It is necessary to place several resin composite layers and light cure each layer separately (the incremental technique) to restore deep cavities with the use of conventional resin composites. Therefore, the incremental resin composite technique is a time-consuming procedure. To overcome such a problem, a different type of resin composite has been introduced, which is called bulk-fill resin composite. The unique advantages of this type of resin composite are the feasibility of placing it in 4-mm thick pieces and polymerize these pieces in one step. These resin composites are composed of modified monomers and fillers that have a high capability to conduct light. ¹

One rationale for limiting the thickness of the layers to 2 mm in conventional resin composites is to allow adequate light to penetrate into the material for initiation and completion of polymerization, convert maximum amount of monomers to polymer, and increase the degree of conversion (DC), which affects the mechanical and clinical properties of resin composites.

Light curing of resin composites might exert thermal stresses on the tooth pulp. ² An increase in the temperature during photopolymerization depends on various factors, including light intensity, the chemical composition of the resin, thermal conduction properties of resin composite, DC of resin composite, the depth of the cavity, the thickness of the restorative material, and the duration of the exposure to light. ² , ³ , ⁴ A study on the amount of increase in temperature within resin composite during polymerization has shown that in bulk-fill resin composites, the increase in temperature is higher. ⁵ However, several other studies have reported less shrinkage forces in bulk-fill resin composites compared to conventional resin composites. ⁶ , ⁷ , ⁸ Soft-start polymerization is a polymerization technique that decreases the shrinkage stresses and results in a decrease in temperature increase. ⁹ , ¹⁰

Given the clear advantage of bulk-fill resin composites in saving time and costs, it appears it is necessary to evaluate thermal changes resulting from photopolymerization and the heat transferred to the pulp and also to determine DC as a parameter affecting the mechanical properties and biocompatibility of these resin composites. Therefore, the aim of the present study was to evaluate thermal changes, due to polymerization, beyond dentin and DC of two types of bulk fill and conventional resin composites using soft start and continuous high light-curing mode.

Specimen preparation

A total of 56 sound human third molar teeth were used for the purpose of this in-vitro study. The teeth were stored in distilled water up to the day before the study. First, the occlusal surfaces of the teeth were made flat with the use of a disc to achieve better adaptation of the light-curing unit. Then, a Class I cavity, measuring 4 mm in mesiodistal and buccolingual dimensions and in depth, was prepared with the use of a cylindrical diamond bur (tizkavan, Iran). Then, the tooth crowns were removed at a distance of 5 mm from the flattered occlusal surface.

The samples were divided into four study groups (n = 14) in terms of the resin composite type and light-curing mode as follows Figure 1. Figure 1

Schematic of study groups in terms of the resin composite type and light.curing mode

Table 3presents the means, standard deviations, P value, and 95% confidence intervals of thermal changes with different resin composite placement and light-curing modes.{Table 3}

There were no significant differences in the DC of the conventional and bulk-fill resin composites between the two light-curing modes. However, DC was significantly different between the two resin composite types Table 4. The conventional resin composite exhibited the maximum DC with the use of soft-start light-curing mode (78.37%) and the bulk-fill resin composite exhibited the minimum DC (67.97%) with the use of continuous high light-curing mode Table 4.{Table 4}

Two-way ANOVA showed that when DC was compared, the cumulative effect of light-curing mode and resin composite type was not significant. The effect of light-curing mode on DC was not statistically significant (P = 0.144); however, the effect of the resin composite type was statistically significant (P < 0.001).

Since the temperature increases after the imitation of photopolymerization of resin composite, resulting in injuries to the dental tissues, the present study was conducted to provide laboratory data on thermal changes within resin composite and the pulp chamber during photopolymerization. In addition, the effect of composite placement technique (the incremental technique vs. bulk-fill technique) on the temperature rise was evaluated with the use of soft start and continuous high light-curing mode.

This explorative, experimental, in-vitro study was done at room temperature (25°C ± 2°C). Jafarzadeh-Kashi mentioned that the bonding agents might be cured more efficiently at human body temperature than at room temperature, although the Scotchbond MP was not significantly affected by the temperature. ¹² However, such an effect might not be applicable for resin composites due to their higher thickness compared to thin layers of bonding agents.

Based on the results of the present study, soft-start light-curing mode resulted in a greater temperature rise, which might be due to the greater energy of this mode compared to the continuous high mode. Based on the results of studies by Looney and Price, the differences in the energy produced by light-cured devices are an important factor for differences in temperature increase with the use of different light-curing modes. Lower energies delivered by light-curing modes will result in less temperature rise. ¹³ Several studies, consistent with the present study, have shown that when the energy produced by a light source increases, there is an increase in the temperature too. ² , ¹⁴ , ¹⁵

In the present study, the mean ΔT _dwas significantly higher than the mean ΔT _twith the use of both light-curing modes. In a study by Aguiar et al., too, the mean temperature rise in all the groups without resin composites was higher than that in resin composite groups with all the three thicknesses of dentin, consistent with the results of the present study. ¹⁵

While Lloyd and Hansen and Asmussen reported that the curing process of resin composite creates heat and ¹⁶ , ¹⁷ the results of the present study showed that resin composite layers serve as insulators, decreasing the transfer of energy than what is produced. In a study by Kim et al., too, the temperature rise measured in the depth of resin composite decreased with an increase in the thickness of resin composite, which was attributed to the decrease in the penetration of light. ⁵

A comparison of ΔT _tvalues showed that the ΔT _tvalue of the first layer of conventional resin composite was higher than that of bulk-fill resin composite. Higher ΔT _tvalue might be attributed to the thickness of conventional resin composite (2 mm) compared to that of bulk-fill resin composite (4 mm).

Garoushi et al. showed that an increase in the thickness of bulk-fill resin composite from 1 mm to 4 mm resulted in a significant decrease in the penetration of light. ¹⁸ In addition, higher DC of conventional resin composite compared to bulk-fill resin composite, resulting in a greater exothermic reaction, might be another reason for higher ΔT _tof the first layer of conventional resin composite compared to bulk-fill resin composite.

In contrast to the results of the present study, in a study by Kim et al., the temperature rise resulting from a 20-s irradiation of bulk-fill resin composite was higher than that of the first layer of conventional resin composite, ⁵ which might be attributed to the use of bulk-fill resin composite with high translucency and high resin content. ¹⁸ , ¹⁹ It appears this increased translucency and the resultant prevention of light from reaching the depth of the cavity by the composite layer might have a greater role in these changes.

In the present study, in order to simulate the clinical situation, the second layer of conventional resin composite was placed immediately after light curing of the first layer before the temperature returned to normal after light curing the first layer. However, the temperature rise resulting from photopolymerization of the second layer with the use of both light-curing modes was not higher than that of the first layer and the temperature during photopolymerization of the second layer occurred more slowly, and the peak of thermal changes was less than that of the first layer, consistent with the results of a study by Kim et al. ⁵ It appears the reason was the presence of 2 mm of resin composite in the first layer, which served as a thermal insulator for the heat resulting from photopolymerization.

In addition, in the present study, the resin composite placement technique (incremental or layering technique vs. bulkfill) affected DC, but light-curing mode did not show such an effect.

In the incremental technique, resin composite receives higher total energy. Each 2-mm thick layer receives 12 j/cm ²and 13.5 j/cm ²energy with the continuous high- and soft-start light-curing modes, respectively, while with the bulk-fill technique, this amount of energy should cure the whole mass of resin composite with 4-mm thickness. Therefore, the amount of light penetrating the 4-mm thickness decreases, resulting in a decrease in DC. The results of the present study were consistent with those of a study by Chang et al. ³ Khaksaran posed a higher light intensity is expected to increase the temperature more since more photons are absorbed by the unit of area on a tooth tissue. Besides, it might be capable of inducing more polymerization due to warming material and reducing its viscosity and therefore radical mobility, as well as increasing the collision frequency of unreacted active groups and radicals. ²⁰

Although DC of bulk-fill resin composite was less than that of conventional resin composite, it was at the accep[table 55]% level, ²¹ which might be attributed to the increased translucency of bulk-fill resin composites.

The bulk-fill resin composite used in the present study had hybrid particles. In this context, an increase in filler size results in a decrease in the specific surface area between the filler and the organic matrix, decreasing light scattering and increasing the curing depth. ²² In addition, based on the claims of the manufacturer, this bulk-fill resin composite contains Ivocerin, which is a germanium-based initiator, with a higher curing activity compared to camphorquinone. Although Silikas et al. reported that the minimum DC necessary to prevent occlusal surface abrasion is 55%, ²¹ an adequate DC for proper clinical performance is yet to be determined and many studies have reported that DC is directly correlated with mechanical and biocompatibility properties.

The temperature rise resulting from the irradiation of the remaining dentin was higher than the final heat of polymerization with the use of both Tetric N-Ceram bulk fill and Tetric N-Ceram conventional resin composites

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.