الفهرس 
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Abstract
The aim of this study was to evaluate vertical marginal gap of CAD-CAM milled PEKK as a framework material to be veneered with CAD-On lithium disilicate glass ceramic as compared to milled zirconia framework veneered with CAD-On lithium disilicate glass ceramic.
Methodology: 14 freshly extracted molars were embedded in epoxy blocks. A uniform 1.5 mm axial reduction (100 taper) and 1,5 mm occlusal reduction with a 3600
deep chamfer finish line (1mm thickness) was prepared on all teeth. Scanning and designing of the restorations were done and the final design was split in the software into core and veneer layers to be milled separately. Fusion of the veneer to the core was the only manual step done in crown construction; it was done using low fusing glass material (IPS e.max Crystall./ Connect) in the control zirconia group and using
adhesive resin cement (multilink) in the PEKK group as recommended by the manufacturer. All the crowns were adhesively cemented on their corresponding preparations using Duo-link adhesive resin cement. All the specimens were subjected to thermo-mechanical fatigue loading for 150,000
cycle simulating one year’ clinical service .Vertical marginal gap was assessed using stereomicroscope of the two groups before and after thermomechanical aging for all the specimens.
Results: PEKK group recorded statistically significant (P<0.05) higher marginal gap mean value
(59.97±2.541 µm) than Zirconia group (41.79±4.424 µm). Regardeless to material type ,totally it was found that after mechanical aging the marginal gap mean value was higher (54.36±13.73 µm) than before (47.39±10.88 µm) and this was statistically non-significant (P=0.1741 > 0.05) as indicated by
two way ANOVA followed by pair-wise Tukey’s post-hoc tests.