## 4 Discussion
In addition, similar results were obtained by [PERSON] et al. [29] and [PERSON] et al. [30]. By the contrary, [PERSON] et al. [31] found that the oblique layering technique had the most leak-free margins when the proximal box ended on enamel. [PERSON] et al. [32] showed that there are no significant differences between the level of shrinkage stress at the tooth-restoration interface when these two techniques of resin composite placement are used. However, [PERSON] et al. [33] observed that the highest degree of cusp flexion occurs in the horizontal technique followed by the oblique one. Other studies, like those that belong to [PERSON] et al. [34] or [PERSON] et al. [35] found that the incremental layering technique leads to a significantly less cuspal deflection when compared to bulk filling technique, however, when comparing the horizontal with oblique layering techniques, there is no significant difference. AFM seemed to be a useful tool for the assessment of marginal adaptation. The AFM uses a software with great mathematical accuracy for the mapping and characterization of surfaces [36; 37] and provides 3D high-resolution offering quantitative data related to the length of marginal gaps. Furthermore, the samples to be investigated by AFM do not require special preparations (such as metallization or fixing) which reduces the occurrence possibility of the artifacts that could affect the analysis [36; 38]. According to SEM images analysis of the Score 0 samples, the increased adhesion of the composite to the dental structures can be explained by the existence of a clearly defined hybrid layer on the entire interface in all specimens treated in this study. The granular appearance of the adhesive is due to the SiO\({}_{2}\) filling content of the adhesive. The formation of a homogeneous hybrid layer means the closure of any communication path between the pulp chamber and the outside, meaning a perfect sealing of the dental wound. Regarding the SEM images analysis of the samples with a score higher than zero, the cracks can be due to the low bond strength at the composite/dental structure interface, which could not face the resin polymerization shrinkage and could not provide an adequate interfacial seal for these samples. These issues can, also, be due to the drying of the tooth sections in the air while manipulating or the dehydration under vacuum during SEM analysis. However, because these gaps or cracks were noticed mainly when horizontal layering technique was used and since all samples were treated in the same way, they can be attributed to the low adhesion between the composite and dental structure. It is not a novelty that the hybrid layer is the most important element in adhesive-dentin bond strength [39; 40]. It is formed by resin that covers the collagen fibrils. The bonding agent needs to uniformly penetrate the collagen system and be completely polymerized in order to generate high bond strength. Considering the EDX analysis, the silica and aluminum in the adhesive layer and dentin is the result of the presence of nanofiller in their composition, and further confirms that resin tags were produced by the adhesive systems. EDX analysis also revealed increased quantities of silica and aluminum in the composite structures, suggesting the presence of an inorganic filler in the material. The most of the composites contain methacrylate resins and the polymerization process is initiated by light. Almost the whole polymerization process occurs in the first minutes after irradiation [41]. In order to ensure the polymerization for all the restorative material, increments of maximum 2 mm thickness are applied, so the light will penetrate and activate the polymerization in depth [42]. To assess the efficacy of polymerization in composite resins, some laboratory analysis are documented in the literature.