الفهرس 
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Abstract
The functions of dental adhesives is to promote conservation of tooth structure and enable minimally invasive dentistry, reinforcement of weakened dentin or enamel, reduce marginal staining, reduce microleakag and may also reduce postoperative sensitivity when used appropriately. Manufacturers are constantly introducing new adhesive systems with claims of simplicity in use, improvement in their composition and ability to bond to tooth structure. Bond strength measurement is one of the most common methods for evaluating the adhesive properties of restorative materials. Various mechanical methods, such as tensile, micro tensile, flexural, shear, and inplane shear tests have been used to assess bond to dental substrate. the most common and reliable tests for evaluation of the bond strength are micro-tensile and micro-shear tests. The micro shear test is normally considered easy to execute, where a knife-edge-chisel or an orthodontic wire loop are closely positioned at the adhesive interface, generating a shear (parallel) stress at the edge of the bonding area . In contrast, the micro-tensile test generates perpendicular stress to the adhesive interface, which is said to evaluate the bond strength between the restorative material and the tooth substrates in a more reliable way, as the stress is more uniformly distributed. Three adhesive systems were used in this study which were Futurabond U adhesive, Tetric NBond Universal and Swiss TEC SL Bond. Sixty freshly extracted caries free sound human third molar teeth were used and prepared by removal of occlusal enamel and exposure of the coronal dentin. Then they were divided into three groups of twenty teeth according to the tested adhesive system used. Each group was subdivided into two subgroups of ten specimens for each subgroup according to the test to be applied (micro shear or micro tensile): group I: Using Futurabond U adhesive (FBU). group II: Using Single Bond Universal adhesive (SBU). group III: Using Swiss TEC SL Bond. Subgroup A: Micro tensile strength tested specimens. Subgroup B: Micro shear strength tested specimens. And the application of each adhesive system was according according to the manufacturer’s instructions, then the composite resin was applied to the specimens and prepared to provide rectangular beams 1 x 1 mm with length of 6mm ( 3 mm for composite and 3 mm for tooth structure) for μTBS testing and a cylindrical beam of a 2 mm2 diameter of composite over the occlusal dentin for μSBS testing. The specimens were subjected to thermo cycling between 5oc and 55oc for 5000 cycles. The dwell time and transfer time were 30 seconds and 10 seconds, respectively which correspond to 6 months of clinical evaluation. The specimens then were mounted in an Instron universal testing machine using a specially prepared attachment and stressed until failure then the results were statistically recorded by One-way ANOVA test between main groups and subgroups. Also mode of failure were inspected under binocular stereomicroscope at 40x magnification to determine the mode of failure of each specimen and Chi square was carried out between failure modes. It was found that the μ-tensile bond strength mean ±SD values recorded for Gr_I adhesive (futurabond u) were (25.31±5.2 MPa) with minimum value (14.58 MPa) and maximum value (36.48 MPa). The μ- tensile bond strength mean ±SD values recorded for Gr_II adhesive (Tetric n universal bond) were (26.7±7.52 MPa) with minimum value (13.08 MPa) and maximum value (44.68 MPa). Also, the μ-tensile bond strength mean ±SD values recorded for Gr_III adhesive (Swiss TEC SL Bond) were (38.01±8 MPa) with minimum value (15.54 MPa) and maximum value (50.82 MPa). Also It was found that the μ-shear bond strength mean ±SD values recorded for Gr_I adhesive were (7.09±1.87 MPa) with minimum value (4.58 MPa) and maximum value (9.26 MPa). Meanwhile the μ-shear bond strength mean ±SD values recorded for Gr_ II adhesive were (11.59±1.82 MPa) with minimum value (10.15 MPa) and maximum value (14.7 MPa). Also, the μ-shear bond strength mean ±SD values recorded for Gr_ III adhesive were (12.07±2.08 MPa) with minimum value (9.65 MPa) and maximum value (15.14 MPa). And according to the mode of failure, group I showed 30.77 % adhesive failure and 69.23% mixed mode of failure for micro tensile bond strength test. And 35% adhesive mode of failure and 65% mixed mode of failure for micro shear bond strength test. group II presented 24.44% adhesive failure, 11.11% cohesive mode failure and 64.44% mixed mode of failure for micro tensile bond strength. Summary and Conclusi on 104 And 33% adhesive mode of failure and 67% mixed mode of failure for micro shear bond strength test. Finally group III showed 15.39% adhesive failure, 15.38% cohesive mode failure and 69.23% mixed mode of failure for micro tensile bond strength. And 25% adhesive mode of failure and 75% mixed mode of failure for micro shear bond strength test.