Strength and Translucency of Lithia-Based Glass-Ceramic CAD/CAM Blocks Varied

Objectives: High-strength silica-based glass-ceramics dominate the indirect restoration market. These materials combine strength, esthetics, and ease of use. The initial commercial material was brought to market in 2005 (IPS e.max). Additional materials and manufacturing processes followed, but these are less documented, potentially leading to disparate patient-care outcomes. The purpose of this study was to investigate the strength and optical properties of a variety of heterogenous, high-strength silica-based materials.
Methods: Bars, 14x4x1 ± 0.2 mm, were milled from 5 commercially available CAD/CAM blocks, shade A2/HT (Table 1) (N=25). Milling was performed (Craft 5x, DOF) to simulate fabrication effects on restorations’ intaglio surfaces. After milling, the two-step CAD/CAM specimens were crystalized per manufacturer’s recommendations. Bars were hand-polished to 2,000 grit SiC abrasive paper. Three-point flexural strength was measured using a universal testing machine. Light transmission for 14x14x2 ± 0.1mm specimens, sectioned using a water-cooled slow-speed saw (Isomet, Buehler), was measured in spectrophotometer (X-rite Ci-7600). CIE L* a* b* color coordinates, whiteness index and contrast ratio were measured against white and black backgrounds for specimens 2 ± 0.1mm thickness (N=25). Means, standard deviations (SD), and coefficients of variance [CV] were calculated. One-way-ANOVA and multiple comparisons tests were used to identify differences in physical properties (P>0.05).
Results: One-way-ANOVAs indicated that differences among materials existed with respect to all physical properties measured: strength; CIE L, a, b, C, and h color spaces; whiteness index; contrast ratio; and absolute light transmission (p<0.05) (Tables 1 and 2). One material had a coefficient of variance for strength that was approximately an order of magnitude lower than for the other materials, indicating a high degree of predictability.
Conclusions: Five commercially available high-strength silica-based glass-ceramics differed in flexural strength, and in 8 optical properties even though they were all nominally of the same VITA shade and translucency. Caveat emptor.