الفهرس 
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Abstract
In some clinical situations, the length of either a prepared tooth or an implant abutment is shorter than ideal, and 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.
The aim of this study is to evaluate the effect of two different incisal veneering porcelain thickness on the fracture resistance of anterior all ceramic CAD/CAM zirconia crown system as compared with the conventionally used metal ceramic crown system. It also evaluated the mode of failure occurred.
A metal die was fabricated to simulate a maxillary central incisor prepared for both all ceramic and metal ceramic crowns, from which twenty epoxy resin dies were reproduced. The twenty specimens were randomly allocated into two equal groups (I and II) each of ten according to the crown material.
Group I: CAD/CAM zirconia all ceramic crowns
(coping thickness of 0.5mm).
Group II: Nickel chromium metal ceramic crowns
(coping thickness of 0.5mm).
Each group was subdivided into two subgroups according to the incisal veneering porcelain thickness.
Subgroup Ia, IIa: The incisal veneering porcelain thickness was 1.5mm.
Subgroup Ib, IIb: The incisal veneering porcelain thickness was 3mm.
CAD/CAM zirconia all ceramic and metal ceramic crowns were fabricated on the prepared dies with standardized dimensions and designs using standardized method according to manufacturer’s instructions. All crowns were then adhesively luted with resin based cement (Multilink cement system).
Subsequently, the specimens were subjected to thermal cycling and cyclic loading corresponding to six months of clinical service. Then, they were loaded until fracture using universal testing machine to indicate the fracture resistance for each crown material in each veneering thickness, then, statistical analysis was carried out.
The results showed that the fracture resistance of nickel-chromium metal ceramic group was significantly higher than that of CAD/CAM zirconia all ceramic group. Also the fracture resistance of crowns with 1.5mm incisal veneering thickness was significantly higher than those with 3mm 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).