الفهرس 
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Abstract
The purpose of this research was to assess the fracture resistance and µSBS of lithium disilicate CAD veneers at different die spacer thicknesses.
Twenty-four extracted sound maxillary premolars were used in this study. Eighteen teeth had ceramic veneer restorations, and another six had ceramic microdiscs, each with three microdiscs on its buccal surface, making a total of thirty-six ceramic specimens. The teeth were placed in self-cure acrylic resin in a custom-made copper mold with a diameter of 14 mm, and 20 mm length exposing the occlusal surface upwards. The buccal surfaces were prepared with depth-limiting and tapered diamond burs 0.5 mm into enamel, with 1 mm extending over the buccal cusp. The specimens were randomly assigned to 2 groups regarding the mechanical test used: group I (n=18), fracture resistance; group II (n=18), microshear bond strength. Each group was then subdivided into 3 groups, according to the assigned die spacer thicknesses: 20 µm, 40 µm, and 100 µm. The ceramic veneers and microdiscs were designed using Exocad software. They were then milled using CEREC inLab MC XL CAD/CAM milling machine. Single Bond Universal, RelyX Ceramic Primer, and RelyX Veneer cement (3M ESPE, Deutschland, Neuss, Germany) were used in the cementation of the specimens on the buccal surface of the prepared teeth. All specimens were aged by thermocycling for 2,500 cycles in water bathes with a temperature range of 5-55°C with a dwell time of 15 seconds in each bath. The veneers were then subjected to fracture resistance testing at a speed of 0.5 mm/min using a 2 kN maximum load using a 6 mm in diameter ball perpendicular to the buccal cusp tip of veneers. The microdiscs were subjected to SBS test using a mono-beveled chisel which fell at the tooth and restoration interface at a cross-head speed of 0.5 mm/min. Stereomicroscopic assessment at 18× magnification of the failure types was performed. The results were recorded, tabulated, and statistically analyzed.
For group I, the 20-µm group (subgroup A) showed the highest fracture resistance, while the 100-µm group (group C) showed the lowest. However, one-way ANOVA (p≤0.05) indicated no statistically significant difference among the three groups.
Chi-square test indicated a significant difference in the distribution of failure modes. Tukey’s post hoc test indicated the difference to be present between groups A and C. group A showed significantly higher adhesive failure (83.3%) than group C (0%).
For group II, one-way ANOVA (p≤0.05) demonstrated a significant difference among the three groups. Tukey’s post hoc test indicated a significant difference between groups A (20 µm) and C (100 µm) and between groups B (40 µm) and C. No significant difference was found between groups A and B. There was no significant difference in the distribution of failure types for the three groups. It was concluded that different digital die spacer thicknesses do not significantly affect the fracture resistance of lithium disilicate CAD veneers, however, an increase of thickness weakens their microshear bond strength.