## 4 Discussion  
According to collected data, the first null hypothesis was rejected: results showed that the use of Panavia V5 cement led to higher \(\upmu\)TBS values at T0 when compared to Bifix QM and Estecem. The second null hypothesis, on the other hand, was accepted, as aging time did not significantly influence bond strength over dentin. Many different tests were conducted to analyze bond strength of luting cements: tensile bond strength (TBS), shear bond strength (SBS), and \(\upmu\)TBS. According to literature, both the TBS and the SBS test were not considered in the present study as they are less indicated for evaluating adhesive interface: the TBS could be affected by the specimen geometry and the unequal distribution of the interfacial stress [20], while the SBS is supposed to stress more the material itself rather than the hybrid layer, supported by a more frequent cohesive failure pattern observed in SBS samples [21,22]. As stated by [PERSON] et al. [16]\(\upmu\)TBS is the most reliable for testing such interface, giving therefore more frequently adhesive failures: this is particularly true when using the Ciucchi's jig for fixating beams to be tested, in order to facilitate manipulation and granting more reliable results [18]. In the present study, in accordance with previous \(\upmu\)TBS studies, a prevalence of adhesive fractures was obtained, proving that the adhesive interface was the area where tensile stresses were concentrated. Manufactures always suggest a combined use of a particular adhesive system to their luting cement, stating better clinical performances. The three luting systems selected for the present study were tested following these guidelines. Results showed that Panavia V5 had a significantly higher \(\upmu\)TBS values at T0 when compared to QM and ES. These results could be explained by the different adhesive protocol and luting cement formulation. The purity of 10-MDP functional monomer composing adhesive and luting systems could lead to different chemical reactions with tooth hydroxyapatite, creating strong insoluble calcium salts [23]. Its highly stable bonding potential could correlate to three reported mechanisms: (1) micro-mechanical interlocking achieved through 10-MDP's etching capacity, and (2) ionic interaction with HAp along with (3) stable monomer-Ca salt formation (nanolayering) [24]. As demonstrated by [PERSON] et al. in 2015 [25], 10-MDP produced by Kuraray showed better bond strength values due to its chemical composition as well as its lower pH, making it capable of a stronger demineralization of dentin itself. On the other hand, another possible explanation may be the presence of chemical kinetics and the use of a separate and fresh DC activator in the adhesive that could increase the effectiveness of Panavia V5 group. The DC activator used for the Panavia V5 method was probably more influential on chemical polymerization compared with the one used in the other cements  
Figure 4: A mixed failure, with a prevalence in adhesive pattern, observed in a specimen treated with Estecem and stored in artificial saliva for 1 year. tested, because, according to literature, this effect is material-dependent [26]. Adhesive dualization grants the system a first polymerization, with a chemical mechanism, that is then completed by light curing the adhesive and the cement. [PERSON] et al. showed that bond strength is statistically influenced by DC and the highest bond strength was obtained when adhesive and luting cement were separately cured [27]. The chemical activation of the adhesive, as performed in the present study, and a chemical conversion of the dual-cure luting cement, followed by photoactivation, probably led to a higher conversion degree of the adhesive-cement complex, which can contribute to justifying the higher bond strength obtained with Panavia V5. This is corroborated by [PERSON] et al.