How do you recommend using LED curing lights with various composite materials for optimal polymerization?
Gary Alex, DMD
Light-curing units (LCU) are an essential component of many current restorative, prosthetic, and orthodontic procedures and are one of the most important pieces of equipment in any dental practice. In fact, the longevity and predictability of many (if not most) current restorative procedures is wholly predicated on dentists’ ability to bond various materials to the tooth via light-initiated polymerization of resin-based restoratives, cements, and adhesives. With very few exceptions, virtually all light-cured materials used in dentistry employ free-radical addition polymerization reactions that are initiated with a LCU. While several types of LCUs are currently in use, including quartz-tungsten halogen (QTH), plasma arc (PAC), argon lasers, and light-emitting diode (LED), it is the LED lights that appear to have emerged as the LCU of choice for most dentists. LEDs have many positive attributes compared to their QTH counterparts, including using less power, being lighter-weight with a smaller profile, being quieter, having less vibration, being cordless (when battery-powered), and being easily portable. LED lights are also affordable, with most in the $500 to $1,800 price range.
Simplistically stated, LED curing lights use negatively and positively charged semiconductors (typically indium gallium nitride forming a unidirectional diode) that, when subjected to an electrical charge, produces light in the blue spectral range (generally wavelengths in the 440-nm to 500-nm range) that emerges from the light tip of the LCU. This enables dentists to light-polymerize various resin-based materials provided that the wavelength of the light emitted from the LED matches the wavelength of light required to initiate the chemical reactions of the photoinitiator(s) used in whatever resin-based material is being polymerized.
Many dentists have a misconception that LED units emit no heat. This is incorrect; some of the higher-output LED units can cause soft-tissue burns or pulpal damage if used incorrectly. Some also believe that LCU intensity (measured in mW/cm2) is the most important factor in selecting a particular LCU. While light intensity is important in terms of light penetration and depth of cure, what may be more important is that the energy being delivered is compatible with the photoinitiating system of the resin material being polymerized. There can be a big difference between total energy delivered and useful energy delivered when evaluating some LCUs. Indeed, much of the energy emitted from some LCUs is at wavelengths that may not be entirely compatible with the photoinitiating system of the resin being polymerized.
The vast majority of light-polymerizable resin-based materials used in dentistry employ camphorquinone (CQ)/amine photoinitiating systems. The absorption spectrum of CQ (the wavelength of light that induces it to react) is between 400 nm to 500 nm with a peak absorbance value of around 468 nm. It is somewhat serendipitous that the wavelengths of typical blue light LEDs matches almost perfectly the peak absorbance value of CQ, which is one of reasons they were chosen for dental applications.
As mentioned, blue-light LEDs generally emit light in the 440-nm to 500-nm range, which makes them ideal for curing the CQ-initiated light-cured materials so prevalent in dentistry. Where some LEDs fall short is when initiators other than CQ are used, as is the case with some of the bulk-fill, “bleach,” and “extra-white” composites that are currently in use. Although it is used in very low concentrations, the inherent yellow color of CQ (it is a yellow powder) can make it problematic when used in some of the lighter-shaded composite materials due to its yellowing effect.1 For this reason, and also to increase the depth of cure in some bulk-fill composites, other photoinitiators are used by some manufacturers in lieu of, or most often in conjunction with, CQ. The absorption spectrum of many of these other initiators is on the fringe of, or just outside of, the wavelength of most of the LED lights currently in use. In such situations a LCU with a broader wavelength emission is desirable.
For example, Lucirin TPO is one of several photoinitiators (the others being CQ and Ivocerin® [Ivoclar Vivadent; www.ivoclarvivadent.us]) used in the bulk-fill composite Tetric EvoCeram® (Ivoclar Vivadent) and has an absorption spectrum of 340 nm to 420 nm, which is below the lower end of the wavelengths emitted by many blue-light LEDs. If a LED with a wavelength emission of 440 nm to 500 nm is used to light-cure this material, the Lucirin photoinitiator will not, or be minimally, activated because it did receive the necessary wavelength of light to cause it to react. The composite will still polymerize because the other two photoinitiators used in Tetric EvoCeram (CQ and Ivocerin with an absorption spectrum of 400 nm to 500 nm and 360 nm to 455 nm, respectively) are within the effective wavelength of the LED, but any potential benefits of the unreacted Lucirin photoinitator will not be realized. Indeed, one advantage traditional QTH curing lights have over some LED curing lights is a broader wavelength emission that is compatible with the absorption spectrum of these lower-wavelength photoinitiators. Many researchers still use these broad-spectrum QTH lights for these reasons.
To deal with this issue, many manufacturers have developed LEDs with expanded wavelength capabilities. These so-called “polywave,” “broadband,” and “multiwave” LEDs typically make use of additional LEDs that emit light at different wavelengths to expand the overall wavelength of emitted light. LEDs in this category include UltraLume and VALO® (Ultradent; www.ultradent.com); The Light and The Light 405 (GC America; www.gcamerica.com); SmartLite® Max (Dentsply; www.dentsply.com); Bluephase® G2, Bluephase® 20i, and Bluephase® 201 Style (Ivoclar Vivadent); and the Translux 2 Wave (Heraeus http://heraeus-kulzer.com). Broadband LEDs of this nature help ensure adequate polymerization of resin-based photosensitive materials that use photoinitiators other than CQ. Indeed, studies show that the use of polywave LEDs significantly improves the degree of conversion and hardness of photosensitive materials containing Lucirin photoinitiators.2
Another misconception that some dentists have is that using very high-intensity LCUs (often delivering 2,000 mW/cm2 or more at the light tip) for very short periods of time is preferable because it saves time. This is based on the belief that the physical properties (the degree of conversion) of a composite will be the same as long as the total amount of energy (joules) put into that composite is the same whether that energy is put in quickly for short periods of time with a high-intensity LCU, or slowly over longer periods of time with a lower-intensity LCU. In other words, suppose a composite requires 10 total joules of energy to completely polymerize. In principle, this could be achieved by using a high-intensity LCU delivering 2,000 mW/cm2 if it is left on for 5 seconds (2,000 mW/cm2 X 5 seconds = 10 J/cm2). This could also be achieved by using a lower-intensity light delivering 500 mW/cm2 but it would have to be left on for a longer period of time (in this case 20 seconds) in order to deliver the same total energy (500 mW/cm2 X 20 seconds = 10 J/cm2). So, if the physical properties of a composite is the same as long as the total energy delivered is the same, whether that energy is put in quickly in a short period of time or slowly over a longer period of time, then it seems to make sense to put energy in as quickly as possible because it saves time. But is this really true? The fact is many studies report a lower degree of conversion when high-intensity lights are used for shorter time periods even when the total energy delivered is the same.3 In 2014, a symposium on light-curing hosted by Dr. Richard Price, an expert in polymerization kinetics, was held at Dalhousie University in Halifax. A consensus statement from this symposium advised caution when using high-intensity LCUs in the range of 1,500 mW/cm2 to 2,000 mW/cm2 that advocate extremely short exposure times. The danger is that high-output LCUs “may not adequately cure all resin-composites to the anticipated depth when using such sort exposure times.”4