## 4 Discussion  
We aimed to evaluate the shear bond strength of three different self-adhesive resin cements to zirconia using different priming agents. The results showed that the priming agent, cement type, and their interaction had a significant effect on shear bond strength to zirconia. Therefore, the null hypothesis that different priming agents have no significant effect on the shear bond strength of self-adhesive resin cements to zirconia was rejected. In the control group (no priming agent), most of the specimens (24/30) failed during the artificial aging, and the surviving specimens (6/30) showed the lowest bond strength among all groups. Our findings supported the concept that air-abrasion (mechanical adhesion) without a priming agent (chemical adhesion) is not enough to achieve adequate bond to zirconia [12]. Therefore, the claim that these cements are able to bond to zirconia without priming was also rejected. To achieve an adequate bond strength to zirconia, a phosphate functional monomer is essential [3; 11], which is present in all priming agents used. However, it appears that they are not equally effective. Regardless of the type of self-adhesive resin cement used, the universal ceramic primer MN achieved significantly higher bond strength values (10.30-16.46 MPa) compared to AB (5.6-13.41 MPa) and ZP (5.39-7.07 MPa). One possible explanation is that different functional phosphoric acid and methacrylate groups have different resistance to hydrolysis, which resulted in variable bond strength to zirconia [13]. Another reason could be attributed to the presence of silane in the composition (as the case \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Priming Agent**} & \multicolumn{4}{c}{**Fracture Mode**} & \multirow{2}{*}{**Chi-Squared**} & \multirow{2}{*}{_p Value_} \\ \cline{2-2} \cline{5-6}  & & **Adhesive** & & & **Mixed** \\ \hline ZP & 19 & 63.33\% & 11 & 36.67\% & & \\ \hline AB & 25 & 83.33\% & 5 & 16.67\% & & \\ \hline MN & 2 & 6.67\% & 28 & 93.33\% & & \\ \hline \hline \end{tabular} \({}^{*}\)Significant at \(p\) ¡ 0.05.  
\end{table} 
Table 7: Chi-square test, effect of priming agent on the fracture mode. Figure 3: Failure mode inspected under a light stereomicroscope at 20\(\times\) magnification. (**A**) Classic presentation of adhesive failure specimen of ABT. (**B**) Mixed failure specimen of MNRx. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Cement Type**} & \multicolumn{4}{c}{**Fracture Mode**} & \multirow{2}{*}{**Chi-Squared**} & \multirow{2}{*}{_p Value_} \\ \cline{2-2} \cline{5-6}  & & **Adhesive** & & & **Mixed** \\ \hline T & 17 & 56.67\% & 13 & 43.33\% & & \\ \hline A & 15 & 50.00\% & 15 & 50.00\% & 0.62 & 0.732520 \\ \hline Rx & 14 & 46.67\% & 16 & 53.33\% & & \\ \hline \hline \end{tabular} \({}^{*}\)Significant at \(p\) ¡ 0.05.  
\end{table} 
Table 8: Chi-square test, effect of cement type on the fracture mode. for MN), which improves the wettability of zirconia and bonding to the resin cement [14]. These properties of MN agent were reflected in the failure mode where most of the samples showed mixed failures. The findings of this study regarding (ZP) were contrary to the expectation. Although zirconia primer ZP has 10-MDP, the bond strength of zirconia primer ZP showed the lowest values with all cements. It has been reported that the carboxylic acid group could destabilize the bond between 10-MDP and the methacrylate monomers of the resin cement leading to a weak bond [14]. Similar results were reported by [PERSON] et al. [15] who found that AB had higher bond strength to zirconia than ZP using multilink universal adhesive resin cement. Also, the differences in 10-MDP concentrations and primer initiation systems might influence the quality and durability of the bond. The failure mode of both AB and ZP were primarily adhesive which coincides with low bond strength adhesion. Recently, multipurpose (Universal) adhesives were introduced to enhance bonding to zirconia, glass ceramic, and metals [17; 18]. [PERSON] et al.