Are All Light-Curing Units Suitable to Polymerize all Resin Composites?
The use of dental adhesives, resin composites, and resin cements is almost a daily occurrence in dental practices. These materials are, in their majority, light activated to produce polymerization and setting. Optimal polymerization results in increased physical properties, reduction of leachable components from the resin, increased wear resistance, increased color stability, and greater bond strengths. The process of light polymerization is complex and is affected by several variables, which include the light curing unit, the restorative material, and the operator.
Light curing units are not all equal. Even though the blue light may appear to be the same, the composition of light output varies greatly among brands of curing lights. Lights can deliver different radiant power and irradiance, wavelength emission or spectral emission, and degrees of beam homogeneity. In addition, the effect of distance on the irradiance received by the resin may vary considerably.
The irradiance value is often inaccurate when measured by handheld radiometers or the radiometers built into light-curing units, and it is inadvisable to compare the irradiance from different curing lights when it has been measured using a dental radiometer. When measured using a laboratory-grade meter, the irradiance received by the resin (not the output at the light tip) should be no less than 400 mW/cm2, which, with a concomitant 40-second exposure, will deliver 16 J/cm2 of radiant exposure (energy) to the resin. This is considered sufficient to effectively polymerize a 2-mm increment of most resin composites. Irradiances of less than 400 mW/cm2 require an increase in the exposure time to deliver sufficient radiant exposure.
The term “spectral emission” refers to the wavelengths of light emitted by the light-curing unit. QTH (quartz-tungsten-halogen) lights emit a broad spectrum of wavelengths, thus polymerizing all resin composites in the market. Most LED (light-emitting diode) lights produce only a narrow spectral emission, so it is possible that little light of the wavelength absorbed by some photoinitiators is delivered; thus some LED lights may fail to optimally polymerize resins that contain non-standard photoinitiators. Newer “polywave” LEDs are offered in an attempt to solve this problem. However, some of these units deliver an inhomogeneous light output, so it is possible that only some areas of the resin benefit from the broader spectral emission and are optimally polymerized.
Material Selection and Technique
The resin composite material being used is as important as the light source. All materials require different amounts of energy and may require several different wavelengths to optimally polymerize. Unfortunately, most manufacturers do not specify the radiant exposure (energy) and wavelengths require to optimally polymerize their materials.
Additionally, the practitioner needs to consider other factors when polymerizing resin-based materials—an increased distance from the light tip to the restorative material will decrease the irradiance received, infection control barriers will reduce the light output, and darker or opaque shades of composites require longer curing times.
In summary, the answer to the question, “Are all light curing units suitable to polymerize all resin composites?” is no. Our recommendation for the dental practitioner is to know the output from their curing light and match the spectral emission from their light-curing unit with the absorption requirements of their resin composite. To accomplish this goal, light-curing unit manufacturers need to specify the power output (Watts) from the light, the wavelengths emitted, the spectral radiant power, the light beam profile from the curing light, and the effect of distance from the light tip on the irradiance (Watts/cm2) received by the resin. Resin composite manufacturers need to specify the wavelengths of light that will best polymerize their resin and how much radiant exposure (Joules/cm2) is required to adequately polymerize the maximum thickness of resin they recommend the dentist to use.
About the Authors
Marcos A. Vargas, DDS, BS, MS, is a professor in the department of family dentistry at the University of Iowa. His primary areas of interest include dental materials, including glass ionomers, dentin bonding, and composite resins, along with esthetic dentistry. Dr. Vargas lectures nationally and internationally on cosmetic dentistry and is an active member of the Academy of Operative Dentistry, the American Dental Association, and the International Association for Dental Research. He maintains a part-time private practice at The University of Iowa College of Dentistry in Iowa City, Iowa.
Richard B. Price, BDS, DDS, MS, PhD, is division head and professor in the department of clinical dental sciences, divisions of fixed prosthodontics and comprehensive dentistry at Dalhousie University in Halifax, Nova Scotia, Canada. He is active in researching dentin bonding systems and adhesive dentistry, and has spent more than 15 years building world-class expertise in the design, evaluation, and use of dental curing lights. He is currently investigating the longevity of posterior resin composite restorations, the light output from dental curing lights in dental offices, the existence of any “blue-light” hazard, and the benefits of fast vs. slow curing of dental restorations. Dr. Price is also a practicing prosthodontist in the Dalhousie Dentistry Faculty Practice.