Effect of Repeated Firings on Biaxial Flexural Strength of Different CAD/CAM Lithium Disilicate Reinforced Materials in Two Different Thicknesses

Abstract

Purpose: To evaluate the biaxial flexural strength of four CAD/CAM lithium disilicate reinforced glass ceramic; IPS e.max CAD (EX) and Amber Mill (AM) as “lab-side”, and Initial LiSi Block (LS) and n!ce (NC) as “chair-side” according to the effect of two thicknesses and repeated firings. Material and methods: CAD/CAM blocks were prepared according to ISO 6872-2015. Four hundred and eighty (n=480) discs were prepared in total with a diameter of 12.00 mm (±0.02). Each selected material (n=120) was divided into two thicknesses: 1.00 mm (±0.03) and 0.5 mm (±0.02), having 60 discs for each thickness in each material. The specimens were subdivided according to firings: baseline (BL), one firing (1F), three firings (3F), and five firings (5F). The firings cycles were performed according to the manufacturers’ instructions. The biaxial flexural strength test (piston-on-three-ball) was performed according to ISO 6872-2015 using a universal testing machine. Biaxial flexural strength data were analyzed using parametric tests: three-way and one-way ANOVA (α=0.05) with Bonferroni post-hoc test. Weibull analysis was used to calculate the Weibull Modulus and Characteristic Strength to create Weibull plots and likelihood contour plots. Results: The biaxial flexural strength of the materials differed from each other (EX=AM>NC>LS). A significant difference was found between the firings, regardless of the thickness, and the general ranking of firings was (3F>5F>1F>BL). Higher thickness (1.00 mm) presented a higher biaxial flexural strength value. Higher Weibull modulus and characteristic strength values were observed with lab-side vs. chair-side materials. Conclusions: Repeated firings significantly affected the biaxial flexural strength of EX, AM, LS, and NC CAD/CAM lithium disilicate materials. The biaxial flexural strength increased with increased thickness. Lab-side materials (EX and AM) have a lower probability of failure than chair-side materials (LS and NC).