الفهرس 
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Abstract
This study was carried out to evaluate and compare the color change and fracture strength of two different ceramic materials with different thicknesses cemented to tooth structure with resin cement.
Thirty samples of lithium disilicate HT A3 were studied; they were divided into two groups (15 samples for each) according to the process of fabrication into IPS e.max CAD and IPS e.max press. Each group was subdivided into three groups according to thickness 0.5mm, 1mm and 1.5mm (5 samples for each).
Occlusal enamel of thirty human premolar teeth was removed to expose underlying yellow dentin and create a smooth uniform surface. Each tooth was embedded in autopolymerizing resin block using a specially designed split copper mold. Each group of samples was fabricated using specially designed Teflon mold with different thickness (0.5mm, 1mm and 1.5mm) and diameter of 8mm.
Wax patterns were formed and the lithium disilicate discs were fabricated using heat press technique and milling CAD/CAM technique according to manufacture instruction. Each sample was cemented to its represented tooth using an adhesive resin cement universal A3 shade. Each sample was preserved in distilled water for 3 days. Finally 5 samples for each thickness were tested for color change and fracture strength.
To test the color change, a digital spectrophotometer (vita esayshade) was used. Blue and gray background was established as a neutral background. CIE LAB parameters were measured and the average values of the color differences (∆E) were calculated.
To test the fracture strength, a universal testing machine (Instron) was used. Indenter with diameter 5mm was prepared to allow fixation of it on the occlusal surface of samples at vertical angle. A compressive load was applied at a crosshead speed of 1 mm min until fracture. The maximum load at which the specimens fractured or cracked was recorded in Newton’s.
Data were collected in the form of tables of mean values and standard
deviations. The collected data were then statistically analyzed.
The results had shown that concerning color change, statistical significant differences were detected among the two groups. CAD showed lower color change compared with IPS e.max press. At the same time, there was significant difference within the same groups with different thicknesses represented as 0.5mm showed higher color change compared with other thicknesses.