الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
With the rise in demand for esthetic restorations, a shift to all-ceramic restorations has occurred. In comparison to other glassy materials or particle filled glasses, zirconia emerges as a useful choice because of its superior mechanical qualities. However, it lacks the translucency and aesthetics of glass ceramic restorations. Another problem of zirconia is its phase instability, which causes low-temperature degradation (LTD), i.e., ageing, caused by moisture exposure at slightly increased temperatures.
Particle size reduction, increasing yttria content, adding Al2O3, and modifying the chemical production pathway to get ZrO2 raw particles are all techniques to lower the risk of ageing in 3Y-TZP. Different varieties of medical grade zirconia are used for dental all-ceramic fixed prosthetic restorations, which can be identified by their chemical composition, particularly the presence of the stabilizer Y2O3. Translucency vs strength maps recently indicated that 4 mol% and 5 mol% Y2O3 partially stabilized zirconia bridge the gap between standard 3 mol% yttria stabilized zirconia (3Y-TZP) and lithium-disilicate.
Recent advancements in milling technology advocate zirconium and titanium as the materials of choice for fabricating patient-specific abutments. Titanium has long been the chosen material due to its high strength, resistance to distortion, and ability to construct a one-piece abutment. To solve the esthetic problem of titanium abutments and assure improved aesthetic outcomes for implant supported fixed partial dentures, all ceramic abutments constructed of yttria-stabilized zirconia oxide have been developed. These abutments have optically attractive properties, low corrosion potential, and great biocompatibility as compared to metal abutments.
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The goal of this study was to compare the effects of three distinct types of implant abutment materials: 1 metallic (titanium grade V) and two types of ceramics (zirconia of medium translucency shade A2 and zirconia of low translucency shade A2) and accelerated ageing on the color stability of two different types of monolithic super-translucent zirconia (Precolored monochromatic and Multilayered).
Two different types of super-translucent zirconia (group 1 – Precolored CubeX zirconia -4mol% Y2O3) & (group 2 – Multilayered CubeX zirconia-5mol% Y2O3) was used, both of shades A2, each type represents a group of 24 specimens, to compare the differences between the precolored monochromatic and the multilayered types. All specimens was milled to produce disc shaped specimens (10mm diameter x 1mm thickness) using CAD/CAM milling machine. Each group was then subdivided into three subgroups according to the substrate material (8 specimens each): 1 metallic (titanium grade V) and two types of ceramics (IPS e.max ZirCAD MT A2- 4mol%Y2O3- and IPS e.max ZirCAD LT A2- 3mol% Y2O3-) disc-shaped to simulate commonly used implant abutment materials. After cementation of specimens their color and their crystal structure. Then, specimens were subjected to accelerated ageing in a steam autoclave for 5 hours at 134°C, 0.2 MPa, after which they were tested again for their color and crystal structure to be compared with the results collected before ageing.
Color Analysis
The color of the 24 specimens of each group were measured against the three different substrates using Vita Easy shade before ageing. Data from the spectrophotometer were recorded as the CIELab color coordinates ΔL, Δa and Δb.
The mean ΔE and standard deviation for all the studied groups and the p value for each group were compared with each other. The results of the specimens having same substrates but different crown materials were also compared. Statistics suggested significant differences
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in group II between subgroup A and the other 2 subgroups, (p<0.001) and between subgroup C in the 2 groups, (p<0.007) and no significant difference between remaining groups.
X-ray diffraction analysis
All specimens were subjected to x-ray diffraction before ageing revealing that ZirCAD LT (3mol% Y2O3), ZirCAD MT (4mol% Y2O3) & Precolored CubeX (4mol% Y2O3) have similar patterns representative of the tetragonal phase, with the characteristic peaks of tetragonal zirconia. While that of Multilayered CubeX (5mol%Y2O3) the tetragonal doublets almost fuse together in single, large peaks.
X-ray diffraction was done again after cementation and ageing of each subgroup revealing the characteristic peaks of monoclinic zirconia, in addition to the previous tetragonal peaks.
Phase composition
The data obtained from the XRD was analyzed by the Rietveld refinement method using MAUD program to quantify the amount of tetragonal and monoclinic phase in each subgroup tested before and after ageing. The quantified results were confirmation of the XRD patterns indicating that as the specimens underwent ageing, the tetragonal to monoclinic phase transformation increased, increasing the amount of the monoclinic phase in the specimens.