A survey of the efficiency of visible light curing units

Polymerization efficiency of LED curing lights

PURPOSE

The purpose of this study was to compare the curing efficiency of three commercially available light-emitting diode (LED)-based curing lights with that of a quartz tungsten halogen (QTH) curing light by means of hardness testing. In addition, the power density (intensity) and spectral emission of each LED light was compared with the QTH curing light in both the 380- to 520-nm and the 450- to 500-nm spectral ranges.

MATERIALS AND METHODS

A polytetrafluoroethylene mold 2 mm high and 8 mm in diameter was used to prepare five depth-of-cure test specimens for each combination of exposure duration, composite type (Silux Plus [microfill], Z-100 [hybrid]), and curing light (ZAP Dual Curing Light, LumaCure, VersaLux, Optilux 401). After 24 hours, Knoop hardness measurements were made for each side of the specimen, means were calculated, and a bottom/top Knoop hardness (B/T KH) percentage was determined. A value of at least 80% was used to indicate satisfactory polymerization. A linear regression of B/T KH percentage versus exposure duration was performed, and the resulting equation was used to predict the exposure duration required to produce a B/T KH percentage of 80% for the test conditions. The power densities (power/unit area) of the LED curing lights and the QTH curing light (Optilux 401) were measured 1 mm from the target using a laboratory-grade, laser power meter in both the full visible light spectrum range (380-780 nm) and the spectral range (between 450 and 500 nm), using a combination of long- and short-wave edge filters.

RESULTS

The emission spectra of the LED lights more closely mirrored the absorption spectrum of the commonly used photoinitiator camphorquinone. Specifically, 95% of the emission spectrum of the VersaLux, 87% of the LumaCure, 84% of the ZAP LED, and 78% of the ZAP combination LED and QTH fell between 450 and 500 nm. In contrast, only 56% of the emission spectrum of the Optilux 401 halogen lamp fell within this range. However, the power density between 450 and 500 nm was at least four times greater for the halogen lamp than for the purely LED lights. As a result, the LED-based curing lights required from 39 to 61 seconds to cure a 2-mm thick hybrid resin composite and between 83 and 131 seconds to adequately cure a microfill resin composite. By comparison, the QTH light required only 21 and 42 seconds to cure the hybrid and microfill resin composites, respectively.

CLINICAL SIGNIFICANCE

The first-generation LED-based curing lights in this study required considerably longer exposure durations than the QTH curing light to adequately polymerize a hybrid and a microfill resin composite.

Optical power outputs, spectra and dental composite depths of cure, obtained with blue light emitting diode (LED) and halogen light curing units (LCUs)

OBJECTIVE

To test the hypothesis that a prototype LED light curing unit, (LCU), a commercial LED LCU and a halogen LCU achieve similar cure depths, using two shades of a camphorquinone photoinitiated dental composite. To measure the LCUs' outputs and the frequency of the LED LCU's pulsed light, using a blue LED array as a photodetector.

DESIGN

Cure depth and light output characterisation to compare the LCUs.

SETTING

An in vitro laboratory study conducted in the UK.

MATERIALS AND METHODS

The LCUs cured A2 and A4 composite shades. A penetrometer measured the depth of cure. Analysis was by one-way ANOVA, two-way univariate ANOVA and Fisher's LSD test with a 95% confidence interval. A power meter and spectrograph characterised the LCUs' emissions. A blue LED array measured the pulsed light frequency from an LED LCU.

RESULTS

Statistically significant different LCU irradiances (119 mW/cm2 to 851 mW/cm2) and cure depths (3.90 mm SD +/- 0.08 to 6.68 mm SD +/- 0.07) were achieved. Composite shade affected cure depth. A blue LED array detected pulsed light at 12 Hz from the commercial LED LCU.

CONCLUSIONS

The prototype LED LCU achieved a greater or equal depth of cure when compared with the commercial LCUs. LEDs may have a potential in dentistry for light detection as well as emission.

High power light emitting diode (LED) arrays versus halogen light polymerization of oral biomaterials: Barcol hardness, compressive strength and radiometric properties

The clinical performance of light polymerized dental composites is greatly influenced by the quality of the light curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing light output with time. This may result in a low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue light emitting diode (LED) LCUs have the potential to polymerize dental composites without having the drawbacks of halogen LCUs. Since these studies were carried out LED technology has advanced significantly and commercial LED LCUs are now becoming available. This study investigates the Barcol hardness as a function of depth, and the compressive strength of dental composites that had been polymerized for 40 or 20s with two high power LED LCU prototypes, a commercial LED LCU, and a commercial halogen LCU. In addition the radiometric properties of the LCUs were characterized. The two high power prototype LED LCUs and the halogen LCU showed a satisfactory and similar hardness-depth performance whereas the hardness of the materials polymerized with the commercial LED LCU rapidly decreased with sample depth and reduced polymerization time (20 s). There were statistically significant differences in the overall compressive strengths of composites polymerized with different LCUs at the 95% significance level (p = 0.0016) with the two high power LED LCU prototypes and the halogen LCU forming a statistically homogenous group. In conclusion, LED LCU polymerization technology can reach the performance level of halogen LCUs. One of the first commercial LED LCUs however lacked the power reserves of the high power LED LCU prototypes.

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

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.

Effect of curing method and curing time on the microhardness and wear of pit and fissure sealants

OBJECTIVE

To evaluate the effects of a light source, polymerization time and storage time on the microhardness and wear of pit and fissure sealants.

METHODS

Five commercial pit and fissure sealants (Fissurit F [FF], Teethmate F1 [TF], Apollo Seal [AS], Concise [CC], and Ultraseal XT Plus [US]) were used. Specimens were cured with a conventional visible light curing unit (Curing Light XL 3000) for 10, 20, 30, 40s or with a plasma arc light curing unit (Apollo 95E) for 3, 6, 9, 12s. The specimens were kept dry in light-shielded bottles at 37 degrees C for 1 week, then half of them were thermocycled. The rest of them were stored in distilled water in light-shielded bottles for another 30 days, which were kept in an incubator at 37 degrees C, followed by thermocycling. Microhardness and wear of the specimens were measured.

RESULTS

Similar degree of microhardness was achieved with the shorter curing time with the plasma arc light curing unit as with the conventional visible light-curing unit. With conventional visible light curing, the microhardness of the top surface was higher than that of the bottom surface (P<0.05). With plasma arc light curing, the microhardness of the top surface was higher than that of the bottom surface for AS and CC, but for FF, TF and US, the microhardness of the top surface was lower than that of the bottom surface, except in the 3-s curing of US. For FF, AS, CC and US, wear in the 6s curing with plasma arc light was similar to or less than that of the 30s curing with conventional visible light, but for TF, wear of the 9s curing with plasma arc light was similar to that of the 20s curing with conventional visible light. After storage in distilled water for 30 days followed by thermocycling, there was a tendency towards a decrease in microhardness and an increase in wear. There was a significant negative correlation between microhardness and wear (P<0.01).

SIGNIFICANCE

The tested curing methods differed significantly in their curing capacity. This study suggested that a plasma arc light curing unit needs shorter curing time than a visible light curing unit to achieve similar mechanical properties of the pit and fissure sealants tested.

In vitro cytotoxicity of a composite resin and compomer

AIM

This work was designed to investigate the potential cytotoxicity of two of the newer dental restorative materials. Spectrum composite resin and Dyract AP compomer.

METHODOLOGY

Cultured human endothelial cells (ECV-304) were exposed to each of the restorative materials through a 70-microm dentine barrier to simulate the in vivo clinical situation. Cell viability was measured by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide) assay and lactate dehydrogenase release assay. The effects of different extents of light-curing were also examined by microscopic examination of stained human promyelocytic leukemia cells (HL-60). Caspase-3 activation was determined as a measure of apoptotic cell death.

RESULTS

Assessment of cellular viability indicated that both materials cause cell death, with Spectrum being the more toxic. The cytotoxicity was considerably increased in the absence of the dentine barrier. Direct exposure to Spectrum for 12 h resulted in the death of 69% of the cells after full light-curing (78% of total death was by apoptosis) and 96% after partial light-curing (73% of total death was by necrosis). Assessment of caspase activation, in the absence of the dentine barrier, showed that longer curing-times resulted in an increase in the proportion of the cells dying through apoptosis, rather than necrosis, for both materials tested.

CONCLUSIONS

These results indicate the restorative materials to be potentially toxic, particularly if the degree of light-cure is inadequate.

The use and maintenance of visible light activating units in general practice

AIM

The present study to investigate the use, care and maintenance of light units in everyday clinical practice was undertaken to complement light unit emission surveys, with a view to developing a protocol for light unit use and care in everyday clinical practice.

METHOD

The investigative work comprised a survey of selected practices in the Blackburn area with follow-up practice visits to examine light units in situ, and to glean additional information in respect of light unit use and care in the practice environment.

RESULTS

Completed questionnaires were returned by 54 of 77 selected practices--a 70% response, including information in relation to 164 light units. Subsequently, 100 (61%) of these light units were examined in 42 practices according to a standardised protocol. The use and care of the light units included in the study was found to be very variable. In addition to finding that 28 (28%) had inadequate light output (<300 mW/cm2), many of the light units were found to be damaged or repaired (47, 47%). Thirty five (35%) of the light units inspected were found to have varying amounts of material adherent to the light guide exit portal.

CONCLUSION

It is concluded that practitioners should address practical aspects of their increasing reliance on light units, and to this end, guidance is offered on visible light curing and the care and maintenance of light units.

Hardness evaluation of a dental composite polymerized with experimental LED-based devices

OBJECTIVE

The main goal of this study was the hardness evaluation of a composite resin cured by five LED (Light Emitting Diodes) based devices and a comparison with a conventional curing unit. The hardness test was used to compare the efficacy of both types of light source.

METHODS

The LED-based devices were made employing an array of LEDs (Nichia Chem. Ind., Japan) emitting light peaked at 470nm. Composite resin (Z100, shade A3) was cured for 20, 40, 60, 120 and 180s with each LED-based device and for 40s with the halogen lamp. The composite samples were prepared with 0.35, 1.25 and 1.8mm of thickness. Five samples of each set of parameters were done. The hardness evaluation was performed at the non-illuminate surface with three indentations for each sample.

RESULTS

All the samples cured by the LED-based devices showed inferior hardness values when compared with the halogen lamp at the typical curing time (40s). The L6 (device composed of six LEDs) was the most efficient one of the LED-based devices. Its obtained irradiance was 79mW/cm(2), whereas the halogen lamp irradiance was of 475mW/cm(2). For the L6 device here presented, longer exposure times or a thinner resin layer are required to achieve reasonable hardness values.

SIGNIFICANCE

Besides the difference of irradiance when compared with halogen lamps, LED-based devices show to be a promising alternative curing instrument. Further development in instrumentation may result in devices even more efficient than conventional lamps.

Effect of Distance on the Power Density from Two Light Guides

PURPOSE

This study determined the effect of distance on the power density from standard and Turbo light guides (Demetron/Kerr, Danbury, Connecticut).

MATERIALS AND METHODS

Power density was measured from 0 to 10 mm away from the tip of standard 8-mm curved light guides and 13/8-mm Turbo curved light guides. To determine the effect of distance on power density, a polynomial regression line was fitted. The Kolmogorov-Smirnov (K-S) statistic and the Wilcoxon rank sum (WR) tests were used to determine if there was a difference in the rate at which the power density decreased for the standard and Turbo light guides as the distance from the tip increased. Photographs of the light dispersion from each tip were also taken.

RESULTS

At 0 mm, the mean (+/- SD) power density from the two standard light guides was 743 +/- 6.1 mW/cm2 and from the four Turbo light guides was 1128 +/- 22.1 mW/cm2. As the distance from the tip of the light-guide tip increased, the power density decreased, but the rate of decrease was greater from the Turbo light guides than from the standard light guides. At 6 mm the power density from the standard light guides fell to 372 mW/cm2 (50% of the original value) and the power density from the Turbo light guides fell to 263 mW/cm2 (23% of the original value). Both the K-S statistic and the WR sum test indicated that the distribution of light intensities was significantly different from the two light guides (WR p-value = .0246, K-S p-value < .0001). The two estimated polynomials intersected at 3.66 mm, and the 95% prediction intervals intersected at about 2.8 and 4.8 mm. Therefore, beyond 5 mm away from the tip of the light guide, the standard light guides gave higher power density readings than the Turbo light guides. Photographs showed that the light dispersed at a wider angle from the Turbo light guides than from the standard light guide.

CLINICAL SIGNIFICANCE

The design of the light guide of a light curing unit affects light dispersion, power density, and ultimately the dentist's ability to properly cure composite. For these reasons, manufacturers should report the power density at the tip of the light guide and 6 mm from the tip of the light guide, since significant differences exist between light guide designs.

Depth of cure and compressive strength of dental composites cured with blue light emitting diodes (LEDs)

OBJECTIVE

The primary objective of this pilot study was to test the hypotheses that (i) depth of cure and (ii) compressive strength of dental composites cured with either a light emitting diode (LED) based light curing unit (LCU) or a conventional halogen LCU do not differ significantly. The second objective of this study was to characterise irradiance and the emitted light spectra for both LCUs to allow comparisons between the units.

METHODS

Dental composite (Spectrum TPH, shades A2 and A4) was cured for 40 s with either a commercial halogen LCU or a LED LCU, respectively. The LED LCU uses 27 blue LEDs as the light source. The composites' depth of cure was measured for 10 samples of 4 mm diameter and 8 mm depth for each shade with a penetrometer. The results were compared using a Student's t-test. Compressive strengths were determined after 6 and 72 h, for six samples of 4 mm diameter and 6 mm depth for each shade after being polymerised for 40 s from each end of the mould. Groups were compared using a three way ANOVA.

RESULTS

The conventional halogen LCU cured composites significantly (p < 0.05) deeper (6.40 mm A2, 5.19 mm A4) than did the LED LCU (5.33 mm A2, 4.27 mm A4). Both units cured the composite deeper than required by both ISO 4049 and the manufacturer. A three way ANOVA showed that there were no significant differences in the compressive strengths of samples produced with either the LED LCU or the halogen LCU (p = 0.460). Significant differences in compressive strength of samples stored for 6 and 72 h (p = 0.0006) and of samples of different shades (p = 0.035) were found as confirmed by the three way ANOVA. The light spectra of both units differed strongly. While the halogen LCU showed a broad distribution of wavelengths with a power peak at 497 nm, the LED LCU emitted most of the generated light at 465 nm. The LED LCU produced a total irradiance of 350 mW cm-2 whereas the halogen LCU produced a total irradiance of 755 mW cm-2.

SIGNIFICANCE

The results showed that both units provided sufficient output to exceed minimum requirements in terms of composites' depth of cure according to ISO 4049 and the depth of cure and the composites' compressive strength stated by the manufacturer. Compressive strengths of dental composites cured under laboratory conditions with a LED LCU were statistically equivalent to those cured with a conventional halogen LCU. With its inherent advantages, such as a constant power output over the lifetime of the diodes, LED LCUs have great potential to achieve a clinically consistent quality of composite cure.

Dental composite depth of cure with halogen and blue light emitting diode technology

OBJECTIVES

To test the hypothesis that a blue light emitting diode (LED) light curing unit (LCU) can produce an equal dental composite depth of cure to a halogen LCU adjusted to give an irradiance of 300 mWcm-2 and to characterise the LCU's light outputs.

MATERIALS AND METHODS

Depth of cure for three popular composites was determined using a penetrometer. The Student's t test was used to analyse the depth of cure results. A power meter and a spectrometer measured the light output.

RESULTS

The spectral distribution of the LCUs differed strongly. The irradiance for the LED and halogen LCUs were 290 mWcm-2 and 455 mWcm-2, when calculated from the scientific power meter measurements. The LED LCU cured all three dental composites to a significantly greater (P < 0.05) depth than the halogen LCU.

CONCLUSIONS

An LED LCU with an irradiance 64% of a halogen LCU achieved a significantly greater depth of cure. The LCU's spectral distribution of emitted light should be considered in addition to irradiance as a performance indicator. LED LCUs may have a potential for use in dental practice because their performance does not significantly reduce with time as do conventional halogen LCUs.