Influence of shading technique on mechanical fatigue performance and optical properties of a 4Y-TZP ceramic for monolithic restorations.

This study aimed to evaluate the effect of shading procedures on the fatigue performance and optical properties of an yttria-stabilized tetragonal zirconia polycrystal ceramic (4Y-TZP - stabilized by 4 mol% YO). Disc-shaped specimens were produced at pre-sintered stage and randomly allocated into 5 groups (n=15) considering the shading technique: Ctrl (no pigmentation); Brush 1× and Brush 3× (brushing on 1 or 3 applications, respectively); Immer 2min and Immer 4min (immersion for 2 or 4 min, respectively). Following each pigment application, the specimens were dried in accordance with the manufacturers' guidelines (15 min in a stove at 70 °C) and sintered in a specific furnace. Biaxial flexural fatigue test followed a step-stress approach (initial strength: 200 MPa; step-size: 25 MPa; 10.000 cycles/step; frequency: 20 Hz). Optical measurements were conducted in a spectrophotometer following the CIEDE2000 parameters. Roughness, topography, crystalline grain size, crystalline phase content and fractography analysis were also performed. The flexural fatigue strength (FFS) and the number of cycles to fracture (CFF) were statistically affected by pigmentation techniques (mean FFS in MPa/mean CFF): Ctrl (446.7/105,619) > Brush 1 × (436.7/102,854) ≥ Brush 3 × (405.0/89,962) ≥ Immer 2 min (395.0/85,103) ≥ Immer 4 min (383.3/81,382), although the exposure intensity (increase in number of applications or in application time) to the pigment had no effect (Brush 1 × = Brush 3 × ; Immer 2 min = Immer 4 min). Regarding optical properties, shading techniques similarly affected translucency and opalescence parameters, regardless of the exposure intensity to the pigment (Brush 1 × = Brush 3 × = Immer 2 min = Immer 4 min > Ctrl), whereas the exposure intensity to the pigment only promoted relevant alterations in the color change parameter (ΔE) for the brushing technique (Brush 3 × > Brush 1 × ). No difference was observed in crystalline phase content, topographic pattern or roughness, although an increase in the zirconia grain size could be detected. Thus, the shading technique might affect the fatigue behavior and promote optical changes in a 4Y-TZP ceramic.