## 4 Discussion  
Microleakage at the tooth-restoration interface is considered a major factor in the longevity of dental restorations [28]. The consequences of microleakage can be diverse, such as marginal pigmentation, the early rupture of restorations, secondary caries at the tooth -restoration interface, postoperative sensitivity and the inflammatory pathology of the pulp organ [8; 29]. The integrity of the marginal seal is therefore essential to increasing the longevity of restorations [3; 30]. The integrity and internal adaptation of the restoration to dental walls is compromised when microleakage occurs, normally resulting from the polymerization contraction of the restorative material. It is, therefore, considered, among  
Figure 2: Visual representation of mean values and standard deviation of total counts in each group and significance level between groups: **\(p\) < 0.01; ***\(p\) < 0.001; ns—meaningless. Group 1: Surefill One(tm); Group 2: Spectra(tm) ST HV composite; CPM: counts per minute. other factors, as the most common cause of failure to direct restorations in posterior teeth [3, 5, 8]. Microleakage can be studied through several methods, such as the use of dyes, glucose penetration, bacterial or toxin infiltration, protein microleakage, electrochemical microleakage or scanning electron microscopy [31, 32, 33, 34]. The most common methods in the evaluation of microleakage, in recent studies, are infiltration with a dye and bacterial infiltration. However, penetration with the methylene blue dye has some limitations, such as the need to destroy the samples, the subjective evaluation of the results by a specialized operator and the semi-quantitative nature of the microleakage measurement [35]. The methodology that uses bacterial infiltration, although more clinically and biologically relevant when compared to penetration with dye, can only be applied to materials that have antimicrobial activity for the specific type of bacteria used and usually presents a qualitative rather than quantitative assessment [32]. The present study was designed to evaluate the ability of marginal sealing and the consequent microleakage of different restorative strategies/materials, using an uncommon radioactive isotopes technique. Spectra(tm) ST HV nanohybrid composite resin (Dentsply-Sirona, Konstanz, Germany) and the bioactive resin Surefill One(tm) (Dentsply-Sirona, Konstanz, Germany) were used in dental restorations with standardized cavities, using a solution of sodium pertechnate (\({}^{99\text{m}}\)Tc). The use of radioactive isotopes is a method with a quantitative and non-destructive nature, allowing the measurement of microleakage of the same specimens at different intervals, for long periods and without the need to destroy the samples, in contrast to the two methods mentioned above [36]. It also allows the quantitative measurement of microleakage, detected quickly, even when present in very small concentrations, representing an advantage over the penetration method with a dye [37, 38]. The adopted method showed lower mean values for the negative control group and higher mean values for the positive control group, with a statistically significant difference (\(p=0.027\)), thus validating the selected methodology. Polymerization shrinkage is a very complex phenomenon, reliant on the conditions of the cavity margins due to the difference in composition of dental substrates; the amount of material in the polymerization reaction; and the formulation and setting reaction mechanism of the selected material [3, 39]. The reduction of polymerization shrinkage has been one of the focuses in the development of new materials [8, 40]. However, different restorative materials have different formulations and, consequently, different polymerization shrinkages, which makes their \"side by side\" comparison complex [3, 40]. The shrinkage process essentially occurs due to the conversion of monomers into a polymer network, replacing van der Waals bonds by stronger covalent bonds.