الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 167 |  |  |
| | | from | 167 | | |
Abstract
Summary
Summary
In the last years, manufacturers have made admirable efforts to produce dental ceramics replicating both the mechanical and esthetic properties of natural teeth. This resulted in launching numerous types of dental ceramics. Ultimately, a dental ceramic material should meet two principal requirements: outstanding mechanical property and ideal esthetics. A ceramic with these two essential properties functions properly in the oral environment while mimicking a tooth-like appearance.
The present study was designed to assess the effect of variation in sintering parameters on the optical properties and microstructure of ultra-translucent multi-shaded cubic zirconia in order to detect changes in color, translucency, opalescence and chromaticity in addition to variations in microstructure and phase composition that might affect the daily clinical applications.
Ultra-Translucent Multi-Layered (UTML) cubic zirconia blanks were wet sectioned with dimensions 20% larger than the desired final dimension to compensate for the sintering shrinkage using a low speed cutting saw. Each blank was vertically sectioned into serial slabs. Each slab was vertically divided into blocks. So that, the sectioned zirconia blank has perpendicular cuts in order to obtain blocks with dimensions (13 X 13 mm). Each sectioned zirconia block is composed of 4 layers (Enamel, Transition 1, Transition 2, Dentin). Within each block, the plates were sectioned from the middle of each layer with thickness of 1.3 mm that will reach 1 mm after the sintering procedure. After that, the plates were placed inside the sintering furnace to be sintered according to the proposed sintering parameters.
The samples microstructure was evaluated using SEM and phase composition was evaluated using XRD. SEM was used to investigate the surface topograghy of the UTML plates. One specimen from each subgroup was evaluated using 24000X magnification. To determine the crystalline phases, one randomly selected specimen from each subgroup from each layer was examined. Scans were performed with an X-ray diffractometer by using Cu-Ka radiation at 40 mA, 40 kV for 2-theta angle in the range of 20-90° at a scan speed of 2°/min.
Prior to the optical properties measurement, sandblasting was performed using 50 µm AL2O3, 10 sec, 1 cm distance and 2.5 bar pressure in a circular direction. Then, the plates were ultrasonically cleaned in distilled water for 5 minutes, air dried and stored in a plastic box. The specimens were then glazed with CERABIEN™ ZR FC Paste and fired in the ceramic furnace. Then, the dimensions of all the specimens were confirmed to be accurate within 0.1 mm with a digital caliper.
The L*, a* and b* values were recorded according to the CIE L*a*b* color system for each specimen on each background (white & black) using a spectrophotometer. The optical properties including color difference, translucency, opalescence, and chromaticity were evaluated.
The color change showed no statistically significant difference between the values of the two sintering cycles except in the dentin layer which might be due to the greater amount of pigments present in the dentin layer. The color change values regarding the sintering cycles were considered clinically undetectable except for the enamel and T1 layer with the normal cycle parameters they were within the clinically acceptable range.
Our results showed a statistically significant difference between mean color change values for different sintering temperatures with the lowest values found with the (1560 ºC) sintering temperature which was the recommended temperature by the manufacturer for the speed sintering cycle. Most of the color change values were considered clinically undetectable except for the enamel (with 1500, 1600, 1510 and 1610 ºC) and T1 layer (with 1500, 1600, and 1610 ºC), they were within the clinically acceptable range.
Generally, the results have shown that normal sintering cycle had superior effect on the translucency and opalescence than the speed cycle. Moreover, as the sintering temperature increases or decreases, the translucency and opalescence values increase or decrease respectively.
Also, the normal sintering cycle showed higher chromaticity values within the enamel and transitional (1) layers than the speed sintering cycle. On the contrary, the normal sintering cycle showed lower chromaticity values within the dentin and transitional 2 layers than the speed sintering cycle. When it comes to the sintering temperature, increasing the temperature led to higher chromaticity values within each layer, while decreasing the sintering temperature led to lower chromaticity values.
As for the SEM results it was found that decreasing the sintering temperature, decreased the average grain size and on the contrary, increasing the sintering temperature, increased the average grain size. When we compare the sintering cycles, the average grain size in speed sintering cycle was smaller than that of the normal cycle. While the XRD analysis showed no obvious changes with variations in sintering parameters.