STATEMENT OF PROBLEMRecently Yttrium-stabilized tetragonal zirconia polycrystal (Y-TZP) has been introduced due to superior flexural strength and fracture toughness compared to other dental ceramic systems. Although zirconia has outstanding mechanical properties, the phenomenon of decrease in the life-time of zirconia resulted from degradation in flexural strength after low temperature aging has been reported.PURPOSEThe objective of this study was to investigate degradation of flexural strength of Y-TZP ceramics after various low temperature aging treatments and to evaluate the phase stability and micro-structural change after aging by using X-ray diffraction analysis and a scanning electron microscope (SEM).MATERIAL AND METHODSY-TZP blocks of Vita In-Ceram YZ (Vita Zahnfabrik, Bad Säckingen, Germany) were prepared in 40 mm (length) × 4 mm (width) × 3 mm (height) samples. Specimens were artificially aged in distilled water by heat-treatment at a temperature of 75, 100, 125, 150, 175, 200, and 225℃ for 10 hours, in order to induce the phase transformation at the surface. To measure the mechanical property, the specimens were subjected to a four-point bending test using a universal testing machine (Instron model 3365; Instron, Canton, Mass, USA). In addition, X-ray diffraction analysis (DMAX 2500; Rigaku, Tokyo, Japan) and SEM (Hitachi s4700; Jeol Ltd, Tokyo, Japan) were performed to estimate the phase transformation. The statistical analysis was done using SAS 9.1.3 (SAS institute, USA). The flexural strength data of the experimental groups were analyzed by one-way analysis of variance and to detect statistically significant differences (α= .05).RESULTSThe mean flexural strength of sintered Vita In-Ceram YZ without autoclaving was 798 MPa. When applied aging temperature at below 125℃ for 10 hours, the flexural strength of Vita In-Ceram YZ increased up to 1,161 MPa. However, at above 150℃, the flexural strength started to decrease. Although low temperature aging caused the tetragonal-to-monoclinic phase transformation related to temperature, the minimum flexural strength was above 700 MPa.CONCLUSIONThe monoclinic phase started to appear after aging treatment above 100℃. With the higher aging temperature, the fraction of monoclinic phase increased. The ratio of monoclinic/tetragonal + monoclinic phase reached a plateau value, circa 75% above 175℃. The point of monoclinic concentration at which the flexural strength begins to decrease was between 12% and 54%.
PURPOSEThis study investigated the effect of amount of thickness reduction on color and translucency of dental monolithic zirconia ceramics.MATERIALS AND METHODSOne-hundred sixty-five monolithic zirconia specimens (16.3 mm × 16.3 mm × 2.0 mm) were divided into 5 groups (Group I to V) according to the number of A2-coloring liquid applications. Each group was then divided into 11 subgroups by reducing the thickness up to 1.0 mm in 0.1-mm increments (Subgroup 0 to 10, n=3). Colors and spectral distributions were measured according to CIELAB on a reflection spectrophotometer. All measurements were performed on five different areas of each specimen. Color difference (ΔE*ab) and translucency parameter (TP) were calculated. Data were analyzed using one-way ANOVA and multiple comparison Scheffé test (α=.05).RESULTSThere were significant differences in CIE L* between Subgroup 0 and other subgroups in all groups. CIE a* increased (0.52
PURPOSEMarginal fit is a very important factor considering the restoration's long-term success. However, adding porcelain to copings can cause distortion and lead to an inadequate fit which exposes more luting material to the oral environment and causes secondary caries. The purpose of this study was to compare the marginal fit of 2 different all-ceramic crown systems before and after porcelain veneering. This study was also intended to verify the marginal fit of crowns originated from green machining of partially sintered blocks of zirconia (Lava CAD/CAM system) and that of crowns obtained through machining of fully sintered blocks of zirconia (Digident CAD/CAM system).MATERIALS AND METHODS20 crowns were made per each system and the marginal fit was evaluated through a light microscope with image processing (Accura 2000) at 50 points that were randomly selected. Each crown was measured twice: the first measurement was done after obtaining a 0.5 mm coping and the second measurement was done after porcelain veneering. The means and standard deviations were calculated and statistical inferences among the 2 groups were made using independent t-test and within the same group through paired t-test.RESULTSThe means and standard deviations of the marginal fit were 61.52 ± 2.88 µm for the Digident CAD/CAM zirconia ceramic crowns before porcelain veneering and 83.15 ± 3.51 µm after porcelain veneering. Lava CAD/CAM zirconia ceramic crowns showed means and standard deviations of 62.22 ± 1.78 µm before porcelain veneering and 82.03 ± 1.85 µm after porcelain veneering. Both groups showed significant differences when analyzing the marginal gaps before and after porcelain veneering within each group. However, no significant differences were found when comparing the marginal gaps of each group before porcelain veneering and after porcelain veneering as well.CONCLUSIONThe 2 all-ceramic crown systems showed marginal gaps that were within a reported clinically acceptable range of marginal discrepancy.
In-Sung Yeo and Seung-Ryong Ha, Effects of airborne-particle abrasion protocol choice on the surface characteristics of monolithic zirconia materials and the shear bond strength of resin cement, Ceramics International, http://dx. AbstractThis study evaluated the effect of several airborne-particle abrasion protocols on the surface characteristics of monolithic zirconia and of protocol choice on the shear bond strength of resin cement. 375 bar-shaped (45×4×3mm) and 500 disc-shaped (Ø9×1mm) monolithic zirconia specimens were divided into 25 groups, respectively. All specimens were abraded with one of three different sizes of alumina particles (25, 50 or 125 μm), two different pressures (2 or 4 bar), two distinct application times (10 or 20s) and two distinct incidence angles (45 or 90°). The bar-shaped specimens were used for 3-point bending test; Weibull parameters were calculated and transformed monoclinic phase (X M ), surface characteristics were examined. The disc-shaped specimens were used to determine the shear bond strength of resin cement before and after thermocycling. All data were analyzed using 4-way ANOVA and a multiple comparison Scheffé test (α=.05). The particle size, pressure and time significantly affected the flexural strength, while the incidence angle was insignificant. The X M and surface roughness were proportional to the size, pressure, time and incidence angle. The Raman spectrum analysis showed a higher proportion of the monoclinic phase as the depth of the specimen was closer to the abraded surface.In bonding with resin cement, the highest shear bond strength after thermocycling was obtained by the abrasion with 50μm particles at 4bar for 20s, regardless of incidence angle. Surface treatment of monolithic zirconia with 50μm particle at 4bar for 20s at either 45° or 90° incidence angles is recommended.
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