الفهرس 
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Abstract
Assessment of the influence of bulk-fill resin composite restorative systems on fracture resistance and microleakage of maxillary premolars with class II mesio-occluso-distal (MOD) cavity preparations immediately and after 6-months storage compared to incremental- fill composite. For fracture resistance test, 70 sound maxillary human premolars were assigned into 5 groups (n =14). Teeth in the first group were left intact and tested as unprepared positive control.Teeth in the remaining four groups were prepared with MOD cavities. Teeth in one of the four groups were not restored and tested as negative control.The remaining three groups were restored with bulk-fill composites and a conventional composite. All groups were stored in artificial saliva at 37 °C and thermocycled .The specimens were loaded occlusally in a universal testing machine.Half of the specimens in each group was tested after 24 hours whoever the remaining half was tested after 6-months of storage. Regarding to microleakage test, 42 premolars selected and received MOD preparations.The prepared teeth assigned into three groups (n =14) and then restored with the three types of resin composites used for fracture test. All the specimens were thermocycled and restored in artificial saliva at 37 °C.Half of the specimens were tested for microleakage after 24 hours whoever the remaining half was tested after 6 months using dye penetration technique.A statistically significant difference in cuspal fracture resistance and microleakage was found between teeth restored with bulk-fill composites and those teeth restored with incremental-fill composite. So bulk-fill composite restorations had higher cuspal fracture resistance and marginal seal than those with incremental-fill composite restorations.