الفهرس 
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Abstract
Statement of the problem : Adequate restoration of endodontically treated teeth (ETT) begins with an appropriate understanding of their physical and biomechanical properties. There is a positive correlation between the residual amount of tooth structure and fracture resistance. Changing concept and design of all-ceramic restorations were suggested for nonvital teeth to improve longevity of ceramic restorations. Aim of the work: to study the influence of different prosthetic materials and techniques for restoring damaged endodontically treated premolars. Materials and methods: Forty-eight permanent human mandibular premolars were treated endodontically in a conventional manner. Thirty-two premolars were decoronated 17 mm from the apical end of the root toward the crown to be used for post & core and endocrown restorations, while the other sixteen premolars were left for sharonlay group. All selected premolars were mounted in epoxy resin block up to 2mm below the CEJ. All premolars were divided into three main groups (n=16) according to type of preparation. group Ι: Premolars were prepared for fiber posts, composite cores (GP&C), and conventional all-ceramic crowns (control group). group ΙΙ: Premolars were prepared for endocrown restorations (GEC). group ΙΙΙ: Premolars were prepared for sharonlay restorations (GSH). Each main group was subdivided into two subgroups (n=8) according to type of prosthetic material: subgroup A : zirconia ceramic (Zenostar) and subgroup B: Silica-based glass ceramic (Celtra Duo). Post space was prepared using pesso reamer to the length of 12 mm from the preparation margins. The post was luted with self-adhesive resin cement (Nova Resin) followed by composite core build up. The specimens were prepared to receive all ceramic crown with 1mm chamfer finish line. Endo crown Group: the pulp chamber of each premolar was prepared and adjusted to be 5 mm from cavosurface margin. ΙΙΙ) Sharonlay Group: mesio-occluso-distal (MOD) onlay cavity preparation was performed, and post space was prepared with 7mm radicular extension. Impression taking, scanning, and designing of restorations were performed. Each restoration was luted to their corresponding specimen. All specimens were stored in distilled water bath then followed by mechanical loading 240,000 mechanical cycles. All specimens were loaded in a universal testing machine until failure occurred. This study was performed on thirty patients. selected patients were divided into three main groups (n=10) according to the remaining tooth structure after endodontic treatment. group Ι: Patients received fiber post and composite core with conventional all-ceramic crown (control group) (GP&C). group ΙΙ: Patients received endocrown restoration (GEC). group ΙΙΙ: Patients received sharonlay restoration (GSH). Each group was subdivided into two subgroups (n=5) according to type of used prosthetic materials: subgroup A: Zirconia ceramic (Zenostar) subgroup B: Silica-based glass ceramic (Celtra Duo). Preparation and obturation of premolars were performed as previously mentioned in vitro part of the study. After cementation, patients were evaluated clinically and radiographically immediately, at 6 weeks, 3 and 12 months. Results: Regarding to material type, the highest mean value of maximum load (Fracture strength) was found in Zenostar (2391.6± 641.3). However, the highest value for Celtra Duo was (1063.9± 403.2). Regarding to the design, endocrown restorations recorded the highest mean fracture load value (2391 ± 641.3) followed by sharonlay (1960 ± 498.7) and the least value was recorded by conventional crown supported by post and core (1418.5 ± 506.7). All restorations’ designs were acceptable clinically regarding marginal adaptation, anatomic form, surface texture and color match. All used materials (Zenostar & Celtra Duo) and designs (sharonlay, endocrown, crown) showed no difference for 12 months follow up. Conclusion: Within the limitation of this study, it could be concluded that, endocrown had highest mean fracture resistance followed by sharonlay and full crown had the least mean fracture resistance. Similarly, Zenostar represents greater fracture resistance than Celtra Duo.