Effect of Different Sintering Cycles on the Flexural Strength of Full Contour Monolithic Zirconia – An In-Vitro Comparative Study

BACKGROUND Monolithic zirconia restorations are known to exhibit good mechanical properties and acceptable aesthetics. During the fabrication of zirconia prostheses, the sintering process is the most critical step which determines the performance of the final product. However, a lot of issues with zirconia restorations were related to sintering cycles which can affect the crystalline content and grain growth that can affect its mechanical properties. The purpose of this study was to evaluate and compare the effect of different sintering cycles on the flexural strength of full-contour monolithic zirconia. METHODS In this in-vitro study, a total of 44 bar-shaped zirconia samples (Amann Girrbach Ceramill® CAD/CAM Material – ZOLID HT+) of final dimensions 20 x 4 x 2 mm were fabricated using Amann Girrbach Ceramill mind – computer-aided designing software and Amann Girrbach Ceramill® motion2 – computer-aided milling unit. These samples were randomly allocated into four groups with 11 samples in each group namely, Group 1, 2, 3 and 4 (Group 1: Long sintering cycle, Group 2: Intermediate sintering cycle, Group 3: Short sintering cycle, Group 4: Ultra-short sintering cycle) and sintered under the respective sintering cycles. The Ceramill® therm (Amann Girrbach) sintering furnace provided 3 sintering cycles ranging from the longest group-1 (19 hours), to the short sintering cycle group-3 (8 hours), with an intermediate sintering cycle group-2 (15 hours). The zircon (KDS) sintering furnace provided an ultra-short sintering cycle group-4 (2 hours). The testing of flexural strength was performed using a three-point bend test on a universal testing machine. Statistical analysis of the collected data was performed by one-way ANOVA using SPSS software. RESULTS One-way ANOVA showed no statistically significant difference in the flexural strength of full-contour monolithic zirconia sintered under the four different sintering cycles. (P > 0.05). CONCLUSIONS The flexural strength of full-contour monolithic zirconia was influenced to some extent by various sintering cycles. The longest sintering cycle (Group 1) showed the least flexural strength whereas the intermediate sintering cycle (Group 2) showed the highest flexural strength. Also, the ultra-short sintering cycle (Group 4) of 2 hours exhibited flexural strength comparable to the rest of the groups with longer sintering cycles, thus making the cycle a viable alternative to the conventional sintering cycles, thereby making the ‘tooth-in-a-day‘ possible for the patients.