Direct Composite Resins
The most critical improvements have come in the filler particles.
Gary M. Radz, DDS
For nearly 60 years, the use of composite resins has increased along with patients’ growing demand for dental composites. This demand has been driven in large part by patients looking for an esthetic alternative to repair carious lesions or traumatized teeth. Additionally, there is a segment of the patient population that has concerns regarding the presence of mercury in amalgam restorations. These patients are very interested in a non-mercury alternative.
The work of Buonocore1 demonstrated how teeth could be acid-etched, which created the potential to bond various materials to tooth structure. This opened the door to many possibilities in dentistry. Bowen’s development of the composite resin material2 provided dentistry with a material that could be combined with Buonocore’s etching discovery to introduce a tooth-colored restorative material that would eventually rival amalgam.
Before these discoveries, dentistry was limited to the use of silicate cements and acrylics to provide a directly applied tooth-colored restoration. These materials were quickly discarded as composites were developed, studied, and improved. Initial composites, however, had several weaknesses.
One problem with early composites is that they were self-cured materials. Dentists had no control over the setting of the material. So, depending on the clinician and the clinical situation, the material would set up too fast or too slowly. The dentist was essentially left to the mercy of the material’s set time as determined by its chemistry.
Another problem was that filler particles in early composite resins were too large. Early filler particles were 10 µm in size; this led to plucking and excessive wear of the material.3 These early materials did not have an acceptable clinical lifespan. In addition, the number of shade options was limited, and creating a truly esthetic resin restoration was challenging with such a small number of shades to choose from. These problems have, however, been addressed through the years. With the development of light-activation technology, the dentist is now able to control the setting of composite resins, and today has more control in determining when the composite reaches its final set.
Today’s composites are typically keyed to the VITA® shade guide (Vident, www.vident.com), and have dozens of shade options that include opaques and translucent shades. This allows the dentist to work chairside in a manner similar to how a ceramist works with porcelain.4 The end result is that today’s dentists have the ability to create highly esthetic composite restorations that can disappear within the tooth structure.
The most critical improvements in composite resins have come in filler particles. In the early years, filler particles in composite resins were 10 µm in size. The largest particles present in today’s composites are less than 1 µm in size. On the low end, nano particles (5 nm to 75 nm)5 have recently been introduced into the composite matrix. Modern composites are a mixture of particles ranging from 1 µm to 0.1 µm in size, depending on each manufacturer’s composite “recipe.” This reduction in particle size means that materials now have low wear properties, significantly lower polymerization shrinkage, the ability to maintain a high polish, and improved handling properties.
Composite resins are a popular and successful restorative option in dentistry today. Although the effort to create the “perfect” material is ongoing, significant improvements have recently been made in composite resin systems. The following is a discussion of the materials currently available, and highlights some recent developments that may point to where resin systems are headed in the future.
Current Composite Resin Systems
Microfilled composites were introduced in the 1980s,5 and initially were very popular. With very small filler particles (0.02 µm to 0.04 µm), microfilled composites were translucent and very polishable, which made them the first highly esthetic composite resin material. However, the small particle size translated to a lack of strength. Bulk fractures were common, and microfilled composites were quickly found to be undesirable for use in the high-stress areas in the posterior region, or as a Class IV material in the anterior.
Microfilled composites are still available today (Renamel®, Cosmodent, www.cosmedent.com; Durafill® VS, Heraeus, www.heraeus-dental-us.com). They are typically used to take advantage of their low-wear characteristics and excellent polish. Microfills are most commonly used today as a veneering material, acting as the last thin layer placed over a microhybrid composite. However, the use of microfills is steadily decreasing as microhybrids continue to improve.
The original hybrid composites were an attempt to take the small particles from microfills and combine them with the stronger “macro” filled composites of the time to create a more ideal material. While a good first step, the original hybrid materials lacked the esthetics and polish to replace microfills as an anterior restorative material, and succeeded only in becoming a better posterior material.
Microhybrid composites evolved from hybrid composites. While the “hybrid” concept had merit, decreasing the size of the largest particles was necessary. The largest particles found in today’s microhybrid composites are less than 1 µm. Practitioners have discovered it is possible to maintain the strength of the hybrid materials, but the esthetics have significantly improved.5 The microhybrids also demonstrated excellent physical characteristics and improved handling. Microhybrids have proven successful for both anterior and posterior restorations, and have been considered a “universal” composite material for more than a decade (Figure 1 and Figure 2).
Condensable composites are a variation of microhybrids. These composites were developed in an effort to create a material with a “feel” more like amalgam, and to make the Class II composite restoration more predictable. This was usually accomplished by increasing the filler load to create a denser, more condensable material. These materials were frequently marketed as “amalgam replacements.” While still available on the market (QuiXX®, DENTSPLY International, www.dentsply.com; ALERT, Pentron Clinical Technologies, www.pentron.com; Solitaire® 2, Heraeus), these have not been widely accepted.
The introduction of advanced matrix systems, which allow dentists to create a clinically acceptable Class II composite in a more predictable manner, have helped stunt condensable composites’ growth in popularity. The V3® system (Triodent, www.triodent.com) and the Composi-Tight 3D™ (Garrison Dental, www.garrisondental.com) are both capable of helping dentists achieve excellent clinical results. The systems are similar in that they use an anatomically contoured sectional matrix that is placed interproximally and secured with a robust “ring” or clamp to provide great adaptation of the matrix and create a separating force. Figure 3 shows the V3 system in place; note the thick, robust ring that creates the adaptation and separation. Figure 4 demonstrates the final restoration, which has excellent anatomical contour and an ideal proximal contact. The Garrison system is similar in design and can achieve similar results (Figure 5).
Nanofilled composites, consisting of nanomers (5 nm to 75 nm particles) and “nanocluster” agglomerates as the fillers, were recently introduced. Nanoclusters are agglomerates (0.6 µm to 1.4 µm) of primary zirconia/silica nanoparticles (5 nm to 20 nm in size) fused together at points of contact, and the resulting porous structure is infiltrated with silane.3 The nanofilled composites present mechanical and physical properties similar to those of microhybrid composites, but perform significantly better in terms of polish and gloss retention.6 A primary example of nanofilled composites is Filtek™ Supreme Plus (3M ESPE, www.3mespe.com). However, several manufacturers are now incorporating nano-sized particles into their formulations, resulting in the creation of yet another category, the “nanohybrid” composites. Some examples: Herculite Ultra®, Kerr Corporation, www.kerrdental.com; EsthetX® HD, DENTSPLY Caulk, www.caulk.com; Venus® Diamond, Heraeus; IPS Empress® Direct, Ivoclar Vivadent, www.ivoclarvivadent.us; Grandio®, Voco America, www.voco.com; Aelite™ Aesthetic Enamel, BISCO, www.bisco.com; Clearfil Majesty™ Esthetic, Kuraray America, www.kuraraydental.com; and Artiste®, Pentron Clinical Technologies.5
Nanofilled composites appear to be the immediate future of composite resins. They have demonstrated excellent strength and wear properties, have favorable handling characteristics, and are highly esthetic. Nanofilled composites have proven to be a commercial success, which would indicate at least some level of clinical acceptance.
Polymerization shrinkage has been attributed to microleakage and sensitivity, and has been a concern with composite resins.7,8 Most current microhybrid and nanohybrid composites have a polymerization shrinkage that ranges from 2% to 3.5%. While significantly better than composites of the past, the end goal of composite resin manufacturers is to minimize shrinkage to the greatest possible extent.
About 4 years ago, a composite resin that could have the potential of exhibiting less than 2% shrinkage was made commercially available for the first time. Using new chemistry that differed from the Bis-GMA chemistry that other composites were based on, 3M ESPE introduced Filtek™ LS. Based on silorane chemistry, this material has shown polymerization shrinkage rates of less than 1%. While a significant breakthrough, this material has not been universally embraced. One explanation for this is that the material can only be used with its own unique bonding agent, which has attributed to its lack of acceptance.
Septodont (www.septodont.com) recently introduced N’Durance®, a low-shrinkage material. This composite uses dimer-based chemistry to create a material with a high conversion rate and polymerization shrinkage less than 2%. Although a unique chemistry, the restorative material can be used with any commercially available bonding system.
DENTSPLY Caulk is the most recent entrant in the market, with SureFil® SDR®. The product’s polymerization shrinkage, however, remains similar to other popular composites. The company claims to use a “polymerization modulator” to reduce the stress that occurs with polymerization shrinkage, thereby addressing the concerns of shrinkage and sensitivity. This is an interesting concept, and the material has drawn notable interest from the marketplace.
These are just some examples of the latest developments in the dental materials industry, as research and development teams at many companies continue to search for new chemistry to further improve composite resin restorations.
The newest and perhaps most intriguing development in composite resins is the emergence of self-adhesive composites. Flowable composites have existed in the composite resin market for more than 15 years, and share many of the characteristics of microhybrid composites. But, because they are less highly filled, these composites are flowable in nature. This allows flowable composites to be very useful restorative materials in the right situation.
Kerr and Pentron Clinical Technologies have recently introduced self-adhesive composites; Vertise™ Flow and Fusio™, respectively. Using technology based on self-etching bonding agents, these companies have developed composite resin materials that require no bonding agent at all. Rather, the bonding agent is incorporated into the restorative material itself (Figure 6).
This represents an interesting advancement toward creating a more “ideal” composite resin. Clinical research on these products is ongoing, but the progress made thus far is promising. If these materials do prove successful, the obvious next step would be to create a conventional microhybrid or nano-hybrid restorative material using this technology.
Composite resins will continue to be a part of modern restorative dentistry. As new chemistries are created and composite resins continue to improve; their clinical use will only increase over time.
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2. Bowen RL. Development of a silica-resin direct filling material. Report 6333. Washington: National Bureau of Standards. 1958.
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8. Nikolaenko SA, Lohbauer U, Roggendorf M, et al. Influence of C-factor and layering technique on microtensile bond strength to dentin. Dent Mater. 2004;20(6):579-585.
About the Author
Gary M. Radz, DDS
Associate Clinical Professor
University of Colorado School of Dentistry