## 4 Discussion
[22], higher bond strength was found only for the commercial adhesive that contained a smaller size of filler particles (5 nm), justified by the increase in the resistance of the hybrid layer, and associated with the penetration of the particles of charge together with the monomers between the collagen fibrils (12 nm). Contrary to the present study, other adhesives with larger particles were not able to promote an increase in bond strength, in agreement with other authors who did not find better bonding results for adhesives containing filler particles [47; 48]. In the study by [PERSON] et al. [22], it was observed that larger-sized cargo particles or cargo agglomerates would be trapped only in the adhesive layer, which would not have been sufficient to increase the bond strength. It is important to note that these contradictory studies [22], refs. [47; 48] analyzed commercial materials and there may be differences in the composition of the tested adhesives, in addition to not having any reference regarding the amount of filler particles present in them, which could have influenced the different results found. The use of bulk-fill and conventional composites in this study was based on differences regarding the polymerization shrinkage [49] of both and, consequently, on the behavior that they could develop related to the shrinkage stresses generated at the bonding interface in cavities of high cavity configuration factor (Class I). Regarding the bond strength, the different types of composites did not influence the results obtained. It is noteworthy that both were applied according to the manufacturers' recommendations, that is, the conventional one by the incremental technique and the bulk-fill by the single-increment technique, which may have compensated for the fact that the conventional composite presented a very high polymerization shrinkage, higher than bulk-fill [49]. Other studies [50; 51] also found similar bond strength values between bulk-fill and conventional composites when applied by the recommended techniques. Regarding the occlusal and mesial marginal gaps observed at the tooth-restoration interface, it was found that the conventional composite showed a higher percentage of cracks when the adhesive without filler particles was used, while for the bulk-fill composite, the type of adhesive used did not influence the results. As verified in previous studies, the increase in the inorganic content in polymeric materials could reduce the polymerization shrinkage [52]. Although it was not the focus of the present study to verify the polymerization shrinkage of the materials used, it can be assumed that the greater shrinkage of the adhesives without filler particles, added to the one that occurred with the conventional composite even when the incremental technique was used, has been a determining factor for the increase in tensions at the bonding interface, leading, consequently, to the occurrence of larger marginal gaps that have culminated in the statistical difference in this analysis, although this fact did not influence the bond strength. In fact, greater material polymerization shrinkage has been associated with greater adhesive failures and maladaptation at the bonding interface between composite and tooth structure [12; 53]. Although no statistical significance was observed for the bulk-fill composite, a gradual decrease in the percentage of marginal cracks was also observed as the inorganic content of the adhesive increased. In the present study, it is convenient to emphasize that only two composites and one filler particle size were studied, and it is essential to carry out additional studies with other experimental conditions to certify the benefits of the presence of inorganic particles in commercial adhesive systems.