## 4 Discussion  
The introduction into clinical practice of build-up and indirect restorations made of composite resins requires the knowledge of adhesion mechanisms, especially when the surfaces are made of already polymerized composite [26]. This analysis can also be linked to the necessity of following a conservative approach in order to increase the longevity of direct restorations and the possibility of further repairing processes [27]. The treatments used most often to enhance the adhesive's action are chemical and mechanical surface roughening [14; 18; 19; 21; 26]. In this study, two mechanical and one chemical technique of surface conditioning and their combination were used. The mechanical techniques consisted of surface roughening by the action of a diamond bur or sandblasting with Al\({}_{2}\)O\({}_{3}\). The chemical conditioning technique instead consisted of an etching procedure on the samples' surface with 35% orthophosphoric acid for 30 s or 60 s [28]. The images obtained with SEM and three-dimensional topography with an optical confocal microscope in all experimental groups agree, showing that among samples of the same groups the surface treatments cause substantially comparable morphological alteration. This highlights how the response of the same type of material to various techniques does not change and is therefore predictable. The control group (treated exclusively by polishing with abrasive discs) presented a smooth, homogeneous surface without any irregularities or microcavities. The surfaces roughened by a bur were irregular, with a limited number of microcavities distant from each other and with parallel streaks resulting from the action of the bur. The sandblasted samples showed greater irregularity and a high number of microcavities. The comparison between the observations for these latter groups, according to previous studies, confirms the hypothesis that sandblasting with Al\({}_{2}\)O\({}_{3}\) is the most suitable treatment for increasing the micromechanical retention of composite surfaces [29]. In our research, the treatment of etching alone, regardless of its duration, does not cause a significant improvement of composite roughness. Furthermore, there are not any substantial modifications on the surface even when the etching action is carried out on a previously mechanically conditioned surface. This outcome is independent of the duration of acid application. The observational data, therefore, seem to exclude any positive impact of the etching, according to [PERSON] et al., who stated that 35% orthophosphoric acid is not able to cause significant alterations to the resin filler. This component can be modified only by more aggressive acids, such as hydrofluoric acid [28]. The morphological findings (SEM and confocal) are confirmed by the profilometric analysis quantifying the average roughness (R\({}_{\text{a}}\)) of the experimental groups. The profilometric analysis (Figure 2) shows variable roughness among the experimental groups. The color scale from red to blue expresses the difference in height at different points of the same sample. To understand these data it is important to note that it is almost impossible to obtain a perfectly flat sample by hand, and this is particularly evident in the A, B and C groups, where the analysis shows the lowest R\({}_{\text{a}}\) values but red and blue areas are visible. This color expression afforded the researchers a visual way of understanding the surfaces' pattern and without comparing their height. Groups A, B and C obtained the lowest R\({}_{\text{a}}\) values; moreover, there were no significant differences in the average roughness between surfaces treated with etching only and the control group. Even when the effect of the etching on milled or sandblasted surfaces was evaluated, the variation in roughness was not statistically significant. The concordance between the observations shows how chemical treatment does not change the roughness of a smooth or previously mechanically roughened surface.