Temperature variation caused by high-intensity LED curing lights in bovine dentin
By Fernanda Brandao Mollica, DDS, MSc
Melissa Aline Silva, DDS
Maria Amelia Maximo De Araujo, DDS, MSc, PhD
Maria Filomena R. Lima Huhtala, DDS, MSc, PhD
Ivan Balducci, DDS, MSc
Featured in General Dentistry, July/August 2009
Pg. 342-347
Posted on Thursday, July 02, 2009
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This study sought to evaluate the temperature variations in bovine dentin when cured with high-intensity LED appliances and quartz-tungsten-halogen (QTH) appliances. Forty-five slices of bovine dentin (0.7 mm thick) were divided into three groups. Temperature variations were measured during polymerization of the adhesive (10 seconds), during polymerization of the resin composite (40 seconds), and 24 hours after the resin composite polymerization. The data were submitted to the ANOVA repeated measures test, which showed a statistically significant difference in the interaction factor (p = 0.0001). Tukey’s test (p = 5%) revealed that the SmartLite PS LED appliance caused a significantly higher temperature increase than the other appliances following polymerization of the adhesive, that both LED appliances produced significantly greater temperature increases than the QTH curing light during polymerization of the resin composite, and that the SmartLite PS produced the greatest temperature increase after 24 hours.
Received: June 16, 2008
Accepted: August 1, 2008
The intrapulpal temperature increase generated during restorative treatments has been a matter of concern among dentists, since it could cause irreversible damage to the pulp tissue. Concerned about the effect of heat on pulp vitality, Zach and Cohen conducted a study on monkey teeth and found that increasing intrapulpal temperature by 5.6°C caused necrosis in 15% of the teeth.1
During a restorative procedure involving resin composite, heat is generated from the exothermic reaction of the resin composite polymerization reaction and from the energy from the light sources.2 The quartz-tungsten-halogen (QTH) appliance is the most common appliance for photocuring resin composites; it generates light by heating a tungsten filament electrically to produce extremely high temperatures.3 Its lamp produces a broad spectrum of wavelengths, ranging from 400–500 nm.4 An internal filter removes the majority of radiation outside the blue range, particularly that in the red and infrared ranges, reducing the heat generated.4,5
In addition to generating considerable heat, the light intensity of QTH appliances diminishes over time, due to various factors that include voltage fluctuations, lamp deterioration, reflector or filter deterioration, contamination of the light polymerizing tip, effects of disinfection procedures on light transmission, and malfunctioning of the light conductor fibers.4,6
To resolve some of the problems inherent to QTH appliances, LED-based appliances were developed for photocuring resin composites. The first LEDs appeared in the 1960s and initially emitted light in the red, yellow, and green spectra. Only in the 1990s did LEDs begin to emit blue light, thus achieving a sufficient light intensity to be used in photocuring appliances.7 These appliances have semiconductors that generate blue light when submitted to electric current.8 As the light emitted is specific to the semiconductor used in the circuit, LED appliances produce a narrow wavelength band that is chosen for the express purpose of exciting the photoinitiators commonly used in resin composites—particularly camphoroquinone, which achieves
LEDs last for thousands of hours, while a conventional QTH lamp lasts for only 30–50 hours; in addition, LED appliances convert electricity into light in the adequate wavelength more efficiently.4 Moreover, LED appliances generate less heat to the dental structure than QTH devices.10
Although they generate little heat, the first generation of LED appliances did not perform as well as conventional QTH appliances.9,11,12 These appliances emitted a very narrow wavelength spectrum (450–470 nm) and were not effective for camphoroquinone or for polymerizing resin composites containing certain co-initiators.13-15 Other studies also verified low polymerization efficiency among this generation of LED appliances when compared with QTH appliances.9,11
The second generation of LED appliances provided more light power but did so with a narrower wavelength of light emission than the QTH appliances; in theory, that higher power meant that they should offer better performance and shorter polymerization times.16-18
Third-generation LED appliances use a combination of LEDs to produce a broader light emission spectrum; as a result, they should polymerize more resin composites compared to second-generation LED appliances.18
According to the literature, these more powerful appliances could generate more heat.19,20 While manufacturers may claim that their LED appliances do not generate heat, studies are needed to prove such claims so that these appliances can be used with complete safety. This study was conducted to assess the amount of heat generated in bovine dentin by high-intensity LED curing lights.
Materials and methods
Bovine mandibles were obtained at a slaughterhouse and 45 incisors were extracted and cleaned using periodontal curettes. The root apexes were sectioned so that the remaining pulpal tissue could be removed later using endodontic files, while 2 mm of the incisal teeth edges were removed using a high-speed lathe (Kohlbach). The teeth were stored in closed receptacles that contained distilled water and frozen at –18°C until they could be used.
The tooth roots were embedded in self-polymerizing acrylic resin (Jet, Classico), using a silicone matrix (Silibor, Classico) so that they could be fixed to the Labcut 1010 cutter (Extec). To obtain dentin slices, the Labcut 1010 cutter was used to make serial longitudinal cuts in the mesiodistal direction of the vestibular face of the dentin. Each slice was cut to a thickness of 0.7 mm, which was confirmed with a thickness meter.
For each tooth, only the vestibular slice closest to the pulp chamber was selected, corresponding to medium dentin depth. The selected dentin slice was demarcated with a pencil and cut with a high-speed cylindrical diamond tip (1093 diamond tip, KG Sorensen), so that all slices measured 10 mm x 7 mm.
After preparation, the slices were divided into three groups (n = 15). One group would use a QTH curing light (XL 3000, 3M ESPE), the others would use one of two high-intensity LED appliances: SmartLite PS (Dentsply International) or Radii (SDI North America) (see Table 1).
Temperature variations were recorded by means of a type-T thermocouple [Copper (+) x Constantan (-)] connected to a data acquisition system model (ADS 2000 IP, Lynx Tecnologia Electronica), which in turn was connected to a computer.
A device was made to measure temperature variation. This device included a self-polymerizing acrylic resin base with a central orifice to guide passage of the thermocouple and support the dentin slice; a 1 mm thick slice of Teflon, containing a central orifice that served as insulation against the heat from the acrylic resin base on which the dentin slice was placed; and a two-piece Teflon unit with a central orifice (2 mm thick and 5 mm in diameter) supported on the dentin slice, serving as a matrix for inserting the resin composite in a single increment. Dentin slices were interposed between the Teflon.
Temperature variations were recorded through all of the dentin slices before any restorative procedures were performed; this confirmed that there were no structural variations that could affect the temperature variations. The mean temperature variation was 1.66 (±0.64)°C.
Before the bovine dentin slice was mounted in the device, the surface that would receive the resin composite increment was etched for 15 seconds with 37% phosphoric acid. At that point, the samples were rinsed for 30 seconds with an air-water spray and dried with a cotton ball. The slice was placed between two pieces of Teflon and fixed with an adhesive. To facilitate heat transfer from the dentin slice to the thermocouple tip, a thermoconductive paste (Implastec, Votorantim) was placed in the central orifice of the acrylic resin base and around the end of the thermocouple.
Next, a dental adhesive (Prime & Bond NT, Dentsply Caulk) was applied, following the manufacturer’s recommendations. The adhesive was photocured for 10 seconds; during that time, the temperature transmitted to the dentin slice was measured and recorded at every second. A resin composite (Esthet-X, Dentsply Caulk) was inserted and condensed (using an insertion spatula and amalgam condenser) up to the top limit of the Teflon matrix in a single increment. A polyester strip was placed over the resin composite and Teflon matrix. This unit was photocured for 40 seconds, supporting the tip of the light polymerizer. An air conditioner was used to maintain an ambient temperature of 23 ± 1°C.
Temperature values were recorded at every second during three stages: during light polymerization of the adhesive; during light polymerization of the resin composite; and 24 hours after polymerization of the resin composite, using the AqDados program. Temperature variation values corresponding to the
Results
The ANOVA test of repeated measurements found statistically significant differences from the interaction of factors (see Table 2). Chart 1 illustrates the differences among the conditions with each light source. After performing ANOVA, Tukey’s test was applied (see Table 3) to compare the means of the nine experimental conditions.
Tukey’s test observed that the SmartLite PS caused a significantly greater temperature increase than the other lights during the initial adhesive polymerization stage. In the resin composite polymerization stage, both LED appliances produced significantly greater temperature increases than the conventional light source. After 24 hours, the temperature increases from all three appliances were significantly lower than those produced during the resin composite polymerization stage, differing statistically among them. The SmartLite PS produced the greatest increase.
Discussion
Dentists must be alert to the potential for pulp damage due to photocuring adhesive and resin composite in deep cavities.10,19 As the thickness of the remaining dentin diminishes, pulp aggression increases.21 In the majority of studies that utilized dentin disks or even cavity preparations in teeth, the remaining dentin had a minimum thickness of 1 mm.19,21,22 However, recent studies have reduced the minimum amount of remaining dentin thickness that must be present to prevent pulp injury.
According to Murray et al, cavities with a remaining dentin thickness of more than 0.5 mm resulted in minimal injuries to pulpal tissue, while restorations with resin composite were the most capable of stimulating the formation of reactional dentin, second only to the use of calcium hydroxide cement.23 Based on their report, dentin slices with a thickness of 0.7 mm were used in this study, which is more than the minimum thickness required to avoid pulpal injury.23
It was decided to use a polyester matrix over both the resin composite and the light tip touching the composite, because heat conductivity is reduced when there is space between the restoration and the tip of the appliance.24 Clinically, placing the light tip of the appliance directly on the restorative material and/or tooth is not recommended; however, this approach made it possible to test the worst possibilities that could occur clinically and enabled a comparative analysis among the appliances. It also has to be noted that since the present study was conducted in vitro, the dissipation of heat due to contact with the surrounding tissues was not considered.24 Loney and Price affirmed that a polyester matrix significantly reduces light intensity, which could be a risk factor when restoring posterior teeth in vivo, leading to an increase in pulp temperature.24
In the present study, it was decided to use a data acquisition system through microcomputers, which would make the temperature variation measurements much more efficient and reliable when compared with traditional methods. Moreover, the use of programs intended for the acquisition and analysis of signals collected by the data acquisition system had the advantage of producing data ready for statistical analysis and export to other programs. The program AqDados demands an adequate configuration in accordance with the type of thermocouple and comparisons of tension and temperature that were used in this study.
The thermocouple type T [Copper (+) x Constantan (-)] was chosen for this study because it is more appropriate for measuring small temperature variations, providing a smaller temperature range (0–370°C) and a margin for error of ±0.5°C.
Various studies have proven that first-generation LED curing lights generated less heat than established QTH appliances.21,25-27 According to Asmussen and Peutzfeldt, these smaller increases in temperature result from the fact that this generation of LED appliances was less powerful than the present ones.20 The first-generation LED appliances had less power and required longer light activation times to polymerize resin composite adequately, resulting in a longer clinical procedure; by contrast, present-day LED appliances are more powerful and may produce greater heat emission and polymerization depth.9,17,28
Zach and Cohen’s in vivo study of Rhesus monkey teeth found that an increase of 2.7°C in intrapulpal temperature created a response of a reversible nature, an increase of 5.6°C was capable of causing necrosis in 15% of the teeth, and an increase of 16.6°C caused necrosis in 100% of the teeth.1 Although the actual critical temperature required to cause pulp damage is a source of debate, it has been suggested that pulp temperature variations must be as low as possible.29
In the present study, using the SmartLite PS to photocure the adhesive produced a significantly greater temperature increase compared with the other two appliances. This result contrasts with a 2004 study that reported no statistically significant difference during polymerization of the adhesive, even when high-intensity light sources were used.22
In terms of resin composite polymerization, both LED appliances produced significantly greater temperature increases than the conventional light source. There was no statistically significant difference between the LED appliances, which is in keeping with previous articles that have defended the idea that more powerful LED appliances must generate more heat.20,29
As the adhesive was photocured for only 10 seconds (in accordance with the manufacturer’s recommendations), it is difficult to affirm that the temperature increases produced during its photocuring were actually smaller than those produced during resin composite photocuring. Shortall and Harrington suggested that photocuring adhesive produces greater thermal injuries compared with photocuring resin composite, because the temperature increase in the empty cavity exceeded the increase produced when photocuring 2 mm of resin composite.30 Ozturk et al reported a
After 24 hours, there was a statistically significant difference in terms of temperature increase, regardless of the appliance used. The SmartLite PS produced the greatest temperature increase, followed by the Radii and the XL 3000.
According to the literature, the light source contributes most to the temperature increase during the polymerization of resin composites; when light intensity is increased, temperature must increase as well.30,31 As there was a statistically significant difference in temperature increase between the two periods of light activation in this study (that is, during photopolymerization and after 24 hours), it is believed that the resin composite’s exothermic reaction itself is a factor in temperature increase. Shortall and Harrington found smaller temperature variations when the resin composite was irradiated for a second time after polymerization. LED appliances caused the greatest temperature variations in dentin—during photopolymerization of the resin composite and after 24 hours—which suggests that these high-intensity appliances generate more heat than the conventional appliances.
It also must be considered that the temperature variations found in in vitro studies must be greater than those reported in vivo, since there are no oral tissues to dissipate the heat.24 Under clinical conditions, increases in temperature are reduced by the blood circulating in the pulp chamber and the movement of fluids in the dentinal tubules.32 In addition, the surrounding periodontal tissues can limit the intrapulpal temperature increase via heat transfer by convection.19
Conclusion
Dentists must be aware that high-intensity LED curing lights may produce temperature increases that can damage pulp irreversibly. Given the various factors that affect the pulp during restorative treatments, an increase in temperature may contribute to irreversible damage.
Based on the methodology used, the present study shows that the temperature increase in dentin was directly proportional to the light intensity of the LED appliances. The high-intensity LED appliances produced significantly greater temperature increases in dentin than the QTH appliance, both during photopolymerization of resin composite and after 24 hours.
Acknowledgements
This study was approved by the Ethics Committee on Research Involving Animals (Protocol No. 005/2007), Dentistry Faculty of Sao Jose dos
Author information
Dr. Mollica is a PhD student, Department of Restorative Dentistry, Sao Jose de Campos,
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Manufacturers
Classico, Sao Paulo, SP, Brazil; 55.11.3022.2588
Dentsply Caulk,
Dentsply International,
Extec,
KG Sorensen, Barueri, SP, Brazil; 55.11.4197.1700
Kohlbach, Jaragua do Sul, Santa Catarina, Brazil; 47.3481.3800
Lynx Tecnologia Electronica, Sao Paulo, SP, Brazil; 55.11.3839.5910, www.lynxtec.com.br
SDI North America, Bensenville, IL; 800.228.5166, www.sdi.com.au
Votorantim, Sao Paulo, SP, Brazil; 55.11.3061.9596, www.votorantim.com.br
3M ESPE,
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General Dentistry, July/August 2009 , Volume 57 , Issue 2
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