstered by the 10-MDP (methacryloyloxy-decyl-dihydrogen-phosphate) monomer, which serves as the primary adhesive in universals, as it is very hydrophobic. Hydrophobic materials are less susceptible to problems that arise from moisture—and, since water is a big contributor to bond failure, this gives universal adhesives a leg up over previous generations.

The 10-MDP also contributes to universals’ ability to bond to a wide range of materials. This versatility means they can be used in a variety of clinical situations, while also simplifying inventory. The ease of use provided by fewer bottles and less mixing is unparalleled, which leads to more streamlined procedures.

Because universals only require one application, they are also more resistant to contamination than previous generations. Systems like the fourth-generation offerings (which were two-bottle, total-etch systems) took longer due to the multiple steps required for application. This leads to an increased window for contamination that can lead to adhesive failure.

Essentially, universal adhesives offer increased bond strength-to numerous materials-with reduced procedure time. They are indisputably a great, efficient addition to any dentist’s arsenal. But while universal adhesives have many applications, are they really universally indicated?

“I don’t think there’s a particular set situation where you couldn’t use a universal, but there are things to take into consideration,” Dr. Lineberry says. “For instance, when I’m bonding veneers in place, I don’t want something that’s going to be thick, which can happen with some universals. So, I’m very conscientious of the overall thickness and the viscosity of the material. That’s really one thing that I think about.”

“I also avoid universals when I’m bonding temporary veneers in place,” Dr. Lineberry continues. “One technique we use is spot etch, so some people will coat the preps with an older-generation bonding agent that obviously doesn’t have self etchant in it.”

Clinicians should always take etching into account when deciding on universal adhesives. Several studies have reported that, when bonding to enamel, universal adhesives have better bond strength when first etched with phosphoric acid.

 Since universals are less acidic and have a lower pH than prior generations, they struggle to provide the necessary surface texture when bonding with enamel. Thus, researchers have concluded that universal adhesives are most effective when using a total-etch technique with phosphoric acid.

 Additionally, most single-bottle systems (both total-etch or self-etch have been reported to have lower clinical success than two-bottle systems that possess a separate hydrophobic bonding agent.

This shouldn’t discourage clinicians from utilizing universal adhesives, however.

“Generally, I’m in support of universals-I’ve used Scotchbond Universal in my practice for the past three years with good results and there are a lot of other great products on the market—so there’s definitely a time and a place for them,” says Dr. Lineberry.

But there’s also a place for other generations of bonding agents.

“I don’t think I’d get rid of previous generations because some of these are tried and true,” Dr. Lineberry says. “They have a proven track record of how well they work. I’ve got patients that have composites that were placed with an older generation composite or bonding agent that are 15, 18, 20 years old, and still doing well. So, as the old saying goes, newer is not always better. We’re seeing good, promising results with the universals, but it’s not like we have a lot of history with them.”

Dr. Goodchild agrees, and also adds that the efficacy of a product varies from practitioner to practitioner-and there’s no one-size-fits-everyone product.
	
“When considering which system is most effective, it’s definitely time and place,” Dr. Goodchild says. “But I also feel like dentists are creatures of habit and subscribe to the ‘what works best in my hands’ argument. So, people will say, ‘Well, I’ve been using this for 5 or 10 or 20 years, so I know it’s good.’ Well, it doesn’t mean it’s good, it just means it’s worked for you-it could work differently in someone else’s hands.”

It may be easy to just reach for the universal adhesive when doing a procedure-after all, they’re easy to use, speed up procedure time and get you on to your next patient quicker. But because they are so simple, it can be easier to make errors.

“It’s definitely easier to get caught on autopilot with universals,” Dr. Lineberry says. “They simplify techniques, so you can become complacent about some of these things for sure.”

It’s also paramount to remember that not every universal is the same. There are many different formulations of universal adhesives-some still recommend using a dedicated primer or separate activator, for example—so it’s important to know how the product you are using functions.

“I don’t see any major significant clinical drawbacks with universals,” Dr. Lineberry says. “The key is to just know your product and what works. For instance, I’ve found that the product I prefer doesn’t bond well with all dual-cure buildup and core materials. So, you need to know that; you need to know that you need to mix it with a dual-cure agent or activator. If you don’t, obviously it’s not going to bond well. So, that’s one important part. You need to know those things because sometimes people just assume it’s a universal bond so it just sticks to everything, but that’s not always the case.”

Not only is it important to know what works and what doesn’t, you need to know how to implement it properly. Reading the manufacturers’ specific guidelines for a particular product can mean the difference between bond success and the patient being back in your chair in short order.

“I always fall back on following directions,” Dr. Goodchild says. “There are a lot of smart chemists working on this technology, and to think that we then, as users, can know better than the chemists who designed the product-well, we really need to follow directions to get the best outcome.”

When it comes to the bottom line, universal adhesives have shown great results, but may not be right for every situation. However, if instructions are followed, indications understood and proper implementation completed, they can provide universally successful results.

		De Goes MF, Shinohara MS, Freitas MS. Performance of a new one-step multi-mode adhesive on etched vs non-etched enamel on bond strength and interfacial morphology. J Adhes Dent. 2014;16:243–250.

		Lawson NC, Robles A, Fu CC, Lin CP, Sawlani K, Burgess JO. Two-year clinical trial of a universal adhesive in total-etch and self-etch mode in non-carious cervical lesions. J Dent. 2015;43:1229–1234.

		Peumans M, De Munck J, Mine A, Van Meerbeek B. Clinical effectiveness of contemporary adhesives for the restoration of non-carious cervical lesions. A systematic review. Dent Mater. 2014;30:1089–1103.

Universal adhesives may not be indicated for all situations.


Are universal adhesives truly universal?

Every dentist wants to create a bond that won’t fail—a strong bond that will last for decades and not cause any short-term post-op sensitivity or long-term problems.

While there are many factors-such as microleakage risks, bond durability, and degradation-that play into adhesive success, it goes without saying that bond strength is a major contributing factor in cement and adhesive selection. But are there problems that arise from a strong bond? In today’s world, can a cement create a bond that is, in fact, too strong?

The answer? Well, that depends, according to Dr. Jason Goodchild, director of clinical affairs at Premier Dental.

“When it comes to permanent cements for indirect restorations, I don’t know that we have a cement that is too strong now,” Dr. Goodchild says. “I think that the bonds you can generate with currently available products probably don’t qualify as too strong. However, on the direct side, you could probably make a claim around a bond being too strong. Some of the bonds generated on the direct side can be stronger than the actual tooth.”

The biggest bond-strength concerns usually come into play with direct restorations.

“One of the things people get concerned about when it comes to bond strength with direct restorations is shrinkage,” says Dr. Jeff Lineberry, a general dentist practicing in Mooresville, North Carolina. “We get composites and resins, and due to the way they’re placed, we get too much volumetric shrinkage. And then you get cuspal deflection and then of course in some cases you can induce some fractures. So, that’s a reality we have to watch out for.”

As a composite is light cured in a direct restoration, it contracts and shrinks, putting tension on the bond and surrounding tooth. In resin restorations, it’s common to have polymerization shrinkage of up to six percent.1 This shrinkage can result in debonding, marginal gaps and staining, cuspal movement, and microcracking of the restoration-or more critically, microcracking of the tooth. As a composite shrinks under the curing light, it pulls away from the tooth, putting so much pressure on the tooth substrate that it, instead of the bond, can crack. 

“The problem is when the bond is stronger than the tooth,” Dr. Goodchild says. “The bond between parts of your teeth may be 16-18 Mpa, but we can generate bonds that are over 30 MPa, which means you have two things happening: You have a tooth trying to hold itself together and a filling trying to bond to the tooth. It can create shrinkage stress and the whole phenomenon around post-operative sensitivity. You really do have to be concerned about polymerization shrinkage, as microcracking can become a real problem.”

The big risk with indirect restorations is that the bond might be stronger than the tooth itself. When researchers test bond strength, they often use a test known as macro-strength bond strength (SBS) testing. This test bonds two materials (such as a tooth and a crown) with an adhesive and loads them in shear until it fractures. One troubling thing researchers have discovered from this testing is that the tooth substrate will sometimes fracture before the adhesive.

While having a strong bond in an indirect restoration is key to longevity, it shouldn’t come at the expense of the tooth. If, under stress, the tooth substrate fractures before the adhesive, you’ve got even bigger problems. So, do you need to be overly concerned when selecting a product that boasts high bond strength? Not really, says Dr. Goodchild.

“When it comes to the indirect side, I’m not sure we have cements that are too strong,” Dr. Goodchild says. “There are failure risks with any product.”

Related Reading: How bond strengths are measured

While a cement itself may not be too strong, other things can contribute to potential bonding issues.

“There are other things to take into account, beyond just the cement,” Dr. Goodchild says. “For example, when you consider retentiveness of a final crown, you also have to consider the prep. If you have a really long prep and a really parallel prep, and you cement it with resin cement, obviously that thing is going to be a bear to get off-you’re never getting that off without cracking it to pieces.”

“I do think when we start talking bonding indirect restorations in place, there’s a fine line of what’s enough and what’s too much,” Dr. Lineberry adds. “I think that’s where you have to take into [account] retention resistance and being able to remove it if you need to.”	

It’s also important to take into consider bond degradation when talking about bond strength. Naturally, a bond will lose strength over time, so it’s a balancing act to create a bond that will last for years but not jeopardize the tooth. While a bond may be extremely strong initially, it may not be as durable down the road.

“Obviously, bonds degrade over time,” Dr. Goodchild explains. “No bonds stay stable forever. So, let’s start as high as we can, so that as we degrade, it’ll still be good for a long period of time-rather than start at a moderate level and just hope it stays that way. So, that’s always been the thinking is that we want to start as high as we can, knowing that the bond degrades over time.”

Dr. Lineberry agrees but takes a more nuanced approach to the compromises between strength and degradation.

“The better question is, are we more concerned about bond strength or bond durability? To me, bond durability is key,” Dr. Lineberry says. “You get these matrix protease inhibitors where we get matrix proteins that are breaking down the bonds. So, if you’ve got a bond that’s 100 MPa but in six months you retest it and it’s down to 1 MPa, that doesn’t do you much good. Now, if it’s at 20 MPa and it’s still well bonded in 10 years, that’s kind of really what we’re looking for.”

In the end, a successful, stable bond is more important than sheer strength.

“I don’t think of a bond as too strong,” Dr. Lineberry concludes. “But there are things you need to take into consideration when bonding to ensure long-term success.”

	Soarees, CJ, Faria-e-silva, AL, Rodrigues, MP, Vilela, AB, Pfeifer, CS, Tantbirjon, D, & Versluis, A. Polymerization shrinkage stress of composite resins and resin cements – What do we need to know? Brazilian Oral Research, (2017). 31(Suppl. 1), e62.

Everyone wants a strong bond, but is can there be too much of a good thing?


Can a bond be too strong?

	Finding the best way to achieve strong bonds on tooth structure is a continuously winding road dating back decades. The solution for adhesion to enamel came in 1955, when Michael Buonocore, DDS, discovered that acid etching enamel with 85 percent phosphoric acid improved acrylic bond retention.

	While enamel bonding didn’t become widely used until approximately two decades later, it revolutionized restorative dentistry. Today, phosphoric acid is still widely used (in 35-40 percent concentrations) to etch enamel to enable retention of resin-based adhesives. With just a few simple steps, a reliable enamel bond can be created.

	“You can clearly say that bonding to enamel is easier than bonding to dentin,” Dr. Jason Goodchild, director of clinical affairs at Premier Dental, says. “We’ve known that since the 1950s-that the best bond occurs on enamel.”

	But what about adhesion to dentin? This has been a greater challenge and has proven to be more difficult and less predictable.

	“Bonding is so technique sensitive, and enamel and dentin are two very different surfaces,” Dr. Goodchild says. “Enamel is highly inorganic and has very little organic material. Dentin is 30 percent collagen and 20 percent water. This makes dentin a hydrated tissue, so you’re trying to bond-and bonding is hydrophobic-to a watery substance, which is inherently difficult.”

	While achieving reliable bonds on dentin can be daunting, understanding the differences between enamel and dentin-and the challenges they create-can assist clinician’s in successfully executing consistent results.

	While enamel is composed of approximately 92 percent inorganic hydroxyapatite, dentin is only around 50 percent inorganic.

 Dentin’s structure is also more complex than that of enamel. Dentin contains fluid-filled tubules, and the hydroxyapatite crystals in dentin are not regularly arranged (as in enamel) but are random in distribution. In addition to being more complex in structure than enamel, dentin’s composition-and subsequently, its peremeability-varies by the depth and region of the tooth. These variations make dentin must less predictable and uniform than enamel, thus making it harder to ensure reliable bonds.

	The success of a dentin bond relies heavily on the successful creation of the hybrid layer. The hybrid layer is the highly organic interface between the dentin substrate and the adhesive resin, a hydrophobic, acid-resistant collagen network that is exposed by etching.

 To create the hybrid layer, mineral components in the dentin are replaced by resin monomers, forming a polymer-collagen biocomposite. Dentin bonding can be achieved with etch-and-rinse or self-etch adhesives, but both techniques share a common goal: To remove the smear layer and minerals to expose the collagen network of the dentin, creating an avenue for the adhesive resin to infiltrate the collagen matrix and form the base of adhesion for the restoration.

	Removing the smear layer is a particularly important step for forming a hybrid layer, as it can fill the dentinal tubules and form smear plugs, decreasing dentin permeability by up to 90 percent.

 This can prevent the adhesive resin from permeating into the dentinal tubules, posing a threat to bonding.

	However, even with effective smear-layer removal and hybrid-layer formation, variations in dentin structure, dentinal tubule permeability and moisture can affect bonding, making achieving uniform bonds a challenging endeavor. These physiological variables, combined with limited procedural time, varying polymerization rates, and biodegradation of resin-dentin interfaces can lead to unpredictable results.

	“Whether we are talking about fillings or restorations on the direct side, or crowns on the indirect side, most of that bonding is into dentin, which creates challenges,” Dr. Goodchild says. “Enamel is easy-you etch enamel, you create an etching pattern, and then you have resin microtags and it locks itself in. That’s pretty simple.

	“But dentin is not so easy,” he continues. “The challenges of dentin include: Do we etch or not? How long do we etch for? What level of dryness or wetness does the dentin need? Plus, application times-is it one coat or multiple coats? There are so many factors involved in that process. There are so many things to consider.”

	With four categories of dentin adhesives, there are indeed many factors to take into account. Falling under the etch-and-rinse and self-etch strategies, the categories include three-step etch-and-rinse; one-bottle etch-and-rinse; self-etch primers; and self-etch (or all-in-one, universal adhesives).

A closer look at what makes dentin bonding so much more complex than enamel bonding.