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

OBJECTIVE To evaluate the impact of high-speed compared to conventional sintering protocols on the color difference (ΔE00) and relative translucency (RTP00) of multi-yttria-layered 3Y/5Y-partially stabilized zirconia (PSZ) in various zones (incisal, transition, and core).

MATERIALS AND METHODS 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 (ΔE00) 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 ΔE00 = 0.8 and ΔE00 = 1.8, respectively, with RTP00 PT = 0.62.

RESULTS High-speed sintering resulted in color changes that were greater than the AT (ΔE00 = 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).

CONCLUSIONS 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.

CLINICAL SIGNIFICANCE 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.