الفهرس 
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Abstract
This study was performed to evaluate the effect of gamma radiation on the microshear bond strength and nanoleakage in Er,Cr:YSGG laser prepared cavities as well as microhardness of two different restorative materials (Nanocomposite and Nano-glass ionomer) and enamel in two different storage periods (24 hrs and 6 months). Micro-shear bond strength test: Enamel was removed from all specimens to expose flat dentin surfaces then the flattened dentin surfaces were uniformly and perpendicularly irradiated with Er,Cr:YSGG laser. The prepared teeth were divided into two main groups according to the restorative material used. Two vinyl Tygon tubes were placed on each dentin surface where the restorative materials were packed. Each group was subdivided into two subgroups according to whether or not subjected to gamma radiation. Each subgroup was further divided into two classes according to the aging periods and stored in artificial saliva. Micro-shear bond strength assessment was performed by Instron universal testing machine. Nanoleakage test: Class V cavities were prepared with the same laser in the buccal surfaces of the selected teeth then classified as in micro-shear bond strength test. The specimens were prepared to be immersed in a 50٪ ammonical silver nitrate solution for 24 hrs in dark chamber then a photo-developing solution for 8 hrs under a florescent light. Finally, the specimens were longitudinally sectioned in bucco-lingual direction with a diamond saw in a cutting machine. Scanning and EDAX quantification were performed for each specimen. One randomly selected specimen from each group was obtained from the other halves of the longitudinally sectioned teeth and prepared for SEM analysis. Representative photomicrographs were taken at magnifications of × 500, × 1000, and × 1500. Enamel and restorative materials’ microhardness test: For the enamel microhardness test, gamma irradiation procedure was performed for half of the specimens then each group was further subdivided as the other tests. The teeth were mounted in acrylic molds and the buccal enamel surfaces were flattened. The teeth were mesiodistally sectioned using diamond saw in a cutting machine. For the materials microhardness test, specimens were prepared from both materials in disc shaped Teflon molds then classified as mentioned before. Surface microhardness was measured with a Digital display Vickers microhardness tester. Micro-shear bond strength test results: -There were statistically significant differences between the control and gamma irradiated groups where the gamma irradiated groups showed lower values than the control groups in the nano-glass ionomer groups and vice versa in the nano-composite groups stored for 6 months while that stored for 24 hrs, showed no statistically significant difference. -Nano-composite restorative material showed higher bond strength values than nano-glass ionomer. Nanoleakage test results: -There was no statistically significant difference between the control and gamma irradiated groups except for the nano-composite control group stored for 6 months which showed higher nanoleakage mean value than the gamma irradiated group. -Nano-composite group stored for 24 hrs showed the lowest nanoleakage mean value while the nano-glass ionomer stored for 6 months showed the highest value. Enamel microhardness test results: -There was no statistically significant difference between the control and gamma irradiated groups at 24 hrs while at 6 months, the gamma irradiated group recorded higher microhardness value than the control group.
Restorative materials’ microhardness test results: -There was no statistically significant difference between the control and gamma irradiated groups. -Nanocomposite gamma irradiated group stored for 6 months recorded the highest mean value while the nano-glass ionomer control group stored for 24 hrs recorded the lowest value. Scanning electron microscopy representative photomicrographs: -Nanocomposite group stored for 24 hrs showed absence of gaps at resin dentin interface and irregular interrupted thin hybrid layer in both control and gamma irradiated groups with no distinct morphological differences between them. -Nano glass ionomer control group of the same aging period showed gap formation at the dentin restoration interface while gamma irradiated group showed funnel shaped resin tags of variable lengths and irregular boundaries. -Both restorative materials stored for 6 months showed crystallization formation along the dentin restoration interface in the control group rather than the gamma irradiated group. Conclusions: Under the conditions of the present study, the followings are evident: 1- Therapeutic dose of gamma radiation had minimal effect on the microshear bond strength, nanoleakage and microhardness of nanocomposite. 2- The microshear bond strength and microhardness of nanoglass ionomer were adversely affected by gamma radiation while nanoleakage is not affected. 3- Gamma radiation had positive effect on enamel microhardness. 4 - Distinct ultra-morphological finding was evident in the aged control group in both restoratives. 5- Storage had an enhancement effect on microshear bond strength of nanoglass ionomer control group and microhardness of both nano glass ionomer control and gamma irradiated groups. 6- Storage had a deteriorating effect on nanoleakage of all tested groups, microshear of nanocomposite control group and both enamel microhardness groups. Recommendations: 1- Further in-vivo studies are needed to evaluate the effect of radiotherapy on the properties of dental structure and restorative materials over longer aging periods. 2- Nanocomposite is more suitable as a restorative material for cancer patients who receive radiotherapy than nanoglass ionomer. 3- A separate acid etching step is recommended to improve the resin dentin bonding in cavities irradiated with Er,Cr:YSGG laser. 4- Further investigations are required to monitor the durability of nanoglass ionomer restoration in different aging periods