الفهرس 
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Abstract
Microleakage:Nanofilled composite resins exhibited lower degree of microleakage than hybrid composite resins but the difference was not significant.Wear resistance : Nanofilled composite resins exhibited higher wear resistance values than hybrid composite resins and the difference was highly significance.Fracture toughness: Nanofilled composite resins exhibited similar values to hybrid composite resins and the difference was not significance.Flexural strength: Nanofilled composite resins exhibited similar values to hybrid composite resins and the difference was not significance.Depending on the results and within the limitations of this study, the following can be concluded:1- There was no significant difference among the materials tested regarding the icroleakage. 2- There was no significant difference among the materials tested regarding the fracture toughness.3- There was no significant differences among the materials tested regarding the flexural strength.4- There was a highly significant difference among the materials tested regarding the wear resistance. Nanofilled composite resin materials exhibited high wear resistance than hybrid composite resin materials. Through the results of the present investigation and as a result of similarity of values of microleakage, fracture toughness and flexural strength between nanofilled and hybrid composite resins and as a result of superiority of wear resistance of nanofilled than hybrid composite it is recommended to use nanofilled composite resins instead of conventional hybrid composite restorative material in restoring posterior teeth subjecting to higher masticatory forces because of the higher resistance of the former to wearing action, and good mechanical properties. More studies are needed to confirm the laboratory findings.