## 4 Discussion
The SEM micrographs and optical microscopy images of most sections revealed well-adapted, continuous adhesive interfaces underneath the restorations. Occasionally, disruption of the adhesive interface was found. These observations are sustained by others [26]. In our study, one-step self-etch adhesives were used as recommended by the manufacturers, although they probably are not the best option for restoring NCCLs [6; 44]. The literature data report that the combination of primer and adhesive in the same bottle tends to decrease the bonding strength, thereby influencing the hybrid layer durability [45]. As a result, the survival of the restorations is affected. However, both adhesives used in the present study are classified as mild acidic adhesive systems associated with an increased bonding efficiency in comparison with the more acidic adhesives from the same class [6]. In the present study, well-formed more electron-dense structures of 5 to 10 \(\upmu\)m penetrating into the dentin and resembling to resinous tags were observed for both adhesives. Usually, mild self-etch adhesives demineralize dentin only very shallowly and remove incompletely the smear layer and smear plugs from the dentine tubuli, providing a submicronic hybrid layer and resinous tags of about 2 \(\upmu\)m [46]. The increased dimensions of resin tags observed by the present study may be related to the increased demineralization performance of current self-etch adhesives and to other influencing factors, such as agitation during application, thickness of the smear layer, and viscosity and wetting characteristics of the products [47]. However, it seems that that the thickness of the hybrid layer and the presence of resin tags do not significantly influence the bonding performance of the adhesives [48]. In this study, the cervical locations of the cavity margins did not influence the adhesive performances of the materials. No differences were highlighted between cervical and occlusal microleakage ratios as well. The data in the literature on this issue are inconsistent [37; 49]. An increased cervical leakage may be due to the irregular prismatic structure of the cervical enamel [50] or to the presence of cervical cement [51] that induces the formation of rare resin tags and thus a weaker bonding area [51]. One limitation of this study is related to the investigated sample size. The variability of the measurements is very high, and thus, small differences between groups could not be detected. The investigation on a larger number of repetitions must be conducted to validate the results of the present study. Although conventional classical class V cavities are usually designed to test composite adhesion [43], the present study prepared wedge-shaped cavities based on the clinical reality that reports increasingly common such clinical situations [52]. The dual localization of the lesions at the cervical level, above or beyond the enamel-cement junction, resulted from the still unperfected clinical problem of the uncertain adhesion in the cervical area [50; 51]. For better observing the materials' adhesive behavior at cervical margins, the present study provided constant high bond strength at the occlusal part of the restorations by preparing an enamel level [53] and selective enamel etching with 35% phosphoric acid [6]. Systematic standardization was performed so as to minimize variability in this experiment. Consequently, the authors were committed to a very precise and detailed restoration technique for obtaining a minimal polymerization shrinkage and better marginal adaptation [8; 54]. Thorough analysis of the restorative steps as derived from clinical recommendations associated with the preparation of a specific form of cervical defects constitutes an element of originality of our study.