The evolution of dental materials

March 14, 2018

Examining the past, present and future of composites.

Most dental professionals would agree that the use of composites and adhesive materials have changed the dental practice. But as technology develops and results in new materials, futuristic possibilities arise that could change medicine. Could dental materials one day be more than a Band-Aid for the tooth? Glass ionomers release fluoride to strengthen the health of the teeth, but perhaps future materials will go far beyond, using the platelet-rich fibrin that is mostly used in implant cases to encourage regrowth for decayed teeth. One day, dental filler materials could even produce antibiotics for the oral cavity, as one professional I interviewed suggested.

Before we get too far ahead of ourselves, let’s review where dental restorative materials have been, where they’re heading now and where they could end up in the future.

A shift from rudimentary materials to resin composites and glass ionomers

Rudimentary restorative materials, such as amalgams, have been commonly used since the 1800s, although their use has been documented to go as far back as A.D. 659, when they were used during the Tang Dynasty in China.

To the relief of dental professionals everywhere, amalgam restorations met some competition when resin composites arrived on the scene in the 1960s.

There were a few kinks to work out in terms of looks and durability, and once microfilled composites came out around 20 years later, they were much more successful, but they were also technique sensitive. By the early 2000s, resin composites had been drastically improved and were suitable for long-term use in posterior restorations.
Unfortunately, the first decade of the 2000s brought little improvement to the resins that hold the fillers together, says Larry Clark, director of clinical affairs and marketing at Pulpdent.

In fact, he argues that it was only in the last eight or nine years that resins started to undergo improvements.

“Before the resins started to improve, it was kind of like cement,” Clark says. “The rain would come and wash away the cement, and you’d get pits and holes in your roads and sidewalks. Composites in the ’70s and early ’80s were failing like that. Now, the resins are more durable and the fillers come in all shapes, sizes and dimensions.”

One major benefit of composite resins and glass ionomers (and the combinations in between), aside from nicer looking teeth and happier patients, is that those materials have inspired innovation in the field of restorative dentistry. Today, restorative materials have evolved to be “more esthetic and wear resistant, easy to use and place, feature superior wear resistance and more accurately match natural dentition,” says Brendan Steidle, product marketing manager at GC America. “Typically, the dentist and patient want something that matches natural dentition so as not to indicate that there were any caries or damage to the tooth. So, anything that helps the restoration look more like natural teeth is something that both the dentist and the patient are going to be interested in. That’s where the industry has been going - dentists want that combination of esthetics and strength. Resin composites are able to offer both of those and blend in with the natural dentition, so it doesn’t stand out.”

The improvements made to resins in the last decade include the ability to bond to the fillers so that they don’t come out of the tooth. “After that, it’s about controlling shrinkage and being able to have less water absorption,” Clark says.

“In the last few years, smarter filler compositions have significantly impacted handling, the gloss properties and the wear resistance of the materials,” adds Melanie Dietrich, restorative business unit manager at COLTENE.
The solution to the finicky restorations of the past was twofold: to improve the physical properties of the fillers and adhesives and to identify the best techniques for use, says John Fundingsland, professional services manager at 3M Oral Care.

“Over the last decades, composites have developed immensely and are today’s material of choice in minimally invasive, direct restorative therapy,” Dietrich adds.

While there are many factors that determine the long-term health of the restoration, they are theoretically designed to last a lifetime, according to Steidle.

“Dental professionals really like resin composites because of that adaptability and esthetic,” he says. “Over time, resins tend to shrink, but technological advances in development of composites provides less polymerization shrinkage over time. So, you have better adaptation at placement, and then when you have a recall a year later or even five years later, these restorations often look as good as when they were placed.”

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A trend toward simplification

Now that restorative materials have improved to be stronger and more esthetic than ever, the industry is experiencing what many call a trend toward simplification.

“If you look back to the late ’90s through the early 2000s, a lot of companies were launching comprehensive shading systems in terms of composite technology,” says Danny Vezmar, director of marketing for restorative and preventive at Dentsply Sirona, pointing to systems with up to 25 or 36 shades. “But the trend we’ve seen is toward simplification in the office. We’re finding that with today’s filler and resin technologies, you can condense the number of shades that a doctor has to worry about. Now, one shade of material may cover multiple VITA shades.”

According to Dietrich, manufacturers and general practitioners have realized that they can get quality esthetics with fewer shades, so the trend to simplify continues.

A focus on handling

Because most products available advertise esthetics, strength and clinical performance, there has also been a shift toward focusing on how the material handles in the dentist’s hands - its workability, as it’s often called.

“People would tend to have conversations like, I’ve got 3 MPa higher fracture toughness than somebody next to me,” Vezmar says. “At some point, you reach a point that it’s not clinically relevant to have those conversations anymore because everybody’s putting materials out that provide really good physical properties in terms of strength.

So, there is a focus now on what’s important to the dentist when they’re applying the material: how it feels in the hands, if it’s sticky, and its adaptability to the tooth in terms of shading and manipulation.”

Since dentists can now feel comfortable with the clinical performance of most of the restorative materials on the market, Vezmar says they’re looking more into which materials feel the best to work with. And that’s something that marketing teams like Vezmar’s are trying to tap into.

“We still go through all of the physical property testing to confirm that we are where we need to be from a clinical performance perspective,” he explains. “But the conversation about the materials is shifting toward what’s important to the dentist in terms of application, how easy it is to place, if it sticks to the instrument, if it stays where it’s placed without slumping, if it can be carved easily, what the final result looks like, if it polishes nicely and maintains that polish over time, and if it avoids staining."

Developing a new product

When developing a new product, every company we spoke to said it considered the needs of the dentist foremost.

“The one characteristic dentists want most often is esthetics,” Steidle says. Next on the list include ease of use, radiopacity, the abilityto match to dentition, durability and wear resistance.

“Clinical performance is always going to be one of the No. 1 things dentists are looking for, but it’s also about how it feels when they’re placing it,” Vezmar says. He adds that the questions posed above in terms of polish, adaptability and shading are what drives the Dentsply Sirona portfolio.

“A product also must fit into the clinician’s protocol,” Fundingsland says. “You can have the most innovative technology in the world and not have a successful product if the technique is not accepted by the clinical team.”

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What makes a restoration fail

If a dentist has a problem, nine times out of 10 it’s because he or she didn’t follow the specific technique advised by the manufacturer of the materials. That’s why it’s important that dentists continue to educate themselves about the materials available and the techniques required, Clark says.

“If they would just read the instructions for use, we’d all be better off,” he says. “I’m very blunt about that in my lectures and how we train people. All the products are really good, so the problem isn’t the product, for the most part. It’s just how they’re placed. The variables that take place in the operatory dictate the success of the composite, in my opinion. And 99 percent of the variables exist with the clinician.”

Clark admits that the whole reason variables in the procedures exist to cause problems is the fault of the manufacturers who created specific technique guidelines to overcome sensitivity. That was before they realized that having a dentist complete two or three steps before doing the bonding could negatively affect the outcome of the bond.

On top of the variables, there are a litany of other issues that determine the success of a bond, including how the tooth is prepared and the adhesive itself.

Clark says that the success of the restoration - in terms of what the patient should be concerned about - is often challenged by the leakage of bonding agents. Once bacteria make their way into the restoration, failure is a near certainty.

“There has always been an assumption that because you had bonding, you had a seal,” he says. But that was not true, especially in dentin. With enamel, you get a better bond and seal. It’s the most dependable substance you can bond to in the tooth. Dentin, on the other hand, is filled with lots of water coming from inside the tooth. That water can be detrimental to the adhesive layer as much as anything else.”

Water will cause the adhesive to break down over time. And despite the fact that the restoration will still be intact, there will be leakage underneath it.

“That’s what David Paschley and Franklin Tay would call ‘water trees,’” Clark explains. “As the dentinal fluids come up, they permeate into the hybrid layer and start to degrade it with their fluids, ultimately breaking it down.”

He says that Pulpdent tries to educate dentists on these challenges to make their restorations more successful.

“One of the things that’s made me successful was that I not only knew the materials, I knew how to use them,” he says. “I think that’s more what the industry needs: more hands-on courses and less opinion.”

What makes a restoration successful

To get restoratives right from the get-go, they should offer high levels of workability.

"Dentists don’t want to invest time to make a composite material work - the material needs to work for them,” Dietrich adds. “A successful material delivers exactly that. Despite all the filler evolution, we still see that ideal handling properties are a prerequisite for a successful composite.”

“You must be able to handle the composite in order to place it successfully,” Fundingsland says. “That being said, composites have to meet standards of esthetics, physical properties and wear to be successful. Marginal seal is critical to preventing secondary decay.”

The ultimate test for any material, Vezmar says, is clinical performance.

“Materials all have unique properties, but the true test is what a material placed today will look like at a six-month recall or five years later,” he says. “How does it stand up over time? You have to consider the factors that are important to the clinician when they’re placing it - handling, polish and shade selections - but then you also have to consider the longevity.”

“We think you can only measure success of a composite restoration by virtue of what happens with the patient, not by sales,” Clark adds. “Everybody likes to say those things, but if that’s the case, then why have we insisted on sticking with the same bonding techniques for the last 20 years? Those are the reasons failure is occurring.”

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The future of restorative materials

We’ve talked about the improvements that have been made to restorative materials so far, and the future is a lot of the same: simplifying the process and getting them to be easier to work with, more durable, more esthetic and more predictable.

“From a technical standpoint, I would anticipate continued efforts to reduce shrinkage and the resulting stress on the tooth that shrinkage causes,” Fundingsland says. “Improvement in strength could increase the indications for composites as well as their longevity. Esthetics already rival ceramics; in the future, esthetic results will be easier to attain.”

“I think continued efforts toward simplification of the systems,” Vezmar adds. “What we’re trying to focus on is not just the composite part of the procedure, but what the procedure looks like. What solutions can we provide to make the composite placement procedure more predictable? For example, we focus on our Class II solution, which implements everything from a sectional matrix system through the finishing system on the end just so we can have a consistent conversation and help that clinician have a predictable result, time after time.”

Getting a seal at the margins helps prevent secondary decay, but what about materials that can interact with the tooth structure? The ideal would be to have a material that could chemically bond to the tooth to strengthen it and inhibit further decay.

“Better bonding interfaces or discouraging the microbes responsible should help address secondary decay,” Fundingsland explains.

Biofunctionality

Getting restorations to be better at what they already do is just half the battle. On the other side of the coin is getting them to be biofunctional - a popular topic in restorative dentistry today and one that will be highlighted at the Nordic Institute of Dental Materials meeting this summer, where Clark will be presenting his topic on bioactive materials.

“These are researchers from all over the world and the whole focus is on the success and the future of dental materials,” Clark says. “The whole focus of this meeting is bioactivity. Bioactive materials arebecoming more of an interest in the research community as well as with manufacturers.”

“You need to have a more hydrophilic material to allow water or ions to transfer, or diffuse,” he continues. “You can’t have biochemistry without water. In the early days, when Pulpdent came up with the first hydrophilic resin in 2002, the idea was that the resin was more moisture-friendly, so we thought that would be a good thing from a technique standpoint, that it was OK if there was a little moisture there.”

It wasn’t easy to convince the dental community of their approach, however. Everyone was used to hydrophobic materials, which required a dry surface. Manufacturers have since realized that they can find a balance between hydrophilic and hydrophobic materials and will engineer materials to reflect that.

“Now, we understand that we need to think of the balance, especially if we try to introduce minerals into the resins, which Pulpdent and other companies in Europe are doing,” Clark says. “We’re starting to integrate calcium and phosphate into the resins. If they’re going to have continual ion release, like the tooth, then we need to make sure water can move in and out so there is a charge and recharge of these ions.”

Clark says that the importance of integrating biofunctional materials in the future is because the current process is based on repairing nature with plastics. He uses a slide in his lectures of a tree that was hit with lightning and repaired with cement as a metaphor for the current restorative protocol of repairing living tissue with materials it can’t identify with.

“That’s the importance of bioactivity for the future,” Clark says. “We’re trying to introduce minerals that the tooth can identify with and be a part of the process of the restoration rather than being totally ignored, which is what we’re currently doing. If you put a hydrophobic composite into a tooth, the tooth has no ability to identify with it.”

“We have to start thinking about composite materials that function more like the tooth,” he continues. “Enamel does one thing and dentin does a completely different thing, but that’s not what we’re doing. We’re restoring everything with something that’s more like enamel. When you look at composite restorations from a biofunctional standpoint, we’re not natural.”

For Clark and the team at Pulpdent, development won’t stop at bioactive or biomimetic materials. It will extend into biofunctionality - the ability of restorative materials to respond to forces the same way the tooth does. To him, the future of restorative materials lies in their ability to flex in the tooth.

Dentin is not just a shock-absorbing factor for the well-being of the tooth; it also knows how to give and take torsional aspects in biting forces, Clark says. “You’re never biting just up and down; you’re biting sideways. Teeth are twisting, cusps are turning. And when you put a block of hard, hydrophobic plastic in there, it becomes the tooth against the restoration. That’s where bonding agents have started to break down. It’s not just acids and bacteria and fluids that are eating away at them, it’s also the forces that are being attributed from natural mastication.”

He says biofunctionality can mean different things. Often, it’s synonymous with bioactivity, but for Clark and his team, it means the ability to get the material to mimic the function of the tooth.

A great example of one bioactive material with positive effects in the oral cavity is platelet-rich fibrin, or PRF. Studies on platelet-rich fibrin have shown its efficacy in revitalizing necrotic teeth. PRF is also commonly used along with freeze-dried bone in implant cases to regrow bone tissue.

“I think the implant landscape has changed drastically over the last five or 10 years,” says Paresh Patel, DDS, who was originally interviewed for a November 2017 piece on dental implants. “We have so many new biologics that have come on board. Certainly, mineralized freeze-dried bone or demineralized freeze-dried bone or combinations of those products are everywhere. Patient-driven biologics such as PRF are very easy to produce and the cost to get those products is almost minimal at this point.”

With those products, dentists can do guided tissue and bone regeneration to either regrow or preserve bone. This has gotten a lot of people in the industry excited about the possibilities.

“Remineralization and tooth repair over time could be a concept that you could look at in a next-generation composite,” Steidle says. “Those are some of the things on my wish list.”

Not everyone sees hope in bioactive restorative materials, however.

“While we do see great use for bioactive materials in root canal sealers, we see limited potential for this technology in filling materials since releasing bioactive components might weaken the matrix of the composite, compromising its mechanical properties,” Dietrich explains.

If researchers can discover a way for PRF to regrow tissue that was lost to decay without compromising the composite, then that would drastically change restorative dentistry.

Composites and adhesives have already revolutionized the dental practice by offering a more durable and esthetic option to amalgam restorations. Perhaps in the future they will revolutionize oral healthcare and medicine as a whole. It’s possible that it won’t just be about applying a sort of Band-Aid to a decayed tooth. It won’t even be about the bioactivity of releasing calcium and phosphate ions to remineralize teeth. Instead, it will be about introducing a material that immediately allows for full functionality of the tooth again while potentially regrowing the tissue lost to decay. Imagine the implications of perfecting tissue regrowth in the neck and crown of the tooth, seeing all signs of previous decay erased by the first recall appointment. Dentists are already using PRF to seal root canals and regrow tissue. Why not extend it to the rest of the tooth?