Hole Truths

Many of today’s dental restorative materials claim characteristics such as low-shrinkage, bulk-fill and flowable capabilities, and bioactivity. But what does it all mean? We explore these classifications and detail how they work.

Restorative dentistry has come a long way from the days when a silver filling was a silver bullet for all sorts of dental catastrophes. Today, scores of new restorative materials are being used in direct restorations, not only to fill cavities but in some cases to actually rebuild tooth structure. And the innovations show no signs of slowing down.

But before you can comprehend the issues surrounding material characteristics, it’s important to understand the materials themselves. A number of modern-day materials are various iterations of composite resins. Other materials such as glass ionomers (GIs), resin-modified glass ionomers, and proprietary preparations are also popular for a variety of applications.

A TOUCH OF GLASS

GIs have been used in restorative dentistry since the 1970s and have undergone a number of evolutions. They are employed as low-viscosity luting cements or high-viscosity restoratives for buildups, and provisional and permanent restorations in both primary and permanent teeth. Typically composed of a water-soluble polyalkenoic acid, strontium or calcium, and fluoroaluminosilicate (FAS) glass powder, GIs release fluoride ions, which helps to foster remineralization of tooth structure.

GIs offer scores of other benefits, including low cost, minimal shrinkage, a coefficient of thermal expansion that nearly matches that of natural dentition, and the ability to chemically adhere to tooth structure. But, on the downside, GIs do not fully cure for up to a week, leaving them vulnerable to water absorption. They are also known for problems with retention, and for lacking polishability and compressive strength when compared to composite resin. For these reasons, methacrylate-based resin was added to GIs in the 1990s. Resin-modified glass ionomers (RMGIs) are light-curable, which addresses the risk of washing out by taking up water. They also boast improved strength and esthetics, while continuing to offer fluoride release.

Some experts believe that GIs have seen their heyday in the U.S. Chicago-based practitioner Lou Graham, DDS — an internationally recognized author, lecturer, and founder of the Catapult Group, a continuing dental education company — however, would disagree. “GIs are alive and well,” he says, adding that a number of manufacturers continue to up the game of GI cements, which also have a role in direct restorative dentistry.

For example, according to prolific author and lecturer Gregori Kurtzman, DDS, a practitioner based in Silver Spring, Maryland, “Some of the companies that have offered GIs for decades have continued research into this class of restorative material.” The result of this, he says, is better durability and wear resistance.

Giomers are another type of material related to this category, but also somewhat in a class of their own. These materials comprise a resin matrix that contains surface pre-reacted glass (S-PRG) fillers, said not only to increase strength and wear characteristics, but also to support enhanced ionic exchange and improved esthetics. Giomers are said to help counteract bacteria, especially in areas next to the gumline. But while giomers are related to GIs, they also owe some of their lineage to composite resins.

THE COMPOSITE CLASS

Composite resin materials used in dentistry have solved numerous problems encountered in placing direct restorations with other types of materials. Because they adhere to tooth structure via chemical reaction — often through the use of etchants and bonding agents — they do not require excessive amounts of tooth structure removal, unlike amalgam. They also beat the heck out of amalgam in the esthetics department in that they are tooth colored. And GIs and RMGIs can’t match their strength, translucency and polishability. But they can be technique sensitive and suffer from physical characteristics that can prove challenging.

When it comes to composite resins, Robert Lowe, DDS, a widely recognized authority on materials and a practitioner based in Charlotte, North Carolina, stresses the importance of following manufacturer instructions. “Composites cannot be placed like amalgam,” he says, “but that’s how many clinicians tend to place them, because they are both direct filling materials. It’s important to read instructions and follow proper protocol.”

Composite resin has become a “go-to” restorative material in countless practices, but Lowe says that where composites are concerned, the tide is turning. “We are continuing to come up with ‘newer, sharper razors,’ and really pushing the envelope on composite materials. However, bioactive dental materials is an area where, in my opinion, more focus needs to be placed.”

Nonetheless, Graham notes that advancements in composites, in general, continue. For instance, he states that one unique launch this past year was a methacrylate-free composite formulation. The addition of low-shrink formulations and other products such as giomers are also unique in this category.

In Kurtzman’s opinion, composite resins are becoming stronger, with manufacturers focusing on flexural strength versus compressive strength. “Tooth structure under load (functioning) microflexes,” he explains. “This microflexure is what will break down the bond with the underlying tooth structure over time. Due to an increase in flexural strength, the resin is able to bend more before it fractures, and is able to bend as the tooth does to better maintain the bond.” But he adds that the ultimate goal is to develop a resin with zero shrinkage.

Graham agrees, adding, that proper isolation, etching, bonding, air drying, light curing, and utilizing the best materials based on the clinical situations are all essential. “But composites truly are challenging, and issues of shrinkage, incomplete curing and microleaking all continue today. However, an excellent dentist can make the majority of materials work well in their hands, and that is the key,” he notes.


Point of Sale | Filling the Void

  • Materials that offer low shrinkage help to ensure that restorations won’t fail due to marginal leakage.
  • Bulk-fill materials help clinicians save time and lessen the chance for technical mishaps.
  • Flowable materials are better able to adapt to tooth anatomy, which increases the odds for restorative success.
  • Glass ionomers remain popular in some camps for their ability to release fluoride ions.
  • Bioactive materials support the concept of minimally invasive dentistry by repairing carious lesions rather than simply plugging a hole.

Filling in the blanks

Physical characteristics of today’s composite resins can be influenced by filler load and through chemical modifications. Both tweaks can lessen polymerization shrinkage. This is perhaps most evident in bulk-fill materials, which can range from highly filled, high-viscosity packable formulations to lesser filled, low-viscosity flowables. “Bulk-fill composites cut down on the number of steps involved in placing restorations, saving time and possibly reducing issues related to technique sensitivity,” says Lowe. He adds that formulations with low-shrinkage properties can help eliminate deleterious effects related to polymerization shrinkage, such as marginal leakage. The combined characteristics of this type of material can add great value.

“Bulk fill isn’t a new concept, but there has been greater demand from practitioners to be able to simplify completing restorations through bulk filling. And manufacturers have listened,” says Kurtzman. He explains that incremental placement and curing of material was long advocated in a deep restoration to lower material shrinkage. “But,” he says, “as materials have been developed that offer lower shrinkage, it’s been possible to apply this to bulk-fill materials.”

Kurtzman notes that the other aspect affecting bulk-fill materials is depth of cure. “A curing light at best will only cure to a depth of 4 mm,” he explains. “So when a proximal box is part of the preparation, the depth from the bottom of the box to the top of the occlusal surface may be 6 mm or even 10 mm, or more. Using a material that is light-cure only limits when a material can be used for these situations and still may require the restoration be placed in two or more increments to properly cure all of the resin. Dual-cure resins have overcome these factors, allowing placement into any depth. Light is used to cure the top 4 mm and the remainder self-cures over a 3- to 4-minute period, ensuring complete curing of the bulk-placed resin.”

Kurtzman reports that a drawback to dual-cure resins involves getting them to flow out of the automix tips. “They do not have the same body as light-cure resins prior to setting. This means that anatomy cannot be fully placed prior to setting of the resins, as can be accomplished with light-cure resins.”

Flowable, or low-viscosity composite resin is often used in lining cavities because of its ability to flow into all the nooks and crannies of tooth structure, often via injection. This would be tough to accomplish with packable material. Flowables are typically used in nonload-bearing locations, such as the anterior, and in tough-to-reach areas. Unfortunately, flowables have tended to suffer the most from shrinkage, due to their light filler loads.

Kurtzman, however, says that flowable composites have become more durable, allowing them to be used in more clinical situations, especially when a surface will be under wearing forces. He anticipates further innovations will include the addition of self-adhesive properties into these materials, as well as an infinite cure to permit a true bulk fill without the potential for gaps at the margins.

BIOACTIVE FORMULATIONS

But perhaps the need for deep restorations will eventually become a thing of the past with the advent of bioactive materials. How the term “bioactive” is applied to dental materials is still up for debate. For instance, some do not view GIs as bioactive, even though they initiate ongoing fluoride ion exchange. For them, a material must precipitate apatite, the primary mineral that makes up tooth structure, in order be classified as such.

Caries is a bacterially based transmissible disease, so it makes sense that the best way to fill cavities in enamel and dentin is with a remineralizing material versus a space-filling inert material. “The key to long-term success involves minimizing bacterial issues and promoting remineralization of tooth structure,” notes Graham. In fact, he anticipates bioactivity playing increasingly greater roles in all sorts of dental restorative materials. “The term ‘bioactive glass,’ I am careful about,” Graham says, “because a lot of materials claim such. The more important question is whether there is true bioactivity.” He cites a number of products that are offering important advancements, such as demonstrating remineralization. “To me,” Graham adds, “this is the future.”

Indeed, research and development in the area of bioactive dental materials is full steam ahead. “Bioactive materials are a growing area of restorative materials as manufacturers seek those that can, in a sense, help teeth to ‘heal’ themselves when acidic attacks present that will break down restorative margins and lead to restorative failures,” says Kurtzman.

And while Lowe says he does not think that one magic material exists, he, too, believes that bioactive materials are the future. “They don’t just fill a cavity but actively work to repair tooth structure,” he explains. Lowe relates that today’s definition of ”bioactive” needs to include precipitation of apatite crystals in the presence of moisture. “These formulations release not only fluoride but also calcium and phosphate,” he says, predicting that further advances in these materials will improve the efficacy of the product, the speed with which they can be placed, and the esthetic result.


LEXICON

Coefficient of thermal expansion: The degree to which a structure expands and contracts in response to thermal stimuli.
Luting cements: Used to attach prosthetics such as crowns to tooth structure.
Proximal: Surfaces of teeth that are adjacent to neighboring teeth.
Proximal box: A preparation for filling that includes the frontand back tooth surfaces.


Says Graham, “There are lots of exciting changes going on and one can expect the following: Greater roles for antimicrobials embedded into bonding agents, liners, bases and composites, and greater roles for bioactivity in all of the above along with cements, root canal sealers and more.”

Kurtzman says he foresees further gains ahead for direct restoratives. “I think there will be advancements in the physical properties of these materials to make them more toothlike and bioactive in how they handle future acidic insults to the restorative margins.”

Each proprietary bioactive material available offers a unique formulation that puts it in a class of its own. “The use of some of these materials,” says Lowe, “might be called ‘hero-adontics,’ as in some cases, they can provide a ‘last-ditch’ way of prolonging the ‘life’ of the tooth. And they might prove the best answer for patients who ‘don’t know which end of the toothbrush has the bristles.’”

With the continuing parade of newer, smarter materials, restorations are getting quicker, easier, more predictable and less expensive than the sometimes hours-long drill sessions of the past. And such efficient and minimally invasive hole-filling, -repairing and -prevention measures are good for your customers, their customers, and everyone’s bottom line. And that’s the truth.

 

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Featured photo by STEPHANHOEROLD/E+/GETTY IMAGES PLUS

From MENTOR. October 2017;8(10): 32-34.

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