الفهرس 
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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 properties and 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.