الفهرس 
problems to appear with need of dental solvingOnly 14 pages are availabe for public view
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Abstract
The tilted molar abutment presents a prosthetic problem in form of achieving a
common path of insertion for a fixed dental prosthesis. When tooth preparation alone
cannot solve the problem, the mechanical solution of non-rigid connector is available
and must be considered for its offering a potential for rotational and resilient movement
between the prosthesis and the abutment teeth. All ceramic fixed dental prostheses have
become more widely used in clinical practice as a result of their high aesthetic potential
and their excellent biocompatibility propertiesand recently improved strength, fracture
toughness. Esthetic and mechanical characteristics of zirconium make it such an
attractive metal to work with. As the most common problem associated with veneered
zirconia restorations is still veneer chipping, using high-performance monolithic
translucent zirconium oxide ceramics in CAD/CAM systems is expanded.
This study aimed to evaluate the fracture resistance of differently designed
CAD/CAM fabricated non rigid connectors for zirconia bridges in form of two
different sizes of the diameter and the attachment projection length under simulating
ageing conditions as thermocycling and fatigue loading.
To conduct this study, a total of eighty bridges with a precision attachment
between the second premolar retainer and the pontic were constructed. They were
divided according to the material into two main groups (40 bridges for each):
- group I (40 samples): Base metal bridges (control group).
- group II (40 samples): Monolithic zirconia bridges.
According to the precision attachment body diameter, both types of the bridges
were divided into two subgroups (20 for each):
- Subgroup (A): A large body precision attachment diameter (2 mm).
- Subgroup (B): A small body precision attachment diameter (1.5 mm).
Each subgroup was further divided into another two divisions according to the
precision attachment projection length (10 for each):
- Division (1): With a long precision attachment projection (3.5 mm).
- Division (2): With a short precision attachment projection (2 mm).
A lower master model of an ivorine lower second premolar of 0° angulation and
tilted lower second molar of 25° angulation representing missing lower first molar were
created. Teeth preparations were standardized to receive full coverage all- ceramic
restoration.
Eighty half-arch impressions were made for the prepared teeth in which epoxy
resin material was poured forming eighty standardized master replication dies on which
the bridges were cemented.
Fabrication of zirconia bridges took many phases (Acquisition, Model, Design and
Milling) using CEREC CAD/CAM system followed by sintering at 1580°C in a special
furnace for 9 hours.
Fabrication of metal bridges were started with fabrication of resinous wax bridges
that took the same phases of zirconia bridges using CEREC CAD/CAM system,
followed by investment, burning out then centrifugal casting of the alloy.
Before bridges cementation, the epoxy resin abutments, the fitting surfaces of the
retainers for zirconia and metal bridges were sandblasted. Each bridge was cemented
using a self-adhesive resin cement (RelyX Unicem).
The samples were tested as following:
1-Measurement of fracture resistance of half of the samples in each group before
ageing was done using a universal testing machine.
2-Artificial ageing program of the cemented bridges for other half of the samples in
each group was done in form of 15000 thermocycles and 240,000 fatigue loading
cycles.
3-Measurement of fracture resistance of aged samples using a universal testing
machine was done.
4-Post testing digital microscope was used for tested samples to detect the mode of
failure and position of fracture.
5- Under scanning electron microscope, fractured samples were imaged to analyze the
fractured segments.
The results of the present study showed that material, attachment dimensions and
ageing as well as the interaction between the three variables had a statistically
significant effect on the mean fracture resistance.
There were significant differences in the failure load before and after ageing for
zirconia samples, but not for metal samples. Zirconia bridges were significantly weaker
than metal bridges from all tested groups.
After ageing either using zirconia or metal, 2x2 mm attachment dimensions
showed the statistically significantly highest mean fracture resistance. Attachment
dimensions 2x3.5 mm showed statistically significantly lower mean value. There was
no statistically significant difference between 1.5x2 mm and 1.5x3.5 mm dimensions;
both showed the statistically significantly lowest mean fracture resistance values.