الفهرس 
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Abstract
Summary
This study aims to evaluate the effect of different scanning technologies and scanning techniques on the retention and internal fit of all ceramic crowns constructed with different convergence angles.
Two stainless steel dies were machined to the dimensions of prepared upper first premolar to receive all ceramic crown with two different convergence angles 12ﹾ and 20ﹾ degrees.
The dies were duplicated into 42 epoxy resin dies, 21 dies with 12 degrees convergence and the other 21 with 20 degrees convergence.
Each 21 dies were divided into three groups seven dies each. First group was scanned using Omnicam intraoral scanner. Then polyether impressions were taken for the other groups. Second group impressions were scanned using Identica blue hybrid extraoral scanner. Third group impressions were poured into gypsum models and the models were scanned using the same extraoral scanner.
All the scanned data were exported in STL file formats to the design software.Crowns were designed using Exocad 2016 software with fixed parameters for all the groups. Wings were added on the proximal surfaces of the crowns for holding the loops to measure retention.
The designed crowns were milled in ultra-translucent zirconia Anterior Bruxzir using Vhf S1 5-axis milling machine then dried and sintered. The designed crowns were milled in enlarged size to compensate for sintering shrinkage.
The internal fit was measured using the replica technique. The intaglio of the crowns were filled with light body silicone impression material then seated with firm hand pressure on the corresponding abutments followed by pressure 20N for 10 minutes. The crowns were gently removed and heavy body silicone was used to remove the films and prevent their distortion during the cutting process.
The replicas were sectioned buccolingually and scanned using light microscope and their thickness was measured with the help of compatible software. Nine measuring points for each section with total of eighteen points for each crown were measured.
The crowns were then seated and cemented. All crowns in all groups were cemented with resin cement. During cementation static finger pressure was applied then the specimens were axially loaded with a 5kg load using a specially designed device.
Universal testing machine was used to measure retention. A special chain holding the crown wings to ensure even distribution of pulling tensile forces using a locking mechanism. The cemented crowns were pulled off along the path of insertion with a crosshead speed of 0.5 mm/minute. The forces required for dislodgment of the crowns were recorded in N.
Two way-ANOVA was used to study the effect of different degrees of convergence and impression techniques on mean retention (N) and Internal fit (µm) between groups.
Tukey’s post-hoc test was used for pair-wise comparison between the means when ANOVA test is significant. One Way-ANOVA was used to study the effect of different degrees of convergence, or impression techniques on mean retention values (N) and Internal fit (µm) within each subgroup separately.
Results showed better internal fit for crowns on 20 degrees convergence than those constructed on 12 degrees. Also intraoral scanner showed better significant internal fit than extraoral scanner. Best internal fit was found with crowns from intraoral scanning of the 20 degrees convergence dies.
Retention of crowns on 12 degrees were significantly better than those on 20 degrees.
The retention was not significantly affected by the scanning technique.