الفهرس 
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Abstract
The restoration of endodontically treated teeth is a major concern in dentistry. These teeth present a higher risk of biomechanical failure than the vital teeth. Excessive removal of tooth structure during mechanical instrumentation of the root canal system, mechanical pressure during obturation, lack of cuspal protection and large restorations can weaken the tooth resulting in complete or incomplete fractures. Clinically, the anterior teeth are placed at an angle to the occlusal plane; the forces are therefore not directed along their long axes. This makes the teeth susceptible to fracture when an unfavorable directional load is applied. Hence, it is advised to give resistance to the endodontically treated anterior teeth by preserving as much coronal dentin as possible and the flexural behaviour of posts should be carefully considered.
A ferrule with 1 mm of vertical height was shown to double the resistance to fracture versus teeth restored without a ferrule. An increase in the success rate of the endodontically treated teeth has been noted when the length of the post is equal to or greater than the crown length. A higher failure rate is seen when the post length is too short. With recent developments of adhesive techniques and ceramic materials, the advantage of adhesive restorations is that a macroretentive design is no longer a prerequisite if there are sufficient tooth surfaces for bonding. Endocrowns strictly follow this rationale owing to a decay-oriented design concept. The monoblock foundation of this technique utilizes the available surface in the pulp chamber to obtain stability and retention of the restoration through adhesive bonding.
Chairside CAD/CAM restorations offer an alternative solution to old ones. a recently developed nano-ceramic restorative material is a unique CAD/CAM block based on the integration of nanotechnology and ceramics.
The aim of this in vitro study is to evaluate the effect of extension of endocrown in the root canal and the ferrule effect using CAD/CAM nanoceramic composite blocks on the fracture resistance of anterior endocrowns.
This study was carried on 28 extracted human anterior teeth. The samples were randomly divided into two groups according to endocrown extension inside the canal (long or short), and two subgroups according to presence or absence of ferrule (n = 28). Teeth were decoronated, cleaned and shaped using crown-down technique and obturated using lateral condenstation technique.
Computer numerical control machine (CNC) was used to prepare the ferrule in the two groups containing ferrule effect, and also was used to prepare the long extension of the endocrowns in the two groups containing long endocrowns extensions. The short endocrown extension was prepared using stone tapered, 80-um abrasive grit size diamond rotary cutting instrument. Copious water coolant was used in all groups.
Scanning and milling of the Lava Ultimate blocks was done by cerec machine software version 4.3..1. Finishing and polishing of the restoration were made with low speed fine grained diamond and a softlex disc.
Dual-curing universal composite-based luting system with ceramic bond and self etch bonding agent was in the cementation process. Fracture resistance was tested using a universal testing machine and maximum loads were obtained. Failure modes were analyzed using a digital microscope for all samples.
Results showed significant effect on the fracture resistance by means of ferrule effect and the interaction between the ferrule effect and the endocrown, however the extension of the endocrown into the canal was not significant.
The analysis of the failure modes showed mostly repairable fractures in all groups however non-repairable was present in only two samples, one in group 2A (long extension endocrowns with ferrule effect) and the other one in group 2B (long extension endocrowns without ferrule).