الفهرس 
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Abstract
The present study aims to evaluate the effect of different occlusal preparation designs on the marginal adaptation and internal fit 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 internal fit was measured using the replica technique. The intaglio of the crowns were filled with light body silicone impression material then seated with firm hand pressure on the corresponding abutments followed by pressure 20N for 10 minutes. The crowns were gently removed and heavy body silicone was used to remove the films and prevent their distortion during the cutting process.
The replicas were sectioned buccolingually and scanned using light microscope and their thickness was measured with the help of compatible
software.
Nine measuring points for each section with total of eighteen points for each crown were measured.
The crowns were then seated and cemented. All crowns in all groups were cemented with resin cement. During cementation static finger pressure was applied then the specimens were axially loaded with a 5kg load using a specially designed device.
The vertical marginal gap was measured for all crowns using a digital microscope fitted on a precision stand.
Two-Way ANOVA test was used to examine the effect of the different occlusal preparation designs on the marginal and internal fit of monolithic lithium disilicate crowns and translucent zirconia crowns constructed on endodontically treated lower molars.
The results revealed that the flat occlusal preparation showed the best
marginal and internal fit for both lithium disilicate crowns and translucent zirconia crowns, followed by reduction in two planes then the anatomical reduction.
The lithium disilcate crowns showed better marginal and internal fit than the translucent zirconia .
The best marginal and internal fit was recorded for the flat occlusal preparation of the lithium disilcate crowns.