## 3 Results
### Atomic Force Microscopy Analysis  
For Group A, the fine microstructure of the Composite (C) is presented in Figure 3a. The topographic image reveals a scaly morphology. It is formed up by some submicron boulders (having diameter around 600 nm) very well bonded together with a nanostructured material which is formed by well stacked nanoparticles of about 60 nm. Overall, in Group A, the restorative material proves to be very compact assuring an optimal bulk filling of the cavity. The tri-dimensional image shows a relatively smooth surface having a Ra = 45.6 and Rq = 57.9. Fact deals within the high level of compactness inside of the material. The composite--enamel (C-E) interface is presented in Figure 3b. The left side of the topographic image is the enamel featuring a smooth and very compact surface and the right side of the image belongs to composite. Its morphology is similar with the one observed in Figure 3a. The middle vertical of the image in Figure 3b represents the interface. It features a very compact structure with less presence of submicron formation but richest in nanostructured compound which is very well attached to the hydroxyapatite nanoparticles within the enamel. The interface is more visible in the tri-dimensional image as a gap filled with a compact material which assures a strong cohesion between restorative material and enamel. The average thickness of the bonding layer is about of 5 um. The presence of the bonding interface between composite and enamel leads to a surface roughness increasing of Ra to 315.25 nm and Rq to 412.0 nm (Table 2). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Microleakage** & **Mean** & **Standard Deviation** & \(p\)**-Value** \\ \hline Group A & 1 & 1.1427 & \\ Group B & 2 & 1.3304 & 0.723 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Microleakage in the oblique (Group A) and horizontal (Group B) layering technique. Figure 3: AFM 2D topographic images for Group A samples: (**a**) composite (restorative material), (**b**) composite—enamel interface, and (**c**) composite—dentin interface. Tri-dimensional images are given next to 2D topographic images. C-composite, E-enamel, D-dentin. The composite--dentine (C-D) interface is observed by AFM in Figure 3c on the diagonal from left down corner to the right upper corner, the dentine appears on the left upper corner and composite on the right lower corner. The adhesion layer also has a thickness of about 5 \(\upmu\)m and is filled up with fine nanostructured bonding material which assures a very good adhesion of the restorative material to the hydroxyapatite nanoparticles from the dentin structure. The tri-dimensional image features the adhesion layer as a well-formed furrow along to the interface proving the strong cohesion between restoration material in Group A sample and dentine. The smoothness of the bonding observed in this case leads to a less increased roughness: Ra to 240.25 nm and Rq to 300.75 nm (Table 2). Although Ra and Rq values are higher for C-E interface than C-D, the differences are not statistically significant (\(p>0.05\)). For Group B, Composite (C) has a particular morphology as seen in the topographic image in Figure 4a. It is based on a submicron granular material with boulder aspect and diameters varying in a wide range between 200 to 500 nm consolidated in a compact block with a nanostructured material having fine rounded units with diameter of about 80 nm. Such compact area is observed in the upper side of the topographic image in Figure 4a. The lower side of the image features a pore occurred in the restorative material having a diameter of about 1.3 \(\upmu\)m and a dendritic-irregular border. The pore is formed in the boulder reach areas, perhaps due to a local lack of nanostructured material. Overall, the composite assures a good in bulk filling of the cavity and features a good cohesion despite the observed pores. The topographic image in Figure 4b captures the interface between enamel (E) (low side of image) and Composite (C) in the upper side of the image.