## 4 Discussion  
The outcome measure employed in the investigation was the extent to which each preparation's design could resist fracturing. The results show that, excluding inays with a 75% ICD, the fracture resistance between the various experimental groups did not noticeably differ. Therefore, it was possible to partially reject the null hypothesis, that the fracture resistance and mode of failure of premolars restored with zirconia inays and onlays are not significantly impacted by variations in cavity-preparation designs. The common clinical concern of broken and cracked teeth is approximately the third-most prevalent cause of tooth loss. Breakage and cusp loss can result from large restorations and severe carious lesions [18]. Regarding preliminary designs for preparations, although opinions on the best way to lessen cuspal fracture differ, there is a definite correlation between fracture strength and the amount of lost dental tissue [19]. The mentioned result is consistent with what [PERSON] et al. [20], [PERSON] et al. [21], and, most significantly, [PERSON] et al. [18] found: that the behavior of teeth whose preparation involved a greater removal of the dental structure affected fracture resistance. Even when oxides are added for reinforcement, indirect ceramic restorations nonetheless have a high modulus of elasticity and a large concentration of stress within the main body of the restoration [22; 23; 24]. The study's findings also support [PERSON] et al. 's [22] call to further investigate constraints associated with the design of inlay cavities. Those authors examined the fracture resistance of natural teeth, inlays, and onlays and found that none of the groups that underwent testing had significantly different fracture resistances [25; 26; 27]. Furthermore, the depth of preparation and the width of the isthmus have an impact on the cuspal deflection and, consequently, the fracture strength [18; 28]. To maintain dental tissue, it is advised to employ a preparation that is defect-oriented [29]. Despite this, it was suggested that superior clinical studies be conducted to confirm the results obtained in the lab [30]. A noteworthy observation is that group 6, one of the study's sample groups, showed the best fracture resistance results compared to the other five groups. This group's onlays had palatal functional cusp reductions of 2.0 mm and buccal nonfunctional cusp reductions of 1.5 mm This finding indicates an additional benefit that the planned design of ceramic onlays may provide. Additionally, the production of onlays with 100% ICD covering all cusps is associated with a decreased risk of ceramic fracture, based on the logistic extrapolation of the data found in this study. However, the results of this study indicate that there is no significant difference in the fracture strength of ceramic restorations made with and without different cavity-preparation designs. This means that different designs did not affect the fracture strength of the restorations. This is a preliminary decision based on the results of the clinical \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Preparation Design** & \multicolumn{4}{c}{**Mode of Fracture (Number of Specimens)**} \\ \cline{2-5}
**Group** & **I** & **II** & **III** & **IV** \\ \hline
1 & - & - & 5 & 5 \\
2 & - & - & 6 & 4 \\
3 & - & - & 4 & 6 \\
4 & - & - & 2 & 8 \\
5 & - & 1 & 7 & 2 \\
6 & 1 & - & 8 & 1 \\ \hline \hline \end{tabular}  
* I: Ceramic extensive failure; II: ceramic cohesive failure; III: fracture within the ceramic and tooth structure while the root is preserved; IV: longitudinal fracture of ceramic and tooth structure while the root is involved.  
\end{table} 
Table 4: The failure mode of experimental groups. practitioner. If it is decided that an indirect restoration is the best course of action, the clinician will choose the geometry of the cavity preparation [31; 32]. However, because there is a chance of obtaining false positives when implementing our findings in clinical settings, caution is advised.