الفهرس 
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Abstract
Purpose: Study the effect of using two impression materials of different stiffness with three impression trays of different flexibility on the casting accuracy of cast metal copings. Materials and Methods: The mandibular right first molar of a typodont model mounted to a semi-adjustable articulator was prepared for a full coverage cast metal coping. Thirty impressions were made with metal dual-arch impression trays, plastic dual-arch impression trays and a custom made acrylic resin impression tray using polyvinyl siloxane and polyether impression materials. Each impression was cast in type IV improved dental stone and the occlusogingival, mesiodistal and buccolingual dimensions of the obtained dies were measured using a traveling microscope and the prepared tooth served as a control. Wax patterns were fabricated on each stone die then cast into metal copings. Each metal coping was fitted on the prepared tooth and examined for vertical marginal gap accuracy using the travelling microscope. One way ANOVA followed by pair-wise tukey’s post-hoc tests were performed to detect the significance between groups, then separate student t-tests to detect the significance between subgroups. Results: The removable stone dies and cast metal copings obtained from the plastic dual-arch impression trays showed the greatest distortion in the occlusogingival, mesiodistal, buccolingual and vertical marginal gap measurements and the metal-dual arch impression trays recorded the least changes in all measurements. Conclusion: The increased rigidity of the impression trays as well as the increased stiffness of the impression materials produce more accurate dies and cast metal copings