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Objective: To investigate the effect of artificial aging on the shade of two types of monolithic translucent zirconia crowns, manufactured using two different CAD/CAM systems. Materials and methods: A sound human maxillary first premolar was selected to receive all ceramic crown preparation. The prepared premolar was then duplicated to produce twenty epoxy dies divided into two groups (n=10) according to the constructed crowns.. Twenty Standardized monolithic translucent zirconia (10 Prettau, & 10 Incoris TZI crowns) were CAD/CAM fabricated onto the epoxy dies and cemented using Panavia F 2.0. Vita Easy-Shade digital spectrophotometer was used to measure the color shade differences (ΔE) value between the selected color shade (A3 classical shade) and the color shade of the cemented CAD/CAM crowns before and after aging. The samples were aged under standard autoclave conditions for ten hours. XRD analysis was used to analyze the microstructure of both ZrO2 types before and after accerlated aging. SEM was done to examine the surface changes before and after aging. Results were tabulated and statistically analyzed using Kruskal-Wallis test and post hoc analysis . Results: The results showed that the Prettau crowns had higher ΔE (4.26 ± 0.51) values than that of Incoris TZI crowns before (3.56 ± 0.28) aging. Also after aging Prettau showed higher ΔE (5.6 ± 0.47) than that of Incoris TZI (4.36 ± 0.34). Statistical analysis showed statistical significant difference between the groups. Conclusions: This study concluded that Incoris TZI crowns better color shade results than Prettau crowns before and after aging. (468) Tarek Abdel Hamid, et al. E.D.J. Vol. 64, No. 1 special oxides such as yttria (Y2O3), ceria (CeO2), calcia (CaO), magnesia (MgO) and alumina (Al2O3) fully or partially stabilizes the tetragonal phase of zirconia at room temperature. This transformation toughness reaction enables the material to stop crack propagation by absorbing the crack energy and increasing the crystalline volume that prevents crack growth1. Clinical experience of zirconia based restorations revealed a problem of porcelain separation or chipping which occurs due to the weak bond strength between the porcelain veneer and the underlying zirconia core2 that led to the occurrence of solid monolithic zirconia crowns (non-veneered zirconia).3 Nevertheless, despite having the transformation toughness as an advantage, it is evident that zirconia have a drawback named low temperature degradation (LTD) in the presence of moisture. In this kinetic phenomenon the polycrystalline tetragonal phase transforms into monoclinic (t-m phase transformations) over a temperature range of 60300 ̊C depending on the stabilizer, its concentration and the grain size of the ceramic.4 Several hypotheses were proposed to explain the cause-and-effect relationship between the presence of water and the t–m phase transformations. According to Sato and Shimada5, surface chemisorbed water induces the breakage of Zr–O–Zr bonds and the formation of Zr–OH, thereby resulting in stress corrosion. An-other hypothesis postulated that water promoted the formation of Y(OH)3, thus depleting the stabilizer Y2O3and triggering the t–m transformation.6 The latter assumption, however, has since been disproved.7 In an alternative explanation, the phase trans-formation was attributed to internal stresses associated with the diffusion of water radicals within the zirconia lattice, by an oxygen vacancy diffusion mechanism8,9 Although (LTD) might roughen surfaces, cause wear and decrease the hardness and strength of Y-TZP restorations10, only little information is available regarding low temperature aging of Y-TZP dental ceramics and its effect on the shade of the final restoration.11 So the aim of this study was to evaluate the effect of artificial ageing on the shade of two types of monolithic translucent zirconia crowns, manufactured using two different CAD/CAM systems The null hypothesis tested was: 1. No difference in shade reproduction on both types of zirconia restorations 2. There no effect of artificial ageing on the shade of both zirconia restorations

The effect of low temperature degradation on color stability of two types of monolithic translucent zirconia crowns