Effect of Multiple Firing Cycles on Shear Bond Strength Between Zirconia and Lithium Disilicate

BACKGROUND Indirect ceramic restorations often need multiple firings to match the shade of natural teeth or need after-correction and ceramic addition during the clinical trial stage. Many studies have examined how multiple firings affect the mechanical characteristics of zirconia-veneered prostheses. The effect of firing number on adhesion between these core and heat-pressed lithium disilicate veneering ceramics is unclear. This study aimed to evaluate and compare the effects of repeat firings on the planar interface shear bond strength (SBS) using the Schmitz-Schulmeyer test between zirconia core and veneering heat-pressed lithium disilicate. MATERIAL AND METHODS Forty standard-sized rectangular zirconia framework specimens were fabricated using the Schmitz-Schulmeyer test, a test to assess bonding quality, and categorized into 4 groups: control, 1 firing cycle, 2 firing cycles, and 3 firing cycles. The additional firing cycles aimed to simulate clinical scenarios. SBS was tested using a universal testing machine. RESULTS The study found that mean SBS values were similar across experimental groups, with the control group showing slightly higher values. Analysis using ANCOVA revealed no significant difference in mean SBS between groups, but maximum load significantly influenced SBS. Regression analysis and Pearson correlation confirmed the significant relationship between maximum load and SBS (P<0.0001). CONCLUSIONS There was no apparent effect on the SBS from increasing the firing from 1 to 3 cycles, indicating that the study's hypothesis was not supported. The mode of failure, which was largely adhesive, did not tend to vary as the number of firing cycles rose, despite the fact that some samples in all groups showed mixed failure.