Optical Properties of Multi-Yttria-Layered Zirconia: Influence of "High-Speed" Sintering Protocol

To evaluate the impact of high-speed compared to conventional sintering protocols on the color difference (ΔE ₀₀) and relative translucency (RTP₀₀) of multi-yttria-layered 3Y/5Y-partially stabilized zirconia (PSZ) in various zones (incisal, transition, and core).

Using a 5-axis milling machine (PrograMill PM7), 60 specimens (14 mm diameter × 1 ± 0.1 mm thickness) were made from the incisal, transition, and core zones of multi-yttria-layered 3Y/5Y-PSZ (IPS e.max ZirCAD Prime, Ivoclar Vivadent). Specimens were randomly allocated to conventional sintering (9 h) or speed sintering (15 min) in a programmable furnace (Programat CS6). Using a spectrophotometer (Ci7600, X-Rite), spectral reflectance over black and white backgrounds was measured. The CIEDE2000 formula was used to calculate the color difference (ΔE ₀₀) and RTP₀₀. Data were analyzed via two-way ANOVA and Tukey's post hoc tests (α > 0.05). Perceptibility (PT) and acceptability thresholds (AT) were set at ΔE ₀₀ = 0.8 and ΔE ₀₀ = 1.8, respectively, with RTP₀₀ PT = 0.62.

High-speed sintering resulted in color changes that were greater than the AT (ΔE ₀₀ = 1.8) in the incisal (3.1 ± 0.47) and transition (1.8 ± 0.22) zones. The core zone, on the other hand, remained stable (1.4 ± 0.11; p < 0.001). Conventional sintering produced much higher RTP₀₀ in all zones (p < 0.001). The incisal zone subjected to conventional sintering demonstrated the highest translucency (10.09 ± 0.43).

Speed sintering significantly reduces the optical properties of multi-yttria-layered zirconia, especially in the esthetically crucial incisal zone, resulting in clinically unacceptable color shifts and decreased translucency relative to conventional sintering methods.

While speed sintering significantly reduces time in the digital workflow, it may negatively impact the final aesthetic outcome of multi-yttria-layered zirconia restorations by compromising optical properties and reducing translucency in the incisal and transition zones. When maximum aesthetic integration is needed for anterior restorations, clinicians and lab technicians should prioritize conventional sintering.