Effects of sintering protocol and dipping time on the optical properties of monolithic zirconia

Statement of problem: The use of monolithic zirconia for restorations has become popular. However, the effects of the coloring process and sintering procedure on the optical properties of monolithic zirconia remain unclear. Purpose: The purpose of this in vitro study was to evaluate the effects of the different sintering protocols and dipping times on the CIELab color parameters, color difference (ΔE00), translucency parameter (TP00), opalescence parameter (OP), and contrast ratio (CR) of monolithic zirconia. Material and methods: One hundred and fifty monolithic zirconia specimens were prepared and divided into 5 groups according to different dipping times in A2 shade coloring liquid: no immersion (D0), 3 minutes (D3), 5 minutes (D5), 7 minutes (D7), and 9 minutes (D9). The specimens were further divided into 3 subgroups according to different sintering protocols (n=10): classic (C); speed (S); and super-speed sintering (SS). The CIE L∗a∗b∗ values, ΔE00, TP00, OP, and CR values were determined with a spectrophotometer. The data were analyzed with 2-way ANOVA and Bonferroni post hoc tests (α=.05). Results: The effects of the sintering protocol, dipping time, and interaction of the sintering protocol and dipping time were significant for the L∗, a∗, b∗, ΔE00, TP00, OP, and CR parameters (P<.001). The ΔE00 values of the SS groups were significantly higher than those of the C and S groups for all the dipping times assessed. However, only the CD3 and CD5 groups had ΔE00 values below the acceptability threshold (<1.8). Coloring significantly reduced TP00 values and significantly increased OP and CR values in the colored groups compared with those in the D0 groups. The C and S protocols for different dipping times showed similar optical properties in TP00, OP, CR, and ΔE00 (excluding the D0 and D9 groups) compared with the SS protocol. Conclusions: Different sintering protocols combined with various dipping times significantly affected the optical properties of monolithic zirconia. However, C and S sintering with a 3-minute and 5-minute coloring process yielded more favorable optical results than SS sintering and prolonged dipping times.