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INTRODUCTION: The natural white color of zirconia together with the veneer chipping problem has led to the development of tooth colored monolithic (full-anatomic) zirconia. In order to optimize esthetics shading of monolithic zirconia were done by two main approaches, either by powder mixing method or by infiltration technique. The influence of infiltration technique on mechanical properties of zirconia has been reviewed previously, while the influence of powder mixing method on the mechanical properties of shaded zirconia, specially the biaxial flexural strength has not been widely studied before. OBJECTIVES: To evaluate and compare the biaxial flexural strength and the crystal structure of shaded CAD/CAM monolithic translucent zirconia by powder mixing method with un-shaded ones. MATERIALS AND METHODS: Twenty fully sintered discs of un-shaded [T 0] and shaded [T S] monolithic translucent zirconia of diameter 12 mm and thickness 1 mm has been divided into 2 parallel groups of 10 discs each. Their biaxial flexural strength was measured using a ball-on-ring test fixture and universal testing machine at room temperature. Statistical significance was measured using One-Way ANOVA test. Then crystallographic analysis was done for both groups using X-ray diffraction. Finally the nature of failures of the tested specimens was examined using confocal laser microscopy. RESULTS: Biaxial flexural strength test showed no significant difference between un-shaded and shaded specimens. X-ray diffraction spectrum only showed crystals of tetragonal zirconia with no evidence of neither monoclinic zirconia crystals nor coloring oxides crystal phases. Confocal laser microscopy images of both groups showed the classic fracture patterns of monolithic zirconia materials. CONCLUSIONS: shading of zirconia blocks by powder mixing methods does not significantly (P≥0.05) affect either biaxial flexural strength or crystallographic structure of monolithic translucent zirconia.

INTRODUCTION Veneer chipping is a common failure of zirconia based restorations, especially in the presence of high occlusal loads. Modifying zirconia design may influence veneer chipping. OBJECTIVES: Purpose of this study was to evaluate fracture resistance of complete and partial contour zirconia crowns. MATERIALS AND METHODS: Twenty standardized epoxy resin dies, prepared to receive zirconia based crowns, were divided into 4 groups, 5 specimens each. Zirconia framework designs were: (a) Full contour zirconia crown (FC), (b) Partial contour zirconia coping with buccal veneering (PC), (c) Anatomically reduced zirconia coping with palatal and proximal collar (ARD) and (d) half mm zirconia coping without collar (CON) (control group). Dual scanning technique was done for the test groups to fabricate zirconia copings with the use of standardized resin patterns, while normal scanning was made for the control group. After milling and sintering of zirconia specimens, veneering was done for all desired surfaces. All crowns were adhesively luted to their corresponding dies, and subjected to thermal cycling and mechanical loading corresponding to one year clinical service. Single load to fracture was applied to all the specimens. Type of failure was recorded for each specimen, investigated visually then by using Stereomicroscope and SEM. RESULTS: Complete and partial contour zirconia restorations showed high resistance to fracture. The presence of collar was insignificant for increasing fracture resistance, but had an influence on the fracture extension toward the proximal surface of the veneered restorations.

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