الفهرس 
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Abstract
This study aimed to evaluate degree of conversion, Knoop micro hardness, three point flexure strength ,Young’s modulus of elasticity, depth of cure, Finite element analysis of MOD composite restorations .A total of ninety composite specimens were prepared and divided into main three groups I,II and III . according to the restorative materials used. The specimens of each group of the above mentioned were divided into three main subgroups (n=10) cured with these LED, conventional and soft start QTH curing units .For clinical study ninety carious posterior teeth from thirty patients were divided as was done in laboratory part. Each restoration was clinically evaluated immediately following finishing and polishing (BASELINE) after 6 months and one year, by two independent examiners using (USPHS) modified criteria . Based on the findings of this study, the following conclusions can be made:-The knoop hardness value of composite specimen decreased significantly as the overlay thickness of the specimen increased. There was a positive correlation between micro hardness, depth of cure and degree of conversion of all composite resins used. Both the light source and the resin composite played an important role in ensuring adequate polymerization, and hence the ultimate physical and mechanical properties and longevity of these composite restorations.