Resin composites have been introduced as an esthetic restorative material26 and are increasingly used in place of amalgam for the restoration of teeth suffering from carious lesions.27 According to the AAPD,28 there is strong evidence supporting the use of resin composites for restoration of Class I cavities in primary teeth. However, polymerization of light-cured materials can cause an increase in pulp chamber temperature, negatively affecting the health of the pulp.29 Studies have suggested that the temperature rise during the polymerization of resin composites depends on the heat generated by curing light and the exothermic polymerization of resin composites.30 Savas et al29 found that heat generated by curing light is the crucial factor causing a temperature rise in the pulp chamber. The aim of the current study was to examine the effect of three different curing modes of a high-powered LED LCU on the temperature rise under human primary teeth dentin with two different thicknesses during resin composite application procedures regardless of the heat generated by the exothermic polymerization reaction.
In an attempt to simulate the pulpal microcirculation, previous studies have inserted a thermocouple into the pulp chamber through the root; therefore, the exact position of the thermocouple is unknown.6 Moreover, the radiography technique that has been used to determine the thickness of the dentin above the thermocouple cannot accurately determine it,2 especially when the thickness of the dentin is 1 mm or less. The technique of temperature measurement presented in the current study contributed to the deletion of many variables. As the same cavity with the same depth and shape was used, the amount of composite placed each time was similar. According to ISO/TS 11405:201523 it is not possible to standardize the composition and structure of the teeth, which may influence the temperature rise.22 To eliminate this variable, only one dentin disc obtained from one primary tooth was used to perform the experiments. Moreover, a primary lower second molar was chosen to obtain the dentin disc, in light of previous studies that have indicated that the lower second molar is the most vulnerable primary tooth to be affected by caries.31 The thickness of the resin composite selected was 2 mm to be clinically practical.32 Additionally, a one microhybrid resin composite with shade A3 was used to eliminate any possible variation in thermal conductivity.33 Hannig & Bott6 stated that the presence or absence of the bonding agent will not affect the temperature rise accompanying the polymerization of the resin composite. The experiments were carried out without applying the bonding agent to facilitate repetition of experiments on the same dentin disc. Loney et al34 demonstrated that a thinner thickness of remining dentin increases temperature changes. Accordingly, we decided to perform our study on two thicknesses of dentin that simulate the thickness of the remaining dentin in the deep cavities. Leprince et al35 stated that the exposure time has an important effect on the temperature rise value. Therefore, we standardized the exposure time during stage 1 curing (10 s) and during the polymerization of the composite resin (20 s) according to the manufacturer’s instructions.36
A K-type thermocouple was used to measure temperature in the present study based on previous studies, which have suggested that it is an appropriate technique to measure temperature changes at a specific point.37 In the current study, a high-powered LED was used to cure resin composites placed in direct contact with dentin. The light guide tip was also in direct contact with the resin composite. In addition, the thickness of the dentin disc was little. Therefore, this study represents a worst-case situation for temperature rise during polymerization of resin composite in primary teeth.
Zach & Cohen4 stated that 5.5°C is the critical limit that allows continued vitality of the pulp. In the current study, in stage 1 of curing with 0.5 mm thick dentin, the high-powered LED produced an excessive increase in the temperature above the critical temperature value regardless of the curing mode used. With the dentin thickness of 1 mm, exposure to the standard and ramp modes also led to a temperature rise above the critical value, while exposure to the pulse mode resulted in a rise in temperature below the critical value. Our results confirm the findings of previous studies that have suggested that bonding agents should not polymerize with high intensity light.38 Regarding this aspect, it should be taken into account that the high temperature rise values recorded in this study may be a potential risk factor that prevents the continuation of the vitality of the dental pulp when the remining dentin is of little thickness.
Exposing the dentin to the pulse mode resulted in a statistically significant lower temperature rise than the other two modes, which was probably due to the pause phases between the irradiation phases. We found it difficult to compare the results of the stage 1 curing with the previous literature due to the lack of studies that dealt with the thermal properties of binding agents.39 However, our result agreed with the work of Dogan et al17 who recorded significantly lower temperature rise values for pulse mode compared with standard mode in a study performed on human permanent teeth dentin with thicknesses of 0.5 and 1 mm.
It is interesting that we observed a decrease in the temperature rise values recorded during the polymerization of the resin composite compared to those recorded during stage 1 curing (polymerization of bonding agents). This may be explained by the ability of the composite resin to reduce the penetration of light into the dentin.32 The peak values recorded during resin composite polymerization were lower than 5.5°C under all conditions. This could be attributed to LEDs not generating infrared rays.11 Based on the results of the current study, we can suggest that it is possible to use high-powered LEDs as a safe alternative to traditional LEDs for polymerization of the composite resin in primary teeth with similar clinical situations.
The lowest temperature rise under the dentin disc during polymerization of the resin composite was recorded with the pulse mode. These data were in agreement with those of Hubbezoglu et al15 who studied the effect of standard, soft-start, and pulse modes on the temperature rise during the polymerization of six resin composites. They observed that pulse mode gave lower temperature rise values than other modes. However, the values recorded were lower than those of the current study, which could be due to the short exposure time (10 s) they used.
Szalewski et al40 measured the temperatures following polymerization of resin composites with seven curing modes (four continuous modes with exposure times of 3-5-10 and 20 s, two pulse modes with exposure times of 5 and 10 s, and soft-start mode). an acrylic resin matrix was used to apply the resin composites. When one exposure time was used (5, 10 s), they found that the continuous mode and pulse mode produced similar temperature rise values (14, 15.5 ℃ respectively). These results do not agree with those of our study, which may be because that study neglected the effect of dentin on temperature rise. Another study revealed no significantly higher temperature rise for pulse mode than standard mode during polymerization of bulk-fill composite placed in a cavity prepared in Teflon blocks.20 We can explain the difference between our results and the results of that study by the difference in the value of the radiant exitance of the LCUs used. The authors used two different radiant exitances for standard mode (600 mW/cm2) and pulse mode (1200 mW/cm2), while the same radiant exitance was used in the current study. The temperature rise during polymerization of the composite resin is related to the degree of conversion,41 and therefore, the lower values of temperature recorded with pulse mode can be clarified by the failure of the pulse mode to achieve the same degree of polymerization obtained with standard mode.
Polymerization of the resin composite with ramp mode produced no significant lower temperature rise than standard mode. These results disagreed with the work of Al-Qudah et al18 who recorded significantly lower temperature values when using Optilux 501 (Kerr, Peterborough, UK) in ramp mode than in standard mode. This can be related to differences in the exposure time between the ramp mode (20 s) and the standard mode (40 s) they used.
The second part of the current study investigated the effect of dentin thickness on the recorded temperature rise. Studies have suggested that dentin thickness is a critical factor that influences the amount of temperature rise in the pulp chamber42 because of the low thermal conductibility of this normal structure.43 The present study confirmed this because differences were observed in the temperature rise between dentin thicknesses. Our study found that the temperature rise of dentin with a thickness of 1 mm was significantly lower than that with a thickness of 0.5 mm during stage 1 curing in samples exposed to ramp mode or pulse mode. However, there was no statistically significant difference in the temperature rise between dentin thicknesses during polymerization of resin composite regardless of curing mode (p > 0.05). Guiraldo et al42 evaluated the temperature rise in the pulp chamber during polymerization of Filtek P90 silorane-based composite placed in a cavity prepared in bovine teeth with two remining dentin thicknesses (0.5 and 1 mm), and they found a significantly higher temperature rise in 0.5 mm dentin thickness (40.07ºC) than 1 mm dentin thickness (39.61ºC). These results disagree with those of the present study and may be due to the different temperature curves of silorane-based composites when compared with those of dimethacrylate-based composites.44
The first null hypothesis of the present study has been rejected that there were no significant differences in temperature rise among curing modes due to the statistically significant difference observed in temperature rise among curing modes. Regarding the thickness of the dentin, a statistically significant difference was found between the 1 mm dentin thickness when compared with the 0.5 mm thick groups in samples exposed to ramp mode and pulse mode during stage 1 curing; therefore, the second null hypothesis has been partially rejected.
This study presented an in vitro experiment to measure the temperature rise accompanying the procedures of applying resin composite, but this study has several limitations that do not allow its results to be applied directly to clinical conditions such as blood microcirculation and the cellular and intercellular matrix of pulpal tissue, which acted as a refrigerant to heat.45 Even though this study has limitations for being performed without applying bonding agents, the results can be considered clinically relevant.
Further studies should be performed to confirm the safety of the fast curing mode of high-powered LEDs during the polymerization of resin composites.