Grasping the Lithium hype: Insights into modern dental Lithium Silicate glass-ceramics

Objectives: Lithium-based glass-ceramics are currently dominating the landscape of dental restorative ceramic materials, with new products taking the market by storm in the last years. Though, the difference among all these new and old products is not readily accessible for the practitioner, who faces the dilemma of reaching a blind choice or trusting manufacturers' marketing brochures. To add confusion, new compositions tend to wear material terminologies inherited from vanguard dental lithium disilicates, disregarding accuracy. Here we aim to characterize such materials for their microstructure, crystalline fraction, glass chemistry and mechanical properties. Methods: Eleven commercial dental lithium-based glass ceramics were evaluated: IPS e.max (R) CAD, IPS e.max (R) Press, Celtra (R) Duo, Suprinity (R) PC, InitialTM LiSi Press, InitialTM LiSi Block, Amber (R) Mill, Amber (R) Press, N!CE (R), Obsidian (R) and CEREC TesseraTM. The chemical composition of their base glasses was measured by X-Ray Fluorescence Spectroscopy (XRF) and Inductive Coupled Plasma Optical Emission Spectroscopy (ICPOES), as well as the composition of their residual glass by subtracting the oxides bound in the crystallized fraction, characterized by X-Ray Diffraction (XRD) and Rietveld refinement, and quantified accurately using the G-factor method (QXRD). The crystallization behavior is revealed by differential scanning calorimetry (DSC) curves. Elastic constants are provided from Resonant Ultrasound Spectroscopy (RUS) and the fracture toughness measured by the Ball-on-Three-Balls method (B3B- K Ic). The microstructure is revealed by fieldemission scanning electron microscopy (FE-SEM). Results: The base glasses showed a wide range of SiO2 /Li2O ratios, from 1.5 to 3.0, with the degree of depolymerization dropping from 1/2 to 2/3 of the initial connectivity. Materials contained Li2SiO3+Li3PO4, Li2SiO3+Li3PO4+Li2Si2O5, Li2Si2O5+Li3PO4+ Cristobalite and/or Quartz and Li2Si2O5+Li3 PO4+LiAlSi2O6, in crystallinity degrees from 45 to 80 vol%. Crystalline phases could be traced to their crystallization peaks on the DSC curves. Pressable materials and IPS e.max (R) CAD were the only material showing micrometric phases, with N!CE (R) and InitialTM LiSi Block showing solely nanometric crystals, with the rest presenting a mixture of submicrometric and nanometric particles. Fracture toughness from 1.45 to 2.30 MPa root m were measured, with the linear correlation to crystalline fraction breaking down for submicrometric and nanometric crystal phases. Significance: Dental lithium-based silicate glass-ceramics cannot be all put in the same bag, as differences exist in chemical composition, microstructure, crystallinity and mechanical properties. Pressable materials still perform better mechanically than CAM/CAM blocks, which loose resistance to fracture when crystal phases enter the submicrometric and na-nometric range (c) 2021 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

Automated summary

Lithium-based glass-ceramics are widely used in dental restorative materials. This study characterized the microstructure, crystalline fraction, glass chemistry, and mechanical properties of eleven commercial dental lithium-based glass ceramics. The results showed significant differences in chemical composition, microstructure, crystallinity, and mechanical properties among these materials. The findings provide valuable information for dentists to make informed choices.