Light Polymerization
There are many different technologies available and several
guidelines to follow to ensure an effective curing process.
By Eduardo Mahn, DMD, DDS, Dr. med. dent.
As composites have continued to be optimized, significant differences in physical, mechanical, and clinical performances between the available systems have lessened. Yet, despite all the improvements, one constant remains: direct composites need to be light-polymerized. Clinicians need to understand the principles of light-curing because unbound monomers are cytotoxic,1,2 and improperly cured composites are less biocompatible.3,4 Composites that are inadequately irradiated, in both quantity and quality, have been found to result in:
• increased bacterial colonization,5 which reduces bond strength.6
• inferior physical properties.7-9
• excessive wear and possible bulk fracture.10,11
• postoperative sensitivity due to dissolution of uncured resin.
• recurrent caries at the interface (Figure 1 and Figure 2).12
Presently, there are four technologies available to cure composites. Once the light source is chosen, the clinician should consider several factors to ensure that the composite is being cured satisfactorily. This article analyzes the various current technologies, their strengths and weaknesses, and the relevance of following certain protocols to ensure proper polymerization rates.
Different Paths… Same Destination
An extensive array of curing lights, even within each technology, is available in today’s market. The following review of the characteristics of these curing lights highlights their benefits and drawbacks.
Halogen Lights
A halogen lamp is an incandescent lamp with a tungsten filament contained within an inert gas and a small amount of a halogen such as iodine or bromine. The light of a tungsten-halogen curing unit is produced when the thin tungsten filament, working as a resistor, is heated by the current to temperatures of around 2727°C. When the filament becomes incandescent, it emits electromagnetic radiation in the form of visible light as well as a large amount of infrared radiation, which creates the need for a filter. As high temperatures are produced in the process, heat elimination is indispensable, which, in turn, creates the need for the use of a potentially noisy fan to cool the system. Furthermore, the lifetime of the halogen bulb is limited, enabling approximately 6 months of clinical use.
Nonetheless, halogen lights are considered reliable and cure all composite materials within a reasonable period of time. They can be purchased at relatively low acquisition costs, and burned-out bulbs are relatively inexpensive to replace. What many clinicians disregard is that operating these lamps at a higher voltage greatly reduces their lifespan. A 6% increase in voltage reduces the operating life by 50%. A small reduction in voltage provides a longer lamp life; however, a reduction of more than 10% may drastically shorten lamp life as the halogen cycle cannot operate efficiently under this condition. Differences in voltage also greatly impact the efficacy of the lamps.13 While this may be irrelevant in industrialized countries, it should be strongly considered in emerging economies where the electric supply may be unreliable.
Plasma Arc Curing (PAC) Lights
These lights (noble gas discharge lamps, mainly xenon) experienced great popularity when they were introduced in 1998 because they were said to minimize curing times without affecting the mechanical properties of the cured materials. Claims of being able to cure up to 5 mm of any composite in 3 seconds convinced many practitioners to purchase them. On average, however, the recommended exposure time to cure an increment of a light-colored composite was between 3 and 5 seconds.14 In addition to their initial high prices (around three times more than a power quartz tungsten halogen (QTH) or modern light-emitting diode (LED)), maintenance is also expensive. One of the problems encountered with PAC lights, as well as with some LED lights and argon lasers, is that they might not cure a number of composites, adhesives, and protective varnishes because of an incompatibility between the photoinitiator of these materials and the light emission wavelength.15 Even if the photoinitiator is compatible with the wavelength range of the emitted light, several studies have shown that the short curing times recommended by the manufacturers are insufficient for many composite materials.14-16
Laser
In the author’s opinion, the argon laser is ideal for the scientific examination of light-curing mechanisms, as individual wavelengths can be specially adjusted, performance can be constantly maintained, and the light-transmission optics are flexible. Because of its high performance and focused light beam with a diameter of 1 mm to 3 mm, the argon laser produces a light intensity that is much higher than other technologies available.
The main advantages of lasers include the possibility of setting the brightness of the light to manufacturers’ specifications, they do not use filters because their light is monochromatic, and they emit a collimated (focused and nondivergent) beam that is more consistent over distance.17-19 Unfortunately, the technology is not free of disadvantages. Lasers are heavier, bigger, less portable, and far more expensive than the other curing devices.17
Although fast curing can be reached which, consequently, leads to short inhibition times, a rapid temperature increase and considerable shrinkage stress20 are the consequences. In vitro investigations have shown that the quality of the margins is poorer when higher intensities are used during polymerization.21-23
Although better physical properties, such as hardness and strength, were obtained with argon lasers in earlier comparisons involving conventional devices,24 later investigations could not confirm that argon lasers achieve better polymerization results or greater depth of cure with comparable intensities.25,26
Light-Emitting Diode Lights
The use of LED technology to polymerize dental materials was introduced more than 14 years ago.27 The main advantages of this technology include moderate costs, the improved light intensity of second-generation LED lights (with a corresponding reduction of the curing time), practical features (eg, smaller, more portable and cordless lights), and the long service life of diodes (approximately 1000 to 2000 hours, which is equivalent to roughly 5 years of clinical use) (Figure 3). Several studies have shown that LED lights are able to polymerize composite materials to a depth of cure with compressive strengths28 and flexural strengths29,30 that are statistically equivalent to the values obtained in composite materials cured with conventional halogen lights using the same light intensity.
The first and second generations of LED lights were not able to cure some materials because of incompatible wavelength ranges. These lamps used only one type of LED. LEDs have the ability to emit in different frequencies (colors), and, recently, some manufacturers added polywave LEDs. By combining LEDs, the generated spectrum can imitate that of QTH light, with the only disadvantage being that the output is less continuous, which could be compensated with minor movements of the lamp. Evidence about this issue is scarce; further investigations are required. This is how the latest generation of LED lights achieves an emissions peak of 410 nm in the wavelength range.31 As the cost of LED lights continues to decrease (they currently cost about the same as halogen lights but have a longer light source life) the technology, which features good portability (battery operation), high light intensity and, with its third generation, an improved wavelength range, is presently offering the clinician a well-balanced combination of positive features.
Ensuring the Polymerization of a Composite
The device used to polymerize composites is, of course, not the only factor affecting the final outcome. How the device is used can be even more influential. In a recent study, Price et al discovered that some clinicians deliver only 20% of the energy achieved by clinicians using the same curing light and in the same location.32
In attempting to ensure the polymerization of a composite the clinician faces a basic and relentless fact: there is no way to test if the composite is perfectly cured, after in vivo placement, without the extraction of the tooth and a laboratory analysis. However, by knowing a few details and controlling a few factors, one can achieve a reliable curing process.
The first question to address is determining how much energy a composite needs in order to be cured. Depending on several factors, between 6 J/cm2 and 36 J/cm2 has been reported to be enough to adequately cure a 2-mm thick increment of resin.33 In another study,31 most of the composites were adequately cured with 17 J/cm2. To simplify calculations and make them suitable for daily use and to provide an acceptable “safety range” for composites used in most clinical situations, 20 J/cm2 seems to be a safe and reasonable amount.34 This amount of energy is achieved by curing for 20 seconds with a 1000 mW/cm2 light.
Another question to consider is what would happen if a more powerful lamp was used? Could this reduce the curing time? Or, would less power output increase the time to compensate?
The polymerization depth (D) is generally expressed by the formula D = c1 x log (c2 x L x T)). The factors c1 and c2 contain data regarding the concentration of initiators and the absorption coefficients of the composite. A number of researchers were able to prove the logarithmic relationship between the curing depth and the light intensity as well as between the curing depth and exposure time shown in this formula.35,36 For the clinical practice, this formula shows that in cases in which the light intensity is increased by a certain factor, the exposure time can be decreased by the same factor.
The next problem clinicians face is to find the irradiance of the curing unit. Curing lights often deliver insufficient irradiance. Independent from the technique being used and the care the clinician is taking during the process, insufficient irradiance will lead to inadequate polymerization even after recommended curing times.37,38 Some producers of curing units may exaggerate their products’ irradiance values, so the clinician must corroborate these numbers with values given in the product literature or in tests such as Professional Product Reviews done by the American Dental Association (ADA),39 for instance. The clinician could also try to measure the irradiance with a radiometer, keeping in mind that the absolute numbers will not necessarily be accurate, but the comparison between different lamps will be useful.
One other factor to consider is the decrease of irradiance or intensity of the light with distance. The more collimated or channeled the light is, the less it diverges and spreads as it is moved away for the area to be cured. This is especially critical for curing lamps without light guides (eg, LEDs mounted at the tip of the lamp) or units using so-called “turbo” light guides. The distance between the tip of the light guide and the cavity floor of a typical Class II preparation was reported by Price et al40 to be 6.3 mm (standard deviation ± 0.7 mm) (Figure 4 and Figure 5).
In an approach that reflected real-life conditions more appropriately, Meyer et al29 observed a decrease between 33% and 83% at 10 mm, depending on the curing light and the light guide in particular. These results were confirmed in another study carried out by Price et al.41 In the last Professional Product Review by the ADA in 2009, an irradiance decrease of as little as 19% to as much as 68% between 2 mm and 9 mm distance from the target was measured.
Another factor that the clinician should consider is the diameter of the light guide. Ideally, curing units should have a diameter of 10 mm to cover the area to be cured in most of the indications, as can be seen in the clinical case depicted in Figure 6, Figure 7, Figure 8 and Figure 9<.
Conclusion
Bear in mind that once the right unit is purchased, proper technique during the polymerization process overrides other factors. As discussed, Price et al22 discovered that some dentists deliver only 20% of the energy achieved by others using the same curing light and in the same location.
Depending on the material and situation, a wide range of energy will be necessary to cure a composite properly. Twenty seconds with a powerful modern lamp (with ideally more than 1000 mW/cm2) seems to be safe for most composites and indications when direct curing is done. Unique situations, like very deep cavities, trans-tooth curing, and especially opaque composites or the curing of resin cements through indirect ceramic veneers, onlays, or crowns, will require more curing time.
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About the Author
Eduardo Mahn DMD, DDS, Dr. med. dent.
Consultant for Restorative and Aesthetic Dentistry
Samaya Clinic, Jeddah, Saudi Arabia</p>
