###### Abstract  
This in vitro study evaluated the influence of restoration design (bi-layered vs. monolithic) and manufacturing technique on the marginal discrepancy and internal fit of 3-unit zirconia fixed dental prostheses (FDPs). Mandibular second premolars and second molars were prepared as abutments in a 3-unit zirconia bridge to develop four groups (\(n=10\) FDPs): MZ: Monolithic zirconia FDPs, ZL: zirconia framework veneered by the hand-layering technique, ZP: zirconia framework veneered by the heat-pressed technique, and CAD-on: zirconia framework veneered by CAD/CAM lithium-disilicate glass-ceramic. All the zirconia FDPs were cemented to their corresponding die replicas using dual-cure resin cement and were subjected to compressive cyclic loading at a load range for half a million cycles using a universal testing machine. FDPs were sectioned mesiodistally to measure the marginal gap and internal fit using scanning electron microscopy. The measurements were taken at pre-assigned points of each abutment. Data were statistically analyzed via a Kruskal-Wallis test (\(\alpha=0.05\)). No significant differences were found between the monolithic and bi-layered zirconia groups in terms of the marginal discrepancy. However, there was a significant difference in the marginal gap between the zirconia groups. The marginal gap between monolithic and bi-layered zirconia FDPs was within the clinically acceptable range (<100 \(\upmu\)m). Comparable mean values of the marginal gaps of 3-unit monolithic and veneered zirconia FDPs were found. Therefore, the FDP design and veneering methods did not affect the marginal discrepancy. However, the mean internal gap varied among the experimental groups. As the current in vitro investigation demonstrated equivalent mean values of marginal gaps of both 3-unit monolithic and bi-layered zirconia FDDs, the use of monolithic 3-unit zirconia FDPs would be a viable alternative fabrication technique. marginal; internal; zirconia; CAD/CAM; prostheses 20  
opacity [4]. Tooth-colored and highly translucent zirconia has been developed and increasingly utilized in full-contour (monolithic) single crowns and multi-unit restorations [5; 6]. Increased translucency in zirconia was achieved by reducing the alumina additive to 0.05% by weight while maintaining the yttrium oxide stabilizer at 3 mol% (equivalent to 4.5-5.6 wt%). Further improvement in translucency was observed when the yttrium oxide content exceeds 4 mol% (less than 10 wt%), in conjunction with an increase in cubic phase zirconia. These changes facilitate the use of zirconia as a monolithic restoration material without an additional veneer layer [7]. Survival and success rates of the zirconia restorations were evaluated according to their mechanical and biological outcomes. Several studies indicate that the most common mechanical failure of zirconia-based restorations is chipping (cohesive failure) within the veneer layer [8; 9; 10; 11; 12]. In addition to the mechanical complications, biological complications including periodontal problems, loss of vitality, and secondary caries have been reported [13; 14]. A systematic review indicated that secondary caries and loss of vitality result from an improper fit of all-ceramic restorations [13]. A well-fitted margin is essential for reducing plaque buildup, preventing recurrent caries, and potentially prolonging the restoration's lifespan [15]. Research studies suggest that a marginal opening of 50 \(\upmu\)m to 100 \(\upmu\)m is clinically acceptable [16; 17]. However, the quality of the restoration fit can be affected by factors like impression, fabrication, and luting thickness. A study by [PERSON] et al. [18] found that increasing the thickness of the resin cement decreased the failure load of feldspathic porcelain crowns. This is because a thicker layer of cement can result in a higher concentration of stress [19]. Additionally, research by [PERSON] et al.