## 4 Discussion
[19] tested bonding to zirconia using two different universal adhesive systems. The findings indicated increased bond strength in comparison to control groups (no priming agent). [PERSON] et al. [15] reported that AB had better performance than ZP and the failure mode was predominantly adhesive failures. In the presented study, it was only successful to achieve better bond strength to zirconia when combined with conventional self-adhesive resin cement. However, failure mode was predominantly adhesive failures. In composition of this bonding agent (AB), presence of hydroxyethyl methacrylate (HEMA) which has high water sorption might increase the bonding degradation [20]. The current results showed that the bond strength of the conventional cement Relyx was superior to the bioactive cements. In disagreement, [PERSON] et al. [28] reported that TheraCem showed higher bond strength to enamel, dentin, and zirconia than conventional self-adhesive resin cement. [PERSON] et al. [29] found that TheraCem had a higher bond strength to zirconia than a resin-modified glass-ionomer cement and conventional self-adhesive resin cement. In addition, [PERSON] et al. [5] showed that the bond strength of self-adhesive resin cement containing MDP had higher values than conventional self-adhesive resin cement. These contradictory results may be attributed to the different materials used in these studies, which may have affected the quality of the covalent bond between zirconia and the monomer of the different self-adhesive resin cement [37]. In addition, some studies have not subjected the samples to thermocycling. The degree of polymerization of the resin cements and their composition directly influence the ability of moisture tolerance after thermocycling [38]. The control group showed adhesive failures. Hence, the application of the ceramic primer or bonding agent containing functional monomer in the composition is recommended to achieve a high bond strength [9]. AB and ZP groups showed both mixed and adhesive failures. However, the performance of MN was reflected in the failure mode. Most of the failures were mixed failures except two specimens with showed adhesive failures [14]. However, the difference between the studies might be related to multiple factors. First, the exact composition and percentage of each material are not declared completely by the provider which might influence the bond strength between different MDP containing cement, ceramic primer, or universal adhesive agent. Second, the use of artificial aging techniques, such as thermocycling, can replicate the oral environment and stress the resin-zirconia contact; however, few studies employ this step. This study included multiple Limitations. First, using shear bond strength instead of microtensile bonds strength which allow to perform uniform stress distribution on the bonding surface. However, shear bond strength is most common to assess ceramic bonding, easier to perform, and provide the researcher with a group ranking [2]. Second, thermocycling is used for aging, but other factors such as mastication load and fluctuation of the pH level might considerably increase the degradation of the bond. Therefore, the authors recommend further studies to evaluate other important mechanical and physical properties in order to complete the knowledge about various zirconia-based materials [39; 40; 41; 42].