الفهرس 
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Abstract
Objective: to evaluate the effect of thermal aging on mechanical properties of different types of nickel titanium orthodontic wires. Materials and Methods: A total of 120 NiTi wire specimens were used in this study and equally devided into 3 groups according to type of wire; group I : Supper elastic NiTi wire, group II : Thermo-activated NiTi wire, and group III: Copper NiTi super elastic at 27oC wire (27oC CuNiTi). Each group was subdivided into 4 equal subgroups 1, 2, 3, and 4.In subgroup 1 specimens were not subjected to thermal cycles (control group). In subgroups 2, 3, and 4 specimens were subjected to 833, 1666, and 2500 thermal cycles respectively. Three point bending test was used to evaluate load deflection and modulus of elasticity for all studied wires. The data were analyzed using analysis of variance (ANOVA test) and Post-hoc Tukey test. Results: There was a significant increase in load deflection of all studied NiTi wires at 1.5mm deflection, and thermo-activated and 27oC CuNiTi at 0.5mm deflection (p<0.05). This increase was significantly pronounced with increase the number of thermal cycles (p<0.05). Also, thermal cycling significantly increased the modulus of elasticity of thermal activated NiTi wires (p<0.05).Conclusion: Thermal cycling had a more pronounced effect on the mechanical properties of thermal activated NiTi wire than the other studied wire types. Increasing the number of thermal cycles negatively affect the load deflection force of all studied NiTi wires and the modulus of elasticity of thermo-activated NiTi wires.