Composite technology has changed the practice of restorative dentistry significantly since its introduction in the late 1960s. While many composites continue to be formulated from Bis-GMA resin, the composition and distribution of filler particles have changed dramatically. The development of new fillers with particle sizes in the range of 0.01 to 0.10 μm has led to composites with improvements in esthetics, polymerization shrinkage, strength, wear resistance, and handling. More recently, with the growing concern of polymerization shrinkage, companies have developed low-shrinkage composites.
This issue of THE DENTAL ADVISOR will focus on microfills, microhybrids, nanofills, and nanohybrid composites. New techniques and innovations will be discussed, as well as clinical tips. Clinical advantages and limitations of low-shrinkage composites will be discussed as well.
Types of composites
The most commonly used classification system for composite resins considers the distribution and average particle size of a given composite’s filler load. Most resin composites can be classified into three main categories: microfills, microhybrids, and nanofills.
MICROFILLS
Particle size: 0.04 to 0.1 μm
Main characteristics: High polish that can be achieved and maintained over time and excellent enamel-like translucency.
Clinically: Microfilled composites work best for anterior restorations, where esthetics is a main concern. Their small particles allow for high luster polish; however, because they are less filled than any other composite, they do lack strength. Many dentists use microfills as an “enamel” layer placed over stronger microhybrid resin.
Indications: Anterior restorations, cervical abfraction lesions
Contraindications: Should not be used in heavy stress-bearing areas.
MICROHYBRIDS
Particle size: 0.4 to 0.6 μm
Main characteristics: High strength and most closely simulate dentin. They are the most opaque of all composites, and therefore, great for supporting a microfill layer. They can be finished and polished well, but lack long-term smoothness and gloss.
Clinically: Microhybrids have the needed strength and wear for posterior composites, while having the polish and esthetic potential for anterior restorations.
Indications: Microhybrids are considered a universal composite that can be used in anterior and posterior teeth.
NANOFILLS
Particle size: 0.01 to 0.10 μm
Main characteristics: Nanoparticles are the smallest particles used in conjunction with composite resins. While conventional particles are generated from ball-milled or ground from larger-sized particles into smaller ones, nanoparticles are built up on the molecular level. There are two types:
- Nanofills — contain nanometer-sized particles throughout the resin matrix
- Nanohybrids — consist of larger particles surrounded by nanometer-sized particles
Clinically: Unlike microhybrids, nanofills have a lifelike, opacious translucency that is very natural in appearance. When compared to microhybrids, nanofills will maintain better surface smoothness and polish and are great for the single-product user.
Indications: Posterior restorations, lower anteriors, composite veneers, building up incisal edges, Class IV restorations, and when restoring or increasing cuspid rise.
Ideal characteristics of composites
- Low polymerization shrinkage
- Highly esthetic
- Good depth of cure
- Low ambient light sensitivity
- Color stability
- Good handling characteristics
- Adequate translucency
- High polishability and longevity
- High strength and wear resistance
- Biocompatibility
- Radiopacity
Low-shrinkage composites
Dental composite products undergo shrinkage during curing due to the conversion of monomers to polymers. This shrinkage is proportional to the conversion efficiency and can contribute to shortcomings in restorative procedures; i.e., marginal leakage, discoloration, tooth fracture, dental decay, and gross failure. Numerous approaches to minimize cure-shrinkage ratios have been investigated. These include various curing light intensities and ramping of light application and duration of application; particle wettability, size, shape and surface area; pre-polymerized filler particles, and additions to the basic Bis-GMA/TEGDMA resin systems. Examples of low-shrink composites with new monomers are GC Kalore (GC America) and N’Durance (Septodont).
Improvements in polymerization methods have produced longer wearing, harder, denser, restorations with expanded use potential and improved predictability. As always, precise attention to each manufacturer’s instructions and proven restorative techniques are required to maximize the potential of each product.
Self-adhesive composites
With the recent market entries of self-etching resin cements and adhesives, there has also been a push to develop a resin composite system with self-etching capability. Currently, the only self-adhesive composite on the market is Fusio Liquid Dentin (Pentron Clinical Technologies). Fusio Liquid Dentin is a self-adhering flowable composite that bonds to both dentin and enamel without a separate adhesive. It shares the same indications as a traditional flowable composite, but also serves as a dentin replacement material or a self-adhesive liner.
To read more, including information on Matrix Systems, low-shrinkage composites, self-adhesive composites, and polishing systems, visit www.dentaladvisor.com.
