الفهرس 
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Abstract
preparation design of endodontically treated molars on the stress distribution and fracture resistance of ceramic crown-tooth system.
A total of twenty lower molars were endodontically treated. Core build-up was then carried out using composite restoration and all teeth were prepared to receive all-ceramic crowns. Teeth were divided into two groups according to the occlusal preparation design as follows:
group I (T): Prepared teeth with two planes occlusal surface.
group II (F): Prepared teeth with flat occlusal surface.
All samples’ crowns were constructed from lithium disilicate ceramic blocks (IPS e.max CAD) using the CEREC CAD/CAM system.
All samples were subjected to fracture resistance test on a universal testing machine. All the data were collected, tabulated and statistically analyzed. Microscopic examination of fractured samples was carried out using the stereomicroscope and the scanning electron microscope to determine modes of failure.
Stress distribution was evaluated by 3D Finite element analysis test. 3D digital model was generated by CBCT scanning and Reverse engineering. The components of the model were assembled using Solidworks software and then imported in ANSYS software to carry out the analysis.
The results of fracture resistance test showed that group II recorded statistically non-significant higher fracture resistance mean values (3107.2± 604.9 N) than group I mean values (2962.6 ±524.27 N) as indicated by student t-test (t=0.55, p= 0.5984>0.05). Also, group II resulted in more favorable failure mode as compared to group I.
The results of 3D finite element analysis showed that both preparation designs yielded low Von-Mises stresses within the factor of safety when subjecting the models to average bite force of 500 N. The stress distribution among different layers of the model differed, group I (Two planes occlusal preparation design) developed lower stresses in the center of the crown and the core, and the stresses increased upon moving apically towards the root with greater influence on the mesial half of the model. While group II (Flat occlusal preparation design) developed higher stresses in the center of the crown and the core, and the stresses decreased upon moving apically towards the root with greater influence on the lingual half of the model.
Finally, it was concluded that the flat occlusal preparation design having comparable fracture strength to the two planes occlusal preparation design and more favorable fracture behavior can be used for the preparation of the endodontically treated molars.