Effect of sintering parameters on phase evolution and strength of dental lithium silicate glass-ceramics

Comparative Invitro Testing of the Tensile Bond Strength Under Artificial Aging Between Different Lithium Disilicate Ceramics to Composite Substrate: A Novel Methodology

Objective The purpose of this study is to compare the tensile bond strength values to composite substrate pre- and post-aging between IPS E.max CAD and Initial LiSi. Methods The study utilized four blocks of IPS E.max CAD LT/B1 C14 (Ivoclar Vivadent, Liechtenstein, Germany) (referred to as E) and four blocks of Initial LiSi LT/B1 (GC, Tokyo, Japan) (referred to as L). These blocks were milled to produce 76 ceramic bars measuring 2 mm × 2 mm × 10 mm (E = 38, L = 38/n = 19). After acid etching with hydrofluoric acid (BISCO, Schaumburg, IL, USA) and silane application (BIS-SILANE, BISCO), the specimens were embedded in putty (Express STD, 3M, Decatur, AL, USA) to create a mold for the resin cement (RelyX U200, 3M). Subsequently, one group of each brand underwent mechanical tensile testing (E0 and L0), while the other groups were subject to tensile testing after artificial aging involving 500 thermal cycles between 5 and 55°C (E5 and L5). The mean tensile strength for each group (E0, E5, L0, and L5) was determined using the Brown-Forsythe one-way ANOVA and Tamhane's post hoc tests. Results Initial LiSi showed a superior pre-aging mean (11.7 MPa). However, both materials had identical post-aging means (7.6 MPa). There were no statistically significant differences, except between the dependent Initial LiSi groups (L0-L5). Most failure modes were mixed (cohesive cement and adhesive). There were no cohesive failures on the cement side except in three specimens of Initial LiSi post-aging. Conclusion The tested conditions have shown that Initial Lisi exhibited the highest pre-aging mean; however, it exhibited inferior bond stability under aging conditions compared to IPS E.max CAD. Analyzing the microstructure before and after aging may provide insights into the greater decrease in bond strength observed in the Initial LiSi specimens.

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.

Biosafety and chemical solubility studies of multiscale crystal-reinforced lithium disilicate glass-ceramics

Lithium disilicate (Li2 Si2 O5 ) glass-ceramics are currently a more widely used all-ceramic restorative material due to their good mechanical properties and excellent aesthetic properties. However, they have a series of problems such as high brittleness and low fracture toughness, which has become the main bottleneck restricting its development. Therefore, in order to compensate for these shortcomings, we propose to prepare a reinforced glass-ceramics with better mechanical properties and to test the biosafety and chemical solubility of the material. Li2 Si2 O5 whiskers were synthesized by a one-step hydrothermal method, and multi-scale crystal-enhanced Li2 Si2 O5 glass-ceramics were prepared by reaction sintering. The biosafety of multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics was investigated by in vitro cytotoxicity test, rabbit pyrogen test, mice bone marrow micronucleus test, skin sensitization test, sub-chronic systemic toxicity test, and chronic systemic toxicity test. Additionally, the chemical solubility of multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics was investigated. The test results showed that the material was non-cytotoxic, non-thermogenic, non-mutagenic, non-sensitizing, and non-systemic. The chemical solubility, determined to be 377 ± 245 μg/cm2 , complied with the ISO 6872 standard for the maximum solubility of ceramic materials. Multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics' biosafety and chemical solubility met current normative criteria, and they can move on to mechanical property measurements (such as flexural strength test, fatigue life test, friction and wear property study, etc.) and bonding property optimization, which shows promise for future clinical applications.

Effect of sintering on the translucency of CAD-CAM lithium disilicate restorations: A comparative in vitro study

PURPOSE

The available independent data on the translucency of novel pre and fully sintered chairside computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate are limited. This comparative in vitro study evaluated the translucency degree of pre and fully sintered chairside CAD-CAM lithium disilicate crowns after optional, required, and additional firing processes.

MATERIALS AND METHODS

One hundred and five maxillary left central incisor crowns manufactured by three different CAD-CAM lithium disilicate brands shade A1 were assigned into seven groups as follows (n = 15): (1) Straumann n!ce without sintering; (2) Straumann n!ce with one additional sintering process; (3) Straumann n!ce with two additional sintering processes; (4) Amber Mill with one sintering process; (5) Amber Mill with two sintering processes; (6) IPS e.max CAD with one sintering process; and (7) IPS e.max CAD with two sintering processes. The translucency of all crowns was evaluated with a color imaging spectrophotometer. All statistical analyses were performed using statistical software. A standard level of significance was set at α < 0.05.

RESULTS

All the milled crowns presented different degrees of translucency, and additional sintering processes altered it. IPS e.max CAD with two (4.33 ± 0.26) and one (4.01 ± 0.15) sintering processes displayed the highest translucency, whereas Straumann n!ce with no sintering process provided the lowest value (2.82 ± 0.16).

CONCLUSIONS

The translucency of chairside lithium disilicate single-unit full-coverage restorations manufactured with subtractive technology was significantly influenced by the brand and the number of sintering processes. The traditional presintered IPS e.max CAD and the fully crystallized glass-ceramic Straumann n!ce considerably increased the translucency after one additional firing process, whereas Amber Mill decreased its translucency.

Evaluation of Glazing and Polishing Systems for Novel Chairside CAD/CAM Lithium Disilicate and Virgilite Crowns

OBJECTIVE

The purpose of this study was to evaluate the effectiveness of glazing, two zirconia, and two lithium disilicate polishing systems on surface roughness of a CAD/CAM lithium disilicate and virgilite ceramic with atomic force microscopy (AFM) and visual assessment performed by dental students and faculty.

METHODS AND MATERIALS

Sixty maxillary right central incisor crowns made of a novel chairside CAD/CAM lithium disilicate and virgilite (CEREC Tessera) were milled for glazing and polishing. The crowns were divided into six groups: no polishing/glazing provided (NoP/G); glazed (GZ); glazed and polished with Brasseler Dialite LD Lithium Disilicate (DiLD); glazed and polished with Meisinger Luster Lithium Disilicate (LuLD); glazed and polished with Brasseler Dialite ZR Zirconia (DiZR); and glazed and polished with Meisinger Luster Zirconia (LuZR). Surfaces were scanned with AFM to measure roughness (Ra) and root mean square roughness (Rq) and generate micrographs. Crowns were visually assessed by 10 dental students and 10 dental school faculty members to determine clinical acceptableness.

RESULTS

Glazing and all polishing kits significantly reduced Ra and Rq compared to no polishing/glazing. No significant Ra differences were found between glazing and all polishing kits (p>0.05). DiZR significantly reduced Rq compared to other groups (p<0.05). Visual assessment showed that GZ, LuLD, and DiZR were the most clinically acceptable crowns.

CONCLUSION

Polishing and glazing considerably improve the surface smoothness of maxillary central incisor crowns fabricated out of a chairside CAD/CAM lithium disilicate and virgilite ceramic. Altogether, zirconia polishing systems provided smoother and more clinically acceptable surfaces than the lithium disilicate kits.

Surface fractures in pre-crystallized and crystallized zirconia-containing lithium silicate glass-ceramics generated in ultrasonic vibration-assisted machining

Machining-induced surface fractures in ceramic restorations is a long-standing problem in dentistry, affecting the restorations' functionality and reliability. This study approached a novel ultrasonic vibration-assisted machining technique to zirconia-containing lithium silicate glass-ceramics (ZLS) and characterized its induced surface fracture topographies and morphologies to understand the microstructure-property-processing relations. The materials were processed using a digitally controlled ultrasonic milling machine at a harmonic vibration frequency with different amplitudes. Machining-induced surface fracture topographies were measured with a 3D white light optical profilometer using the arithmetic mean, peak and valley, and maximum heights, as well as the kurtosis and skewness height distributions, and the texture aspect ratios. Fracture morphologies were analysed using scanning electron microscopy (SEM). The surface fracture topographies were significantly dependent on the material microstructure, the mechanical properties, and the ultrasonic machining vibration amplitudes. Larger scale fractures with higher arithmetic mean, peak and valley heights, and kurtosis and skewness height distributions were induced in higher brittleness indexed pre-crystallized ZLS than lower indexed crystallized ZLS by conventional machining. Conchoidal fractures occurred in pre-crystallized ZLS while microcracks were found in crystallized state although brittle fractures mixed with localized ductile flow deformations dominated all machined ZLS surfaces. Ultrasonic machining at an ideal vibration amplitude resulted in more ductile removal, reducing fractured-induced peaks and valleys for both materials than conventional processing. This research demonstrates the microstructure-property-processing interdependence for ZLS materials and the novel machining technique to be superior to current processing, reducing fractures in the materials and potentially advancing dental CAD/CAM techniques.

Edge chipping damage in lithium silicate glass-ceramics induced by conventional and ultrasonic vibration-assisted diamond machining

OBJECTIVES

Diamond machining of lithium silicate glass-ceramics (LS) induces extensive edge chipping damage, detrimentally affecting LS restoration functionality and long-term performance. This study approached novel ultrasonic vibration-assisted machining of pre-crystallized and crystallized LS materials to investigate induced edge chipping damage in comparison with conventional machining.

METHODS

The vibration-assisted diamond machining was conducted using a five-axis ultrasonic high-speed grinding/machining machine at different vibration amplitudes while conventional machining was performed using the same machine without vibration assistance. LS microstructural characterization and phase development were performed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. Machining-induced edge chipping depths, areas and morphology were also characterized using the SEM and Java-based imaging software.

RESULTS

All machining-induced edge chipping damages resulted from brittle fractures. The damage scales, however, depended on the material microstructures; mechanical properties associated with the fracture toughness, critical strain energy release rates, brittleness indices, and machinability indices; and ultrasonic vibration amplitudes. Pre-crystallized LS with more glass matrix and lithium metasilicate crystals yielded respective 1.8 and 1.6 times greater damage depths and specific damage areas than crystallized LS with less glass matrix and tri-crystal phases in conventional machining. Ultrasonic machining at optimized amplitudes diminished such damages by over 50 % in pre-crystallized LS and up to 13 % in crystallized LS.

SIGNIFICANCE

This research highlights that ultrasonic vibration assistance at optimized conditions may advance current dental CAD/CAM machining techniques by significant suppression of edge chipping damage in pre-crystallized LS.

The effect of prolonged holding time on the mechanical property and microstructural property of lithium disilicate glass-ceramic

Repeat firing produces uncertainty about stabilizing lithium disilicate glass-ceramic (LDGC) material properties, even though prolonged holding time can enhance the mechanical property of LDGC during a single firing cycle. However, the effect of prolonged holding time and repeat firing on the mechanical property and microstructure of LDGC is not fully understood. In the present study, three groups of LDGC material were created: (i) extension of holding time (7 vs. 14 vs. 28 min) at 780-800 °C; (ii) holding time extension (7 vs. 14 min) and dual sintering at 800-820 °C, respectively; (iii) dual sintering with prolonged holding time (7 vs. 14 min) at 820-840 °C. The nano-indenter test revealed that prolonged holding time (14 and 28 min) promoted the enhancement of LDGC hardness and Young's modulus. X-ray photoelectron spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy confirmed that prolonged holding time increased and stabilized LD phase in LDGC, as well as induced residual compressive stress. Scanning electron microscopy showed that prolonged holding time increased LD crystal grains homogeneously and facilitated LDGC to form dense interlocking structure without enlarging crystal size grains significantly. In contrast, LDGC that dual sintered alone at 820-840 °C possessed inferior mechanical properties, coupled with heterogeneous crystal phases, residual tensile stress, and melted crystals grains in the porous microstructure. Interestingly, these deteriorated properties of LDGC caused by dual sintering alone could be counteracted by prolonging the holding time. Nevertheless, the LDGC materials displayed an excellent biocompatibility throughout the study. This study identified that prolonged holding time during repeated firing cycles stabilized LD phase and crystal grain size of LDGC, thus enhanced the mechanical properties, which provided a new insight to extend the repeat fired restoration longevity of LDGC. Graphical abstract.

Fracture Resistance of Sintered Monolithic Zirconia Dioxide in Different Thermal Units

The purpose of this study was to compare the fracture strength of monolithic zirconium dioxide subjected to a sintering process in two different furnaces: InFire HTC Speed and CEREC SpeedFire. Methods: Forty restorations were designed and machined using a computer aided design / computer aidded machine (CAD/CAM) system. The restorations were randomly assigned to two groups of 20 samples each, Group 1 for the SpeedFire furnace (fast sintering) and Group 2 for the InFire furnace (slow sintering). Each of the crowns was subjected to a maximum compression load recorded in Newtons (N) and a displacement control with a speed of 1 mm/min. Results: Group 1 presented an average of 1222.8 N and a standard deviation of 136.91 N. Group 2 obtained a mean of 1068.5 N and a standard deviation of 316.39 N. Conclusions: There were no significant differences between the two groups, and the mechanical strength of the material was not affected, which would imply a saving of clinical and laboratory time when performing rapid sintering on monolithic translucent zirconium dioxide restorations. However, rapidly sintered restorations have limited reliability.

Investigation of indentation size effect and R-curve behaviour of Li2O-SiO2 and Li2O-2SiO2 glass ceramics

Indentation size effect (ISE) and R-curve behaviour of Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics are investigated using micro-indentation and indentation-strength (IS) techniques, respectively. Vickers micro-indentations were applied on both materials at the load of 0.10-19.6 N to determine the load influence on the measured hardness. For the IS-measured fracture toughness, the load ranged from 1.96 to 19.6 N. The hardness decreased with increasing load by 20% and 18% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, indicating the ISE behaviour on both materials. The fracture toughness increased with the load by 27% and 59% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, signifying the R-curve behaviour. The ISE behaviour of both materials was analysed using the Meyer's, Hays-Kendall (HK), proportional specimen resistance (PSR), Nix-Gao (NG), modified PSR (MPSR) and elastic plastic deformation (EPD) models while the R-curve behaviour was analysed by the fractional power law. The Meyer's index of both materials was less than 2, strongly confirming the ISE existence. The HK, PSR and NG models were only suitable to determine intrinsic Vickers hardness for Li₂O-2SiO₂ glass ceramic while the MPSR and EPD models were successful for both materials. The fractional power law gave higher R-curve steepness for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics. Also, material and brittleness indices predicted, respectively, higher quasi-plasticity and better machinability for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics indicating superior performance in the former to the latter. Finally, this study presents a new significant insight into the micro-mechanisms of fracture tolerance behaviour of these glass ceramics which is critical to their functional performance as structural ceramics.

Resistance to Fracture of Lithium Disilicate Feldspathic Restorations Manufactured Using a CAD/CAM System and Crystallized with Different Thermal Units and Programs

The aim of this study was to determine the resistance to fracture of feldspathic restorations with lithium disilicate and crystallized with different ovens and programs. Methods: Sixty monolithic restorations (LD) (EMAX CAD™ LT, Ivoclar-Vivadent™) were designed with the same parameters and milled with a CAD/CAM system (CEREC SW 5.1, CEREC MCXL, Dentsply-Sirona™, Bensheim). Each restoration was randomly assigned by randomization software (RANDNUM) to one of the three groups: (a) (NF) Oven P310 (Ivoclar, Vivadent) normal crystallization program, (b) (FF) Ivoclar P310 oven (Ivoclar-Vivadent™) rapid crystallization program, or (c) (SF) SpeedFire oven (Dentsply-Sirona™). Results: There were statistically significant differences between the groups (ANOVA, p < 0.05). The NF and FF groups showed the highest values of resistance to fracture, with statistically significant differences with the SF group. Conclusions: Using a furnace from the same dental company with predetermined programs from the material manufacturer, as well as using a predetermined program for rapid crystallization, has no effect on fracture resistance, and would save clinical time when performing ceramic restorations with lithium disilicate, while keeping their mechanical properties.

Surface Characterization and Optical Properties of Reinforced Dental Glass-Ceramics Related to Artificial Aging

The development of various dental glass-ceramic materials and the evolution of novel processing technologies lead to an essential change in the clinical and technical workflow. The long-term success of a dental restoration treatment is defined by its durability, which is directly influenced by the oral environment. This study's purpose was to evaluate the artificial aging behavior of nanostructured, respective microstructured ceramics related to surface topography, roughness, and optical properties. Six monolithic restoration materials were selected: milled lithium disilicate glass-ceramic (LDS-M) MT (medium translucency), hot-pressed lithium disilicate glass-ceramic (LDS-P) MT and HT (high translucency), milled zirconia-reinforced lithium silicate ceramic (ZLS-M) MT and hot-pressed zirconia-reinforced lithium silicate ceramic (ZLS-P) MT and HT, resulting n = 96 surfaces. All the samples were artificially aged by thermal cycling, and all investigations were made before and after thermal cycling. In terms of optical properties, differences recorded between ZLS and LDS ceramics are not significant. Thermal cycling increases the translucency of ZLS and LDS glass-ceramic materials significantly, with the most harmful effect on the pressed and polished samples. Micro- and nano roughness are significantly influenced by in vitro aging and a negative correlation was recorded. Glazed samples are characterized by significant rougher surfaces for all types of materials. On nanolevel, ZLS materials are significantly smoothed by thermal cycling.

Glass-Ceramics in Dentistry: A Review

In this review, we first briefly introduce the general knowledge of glass–ceramics, including the discovery and development, the application, the microstructure, and the manufacturing of glass–ceramics. Second, the review presents a detailed description of glass–ceramics in dentistry. In this part, the history, property requirements, and manufacturing techniques of dental glass–ceramics are reviewed. The review provided a brief description of the most prevalent clinically used examples of dental glass–ceramics, namely, mica, leucite, and lithium disilicate glass–ceramics. In addition, we also introduce the newly developed ZrO₂–SiO₂ nanocrystalline glass–ceramics that show great potential as a new generation of dental glass–ceramics. Traditional strengthening mechanisms of glass–ceramics, including interlocking, ZrO₂–reinforced, and thermal residual stress effects, are discussed. Finally, a perspective and outlook for future directions in developing new dental glass–ceramics is provided to offer inspiration to the dental materials community.