Tipping Point: An Update on Curing Lights, Resin Composites, and Matrix Systems

Dental light-curing units can be broadly divided among quartz-tungsten-halogen (QTH) lights and light-emitting diode (LED) lights. QTH lights contain a quartz bulb housing a tungsten filament in halogen gas. These bulbs emit a broad spectrum of visible light and heat, and a filter is used to limit the light output to within 200 nm to 550 nm. Due to the large amount of heat produced by the filament, a cooling fan is added to these lights. The high voltage necessary for the bulb requires these lights to be plugged into a power source. In addition, the output of the bulb will decrease with time, which requires bulb replacement. These drawbacks of QTH lights were addressed with the development of LED lights, which are cordless, are smaller (due to the absence of a fan), and have a stable light output over time. LED lights, in contrast to QTH bulbs, are fabricated to emit a narrow wavelength of light specific to the photoinitiators in dental composites. Initial LED curing lights contained an array of blue LED bulbs that were similar to the blue LED lights seen in a string of Christmas lights. Current-generation LED lights feature a chip, which has a higher surface area for emitting light. These chips are housed either in the body (or handle) of the light with a removable fiberoptic light guide to carry the light to its tip, or the chips can be housed directly in the tip of the light. The advantage of a removable fiberoptic light guide is that it separates the heat-generating LED chips from the tooth, and it allows replacement of the light guide if it is damaged. The advantage of housing LED chips in the tip of the light is that no light energy is dissipated in the light guide.

When evaluating an LED dental curing light, several parameters should be considered; these include power output, spectral output, beam profile, and beam parallelism. The radiant emittance, commonly known as the power output, is a measure of the light energy delivered per unit time per surface area of the light tip (mW/cm2). A more powerful light (higher radiant emittance) will be able to activate more of the composite’s photoinitiator and penetrate deeper into the composite. In other words, it will require a shorter cure time and produce a deeper depth of cure.² However, the benefits diminish as power increases above a certain value, and current composites still require a minimum curing time.³ Power output should be monitored using a dental radiometer. If the values decrease with time, the light tip should be inspected for remnant composite or surface damage. Dental radiometers, however, are not accurate tools for comparing brands of lights, as they are all sensitive to different wavelengths of light. A light tested with one radiometer may perform drastically different when it is tested with another radiometer.⁴

Another differentiating feature of lights is their spectral output, or the wavelengths of light they emit. Most dental curing lights contain an LED chip that emits blue light around the 465-nm wavelength. Polywave lights contain additional chips that emit violet light around the wavelengths of 445 nm or 405 nm.

The beam profile refers to the localized power and spectral emittance over the tip of a curing light. Beam profile can be measured by imaging the tip of a curing light and mapping the areas that emit more or different colors of light. Lights with a non-homogenous beam profile have been shown to undercure composites in areas where the light was weak or the wrong color.⁵ To reduce the effects of a non-homogenous beam, the curing light can be waved in small movements to ensure the entire restoration receives adequate light.

Beam parallelism refers to light rays exiting the curing light in the same direction without spreading outward. Beams that spread significantly after exiting the light tip will show a significant reduction in power with an increased distance from the light tip.⁶ Beam parallelism can be demonstrated by shining a light on the edge of a piece of paper and observing its glow on the face of the paper. A parallel beam will produce an even column of light. Some new lights contain a collimating sphere to ensure that light rays exiting the light tip are parallel and will not spread as far as 10 mm from the light tip. A parallel light beam will assist the clinician by compensating for situations in which it is difficult to position the light tip on the composite restorations. Some lights have turbo tips, which are light guides that taper from a larger circumference where they exit the body of the light to a smaller circumference at the light tip. Turbo tips can increase the power output from a light; however, they will also cause the light beam to diverge and reduce the light energy at distances away from the tip.⁶ Therefore, it is particularly important to keep the light tip as close as possible to the restoration when using a light with a divergent beam or a turbo tip.

Purchasing a high-performing light, however, will not guarantee adequate polymerization, as poor technique can drastically reduce the light energy received by the composite. The distance of the curing light from the restoration will significantly decrease the energy received by the composite.⁷ Therefore, the clinician should take time to position the face of the light tip as close as possible to and parallel with the restoration prior to curing. If the light cannot be positioned close to the restoration due to the location of the restoration or the anatomy of the patient, the curing time should be increased.

Two questions often put forth by clinicians are “How strong does my light need to be to cure my composite” and “How long do I need to cure my composite?” There are no generic answers; rather, all composites require a specific energy for optimum polymerization (EOP). The filler concentration, filler type, shade, resin type, and photoinitiators present in a composite may all affect its EOP.⁸ The EOP can be determined by measuring various physical and chemical properties of the composite after it has received incremental amounts of light energy (either by varying curing time or light output). Therefore, the time required to cure a composite to achieve its optimal properties will be related to the power output of the light, or, stated in another way, lights with lower power output will require more curing time. The manufacturer does not provide the EOP for each brand of composite. The instructions for use of the composites will generally give a curing time for high power lights (typically over 1000 mW/cm2) and a longer curing time for less powerful lights. The checkMARC® (Blue Light Analytics, checkmarc.net) system was developed to get an exact match between the EOP of the composite and the output of a curing light. The unit has a light radiometer connected to a software package containing the EOP for many of the major brands of dental composites. This unit will inform the clinician of the exact amount of curing time required for their composite with their curing light.

Another concern regarding composite and light compatibility is related to the spectral output of the curing light and the peak absorbance of the photoinitiators in the composite. Monowave LED lights contain LED chips, which produce a blue-light output at 465 nm to match the peak absorbance of 468 nm for the most commonly used photoinitiator, camphorquinone (CQ). Other photoinitiators, such as phenylpropanedione (PPD) and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO, peak absorbance = 350 nm to 410 nm), have been added to translucent and bleach shades of composite to avoid the yellow tint associated with using CQ. To activate those photoinitiators, polywave lights were developed with violet LED chips at 445 nm and 405 nm. A new germanium-based initiator (Ivocerin™, Ivoclar Vivadent, ivoclarvivadent.us) with a peak absorbance at 408 nm was added to a bulk-fill composite. The use of a polywave light, however, was not necessary to achieve a 4-mm depth of cure with this material.⁹ Of the composites on the market, the only composites that benefit from polywave curing lights are translucent and bleach shade composites containing PPD or TPO.

Some manufacturers have also employed strategies for improving curing with translucent matrix bands and rings or matrices with perforations. Little evidence exists to show the advantages of these systems.¹⁰ Practically transparent sectional matrices can be cumbersome to place and contour, as they are not as firm or malleable as metal matrices.

Nathaniel C. Lawson, DMD, PhD

Director, Division of Biomaterials, Department of Clinical and Community Sciences, University of Alabama at Birmingham School of Dentistry, Birmingham, AL

Augusto Robles, DDS, MS

Director of Operative Dentistry, Division of General Dental Sciences, Department of Restorative Dentistry, University of Alabama at Birmingham School of Dentistry, Birmingham, AL