Strength Verification of Novel Lithium Silicate Glass-Ceramic Restorative Materials

Objectives: A wide variety of strength values have been reported by different laboratories for lithium silicate glass-ceramic materials that recently became available for use in fixed dental prostheses. The purpose of this study was to perform a side-by-side comparison of strength distributions when tested according to the ISO 6872 standard.
Methods: Disk-shaped specimens (n=36, 14 mm OD, 1.4 mm thickness) were prepared from Cerec Tessera (T, Dentsply Sirona), Celtra Duo (CD, Dentsply Sirona), Initial LiSi Block (LS, GC), and e.max CAD (CAD, Ivoclar Vivadent). The disks were ground to 1.2 mm thickness under water irrigation using a 30-micron nickel-bonded diamond disk at 150 rpm and 200 g load and polished using 15-micron diamond plastic films at 250 rpm and 125 g load. The specimens were placed in a biaxial flexure fixture (piston-on-three-ball) and loaded at 1 mm/min until fracture. The load at fracture was recorded for each specimen, and the strength was calculated according to ISO 6872.
Results: The table shows the means (and standard deviations) for final dimensions and biaxial flexure strength as well as Weibull parameters (and their 95% confidence intervals) for each material. Thickness values were noted to be similar for all materials and were more tightly controlled than required by ISO 6872. A similar observation was made for the diameters. The strength data passed a normality test (Shapiro-Wilk, p=0.10) but failed an equal variance test (Brown-Forsythe, p<0.001), so one-way Kruskal-Wallis ANOVA followed by pairwise testing using Dunn’s test was used to compare the strength values between materials. Materials that share the same lowercase letters in the strength row of the table are not statistically different.
Conclusions: CAD showed the greatest strength and least variability in strength. T showed the most variability in strength and was statistically tied for lowest strength with LS.