Purpose: the study objective was to evaluate the effect of modifying digital occlusal spacers on the marginal gap of full monolithic zirconia-reinforced lithium silicate crowns by using 4- axis machine. Materials and Methods: Forty-five acrylic teeth with preparation for full crowns were divided into three groups with different occlusal digital spacers (60,90,120 μm). All forty-five abutments were scanned by an intra-oral scanner and a 4-axis machine grinned zirconia reinforced lithium silicate crowns. Each crown was cemented to the abutment by using universal resin cement and for each unit (crown and abutment) a scanning electronic microscope was used to assess the marginal gap. For comparing the marginal gap between the three groups a kolmogorov-smirnov test was performed on the study variables indicated a normal distribution (p>0.05) followed by a one-way ANOVA with post-hoc Bonferroni test (α=.05). Results: The total mean marginal gap for 120 µm (101.38 ±17.94 µm) occlusal spacer was found significantly higher compared to the 90 µm (72.84 ±27.21 µm) and 60 µm (60.16 ± 29.34 µm) (p<.05). There was no significant difference between the 90 μm (72.84 ±27.21 µm) and 60 μm (60.16 ± 29.34 µm) groups (p=0.533). Conclusions: Clinically acceptable marginal gap (<120 µm) was obtained for all three occlusal spacers (60,90,120). As the digital occlusal spacer reduces the marginal gap reduces as well. Occlusal spacer of 120 µm showed a significantly higher marginal gap compared to 90 and 60 µm. The digital occlusal spacer which should be favorable for zirconia-reinforced lithium silicate crowns is 60 µm.
