الفهرس 
STATEMENT OF THE PROBLEMOnly 14 pages are availabe for public view
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Abstract
Increased demand for achieving the best esthetics in dentistry has led to an increase in the use of ceramic, hybrid ceramics and tooth colored composite resin restorations.(99) In particular, the CAD/CAM ceramics have enjoyed growing popularity.(100) The final restoration color may be influenced by the thickness and translucency of the veneer restoration.(56) In addition, the color of supporting structure or luting agent may also influence the final color of veneer restorations.(101) Shade selection of thin restorations like laminate veneers is considered a challenge. With the introduction of new ceramic materials, the no-preparation thin veneer concept has received more attention and the challenge increased.(50)Two of the new ceramic materials indicated for thin laminate veneers were used in this study, Vita Enamic which belongs to a new class of ceramics, known as “hybrid-ceramic” It consists of a hybrid structure with two interpenetrating networks of ceramic and polymer. And Vita Suprinity which is a zirconia (10 % by weight) reinforced lithium silicate, in order to obtain the world‘s first zirconia reinforced lithium silicate ceramic (ZLS).ceramic (ZLS).
Perception of color involves the participation of three factors that can effectively exist, the light source, the object being viewed and the observer viewing the object. In 1936, Munsell described the three dimensions of color to opaque objects: hue, chroma and value then the International Commission of I’Eclairage (CIE) has developed some methods to express the spectral curves in a numerical form known as CIEL*a*b*. Differences in perceived color (ΔE) can be determined using the CIELAB coordinates. This system is widely used in dental research and is useful for calculating color differences. An essential requirement of an all ceramic restoration is its masking ability, the absence of which will allow the unacceptable display of a discolored tooth or metal post beneath such a translucent restoration. The masking ability of an all-ceramic restoration can be determined by measuring the color difference (ΔE) between a uniform thickness of the material on black and white backgrounds (4). If the masking ability of a system is perfect, there will be no color difference (ΔE*=0). However, the color difference of a restoration could still be regarded as visually acceptable if ΔE* was less than or equal to 5 units.(97) . It is calculated using any instrument capable of quantitatively measuring visible light intensity like spectrophotometers.(102) Spectrophotometers are amongst the most accurate, useful and flexible instruments for color matching in dentistry.(24) They estimate the color by measuring the intensity of the reflected light in all visible wavelengths. The measurements obtained by the instruments are frequently keyed to dental shade guides and converted to shade tab equivalent.(25) In a study, it was found that spectrophotometers offered a 33% increase in accuracy and a more objective match in 93.3% of cases.(27)Newly introduced all-ceramic restorations have been advocated for superior esthetics with small thicknesses, but there is a lack of information on how to mask the color of the underlying discolored dental substructure to achieve best esthetics. The Aim of this study is to evaluate the effect of veneer material (Vita Enamic, Vita Suprinity), degree of ceramic translucency (High translucent , Translucent) and Luting cement shade (A1, Opaque white) on masking the color of the underlying discolored substructure without increasing the thickness of the restoration (0.5mm).Sampling: in this study we used two groups of materials group E: Vita Enamic n=28 and group S: Vita Suprinity n=28, each group was later subdivided into two subgroups according to the degree of translucency subgroup A: High translucency (n=14) and subgroup B: Translucent (n=14), then each subgroup was subdivided into 2 subdivisions according to shade of resin cement Subdivision I: (A1) shade of resin cement (n=7) and Subdivision II: (opaque white) shade of resin cement (n=7) Methods: blocks of the two types of the veneer materials {Vita Enamic (1M2 shade) and Vita Suprinity (A1 shade)} were sectioned horizontally in a sawing machine (Isomet saw 4000) in order to obtain the required thickness (0.5mm). To simulate the color of a dark underlying dental substructure, background resin composite discs were made with shade (C3) then the disc shaped specimens with varying translucencies(HT,T) were cemented on the dark background resin composite discs using two shades of resin cement (A1, Opaque white). Measurements: masking abilities of the specimens before cementation were determined by measuring ∆E of the specimens using the Easyshade Advance 4.0, after placing them on a white background and then on a black background, then the following equation was used: ∆E = { (Lw-Lb)2 + (aw-ab)2 + (bw-bb)2 }1/2 where Lw, aw, bw = Color of the specimens over the white background and Lb, ab, bb = Color of the specimens over the black background, then the masking abilities of the specimens after cementation were determined by measuring ∆E from A1 of all specimens by setting the Easyshade to the restoration mode and the shade A1 was selected. Results of masking ability before cementation revealed than none of the 0.5mm veneers could mask the color of a black background, and that there is a significant increase in ΔE and decrease in masking ability from Suprinity to Enamic, it also revealed significant increase in ΔE from T samples to HT samples in each material. As for the results of masking ability after cementation, it revealed no significant difference in the ∆E from A1 between the two materials, it also revealed that the type of ceramic translucency was the most effective factor in masking the underlying color which is then followed by the cement shade type. Enamic T and Suprinity T samples cemented by opaque cement had the highest masking ability and gave measurements of ∆E from A1 as low as 3.4 with mean equals to 3.65 which could be considered clinically acceptable results in masking the underling discolored substrate. As for the results of masking ability after cementation, it revealed no significant difference in the ∆E from A1 between the two materials, it also revealed that the type of ceramic translucency was the most effective factor in masking the underlying color which is then followed by the cement shade type. Enamic T and Suprinity T samples cemented by opaque cement had the highest masking ability and gave measurements of ∆E from A1 as low as 3.4 with mean equals to 3.65 which could be considered clinically acceptable results in masking the underling discolored substrate. The degree of translucency of thin veneers and the use of try-in pasts must be taken into consideration during shade selection especially with dark prepared teeth.