The Incredible Bulk

Bulk-fill composites are fairly new, but they are already showing much promise as solid alternatives to amalgam for posterior restorations

By Rebecca Stone

They may not have anything to do with mild-mannered scientists or newspaper reporters transforming themselves into superheroes, but "bulk-fill" materials are coming to the rescue of many time-squeezed dental practitioners. Regarded as alternatives to amalgam in posterior restorations, these types of composites seem up to the challenge — so far, at least.

Why is this a big deal? While amalgam has a long history of being the material for posterior fillings, it's common knowledge that, as successful as they've been, they are under fire on several fronts. As a result, dentists are turning to composites. And for many, this seems to be working out well… except for one thing: time. Con ven tional composites used in posterior restorations come with their own set of issues — time being at the forefront. To address this concern, manufacturers have come up with bulk-fill composites. But to understand the value of these materials, it's helpful to take a step back for perspective.

Since their introduction in the 1960s, composite resins have proven to be successful dental restorative materials, especially in anterior work. Posterior restorations, however, continued to reside in amalgam territory, as composites were not deemed strong enough to withstand compressive forces. While many practitioners still remain loyal to silver, composite alternatives began to see the light of day as posterior choices in the early 2000s.

"Thanks to improvements in chemistry, composites are now plenty strong for posterior placement," asserts Martin Jablow, DMD, a Woodbridge, New Jersey-based cosmetic dentist who's been using them in posterior restorations for several years.

Composites have many things going for them — but for all their advantages, these materials have a downside: They shrink upon curing (or what clinicians refer to as polymerization).

 

lexicon

Bulk-Fill Composite: Generally, a low-shrink formulation with at least a 4 mm depth of cure that can be placed in at least 4 mm increments, often filling the entire cavity.

Double-Bond Conversion: The linking of monomers to form polymer chains.

Monomers: The molecules that make up resin composites, and combine into polymers during curing — or what's technically known as polymerization.

Occlusal: The top portion of a tooth, such as the biting edge of an anterior tooth or chewing surface of a molar.

Polymerization Shrinkage: Dimensional changes in polymer resins triggered by the curing process.

Polymerization Shrinkage Stress: Curing causes composite resins to shrink, which can create gaps and stress where the bonded material interfaces with surrounding tooth structure. This can lead to marginal leakage, caries and fractures.

Proximal Contact: Describes where adjoining teeth "touch" each other, side to side.

 

SHRINK RAP

Under a curing light, resin monomers link to form polymer chains (called double-bond conversion). In the process, they shrink — to what degree depends on a variety of factors, including depth of cure, curing light power settings, light position and technique, and the duration of light exposure.

Shrinkage generates increased tension within the material, and this can result in a host of problems, from marginal leakage and secondary caries to post-op sensitivity and restoration failure. The good news is that advances in chemistry have resulted in composites with reduced shrinkage rates — although it's still true that the thicker the layer of composite, the more likely it is to cause shrinkage stress. To compensate, in Class I and Class II posterior restorations, clinicians have traditionally placed and cured the material in a series of layers — one layer at a time — each not exceeding 2 mm. Such restorations are often built up to within 2 mm of the occlusal surface, at which point another type of composite is placed as a cap for esthetics and strength. Problem solved? Not quite.

TIME IS MONEY

The downside of this happy little scenario is that it is time consuming. Not only must the tooth be prepped, isolated, etched and bonded, but after all the layering and curing, it must also be capped with another composite. And with so many steps involved, there are multiple opportunities for errors in technique. Compare this to amalgam, which can be quickly and simply placed in bulk without bonding systems or caps, and practitioners could begin to wax nostalgic.

It's natural to question how such procedures (especially when performed multiple times a day) can adversely affect not only profit margin, but also outcomes and patient comfort. Is it any wonder that manufacturers are dreaming up ways to address these issues? Although dentists can be slow to adopt new products and techniques, sales representatives can look like superheroes by keeping their clients abreast of the latest developments in timesaving materials.

 

Point of Sale BULK-FILL COMPOSITES

  • Although the category is loosely defined, "true" bulk-fill materials are generally considered to be those that offer a depth of cure of at least 4 mm in one placement.
  • By reducing the number of steps involved in traditional layering techniques, bulk fills offer a significant timesavings per restoration.
  • Easy to place, they also tend to be less technique sensitive than conventional procedures.
  • Self-leveling flowable bulk fills reportedly offer excellent adaption to cavity walls.
  • Manufacturers claim these materials reduce polymerization shrinkage stress that can result in restoration failure.
 

BULK-FILL COMPOSITES

Regarded by many clinicians as one of the most exciting advances in dentistry, bulk-fill composites are making tremendous strides in simplifying posterior restorative procedures. Through advances in chemistry, they offer dentists — and patients — a faster, less technique-sensitive process. While a bonding system must still be used, these new materials can be placed in 4 mm increments — and for some bulk fills, at an even greater depth. For the most part, this allows the team to fill even a deep prep in one or two increments vs. multiple increments, as was previously the case.

Yet, the exact definition of a bulk fill seems elusive.

"To a dentist, 'bulk fill' has always meant filling the cavity with one or two increments of composite material and then light curing," observes Ronald D. Jackson, DDS, FAACD, a Middleburg, Virginia-based general practitioner who also serves as director of the Advanced Adhesive Aes thetic Dentistry and Composite Artistry programs at the Las Vegas Institute for Advanced Dental Studies. Jackson believes that for marketing purposes, manufacturers have broadened the definition to include any material that can be placed in layers greater than 2 mm.

Many practitioners point out, however, that not all low-shrink composites are true bulk fills. Jablow defines a bulk fill as a material that allows "a single placement up to 4 or 5 mm, with appropriate physical properties to withstand compressive posterior forces. Some require capping with another composite for either esthetic or functional reasons."

According to John C. Comisi, DDS, MAGD, who operates a general practice in Ithaca, N.Y., "Bulk fills are composites that have had their chemistry altered to enable them to be placed 'in bulk' in a typical tooth preparation." Traditional composite restorations cannot effectively be placed in bulk, he explains, because they may not cure fully, or could create excessive stress on the composite and surrounding tooth structure during polymerization.

 

Go With the Flow

A good portion of the products being sold as bulk-fill composites are flowables. While flowables are not considered strong enough to form the final occlusal layer, they are widely believed to be the best choice for the base increment. This is because they adapt well to the nooks and crannies of internal tooth structure. Notes Ronald D. Jackson, DDS, FAACD, a general practitioner and lecturer based in Middleburg, Va., "Flowables wet the cavity walls better than high-viscosity materials — which is why most dentists use them as a separately placed liner under composite restorations. Some dentists will use a low-viscosity resin-modified glass ionomer as a liner."

Jackson believes the choice between a bulk-fill flowable or one of the high-viscosity bulk fills is more a matter of preference than science. "Bulkfill flowables placed in 4 mm increments will result in the proximal contacts of Class II restorations being in this low-particle-filled material," he explains. "As a result, some dentists are concerned that it will result in higher wear and flattening of the contact than if the contact was in a highly filled restorative composite. So far, I haven't seen any science on this. On the other hand, when used as large bases (or liners), bulk-fill flowables are not indicated for occlusal contact because they would wear too fast. This is why it is recommended they be 'capped' with a traditional, highviscosity composite material."

 

GENERAL REQUIREMENTS

To be considered a true bulk-fill material, the consensus (if there is one) is that it must: exhibit low shrinkage stress values; offer at least a 4 mm depth of cure; adapt well to internal tooth structure; and be self leveling, which eliminates the need for manipulation that could introduce voids in the form of air bubbles.

In order to cure such a large increment of composite, the material — often a flowable — has been rendered translucent to offer a depth of cure of at least 4 mm. The translucency facilitates polymerization by allowing curing light energy to penetrate the deepest portions of the prep. Flowables are normally capped with a high-viscosity composite, which is finished and polished to complete the restoration.

For busy practitioners, the biggest attraction of using bulk fills is shorter chairtime. "The theoretical benefit is that the materials can be cured in a bulk-filled restoration without needing to layer the composite in the traditional 2 mm increments," Comisi explains, "and they are easy to place. It's also reported they don't generate undo stress on the tooth structure or final restoration."

All the practitioners we spoke with agree that the timesaving aspect of bulk fills is a huge plus. As Jackson notes, "Placing posterior composites is tedious, time-consuming work, so bulk fills help increase efficiency. In addition, fewer layers lessen the chance of voids."

Comisi points out that it will be a while until we know how these new materials will affect tooth structure in the long term. "They are claimed to completely cure during polymerization, and do not exhibit shrinkage that can contribute to restoration failure. But the full story on this is yet to be seen," he says.

Jablow believes a remaining challenge will be to understand the physical properties of various bulk-fill composites. Such an understanding, he feels, will help dentists decide which materials to use, when, and how to place them for optimum outcomes. "Like other aspects of dentistry, it boils down to picking the right material for the patient," he adds.

In current bulk fills, there's a trade off between depth-of-cure characteristics and esthetics. As Jablow explains, "Due to the translucency that's required to achieve the appropriate depth of cure, the esthetic properties of bulk fills are not the same as standard composites." In fact, this translucency has been reported to lend a gray cast to the restoration. For this reason, in addition to added strength, a highly filled composite cap makes sense.

WHAT'S NEXT?

You can be sure that manufacturers are not resting on their laurels in the quest for ideal materials. It's all about making things simpler, easier and, ultimately, better for the practitioner and patient. And while dental professionals may still be waiting for definitive word on how bulk fills will perform over the long haul, Jackson notes that, in general, "Today's composite resins offer excellent physical properties and performance. Dentists have asked for a bulkfill material for years — and now we have it. I suppose the next logical advancement would be to make the materials self-adhesive, thus eliminating another step in the placement process."

Jablow is equally optimistic. "These materials are the wave of the future — this is where it's heading," he says. "Eventually, I think dentists will able to choose a single syringe that will be adaptable to most situations."

Who cares about leaping tall buildings in a single bound when you can fill deep preps in a single increment? With today's incredible bulk-fill materials, dentists will feel like superheroes in their quest to rid the world — or at least their practices — of the vil lainous tyranny of endless layering, polymerization shrinkage and marginal leakage. And that makes traditional composites green with envy.

 

 

Bonus Web Content : Amalgam Under Fire

When it comes to posterior restorations, amalgam has long been regarded as the go-to material as far as what it offers in compressive strength and wearability — even bacteriostatic properties — over the life of the restoration. It doesn't require a bonding agent, it can be quickly placed in bulk, it is affordable and it doesn't need another material to serve as an occlusal layer.

But the use of amalgam is under fire on at least four fronts: It contains mercury; it creates residual hazardous waste, which has become a disposal issue for dentists; it requires a fair amount of tooth structure removal in order to create retentive features; and finally, patients increasingly desire restorations that look like real teeth instead of a metallic patchwork. All of which is driving increased demand for composites — both traditional restoratives and the new bulk-fill materials.

 

Bonus Web Content : innovative bulk fills with a different spin

A couple of products have been recently released with a fresh spin on the bulk-fill category. SonicFill from Kerr is a single-step, bulk-fill composite system that is delivered via sonic handpiece. The sonic waves liquefy the material, allowing it to take on flowable characteristics, resulting in excellent adaptation to tooth structure, which helps to eliminate voids. Once the handpiece is deactivated, the material takes on its non-flowable, high-viscosity characteristics once again, which allows sculptability. According to the manufacturer, SonicFill allows bulk fill up to 5 mm, while providing superior strength, low shrinkage and high depth of cure. In addition, it can be left as the final occlusal cap, and then finished and polished.

Another innovation comes from Septodont. Its Biodentine is a biocompatible, bioactive bulk filler that can be used as a dentin replacement. Said to have the same mechanical properties as natural dentin, it's designed to preserve pulp vitality, promote pulp healing, and help remineralize dentin. The manufacturer reports that is self-curing and can be used with any bonding system. Available in one shade, it must be topped with a composite.

 
MENTOR April 2012, 3(4): 18-23.

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