الفهرس 
History of ceramics:Only 14 pages are availabe for public view
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Abstract
This in vitro study was carried out to compare fracture resistance of
different ceramic restorations. 64 ceramic crowns were fabricated. They were divided into 4 equal groups according to the type of ceramic used (16 each). group I: lithium disilicate (e.max CAD). group II: Zirconia reinforced lithium silicate (celtra duo). group III: monolithic zirconia (prettau zirconia). group IV: bilayered zirconia (zirconia substructure veneered with feldspathic
porcelain). Models of prepared teeth #14 (upper right first premolar) were used as abutments. The tooth model has standard preparation criteria for all ceramic crown. Abutments were mounted in an acrylic blocks. Each tooth was scanned by digital scanner.
Designing the crowns was done with standard measurements: 1.5 mm at
the buccal cusp and 2 mm at the palatal cusp by using Exo Cad 2016 software. For group IV (bilayered zirconia), the design was set to produce a 0.5 mmthick
zirconia coping. Ceramic blocks were introduced to the milling machine.
Wet milling was done for the lithium disilicate and zirconia reinforced lithium silicate blocks, while dry milling was done for zirconia blank. The restorations were Cleaned and dried. The final restorations were tried to examine their fit.
Lithium disilicate crowns were crystallized in an Ivoclar Vivadent
ceramic furnace. Zirconia restorations were sintered in the sintering oven.
Then all monolithic restorations were glazed and stained to reach the desiredshade. Then glaze fire was done. For group IV (bilayered zirconia), to standardize the veneering thickness and contour, a silicone index of a previously milled full anatomic monolithic lithium disilicate crown was made. This index was used to guide the veneer
build up to get standard thickness and external dimensions. The feldspathic
porcelain was mixed and applied to the zirconia cores with layering technique
then it was fired according to manufacturer’s directions. Zirconia fitting surface was airborne-particle abraded with Al2O3 (50μ).
While for e.max CAD and Celtra Duo, the inner surface was etched with
hydrofluoric acid gel and rinsed with water and dried. Then silane was applied to the etched surface followed by air thinning. Self-adhesive resin cement was mixed and applied to the fitting surface of the crowns. Each restoration was seated on its corresponding abutment. Excess cement was removed. The specimens were stored at room temperature in distilled water till they were tested. Eight sample specimens from each group were thermocycled 500 times between the 55 ± 1°C and 5 ± 1°C respectively, with a dwell time of 30 seconds in each bath and a lag time 10 seconds. Fracture resistance was tested for the specimens before and after thermocycling using a computer controlled Universal Testing Machine. The load required to cause fracture was recorded in Newton. Mean value for each group was calculated before and after thermal aging, and differences between groups were tested for statistical significance.
One fractured specimen from each group was scanned by using scanning
electron microscopy (SEM) to determine the failure mode. Before thermocycling it was found that the highest fracture resistance mean value recorded with monolithic zirconia group (1981.58 N) followed by lithium disilicate group (1457 N) then bilayered zirconia group (1423.68 N) while the lowest fracture resistance mean value recorded for zirconia reinforced lithium silicate group (1018.6 N) and this was statistically significant.
The four tested groups showed statistically significant lower fracture resistance mean values after thermal aging than before thermal aging. The fractography showed that surface defects were the main origin of fractures in glass ceramic groups. While monolithic zirconia showed
cementation internal surface cracks (radial cracks).