February 2013, Volume 9, Issue 2
Published by AEGIS Communications
Question: What is the latest thinking on fast-curing composites?
“Cures all composites in 5 seconds” is not an uncommon claim. However, clinicians beware; in these ads, the manufacturers make no claim as to how well that composite is cured and to what depths. These ads should be read more closely to examine not only what they say and imply, but also what they do not say. It is one thing to cure the top, irradiated composite surface—something easily done with a blue-LED keychain; however, it is another thing entirely to gain adequate cure at the bottom of a composite increment.
Perhaps an analogy to cooking a steak is in order.
Many people like their meat “flame broiled”—meaning that the outside surface of the beef is seared to help lock in all the flavorful juices. Usually the middle of the meat is only slightly cooked—just short of needing an anticoagulant.
There are many similarities between transference of radiant heat energy and radiant light energy involved both in cooking food and curing composites. When cooking, the goal is to provide a sufficient amount of heat over a specified amount of time to cook (heat-treat) the item to the desired extent. Thus, heat (radiant power) is applied over a specified amount of time. Tables found online and in cookbooks base cooking time for the desired amount of doneness—ie, “cure”—on the thickness of the meat and offer parameters based on external temperatures. For example, the instructions for rare-cooked steak may say, “Cook a ¾-inch-thick steak at 325°F to 350°F for 4 to 5 minutes on each side, over an open flame.” However, in reality, the key to cooking the steak to the desired doneness should instead involve monitoring the meat’s inside temperature, where the heat is acting locally over a given amount of time. The heat energy used to cook the steak is the mathematical product of the heat applied to the different levels of thickness and the time spent on the grill.
A piece of meat cooked to the “rare” level, when picked up, is quite limp and red, but if left for longer at the same temperature to be “medium,” it becomes darker on the outside, light brown on the inside, and has a firmer consistency than the one that was “rare.” Left even longer at the same heat level, the result is one solid, hard piece of food that is charcoal black on the outside and also very dark inside.
While a steak can be turned over to “cure” both the top and the bottom, that is not the case with curing the bottom composite surface. To have a composite that is “well-cured” throughout, a sufficient level of light energy must be supplied at the bottom surface; the top will take care of itself. Using a fixed amount of irradiance, the only way to gain a composite cure at the bottom is to extend the exposure time, even if using a high-power unit.
Clinicians should be aware that supplying high levels of blue light energy when irradiating a restoration will also deliver energy to the pulp, increasing its temperature, with the possibility of causing pulpitis, post-insertion tooth sensitivity, or perhaps, even a well-done (cooked) pulp. Therefore, in order to supply adequate visible energy for curing and to minimize thermal issues with the pulp, the author recommends that clinicians direct a stream of air over the tooth surface just prior to, during, and shortly after light exposure. This air can be from an air/water syringe, or can also be from a high-speed vacuum tip placed close to the tooth. In my laboratory and from some of my internal, unpublished in vivo data, I have found that this procedure actually decreases intrapulpal temperature rise during exposures.
The act of light-curing dental composites—especially in multiple layers or increments—can be a time-consuming process for the dental staff. The desire for rapid curing is understandable, but there is a cost. It is important to cure composite adequately to optimize the clinical performance of the material. This means that the reaction of monomer to form polymer—ie, the degree of conversion—should be maximized. Degree of conversion is directly dependent upon the total light energy delivered to the composite. The total energy is quantified by the radiant exposure (mJ/cm2)—or the product of curing time(s) and curing light irradiance (mW/cm2). Based on the principle of exposure reciprocity, the same quality material can be achieved with short exposure times and high light irradiance as with long exposure times and low light irradiance. Therefore, it is obvious that reducing the curing time for the sake of convenience requires one to use a light of very high intensity to deliver sufficient photons to activate the photosensitizer and cause polymerization. However, it has been suggested that the reciprocity rule does not hold for all resin composites at very high irradiances and very short curing times due to radical termination reactions that limit conversion. Thus, the shortest curing time possible may be limited, no matter how high-powered the curing light is due to this inefficiency.
Irrespective of the curing efficiency, manufacturers continue to develop curing lights with higher output and composites that are more efficient polymerizers by virtue of enhanced optical properties and modified catalyst composition. Thus, the dentist is provided with systems that claim 5-second or less curing times, with curing lights exceeding 1,200 mW/cm2 to 1,500 mW/cm2 output or greater. To date, minimal evidence exists from clinical studies conducted to directly compare the clinical performance of dental composites cured with very high irradiance and short exposure times vs. lower irradiance and longer exposures—ie, 20 seconds. However, it is generally believed that rapid curing produces higher contraction stresses in dental composites. These stresses can produce deleterious effects, and are a direct outcome of the polymerization shrinkage of the curing composite and other characteristics, such as elastic modulus and ability to flow. Marginal gap formation, reduced bond strength, enamel fractures, and reduced restorative properties all have been identified as negative outcomes attributed to high contraction stress. Thus, the trend with today’s composites is to reduce contraction stress, an outcome that seems at odds with the concept of rapid curing.
Time will tell whether the observations shown in the laboratory—ie, more deleterious effects on composite restorations when cured rapidly—will manifest as greater clinical failures. But for now, it would seem to be prudent to approach rapid-curing methods with appropriate caution.
Fast-curing of dental resins saves valuable office time, and many curing lights include a “turbo” or “plasma” power mode designed to make the light better and faster than previous versions. Unfortunately, there are no long-term clinical trials showing that high-power lights and faster curing times are better for the patient. However, current laboratory research does show that even under ideal conditions, using high-power curing lights for short curing times is likely to result in an under-cured resin.
Resin photopolymerization is a complex reaction that takes time to complete. In the past, when curing lights usually delivered 400 mW/cm2, resin manufacturers often recommended light-curing each 2-mm thick layer of their resin for 40 seconds. Thus, they were recommending that their resin should receive 16 J/cm2. To deliver this much energy in 2 seconds, the curing light would have to deliver 8,000 mW/cm2. Currently no curing light can do this. Most deliver about 1,600 mW/cm2, which means they need to be used for 10 seconds to deliver 16 J/cm2.
However, this is not just about having a high-power curing light in the dental office, it is also about how well the light is both used and maintained. Even if the resin could be cured adequately in 2 seconds on the laboratory bench, how the curing light is used is critical. For example, if the light tip of a high-power curing light is misaligned for 1 second when using the light for only 2 seconds, this reduces the amount of energy delivered to the resin by 50%. In contrast, a tip misalignment of 1 second when using the curing light for 20 seconds will reduce the potential amount of energy delivered to the resin by only 5%. Unfortunately, most dentists have never been taught the importance of proper light-curing technique nor the value of their curing light to the success of their resin restorations. The lack of knowledge of the importance of delivering an adequate amount of light at the correct wavelengths to the resin could explain the high failure rate of resin restorations placed in dental offices compared to controlled trials. It could also explain why every study published has shown that many curing lights in dental offices worldwide do not deliver an adequate light output. Thus, many dentists or their assistants are very likely delivering an insufficient amount of energy to their resin restorations.
In short, current research does not support curing today’s resins using fast, high-power curing lights and light exposure times of less than 10 seconds.
Back in the 1980s, my first unit had a power density of about 150 mW/cm2. Since then, resin composites are more sensitive to light energy, and the light activation units themselves have become far more powerful. Clinically, 40 seconds does seem lengthy for activating a 2-mm layer, and expediency became a popular notion. Expediency was the driving force behind the appearance of plasma arc lights, which were typically of high power densities (> 2,000 mW/cm2).
The promise of “3-second cures” and luxury automobiles were dangled before dental audiences, but there was a downside—resin composite materials shrink when they polymerize. This shrinkage exerts a force on the bonded surfaces and the force is multiplied as the number of bonded walls increases. The polymerization stress exerted is also highly dependent on the rate of polymerization, which is actually more important than the volumetric shrinkage. The faster a resin polymerizes, the greater the stress that is exerted on the bonded interfaces. This can lead to cracking of the enamel, ie, white lines. Such cracking can lead to staining and permeation of oral fluids into the interface. Absolutely all of the literature on this topic agrees that rapid polymerization generates greater magnitudes of stress than does a slower rate of polymerization.
Today, LED activation units are commonplace and they are capable of producing anywhere from 700 mW/cm2 to 3,200 mW/cm2, yet they too must be operated judiciously. Despite the availability of smaller, higherenergy activation units, the principles do not change. When it comes to resin composite polymerization, slower is better for teeth. This author is currently employing a modified “pulse activation” protocol in which the resins are exposed for a brief period of time, later followed by a longer period of exposure. This allows polymerization stress to be lessened, and in turn is less likely to result in damaged margins.
Check out these online articles for more information Curing Lights: Characteristics
The Physics of Light Curing and its Clinical Implications
A Driving Force for Curing Light Advancements
Light-Curing Considerations for Resin-Based Composite Materials
Guidelines for Successful Light-Curing
About the Authors
Fred Rueggeberg, DDS, MS | Dr. Rueggeberg is Professor and Section Director, Dental Materials, College of Dental Medicine, Georgia Regents University, Augusta, Georgia.
Jack L. Ferracane, PhD | Dr. Ferracane is Professor and Chair, Department of Restorative Dentistry, Division Director, Biomaterials and Biomechanics, Oregon Health & Science University, Portland, Oregon.
Richard Price, DDS, MS, PhD | Dr. Price is Professor of Prosthodontics/Biomedical Engineering, Department of Clinical Dental Sciences, Faculty of Dentistry, Dalhousie University, Halifax, Nova Scotia, Canada.
John Kanca, III, DMD | Dr. Kanca is in private practice in Middlebury, Connecticut.