Versatile Composite Resins Simplifying the Practice of Restorative Dentistry

The physical properties of dental composite resins are greatly enhanced by the addition of fillers. This increases the strength and reinforcement of the matrix⁵ while reducing the linear coefficient of thermal expansion. Filler materials for composite resins include ground quartz, alumina, zinc, and zirconium, just to name a few. Fillers can vary in size depending on the manufacturing process. To create a strong bond between the matrix and the filler, a silane coupling agent is used.⁶ Altering the filler component remains the most significant development in the evolution of composite resins.⁷

Classifying Composite Resins

As manufacturers endeavor to increase the amount of fillers in their resins to improve such mechanical and physical properties as compression strength, flexural strength, elastic modulus, coefficient of thermal expansion, water sorption, and wear resistance, several classification systems have been developed. These systems are based on particle size, distribution, and quantity incorporated.⁸ Classifications are more commonly referred to as hybrid, microhybrid, microfill, and the newer nanofill as well as bulk fill. The descriptions of these classifications can vary from system to system.

Hybrid and Microhybrid

The hybrid and microhybrid materials generally have a filler content of approximately 75% by weight. Filler particles can range in size from 1 µm to 3 µm and include silica particles, which generally have a size of 0.04 µm. Hybrid materials exhibit superior tensile strength, reduced polymerization shrinkage, low coefficient of thermal expansion, improved abrasion resistance, and better fracture resistance.⁹ Disadvantages of hybrid materials include their poor polishability and need for maintenance of polish over time.

Microfills

Microfills consist of submicroscopic silica particles that are approximately 0.04 µm in size. Due to the difficulty in wetting these smaller particles, the filler concentration is usually 35% by weight. The smaller particle size enables excellent polishability and the ability to hold a polish over time. These types of materials are significantly weaker than the hybrids and have a higher water sorption, a lack of radiopacity, and decreased fracture toughness. These materials are best used in conjunction with a hybrid material on the lingual surface for strength and in low-stress areas. They are well suited for Class III and Class V restorations as well as for direct veneers.

To try and achieve both strength and polishability in one type of composite, manufacturers have introduced smaller particle hybrid composite resins with average particle sizes of approximately 0.02 µm to 1 µm.⁵ This allows the clinician to implement a single restorative material with all the improved mechanical and physical properties of previous resins. The major drawback of these types of composites is maintenance of polish. The polish is satisfactory initially but tends to lose its luster over time.

Nanocomposites

Nanotechnology, or nanoscience, refers to the research and development of an applied science at the atomic, molecular, or macromolecular levels, which is also known as molecular engineering.¹⁰ The prefix “nano” is defined as a unit of measurement in which the characteristic dimension is one-billionth of a unit.¹¹

While many manufacturers use the term “nano” to describe their composite systems, few are actually true nanocomposites. The nanocomposite is composed of two types of nanofiller components: nanomeric particles and nanoclusters.¹² The nanomeric particles are monodispersed discrete nonaggragated and nonagglomerated nanosized silica particles, 20 nm and 75 nm in diameter. There are two types of nanoclusters. The first consists of spheroidal agglomerates formed by lightly sintering discrete zirconia and silica particles with a primary size from 2 nm to 20 nm. The second type of nanocluster is synthesized from 75 nm discrete primary particles of silica and has broad secondary particle distribution with an average particle size of 0.6 µm.¹³ This type of composite was developed for both anterior and posterior use.

Bulk-Fill Composites

Flowable composites with higher filler content have been developed to allow the material to be used as a dentin replacement. These materials can be used as a single increment up to 4 mm. The material is then covered on the surface with a conventional composite. Not many clinical studies yet exist to substantiate the claimed benefits of these materials. Although dentists anecdotally have been satisfied with the latest sonic fill composite, there are no clinical papers that examine the benefits of this product.

Conservative Chairside Protocol

For everyday practice, composite resins offer many benefits. They enable clinicians to follow a predictable, conservative, and reliable chairside protocol for enhancing patient smiles and restoring worn and decayed tooth structure. Combined with the best adhesive protocols, these procedures can be used successfully for highly esthetic results. Composite resin is one of the most versatile materials in dentistry, and when utilized properly with meticulous care it can perform comparably to porcelain restorations. Proper usage often requires additional training to achieve a master’s level of skill. When used in appropriate situations, and with proper maintenance of polish, these materials should provide strong, long-lasting results.

The ability to be minimally invasive and conserve tooth structure is another significant benefit. Composites are used on a daily basis to restore caries, close spaces, lengthen teeth, cover dark or discolored teeth, and fix fractured teeth. Which type of material to use in a given clinical situation is open for debate.

Often times hybrids and microfills are used in combination to achieve a restorative result that offers optimal physical and mechanical properties. The hybrid material provides strength and opacity, and the microfill delivers the definitive luster and durability of the polish. This incremental layering technique with composite resin results in an optimum depth of cure while reducing the effects of shrinkage and stress forces during polymerization.¹⁴ In addition, the polychromatic effect can be observed when different restorative components of varying refractive indexes, shades, and opacities are stratified.¹⁵ By utilizing an anatomic stratification with successive layers of dentin, enamel, and incisal composite, a more realistic color can be achieved, as can surface and optical characteristics that mimic nature.¹⁶

Clinical Examples

In Figure 1, a clinical situation of a fractured tooth is shown in which the patient presented to the clinician’s office in an emergency. Figure 2 shows the tooth restored on the same day immediately following a multiple layering technique of composite resins. A dentin-like composite material was used to replace the dentin and produce the opacity to block any shine-through, and an enamel-like composite material was used to provide translucency.

Another common clinical situation—and one that is well-suited for composite resin use—is when patients present with diastemata (Figure 3 and Figure 4). The use of composite resins in this instance is minimally invasive and typically is reversible. Usually, no tooth structure needs to be removed, and the procedure is done in one appointment. Orthodontists rely on general dentists to be able to close the unwanted spaces or change the shape of a peg lateral. If the space is small, the material for this type of clinical situation can be either a microfill or nano material, both of which handle well and can provide an excellent final result.

Composite resin can also be used to repair a fracture when a patient presents with, for example, fractured porcelain off of a long-span bridge. Although this type of procedure is not always predictable, when done correctly and with proper control of occlusion, a successful outcome can be achieved. Numerous composite materials are needed when undertaking a repair with metal exposed. The use of opaquers, tints, hybrids, or nanofills along with a microfill for the final layer can provide a beautiful result by blocking out the underlying metal substructure. This can also be an economical solution compared with the cost of a new bridge.

These are just some examples of the multiple uses of composite resin.

Conclusion

Composite resins offer a conservative and cost-effective solution for various clinical situations. Manufacturers continue to improve the physical properties and ease of use of these materials. With numerous newer nano-type materials available, it is the individual clinician’s responsibility to be able to determine which material(s) he or she prefers to use for everyday clinical situations.

About the Author

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