الفهرس 
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Abstract
Summary
Background:
Endodontically treated teeth (ETT), are known to have increased risk of biomechanical failure {1}, owing to the decreased dentin elasticity, water content and loss of structural integrity, which in terms makes them liable to fracture and need a special care during restoring them , therefore it’s restoration became clinically challenging {2}. Multiple restoration designs were proposed including partial and complete coverage restorations with or without post and core build-up material with the aim to attain increased fracture resistance of the ETT such as Post and core and crown, Endocrowns and the overlay system.The increased emphasis on minimally invasive concepts as well as the advancement in adhesive dentistry {5} made endocrownsconsidered to be the gold standards for the restoration of ETT nowdays. Pissis (17) was the forerunner of endocrown technique and described it as the ‘mono-block technique ‘. The overlay is considered a more conservative modification of the endocrown without pulpal chamber extention (24). Despite all these new designs fracture of ETT remained a common clinical occurrence {14}.
Increasing the fracture resistance of the tooth with a more favorable pattern to is one of the main concerns in suggesting new treatment modalities. Several in vitro studies reported increased fracture resistance and much more favorable failure patterns due to better stress distribution for teeth restored with FRC posts, with different restorative systems (35). Overlays are usually fabricated from monolithic adhesive materials such as etchable ceramic containing glass (35) and In this study, the materials of choice were Lithium disilicate (E-max) and Hybrid ceramic (Vita enamic) enabling the biomimetic concept (30).
Objective: The aim of this research was to study the fracture resistance of overlay system for endodontically treated teeth with composite resin core and fiber post and composite resin core using two ceramic materials lithium disilicate and polymer infiltrated ceramic network PICN (Vita Enamic hybrid ceramic).
Material and Methods: 28 extracted molars will be selected with comparable dimensions, access cavity will be made, the molars will be endodontically treated and they will receive 1.5 mm occlusal reduction and will be divided into two groups (N=14). The first group will be filled with composite resin core (C group) ((N=14) and the second group will receive composite resin core with fiber post (F group) (N=14). Each group will further be subdivided into two subgroups (N=7) according to the material of fabrication of the overlay. Subgroup L (lithium disilicate) (N=7) and subgroup P (PICN) (N=7).
Results: Two-way ANOVA showed that fiber post placement did not significantly affect the fracture resistance (p=0.751) even when two different materials were used (p=0.361). Moreover, The mode of failure of the majority of samples made from both materials was the fracture of the restoration only without tooth fracture (Mode of failure A) (p=0.453) with and without fiber post (p=0.169) and the difference was not statistically significant.