## 4 Discussion  
It has been shown that the study of the adhesion mechanisms of resin-based materials is highly dependent on many variables related to material composition, dental substrates, tooth lesion morphology, or treatment protocol. The restoration of NCCLs is still challenging in clinical practice [3,34] due to the complex dental structural modifications of affected surfaces and the proximity of the gingival margin [3]. Moreover, the insufficient bondable enamel in cervical area increases the susceptibility to microleakage and recurrent decay [35]. In NCCL restorations, polymerization shrinkage of resin composites increases the stress on the cervical adhesive interface, possibly developing a high risk of bonding failure. In addition, this process may also result in dental as well as periodontitis relevant complications [35,36]. The literature recommended further research to identify materials and techniques providing the optimal results in NCCL restorations. On account of the microleakage testing conducted in the present study, it can be concluded that a certain amount of microleakage was recorded for all tested materials, which is mostly located in the cervical part of the restorations. It is important to underline that other similar in vitro studies [37] reported analogous results. Most of the samples displayed no microleakage. However, microleakage associated with resin composites is an expected phenomenon [38] and could not be avoided even under ideal laboratory conditions [26,33]. Results concerning the adhesive capabilities of restorative materials failed to reject our null hypothesis, as there was no statistically significant variation in adhesive efficiency. The evaluation of marginal microleakage recorded several ratio differences. Our interpretation of the results is that such ratio differences of microleakage emerged from random variation. Our data are also in agreement with the results of other microleakage studies using flowable composites [39]. However, [PERSON] et al. showed better results of in vitro flowable composite cervical restorations in comparison with conventional composites [40]. The good adhesion recorded for tested materials recommends both types of resin composites in clinical practice to restore NCCLs. The set of four materials chosen in the present study represents an element of novelty. Moreover, to our knowledge, there are no in vitro adhesion studies on such lesions using the low shrinkage BF material. The use of conventional and flow materials from the same manufacturer completes the novelty of the present investigation. Flowable resin composites have been increasingly implemented in practice as an alternative to conventional composites in many clinical situations including the restoration of NCCLs [41] due to their good polishability, relative performant optical properties, biocompatibility, adhesive properties, and reduced low elastic modulus [17]. The low elasticmodulus of flowable composites in comparison with conventional hybrid composites could compensate for the increased polymerization shrinkage stress [42] prevent the failure of the marginal adhesive seal in cervical restorations [43], which could explain the results of our study. The D material has the lowest filler content and a low elastic modulus, but its adhesive behavior was comparable with those of the other materials. The high wettability of the dental surfaces ensuring an intimate contact with the adhesive layer is an advantage of flowable composites in comparison with conventional ones [17]. However, many clinical details related to the manoeuvrability of both flowable and conventional composites, materials' application steps, curing protocol, as well as performance of the isolation field highly influence the restoration fate beyond the composition of dental materials [8; 14; 17]. Controlling moisture during restoration influences the prognosis of class V restorations, but the optimal isolation of the operative field still remains a difficult task [16].