Analysis of intrapulpary temperature variation during the operative and restorative phase in human molars

The aim of this study was to analyze the intrapulpal temperature variation during the operative and restorative phase in human molars. This study was approved by the Research Ethics Committee, registration CAAE 43677521.6.0000.5237 and 24 samples were made from human molars divided into 2 groups, group 1 (n = 12) restored with Bulk Fill composite resin in a single increment and group 2 (n = 12) restored with conventional composite resin in two increments. The temperature was monitored and recorded with a Lutron Tm-902C digital thermostat device during preparation, adhesive system application and restoration. The mean temperature variation in ºC for cavity preparation was -2.07 ± 0.89, application of the adhesive system was 1.12 ± 0.65, restoration with Bulk Fill resin was 2.73 ± 0.72 and restoration with conventional resin was 1.94 ± 0.35. Thus, in the operative phase there was a decrease in temperature in both groups and in the restorative phase the Bulk Fill resin presented a higher temperature compared to conventional resin.


Introduction
Current dentistry aims to preserve pulpal health. During the restorative stage, biological, physical and chemical factors affect the pulp. In deep cavities, the thickness of the remaining dentin is smaller, making it easier for external factors to alter the pulp tissue (Savas et al, 2014).
Different dental procedures can lead to warming of the pulp tissue. This occurs in the exothermic reaction of the photopolymerization of the composite resin, the adhesive system and in the making of the cavitary preparation through the rotating instruments and drills (Braga, 2017;Brito et al., 2017).
The pulp is formed by highly innervated and vascularized connective tissue. The increase in temperature from the operative and restorative phases can be quite aggressive causing damage to the pulp organ, since it has low recovery capacity because it is surrounded by dentin and its vascular supply is only through the orifice of the apex of the dental root (Oliveira et al., 2009;Ramagem, 2018).
Currently, the concern of dentists to maintain biological structures and their functions has been widely used. This gave rise to minimally invasive dentistry. This technique preserves a greater amount of dentin tissue, invading only what is necessary during the removal of carious tissue and in the preparation of cavitary, preserving a greater structure of dental support (Raucci Neto, 2009;Magalhães et al., 2017;Silva Neto et al., 2021).
The drills are effective during preparation, however if they are used incorrectly can harm the pulp irreversibly because, in addition to vibration, it generates a large amount of heat that results in postoperative sensitivity and even pulp death (Oliveira et al., 2009).
Composite resins have undergone major improvements in recent years, along with photopolymerizers and their techniques for activating composites (Awliya, 2007). Chosen in most of cases for tooth restoration, composite resins have the disadvantage of releasing heat during polymerization (Hernandez & Bonilla, 2016). The act of polymerizing a composite resin is a thermophysical method well known among dentists for being present in numerous procedures of their clinical routine.
Polymerization is a reaction that occurs through the covalent bond of resinous monomers where they will be converted into polymers. It is essential that the wavelength in blue color (465 to 470 nm) of the device is reaching the photoinitiator present in the resin composed, in its majority, the camphoroquinone (Martins, 2016).
In the market there are photopolymerizers with different powers and some release a lot of heat, which can cause damage to the pulp of vital teeth (Baratieri, 2010;Brandão & Machado, 2019). The intensity of the light increases the characteristic stresses of the polymerization contraction of the composite resin and produces thermal energy that will dissipate in the dental element, raising the intrapulp temperature (Ramagem, 2018;Martins, 2016;Andreatta, 2015).

Ethical aspects
Considering that human teeth were used throughout the study, a research project was previously submitted to the ethics committee before the execution of the experiment. Being approved on March 18, 2021, with CAAE number 43677521.6.0000.5237.

Experimental design
The project was designed and written considering the recommendations proposed by the CRIS guide (Checklist for Reporting In-vitro Studies) (Krithikadatta et al., 2014). The in vitro study followed a prospective and parallel model with the The primary outcome was the temperature variation during the cavity preparation and as secondary outcomes, the temperature variation during the application of adhesive system and restoration was evaluated. Table 1 describes the composition of materials, according to the manufacturers.

Sample calculation
The sample calculation was based on the primary outcome of the study (temperature variation during the cavity preparation) and performed at G*Power software (Faul, 2007). The ANOVA test of repeated measures within-between interactions was selected, family F. With an effect size of 0.359, power of 80%, significance level of 5%, and considering the number of experimental groups (total of 2 groups), as well as the times of measurements that would be performed (3 moments), resulted in a minimum total sample size of 16.

Sample preparation
Initially, 24 healthy permanent human molars were sectioned in the cervical third of the root with IsoMet 1000 -Buehler (Figure 1), removing the furca region and exposing the pulp tree as shown in Figure 2.  With the specimeter, the coronary portion (enamel and dentin) was measured at the deepest point of the main sulcus to the roof of the pulp chamber of the samples ( Figure 3) and as inclusion criteria those that presented values of 4.0 ± 0.5 mm was selected. Then, the groups were divided into two groups (n=12). Therefore, Figure 4 shows the apparatus assembled to perform the temperature test, with the specimens fixed in a plastic container whose lid has been previously perforated. To make it possible to monitor the temperature constantly, the sensor of the digital thermometer (Tm-902C -Lutron, Pennsylvania, USA) was inserted into the pulp chamber surrounded by a material that allows heat dissipation throughout the pulp chamber (Thermal Paste -Implastec, São Paulo, Brazil). For the assembly to be sealed, the silicone by condensation reaction (Profile -Coltene, Altstätten, Switzerland) was used.  Source: Authors.

Cavity preparation
For the cavity preparation, an electric motor (ELECTROmatic -Kavo, Biberach an der Riß, Germany) configured at 40.000 rpm was used in conjunction with a diamond drill (#3131 -KG Sorensen, Serra, Brazil) marked with the aid of a 3.0 mm ( Figure 5), respecting the measurement of 1 mm of dentin remaining for the samples. Black Class I preparations were performed under constant refrigeration until reaching the desired depth and the temperature was recorded before starting the preparation (P0) and after its completion (P1). Source: Authors.

Application of adhesive system
After the cavitary preparation, the steps of the adhesive system were started making the selective conditioning of the enamel with phosphoric acid at 37% and recording the initial temperature (A0) after the tooth enamel drying stage. Then, the universal adhesive (Ambar -FGM, Joinville, Brazil) was applied in 2 layers with microbrush in an active way in the preparation. In the first and second layers, an air jet was applied at 30 cm from the tooth to volatilize the adhesive solvent, and only the second layer was photoactivated with a curing light (Valo Cordless -Ultradent, Indaiatuba, Brazil) for 20 seconds (1000 mW/cm 2 ) and recorded the temperature peak reached (A1).

Restoration
For restoration, the composites presented in Table 1 were used. FBF group was restored by the single increment technique and the initial temperature of the restoration (R0) was recorded, the composite inserted into the cavity by means of a suprafill spatula followed by photopolymerization with a curing light (1000 mW/cm 2 ) for 40 seconds and recorded at the final temperature peak (R1). The conventional resin, by the incremental technique, was used for restoration of FXT group. The temperature at R0 was recorded before inserting the first increment into the cavity. Soon after, the first layer of the resin was inserted to the middle of the cavity (1.5 mm), and it was polymerized for 20 seconds recording the value of R1. Next, the second increment of the conventional resin was inserted and polymerized for another 20 seconds and thus the maximum of the final temperature reached (R2) was recorded. Figure 6 shows the stages of the operative and restorative phases. Where the difference between FBF FXT groups lies in the technique selected to insert the resin into the cavity and in the number of consecutive photoactivations.

Statistical analysis
Data for each response variable was submitted to the Shapiro-Wilk and Levane tests, indicating homogeneity and normality (p>0,05). Thus, submitted to analysis of variance ANOVA for repeated measures and Tukey's post hoc test of multiple comparisons (= 5%) for detect any significant differences among the groups.

Results
In the course of the study, through statistical analysis, it was found that the mean temperature of the final preparation (P1 = 20.64 ºC) decreased in relation to the initial preparation (P0 = 22.72 ºC), as shown in the Table 2, therefore A>B. Research, Society andDevelopment, v. 12, n. 8, e3912842834, 2023 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v12i8.42834 8 It is important to note that there was a decrease in pulp temperature during the operative phase of the cavity preparation, since the cooling of the high-speed engine was present. This cooling effect is consistent with the general understanding that such instruments dissipate heat and prevent substantial temperature elevations within the pulp chamber (Farah, 2018). Table 3 shows the results regarding the mean temperature recorded during the application of the adhesive system. In this research, the mean initial temperature of the application of the adhesive system was A0 = 20.64 ºC and A1 = 21.77 ºC, with a variation of 1.12 ± 0.65 ºC at the time of application and photoactivation of the adhesive system. Source: Author.
In the case of application of the adhesive system, because it is a thin layer, despite the depth of application, there was a slight increase in temperature with a low variation between the initial and final time of this stage. Table 4 presents the mean temperature values of the restoration with a bulk fill composite (FBF group) while Table 5, the values for the conventional resin (FXT group). Thus, it was verified in Table 4 that the mean temperature in R0 was 21.74 ºC and in R2 was 24.47 ºC. Therefore, there was an increase in temperature (2.73 ± 0.72 ºC) at the time of photoactivation of the composite. In Table 5, the variation in temperature was 1.94 ± 0.35 ºC, being R0 = 21.84 ºC and R2 = 23.78 ºC. In this sense, there was increasing warming in the polymerization steps of the two increments.  Source: Authors.
The difference between the two Tables 4 and 5 lies in the intermediate photopolymerization of the FXT group, since it is a conventional composite resin where it is known that increments of up to 2 mm do not negatively interfere with the longevity of the restoration considering the cavity configuration factor (Factor C) and inherent polymerization shrinkage.
The comparison between the mean values obtained in the restoration with the FBF group and the FXT group is shown in   labeled as "A". The temperature variation (Δr) was distinct between the groups, with FBF ("a") showing a greater variation compared to FXT, labeled as "b". These findings suggest that the FBF and FXT groups may have differing responses to temperature changes throughout the experiment. The sample size was consistent in both groups, with n=12. These findings contribute to understanding the thermal responses of these groups in a specific context.

Discussion
The temperature difference in preparation (-2.07 ± 0.89 ºC) reaffirms the importance of using the cooling system of dental engines to avoid excessive temperature increase (Britto et al., 2018). Depending on the intensity of the aggression will result in different pulp responses, whether it is mild, moderate inflammation or necrosis (Oliveira et al., 2009). The pulp organ presents reversible response under a temperature variation of 2.77 ºC, while temperatures above 5.55 ºC occur loss of pulp vitality (Zach & Cohen, 1965).
In the context of thermal elevation within the pulp chamber, it was observed that the irradiance intensity from the light-curing unit emerged as a predominant factor compared to the exothermic reaction of bulk-fill resins. Even in the presence of an exothermic polymerization reaction, the bulk-fill resins demonstrated insulating properties when exposed to the irradiance from the light-curing unit. Concerning direct pulp capping procedures, a strategy involving a reduction in the number of light activations is recommended to alleviate the period during which the intrapulpal temperature exceeds the 10 degree Celsius threshold. This strategic approach holds the potential to contribute to pulp health promotion within clinical interventions (Nilsen et al., 2020;Oliveira et al., 2022).
The temperature variation found in this study did not have the potential to cause any serious aggression to the pulp (Aranha et al.,2011). Furthermore, a way to reduce the pulp temperature during the application of the adhesive system is through the simulated pulp pressure during photoactivation of the adhesive system (Da Silva et al., 2017).
In the present work it was established that the light emitted, and the thickness of the remaining dentin is directly related to the elevation of the temperature within the pulp organ (Savaris, 2008), where there is a direct relationship between temperature variation, type of light used, thickness of remaining dentin and amount of consecutive photoactivation.
In order to avoid failures in photopolymerizing devices, it is of paramount importance for Dentists to be up to date in relation to the maintenance of their photopolymerizing devices and choose the most appropriate technique for each case (Brandão & Machado, 2019).

Conclusion
Based on the results obtained in the present study, it is concluded that during the operative phase there is a decrease in temperature due to the cooling of the rotation instrument. However, the elements restored with Bulk Fill resin presented higher temperature values than the sample restored with conventional resin, although the temperature variations remained within safe limits, this difference suggests that Bulk Fill resin may have a potential to induce reversible pulp reactions. Drawing from the results, it is suggested to investigate the long-term effects of temperature variations in dental restorations, explore the mechanisms behind temperature differences among resins, assess the clinical impact of these variations, and develop enhanced cooling strategies.