الفهرس 
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Abstract
Metal-ceramic restorations are still used extensively due to their
superior physical properties. Titanium alloys are an attractive alternative to
precious metals to be used as a metal framework. The CAD/CAM
technique was used for fabrication of titanium crowns, allowing adequate
mechanical properties.
According to the amount of possible clearance with the opposing
dentition there three different designs of metal framework.
The aim of this study was to evaluate the effect of different metal
framework designs on the fracture resistance of porcelain fused to machined
titanium crowns versus the widely used nickel-chromium crowns. It also
evaluated the mode of failure.
A metal die was fabricated to simulate a mandibular first molar
prepared for porcelain fused to metal restoration, from which 30 epoxy resin
dies were reproduced. The thirty epoxy dies were randomly allocated into two
equal groups (I and II). group I received 15 porcelain fused to milled titanium
crowns and group II received 15 porcelain fused to nickel-chromium crowns.
Each group was divided into three subgroups according to metal
framework design.
Design I: Metal coping with no collar.
Design II: Metal coping with 3mm lingual collar.
Design III: Metal coping with metal collar extending to cover the occlusal
surface except the buccal cusps to the central groove.
Crowns were fabricated using standardized method according to
manufacturer’s instructions. All crowns were adhesively luted with composite
resin-bonded cement (Multilink cement system).
Subsequently, the specimens were subjected to thermocycling and
cyclic loading. Then, they were loaded until fracture using universal testing
machine to indicate the fracture resistance for each group and each design,
then, statistical analysis was carried out.
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The results showed significant difference in the fracture resistance
between CAD/CAM titanium and nickel-chromium crowns, where nickel –
chromium crowns had a higher fracture resistance than CAD/ CAM titanium
group. The result also showed that design III demonstrated significantly
higher fracture resistance than design I, II which showed no statistically
significant difference, also 50% of specimens of design III represented Mode
I fracture and 50% represented Mode IV fracture.