الفهرس 
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Abstract
Objectives: To evaluate the effect of resin matrix structure on 18-month clinical performance and in vitro cuspal deflection, and marginal adaptation of three different bulk-fill resin-based composites (BFRBCs) in Class II cavities. Materials and Methods:For clinical performance trial, forty patients, each with three Class II restorations under occlusion, were enrolled in this study. One hundred and twenty restorations were placed, 40 for each material: Filtek bulk fill Posterior Restorative/Single Bond Universal, Tetric EvoCeram/ Adhese Universal and X-tra Fil/ Futurabond U. A single operator placed all restorations according to the manufacturers’ instructions. Immediately after placement, the restorations were finished/polished. Clinical evaluation was performed at baseline and at 6 month intervals after placement by two other independent examiners using modified United State Public Health Service (USPHS) criteria. The changes in the USPHS parameters during the two- year period were analyzed with the Friedman test. Baseline scores were compared with those at the recall visits using the kruscal wails test. The level of significance was set at p< 0.05. For in vitro tests, a standardized Class II MOD cavities were prepared in 60 freshly-extracted intact human molars. The teeth were randomly assigned to two experimental groups according to test will be performed (n =30). Three BFRBC materials were used for restorations (Filtek bulk fill Posterior Restorative, Tetric EvoCeram and X-Tra Fil) according to manufacturer’s instructions. Half of the specimens were subjected to cusp deflection test using digital image correlation technique. while the other half, after finishing procedures, was stored in a 37 o incubator for 24 hours, and thermocycled for 1000 cycles (5o -55o). and subjected to marginal adaptation test using scanning electron microscope (SEM). Statistical analysis was performed using one-way analysis of variance (ANOVA) test. The level of significance was set at p< 0.05. Results: Regarding the clinical trial, all restorations showed minor modifications after 2 years. However, no statistically-significant differences were detected between the materials’ performance at baseline and after 2-year for all criteria (P > 0.05). For in vitro cusp deflection test, Tetric EvoCeram Bulk-Fill showed the lowest mean cuspal deflection (7.66 µm) while x-Tra Fil showed the highest mean (9.04 µm). ANOVA indicated insignificant difference among mean values of groups (P> 0.05). In time, no significant difference in marginal adaptation scores was recorded between groups (P> 0.05). No statistically significant difference in cuspal deflection between the groups (P< 0.05). No statistically significant difference in marginal adaptation was found between teeth restored with the three Bulk Fill RBCs. Conclusions: On the basis of the results of the current study, it seems reasonable to conclude that the difference in structure of resin matrices has insignificant effect on clinical performance, cuspal deflection, and marginal adaptation of Class II bulk-fill resin composite restorations.