الفهرس 
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Abstract
This aim of this in vitro to study was to evaluate the effect of different polymerization modes of resin cements and ceramic thicknesses on the color stability of ceramic laminate veneers.
Sixty-four sound extracted permanent maxillary central incisors were used in this study. The root portion of each tooth was removed, and the crown part was inserted into self-cure acrylic resin in a custom made copper mold with a diameter of 14 mm, and 20 mm length exposing the labial surface upwards. The samples were randomly divided into 2 main groups according to the thickness of ceramic disc used. group I (n=32); 0.5 mm ceramic thickness, group II (n=32); 1mm ceramic thickness, then each group was subdivided according to the type of adhesive resin cement used; Light cure (Rely X Veneer®, 3 M ESPE) or Dual cure (Rely X Ultimate ®, 3 M ESPE) for cementation of the ceramic discs into 4 subgroups (group I A, II A and group I B, II B) of 16 specimens each.
Sixty four ceramic discs (7 mm in diameter) were obtained from multilayered glass-ceramic blocks (IPS Empress CAD HT I12 A1) using a microtome precision cutter (Isomet ™ 4000 linear precision saw, Buehler, USA), cutting at a speed of 350 rpm with a water cooling unit attached to the cutter. For group I, 32 discs were obtained of thickness 0.5 mm cemented to group IA with LC cement and group IIA with DC resin cement. For group II, 32 discs were obtained of thickness 1mm, cemented to group IIA with LC cement and group IIB with DC resin cement.
The specimens were submitted to color readings with a digital spectrophotometer against a white background at three different time intervals; after the preparation (the tooth
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substrate alone), after the cementation and polymerization of the veneers and after the accelerated artificial ageing (AAA) process.
All specimens were subjected to thermo-cycling for 1000 cycles in water bathes with a temperature range of 5-55°C with a dwell time of 15 seconds at each temperature.
The color was evaluated according to the color system CIE La*b*.The difference in color was expressed as ΔE and was measured by the following equation.