Statement of Problem. In some clinical situations, the vertical length of either a prepared tooth or an implant abutment is short, while the occlusal clearance to be restored by a porcelain crown is large. Incisal thickness of the veneering porcelain should be considered to prevent mechanical failure of the crown. Purpose. The aim of this study is to evaluate the effect of two different incisal veneering porcelain thickness on the fracture resistance of the anterior all-ceramic CAD/CAM zirconia crown system as compared with the conventionally used metal ceramic crown system. Method. CAD/CAM zirconia all-ceramic and metal ceramic crowns were fabricated on the prepared dies with standardized dimensions and designs using standardized methods according to the manufacturer’s instructions. All crowns were then adhesively luted with resin-based cement (Multilink cement system), subjected to thermal cycling and cyclic loading, and were loaded until fracture using the universal testing machine to indicate the fracture resistance for each crown material in each veneering thickness. Results. Statistical analysis was carried out, and the results showed that the fracture resistance of the nickel-chromium metal ceramic group was significantly higher than that of the CAD/CAM zirconia all-ceramic group. Also, the fracture resistance of crowns with 1.5 mm incisal veneering thickness was significantly higher than those with 3 mm incisal veneering thickness in both groups. Furthermore, there was no significant difference in the fracture mode of the two groups where 50% of the total specimens demonstrated Mode II (veneer chipping), while 35% demonstrated Mode I (visible crack) and only 15% demonstrated Mode III (bulk fracture). Conclusion. High failure load values were demonstrated by the specimens in this study, which suggest sufficient strength of both incisal veneering thickness in both crown systems to withstand clinical applications; however, the fracture patterns still underline the requirement of a core design that support a consistent thickness of the veneering ceramic, and it is recommended to conduct long-term prospective clinical studies to confirm findings reported in the present study.
INTRODUCTION:Zirconia restorations present long-term clinical survival. Their low translucency makes them esthetically less than lithium disilicate (e.max CAD). The newly introduced ultra-translucent (UT) zirconia blocks improved the aesthetic problems of esthetic restorations. OBJECTIVES: To evaluate the translucency of UT zirconia restorations and to assess the effect of tribochemical silica coating (TSC) on its bond strength (BS) to the resin cement, using lithium disilicate as control. MATERIALS AND METHODS: Forty four disc-shaped specimens were fabricated and subjected to two main tests which were translucency test and shear bond strength test. Translucency groups: the specimens (10×0.5mm (n=20)) were divided into UTZT (ultra translusent zirconia) and LDT (lithium disilicate). Shear bond strength groups (SBST), specimens (5mm x 4mm (n=24)) were divided into UTZAA (ultra translusent zirconia treated by air abrasion), UTZSC (ultra translusent zirconia treated by TSC) and LDS ( lithium disilicate that was etched and silanated). RESULTS: Translucency results showed p value <0.001 which indicated higher translucency for the lithium disilicate than UT Zirconia. For SBST the results showed that, (UTZSC) provided the heighest mean BS (22.73 ± 3.91) followed by (LDS) (18.18 ± 1.56) and finally (UTZAA) provided the least BS (13.39 ± 4.31). CONCLUSIONS: Within the constraints of this study, UT zirconia showed less translucency than lithium disilicate. The use of silica coating improved the BS of zirconia.
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