## 4 Discussion  
A proper fit of zirconia FDPs is essential to ensure excellent long-term longevity and performance. Thus, this in vitro study evaluated the marginal and internal fit of monolithic and bi-layered zirconia FDPs. With respect to the marginal discrepancy, no significant difference was observed between the groups; therefore, the null hypothesis could not be rejected. The marginal openings between 50 and 100 \(\upmu\)m are considered to be clinically acceptable [16; 17]. In this study, the marginal gap for monolithic and bi-layered 3-unit zirconia FDPs was within the acceptable clinical range. In contrast, there was a significant difference in terms of internal discrepancy; therefore, the null hypothesis stating that 3-unit zirconia FDPs design and manufacturing techniques affected the internal fit was rejected. The marginal and internal fit of zirconia restorations can be affected by various factors, including impression procedures, fabrication processes, and cementation [29; 30; 31; 32; 33; 34]. The precision of the impression technique affects the fit of the dental restorations. Studies have compared the fitting accuracy of zirconia restorations using digital and conventional impression methods [29; 35]. In a systematic review, [PERSON] et al. [36] concluded that the marginal discrepancy of all-ceramic restorations manufactured using the digital impression method is statistically similar to those constructed using the conventional impression method. In this in vitro study, digital impressions were used to fabricate the 3-unit monolithic and veneered zirconia restorations. The marginal gap ranged between 65.17 \(\upmu\)m and 84.33 \(\upmu\)m. Consistent with our study, [PERSON] and [PERSON] [31] compared the marginal and internal fit of 3-unit zirconia FDPs fabricated using digital and conventional impressions. They revealed that the marginal gaps were 63-76 \(\upmu\)m for digital and conventional impressions, respectively. In contrast, [PERSON] et al. [37] found that the internal gap of 3-unit zirconia FDPs fabricated using digital impression differs from that of conventional metal-ceramic  
Figure 5: Boxplot showing the internal gap measurement at axial, cervical, axio-occlusal, and occlusal areas of zirconia FDPs for both abutments. The same letter indicates significant differences between the groups. restorations. The variation between the results of the current investigation and those of previous studies can be attributed to the performance of different CAD/CAM systems. The veneering technique and material used affect the fitting of zirconia restorations. The influence of the veneering process on zirconia restoration has been investigated in previous studies for both zirconia crowns and FDPs in previous studies [25, 26]. In an in vitro study evaluating the marginal discrepancy of zirconia copings before and after veneering procedures, investigators found a significant increase in the marginal opening after the veneering process [26]. In this study, the marginal gap in the ZL, ZP, and CAD-on groups were 77.25, 69.92, and 68.42 \(\upmu\)m, respectively. The highest mean value of the marginal opening was observed in the zirconia group that was veneered manually (ZL). A possible explanation for this finding could be the influence of the number of firing cycles. Manual veneering usually requires between four and five firing cycles, while between two and three firing cycles are needed for pressed-on and CAD-on techniques. In addition to the number of firing cycles, the coefficient of thermal expansion mismatch between the veneering material and the zirconia framework is another factor. This difference causes stresses in the veneering layer during cooling from the glass transition to room temperature [27]. As veneering methods negatively affect the marginal adaptation of bi-layered zirconia FDPs, information regarding the influence of glazing firing on monolithic zirconia restorations is limited.