Effect of the coloring liquid shade and dipping time on the color, transparency, and flexural strength of monolithic zirconia.

Statement Of Problem: The impact of different coloring liquid shades and dipping times on the color, transparency, and flexural strength of monolithic zirconia ceramics is unclear.

Purpose: The purpose of this in vitro study was to evaluate the effects of different coloring liquid shades (A2, 3M2, and 5M2) and dipping times (no dipping, 30 seconds, 60 seconds, and 90 seconds) on the color difference (ΔE), relative translucency parameter (ΔRTP), and 3-point flexural strength (σ) of monolithic zirconia ceramics.

Material And Methods: Yttria-stabilized zirconia (3Y-TZP, 3 mol%) was cut into Ø16×1.2-mm plates (n=10) and 25×4×1.2-mm bars (n=15), which were colored using 3 shades of coloring liquid at 4 dipping times. Color coordinates were measured on a gray background by using a digital spectrophotometer with an integrating sphere attachment. The color and translucency differences were evaluated using 50:50% perceptibility (PT and TPT) and acceptability (AT and TAT) thresholds. The 3-point flexural strengths of the bar-shaped specimens were measured using a universal testing machine and analyzed using the Weibull distribution to calculate the Weibull modulus (m) and characteristic fracture strength (σ). The data were analyzed with the 2-way ANOVA, Kruskal-Wallis, and LSD post hoc tests (α=.05).

Results: Both shade and dipping time significantly affected the color and translucency of monolithic zirconia (P<.001). The ΔE was above the PT for all groups, with only 3M2-90 and A2-60 being below the AT. The main cause of color differences was the difference in lightness. Only A2 showed ΔRTP below the TPT (A2-30 (ΔRTP=0.26), A2-60 (ΔRTP=0.29), and A2-90 (ΔRTP=0.46)). All experimental groups showed translucency differences below TAT. In addition, only the dipping time had a significant effect on the flexural strength of zirconia (P<.001).

Conclusions: The optical properties of monolithic zirconia ceramics were affected by the shade and dipping time of the coloring liquid. The mismatch in lightness was the main reason for the color difference. The dipping time affects the flexural strength of monolithic zirconia, whereas the shade of the coloring liquid did not seem to influence flexural strength.