Impact of sintering procedure and clinical adjustment on color stability and translucency of translucent zirconia

Statement of problem. Chairside and laboratory adjustments of translucent monolithic zirconia restorations sintered by short cycles are inevitable. Therefore, the optical properties of these materials should be evaluated under conditions similar to those in the oral environment. Purpose. The purpose of this in vitro study was to evaluate the color stability and the translucency changes of translucent monolithic zirconia after the sintering procedure, simulated clinical adjustment, and coffee thermocycling. Material and methods. Eighty monolithic zirconia disks (diameter: 10 mm; thickness: 1 mm) were milled from 4 zirconia brands (Zolid FX Preshaded, Zolid FX White, DD Cubex(2), and DD Bio ZX(2)). Two sintering procedures (standard or speed) were performed for each zirconia brand. The color coordinates and the translucency after sintering, clinical adjustment, and after 5000 cycles in a coffee solution were analyzed with a reflectance spectrophotometer, and the translucency parameters and contrast ratio were measured. The color differences and the translucency variations were calculated for each tested variable and statistically analyzed with 2-way ANOVA and pairwise comparison (alpha=.05). Results. Both clinical adjustment procedure and coffee thermocycling had significant influence on color differences (P<.001). For translucency, 2-way ANOVA showed a significant interaction between sintering procedure and zirconia brand after coffee thermocycling (P<.001) while no significant interaction was recorded after the clinical adjustment procedure (P=.247). For all evaluated groups, the translucency parameter values decreased after either clinical adjustment procedure or coffee thermocycling (P<.001). For tested zirconia, group DD Cubex(2) showed higher translucency parameter regardless of the variables tested. Conclusions. The color and the translucency of the translucent zirconia can be affected by the type of the zirconia brand and the sintering protocol. Furthermore, the color and the translucency were affected by both the clinical adjustment procedure and the coffee thermocycling, but not beyond the clinically acceptable limit of the color difference.