Effect of different sintering cycles on the flexural strength and translucency of CAD-CAM milled monolithic zirconia with different thicknesses

Abstract

Zirconia is the preferred material for posterior teeth subjected to high occlusal forces & its fabrication involves the use of CAD/CAM technology. Sintering conditions play a crucial role in determining the grain size, final product stabilization, and mechanical properties of the material. Limited studies are available to determine effect of sintering cycles on mechanical and optical properties of zirconia. To investigate flexural strength and translucency produced by two different sintering cycles (conventional and high-speed sintering) on three different thicknesses of CAD-CAM milled monolithic zirconia crowns. In vitro study. Zirconia blanks (Dentsply Sirona, Cercon ht diameter 98mm x 12 mm height, Germany) were used to fabricate 30 premolar crowns (N = 30, divided into 2 groups of 15 each) which were constructed in three different occlusal thicknesses (1.5, 2.0, 2.5 mm) using two different sintering cycles: conventional sintering (at 1,450°C for 9 hours 50 minutes) and high-speed sintering (at 1,550°C for 2 hours 55 minutes). Flexural strength and translucency parameter (TP) tested after all crowns were underwent 5,000 times thermocycling test (between 5°C and 55°C). Intergroup comparison done using unpaired ‘t’ test. Intra-group comparison done using Tukey’s post hoc test. The effect of sintering procedure on TP was not statistically significant but effect of thickness on TP was statistically significant. As thickness was reduced from 2.5 to 1.5 mm, translucency was increased but flexural strength was decreased. Sintering monolithic zirconia using conventional sintering cycle rendered high flexural strength than high speed sintering cycle but both groups showed higher values than optimum value of flexural strength of zirconia. The flexural strength increased with the increase of thickness of zirconia crown whereas translucency decreases. Significant differences among flexural strength were also determined between the same thicknesses of the zirconia materials at different sintering programs (p<0.05).