In this study, temperature recording was started prior to light irradiation and continued until the temperature of the dentin disc reached the environmental temperature. The graph in Fig. 3 shows the temperature rise profile caused by different standard modes of the QTH curing unit in 4 radiation cycles (in terms of duration). This graph shows the temperature rise caused by light-curing versus time. Moreover, this graph shows the levels described by Zach and Cohen [22] indicative of the percentage of irreversible pulpitis caused by the pulp chamber temperature rise by 5.5°C and 11.1°C for 10 s. In addition to the maximum temperature rise, the duration of time remaining at high temperature is also important in the rate of damage. Thus, precise assessment of the thermal profile versus time is highly important as well. The temperature of the dentin disc in this study increased by 5.2°C following light curing in the standard mode for 10 s. In this state, the entire thermal profile graph was below both 15% and 60% pulpitis levels. The maximum temperature rise was 7.2°C in the 20 s irradiation cycle. In this state, the graph was above the level of 15% pulpitis for approximately 20 s, but the entire graph was below the level of 60% pulpitis. The maximum temperature rise was 14°C in the 30 s irradiation cycle of the standard mode of the QTH curing unit. The thermal profile in this state indicated that the graph exceeded both 15% and 60% pulpitis levels and was above the 60% level for approximately 16 s. The maximum temperature rise was 15.3°C in 40 s irradiation cycle by the standard mode of the QTH curing unit. The thermal profile in this state exceeded both 15% and 60% levels and remained above the 60% pulpitis level for approximately 28 s (Fig. 3).
Figure 4 shows the temperature rise versus time graph of the bleaching, ramp, and boost curing modes of the QTH curing unit. With regards to the boost curing mode, which has the highest radiation intensity among different modes of the QTH curing unit, the maximum temperature rise was recorded to be 5.9°C. In this state, the entire thermal profile was below the 60% level of pulpitis. Although the maximum temperature in this mode was above the 15% pulpitis level at 5.5°C, the temperature was above this level for only 5 s, which is lower than the 10-s threshold required to cause 15% pulpitis at this temperature according to Zach and Cohen [22]. The dentin disc was subjected to light-curing for 20 s with the ramp mode of the QTH curing unit, and it was observed that the maximum temperature rise was around 8.6°C. In this state, the thermal profile was below the 60% pulpitis level, but it was above the 15% pulpitis level for 18 s. Concerning the bleaching mode of the QTH curing unit, the maximum temperature rise was 13.8°C, which was above both the 15% and 60% pulpitis levels, and remained above the 60% pulpitis level for over 16 s.
Assessment of the data regarding the temperature rise in the dentin disc following light-curing by the LED curing unit revealed that light curing with the pulse mode for 10 s and high modes for 5 s resulted in a temperature rise below 15% pulpitis level. The maximum temperature rise caused by curing with the high mode for 5 s was 4.4°C; while, this rate was 5.3°C for curing with the pulse mode for 10 s. Light-curing with the high mode for 10s resulted in temperature rise above the 15% pulpitis level, and as shown in Fig. 5, the temperature remained at this level for over 26 s but did not reach the 60% pulpitis level. The maximum temperature rise was 9°C in this high mode. The temperature rise caused by curing with the soft mode for 20 s exceeded both the 15% and 60% pulpitis levels and remained above the 60% level for over 13 s. The maximum temperature rise in this mode was 15°C.
Comparison of Figs. 3, 4 and 5 reveals that the maximum temperature rise by the standard mode of the QTH curing unit and soft mode of the LED curing unit was around 15°C. The only difference in this respect is the time spent to reach the maximum temperature level such that it took 40 s with the standard mode of the QTH and 20 s with the soft mode of the LED curing unit. This finding indicates that the generated heat in the specimen was higher in curing by the soft mode of the LED curing unit. Thus, the temperature rise curve alone is not sufficient to study the heat transfer when using different modes of LED and QTH curing units. Therefore, for a more precise comparison of curing with different modes of LED and QTH curing units, the heating rate graphs were drawn separately for each mode by derivation of the temperature rise versus time graphs. The heating rate is a thermal parameter that physically indicates how hot the heat source is, and how efficiently the heat is transferred [34]. Since the same dentin disc was used in all tests conducted in this study, comparing heating rates when using different light-curing modes would indicate the thermal power of the heat sources. Since the extraction of the data with noise would result in extrapolation of the noise rate, it is necessary to filter the data after derivation to eliminate or decrease the rate of noise in the graphs. Figure 6 shows the graph of the heating rate for the standard mode of the QTH curing unit for 40 s with and without the use of an exponential smoothing filter. As shown, the heating rate became positive as the curing started and reached around 0.8°C/s, and then linearly decreased with time, reaching 0.2°C/s upon the termination of irradiation. Subsequently, the heating rate quickly became negative (since the temperature of the specimen was higher than the environmental temperature), which indicates a reduction in the thermal difference between the specimen and the environment over time after the termination of light-curing.
Figure 7 shows the heating rate graph of the ramp mode of the QTH curing unit. As presented in Table 2, the light intensity in this mode increases exponentially to 1000 mW/cm2 in the first 10 s of irradiation. This graph also shows the exponential increase in the heating rate in the first 10 s, reaching the maximum value of 0.9°C/s sometime in the middle of the irradiation time.
Figure 8 indicates the heating rate graph for the soft mode of the LED curing unit. In this state, the heating rate increased almost linearly in the first 10 s and reached the maximum value of 1.2°C/s, remained at this temperature for a couple of seconds, and then slightly decreased before the end of irradiation.
Figure 9 shows the heating rate graph for 10 s of irradiation with the high modes of the LED curing unit. In these modes, the heating rate quickly increased and reached the maximum value of 1.25°C/s, and then decreased until the end of the 10 s period.
Table 3 presents the results regarding the maximum temperature rise, maximum heating rate, and the mean heating rate for each mode of the QTH and LED curing units along with the standard deviation values.
Curing unit
|
Curing mode
|
Time (s)
|
Temperature rise (SD) (°C)
|
Max. heating rate (SD) (°C/s)
|
Mean heating rate (SD) (°C/s)
|
Table 3
Summary of the results regarding the maximum temperature rise, maximum heating rate, and the mean heating rate for each mode of the QTH and LED curing units
QTH
|
Standard
|
10
|
5.2(0.2)
|
0.6(0.01)
|
0.52(0.02)
|
20
|
7.2(0.1)
|
0.7(0.02)
|
0.36(0.01)
|
30
|
14(0.4)
|
0.68(0.01)
|
0.46(0.03)
|
40
|
15.3(0.3)
|
0.8(0.05)
|
0.38(0.01)
|
Ramp
|
20
|
8.6(0.4)
|
0.9(0.03)
|
0.43(0.02)
|
Boost
|
10
|
5.9(0.5)
|
0.78(0.04)
|
0.59(0.05)
|
Bleaching
|
30
|
13.8(0.7)
|
0.81(0.02)
|
0.46(0.06)
|
LED
|
High
|
10
|
9(0.9)
|
1.25(0.05)
|
0.90(0.08)
|
High
|
5
|
4.4(0.8)
|
1.24(0.06)
|
0.88(0.05)
|
Soft
|
20
|
15(0.7)
|
1.2(0.04)
|
0.75(0.03)
|
Pulse
|
10
|
5.3(0.8)
|
0.84(0.03)
|
0.53(0.07)
|
SD: Standard deviation |
Zach and Cohen indicated that temperature rise by 5.5°C for 10 s caused 15% pulpitis [22]. The present results regarding the maximum temperature rise and the thermal profile due to light-curing revealed that the temperature rise caused by the 10 s standard mode of the QTH curing unit and the 10 s pulse mode and 5 s high mode of the LED curing unit was completely below the 15% pulpitis threshold. Although the maximum temperature rise caused by the 10 s boost curing mode was over 5.5°C, the specimen remained at this temperature for less than 10 s. Thus, the thermal damage was still below the 15% pulpitis level defined by Zach and Cohen [22]. This finding highlights the significance of assessment of thermal profile while some previous studies on heat transfer by the curing units only focused on the maximum temperature rise [5, 17-20, 35, 36]. Of different curing modes evaluated in this study, the maximum temperature rise caused by the 20 s ramp mode, and 20 s standard mode of the QTH curing unit, and 10 s high mode of the LED curing unit exceeded the 15% pulpitis level, but it was lower than the 60% pulpitis level in all three modes. The temperature rise caused by the 30 s and 40 s standard mode and 30 s bleaching mode of the QTH and 20 s soft mode of the LED curing unit exceeded both the 15% and 60% pulpitis levels.
For a more precise assessment of the difference between different curing modes, the heating rate was also studied. As the heating rate increases, a temperature rise occurs in a shorter time. Study of the heating rate is important since according to the results of Zach and Cohen [22], pulpal temperature rise by 5.5°C for 10 s and 11.1°C for 10 s would cause 15% and 60% pulpitis, respectively; whereas, another study demonstrated that transient temperature rise by 8.9°C to 14.7°C would not cause pulpitis [23]. Controversy in the results of the abovementioned studies can be due to the different heating rates. In the study by Baldissara et al, [23] it took 160 s to increase the temperature to 14.7°C, indicating a mean heating rate of approximately 0.09°C/s. However, according to the results adopted from the study of Zach and Cohen, the mean heating rate was approximately 1°C/s, and this rate is approximately 10 times the heating rate in the study by Baldissara et al [23].
The present results indicated that maximum value of the mean heating rate was 0.9°C/s, and the maximum heating rate was 1.25°C/s in use of the high mode of the LED curing unit for 10 s; these values were over 10 times the heating rate reported by Baldissara et al [23]. Since the degree of tissue damage due to temperature rise depends on the heating rate [30], the heating rate of the thermal source should be addressed in addition to the temperature rise, as an influential factor in the degree of tissue damage. It has been confirmed that the rate of heating has a profound effect on the degree of cell damage [24]. This finding indicates that in equal maximum temperature rise for a certain specimen, the thermal stimulation with a higher mean heating rate has a higher risk of causing tissue damage.
Since the heating rates in all tested modes of both curing units in the present study were all higher than the heating rate reported by Baldissara et al, [23] and were closer to the mean heating rate in the study by Zach and Cohen [22], it is recommended to pay attention to the risk of thermal damage as described by Zach and Cohen [22], stating that temperature rise of the pulp chamber by 5.5°C for 10 s would cause 15% irreversible pulpitis and temperature rise of the pulp chamber by 11.1°C for 10 s would cause 60% irreversible pulpitis.