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J Am Dent Assoc, Vol 133, No 4, 429-434.
© 2002 American Dental Association
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RESEARCH

COVER STORY
JADA Continuing Education

Curing-light intensity and depth of cure of resin-based composites tested according to international standards



P.L. FAN, Ph.D., RYAN M. SCHUMACHER, D.M.D., KRISTY AZZOLIN, B.S., RICHARD GEARY, B.S. and FREDERICK C. EICHMILLER, D.D.S.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Background. Several factors control the light curing of a resin-based composite: the composition of the composite, the shade of the composite, the wavelength and bandwidth of the curing light, the distance of the light from the composite, the intensity of the curing light and the irradiation time. The authors investigated the depth of cure of several shades of five brands of resin-based composites when irradiated via light in the 400- to 515-nanometer wavelength bandwidth at the International Organization for Standardization, or ISO, recommended intensity of 300 milliwatts per square centimeter. The resin-based composites were irradiated for the times recommended by the products’ manufacturers.

Methods. The authors used a curing light adjusted to emit 300 mW/cm2 in the 400-nm to 515-nm wavelength bandwidth to polymerize five samples of each composite brand type and shade. They measured depth of cure using a scraping method described in the ISO standard for resin-based composites. Depth of cure was defined as 50 percent of the length of the composite specimen after uncured material was removed by manual scraping. The authors determined a mean from the five samples of each composite brand and shade.

Results. Thirteen (62 percent) of 21 composite materials met the ISO standard depth-of-cure requirement of 1.5 millimeters. Six of the eight remaining materials met the depth-of-cure requirement when the authors doubled the irradiation time recommended by the product manufacturers.

Conclusions and Clinical Implications. Curing lights with an intensity of 300 mW/cm2 appear to effectively cure most resin-based composite materials when appropriate curing times are used, which, in some cases, are longer than those recommended by the manufacturers. Dentists should verify the depth of cure of a composite material as a baseline measure, and then check depth of cure periodically to confirm light and material performance. The ISO depth-of-cure measurement method can be used for this purpose.

Several factors affect light curing of a resin-based composite: the material’s composition (including opacity), the choice of photoinitiators and the concentration of the initiators. The peak wavelengths and bandwidth of the curing light, the intensity of the light and the irradiation time also have profound effects on the depth of cure. Although manufacturers determine the composition of the materials and establish recommended irradiation times, they rarely offer any suggestions in regard to the wavelength or intensity of the curing light to be used. As a result, we need to determine the factors that affect light curing and how these factors influence the depth of cure of resin-based composites.

A curing light with an intensity of 300 milliwatts per square centimeter will effectively cure most composite shades within the manufacturers’ recommended times.

Many researchers have measured the intensities of curing lights using radiometers, many of which were designed for dental office use.1–20 The depth of cure of a resin-based composite decreases with decreasing intensity of the curing light.5,7,10,11,14,16,21 An array of methods, such as hardness tests,6,10,14,16,17,21–30 interaction with color dyes,31 translucency changes,27,32 double-bond conversion,11,25,27,33,34 nuclear magnetic resonance microimaging,35 tactile tests,10 penetration tests7,12,18,35,36 and scraping tests,4,5,9,15,22,27,29,30,37–40 have been used to measure the depth of cure of resin-based composites. In addition, many studies of composites have examined the type of curing light, but provide no information about the light intensity or wavelength characteristics.

To address these concerns, the International Organization for Standardization, or ISO, developed a technical specification for measuring the intensity of a curing light.41 It also suggested a minimum intensity of 300 milliwatts per square centimeter in the 400-nanometer to 515-nm wavelength bandwidth at the curing-light tip. In addition, the 2000 ISO standard for polymer-based filling materials requires resin-based composites to have a minimum depth of cure of 1.5 millimeters when irradiated for the manufacturer’s recommended time.42 "Depth of cure" is defined in the specification as 50 percent of the length of the cured composite sample after the soft, uncured portion has been scraped away manually. A specified pass/fail criterion then is used to compare the depth of cure with the 1.5-mm requirement to determine if the materials meet the standard.42

This study investigated whether the depth of cure of several shades of five commercially available resin-based composites met the ISO-defined depth-of-cure requirement when irradiated by a curing light with an intensity of 300 mW/cm2. The study also suggests a method by which dentists can establish the depth of cure of a resin-based composite and periodically verify consistency in regard to the depth of cure.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
We used a curing light (Optilux 400, Demetron, Danbury, Conn.) throughout this study; the input voltage to the curing light was controlled by a variable transformer and was measured with a voltmeter. To generate a calibration curve, we measured curing-light intensity at several input voltages, using the method described in ISO technical specification 10650 for powered polymerization activators.41 The apparatus for measuring the light energy is shown in the figureGo. The detector of light energy (Molectron PM3, Molectron Detector Inc., Portland, Ore.) had a measurement range between 190 nm and 1,100 nm, and the energy readings were displayed on a power meter (Molectron 500D, Molectron Detector Inc.).



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  		Figure. Apparatus for measuring curing-light intensity.

 
Wavelengths. We used a bandpass filter (Schott GG 400, Melles Griot, Irvine, Calif.) that allowed light with wavelengths longer than 400 nm to pass through to the detector to limit the measured intensity to wavelengths above 400 nm. A second filter (Schott OG 515, Melles Griot) allowed light with wavelengths longer than 515 nm to pass through to the detector. The total energy of the curing light between 400 nm and 515 nm was determined by subtracting the energy of light passing through the 515-nm filter from the total energy passing through the 400-nm filter. The intensity of the curing-light tip was determined by dividing this energy difference by the cross-section area (0.8659 cm2) of the curing-light tip.

From a plot of input voltage vs. intensity, we extrapolated the input voltage needed to deliver a curing-light intensity of 300 mW/cm2 for the 400- to 515-nm wavelength bandwidth. The power supply to the curing light then was set to this voltage level, and the intensity was verified according to the ISO test method to determine the depth of cure of the resin-based composites.

Five brands of resin-based composite. We examined depth of cure using five brands of resin-based composites. Because the aim of the study was to survey several composites and not to compare brands, we designated the materials as A, B, C, D and E. Table 1Go shows the shades of each resin-based composite studied and the manufacturers’ recommended irradiation times. The depth of cure for each sample was determined using the method described in the 2000 ISO standard for polymer-based filling restorative and luting materials.42


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  		TABLE 1 DEPTH OF CURE OF RESIN-BASED COMPOSITES IRRADIATED AT 300 mW/cm2* FOR THE MANUFACTURERS’ RECOMMENDED TIME.

 
Stainless steel mold. We placed a stainless steel mold (6 millimeters high and 4 mm in diameter) on a glass slide, which was covered with a polyester film (0.05-mm thick). The mold was filled with the resin-based composite and a second polyester film was placed on the top of the filled mold. We pressed a glass slide against the upper polyester film, thus extruding the excess composite and forming a flat surface. The top glass slide then was removed and the entire assembly was placed on top of a white filter paper.

Irradiation. We then irradiated the sample through the top polyester film, with the curing light set to deliver 300 mW/cm2 (400 to 515 nm) and the light tip in contact with the polyester film. Each resin-based composite sample was irradiated for the manufacturer’s recommended time. At the end of the irradiation period, we immediately removed the composite sample from the mold, and removed the uncured material at the bottom of the sample by scraping it away manually with a plastic spatula.

Using a micrometer, we measured the length of the cured specimen to the nearest 0.01 mm. The depth of cure was recorded as 50 percent of the remaining measured length, as required by the ISO. We used five samples of each resin-based composite shade to determine the material’s mean depth of cure.

ISO depth-of-cure standard. We applied the ISO pass/fail criterion to the individual sample depths of cure to determine if the material met the ISO standard. This criterion required the first three samples to be equal to or greater than 1.5 mm in depth. Thus, we used the first three of our five samples to determine compliance with the ISO pass/fail criterion for depth of cure. When mean depth-of-cure values did not meet the 1.5-mm ISO requirement using the manufacturer’s recommended irradiation time, we prepared additional samples and irradiated them using twice the recommended times, and determined depth of cure using the same methods as those described above.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
Table 1Go lists the depth-of-cure values for the resin-based composite samples irradiated at 300 mW/cm2 for the manufacturers’ recommended times. For all shades of resin-based composites A, C and E, the depths of cure met or exceeded the ISO requirement of 1.5 mm. Two shades of composite D (A2 and D3) met the ISO depth-of-cure requirement. None of the composite B shades met the ISO depth-of-cure requirement.

Table 2Go presents the depth-of-cure values for the samples that did not initially meet the ISO requirement. After doubling the manufacturer’s recommended irradiation times, we found that the remaining two shades of composite D and four (67 percent) of the six composite B shades met the ISO depth-of-cure requirement of 1.5 mm. Only two shades of composite B did not meet the requirement at twice the recommended irradiation time.


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  		TABLE 2 DEPTH OF CURE OF RESIN-BASED COMPOSITES IRRADIATED AT 300 mW/cm2* FOR TWICE THE MANUFACTURERS’ RECOMMENDED TIME.{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Adequate curing of a resin-based composite is paramount to its clinical performance. The photoinitiator used in most composites is activated by light in the 400- to 515-nm wavelength, with 470 nm being the wavelength of peak absorption for the most commonly used photoinitiator.4 To be effective, a curing light must have sufficient energy in the 400- to 515-nm range to effectively activate the mass or increment of composite being irradiated. The light also must be capable of generating sufficient energy density, or intensity, to cure through the entire thickness of the mass or increment.

The ISO technical specification 10650 was based on commercial curing lights available at the time the specification was developed. The suggested minimum intensity of 300 mW/cm2 could be achieved by most curing lights available in the global market at that time (currently, most curing lights available in the United States can achieve this intensity). The results of this study show that 13 (62 percent) of the 21 tested samples met the ISO depth-of-cure requirement of 1.5 mm when cured according to the ISO methods. All but two of the samples that did not meet the ISO requirement at the manufacturer’s recommended irradiation time met the requirement when irradiated for twice the recommended time. Consequently, a curing-light intensity of 300 mW/cm2 should polymerize most composite shades if the appropriate curing time is used.

The ISO scraping test used to determine depth of cure requires minimal instrumentation and can be performed easily in a dental office. The ISO defines depth of cure as 50 percent of the length of the composite specimen after the uncured material is removed with a plastic spatula. Although several researchers have used the total remaining length after uncured material is removed,4,9,15,22,27,29,30,37–40 other studies have shown that the hardness of the cured composite decreases significantly from the top of the specimen toward the bottom.9,29,30,39,40 If the total remaining length were used as the depth of cure, underpolymerization likely would occur and clinical performance could be compromised.

Using the curing lights in their offices, dentists can readily adopt the International Organization for Standardization method to establish the depth of cure of various composite materials used in their practices.

Varying standards. The ISO adopted a more conservative standard, defining the depth of cure as 50 percent of the remaining length.42 Other researchers have proposed alternate standards. For example, Hansen and Asmussen9 suggested that the depth of cure be defined as 55 percent of the remaining length of the scraped specimen. DeWald and Ferracane27 compared the scraped values with those obtained with double-bond conversion, hardness tests and translucency changes as methods to determine depth of cure. Our analysis of their data, however, showed that 50 percent of the scraped length results in similar or more conservative depth-of-cure values than those determined by the extent of double-bond conversion using infrared spectrometry or hardness. Therefore, the ISO method should ensure adequate polymerization of most resin-based composites.40

Application to dental offices. Using the curing lights in their offices, dentists can readily adopt the ISO method to establish the depth of cure of various composite materials used in their practices. Knowing the depth of cure of a particular shade of material would guide them in regard to the thickness of a composite layer that could be adequately cured clinically.

Although the ISO method uses a stainless-steel mold, other mold materials can be used, with the understanding that the depth-of-cure values may differ slightly from those reported for stainless-steel molds.4 Nevertheless, these measurements provide valuable baseline information about the specific depth of cure of different resin-based composite materials used by dentists.

Once a baseline value is established, the dentist can use this method to check the depth of cure periodically to verify the performance of the resin-based composite and the curing light. Although commercial light meters are available, they measure only the intensity of a curing light. Resin-based composites can vary in composition, color and translucency, and curing-light intensity alone does not ensure adequate depth of cure. By using the ISO method to determine the depth of cure for a specific curing light and resin-based composite, dentists can obtain valuable information that can be applied clinically.


   CONCLUSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
The results of this study show that a curing-light intensity of 300 mW/cm2 in the 400-nm to 515-nm wavelength bandwidth, as measured according to ISO technical specification 10650, polymerized 13 (62 percent) of 21 resin-based composites at or beyond the ISO depth-of-cure requirement of 1.5 mm when the manufacturers’ recommended irradiation times were used. At twice the manufacturers’ recommended irradiation times, all but two of the remaining composite shades met the ISO requirement. A curing light with an intensity of 300 mW/cm2 will effectively cure most composite shades within the manufacturers’ recommended times, but some materials may require longer times. We believe it is important for dentists to establish the appropriate curing times for the various resin-based composites used in the dental office and periodically verify the adequacy of the curing light and composite performance.


   FOOTNOTES
 

Dr. Fan is senior director of Research, Division of Science, American Dental Association, and director of the Research Institute, American Dental Association Health Foundation, 211 E. Chicago Ave., Chicago, Ill. 60611, e-mail "fanp{at}ada.org". Address reprint requests to Dr. Fan.


At the time this study was conducted, Mr. Schumacher served as an American Student Dental Association summer extern at the American Dental Association, Chicago. He now is with the Dental Corps, U.S. Air Force, Washington.


Ms. Azzolin is laboratory coordinator, Division of Science, American Dental Association, Chicago.


Mr. Geary is group leader, Council on Scientific Affairs, American Dental Association, Chicago.


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


The authors thank Karen Gomolka for her assistance in preparing the manuscript.


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 DISCUSSION
 CONCLUSION
 REFERENCES
 

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