JADA Continuing Education
Depth of cure and microleakage with high-intensity and ramped resin-based composite curing lights
POONAM JAIN, B.D.S., M.S. and
ADAM PERSHING, D.M.D.
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ABSTRACT
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Background. The authors conducted a study to determine whether high-intensity curing lights in high and ramped intensity modes affect microleakage of resin-based composite restorations and whether different types of resin-based composites meet American National Standards Institute/American Dental Association Specification no. 27 (1993): 7.7 for depth of cure when polymerized using these lights.
Methods. The authors compared five high-intensity lights, three plasma arc lights and two quartz-tungsten-halogen lights in their regular and ramped intensity modes with a quartz-tungsten-halogen 40-second light. The parameters tested were microleakage one month after bonding and curing depth for different resin-based composite types. The authors measured curing depth using a scratch test.
Results. Light curing with Optilux 501 (Kerr/Demetron, Orange, Calif.) for 10 seconds and ADT Power PAC (American Dental Technologies, Corpus Christi, Texas) for 10 seconds resulted in higher microleakage values than light curing with other lights (P < .05). The microhybrid resin-based composite was the only material that met the specification when light cured with all of the lights tested. The flowable resin-based composite did not meet the specification when light cured with all lights tested. Microhybrid resin-based composite had the greatest depth of cure, and flowable resin-based composite had the least depth of cure.
Conclusions. Microhybrid resin-based composite microleakage is affected by some light-curing modes. Different categories of resin-based composites are cured to different depths using high-intensity lights.
Clinical Implications. Light curing with some high-intensity lights compared with halogen lights may result in higher microleakage values. Use caution when light curing flowable resin-based composite with the high-intensity lights. Place increments less than 2 millimeters in depth when using this material.
As the number of resin-based composite restorations placed by dentists each day increases, it is becoming more important to devise new ways of decreasing the curing time and reducing marginal gaps caused by polymerization shrinkage. Recently, resin-based composite curing lights have been developed that have higher intensities and shorter curing cycles which help speed resin-based composite curing. Ramped- and stepped-intensity intensity curing lights are marketed for their ability to "soft-start" polymerization. This creates less stress at the resin-based composite-enamel/dentin joint and, thus, reduces marginal gaps in resin-based composite restorations.1 These curing lights first use a lower-intensity light followed by high-intensity light. There is limited information, however, regarding these curing lights and their effect on microleakage and curing depth for different categories of resin-based composite materials.
It is important to devise new ways to decrease the curing time and reduce marginal gaps caused by polymerization shrinkage.
Some researchers have reported that the use of high-intensity curing lights negatively affects the integrity of the restoration-cavity interface; increases the incidence of restorative margin fracture, enamel margin fracture and marginal openings; increases shrinkage stresses; and results in higher microleakage values.2–6 Other researchers, however, have reported similar polymerization shrinkage values and increased depth-of-cure values when comparing plasma arc lights with regular halogen lights and no difference in marginal adaptation of resin-based composites when comparing plasma arc lights with halogen lights.7–9
Peutzfeldt and colleagues10 reported that the shorter curing times of the high-intensity lights produced physical properties comparable to those achieved with regular halogen lights. Tanoue and colleagues11 reported that applying high-intensity lights increased the surface hardness and water solubility of a prosthetic resin-based composite.
Several researchers have examined ramped-and stepped-intensity curing lights. Burgess and colleagues1 compared soft and hard curing and reported that although the high-intensity curing lights provided rapid curing to resin-based composite increments, the two-step curing cycle (a period of low intensity followed by a period of high intensity) afforded significantly better marginal integrity.
Koran and Kurschner12 found that the two-step approach to light curing does not affect shrinkage, surface hardness or residual monomer concentrations compared with the conventional continuous curing approach if the total irradiation dose is high enough to achieve complete polymerization. Adhesion values may be improved with the two-step approach, which reduces contraction stresses in the cavity during polymerization and preserves marginal integrity.
Mehl and colleagues13 found that the initial cure using decreased light intensity followed by a final cure using high light intensity has no influence on microhardness, but this process increases flexural modulus and flexural strength and yields better marginal integrity. Yap and colleagues14 reported that soft-start polymerization did not significantly reduce polymerization shrinkage and did not affect the effectiveness of cure for the resin-based composite Z100 (3M ESPE, St. Paul, Minn.). Bouschlicher and colleagues15 reported that polymerization shrinkage force and depth of cure were dependent on duration of exposure and not the intensity of the curing light.
Dunn and Bush16 studied the adequacy of light-emitting diode, or LED, light-curing units and concluded that halogen lights produced significantly harder top and bottom resin-based composite surfaces than did LED lights.
Many different light-curing variables may affect the depth of cure of resin-based composites. These variables may be curing equipment factors (for example, bulb or filter degradation, light guide fracture, tip contamination), procedural factors (for example, light tip direction, distance, size of tip) or restoration factors (for example, cavity design, restoration thickness, shade).17 In our study, we looked at the effect of different lights and light-curing modes—for example, short high-intensity curing cycles versus the longer ramped intensity curing cycles—on depth of cure and microleakage. Although several authors have reported on the issue, there is no consensus on the effects of high-intensity and ramped or stepped (soft-start) modes of polymerization on the depth of cure and microleakage of different types of resin-based composites. We compared the microleakage values obtained after using different curing lights with those obtained after light curing restorations with the regular 40-second quartz-tungsten-halogen light (Optilux 401, Kerr/Demetron, Orange, Calif.). We studied the depth-of-cure values to ensure that the two requirements of American National Standards Institute/American Dental Association Specification no. 27 (1993): 7.7 for depth of cure were met.
The purpose of our study was two-fold. First, we evaluated microleakage values in Class V caries restored with a microhybrid resin-based composite light cured with five different lights one month after sample preparation and compared it to microleakage values obtained after curing microhybrid resin-based composite restorations using the regular 40-second quartz-tungsten-halogen light. Then, we investigated if the depth of cure of four categories of resin-based composites cured with six curing lights met the ANSI/ADA specification.
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METHODS
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The curing lights we studied and their specifications are listed in Table 1
. We tested nine curing modes with six curing lights. We used the quartz-tungsten-halogen light that uses 40 seconds of cure time for a 2-millimeter increment of resin-based composite as the control light for this study. A light tip with an 8-mm diameter was used for each curing light.
Microleakage measurement.
We used 90 extracted, noncarious premolars (n = 10) to measure microleakage. We stored the premolars in isotonic saline with 0.2 percent sodium azide until we used them. We prepared box-type Class V caries on the buccal surfaces of these premolars with gingival margins below the cementoenamel junction, or CEJ. The preparations measured 2 mm incisogingivally (1 mm below the CEJ) and 1.5 mm in depth, and they extended from mesial line angle to distal line angle. We beveled the enamel margin with a 0.5-mm bevel at a 45-degree angle. We used a no. 330 bur to cut the entire preparation and a flame-shaped diamond bur to cut the enamel bevel.
We etched the enamel of each preparation with 34 percent phosphoric acid gel etchant (Dentsply/Caulk, Milford, Del.) for 15 seconds and the dentin for 30 seconds. We then removed the etchant by thoroughly rinsing the preparation for 15 seconds. We dried the preparation to remove excess water, leaving the dentin shiny and moist. We applied Prime and Bond NT (Dentsply/Caulk) to the preparations with a gentle rubbing motion and removed excess solvent with a mild air blast. We light cured the bonding agent in each group of 10 preparations with a different curing light or mode (Table 1
). We then restored these preparations with the microhybrid resin-based composite Esthet X (Dentsply/Caulk). Since each preparation was 1.5-mm deep, a single increment of resin-based composite was sufficient to restore the preparation. Therefore, we inserted the resin-based composite in one bulk increment, sculpted and shaped it with resin-based composite filling instruments and light cured it using one of the nine different lights or curing modes listed in Table 1. We held each light tip about 1 mm away from the surface of the restoration being cured.
After completing the restorations, we subjected the premolars to 1,000 cycles of thermocycling, from 5 C to 55 C with a dwell time of one minute in each of the two water baths and 12 seconds of transfer time from one bath to the other. We then stored the premolars in 37 C water for one month.
We coated the surfaces of teeth, apart from the restoration and 1 mm of the surrounding enamel/cementum, with nail polish. We then immersed the teeth in a 2 percent solution of basic fuchsin as the tracer. After 24 hours of immersion, we sectioned the teeth into halves buccolingually, using a diamond blade in a hard-tissue saw.
Three independent examiners evaluated the cut sections for degree of dye penetration using a binocular microscope at a magnification of x 20. In each case, they recorded the most severe degree of dye penetration. Disagreements among the examiners were resolved through discussion, and their consensus was recorded. The ranking system used to score the degree of dye penetration is described in the box.
Depth-of-cure measurement.
We measured the depth of cure for four different resin-based composites using the nine different light-curing modes. The four different resin-based composites were a microhybrid, a hybrid (Prodigy, Kerr), a flowable (AeliteFlo, Bisco, Schaumburg, Ill.) and a condensable (Prodigy Condensable, Kerr). We used Vita shade A2 (Vita Zahnfabrik H. Rauter GmbH KG, Bad Säckingen, Germany) for each resin-based composite.
We measured depth of cure using the scratch test described in ANSI/ADA Specification no. 27 (1993): 7.7. The specification calls for the preparation of three specimens of each material. One-half of the actual value measured for depth of cure then is reported. According to the specification, all three values should be greater than 1 mm. Also, all three values should be no greater than 0.5 mm below the value stated by the manufacturer.
For specimen preparation, we used a 6-mm–thick acrylic block. We cut a 5-mm–diameter hole into the block (Figure 1), placed the block on a glass slab and placed a black vinyl sleeve with a 4-mm lumen diameter inside the hole (Figure 2). We placed resin-based composite inside the vinyl sleeve (Figure 3) and placed a glass slide on top of the resin-based composite. We cured the resin-based composite for the recommended curing time. We placed the curing light tip directly on top of the glass slide (Figure 4). Then we sectioned the vinyl sleeve to remove the cured resin-based composite plug from the acrylic block (Figure 5) and removed all of the soft, uncured resin-based composite material from the bottom of the resin-based composite plug, using a plastic spatula. We then measured the height of the cured resin-based composite plug using a digital micrometer, which was accurate to 0.01 mm, and divided this value by 2. We prepared and tested three samples of each category of resin-based composite.
Data analysis.
We analyzed microleakage data using the nonparametric Kruskal-Wallis test analysis of variance, or ANOVA, which yielded significant differences among the groups. We then used one-way ANOVA to analyze the data, and we obtained similar results. We used Tukey’s Honestly Significant Difference post hoc test to determine the groups with significant differences. We studied depth-of-cure data to evaluate whether all groups met both parts of the ANSI/ADA specification for depth of cure.
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RESULTS
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Microleakage.
The restorations light cured with Optilux 501 (Kerr/Demetron) (10 seconds) and ADT Power PAC (American Dental Technologies, Corpus Christi, Texas) (10 seconds) had statistically significantly (P < .05) higher microleakage values compared with the restorations light cured with the other curing lights, but they were not different from each other (Table 2, page 1220). The restorations light cured with the remaining lights listed in Table 2 did not have statistically significant different microleakage scores.
Depth of cure.
The microhybrid resin-based composite was the only resin-based composite that met both parts of ANSI/ADA Specification no. 27 (1993): 7.7 for depth of cure; the values for all three halved specimens were greater than 1 mm and none of the three values was more than 0.5 mm less than the 2 mm stated by the manufacturer (Table 3, page 1221).
The flowable resin-based composite failed to meet the depth-of-cure specifications when we light cured it with all the nine light-curing modes tested, including the 40-second halogen control light. When we used the Apollo 95 E (Dental/Medical Diagnostic Systems, Woodland Hills, Calif.) (three seconds) curing light, one of the depth-of-cure values was less than 1 mm. When we used all of the other light-curing modes, one or more values were more than 0.5 mm below the 2 mm stated by the manufacturer.
The hybrid resin-based composite met the first part of the specification when we light cured it with all nine light-curing modes. All depth-of-cure values obtained were greater than 1 mm. The hybrid resin-based composite failed to meet the second part of the specification when we light cured it with Virtuoso (Denmat, Santa Maria, Calif.) (three seconds), Apollo 95 E (three seconds), Apollo 95 E (six seconds ramp), Spectrum 800 (Dentsply/Caulk) (10 seconds), ADT Power PAC (10 seconds) and ADT Power PAC (10 seconds ramp), since one or more depth-of-cure values were more than 0.5 mm below the 2 mm stated by the manufacturer.
The packable resin-based composite met the first part of the specification when we light cured it with all nine light-curing modes. All of the depth-of-cure values obtained were greater than 1 mm. The packable resin-based composite failed to meet the second part of the specification when we light cured it with Optilux 501 (10 seconds), Virtuoso (three seconds), Apollo 95 E (three seconds) and Spectrum 800 (10 seconds).
When we plotted depth-of-cure values against time for each of the four resin-based composites, a linear fit of the data was realized in each case (Figure 6, Figure 7, Figure 8 and Figure 9, page 1222). The correlation values were significant for each material (flowable resin-based composite: r = 0.538, P = .004; microhybrid resin-based composite: r = 0.783, P < .001; hybrid resin-based composite: r = 0.59, P < .001; condensable resin-based composite: r = 0.615, P < .001). This indicates that regardless of the curing light used, depth of cure was related linearly to the duration of the light curing.
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Figure 9. Depth of cure versus time for the condensable resin-based composite (r = 0.615, P < .001). Note the linear relationship between depth of cure and time for each of the four resin-based composites.
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DISCUSSION
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In this study, we slightly modified the apparatus for the depth-of-cure test. ANSI/ADA Specification no. 27 (1993): 7.7 calls for a metal mold to be used for specimen preparation. We tried using a stainless steel mold with a 4-mm diameter hole as suggested, but we found it difficult to remove the specimens from the mold after they were cured. Therefore, we devised the black vinyl sleeves with a 4-mm lumen diameter, which fit the acrylic mold. We prepared all of the specimens using this technique.
We used only one representative of each type of light, as the emissions from these light types are supposed to conform to specified wavelength ranges. Therefore, we did not expect significant differences between different representatives of each type of light. The Apollo 95 E light no longer is being marketed. Also, we used only one representative product from each category of resin-based composite material. Therefore, the results cannot be generalized to all of the commercial materials in each category.
Microhybrid resin-based composite was the only resin-based composite material that met both parts of the depth-of-cure specification when we light cured it with each curing light tested. The only resin-based composite/curing light combination to fail to meet the first part of the depth-of-cure specification was the flowable resin-based composite when we light cured it with Apollo 95 E for three seconds. This resin-based composite failed to meet the second part of the specification when we light cured it with every light tested, including the control quartz-tungsten-halogen light.
In general, the microhybrid resin-based composite had the greatest depth of cure, whereas the flowable resin-based composite had the least depth of cure. The composition of a resin-based composite has been shown to affect the depth of cure. The resin-based composites with smaller filler particles are more difficult to cure than are the resin-based composites with larger filler particles. The small particles scatter light more than large particles, which makes it harder for light to penetrate deep into the material and means that greater irradiance or longer exposure times are needed to cure small particle resin-based composites. The ratio of filler to unfilled resin matrix also is important. The higher the proportion of filler particles, the more difficult it is for light to pass through the resin-based composite.18,19
The shade of the resin-based composite may affect its depth of cure by influencing the transmission coefficient. Several studies have found that the more-pigmented resin-based composites, especially the brown shades, are the least curable.20–22 Therefore, in this study, we tested all of the resin-based composites using the same shade (Vita A2).
In general, the ramped and stepped cure modes that provided longer exposure times yielded increased depth of cure.
The two light-curing modes that exhibited higher microleakage values than the others were Optilux 501 (10 seconds) and ADT Power PAC (10 seconds). Restorations light cured with Apollo 95 E and Virtuoso and Spectrum 800, as well as Optilux 501 and ADT Power PAC each in ramp mode, yielded microleakage values similar to those of restorations light cured with the control halogen light. Restorations light cured with Optilux 501 and ADT Power PAC yielded lower microleakage values when we tested the lights using their ramp modes. The exposure time doubled to 20 seconds for Optilux 501 in ramp mode and remained at 10 seconds for ADT Power PAC. Correspondingly, the depth-of-cure values for the microhybrid resin-based composite were higher when we light cured it with Optilux 501 in 20-second ramp mode (2.66 ± 0.01 mm) than when we light cured it with Optilux 501 in its regular 10-second mode (2.55 ± 0.09 mm). It is not clear whether the lower-intensity light to which the resin-based composite was subjected initially with the ramp mode or the increased exposure time was responsible for the lower microleakage values noted for the microhybrid resin-based composite restorations when we light cured it with Optilux 501 for 20 seconds in its ramp mode. It has been suggested that the low-intensity light to which resin-based composites are subjected to initially with the ramped and stepped cure modes may help maintain dentin-resin bonds better by lowering polymerization shrinkage forces.13 However, the fact that the stepped or ramped cure modes generally expose the resin-based composite to longer curing times, which can result in increased depth of cure, may be the reason for the lower microleakage values. When depth-of-cure values for each material were plotted against exposure time, we found a linear correlation in each case.
For the ADT Power PAC, the exposure time remained 10 seconds for the ramp curing mode, and there was no increase in curing depth of the microhybrid resin-based composite when we compared the 10-second ramp light cure with the 10-second regular light cure (2.74 ± 0.17 mm with 10-second light cure versus 2.69 ± 0.14 mm with the 10-second ramp light cure). Therefore, the lower microleakage values may be attributable to the lower-intensity light to which the resin-based composite was subjected to initially.
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CONCLUSIONS
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The restorations light cured with ADT Power PAC and Optilux 501 yielded higher microleakage scores than did the restorations light cured with the other lights (P < .05). The restorations light cured with these two lights in their ramped modes yielded similar microleakage values as restorations light cured with the control halogen light.
The flowable resin-based composite was the only material that cured to less than 2.0 mm when we light cured it with Apollo 95 E (three seconds). Therefore, it failed to meet the first part of the depth-of-cure specification when we light cured it with Apollo 95 E (three seconds). The flowable resin-based composite also cured to more than 0.5 mm below the value stated by the manufacturer when we light cured it with every curing mode tested.
The microhybrid resin-based composite material had the greatest depth of cure. It was the only material that met both parts of the depth-of-cure specification when we light cured it with every light-curing mode tested.
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Dr. Jain is an assistant professor, Department of Restorative Dentistry, Southern Illinois University School of Dental Medicine, 2800 College Ave., Alton, Ill. 62002, e-mail "pjain{at}siue.edu". Address reprint requests to Dr. Jain.
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Dr. Pershing was a senior dental student, Southern Illinois University School of Dental Medicine, Alton, when this article was written. He now is a resident, U.S. Navy Advanced Education in General Dentistry program, Parris Island, S.C.
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FOOTNOTES
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The authors thank Denmat, Santa Maria, Calif.; Dental/Medical Diagnostic Systems, Woodland Hills, Calif.; American Dental Technologies, Corpus Christi, Texas; Kerr/Demetron, Orange, Calif.; and Dentsply/Caulk, Milford, Del. for providing their respective curing lights for this study. The authors also thank Dentsply/Caulk for providing the samples of Esthet X resin-based composite used for this study.
Part of this project was supported by the Dean’s Summer Research program at Southern Illinois University, School of Dental Medicine, Alton.
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ABSTRACT
METHODS
