Composite Materials: Advances Lead to Ease of Use, Better Performance

The development of the epoxy resin molecule in 1951 started a revolution in dental composite technology. For more than 50 years manufacturers have been striving to produce the most ideal composite resin that will simulate the natural tooth. Initial formulations were chemically cured, but their clinical performance was unsatisfactory.¹ In the 1970s the development of photo-curable composite resins changed the landscape entirely.² Light-cured composite resins were more color-stable than the earlier self-cured composites and had smaller filler particles, which improved wear resistance.³ Microfill resins were introduced in the late ’70s and had a submicron average particle size, which resulted in better handling and higher polishability and wear resistance.³ The 1980s brought reduced particle size and increased filler loading, which significantly improved light-cured composite resins for universal use in anterior and posterior restorations.³

Today, research is focusing on nanoparticle composites. These nanofillers are discrete particles with particle sizes below the wavelengths of visible light, which may result in higher filler loading and, therefore, improved physical and mechanical characteristics with reduced polymerization shrinkage.⁴

A Combination of Materials

Composites can be defined as a 3-dimensional (3-D) combination of at least two chemically different materials with a distinct interface separating the components.⁵ The infrastructure of composite resins comprises the organic phase (matrix), the dispersed phase (filler), and the interfacial phase (coupling agent).² Bis-GMA is the organic phase and consists of a high molecular weight monomer system used in approximately 80% to 90% of commercial dental composites.⁶ The mineral component of a composite is termed “the dispersed phase” and has been noticeably improved with the addition of small particles or fillers that vary in size depending on their manufacturing process.⁷ The fillers in dental composites provide strength and reinforcement. The coupling agent connects the resin matrix and the inorganic filler. Alteration of the filler component remains the most significant development in the evolution of composite resins.⁷ Altering the filler component changes the mechanical and physical properties of the composite material. Compression strength, flexural strength, elastic modulus, coefficient of thermal expansion, water absorption, and wear resistance depend on this filler phase.

As stated earlier, nanocomposites utilize filler particle sizes that are on a molecular level. These nanosized particles can be dispersed in higher concentrations and polymerized into the resin system to provide unique characteristics. The filler is the primary determinant of the clinical property of the composite resin. These composites can have different types of filler particles: prepolymerized, finely milled agglomerated nanoclusters; larger (submicron-sized) glass or silica particles in the range of 0.4 m; and individual nanosized particles (0.05 m).

Several years ago a new technology was introduced aimed at changing the chemistry of composites to address the problem related to polymerization shrinkage. Called silorane, these materials are hydrophobic and need to be bonded using a different adhesive system. According to some studies,⁸ silorane’s low shrinkage leads to a lower contraction stress; furthermore, these restorations were shown to have both low water absorption and water solubility. Silorane has been shown to have good mechanical properties.⁹ These materials, though, have not really flourished in the United States due to the limited shades and the need to use a separate adhesive system that is only compatible with the silorane technology. In one clinical study, the marginal quality of the silorane composite was shown to be inferior to that of a nanohybrid material.¹⁰ In another study, silorane did not produce lower contraction stress than other composites.¹¹

Other low-shrinkage materials are available in some composites. Urethane dimethacrylate (UDMA) from DuPont has a relatively high molecular weight compared to bis-GMA. Another company uses a dimer acid monomer that has been shown to have high conversion of carbon double bonds while undergoing lower polymerization shrinkage than bis-GMA–based systems.¹² Yet another company has incorporated a special additive into the photo-initiator system causing the material to be less sensitive to ambient light while being highly reactive to curing lights. This allows a shorter polymerization time for each increment.

Moving Forward

So where does that leave dentistry today in 2013? Increasingly, manufacturer companies are advocating bulk fill for posterior restorations. The newer composites being released are claiming to be able to be cured to a depth of 5 mm in 40 seconds while also possessing less polymerization shrinkage. While these claims may be true there are not many studies that show long-term results. Other popular products are flowable resins that can be bulk filled. These materials are easy to use, but a layer of nonflowable resin should be placed over the top for better wear resistance. More important than polymerization shrinkage is the stress that is produced when they shrink. Flowable resins possess a lower elastic modulus, therefore they produce less stress on polymerization.

Another clever innovation to make composites more flowable is the use of sonic energy. A special handpiece is utilized, and the composite manufacturer claims that the cavity preparations can be filled and polymerized to a depth of 5 mm. Once again, long-term studies are lacking, though the author has spoken with several clinicians who use the sonic technology and said they have been pleased with the results.

To date, there may not be one ideal composite to use in every situation. Manufacturers will continue to improve handling and polishability while lowering stress on polymerization. Competition is good for the industry, but it is important to note that technique will always trump the materials that are used. As the saying goes, “The magic is not in the wand; it is in the magician.” Good technique will always prevail.

REFERENCES

1. Rubinstein S, Nidetz AJ. Posterior direct resin-bonded restorations: still an esthetic alternative. J Esthet Dent. 1995;7(4):167-173.
2. Talib R. Dental composites: A review. J Nihon Univ Sch Dent. 1993;35(3): 161-170.
3. Jackson RD, Morgan M. The new posterior resins and a simplified placement technique. J Am Dent Assoc. 2000;131(3):375-383.
4. Combe EC, Burke FJ. Contemporary resin-based composite materials for direct placement restorations: packables, flowables and others. Dent Update. 2000;27(7):326-336.
5. Phillips RW . Skinner’s Science of Dental Materials. 7th ed. Philadelphia, PA: W.B. Saunders Co.; 1973.
6. Ruyter IE, Oysaed H. Composites for use in posterior teeth: composition and conversion. J Biomed Mater Res. 1987;21(1):11-23.
7. Roulette JF. Degradation of Dental Polymers. Basel, Switzerland: S. Karger AG; 1987.
8. Ernst CP, Meyer GR, Klöcker K, Willershausen B. Determination of polymerization shrinkage stress by means of a photoelastic investigation. Dent Mater. 2004;20(4):313-321.
9. Ilie N, Hickel R. Silorane-based dental composite: behavior and abilities. Dent Mater J. 2006;25(3):445-454.
10. Schmidt M, Kirkevang LL, Hørsted-Bindslev P, Poulsen S. Marginal adaptation of a low-shrinkage silorane-based composite: 1-year randomized clinical trial. Clin Oral Investig. 2011;15(2):291-295.
11. Marchesi G, Breschi L, Antoniolli F, et al. Contraction stress of low-shrinkage composite materials assessed with different testing systems. Dent Mater. 2010;26(10):947-953.
12. Lu H, Trujillo-Lemon M, Ge J, Stansbury JW. Dental resins based on dimer acid dimethacrylates: a route to high conversion with low polymerization shrinkage. Compend Contin Educ Dent. 2010;31 spec iss 2:1-4.

Educational Resources

Clinical Decision-Making for Restoration Replacement or Repair
dentalaegis.com/go/cced390

Resin-Based Composite as a Direct Esthetic Restorative Material
dentalaegis.com/go/cced391

Composite Resin: A Versatile, Multi-Purpose Restorative Material
dentalaegis.com/go/cced392 

Advances in DirectComposite Restorations
dentalaegis.com/go/cced393

Robert Margeas, DDS
Adjunct Professor, Department of Operative Dentistry,University of Iowa College of Dentistry, Iowa City, Iowa;
Private Practice, Des Moines, Iowa