الفهرس 
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Abstract
This study was performed to evaluate the treatment effects and compliance of modified twin block appliance versus conventional twin block appliance in the treatment of growing females with skeletal Class II malocclusion having mandibular deficiency, And to reveal the physical changes of polymers used in this study after irradiation by gamma rays. The total sample consisted of 30 females divided into 2 groups, group 1 was treated by modified twin block appliance (MTB) and group 2 was treated by conventional twin block appliance (CTB). Results revealed that both treatment interventions showed a positive effect on mandibular growth, lower incisors showed no further proclination during treatment using MTB group and this group showed a better compliance for appliance wearing. Concerning the gamma radiation effect on the polymers used, Poly methylmethacrylate showed yellowish color change and increased number of pores on its surface after irradiation while polyethylene vacuum sheets didn’t show any color change but the scratches and pores appeared on its surface using scanning electron microscope.