الفهرس 
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Abstract
This in vitro study evaluated the fracture resistance of four zirconia framework designs; (a) group I, full contour zirconia crown, (b) group II, partial contour zirconia coping with buccal veneering, (c) group III, zirconia coping with palatal and proximal collar and (d) group IV, 0.5 mm zirconia coping without collar (control group).
Twenty standardized epoxy resin dies, simulating all ceramic preparation of maxillary first molar tooth, were divided into 4 groups, 5 specimens each. Standardized resin patterns were fabricated for the three test groups, representing the desired shapes and thicknesses of the zirconia designs. Dual scanning technique was done for the test groups to fabricate zirconia copings, while normal scanning was made for the control group. Zirconia specimens were milled from a pre sintered zirconia blank to the desired shape, and sintered afterwards. Veneering was done for group II, III and IV. Fired specimens were then adjusted by diamond stones and then stained and glazed. All crowns of all groups were checked to have 2mm occlusal thickness.
All crowns were adhesively luted to their corresponding dies, and subjected to thermal cycling and mechanical cyclic loading corresponding to one year clinical service. Single load to fracture was applied to all the specimens. Veneer chipping, cracking or sudden decrease in load was considered as failure. Type of failure was recorded for each specimen, investigated visually then by using Stereomicroscope and SEM.
Complete contour zirconia crowns showed the highest statistically significant fracture load, among other groups. Partial contour zirconia crown design showed higher statistically significant fracture load than both the collared group and the control group. While there was no statistical difference between the collared group and the control group.
SUMMARY
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In conclusion, complete and partial contour zirconia restorations showed high resistance to fracture, and could offer an alternative to porcelain fused to metal restorations. The presence of collar was insignificant for increasing fracture resistance.