الفهرس 
Clinical acceptability of marginal gapOnly 14 pages are availabe for public view
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Abstract
This in vitro study evaluated the vertical marginal gap of CAD/CAM Zirconia based crowns after fabrication stages; the frameworks were designed and machined using Cercon Smart Ceramic System (DeguDent/ Hanau/ Germany), veneered by Cercon Ceram Kiss veneering porcelain, cemented with conventional GIC under constant loading to their corresponding teeth and finally, artificially aged by means of thermocycling and mechanical loading. The assessment of the marginal fit at each stage was performed on extracted human lower molars using the direct view technique with the aid of optical microscope with image software analysis system. Twenty extracted molars were selected. The roots of selected teeth were covered with modeling wax and embedded vertically in auto-polymerizing resin ,so that the resin is extending 2mm of the cement-enamel junction. Prior to preparation, two silicone indices were made for each specimen to aid in grinding the tooth according to preparation guidelines and thus, standardization of frameworks’ and veneering layers’ thickness.Following a standardized preparation protocol, the twenty molars were prepared using diamond tips mounted on under water cooling. The occlusal surface was reduced to a standard thickness of 1.5 mm and an axial reduction of 1.2 mm was made. A circumferential shoulder with rounded angles preparation was completed with a specially-modified surveyor in order to standardize the degree of tapering at approximately 6 degrees for all preparations.