## 4 Discussion  
The rapid spread of the latest generation universal adhesives and their versatility had prompted us to test them on sound and on DI-II-affected teeth. Only in recent years the literature started to talk about these new adhesive systems. Literature on these adhesives is principally oriented to sound teeth, dentin contaminated by metal oxides, NCCL and demineralized enamel [20], but there is no research on adhesion to DI-II-affected teeth. These adhesives showed similar SEM appearance to fourth and fifth generation adhesives on sound dentin and enamel with a thin interface in enamel and a visible hybrid layer in dentin with formation of resin tags in dental tubules. The advantage of the newer universal bonding adhesives is that they can be used in both self-etch and total-etch mode. The behavior of both tested adhesives was found to be effective and similar on both sound enamel (Figure 1A,B) and on enamel affected by DI-II (Figure 3A,B). This was expected since the structure of the enamel affected by DI-II, as previously discussed [18], is identical in quality and structure to the sound one. Even if universal adhesives are capable of adhesion on enamel in self etch mode, the literature agrees that a dedicated etch and rinse step leads to better adhesive performance on the highly mineralized enamel substrate. The choice of using these universal adhesives in an etch-and-rinse mode prepared the enamel surface to be properly wetted by the adhesive helping to achieve chemical and micromechanical bond in enamel. On the other end, on the dental substrate, the etch-and-rinse step prior to adhesive application is not mandatory since the chemical characteristics of the universal bonding systems is capable to deal with the leftover smear layer creating in this case a thicker but less penetrating hybrid layer. Adhesion to dentin is highly affected by its heterogeneity in microanatomy (number and diameter of tubules) and mineralization, leading to a vast variability of bonding strength even in the same dentin of the same tooth [19]. Differences between adhesive interface in sound and DI-II-affected dentin was expected and to reduce  
Figure 4: SEM images of fractures in samples with Universal Bond (**A**) and All-Bond Universal (**B**) on a tooth affected by DI-II. The fractures depicted occurred during sample processing for SEM due to dehydration; this occurs due to the different content of water between enamel and dentin, causing the dentin to shrink and separate from enamel. The way the fracture ends in the composite mass, tells that bonding between dentin and composite is equal or stronger than the natural bonding between enamel and dentin. the number of variables the universal adhesives were used removing the smear layer before adhesion with an etch and rinse technique [9]. The etch and rinse procedure on dentin leads to the increased availability of dentinal tubules that can be infiltrated by the adhesive. The reduction of the presence of water that wets the surface, due to the drying process, increases the quantity of infiltrating resin and therefore the quality of the resin tags. The morphology of the hybrid layer on dentin of the two tested adhesives appears similar to that of the self-etch adhesives and the fourth and fifth generation adhesives reported in the literature (Figure 1C,D) [9]. The morphology of the tested adhesives on DE-II-affected dentin (Figure 3C,D) seems to confirm the possibility of using them in selective etch-and-rinse mode on dentin as was made in our previous works with a fifth-generation adhesive [17; 18]. The adhesive failure found on the DI-II-affected samples treated with a fifth-generation adhesive in a previous work [17] was not found in the samples treated with the two universal nanofilled adhesives tested in this study (Figure 3C,D). The quality of the hybrid layer on DI-II-affected dentin, even if in total absence of resin tags, can be appreciated in Figures 2 and 4.