Abstract
Objectives
Sintering influences the optical properties of zirconia. This study examined the effect
of altering sintering rates on color characteristics of 3, 4, and 5 mol% yttria (Y)-containing
multilayer zirconia.
Materials and Methods
A total of 135 specimens (width × length × thickness = 11.2 × 20 × 1.5 mm) were prepared
from multilayer (cervical [C], middle [M], and incisal [I]) 3Y, 4Y, and 5Y zirconia,
and randomly sintered at regular (RS: 10 °C/min), fast (FS: 35 °C/min), and speed
(SS: 70 °C/min) sintering (n = 15/group). Translucency parameter (TP00), contrast ratio (CR), opalescence parameter (OP), and color difference (∆E00) were evaluated with the CIEL*a*b* system. Microstructure, crystalline (monoclinic
[m], tetragonal [t], and cubic [c]) phases, and surface roughness (Ra) were evaluated
by scanning electron microscopy (SEM), X-ray diffraction (XRD), and 3D-digital topography.
Statistical Analysis
ANOVA and Bonferroni comparisons were determined for significant differences (p < 0.05).
Results
Significant differences in TP00, CR, OP, and ∆E00 of zirconia types, layers, sintering rates, and their interactions were indicated
(p < 0.05). Significant increases in TP00 and ∆E00, but decreases in CR and OP, upon rising the amount of Y (5Y > 4Y > 3Y), region (I > M > C),
and speed sintering (SS > FS > RS) (p < 0.05) were observed. Nevertheless, the color alteration was within an acceptable
threshold (∆E00 ≤ 1.8); Ra values: 3Y > 4Y > 5Y. SEM indicated a larger grain for 5Y > 4Y > 3Y. XRD
indicated higher t-phase in 3Y, whereas higher c-phase in 5Y.
Conclusion
Increasing translucency and color alteration, and decreasing contrast and opalescence
were influenced by Y content (5Y > 4Y > 3Y), region (I > M > C), and sintering rate
(SS > FS > RS). Nonetheless, color alterations were within acceptable limits, suggesting
the speed sintering rate to produce better color characteristics of restoration.
Keywords
color appearance - contrast - sintering rate - translucency - opalescence - zirconia
