## 4 Discussion
This process creates contraction in the restorative material and the maladaptation of the margin, leading to an internal tensional stress on the surrounding tooth structure and consequent microleakage [9, 41, 42]. The magnitude of these stresses depends on several factors, such as the configuration of the cavities but also the effect of light-curing mode of the material [3, 40]. In this study, all cavities had the same dimensions, as they had the same C factor (index between free and adhered faces). The null hypothesis was rejected, because the study resin, Surefill One(tm), did not obtain a lower microleakage rate when compared to other conventional composite resins. Even more, the higher values of microleakage observed in group 1 (Surefill One(tm)) when compared to group 2 (Spectra(tm) ST HV composite) suggest that the new bioactive resin may present greater polymerization shrinkage compared to a conventional nanohybrid composite resin. Dual polymerization was used for Surefill One(tm) (recommended by the manufacturer) which, by itself, can induce a higher polymerization shrinkage stress in cavities with a high C factor and a greater challenge for adhesion to the dentin of the cavity floor, when compared to self-curing techniques [21, 43, 44]. This way, composite resins achieve better control over polymerization shrinkage, reducing micrographs and associated stress, thus managing to produce restorations with less microleakage and consequently greater longevity. However, when restoring cavities of this type, other parameters must be taken into consideration, such as the mechanical properties of the materials and the strength of adhesion to dentin [11; 45]. The adhesion of Surefill OneTM depends mainly on high molecular weight polyacrylic acids, capable of facilitating the hybridization of the smear layer and ionic interactions between the dentin calcium and the carboxylic groups of the MOPOS (Modified Polyacid System), also reported in RM-GICs [11; 14]. Despite containing water in its formulation, this self-adhesive hybrid bulk-fill resin requires some moisture to activate the functional acids [14; 46]. This way dentin must not be completely dehydrated. However, it is much more difficult to control the moisture level in a deep, narrow cavity than on a flat surface, making it difficult to obtain the ideal moistened dentin [15; 47]. The above-mentioned factors can help to justify the higher values of microleakage with statistically significant differences obtained between the test groups (\(p\) = 0.002). Test groups 1 and 2, when compared with the positive control group, did not show values with a statistically significant difference (\(p\) > 0.05), unlike when analyzed with the negative control group where there is a statistically significant difference (\(p\) < 0.001). From this analysis, we can infer that both restorative materials under study are far from what could be considered an ideal restorative material, where microleakage values would potentially be close to null. Additionally, recent research showed that Biomimetic Hydroxyapatite [48] and Casein phosphopeptide-amorphous calcium phosphate [49] compounds showed the deposition of hydroxyapatite on polymeric composite, thus preventing caries on the margins of the composite frameworks. Additionally, the influence of these compounds should be tested in future microleakage investigations. The standardization of in vitro conditions is an essential step to control possible bias factors, optimize the statistical analysis and allow the reproducibility of the study [31; 50]. As recommended by several authors [51; 52; 53; 54], the restorative procedures were performed by the same operator, reducing the associated \"human\" error. However, this study has some limitations: not being an \"in vivo\" protocol, the restorative process ends up not being influenced by several factors present in the oral cavity, such as contaminants (saliva, blood, etc. ), humidity, quality of isolation protocol and access and location of the lesion.