The current clinical trial was aimed to compare the effect of preheating at 50 °C and 70 °C on two bulk fill resin composite restorative materials one containing Bis-GMA (VCB) and the other free Bis-GMA (AFX) on the temperature change of the pulpal floor and restoration surface. The expelled of bisphenol-A out from resinous matrix could enhance its cytotoxic properties. Bisphenol-A glycidyl methacrylate (Bis-GMA) monomer is routinely used in manufactured resin composites (El-Askary et al. 2017). As a result, an alternative to Bis-GMA has been developed, such as Ormocer resin composite (organic modified ceramic). Due to their greater three dimensions cross-linked ceramic polysiloxane monomers, Ormocer composite materials are thought to get low polymerization shrinkage than Bis GMA resin composite restorative materials. (Kalra et al. 2012). Compared to standard dimethacrylate resins, they have decreased or no cytotoxicity (El-Askary et al. 2017).
To improve the handling properties of dental resin-based composites (RBCs), chairside pre-heating has been adopted. It minimizes microleakage and gap formation by lowering the viscosity of composites, resulting in enhanced flowability and marginal adaptability (Yang et al. 2020). The temperature range of 54–68 °C is regarded safe because it does not harm the pulp tissue (Gavic et al. 2015), (El-Deeb et al. 2015), and (Karacan and Ozyurt 2019). Moreover, resin composite can be preheated at 50–70 °C (Trugillo et al. 2004; Prasanna et al. 2007; Uctasli et al. 2008; Silva-Júnior et al. 2018).
K-type thermocouple is the most widely used sensor in temperature measurement, it can measure temperature ranging from − 50 °C to + 1300 °C. It was used by many authors (Daronch et al. 2007; El-Deep et al. 2015; Karacan and Ozyurt 2019; and Erhardt et al. 2020) in laboratory measurement of intrapulpal temperature with preheated composite. They measured the intrapulpal temperature on extracted teeth (in vitro study), and the metal probe of the K-type thermocouple was fitted inside the pulp chamber through the molar root after removing of the pulp tissues. In this clinical trial, they believe that depending on empty pulp chamber it gives no indication what really happens in the temperature of the pulp tissues due to resin composite preheating. Rueggeberg et al. 2010 used a typical photoactivated hybrid resin composite (Esthet-X) at room temperature (23.6 °C) or preheated to 54.7 °C, to assess the temperature of the prepared tooth surface throughout a restorative treatment on only three patients. Temperature data at the tooth pulpal floor were recorded using a customized handheld temperature measuring probe equipped with K-type thermocouples. To our knowledge, this is the first clinical trial to measure the temperature of the pulpal floor of the cavity believing that a higher increase in the temperature due to preheating can affect the pulp tissues causing harm. Similarly, measuring the resin composite temperature after curing might be effective as rapid returning to intraoral temperature is important to avoid pulp affection.
In the current study the whole metal probe was modified and covered with shrinkable non-conductive plastic tube leaving only the tip that touches the pulpal floor for measurement. This was done to overcome the problem that there was no specific device to pass through the restoration measuring only the pulpal floor temperature without measuring the temperature of the preheated resin composite. Also to prevent the probe from sticking to the resin composite while packing. This heat shrinkable tubing is commonly used for electrical and mechanical insulation, sealing, and connecting application (Barth 2005). When heated, heat shrink tubing is a mechanically expanding extruded plastic tube, often made of the thermoplastic materials nylon or polyolefin, that contracts exclusively in one plane (its diameter), (Luo et al. 2014).
In the present study, the mean values of the recorded baseline temperature of the pulpal floor before the bonding procedure ranged from 31.5 to 32.5 °C with no statistically significant difference in all groups and this was consistent with Rueggeberg et al. 2010 where the temperature was 30.5 °C, not 37 °C like the intraoral temperature, while this low temperature 30.5 °C acted to rapidly cool warmed composite but it has negative effect on increasing the viscosity, reducing the potential for composite flow that enhances the restoration adaption to prepared tooth surfaces (Rueggeberg et al. 2010).
The result of pulpal floor temperature on this clinical trial showed that there was significant difference between VCB preheated at 50 °C and AFX during packing and after curing while VCB showed the higher mean value. In the contrary, when both testing materials were preheated at 70 °C, there was a nonsignificant difference between both testing materials either before and after curing (Table 2). The varied matrix compositions seem to be a reason for these findings between VCB and AFX that results in differences in the degree of convergence which lead to exothermic variances between two materials (Al-Qudah et al. 2007). Moreover, increased heat is a sign of a high conversion rate (Knežević et al. 2005). These results were in accordance with (Daronch et al. 2006) whom reported that increasing the conversion of dimethacrylate monomers is caused by increasing polymerization temperature, but only up to a certain point. Following that point, monomer conversion reduces as the temperature rises. This limit is reached near 90 °C for monomers like Bis-GMA or BisEMA. Reactant evaporation and photoinitiator degradation cause a decrease in monomer conversion when the temperature is too high.
Comparing the pulpal floor temperature before (C1) and after curing (C3) to the baseline (C0) temperature, there was significant difference in both testing materials when preheated at 50 °C and 70 °C (Tables 3 and 4). There was a significant increase in the pulpal floor temperature from base line (C0) to (C1), where the highest recorded value of C0 was 32.5 °C and highest recorded value of C1 was 35.25 °C where the increasing was only 2.75 degrees Celsius. These results were in agreement with (Akarsu and Aktuğ Karademir 2019) who reported that the application of heat to tooth structures might result in increasing pulpal floor temperature and can cause varying degrees of pulpal damage. Chiang et al. 2008 reported that the enamel may be considered as a origin of heat because it has the fastest early temperature rise. Furthermore, investigations revealed that enamel and dentin had poor thermal conductivity and diffusivity (Lin et al. 2010). Dentin tubule fluids volume and blood flow rate, as well as the tooth's potential to perform as a heat origin in the occurrence of a temperature change, all have a significant impact on the tooth's thermal properties (Raab 1992). There was a significant increase in the pulpal floor temperature from (C1) to (C3), where the highest recorded value of C1 was 35.25 °C and the highest recorded value of C3 was 38.04 °C where the increasing was only 2.79 °C. The light exposure causes significant intra-pulpal temperature changes, with light energy, exposure length, and light source being some of the characteristics related to greater temperature variations (Yazici et al. 2006; Armellin et al. 2016). All of the curing units resulted in a statistically significant increase in pulpal temperature. In high-energy curing modes, the pulpal temperature climbed by more than 5.0 °C, while in low-energy curing modes it increased by 2.5 °C. Furthermore, researchers showed a strong correlation between high energy density and increased pulpal temperature (Vinagre et al. 2019).
Even after the curing light has been switched off, the temperature of heat energy in the pulp eventually dissipates, which arrives to a higher temperature in the pulp chamber. Dentin has not only the potential to transmit heat energy but also the tendency to preserve it due to its low thermal diffusivity (Chiang et al. 2008). The thermal energy of dentin should only be considered when incremental strands of resin composite are serially positioned in a cavity followed by exposure to the LCU, and it is logical to assume that many exposures to LCU above a short time will lead to higher heat storage in dentin. (Runnacles et al. 2015).
The pulpal floor temperature rises in both testing materials from the baseline (C0) to after curing (C3) was in the range of 4–6 degrees only. The highest recorded value of C1 was 35.25 °C, and the highest recorded value of C3 was 38.04 °C The application of heat to tooth structures might result in varying degrees of pulpal damage. Temperatures between 42 and 42.5 °C are required for reversible dental pulp damage (Pohto and Scheinin 1958). At 5.5 °C, 15% of pulps have necrosis, which is the most well-known temperature threshold associated with pulpal injury. This threshold was observed in a monkey study by Zach (1965), who used a soldering iron at 275 °C to produce temperature variations, which did not correlate to the temperature changes that occur during dental procedures (Erhardt et al. 2020). With temperature rises ranging from 8.9 to 14.7 °C, Baldissara et al. (1997) observed neither clinical or histological pulp injury in people using various techniques. Heat-induced pulp cell degeneration occurs when the pulpal temperature is raised over 42 °C or 43 °C, according to other researchers (Amano et al. 2006; Kitamura et al. 2005). As a result, Jakubinek et al. (2008) and Tunc (2007) concluded that those temperatures were critical for pulp viability. However, the current research results can support all previous studies reported that the greatest temperature change caused by the application of the curing unit.
The findings of this clinical investigation revealed a statistically significant difference in the temperature rises at the Admira Fusion x-tra restoration before and after curing when preheated at 50 °C and 70 °C, while after curing showed the highest mean values. At composite surface, with VisCalor bulk preheated at 50 °C after curing, the mean composite surface temperature increased statistically significantly. There was no statistically significant difference in mean composite surface temperature after curing of VCB preheated at 70 °C. The materials were supplied in two forms where the VCB supplied in compules and AFX supplied in syringe. The preheated compules actual delivery temperature was lower than the heating device's preset temperature. Furthermore, when a resin composite heated at 60 °C and then separated from the heat source, the temperature decreased 50% in 2 min and 90% in 5 min. As a result, to keep the temperature from lowering too far, the operator must intervene quickly. To gain the benefits of increased monomeric conversion, the operator should deliver the material, adjust it, remove excess and shape it as needed, then light-cure it while it is still warm (Daronch et al. 2007).
The degree of rising temperature during photopolymerization is affected by a number of factors, including the type of light curing unit, power output, exposure time, nearness between the tooth and/or composite surface and the light guide tip end, composite shade, and thickness of both the composite material and remaining dentin (El-Deeb et al. 2015). The majority of the temperature rise in the preheated composite was caused by exothermic photopolymerization and heat produced by the LCU (Daronch et al. 2007; Fróes-Salgado et al. 2010). Researchers and physicians have long been concerned about the buildup of heat during photopolymerization.
Significant temperature gains during light curing can be attributed to the increased irradiance and/or a longer exposure period when comparing different forms of LCU (Rueggeberg et al. 2017).
The current study's findings that the hypothesis was accepted as there was a nonsignificant difference in the pulpal floor and restoration temperatures between Bis-GMA containing and free when both preheated at 50 °C and 70 °C.