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ORIGINAL ARTICLE
Year : 2013  |  Volume : 4  |  Issue : 1  |  Page : 1-5

Thermographic analysis of temperature rise in the pulp chamber with LED and QTH light curing units: An in vitro investigation


Department of Conservative Dentistry and Endodontics, Tamilnadu Government Dental College and Hospital, Chennai, Tamil Nadu, India

Date of Web Publication22-Aug-2013

Correspondence Address:
S Uma Maheswari
Flat No. 004, Manish Residency, 28th Main, Sarakki, J.P. Nagar 6th, Phase, Bangalore - 560 078
India
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DOI: 10.4103/0976-433X.116820

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  Abstract 

Aims: To compare the temperature rise in the pulp chamber with light-emitting diode (LED) and Quartz-Tungsten-Halogen (QTH) light curing units by infrared thermography. Materials and Methods: Class V cavity was prepared in 20 freshly extracted maxillary first premolars measuring 2 mm depth, 4 mm width, and 1 mm above the cement enamel junction. The samples were divided into two groups (n = 10). Cavity was etched with 37% phosphoric acid, rinsed and bonding agent was applied over the etched cavity surface on both the groups. In group I the samples were cured with LED curing unit, whereas group II were cured with QTH unit, which was followed by two incremental curing of composite. Thermal emission for QTH and LED (fast mode) light curing units were noted by Fluke Ti32 infrared thermography after 20 s bonding agent curing and first and second increment composite curing for 40 s each. Statistical Analysis: Data were analyzed with Student t-test and Friedman test using SPSS version 11.5 software. Results: The statistical analysis revealed that the temperature rise was significantly minimal with LED fast mode (Group I) in all stages of curing compared with QTH unit (Group II). There is significant rise of temperature during first incremental curing of composite, whereas insignificant increase of temperature during curing of second increment of composite (P < 0.001%). Conclusions: Temperature rise caused due to both QTH and LED light curing units does not result in irreversible pulpal damage and thermography is a viable means of quantifying the temperature changes during photo polymerization.

Keywords: Infrared thermography, light-emitting diode light curing unit, Quartz-Tungsten-Halogen light curing unit


How to cite this article:
Kavitha M, JaiKailash S, Maheswari S U. Thermographic analysis of temperature rise in the pulp chamber with LED and QTH light curing units: An in vitro investigation. SRM J Res Dent Sci 2013;4:1-5

How to cite this URL:
Kavitha M, JaiKailash S, Maheswari S U. Thermographic analysis of temperature rise in the pulp chamber with LED and QTH light curing units: An in vitro investigation. SRM J Res Dent Sci [serial online] 2013 [cited 2021 May 8];4:1-5. Available from: https://www.srmjrds.in/text.asp?2013/4/1/1/116820


  Introduction Top


The rise in temperature, which accompanies visible light curing of resin composite, is caused by both the exothermic reaction process and the radiant heat from the light source. [1],[2],[3] The temperature rise in resin composite materials and surrounding oral tissues during polymerization have been assessed by previous in vitro studies, which have included the use of thermistor, [4] thermocouples, [5],[6] differential scanning calorimetry, [7] and differential thermal analysis. [8],[9] Temperature rises of between 3.3°C and 40.8°C during resin composite polymerization have been observed. But these methods can only measure point locations. These invasive methods contact with the surface under study, which will alter the temperature recording accuracy. The noncontact recording modality, electronic infrared thermography is accurate and sensitive enough to record 0.1°C.

This technique has been used frequently by most academic and scientific disciplines since 1960s. [10],[11],[12],[13] However, within the field of dentistry, its use has been scarce and was mainly due to equipment cost, lack of sensitivity, and image processing of older version of equipment. Today, through technical developments and image processing applications with dedicated image processing software, modern equipment enhance the detection of very minute temperature changes accurately and thus allows for its usage in the field of dentistry. It is considered as more advantageous over other methods of temperature measurement. Hussey et al. [14] utilized this technique for the in vivo measurement of heat generated during visible light curing of resin composite. However, factors such as shade and thickness are difficult to standardize during in vivo studies.

Despite proper use of light curing units, polymerization of light-activated composites results in a temperature rise due to the exothermic reaction process and the energy absorbed during irradiation. Irreversible damage occurred when the temperature within the pulp chamber rose by 5.5°C. [15] The objective of this study was to examine the effect of Quartz-Tungsten-Halogen (QTH) and light-emitting diode (LED) (fast mode) curing light characteristics on the temperature rise associated with composite curing and recording the temperature changes after each step by utilizing the technique of infrared thermography.


  Materials and Methods Top


Materials

LED curing unit: D2000 light curing unit with a light intensity of 900 mW/cm 2 (Apoza Enterprise CO.LTD., Taiwan) with 8 mm diameter light guide tip.

QTH curing unit: Rolence QTH CU 100 A light curing unit having light intensity of 600 mW/cm 2 . (Rolence enterprises, Unicorn Denmart, Taiwan) having the diameter of 8 mm at the light guide tip.

Tetric N bond (5 th generation) bonding agent (Ivoclar Vivadent, Asia).

Tetric N ceram Composite (A3 shade) composite (Ivoclar vivadent, Asia).

Thermography: Fluke Ti32 Infra Red Thermography (Fluke corporation, USA).

Methods

Twenty freshly extracted maxillary first premolar teeth were collected and stored in saline. The teeth were cleaned and mounted in stone block (10 × 10 mm) so that it could be properly seated on the table during thermographic analysis.

Class V cavities were prepared on the buccal side of the teeth with the dimension of about 2 mm depth, 2 mm occluso gingival height, 4 mm mesio distal width, and 1 mm above the cement enamel junction for all the samples. Base line temperature measurement done with FLUKE Ti32 Infra Red Thermography with 0 mm composite [Figure 1].
Figure 1: Thermographic recording of temperature rise of sample

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Thermal measuring system includes:

  • A Fluke Ti32-10060564 thermal camera
  • Standard lens with calibration range −10.0°C to 80.0°C.
The measuring conditions were:

  • Background temperature: 39°C
  • Image range: 37.2°C to 40.2°C
  • Frame rate of sequence: 85 f/s
  • Resolution of the thermal image: 640 × 512 pixels
  • Emissivity: ε = 0.95
  • Distance between the object and the camera: 11 cm.
The samples were divided into two groups, 10 in each group. In Group 1, the samples were cured with LED D 2000 unit (fast mode) and Group 2, the samples were cured with QTH CU 100 A unit. In both the groups the class V cavity was etched with 37% phosphoric acid and rinsed. Then the thin uniform coat of the Tetric N bond (5 th generation bonding agent) was applied on the etched class V cavity of all the samples and curing done with LED curing unit in fast mode for group I and with QTH curing unit for group II for 20 s for all the samples. The real time temperature change was analyzed with Fluke Ti 32 infrared thermography during curing of bonding agent for 20 s for both the groups by a thermographic trainer.

The first increment of about 1 mm thickness A3 shade (Ivoclar Vivadent) composite was placed in the class V cavity and cured for 40 s. This step was followed by placement of second increment (1 mm) A3 shade (Ivoclar vivadent) composite and curing done for 40 s. The temperature change was measured with infrared thermography during curing of each increment of composite for both group I and group II [Figure 2] and [Figure 3].
Figure 2: Thermography image - The average pulpal wall temperature increase with LED unit. (Hotspot -39.5°C)

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Figure 3: Thermography image - The average pulpal wall temperature increase with QTH unit (Hotspot -41.8°C)

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Statistical analysis

The recorded images were subjected to smart view full analysis and reporting software and the data were subjected to statistical analysis. Mean temperature values and standard deviations were calculated for each set of data. Maximum temperature rises among the samples cured by QTH and LED units and the temperature rise after curing of bonding agent and each incremental composite photo curing were analyzed using Student t-test [Table 1] and Friedman's multiple comparison tests [Table 2] to identify statistically homogenous subsets, with significance level less than 0.001%.
Table 1: Statistical analysis for temperature rise


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Table 2: Statistical analysis for stepwise temperature rise between the groups


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  Results Top


[Table 1] and [Table 2] show the mean temperatures and mean changes in temperature from baseline, resulting from the two light curing sources and the two modes of curing. The temperature of the baseline sample (39°C), which is suggested to be a measure of the heat generated by the light sources themselves. The baseline temperature before curing was 39.0°C. The mean maximum temperatures of composite samples recorded during curing were: 42.8°C with D 2000 LED curing unit, and 45.2°C with CU-100A QTH unit. The results are graphically presented in [Figure 4], which shows the mean rise in temperature from base line temperature versus curing unit used in the experiment. The temperature rise after curing the bonding agent for 20 s and curing each increment of composite for 40 s as represented in Graph 2.
Figure 4: Temperature rise from baseline between LED and QTH group in degree celsius

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Student t-test revealed the rise of temperature is 4°C from baseline (39°C) for QTH unit and 2.5°C from baseline for LED unit [Figure 4]. Friedman's multiple comparison test revealed that the rise of temperature is significant (P < 0.001%) while curing the first increment of composite in both curing units and insignificant (P < 0.001%) rise of temperature while curing the second increment of composite in both QTH and LED units [Figure 5].
Figure 5: Temperature rise during curing of bonding agent and during incremental curing of composite between LED and QTH in degree celsius

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  Discussion Top


The technique of infrared thermography allows a noncontact recording of temperature changes during polymerization by measuring the infrared emission from the surface of the curing composite. It gives a two-dimensional representation of the temperatures over the entire surface of interest. [14] This is unaffected by curing lamp blue light, which is in the range of 470 nm. The blue light acts on the initiator system, forming free radicals in the presence of the amine when subjected to the correct wavelength of light. The resin materials are cured by an addition polymerization reaction, through the carbon-carbon double bonds present in the photocurable monomers. Every time a (C=C) bond is broken and converted into (C-C) bond, heat is evolved, resulting in an exothermic polymerization reaction. The infrared thermography demonstrates clearly and accurately the clinical reality of exothermic polymerization of resin composite.

In this study, the Rolence QTH unit CU-100A has light source of 75 W halogen curing unit, cool light with the light intensity of 600 mW/cm 2 and wave length of 470 nm. It generates more heat. D-2000 LED curing light has digital meter of 5 W LED. It has wavelength of 530 nm and 900 mW/cm 2 . LED light generates less heat and makes no noise. It has longer lifespan than conventional halogen light and has three function modes - fast (900-945 mW/cm 2 ) ramp and pulse mode. Hoffman et al.[16] reported that LED curing units are characterized by a relatively narrow emission spectrum and lower heat generation than QTH curing units The higher power density QTH light source caused a greater increase in tooth temperature than a high power LED light. [17]

The samples cured with the CU 100 A QTH curing unit produced higher polymerization exothermic heat than those cured with the D 2000 LED curing unit. The difference in temperature indicated that a large part of the differences in the overall temperature was due to variation in heat produced by the light sources themselves.

According to Peutzfeldt, [18] curing of composite with high intensity light may result in a higher degree of conversion of carbon-carbon double bonds (C=C) and therefore higher exothermic heat.

In the present study, the temperature rise induced during light curing of composite resin using poza D 2000 LED fast mode (group I) light curing unit showed less temperature rise in the pulpal wall (2.5°C) from the base line temperature than CU 100 A (group II) QTH light curing (4°C).

The temperature rise was minimal during bonding agent curing for 20 s from base line temperature in both light curing units. There is drastic temperature rise during curing of first increment of composite in both light curing units. Further there is lesser degree of temperature rise during curing of second increment of composite in both light curing units.

Though the temperature rise was due to more effective polymerization and heat of reaction of samples cured with the high intensity light, a large part was due to heat inherent in the esign of the curing lamp itself. Watts et al. [19] found that during incremental placement of resin composite, the greatest temperature rise at the floor of cavities will result during placement of the first increment. The temperature rise was observed in this study after placement of bonding agents, and an obvious increase in temperature was recorded after first incremental curing. The second incremental composite curing produced a lesser degree of temperature rise, thereby indicating the insulating property of composite material. Here again, the LED units produced little temperature increase when compared with QTH units.

In this study, the greatest temperature rise was observed with the 1 mm-thick composite samples. There was a marked reduction in maximum temperature rises when curing samples thicker than 1 mm. This result is in accordance with findings of McCabe [20] and Lloyd [21] who recorded lower temperature rises with thicker composite specimens. The lower temperature rises with thicker resin samples observed in this study, may be explained by less effective polymerization as the light intensity attenuated as it passed through the composite samples. Light attenuation results from scattering as it passes through resin composite samples. [22] Several investigations have reported that increasing composite thickness has a significant effect in attenuating light intensity and therefore reducing the observed degree of cure. [23],[24],[25]

In the present study composite resin samples were placed in direct contact with the light guide tip that covered the entire surface of the sample. In addition, relatively high intensity light curing units were used to cure the composite resin samples. These were the added factors that favor maximum polymerization and heat of reaction. So, the results observed in this study represent a worst-case scenario for temperature rises during light curing of resin composite. This current study showed a temperature rise of up to 4°C from base line curing of composite samples with QTH curing unit and 2.5°C rise in LED unit.

The tunnel visioned in vitro studies isolate certain parameters that may be far from the clinical reality if all parameters were included. In the clinical situation, the lower intrapulpal temperature rises are expected when curing resin composites because of heat dissipation by pulpal, periodontal and osseous circulation and also thermal stimuli can trigger nervous reflexes and release of vasoactive mediators resulting in arteriolar dilatation and enhanced pulpal circulation. Moreover, the remaining dentin thickness has low thermal conductivity and significantly reduced the observed temperature rise during resin composite photo polymerization. [3]


  Conclusion Top


Within the limitations of this in vitro study, following conclusions were arrived.

  1. The use of infrared thermography to monitor temperature rises during visible light curing of resin composite, revealed that the QTH unit produced higher temperature rise than LED unit for all tested conditions.
  2. The temperature rise in both QTH and LED unit was within the physiological limits and did not produce any pulpal damage and infrared thermography is a viable means of quantifying the change in temperature during polymerization of restorative resin composite.


 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]


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