The Restorative Horizon

Material advances shaping the future of dentistry

Allison M. DiMatteo, MPS

Innovation on the microscale is driving change on the macroscale as science continues to push the boundaries of what is possible with direct restorations. Now, composite resins contain materials such as glass and zirconia that improve the strength and esthetics, and advances in chemistry have allowed for faster polymerization and bulk curing, increasing efficiency. And that's only the beginning.

Although almost all dentists place indirect restorations when indicated, in the majority of general dental practices, direct composite restorations remain the “bread and butter” and most-often-placed restorations. The marketplace for the materials used in these restorations is competitive, which explains why manufacturers have responded well to the needs and desires of practitioners, observes Jack Ferracane, PhD, professor and chair of the Department of Restorative Dentistry at OHSU School of Dentistry. Compared with the direct restoratives available 20, 10, and even 5 years ago, the numerous options available today are easier to handle, demonstrate nearly optimal esthetics, and have shown reasonable wear and mechanical properties.

For example, dental product manufacturers have acknowledged clinicians' needs for flowable composites that work in esthetic areas, where better blending and shade matching are required, says Marcos Vargas, BDB, DDS, MS,professor in the Department of Family Dentistryat the University of Iowa College of Dentistry. To meet these needs, they've developed flowable composites with improved translucency in multiple shades.

Essentially, what has changed in direct restoratives is the size and quantity of filler particles—they're smaller, as in nanosized, and they're present in a higher quantity, explains John Burgess, DDS,adjunct professor at the University of Alabama at Birmingham School of Dentistry. Generally, this means that the materials polish easier and are a little stronger and tougher than microfilled composites; however, those materials with greater volumes of nanoparticles are prone to problems with fracture toughness and will chip more readily.

The filler content in some flowable composites has been increased and the physical properties improved. Some of these “enhanced” flowables (eg, G-ænial™ Universal Flo, GC America, Inc.) have demonstrated similar clinical performance to traditional nanohybrid composites after two years when used to restore small Class I caries, says Gary Alex, DMD. He adds that certain flowables have physical and optical properties that make them suitable for bonding porcelain veneers and that many flowables can be used to repair the margins of old composite restorations as well as to touch up provisionals.

“Nowadays, we have resin composite materials that possess excellent physical properties, work reasonably well, and provide improved longevity, function, and esthetics,” notes Vargas. “Dentists have learned that these materials enable conservation of tooth structure, and they know that they can use them in many situations, such as Class I caries and resin veneers, with high levels of predictability and esthetics.”

In fact, modern direct restorative composites can be used to replace some indirect restorations (eg, inlays, onlays, partial crowns), notes Christian Plath, PhD, from VOCO GmbH,and full-coverage crowns fabricated from composite CAD/CAM blocks are readily available, demonstrating some advantages over conventional ceramics. All of these techniques are minimally invasive and can be performed in one appointment.

However, successfully placing direct restorations requires an understanding of the materials being utilized, as well as the substrate being bonded to, and that they be utilized in the appropriate clinical situation and according to a logical and precise clinical protocol, Alex observes. This is especially important for preventing postoperative sensitivity, which frustratingly, sometimes occurs even when dentists do everything correctly, because teeth do not always respond as expected.

“No direct restorative material or adhesive system can absolutely guarantee the complete elimination of postoperative sensitivity in every case,” Alex cautions. “However, there are steps that dentists can take to minimize postoperative sensitivity (See “Avoiding Postoperative Sensitivity”), and they should critically examine theirown technique to ensure that they are following guidelines for placement, curing, drying, and other aspects of the procedure.”

Additionally, although direct restorative techniques have not changed very much, the range of viscosities and handling properties provided by newer resin composites has enabled clinicians to treat a wider range of indications, observes Vargas. Traditionally, hybrid and microhybrid materials were sticky and slumped during manipulation, which did not facilitate esthetic freehand restorations. Today, in addition to the inclusion of nanofillers, new formulations of these materials have resulted in direct restoratives that are much less sticky and that do not slump during placement, allowing for more control and better sculpting.

Essentially, direct restorative composites always contain resin, filler, and a coupling agent to bind or connect the filler with the resin, explains Rainer Seemann, MBA, PhD, director of clinical affairs for Dentsply Sirona Restorative. Fillers are coated to promote connection with the resin. Other components influence the material's shade and polymerization (eg, chemical or photoinitiators).

“When designing a composite, you're actually playing with the resin, which influences composite shrinkage, flexibility, and shrinkage stress, as well as with the fillers, which can be large or small, porous or solid, and of different shapes,” Seemann says.

“Fillers play one of the most important roles in direct restorative composites in terms of wear resistance, ease of polishability, and polish retention,” explains Timothy D. Dunbar, PhD, advanced product development specialist at 3M ESPE. “These have evolved from microhybrids to microfills to the current nanohybrids and nanofills.”

Historically, Dunbar notes, microhybrids—which are characterized by their large particle size—have been renowned for their strength and application in the posterior, but also criticized for their poor polish retention. Alternatively, microfills have demonstrated better polish retention but lower strength, making them more suitable for anterior indications. Nanohybrids are stronger, but still suffer from poor polish retention; however, newer, mechanically stronger nanofill composites, which contain a filler structure that closely mimics the structure of natural teeth, demonstrate good polish retention over time, Dunbar says.

According to Plath, it is well-documented that true nanohybrid materials overcome the physical limitations of microhybrid materials in terms of filler content, offering strength and lower shrinkage. This is essential for bulk-fill and flowable restoratives to achieve a high depth of cure and low viscosity without sacrificing strength.

Adhesive Advances

Further contributing to expanded options when placing direct restorations has been the emergence of so-called “bulk-fill” composites and “low shrinkage stress” composites, the use of heating units and “sonic” energy to improve flow and adaptation, and the development of better matrix systems to enhance interproximal contacts in Class II restorations, simplified adhesive systems, and “bioactive” materials, Alex elaborates.

“Shrinkage of direct restoratives was once approximately 3% to 5% per volume—a very high rate that led to contraction when the material was polymerized, which subsequently distorted the tooth,” Burgess explains. “For example, the buccal and lingual cusps on an MOD composite restoration would move closer together as the composite polymerized.”

This strain caused constant stress within the tooth at the restorative margins and ultimately led to marginal breakdown on occlusal surfaces that has been clearly observed in clinical trials, Burgess says. Although marginal breakdown remains a major problem with composite resins, new direct restorative materials offer improvements in terms of lower shrinkage and stress.

Scientifically and clinically, today's low-shrinkage/low-stress direct restorative materials make sense, Ferracane says, adding that a lot of in vitro evidence suggests that they are advantageous in terms of properties, marginal integrity, and cuspal deflection. However, he cautions that sufficient clinical evidence is lacking at this point.

“It's very important for clinicians to understand that less shrinkage in a composite does notnecessarily mean that less stress is transferred to the adhesive interface and margin,” Alex cautions. “In other words, it is very possible to have a composite that shrinks less than another, but still induces far more stress at the adhesive interface once polymerized.”

This occurs because photopolymerization is complex and many other factors—not just how much a composite shrinks—determine how much stress is transferred to the adhesive interface. Among these factors are modulus (ie, stiffness), rate of polymerization, cavity configuration, polymerization kinetics, initiator chemistry, gel point kinetics, filler and monomer type, and degree of conversion, Alex explains.

In fact, to increase the depth of cure of today's direct restoratives, the materials have become more translucent, which also contributes to a better chameleon effect when compared with older materials, Burgess says. Older direct composites were much more opaque, whereas newer materials are more translucent to enable better light penetration.

“The result is that these more translucent materials blend better with the surrounding tooth structure, and the chameleon effect causes them to appear extremely tooth-like when cured,” Burgess explains. “Even though some newer materials for posterior applications may only be available in limited shades, because those shades match natural teeth extremely well, the time required for shade selections on posterior teeth is significantly reduced.”

Ferracane says that the need for higher translucency to ensure a greater depth of cure has—to some extent—compromised the esthetics of bulk-fill materials, but that this is not really a problem.

“I can't imagine that the materials specifically formulated for maximum esthetics could become much more esthetic than they already are,” Ferracane admits.

But strength and toughness is a different story. Current formulations of direct restoratives are designed for maximum polishing, esthetics, and abrasion resistance. Ferracane explains that to achieve these characteristics, they rely on very small particle sizes, which isn't the same recipe required to produce the strongest and toughest composites.

“Bigger particles are typically required for composite materials to enjoy all of the available toughening mechanisms,” Ferracane elaborates. “Many years ago, we had stronger, tougher materials, and these are no longer available. I think we could see fewer clinical chips and fractures if we had tougher materials, but we would also have to give up some of these other attributes.”

According to Burgess, the physical properties and material characteristics that clinicians really want in their direct restoratives are high fracture toughness, which suggests that a fracture won't propagate very rapidly, and low shrinkage and good wear resistance, which are essential for posterior restorative procedures.

“Compressive strength doesn't tell you much about the longevity of the restoration that's being created using a particular material,” Burgess says.

The Bulk-Fill Advantage

More dentists are beginning to use so-called “bulk-fill” composite materials. Unlike traditional light-cured composites that are typically designed to be placed in 2-mm increments, bulk-fill composites are designed to be placed in 4-mm to 6-mm increments, at least according to their various manufacturers, Alex observes. When restoring large and deep cavity preparations, placing and light-curing multiple layers of composite in traditional 2-mm increments could take considerable time and also be very technique sensitive, says Alex.

“Assuming they perform as advertised, that makes bulk-fill materials an attractive treatment option for very obvious reasons. When placing direct composite restorations, using bulk-fill composite can save considerable time and reduce the potential for errors, such as the presence of gaps between the layers of the restoration,” Alex says. “If the same or better results can be achieved by placing fewer increments using a bulk-filled composite, then why not do that?”

Scientifically speaking, the 4-mm increment bulk cure claims are a reality, says Ferracane, who notes that virtually all of the products he's tested have lived up to the manufacturer's claims and that most of the literature supports this.

“There is some question about how this property is assessed, but the key factor is probably having the light directly over the material, in near contact, with ample quality and power, and with an unobstructed path to all parts of the restoration,” Ferracane explains. “So, clinicians still have to pay attention to how they are placing/curing the material and know the true depth of the preparation. Bulk-fill materials may require more than one increment to fill a preparation.”

Depending upon the manufacturer, materials designed to be placed in 4-mm to 5-mm increments generally demonstrate good physical properties for posterior restorations, and their depth of cure and shrinkage are equivalent to a 2-mm increment of conventional materials, observes Vargas. However, due to the need for the curing light to reach a depth of 4 mm, 5 mm, or even further for proper polymerization, these new materials tend to be more translucent than their counterparts. In addition, some manufacturers require their bulk-fill material to be polymerized from the buccal and lingual surfaces.

“We've measured different materials and looked at their depth of cure in both tooth molds and metal molds, which is the standard for evaluating depth of cure, and there's not much difference between the two as far as depth of the cure is concerned,” Burgess says. “Photoinitiators have been increased in some materials to enhance curing, and manufacturers have also made them more translucent to facilitate a better depth of cure.”

Composite Composition

Incorporated into today's direct restoratives are various material technologies that are categorized and described with trademarked terminologies. Among them are Giomer (Shofu Dental Corporation), Adaptive Response Technology (Kerr Dental), ORMOCER® (Admira, VOCO; ORMOCER® is a registered trademark of Fraunhofer-Gesellschaft), SphereTEC™ (Dentsply Sirona Restorative), and Smart Contrast Ratio Management Technology (3M ESPE). And there are many more. They can be confusing to understand and add to the already difficult task dentists face when choosing the most appropriate material for a given indication.

“I think it's helpful for clinicians to understand the origins of the terminology and what exactly the difference is between these bulk-fill composites and more ‘conventional' formulations,” Ferracane says. “This is helpful so that dentists don't feel like they're being left behind if they're not using them. Dentists shouldn't feel the need to jump on any new product idea; they should wait until they are comfortable that there is sufficient data/evidence to support the change.”

“These terms are, to a large extent, created by marketing departments to describe what is different about a new composite when compared with the more than 100 others on the market, so there's always an innovative technology behind the terminology,” explains Seemann. “The terminology is intended to make the science and technology more understandable.”

According to Plath, all of the components of direct restoratives have an effect on their physical properties, and it can be fascinating to understand how they correlate. However, he says it is more interesting to discuss what the resulting physical properties really mean for the dentist's working procedure and the clinical survival rate of the restorations produced.

“This is an eye opener for those practitioners looking to broaden their view on the efficiency and effectiveness of their individual approach,” Plath says.

At the end of the day, Seemann says that regardless of what new composite technology is offered, a dentist's judgment of a material based on what they can see and feel will determine their interest. The new descriptive terminology—and the technology itself—become relevant only after a correlation is made between what the dentist sees and feels and the technology making that experience possible.

“A dentist can observe things, such as handling, polishability, adaptability, and absence of stickiness, which lends to an explanation of how and why a new composite filler or resin technology produces certain characteristics, features, and benefits,” Seemann continues. “Of course, it can also work the other way: by stating that there is a new technology that enables direct restoratives to demonstrate certain benefits and then explaining how and why.”

Conclusion

Controlled clinical trials have shown that the annual failure rates of direct composite resin restorations range from around 1.4% to a little less than 2%, says Burgess. Newer materials, however, are going to last significantly longer than their predecessors, he adds.

In fact, there is sufficient evidence and published literature to suggest that—if properly placed in patients without a high caries index and difficult occlusal habits—current direct restorative materials can last between 15 and 20 years, Ferracane says.

“But we know this does not represent the typical case, so there is evidence that the average lifespan is closer to one-half to one-third of this timeframe,” Ferracane clarifies, adding that there are many factors that influence the longevity of direct restorations.

Therefore, there are many factors to consider when selecting direct composite restorative materials. Dunbar admits that consulting resources such as dental material journals can be somewhat daunting for a busy dentist, however, the academic literature and dental conferences can provide good information.

“One of the most important things about information supplied by manufacturers regarding a product is trust,” Dunbar says. “Does the dentist really trust what the manufacturer is saying? By assessing the educational materials that the manufacturer is distributing, dentists can see how the composite is being promoted and what the manufacturer is touting as significant.”

In the future, dentists could see additional advances that produce tougher materials, but Ferracane predicts that they won't be achieved through particulate nanotechnology, because the science has reached a point of diminishing returns. He says it's possible that new resin formulations and other modifications may provide some of the answers for current systems that are not very tough.

As digital workflows (eg, intraoral scanning, 3D printing) continue to infiltrate everyday dentistry, the need to redefine or substitute the classic understanding of “indirect” and “direct” may be necessary in the future, believes Plath, particularly given the synergistic potential of these technologies and direct restorative materials.