الفهرس 
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Abstract
This study was designed to evaluate the effect of different occlusal preparation designs on the fracture resistance of two All - Ceramic crowns restoring endodontically treated teeth Thirty natural sound lower first molars of the same size were endodontically treated then prepared with the same criteria except for the occlusal surface using CNC machine.
Flat occlusal preparation was performed on ten samples, another ten samples were prepared with occlusal plane in two planes and the last ten samples were prepared with anatomical occlusal design.
Thirty full contour crowns were designed on the scanned abutments, using
inLab 3D software (V4.2.5), and then were milled using translucent zirconia blocks and lithium disilicate blocks, 5 crowns of each material for every occlusal preparation design .
The translucent zirconia crowns were milled with an oversize of approximately 25% to compensate for the later sintering shrinkage. The crowns were sintered in high temperature furnace to have maximum density.
While the lithium disilicate crowns were adjusted in the pre-crystallized state and then were glazed and crystallized.
The crowns were then seated and cemented. All crowns in all groups were
cemented with self-adhesive resin cement1. During cementation static finger pressure was applied then the specimens were axially loaded with a 5kg load using a specially designed device.
After cementation, the specimens were screwed to an attachment which was screwed to the base of the machine. The specimens were then loaded vertically on their occlusal surfaces. The loading piston was centered along the long axis of the specimens. The specimens were then loaded in the universal testing machine under a thrust speed of 0.5mm/min.
Two-Way ANOVA test was used to examine the effect of the different occlusal preparation designs on the fracture resistance of monolithic lithium disilicate crowns and translucent zirconia crowns. Strain gauges were adhered to the cervical thirds (1mm above the crown margin) of the buccal surfaces of each crown.
The results revealed that the fracture resistance of translucent zirconia crowns was significantly higher than that for lithium disilicate crowns regardless the type of occlusal preparation design. While in e.max crowns the flat occlusal preparation designs showed the highest fracture resistance followed by occlusal preparation in two planes and the flat occlusal preparation design.
In case of zirconia crowns the flat occlusal preparation design showed significantly the lowest fracture resistance than the other types of occlusal preparation designs. Anatomical occlusal preparation design showed the highest fracture resistance in all groups.
Conclusions:
Within the limitations of this study, the following conclusions could be
drawn:
1- Flat occlusal preparation design is the best type of occlusal
preparation designs for e.max crowns on endodontically treated teeth.
2- Anatomical occlusal preparation design is the best type of occlusal
preparation designs for monolithic zirconia crowns on endodontically treated
teeth.
3- Monolithic translucent zirconia crowns are more resistant to fracture
than monolithic lithium disilicate crowns.
Clinical Recommendations:
For endodontically treated molars, anatomical occlusal preparation
design is recommended for full contour monolithic zirconia crowns
regarding the fracture resistance and flat occlusal preparation design is
recommended while using full contour monolithic e.max crown.