## 4 Discussion
To the best of our knowledge, only one study has emphasized that the glazing procedure does not affect the marginal discrepancy of single monolithic crowns [38]. The marginal fit of zirconia restorations is affected by cementation. A systematic review indicated that cementation dramatically increased the marginal gap value [16]. [PERSON] et al. [38] measured the vertical marginal discrepancy of monolithic zirconia crowns before and after cementation. The marginal gap increased from 38 \(\upmu\)m in the pre-cementation stage to 60 \(\upmu\)m after cementation. This increase could be attributed to the pressure applied during cementation to gain full seating of the crown, which most likely resulted in an uneven cement layer. The measurements at the marginal gap in the current study were performed after the cementation process because all specimens were subjected to a compressive cyclic loading for half a million cycles that represented five years of chewing function. In addition, the measurement method utilized cross sectioning using scanning electronic microscopy, which was not possible prior to the fatigue test. Previous investigations have reported that the largest internal gap occurs in the occlusal area of zirconia FDPs [12, 14]. The acceptable average occlusal gap has been reported to range from 100 to 200 \(\upmu\)m [39]. In the present study, the mean occlusal gap in each group was in the acceptable occlusal range. The lowest mean occlusal gap was revealed as 145 \(\upmu\)m for monolithic zirconia FDPs, whereas the highest was 192 \(\upmu\)m for zirconia FDPs veneered manually. [PERSON] et al. [40] indicated that the internal gap at the occlusal area of zirconia restoration was 204 \(\upmu\)m compared with the occlusal gaps of traditional veneered metal-ceramic and pressed-on metal-ceramic restorations, which were 397 and 285 \(\upmu\)m, respectively. The reasons for obtaining high occlusal and axial gaps could be attributed to defects in the scanner resolutions, which led to reduced scanner accuracy along with milling device limitations [39]. The results of the current investigation showed no significant difference between monolithic and bi-layered zirconia FDPs with respect to the marginal adaptation. The mean marginal opening of the monolithic zirconia FDPs was 74.92 \(\upmu\)m. [PERSON] [41] et al. compared the marginal fit of monolithic 3-unit zirconia FDPs with that of bi-layered zirconia FDPs. The marginal gap was 76.2 (36.3) \(\upmu\)m and 77.4 (38.9) \(\upmu\)m for veneered and monolithic zirconia FDPs, respectively. These results are consistent with the findings of the present investigation. In this study, the marginal gaps for both monolithic and layered zirconia FDPs were within the acceptable clinical range and less than 100 \(\upmu\)m. By contrast, [PERSON] et al. [42] examined and evaluated the fit of highly translucent and regular zirconia manufactured using different CAD/CAM systems (Cercon vs. Ceramill). The authors found a significant difference between the zirconia brands fabricated using different CAD/CAM systems. One possible explanation for the different results of these studies is the use of different CAD/CAM systems. The study findings have practical relevance for dental professionals and laboratories engaged in the manufacture of zirconia FDPs. Understanding how different restoration designs and manufacturing procedures impact fit and marginal discrepancy might help clinical decision making and patient outcomes. The results of this present in vitro study should be interpreted with caution for several reasons. Monolithic and bi-layered zirconia restorations were fabricated under ideal conditions using the same cast model, which does not imitate clinical procedures. In addition, the marginal and internal fits of the zirconia restorations were measured by a cross-sectioning technique using scanning electron microscopy. This technique allows the fit of the restorations at certain points to be measured.