الفهرس 
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Abstract
Objectives: To evaluate the effect of different formulations of base materials: fiber reinforced bulk fill composite, two bulk fill flowable composites and resin modified glass ionomer, layered by nano-hybrid composite and different storage times: 24 hours and 6 months, on the flexural strength of overlying resin composite.
Materials and Methods: The materials used in this study were: nanohybride composite, fiber reinforced bulk fill composite (EverX Posterior), two bulk fill flowable dentine substitutes (Smart Dentine Replacement-SDR and X-tra base) and Resin Modified Glass Ionomer. A total number of 100 specimens were prepared. The specimens were made in addition silicone, polyvinyl siloxane moulds to obtain a dimension of 2mm width x 15mm length x 6mm thickness. Each specimen was made of 4mm dentine substitute, layered by a 2mm nano-hybrid resin composite. The specimens were divided into 5 groups according to the type of dentine substitute used. group 1: Bulk fill flowable resin composite (SDR) layered by nano-hybrid composite. group 2: Bulk fill flowable resin composite (X-tra base) layered by nano-hybrid composite. group 3: Fiber reinforced bulk fill dentine substitute (EverX posterior) layered by nano-hybrid composite. group 4: Resin modified glass ionomer (Fuji II LC) layered by nano-hybrid composite. group 5: Incremental layering of nano-hybrid composite. Those groups were assessed for their flexural strength by 3-point bending test at 24 hours and 6 months. Data was statistically analysed using ANOVA, followed by Tukey’s Post Hoc test. A student t-test for each material was done to compare between the immediate (24 hours) and aged (6 months) groups.
Results: Flexural strength testing after 24 hours, indicated that bulk fill base materials perform better than RMGI and resin composite incrementation. Storage of specimens in water for 6 months, had a statistically significant effect on all tested materials. RMGI showed a statistically significant increase in flexural strength after 6 months, while all other materials tested showed a statistically significant decrease in flexural strength.