الفهرس 
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Abstract
7- Summary
The clinical performance of zirconia-based restorations has been
reported to be compromised by chipping and/or delamination of the
porcelain veneer. This study evaluated four different veneering
techniques of zirconia cores and a monolithic extra translucent zirconia
ceramic in terms of fracture resistance and color reproduction. Shear
bond strength of different veneering materials to zirconia was also
evaluated.
25 transparent epoxy resin dies were made by silicone duplication of a
standardized preparation of a premolar tooth and divided into five
groups (n=5). Four resin die groups were assigned to receive veneered
zirconia crowns while the fifth group was assigned to receive extra
translucent monolithic zirconia crowns. CEREC omnicam was used
for scanning the epoxy resin die and the CEREC software was used to
design a multilayer restoration. Zirconia cores were milled from the
core part of the multilayer design, sintered, then the cores were
veneered as follows; group 1: Fused e.max CAD/CAM veneers,
group 2: Bonded e.max CAD/CAM veneers, group 3: Conventional
ceramic layering, group 4: Composite layering. A finished veneered
zirconia crown from (group 1) was scanned and biocopy design mode
of the CEREC software was used to fabricate the extra translucent
monolithic zirconia crowns (group 5). Thermocycling was undertaken
for all crown samples. The color change (∆E) between the selected
shade and the produced crown shade was measured before and after
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thermocycling using intra-oral spectrophotometer. Fracture resistance
was measured for all crown groups after thermocycling using a
computerized universal testing machine. In order to evaluate the shear
bond strength, pre-sintered CAD/CAM zirconia blocks were cut into
rectangular shaped samples (19×15×2mm) using a low speed cutting
saw and sintered. The sintered samples were divided into four groups
and veneered according to the veneering techniques mentioned before.
Shear bond strength was measured after thermocycling using a
computerized universal testing machine.
Results revealed that zirconia cores veneered with conventional
ceramic layering showed the best color match followed by extra
translucent monolithic zirconia then composite veneered zirconia cores
with no statistically significant difference. Meanwhile, zirconia cores
veneered with fused or bonded CAD/CAM veneers showed the lowest
color match. In terms of fracture resistance, zirconia cores veneered
with fused CAD/CAM veneers showed the highest fracture resistance
which was closely followed by extra translucent monolithic zirconia
with no significant difference. On the contrary, the rest of the veneered
zirconia core groups (groups 2,3&4) showed lower fracture resistances
which were statistically significant than groups(1&5). Results of shear
bond strength test showed that the highest shear bond strength was
attained by zirconia samples veneered with fused lithium disilicate
discs followed by zirconia samples veneered with bonded lithium
disilicate discs. Zirconia samples veneered with conventional ceramic
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or composite discs showed lower shear bond strengths with
statistically significant difference between all groups.