Glass science behind lithium silicate glass-ceramics

OBJECTIVES

Lithium silicate-based glass ceramics have evolved as a paramount restorative material in restorative and prosthetic dentistry, exhibiting outstanding esthetic and mechanical performance. Along with subtractive machining techniques, this material class has conquered the market and satisfied the patients' needs for a long-lasting, excellent, and metal-free alternative for single tooth replacements and even smaller bridgework. Despite the popularity, not much is known about the material chemistry, microstructure and terminal behaviour.

METHODS

This article combines a set of own experimental data with extensive review of data from literature and other resources. Starting at manufacturer claims on unique selling propositions, properties, and microstructural features, the aim is to validate those claims, based on glass science. Deep knowledge is mandatory for understanding the microstructure evolution during the glass ceramic process.

RESULTS

Fundamental glass characteristics have been addressed, leading to formation of time-temperature-transformation (TTT) diagrams, which are the basis for kinetic description of the glass ceramic process. Nucleation and crystallization kinetics are outlined in this contribution as well as analytical methods to describe the crystalline fraction and composition qualitatively and quantitatively. In relation to microstructure, the mechanical performance of lithium silicate-based glass ceramics has been investigated with focus on fracture strength versus fracture toughness as relevant clinical predictors.

CONCLUSION

Fracture toughness has been found to be a stronger link to initially outlined manufacturer claims, and to more precisely match ISO recommendations for clinical indications.

Speed-sintering and the mechanical properties of 3-5 mol% Y2O3-stabilized zirconias

Ever faster workflows for the fabrication of all-ceramic restorations are of high economic interest. For that purpose, sintering protocols have been optimized for use in modern sintering furnaces, the so-called speed-sintering. However, conventional furnaces are still the most widely used equipment to sinter zirconia restorations. In this in-vitro study, we evaluated the feasibility of a speed-sintering protocol using a conventional sintering furnace to sinter different dental zirconias (stabilized with 3 mol% up to 5.4 mol% Y2O3) in comparison to a conventional sintering program. The properties evaluated were Young's modulus, Poisson's ratio, density, biaxial flexural strength, and fracture toughness. We show here that despite differences being dependent on material, the physical and mechanical properties of speed-sintered zirconia are comparable to those obtained by the conventional sintering.

Fatigue Threshold R-Curves for Dental Lithium Disilicate Glass-Ceramics

Chemical and mechanical fatigue degradation in ceramic materials is generally inconspicuous yet ubiquitous, to the effect that clinical fractures still consist of the main cause of failure in all-ceramic restorations. Implications of this span wide, from a reduced survival prognosis for the affected teeth, including more frequent and increasingly invasive procedural interventions, to the financial burden borne by individuals and health care systems. To suffice as an effective corrective, restoration lifetimes need only to be extended so to outlive the patient. That opens a box of problems from a materials science standpoint, entailing inherent deficiencies of brittle materials to resist mechanical and environmental challenges. Efforts in developing more damage-tolerant and fatigue-resistant restoratives go thus hand in hand with understanding intrinsic mechanisms of crack growth behavior under conditions that simulate the oral environment. Here we developed experiments using size-relevant sharp precracked specimens with controlled size and geometry (truncated semielliptical crack in the surface-crack-in-biaxial-flexure method) to establish a relationship between crack size and strength. The tangent method was used to construct envelopes for the quasi-static resistance curves (R-curves), which served as template for deriving residual cyclic R-curve analogs. By means of experimentally obtained stress-cycle curves, lifetime and fatigue parameters were employed within a mechanistic framework to reveal constitutive toughening mechanisms during subcritical growth under cyclic loading in a wet environment. Using 3 modern dental lithium disilicate glass-ceramics, we demonstrate the extent of R-curve degradation up to a threshold of 10 million cycles (~30 y in service) and draw parallels between the scope of fatigue degradation and the size of the microstructural units responsible for toughening mechanisms in glass-ceramic materials. Our results indicate that larger microstructural elements endow glass-ceramics with a higher reaching quasi-static R-curve at the onset but degrading more rapidly to comparable levels of lithium disilicates having submicrometric and nanometric crystal phases.

On the assignment of quartz-like LiAlSi2O6 - SiO2 solid solutions in dental lithium silicate glass-ceramics: Virgilite, high quartz, low quartz or stuffed quartz derivatives?

OBJECTIVES

Here we aim to provide a background on X-Ray Diffraction analysis of quartz-like crystal structures with varying amounts of Al³⁺ and Li⁺ substitution, existing confusions on their nomenclature and its implications for novel lithium silicate glass-ceramics.

METHODS

We reviewed the literature dealing with modifications of the quartz crystal structure and their stuffed LiAlSi₂O₆ derivates, LiAlSi₂O₆ - SiO₂ solid solutions, the terminology of such phases and criteria used to define the structure known as virgilite. Based on this information, we attempted to allocate the quartz-like phases found in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill in the range of LiAlO₂ - SiO₂ solid solutions. For this purpose, their lattice parameters obtained from Rietveld refinement were compared with the lattice parameters of members of the corresponding solid solutions with defined SiO₂ molar fraction found in the literature.

RESULTS

Based on the lattice parameters available for low quartz, high quartz and its stuffed derivatives, including LiAlSi₂O₆ and the mineral virgilite, a plot of the a- and c-parameters vs. the mol% SiO₂ related to LiAlO₂ was constructed with the literature data and the data found for the three dental lithium silicates. As per the definitions of virgilite as either Li_xAl_xSi_3-xO₆, with 0.5 < x < 1 or especially as members of the LiAlSi₂O₆ - SiO₂ solid-solution series with more than 50 mol% LiAlSi₂O₆, the crystal structures in CEREC Tessera^TM, Initial^TM LiSi Block and Amber® Mill failed to fall within the ranges of mol% SiO₂ confined for virgilite.

SIGNIFICANCE

Based on available literature and definitions, the quartz-like phases found in the three dental lithium silicates should be addressed as stuffed (probably low) quartz solid solutions instead of "virgilite". However determined by mineralogical practices, the term "virgilite" for parts of the LiAlSi₂O₆ - SiO₂ solid solution is ambiguous and can be considered as arbitrary.

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® CAD, IPS e.max® Press, Celtra® Duo, Suprinity® PC, Initial™ LiSi Press, Initial™ LiSi Block, Amber® Mill, Amber® Press, N!CE®, Obsidian® and CEREC Tessera™. The chemical composition of their base glasses was measured by X-Ray Fluorescence Spectroscopy (XRF) and Inductive Coupled Plasma Optical Emission Spectroscopy (ICP-OES), 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 field-emission 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 ½ 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® CAD were the only material showing micrometric phases, with N!CE® and Initial™ 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√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 nanometric range.

Toughening by revitrification of Li2SiO3 crystals in Obsidian® dental glass-ceramic

As a predominantly lithium-metasilicate-containing glass-ceramic, Obsidian® (Glidewell Laboratories, USA) has a peculiar composition and microstructure among other dental lithium silicates, warranting an evaluation of the crystallization process to establish relationships between microstructural evolution and mechanical properties. Blocks of the pre-crystallized material were processed into slices measuring 12 × 12 × 1.5 mm³ and subjected to the mandatory crystallization firing by interruption the heating ramp at temperatures between 700 °C and 820 °C (dwell time between 0 min and 10 min). The crystallization peaks of the base and the pre-crystallized glass were obtained by differential scanning calorimetry (DSC). The coefficient of thermal expansion and the glass transition temperature were derived from differential thermal analysis (DTA). X-ray diffraction (XRD) was performed to quantify and characterize the crystal phase fraction, whose microstructural changes were visualised using FE-SEM. The ball-on-three-balls surface crack in flexure method was used to track the evolution of fracture toughness. The microstructural evolution during crystallization firing was characterized by two regimes of growth: (i) the progressive revitrification (dissolution) of the 5 μm-sized Li₂SiO₃ polycrystals manifested at the boundaries of nanometric single coherent scattering domains (CSDs); (ii) the non-isothermal period is marked by an Ostwald ripening process characterized by the growth of the single crystalline structures into 0.5 μm polycrystals. The decrease in the crystal fraction of Li₂SiO₃ crystals from 41 vol.% to 37 vol.% is accompanied by the formation of a small amount of Li₃PO₄ (6 vol.%), maintaining the total crystal phase fraction mostly constant. The K_Ic accompanied the reverse trend of crystallinity, departing from 1.63 ± 0.02 MPa√m at the pre-crystallized stage to 1.84 ± 0.06 MPa√m after 10 min at 820 °C in a linear trend. Toughening appeared counter-intuitive in view of the decreasing crystal fraction and size, to rather relate to the relaxation of the residual stresses in the interstitial glass due to the spheroidization of the initially anisotropic, elongated Li₂SiO₃ crystals into round, nearly equiaxed particles, as let suggest from the disappearance of the extensive microcracking.

Inner marginal strength of CAD/CAM materials is not affected by machining protocol

Purpose

Here we aimed to compare two machining strategies regarding the marginal strength of CAD/CAM materials using a hoop-strength test in model sphero-cylindrical dental crowns, coupled with finite element analysis.

Materials and Methods

Five CAD/CAM materials indicated for single posterior crowns were selected, including a lithium disilicate (IPS e.max^® CAD), a lithium (di)silicate (Suprinity^® PC), a polymer-infiltrated ceramic scaffold (Enamic^®), and two indirect resin composites (Grandio^® Blocs and Lava™ Ultimate). A sphero-cylindrical model crown was built on CAD Software onto a geometrical abutment and machined using a Cerec MC XL system according to the two available protocols: rough-fast and fine-slow. Specimens were fractured using a novel hoop-strength test and analyzed using the finite element method to obtain the inner marginal strength. Data were evaluated using Weibull statistics.

Results

Machining strategy did not affect the marginal strength of any restorative material tested here. Ceramic materials showed a higher density of chippings in the outer margin, but this did not reduce inner marginal strength. IPS e.max^® CAD showed the statistically highest marginal strength, and Enamic^® and Lava™ Ultimate were the lowest. Grandio^® Blocs showed higher performance than Suprinity^® PC.

Conclusions

The rough-fast machining strategy available in Cerec MC XL does not degrade the marginal strength of the evaluated CAD/CAD materials when compared to its fine-fast machining strategy. Depending on the material, resin composites have the potential to perform better than some glass-ceramic materials.