Curing lights are used to activate photoinitiators in restorative materials to initiate polymerization. Photoinitiators are activated by absorbing photons. The change in the molecular structure of the restorative material, or polymerization, occurs as monomers are incorporated into a polymer network.1 The amount of activated photoinitiator depends on the concentration of photoinitiator in the material, the number of photons to which the material is exposed and the energy of the photons. The number of photons and energy of the photons (wavelength) depends on the curing light.1 Photoinitiator activation occurs at specific wavelengths. The most common photoinitiator in dental materials is camphoroquinone, the activity of which peaks between 470 and 480 nanometers.2
Factors affecting polymerization include filler type (size and loading), the effectiveness of the light transmission (for example, light guide tips being free from debris and scratches), thickness and shade of restorative material, exposure time, distance of the light source from the restorative material and light intensity.3 Several types of available curing units have different light intensities and light sources. Light-curing units use halogen-based; light-emitting diode, or LED; plasma-arc; or laser technology. The energy levels range from 300 to more than 1,000 milliwatts per square centimeter.4
Halogen bulbs generate light through the heating of tungsten filaments to high temperatures. A small percentage (< 1 percent) of the energy is given off as light, while most of the energy given off is in the form of heat.1,4 A drawback of halogen bulbs is that this generation of heat causes a degradation of the components of the curing unit over time.5,6 The result can be a decline in the irradiance, which compromises the curing ability of the unit. A study by Barghi and colleagues7 found that 45 percent of light-curing units in 122 dental offices had outputs below 300 mW/cm2. Even when light output is set to 300 mW/cm2, a study by Fan and colleagues8 found that only 62 percent of the resin-based composites tested were adequately cured, with a light intensity of 300 mW/cm2 in the 400- to 515-nm–wavelength range, using the manufacturer’s recommended irradiation times. Ninety percent of the composites cured when twice the recommended irradiation times were used. The recommendation was for dentists to determine the appropriate curing time for the composites used in their dental offices and to occasionally monitor the curing light and composite performance.
LEDs use junctions of doped semiconductors for generating light. The advantages of these units are that the spectral output falls between 400 and 500 nm so that no filters are required.6 Because the energy generated is not in the form of heat, LEDs have a longer lifetime, with little degradation over time.5 Studies have compared the curing ability of the halogen-based vs. LED units. Jandt and colleagues6 reported that the depth of cure and the compressive strength of samples cured with a halogen-based unit were not significantly different from samples cured with an LED. However, Dunn and Bush5 reported that halogen-based units produced significantly harder top and bottom resin-based composite surfaces than did the LED units tested. Similarly, Knezevic and colleagues3 showed that the degree of conversion of carbon bonds, a measure of the amount of polymerization, is higher for materials that are cured using halogen-based curing units compared with LEDs. The downside is that with the higher degree of conversion there is an associated increase in the temperature and polymerization shrinkage of the composite material.9
Plasma-arc lights are made up of two electrodes in a xenon-filled bulb. Current heats the plasma to several thousand degrees Celsius. The heated plasma gives off light and heat (as with halogen bulbs most of the energy is given off as heat, and less than 1 percent is given off as light). The light intensity given off by plasma-arc units is greater than that for halogen-based units, which can decrease curing time up to 75 percent10; but, like halogen, filters are required.1 The increase in light intensity and decrease in curing time with plasma-arc lights have been shown to enhance polymerization shrinkage in some studies,10–12 but not in another.13 Additionally, Millar and Nicholson14 found that restorative materials cured with plasma light have different solubility and water uptake properties compared with materials cured with halogen light. They concluded that this may affect the long-term durability of materials. However, Sfondrini and colleagues15 found no difference in shear bond strength of a composite resin and a resin-modified glass-ionomer cured with plasma vs. halogen light.
Lasers can emit light at specific wavelengths as a result of the excitation of atoms of suitable gases to specific energy levels.1 Because lasers emit light at specific wavelengths, there is no need for filters. Lasers are reported to require less time to adequately polymerize composites.2,16,17 However, these units are large and expensive.
There are many factors that affect curing of dental materials. The most important of these are intrinsic to the material and the light source. There are, however, steps that the dental office can take to maximize the curing process, such as keeping the light tip clean and free of scratches, positioning the light tip in the correct distance from and the correct orientation to the material, maintaining the bulb and filter in good working order, and establishing appropriate curing times for particular materials.
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