الفهرس 
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Abstract
This in vitro study was carried out to evaluate the effect of different light curing distances (0, 2, 4, and 6 mm) on the degree of conversion and surface hardness of top and bottom surfaces of two different bulk-fill flowable resin-based composite materials (FiltekTM and X-tra base®).
A total of 80 resin composite specimens were prepared in a specially designed split Teflon mold of 4 mm diameter and 4 mm thickness. They were divided into two main groups (n=40) according to the type of bulk-fill flowable resin-based composite used. Each group was further divided into four subgroups (n=10) according to light curing distance (0, 2, 4 and 6 mm). Each subgroup was further subdivided into two classes according to the surface assessed for the degree of conversion and surface hardness (top and bottom surfaces of the cured specimen). The 10 samples in each subgroup of each type of bulk-fill flowable resin-based composite was divided into two halves (n=5), half for assessment of degree of conversion and half for surface hardness. The samples were then stored at room temperature for 24 hours. Degree of conversion (DC) assessment of the tested samples was performed utilizing Fourier Transform Infrared Spectrometer (FTIR) apparatus. Microhardness evaluation of samples was performed utilizing Vickers hardness testing. Five hardness measurements were taken across the top and bottom surfaces using a 100 g load for 10 s and a mean Vickers hardness (VH) was obtained. Statistical analysis was carried out using IBM® SPSS® Statistics Version 20 for Windows.
Results of degree of conversion showed that the variable “restorative material” had no statistically significant effect on degree of conversion (P=0.121). While, the variables “curing distance” and “surface” had a significant effect on degree of conversion (P<0.001) and (P=0.002) respectively. Regarding the curing distance, 0 mm showed the highest mean DC% value in relation to (2, 4, and 6 mm). Regarding the assessed surface, the top surface showed the highest mean DC (%) value in relation to the bottom surface.
Results of microhardness showed that the variables “restorative material” and “curing distance had a statistically significant effect on microhardness (P<0.001) and (P=0.002) respectively. Regarding the restorative material, the highest mean value was found in X-tra base. Regarding the curing distance, the highest mean value was found in 0 mm. While, the variable “surface” had no statistically significant effect on microhardness (P=0.211).
Conclusions:
Within the limitations of this study, the following conclusions could be drawn:
Degree of conversion of the studied bulk fill flowable resin composites was found to be inversely proportional to increasing the light curing distance.
Both degree of conversion and depth of cure in terms of microhardness of bulk fill flowable resin composites were affected by material composition.
Microhardness at bottom surface of both tested bulk fill flowable composites was comparable to that of top surface; hence, 4 mm depth of cure that was claimed by their manufacturers, could be achieved.
Recommendations:
1. The distance between the tip of the light source and the restoration surface should be as close as possible for adequate polymerization.
2. Further investigations both in vitro and in vivo, studying the effect of increasing light intensity and prolonging curing time on the tooth pulp.
3. Furthermore, dentists should periodically evaluate the condition of their light-curing units to ensure that they produce maximal light intensity, which achieves optimal polymerization.