Bulk-fill composite resins have been launched in the dental market as a new restorative concept. It is highly filled viscous resin composite restoration which can fill a posterior cavity with depth 4 to 5 mm. Highly viscous bulk-fill resin composite has many advantages over the low viscosity resin composite. It has high filler content, superior mechanical properties and easier in application as it does not need incremental coverage. In addition, it contains superior features such as pre-polymer stress relievers, polymerization modulators and modified high-molecular-weight base monomers to minimize the polymerization shrinkage stresses. Although these changes in composition have been introduced to allow greater conversion at increasing depths, they also potentially enhance the mechanical properties, volumetric shrinkage and other related properties, including the clinical performance of the materials (Ilie et al. 2014). It was agreed that higher fillers content of the viscous resin composite results in increasing stiffness of the material in the pre-gel phase and increase the total volumetric shrinkage stresses. In addition, the polymerization rate of highly viscous bulk-fill composites is low.
Polymerization shrinkage is the main drawback of resin composite (Bis-GMA containing), where stress of polymerization passed to the tooth, causing deformation that manifests as enamel fracture, cracked cusps, cuspal displacement, interface adhesive failure, and restorative material micro-cracking (Giachetti et al. 2006).
Due to these effects, free methacrylate composite resins have developed, since they do not contain typical methacrylate in their formula. This lack of monomers allows for a contraction of less than 1% during polymerization, as well as increased biocompatibility, preventing any harmful consequences (Bacchi et al. 2015). Organically modified ceramics (Ormocer) were developed as Bis-GMA-free resin composite materials have been more recently introduced for direct restorations. Ormocer is the acronym for organically modified ceramic and comprises inorganic–organic co-polymers with inorganic silanated filler particles. It generally showed reduced volumetric shrinkage and shrinkage stress, lower or similar wear resistance, and intermediate strength, fracture toughness, or elastic modulus when compared to conventional resin composites. Until now, only one brand of this recent generation of pure Ormocer composite resin marketed as Admira Fusion-Ormocer (Klauer et al. 2019). It also lacks cytotoxicity associated with conventional monomers, such as Bis-GMA and TEGDMA. A fact that proves to be a great advantage when compared to methacrylate-based composite resins. So, it is considered inert and improves the biocompatibility (El-Askary et al. 2020).
Preheating resin composites have found to reduce the polymerization shrinkage as the increased temperature reduces the viscosity of the material and increases radical mobility. Thereby, the benefits of preheating composites may have an impact on daily restorative procedures as well, with the application of shorter light exposure to provide conversion values similar to those seen in unheated conditions. So that improves adaptation also results in increased polymerization and higher degree of conversion (Baroudi and Mahmoud 2015).
Preheating of the resin composites exhibited significant decrease in film thickness after preheating, thus enhancing flow due to the thermal energy that increases the molecular motion of the monomer chains within the composite and also increases the radical collision frequency, and propagation is allowed to continue for a longer time before the onset of deceleration, increasing conversion and shrinkage (Deb et al. 2011).
To have the advantage of preheating to the resin composite before packing, a bulk-fill BIS-GMA containing resin composite with the thermoviscous technology which has been specially developed for warming up to become less viscous, allowing for application similar to that for a flowable material. As a consequence, the current study was conducted to determine the post-gel shrinkage strain (PGSS) of bulk-fill BIS-GMA containing (Viscalor, thermoviscous resin composite) and bulk-fill BIS-GMA free resin composite (Admira fusion Xtra) preheated at 50 °C and 65 °C in comparison with without preheating at room temperature 23 ± 2 °C.
Both types of the composite resin restorative materials were heated for 5 min after stabilization of each selected heating temperature so as to accomplish the most extreme present temperature (Elolimy 2020). The mold was prepared of the Teflon material that not adheres to the resin composite, thus permitting its free shrinkage. The measurement of polymerization shrinkage strain has been done by using the strain monitoring device that is a simple and available approach to determine the post-gel shrinkage strain. The strain was recorded for only 3 min after curing time, as the polymerization kinetic curves of preheated resin composite restorative materials from 0 to 15 min were equivalent for different preheating times and irradiation durations (Yang et al. 2020).
The results of this study showed that regarding the effect of different resin composite used in the study, there was a statistically significant difference in the mean value of polymerization shrinkage strain between (Group A1) and (Group A2) at the three different preheating temperatures, and the mean value of polymerization shrinkage strain of admira fusion xtra was higher than that of viscalor bulk resin composite.
This result was in agreement with Xu et al. (2020), Monsarrat et al. (2017) and Kournetas et al. (2004), where BIS-GMA containing composite presented the lowest shrinkage strain than BIS-GMA free composite, and Admira fusion showed the highest degree of conversion (DC) and the value of polymerization shrinkage strain was significantly higher than that of other resin composites containing BIS-GMA tested. They attributed that to the chemical composition of the resin matrix that has a significant impact on the magnitude and kinetics of shrinkage strain, as well as the development of elastic modulus. Resin composite containing BIS-GMA has high molecular weight than that of resin composite free BIS-GMA (admira fusion), and the shrinkage strain values of resin matrixes created with high molecular weight (Mw) monomers were lower than those formulated with low Mw monomers (Peutzfeldt 1997).
This result was in discordance with Taubock et al. (2018) and Lins et al. (2019), as the bulk-fill resin composites based on Ormocer (Bulk Ormocer) had the lowest linear polymerization shrinkage and shrinkage force that was attributed to its resin system, which consists of inorganic–organic copolymers rather than traditional monomers (e.g., Bis-GMA, UDMA and TEGDMA).
Concerning of the effect of resin composite preheating temperature, the polymerization shrinkage strain of both types of tested resin composite increased with the increase in preheating temperature as the highest mean value of polymerization shrinkage strain was found at (T3). To promote flow and adaptability, preheating the composites exposed to high temperatures (54 °C or 68 °C) induces volumetric shrinkage (Walter et al. 2009).
These results were in agreement with (El-Korashy 2010) that preheating resin composites before application enhanced its DC while also increased its post-gel shrinkage strain (PGSS). Three concurrent reasons may be responsible for the considerable rise in PGSS of all pre-warmed composite resin groups relative to room temperature groups. First, because of a higher rate of polymerization as a result of preheating and promptly reaching the gel point, there was likely a rapid stress buildup within the composite. Second, due to the increased DC values caused by preheating, the volumetric shrinkage and elastic modulus of the material increases. Third, combined with polymerization shrinkage, the effect of substantial thermal shrinkage of the warmed composite as it cools to room temperature may contribute to the dramatic increase in generated stresses.
This result was in disagreement with (Deb et al. 2011) as pre-warming dental composites improve linear polymerization shrinkage, flow, and the degree of conversion, and also with (Lohbauer et al. 2009) as they found that preheating did not significantly increased the polymerization strain. Also (Yang et al. 2020) reported that preheating had no negative effects due to premature polymerization.
