الفهرس 
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Abstract
To investigate and evaluate the impact of different application strategies of three different resin composites on cuspal deflection, marginal adaptation and microleakage on mesio-occluso-distal (MOD) cavities of premolar teeth. Materials and methods: The resin composite restorative systems used in this study were conventional nano-hybrid resin composite, high viscosity bulk fill resin composite, and medium viscosity dual cure bulk fill resin composite. One hundred and twenty (120) human premolar teeth were subjected to standardized preparation of (MOD) cavities, then divided into three groups to be used for cuspal deflection (n=30), marginal adaptation (n=60), and microleakage tests (n=30). For the cuspal deflection test, the prepared teeth were divided into three sub-groups each restored with one restorative system (n=10) with a single operator, and according to the manufacturer’s instructions. Digital micro-meter was used to evaluate the cuspal deflection. For marginal adaptation test, the prepared teeth were divided into two main sub-groups (n=30) for immediate and delayed evaluation of marginal adaptation, then each sub-group was divided into three divisions (n=10) according to the restorative system. Scanning electron microscope (SEM) was used for marginal adaptation evaluation at (200x). For microleakage test, the prepared teeth were divided into three sub-groups (n=10) each one restored with one restorative system. After storage, teeth were subjected to thermal cycling (5oC-55oC) then immersion in Leishman dye solution, microleakage was evaluated using a stereomicroscope (25x). The collected data were tabulated, coded then statistically analysed using IBM SPSS software (version 22.0, IBM, NY, USA). Results: For the cuspal deflection test, no statistically significant difference between tested material in immediate or delayed evaluation, except bulk fill resin composites showed significant difference (p<0.05) when comparing the effect of artificial aging on each material. For the marginal adaptation test, there was no significant difference between tested materials in the immediate or delayed evaluation, but when comparing the effect of artificial aging on each material, Tetric N Ceram showed significant difference. For microleakage test, there was no significant difference between conventional and bulk fill resin composites, but the two bulk fill resin composites showed significant difference. Conclusion: Based on the results of the current study, bulk fill and conventional resin composites had the same effect on cuspal deflection. Also, marginal adaptation and sealing ability of bulk fill resin composite were not better. Aging process resulted in relief of contraction stresses and reduction of adaptation between tooth structure and restoration.