TRANSLUCENCY AND BIAXIAL FLEXURAL STRENGTH OF MONOLITHIC ZIRCONIA AS AFFECTED BY SINTERING SPEEDS

Statement of the problem: Approaches towards reducing fixed prosthetic restoration fabrication times include; (CAD/CAM) chair side systems in addition to rapid sintering cycles, which can be carried out in minutes rather than hours in order to deliver zirconia-based restorations in one visit .However, limited information is available regarding the effect of rapid sintering on properties of monolithic translucent zirconia.
Aim of the study: The aim of this study was to evaluate the effect of changing the sintering speed (classic, speed and super speed cycles) on translucency and biaxial flexural strength of monolithic translucent zirconia before and after thermodynamic aging.
Materials & Methods: 36 monolithic translucent zirconia discs (10mm diameter×0.8mm thickness) were CAD/CAM fabricated from translucent zirconia blocks(In Coris TZI) and randomly divided into three main groups (n=12 each) according to the sintering speed; group I: samples sintered by classic sintering cycle (sintering at 1510°C for 120 min holding time & 8 hours total cycle time), group II: speed sintering (sintering at 1540°C for 25 min holding time & 2 hours total cycle time) and group III: super speed sintering (sintering at 1580°C for 10 min holding time representing the total firing cycle). In each group, half of the samples were evaluated in the non-aged condition (subgroup1, n=6) while the other half were evaluated after thermodynamic aging (subgroup2, n=6) in a chewing simulator. Translucency parameter (TP) and contrast ratio (CR) were measured using a digital reflective spectrophotometer. Biaxial flexural strength (MPa) was tested using piston on three ball technique in a universal testing machine. Representative sample from each subgroup was analyzed by Scanning electron microscope. Statistical analysis was performed using Two-way ANOVA and Tukey’s post-hoc tests ( P ≤ 0.05).
Results: Two-way ANOVA revealed that sintering speed and thermodynamic aging had a statistically significant effect on mean (TP), (CR).Either before or after aging, classic cycle showed the statistically significantly highest mean (TP) & lowest (CR) values. On the other hand, biaxial flexural strength was statistically significantly affected by the sintering speed only and not by thermodynamic aging; the super speed cycle registered the lowest mean biaxial flexural strength before and after thermodynamic aging.