Effect of Zirconia Core Thickness, Firing Cycle and Veneering Technique on Biaxial Flexural Strength of Veneering Porcelain in Porcelain Veneered Zirconia Restorations

Abstract

Aim: Evaluate the effect of different core thicknesses, firing cycles and veneering techniques on the biaxial flexural strength of veneering porcelain. Materials and methods: One core material, Vita In-Ceram YZ, and one porcelain veneer material, VITA VM9, were used in this study. Vita YZ zirconia blocks were sectioned and sintered to provide slides of 1.65, 3.25, and 6.50 mm thickness. Two techniques were used to fabricate VITA VM9 porcelain veneer; Hand Layered (HL) and Pressed (PR). The assemblies were sintered as per three different firing cycles. Biaxial flexural strength was determined using a universal mechanical tester. Statistical analysis was conducted for all tests using ANOVA and Tukey post hoc test at p ≤ 0.05. Results: In pooling all flexural strength values, for VITA VM9 porcelain, the highest value resulted by using 1.65 mm YZ core thickness (146.10 ± 23.08 MPa) while the lowest value was obtained using 6.50 mm thickness (140.91 ± 27.501 MPa). However, the difference was not statistically significant. The highest value resulted by using Cycle 2 (Heat Rate 20 C/ Min, Hold Time 2 Minutes) (154.34 ± 22.11 MPa), while the lowest value was measured using Cycle 1 (Heat Rate 55 °C/ Min, Hold Time 1 Minute) (127.42 ± 26.62 MPa), and the difference was statistically significant. In pooling all values of (HL) VITA VM9, the result was (142.66 ± 25.87) MPa which was lower than (PR) VITA VM9 (145.87 ± 25.56), but not by a statistically significant amount. Conclusion: The effect of different YZ core thicknesses and veneering techniques on the biaxial flexural strength of VITA VM9 veneering porcelain was not statistically significant. Different firing cycles resulted in a significant effect on the biaxial flexural strength of porcelain veneer. Clinical significance: The use of an appropriate firing cycle can help to enhance the biaxial flexural strength of porcelain veneer in zirconia-based restorations.