الفهرس 
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Abstract
The increased esthetic demands of nowadays patients led to increase in use of lithium silicates glass ceramics in dentistry.
During laboratory procedures glass ceramic dental restorations may undergo multiple firing cycles to improve the esthetic outcome by glazing, staining or surface layering. These firings influence the optical and mechanical properties of material which in turn may affect restorations clinical results.
This in-vitro study was designed to evaluate the effect of multiple firing on translucency and biaxial flexural strength of three different lithium silicate glass ceramic materials.
A total of 72 disc shape specimens (10mm diameter and 1mm thickness) were fabricated by heat pressing of LTA1 shaded ingots. 24 specimens of Celtra press, and the same number of IPS e.max press and Livento press lithium silicate materials.
Each material group were subdivided in to three subgroups (8 samples per group) according to the number of firing cycles. One, three, and five firing cycles’ protocols were constructed, to simulate the firing protocols of the three common laboratory fabrication techniques (staining, cutback, and layering) of glass ceramic restorations.
CIELAB system was introduced in 1978, to defined color in terms of 3 coordinate values (L*, a*, and b*). L* refers to the lightness coordinate and its value ranges from 0 (black) to 100 (white). The values of a* and b* represent the chromaticity coordinates, where a* represents the red (positive value) or green chroma (negative value), while b* represents yellow (positive
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value) and blue chroma (negative value). Since then CIElab color coordinates were extensively used in dental research to measure color.
Color coordinates (CIE L*a*b*) of each specimen were measured against white and black background using carry 5000 UV-vis-NIR spectrophotometer. Each value was measured on five different areas of each specimen starting from the center of specimen by moving 1 mm toward each quadrant direction. Average L*, a*, b* values were used to calculate the needed parameters.
The translucency parameter (TP) of each specimen was obtained from the recorded L*, a*, and b* average values over the black and white backgrounds using the following formula:
TP= [(L*w-L*b)2 + (a*w-a*b)2 + (b*w-b*b)2] ½
Then, specimens were subjected to piston-on-three-ball test in Instron universal testing machine model 3345. Fracture load data of each specimen were used to calculate the biaxial flexural strength using the following equation: 𝛔=−0.2387P(X − Y)𝑏2 𝑋=(1+ν)ln(𝑐2𝑅2)+(1−𝑣2)(𝑟2𝑅2) 𝑌= (1+ν)[1+ln(𝑎2𝑅2)]+(1−ν)(𝑎2𝑅2)
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where αthe maximum tensile stress in mega Pascal (MPa), P is the failure load in Newton (N), ν is the Poisson’s ratio, and other parameters about dimensions are in millimeters (mm).
Two-way ANOVA test was used to study the effect of different tested variables and their interaction on biaxial flexural strength and translucency. Comparison of main and simple effects were done utilizing pairwise t-tests with Bonferroni correction. Spearman rank order correlation coefficient was used to study the correlation between biaxial flexural strength and translucency.
The results showed significantly higher mean value of translucency for Celtra press rather than Emax press, and Livento press respectively. TP values for all materials showed significant decrease with more firing cycles.
Regarding biaxial flexural strength, Emax press showed significantly higher values than Livento press, and Celtra press respectively. Multiple firing cycles had no significant effect on tested materials biaxial flexural strength values.