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J Am Dent Assoc, Vol 137, No 2, 213-223.
© 2006 American Dental Association
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RESEARCH

JADA Continuing Education

Light-emitting diode curing light irradiance and polymerization of resin-based composite



Krishna Aravamudhan, MS, Cynthia J.E. Floyd, MS, Duane Rakowski, BS, Glenn Flaim, BS, Sabine H. Dickens, Dr Med Dent, Frederick C. Eichmiller, DDS and P.L. Fan, PhD


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Background. Light-emitting diode (LED) curing lights are becoming popular; however, questions about their efficiency remain. The authors performed a comprehensive analysis of the properties of resin-based composites cured with LED lights.

Methods. The authors evaluated seven LED lights and one quartz-tungsten-halogen light (control). They measured intensity, depth of cure (DOC), degree of conversion (DC), hardness and temperature rise. They used three shades of a hybrid resin-based composite and a microfill composite, as well as one shade of another hybrid composite.

Results. Two LED lights required additional cure time to reach a DOC similar to that of the control light. DC at the top of the samples was independent of the light used. At 2.0 millimeters, the DC for several LED lights was significantly lower than that for the control light and was correlated strongly to the light’s intensity. The bottom-to-top ratio for hardness of resin-based composites cured by all but one light was greater than 0.80. All LED lights except one had smaller temperature rise than did the control light.

Conclusions. Six of the seven LED curing lights performed similarly to a quartz-tungsten-halogen curing light in curing resin-based composites.

Clinical Implications. While LED curing lights and a quartz-tungsten-halogen light could cure resin-based composites, some resin-based composites cured with LED lights may require additional curing time or smaller increments of thickness.

Key Words: Curing lights; resin-based composite; radiation effects; hardness

The use of light to polymerize dental resin-based composites is a mainstay of operative dentistry. A 2000 survey1 showed that 93.7 percent of all dentists used visible-light curing units. For more than two decades, visible-light curing units have been based on quartz-tungsten-halogen technology. The use of blue light-emitting diodes (LEDs) as an alternative method of light curing was suggested in 1995.2 Prototype LED curing lights were reported to cure resin-based composites with resulting properties similar to those obtained with quartz-tungsten-halogen light-curing units. These properties included depth of cure (DOC),3–5 compressive strength,4 flexural strength,6 hardness,7,8 and degree of polymerization or double-bond conversion.9 However, there also were reports of lower hardness or lesser degree of double-bond conversion in resin-based composites polymerized using some LED curing lights.10–12

Recognizing the potential of LED technology, manufacturers began marketing LED curing lights. With respect to efficiency, the reports on the commercially available LED curing lights were mixed. Likewise, inconsistent results were reported concerning the polymerization-related properties of resin-based composites cured with LED lights such as DOC, hardness, temperature rise, physical strength and degree of conversion (DC). Both the LED curing lights and the resin-based composites used in the evaluation influenced the results.13–32 Some LED curing lights performed similarly to quartz-tungsten-halogen curing lights, while others were reported to be less efficient.

With the introduction of many more LED curing lights to the market, dentists have an array of choices between both LED and conventional quartz-tungsten-halogen curing lights. We conducted a study to characterize commercially available LED curing lights and compare their performance with a commonly used quartz-tungsten-halogen curing light.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
We evaluated seven LED curing lights and one quartz-tungsten-halogen curing light, which was used as a control. The curing lights and their manufacturers are listed in Table 1Go along with the following characteristics: number of LEDs, presence of cord, style (gun or wand), intensity, wavelength, battery minutes per charge, light tip configuration (circular or elliptical), weight, weight distribution, presence of built-in radiometer, mode of operation and audible timer frequency.


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  		TABLE 1 Characteristics of lights tested.

 
In this evaluation, we used all of the curing lights in their standard modes and manufacturer-recommended modes corresponding to a standard cure. We did not use the ramp, fast or pulse modes. We maintained all cordless curing lights at full charge before use. We used corded curing lights at a controlled input voltage of 110 volts ± standard deviation 1 V.

Curing light intensity. We measured curing light intensity using the method described in the International Organization for Standardization (ISO) standard 10650 for powered polymerization activators.33 We reported this measuring method in a previous study.34 We used a power meter and a detector with flat response between 190 and 1,100 nanometers to measure the power from the curing lights.

We used a bandpass filter to allow light with wavelengths longer than 400 nm to pass through the detector, which limited the measured power to light with wavelengths longer than 400 nm. We conducted a second measurement using a second bandpass filter to allow light with wavelengths longer than 515 nm to pass through the detector, which limited the measured power to light with wavelengths longer than 515 nm. We determined the total power between 400 and 515 nm by subtracting the power of light passing through the 515-nm filter from the power of light passing through the 400-nm filter.

We determined the intensity (or power density) of the curing light by dividing the power (400–515 nm) by the cross-sectional area of the curing tip. For light tips with circular cross sections, we calculated the area of the curing tip from the diameter of the light tip using a toolmaker’s microscope. For the one light tip that did not have a circular cross section, we calculated the area using light tip dimensions provided by the manufacturer. We also measured curing light intensity using two commercial radiometers designed for dental curing lights (Optilux Radiometer, Model 100, Kerr Dental, Orange, Calif.; and Cure Rite Visible Curing light meter no. 644726, Dentsply Caulk, Milford, Del.). We conducted five independent measurements for each curing light.

DOC of resin-based composites. To evaluate the DOC achieved by using the LED and quartz-tungsten-halogen curing lights, we used three shades (A1, A3, A4) of a hybrid resin-based composite (TPH Spectrum, Dentsply Caulk) and a microfilled resin-based composite (Heliomolar, Ivoclar Vivadent, Amherst, N.Y.), as well as one shade (A3) of a hybrid resin-based composite (Z100, 3M ESPE, St. Paul, Minn.).

We determined the DOC using the method described in the 2000 ISO standard 4049 for polymer-based filling restorative and luting materials.34,35 We placed a stainless steel mold (6 millimeters high, 4 mm in diameter) on a glass slide covered with a 0.5-mm thick polyester film. We filled the mold with the resin-based composite and placed a second layer of polyester film on top of the filled mold. We pressed a glass slide against the top layer of polyester film to extrude the excess resin-based composite, forming a flat surface. We then removed the top glass slide and placed the entire assembly on top of a piece of white filter paper. We irradiated the sample through the top layer of polyester film with the curing light tip in contact with the polyester film for the time recommended by the manufacturer of the resin-based composite. At three minutes after the start of irradiation, we removed the sample from the mold and removed the uncured material at the bottom of the sample by manually scraping it away with a plastic spatula. We measured the length of the cured sample to the nearest 0.01 mm using a micrometer. We recorded the DOC as 50 percent of the remaining measured length. We measured five samples for each resin-based composite/shade/curing light combination.

We used a Student t test to compare each DOC resulting from the LED curing lights with the DOC resulting from the quartz-tungsten-halogen curing light. When the DOC of a tested resin-based composite irradiated with an LED curing light was statistically lower than that of the same resin-based composite irradiated with the quartz-tungsten-halogen curing light, we repeated the DOC determination by increasing the irradiation time using the LED curing light in 10-second intervals until there was no statistical difference between the DOCs.

In the case of Hilux LED MAX 1 (First Medica, Greensboro, N.C.) curing light, we had to increase the irradiation time for the Hilux LED MAX 1 curing light from 40 to 130 seconds for Heliomolar shade A3 to achieve a DOC similar to that achieved when using the control curing light. Hence, for subsequent resin-based composite/shade combinations cured with the Hilux LED MAX 1 curing light, we did not measure the baseline values (at the manufacturers’ recommended cure times; shown as "Not tested" in the tables). Instead, we started each subsequent combination at a higher cure length than that recommended by the manufacturer.

The authors irradiated the resin-based composites with the light-emitting diode curing lights and the control curing light for the times recommended by the resin-based composite manufacturers.

For resin-based composites that had an initial DOC of more than 3 mm when the 6-mm high mold was used, we repeated the DOC determination process using an 8-mm high stainless steel mold.

Surface hardness. We used the same resin-based composites and shades as we did in our DOC evaluation of surface hardness.

We irradiated the resin-based composites with the LED curing lights and the control curing light for the times recommended by the resin-based composite manufacturers. We measured Barcol hardness of the top and bottom surfaces of the cured resin-based composites using an impressor (Model GYZJ 934-1, Barber Colman, Loves Park, Ill.). We used a 5-mm diameter split brass mold to prepare resin-based composite samples to determine surface hardness. The height of the mold corresponded to the composite manufacturers’ recommended incremental thickness for placement (2 mm for Heliomolar and TPH Spectrum, 2.5 mm for Z100). We placed the split mold on a glass slide covered with a 0.5-mm thick polyester film. We filled the mold with the resin-based composite and placed another layer of polyester film on the top. We pressed a glass slide against the top layer of polyester film to extrude excess resin-based composite. After removing the top glass plate, we placed the assembly on the top of a piece of white filter paper. We placed the curing light on the top layer of polyester film and irradiated the sample for the time recommended by the manufacturer. At three minutes after the start of irradiation, we removed the sample from the mold and determined the surface hardness at three points on each of the top and bottom surfaces. We used a Student t test to compare the ratio of the bottom-to-top hardness for the resin-based composite/shade/light combination with the corresponding ratio for the quartz-tungsten-halogen curing light.

Double-bond conversion. We used Heliomolar shades A1 and A4 to evaluate the double-bond conversion.

We irradiated the resin-based composites with the LED curing lights and the control curing light for the times recommended by the resin-based composite manufacturers. We estimated the double-bond conversion as a function of depth using a near infrared (NIR) spectroscopic technique.36 We inserted uncured resin-based composite into a slotted mold; the slot’s dimensions were 11 mm top to bottom, 2 mm thick and 5 mm wide. We clamped the mold between glass slides. We recorded the net thickness of the resin-based composite (thickness of the assembly minus thickness of the glass slides) and obtained the NIR transmission spectrum of the uncured resin-based composite from 6,500 to 4,500 cm–1.

We filled a second mold of the same dimensions with resin-based composite, clamped it between metal sheets and covered the top surface with a polyester film. We cured the resin-based composite from the top with the curing light for the assigned cure time, while the light tip contacted the polyester film. We collected NIR transmission spectra immediately. We removed the metal sheets and measured and fixed the specimen to a micrometer stage that had a 0.5 mm slot to permit transmission of the infrared beam through the sample at a controlled distance from the top. We inserted the assembly into the sample holder of an infrared spectrophotometer (Nexus 670, Thermo Electron, Madison, Wis.).

After polymerization, we obtained NIR transmission spectra of the resin-based composite at 0.5 mm, 2.0 mm and 3.5 mm from the top. To obtain the conversion of double bonds, we calculated the ratio of the area under the carbon-carbon double bond absorbance at 6,167 cm–1 of the cured resin-based composite specimen at the various depths and compared it with the area of the uncured specimen. In each case, we obtained normalization by dividing the area by the thickness of the uncured or cured specimen. We compared the results of each combination with the corresponding control using a Student t test with a significance level set at .05.

Temperature rise. We measured the temperature rise of Heliomolar shade A1 resin-based composite during irradiation with the curing lights using an apparatus similar to that used for determining the working and setting times described in ISO standard 4049.35 The apparatus consisted of a type K thermocouple inserted 1 mm into the resin-based composite, which was in a polymethylmethacrylate mold 3 mm high and 4.7 mm in diameter. The thermocouple probe was 1 mm in diameter.

We connected the thermocouple to a computer that recorded the temperature every 2.3 seconds. After recording a baseline temperature, we irradiated the resin-based composite for 40 seconds and recorded the corresponding temperature. We determined the temperature rise by subtracting the baseline temperature from the peak temperature. We measured three to four samples for each curing light. We conducted statistical analysis of the temperature rise using one-way analysis of variance (ANOVA) followed by Sidak-Holmes post hoc test.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
Light intensity measured using the ISO standard 1065033 method ranged from 119 to 447 milliwatts per square centimeter (Table 2Go). Corresponding intensity values using both the Optilux and the Cure Rite radiometers were nearly twice the ISO values, and the rankings by curing light intensities differed among all three methods. Except for the L.E.Demetron 1 (Kerr Dental, Orange, Calif.), the intensities measured according to ISO standard 1065033 of all other LED curing lights evaluated were lower than the intensities of the quartz-tungsten-halogen control curing light.


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  		TABLE 2 Intensity of lights tested.

 
The DOCs of all tested resin-based composite/shade combinations irradiated with the L.E.Demetron 1 were statistically higher compared with those irradiated with the control light (Table 3Go). For all of the resin-based composites used in this study, use of most of the other LED curing lights resulted in DOCs that were statistically lower than the DOCs resulting from the quartz-tungsten-halogen curing light used as the control. To achieve the same DOC as that resulting from use of the control light, additional irradiation time ranging from 10 to 70 seconds was needed for some of the resin-based composite/light combinations (Table 4Go, page 219).


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  		TABLE 3 Depth of cure.

 

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  		TABLE 4 Total cure time required to obtain depth of cure similar to that resulting from use of a quartz-tungsten-halogen light.

 
The ratios of bottom hardness to top hardness for three LED curing light/resin-based composite/shade combinations were statistically higher than those for the same resin-based composite/shade combinations irradiated with the control light (Table 5Go, page 220). The ratios for 16 of the resin-based composite/shade combinations irradiated with LED curing lights were statistically lower than those of the same resin-based composite/shade combinations irradiated with the control light. Use of the Hilux LED MAX 1 curing light consistently resulted in lower hardness ratios for all resin-based composite/shade combinations.


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  		TABLE 5 Barcol hardness number.

 
The degrees of double-bond conversion for Heliomolar resin-based composite shade A4 at 0.5 mm and 2.0 mm were not statistically different when the resin-based composite was irradiated with the LED lights or the control light (Table 6Go, page 221). For Heliomolar resin-based composite shade A1, however, use of three LED curing lights resulted in a statistically lower conversion at a depth of 2.0 mm and at 3.5 mm. We found a statistically significant correlation between the conversion at a depth of 2.0 mm and the power density of the lights (Pearson product moment correlation, r = 0.91, P < .002). We did not find a correlation between conversion and power density close to the exit windows of the lights.


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  		TABLE 6 Degree of conversion.

 
At the irradiation times required to achieve a DOC equivalent to that obtained with irradiation using the control light, five of the seven tested LED curing lights had significantly smaller temperature changes compared with the control light (one-way ANOVA, Sidak-Holmes post hoc test versus the control light; P < .05) (Table 7Go, page 222). At 40-seconds irradiation times, we statistically correlated the temperature rise to the power density of the lights (Pearson product moment correlation, r = 0.78, P < .001).


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  		TABLE 7 Temperature rise.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
The intensity of one LED curing light (L.E.Demetron 1) was higher than that of the control light, while the intensities of other LED curing lights were lower than that of the control light when measured using the ISO method. The intensities of two LED curing lights (L.E.Demetron 1 and Ultra-Lume LED 2, Ultra-dent, South Jordan, Utah) were higher than that of the control light when measurements were made using the commercial radiometers. In addition, the relative values and rank orders of intensity differed among all three radiometric methods. We expected this, as radiometers often have different aperture sizes, filters and detectors. Similar observations have been reported for intensity measurements of quartz-tungsten-halogen curing lights.37,38 However, simply comparing the intensities of LED curing lights to a quartz-tungsten-halogen curing light and extrapolating them to predict comparative effective curing of resin-based composites is not appropriate, as the spectral distribution of LED curing lights differs from that of quartz-tungsten-halogen curing lights.21,24
Any attempt to assess efficiency of lights should use a number of resin-based composite/shade combinations.

A more appropriate and practical method to evaluate the effectiveness of LED curing lights to polymerize resin-based composites is to determine the DOC using the scraping method described in ISO standard 404935 for polymer-based filling, restorative and luting material. DOC is influenced by the curing light’s spectral distribution and intensity, as well as the resin-based composite’s shade, degree of opacity and chemical composition. Therefore, any attempt to assess efficiency of lights should use a number of resin-based composite/shade combinations.

The results of our study showed that a number of the tested LED curing light/resin-based composite/shade combinations had statistically different DOCs compared with those of the control light/resin-based composite/shade combinations. We found an obvious difference in DOC between resin-based composite brands when we made comparisons for the same A3 shade. We also observed from these data that Heliomolar resin-based composite has a larger reduction in DOC as the shade becomes darker. In most cases, when the DOC resulting from irradiation using LED curing lights following the manufacturers’ recommended times was statistically lower than that of the DOC resulting from using control light, an increase in irradiation time of 10 to 20 seconds over the cure time recommended by the resin-based composite manufacturer using the LED curing lights resulted in a DOC similar to that of the control light. The only exception to this was the Hilux LED MAX 1 curing light, for which nearly three times the resin-based composite (Heliomolar A3) manufacturer’s recommended irradiation time was needed to achieve DOC similar to that resulting from curing using the control light.

It is important to consider the clinical relevance of some of the statistically different DOC values. Even when one LED curing light (L.E.Demetron 1) consistently resulted in a statistically higher DOC than the control light, the additional DOC was only about 0.1 to 0.5 mm. This additional depth is not likely to be clinically significant at the resin-based composite manufacturers’ recommended 2 or 2.5 mm incremental placement thicknesses for the three resin-based composites used in this study. However, other parameters such as polymerization shrinkage and microleakage may have to be addressed in separate studies. Similarly, for the E-Light (GC America Alsip, Ill.) LED curing light, the use of which resulted in statistically lower DOC in all but one resin-based composite/shade combination, the differences were only 0.1 to 0.3 mm, which also may not be clinically significant. In the case of the Hilux LED MAX 1 curing light, however, baseline DOC values recorded at the manufacturer’s recommended cure time of 40 seconds for Heliomolar shade A3 proved to be much lower than that resulting from use of the control light. For the Heliomolar shades A3 and A4, we had to increase the curing time to 130 seconds to obtain a DOC similar to that resulting from use of the control light. Differences this large could be clinically significant; therefore, it is important to evaluate each curing light/resin-based composite/shade combination individually.

The ratio of bottom-surface hardness to the top-surface hardness of a polymerized resin-based composite often is used to evaluate DOC. A ratio of 80 percent or higher usually is considered an adequate cure. We used 2.0-mm thick specimens for Heliomolar and TPH Spectrum and 2.5-mm thick specimens for Z100, based on the manufacturers’ recommended incremental thickness. The DOC results for Heliomolar shades A3 and A1, as well as all shades of TPH Spectrum, resulting from irradiation with the LED curing lights (except for the Hilux LED MAX 1 curing light) and the quartz-tungsten-halogen control light were greater than 2.0 mm. These results correspond well with the measured hardness ratios, which were all higher than 80 percent. In addition, the results for hardness ratio for the 2.5 mm (Z100) specimens irradiated with the LED curing lights (except for the Hilux LED MAX 1 curing light) and the quartz-tungsten-halogen control light were higher than 90 percent. This hardness ratio corresponds well with the DOC results of 2.7 to 3.45 mm. Furthermore, the ratios for most LED curing light/resin-based composite/shade combinations were not statistically different from the control curing light/resin-based composite/shade combination and, thus, showed similarity to the DOC results obtained using the scraping method described in ISO standard 4049.35

The degree of double-bond conversion for Heliomolar shades A1 and A4 when irradiated with the LED curing lights and the control light showed no statistical difference at 0.5 mm from the irradiated surface. At 2.0 mm from the irradiated surface, the resin-based composite specimens cured by four LED curing lights resulted in DCs that were statistically similar to the resin-based composite specimens irradiated with the control light. The DC at 2.0 mm was about 50 percent of the DC at 0.5 mm. For Heliomolar shade A4 cured with the Hilux LED MAX 1 light, however, conversion at 2.0 mm was approximately 33 percent, whereas hardness could not be measured. The bottom hardness measurements were made on the underside of the sample in a discrete plane. By contrast, DC measurements were made through a 0.5 mm slot, which may have taken into account material cured to a greater degree just above the 2 mm line. We did not, however, notice this phenomenon with other combinations having similar or lower conversion values at 2.0 mm. For the most part, there was similarity in the statistical groupings and rankings between DC, DOC and hardness ratio. This suggests that any of these measures would provide a reliable assessment of curing efficiency for a particular light/resin-based composite combination.

The temperature rise results showed no statistically higher temperature rise when Heliomolar shade A1 specimens were irradiated with any of the LED curing lights compared with the quartz-tungsten-halogen control light. However, temperature rise measurements can be influenced greatly by the experimental setup and probably do not reflect temperatures experienced during clinical curing. We conducted our experiments using the methods and instrumentation described in ISO standard 4049,35 which only offers a method of relative comparison. The ISO method relies on heat transfer largely to air and the surrounding plastic apparatus. The heat transfer within an aqueous environment of hydrated dentin and enamel would be much more rapid, resulting in a smaller temperature change. The results from our experiments showed that temperature rise during LED curing light irradiation of resin-based composites generally is less than that recorded when quartz-tungsten-halogen curing lights are used. The only exceptions were the E-Light at 50-second and L.E.Demetron 1 at 40-second irradiation times, which were less than 1 degree higher than the control light. Considering the record of safe use for quartz-tungsten-halogen curing lights over the past two decades, we would expect a similar level of safety for LED curing lights and the same precautions necessary for eye protection.


   CONCLUSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
The results of our study show that, in general, six of the seven LED curing lights evaluated had similar performance in curing resin-based composites as did a quartz-tungsten-halogen curing light. There were slight differences in the DOC, bottom-to-top surface hardness ratios, degree of cure and temperature rise. These differences varied among curing light/resin-based composite/shade combinations. A dentist could easily establish the DOC of the any curing light/resin-based composite/shade combination using the scraping method. The dentist then could use this information as a reference for periodically checking to ascertain that an adequate cure is achieved consistently over the life of the curing lamp bulb, transformer, battery or LEDs or from resin-based composite to resin-based composite.


   FOOTNOTES
 

Dr. Aravamudhan is the manager, Product Evaluations, Research and Laboratories, Division of Science, American Dental Association, 211 East Chicago Ave., Chicago, Ill. 60611, e-mail "aravamudhank{at}ada.org". Address reprint request to Dr. Aravamudhan.


Ms. Floyd is a research assistant II, American Dental Association Foundation, Paffenbarger Research Center, National Institute of Standards and Technology, Gaithersburg, Md.


Mr. Rakowski is a research assistant I, Research and Laboratories, Division of Science, American Dental Association, Chicago.


Mr. Flaim is a research assistant II, American Dental Association Foundation, Paffenbarger Research Center, National Institute of Standards and Technology, Gaithersburg, Md.


Dr. Dickens is a chief research scientist, American Dental Association Foundation, Paffenbarger Research Center, National Institute of Standards and Technology, Gaithersburg, Md.


Dr. Eichmiller is the managing director, American Dental Association Foundation, Paffenbarger Research Center, National Institute of Standards and Technology, Gaithersburg , Md.


Dr. Fan is senior director, International Science and Standards, Division of Science, American Dental Association, Chicago.


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 DISCUSSION
 CONCLUSION
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