الفهرس 
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Abstract
The aim of our study was to evaluate the effect of different pH values of citric acid solutions (2.5) and (5.5) on surface microharness and surface roughness of Nanofilled and Bulkfill resin composite formulations.
Dental resin composite restorative materials are becomingly the most common direct restorative materials due to the increased esthetic demands. Hence the oral cavity is one of the most challenging environments, thus, these resin composite materials are subjected to a lot of challenges that might deteriorate their physical and mechanical properties.
Specimens from each type of dental resin composite were prepared and divided into two main groups according to type of resin composite. Then each group was further subdivided into 3 subgroups according to the used pH values and time of immersion.
Vicker’s hardness tester was used to record the surface microhardness. Regarding surface roughness, specimens were photographed using USB Digital microscope with a built-in camera (Scope Capture Digital Microscope, Guangdong, China) connected with an IBM compatible personal computer using a fixed magnification of 120X.The images were recorded with a resolution of 1280 × 1024 pixels per image. WSXM software was used to calculate average of heights(Ra) expressed in μm, which can be assumed as a reliable indices of surface roughness.
Results of Microhardness test revealed that, different pH values of citric acid solutions had no statistically significant effect at (P-value =0.812). Also, different time periods had no statistically significant effect at (P-value =0.231). Different resin composite materials had no statistically significant effect at (P-value 0.963). The interaction between the three variables also had no statistically significant effect at (P-value =0.050).
Regarding surface roughness test, the results revealed that different pH values of citric acid solutions had no statistically significant effect at (P-value =0.926). Also, time had no statistically significant effect at (P-value =0.555). While different resin composite materials had a statistically significant effect at (P-value =0.025). The interaction between the three variables also had no statistically significant effect at (P-value =0.480).
Within the limitations of the current study, the following could be concluded:
1. Citric acid solutions of low pH values had a negative influence on surface microhardness of conventional Nanofilled resin composite materials, on contrary to bulkfilled resin composite, which exihibited no change in microhardness values and was not affected by low pH values of citric acid solutions.
2. Short immersion time durations seemed to be not sufficient to produce a significant effects on the Surface microhardness and surface roughness of different resin composite formulations.
3. Bulkfilled resin composite formulation was more stable and less affected by citric acid solutions regarding the surface roughness compared to conventional Nanofilled resin composite.
4. Citric acid solutions of low pH values had insignificant influence on surface roughness values of the two resin composite formulations and both had clinically accepted values.