الفهرس 
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Abstract
This study was conducted to evaluate the effect ofthe thickness of the material being cured and distance from light cure on the relative microhardness of three bulk filling resin composite materials.A total of 90 composite resin samples were prepared for this study. They were divided into three main groups (n=30) according to the bulk fill composite used. Each was further classified into 2 subgroups (n=15) according to the thickness of sample. Each subgroup was further classified into 3 classes (n=5) according to the distance between the tip of the light cure to the under surface of composite samples.Microhardness evaluation of samples from each group used for the experiment was performed utilizing Vickers hardness testing. Five hardness measurements were taken across the top and bottom surfaces using a 500 g load for 15 s.
The results of this study showed that all the three variables tested had influence on the depth of cure of composite resin. Both X-tra base bulk-fill and Filtek bulk fill showed higher bottom to top microhardness ratio compared to TetricEvoCeram Bulk Fill. The two millimeter thickness for all materials showed higher mean bottom to top microhardness ratio than that of the four millimeter. None of the three materials tested exceeded the 0.8 ratio which is considered by authors as the minimum accepted bottom to top microhardness ratio when cured in four millimeter increments. Regarding the three distances evaluated, both the zero and five millimeter showed higher depth of cure than that of the ten millimeter.
Conclusions:
Within the limitations of this study, the following conclusions can be drawn:
1-The depth of cure in terms of relative microhardness of bulk fill composite resin is affected by composite resin material, increment thickness and the distance between light cure tip and composite resin.
2- Two millimeter thick increments achieve better depth of cure than that of the four millimeter for bulk fill composite resins.
3-Increasing the distance between the tip of light cure and composite resin causes a decrease in the depth of cure of bulk fill composite resins.
Recommendations:
Independent investigations studying the effect ofCuring time of bulk fill composite resin on the depth of cure would be beneficial in better understanding their properties and to enhance their clinical results.