Zirconia-reinforced Lithium Silicate Glass-ceramic Research Articles

Investigation of Fracture Strength and SEM Images of Different CAD-CAM Materials Applied to Two Different Inlay Cavities

Introduction: This study aims to determine and compare the fracture strength and failure modes of zirconia-reinforced lithium silicate glass ceramics (ZLS) and yttria-stabilized zirconia-based ceramic MOD and MO inlay restorations. Materials and Methods: Stumps representing the maxillary second premolar were prepared using HyperDent software and CAD/CAM milling units. Thirty-two epoxy resin die models were obtained, with 16 samples in each group. Subsequently, restorations were fabricated using Vita Suprinity (VITA Zahnfabrik, Bad Sackingen, Germany) and IPS e.max ZirCAD CAD/CAM (Ivoclar et all., Liechtenstein) blocks to restore the inlay cavities. The specimens were subjected to aging and then tested for fracture using a universal testing machine. The resulting fractures were classified. Data normality was assessed using the Shapiro-Wilk test, and homogeneity of variances was evaluated using the Levene test. The interaction between restorative material type and cavity surface was tested using two-way ANOVA. Results: The fracture strength of IPS e.max ZirCAD material (mean value: 723.18±57.51) is higher than that of Vita Suprinity ZLS material (689.86±113.61), but this difference is not statistically significant (F=3.46, p=0.073). The group with 3-surface cavities in the tooth material (768.00±60.60) has significantly different fracture strength compared to the group with 2-surface cavities (645.037±71.20) (F=47.18, p<0.001). Conclusions: Having a 3-surface cavity may further enhance the fracture resistance of inlay restorations, and this difference is statistically significant. There is no significant difference in fracture strength among restorative materials.

Comparison of Mechanical Properties Between Zirconia-reinforced Lithium Silicate Glass-ceramic and Lithium Disilicate Glass-ceramic: A Literature Review

Lithium disilicate glass-ceramic (LDS) is increasingly being adopted for use in therapeutic restorative procedures. Concurrently, zirconia-reinforced silicate glassceramics (ZRS) are becoming broadly utilized in dental applications. The purpose of this study was to evaluate and compare the mechanical properties of zirconia-reinforced lithium silicate glass-ceramics and lithium disilicate-based glass-ceramics, with a focus on their application in CAD/CAM technologies. In this review, the researchers conducted a search of the PubMed (MEDLINE) database to identify studies related to LDS and ZRS. This search was limited to articles published in English over a seven-year period, from January 1, 2015, to December 31, 2022. Additional studies were sourced from Google Scholar and through manual exploration. Key published works were identified and included in the literature review. The findings concluded that ZRS exhibits superior mechanical properties, including higher flexural strength, fracture toughness, and hardness, compared to LDS. Furthermore, ZRS combines desirable esthetic qualities with robust mechanical strength, rendering it an excellent material for single tooth aesthetic restorations such as inlays, onlays, crowns, and veneers, applicable to both tooth and implant supports. Currently, there is a notable scarcity of data concerning the mechanical properties and clinical efficacy of ZRS. Therefore, it is imperative to conduct long-term clinical studies to verify the optical and mechanical properties, clinical applications, limitations, and long-term effectiveness of ZRS.

Interfacial fracture toughness of different surface treatments on zirconia-reinforced lithium silicate glass-ceramics.

This study investigated the influence of different surface treatments on unfiring or firing zirconia-reinforced lithium silicate (ZLS) glass-ceramics. Celtra Duo and IPS e.max CAD blocks were cut and process following manufacturer protocols. The specimen surface was treated with seven different protocols. Two ceramic blocks with the same surface treatment were bonded with luting agent and prepared for mini-interfacial fracture toughness tests (mini-iFT). The specimens were tested after 1-week storage. The data was statistically analyzed using two-way ANOVA and Dunnett's T3 comparison (α=0.05). The highest mini-iFT of both Celtra Duo unfired and fired was shown in the HF+S group, which was not significantly different from HF+S+UA. For IPS e.max CAD, the mini-iFT was higher in the groups treated with hydrofluoric acid. Additional adhesive after silane application did not significantly improve bonding effectiveness. Therefore, surface treatment with hydrofluoric acid and silane is recommended for both unfiring and firing ZLS glass ceramics.

The Wear Behavior of Glass-Ceramic CAD/CAM Blocks against Bovine Enamel

The wear of enamel and crown restorative materials often occur by occlusion. The purpose of this study was to evaluate the wear volume between glass-ceramics used for CAD/CAM blocks (lithium disilicate: Initial LiSi block (LIS), IPS e.max CAD (IPS), zirconia-reinforced lithium silicate glass-ceramics: Celtra DUO (DUO), VITA Suprinity (VITS) and feldspar-based glass-ceramics: Vitablocs Mark II (MAK)) and bovine tooth enamel using a two-body wear test, the hardness, three-point bending strength, micro-structure and the element components of glass-ceramics. The data were analyzed using a one-way analysis of variance and Tukey’s multiple comparison test (α = 0.05). IPS and DUO with relatively large size crystal gain had significantly larger abrader wear volumes. Zirconia-reinforced lithium silicate glass-ceramics (DUO, VITS) caused significantly greater wear volume in antagonist enamel. MAK with scale-shape crystals grains produced distinct scratches after wear tests, both in the material itself and in the enamel. A strong correlation between the mechanical properties (hardness, three-point bending strength) and wear volume could not be confirmed. The type of glass-ceramic, size, and shape of the crystal grains affected the wear behavior of the glass-ceramics for CAD/CAM blocks. Therefore, dentists should consider that wear behavior varies with crystal structure, size, and shape in glass-ceramics for CAD/CAM blocks.

Effects of gastric acid and mechanical toothbrushing in CAD-CAM restorative materials: Mechanical properties, surface topography, and biofilm adhesion

ObjectiveTo evaluate the effects of simulated gastric acid erosion combined with mechanical toothbrushing abrasion on the mechanical properties, surface topography, and biofilm adhesion of different CAD/CAM materials. Material and methodsSpecimens of zirconia-reinforced lithium silicate glass-ceramic (ZLS), polymer-infiltrated ceramic network (PICN), feldspathic glass-ceramic (FE), and two nanoceramic resins (RK, RG), were submitted to the following challenges: erosion (E), abrasion (A), erosion combined with abrasion (E + A), or remained untreated (control - C). After challenges, flexural strength was evaluated, while microhardness (KHN) and surface roughness (Ra) were tested before and after treatments. The biofilm adhesion (Streptococcus mutans ATCC 700610, Streptococcus sanguinis ATCC 10556 e Candida albicans MYA 2876) was determined by the counting of colonies forming units per milliliters (UFC/mL) after erosive and abrasive challenges. ResultsFE showed the lowest flexural strengths, while ZLS and RG exhibited the highest, while PICN and RK, had intermediate values. PICN, ZLS, and FE showed lower microhardness after E and E + A challenges than polymer-based materials (RG and RK). FE surface roughness increased after E and E + A challenges and after A and E + A challenges for RK. Biofilm formation after erosive/abrasive challenges was higher on ZLS than FE, RK, and RG, but no different than PICN. RK and RG exhibited the lowest biofilm formation among the groups. Furthermore, E + A challenges held significant changes in the surface of the materials, which were more severe on the surface of glass ceramics and hybrid materials. ConclusionErosive challenges combined with abrasion negatively influenced the mechanical properties and surface topography of most CAD/CAM materials and increased the biofilm adhesion on ZLS. Besides, the severity of the damage is related to the type and composition of each material.

Lithium disilicate and zirconia reinforced lithium silicate glass-ceramics for CAD/CAM dental restorations: biocompatibility, mechanical and microstructural properties after crystallization

ObjectivesThe objective of this study was to define the impact of heating rate on the crystal growth, the mechanical properties, and the biocompatibility of three different kinds of CAD/CAM glass-ceramics treated with a conventional furnace. MethodsLithium disilicate (IPS EMax-CAD, Ivoclar Vivadent) (LS2) and two zirconia reinforced lithium silicate (ZLS) ceramics (Vita Suprinity PC, VITA Zahnfabrik; Celtra Duo, Dentsply Sirona) (ZLSS; ZLSC) were used. The mechanical properties and the crystal growth were evaluated on 42 specimens (n = 14 per group). The thermal treatments recommended by the manufacturers were carried out. All groups were tested for fracture toughness (Ft) and Vickers hardness (Hv). Scanning electron microscope (SEM) images were taken after a slight surface etching with hydrofluoric acid solution (1% for 20 s). Differential Thermal Analysis (DTA) was performed and cellular adhesion with human periodontal ligament stem cells (hPDLSCs) culture was qualitatively assayed. Data were analyzed with Repeated Measurements ANOVA and ANOVA followed by Tukey post hoc test. ResultsThe crystals’ mean size (±SD) after heat treatment was 1650.0 (±340.0) nm for LS2, 854.5 (±155.0) nm for ZLSS and 759.9 (±118.4) nm for ZLSC (p < 0.05 among the groups). As consequence of crystallization, the Hv was 6.1 ± 0.3 GPa for LS2, 7.6 ± 0.7 GPa for ZLSS and 7.1 ± 0.5 GPa for ZLSC (p < 0.05 for LS2 vs ZLSS and ZLSC), while the Ft was 2.2 ± 0.1 MPa m1/2 for LS2, 4.7 ± 0.8 MPa m1/2 for ZLSS and 3.8 ± 0.6 MPa m1/2 for ZLSC (p < 0.05 among the groups). The DTA curves showed a crystallization process for LS2, ZLSS and ZLSC at a temperature range 810–840 °C. The amount of adherent hPDLSCs was superior on LS2 than on ZLS. ConclusionsAll the CAD/CAM materials can be properly crystallized if heat treated following the manufacturers’ instructions. The crystallization process highly depends on temperature. ZLS glass ceramics show significantly inferior crystals dimensions and higher fracture toughness and Vickers hardness than LS2 ceramic. hPDLSCs cultured on LS2 have a superior adhesion than those cultured on ZLS. Clinical significanceThe value of this study relies on the demonstration that a proper heat-treatment of CAD/CAM lithium disilicate and ZLS glass ceramics generates products that are suitable for clinical use . The differences highlightable in mechanical properties and biocompatibility behavior do not affect their successful clinical application.

The effect of thermocycling on microhardness and surface roughness of two zirconia reinforced lithium silicate glass-ceramics

Nowadays, dental glass-ceramics are experiencing remarkable development in terms of aesthetics and hardness. New materials with high fracture resistance were developed and can be obtained by milling ceramic blocks or by heat-pressing prefabricated ceramic ingots.This paper aims to evaluate the effect of thermocycling (10,000 cycles) on the microhardness and roughness of two zirconia reinforced lithium silicate glass-ceramics (ZLS) obtained using two different processes, milling, and heat-pressing ceramic.The tested ZLS ceramics behaved differently to the aging and surface treatments. The milled ZLS ceramic was more affected by the aging process. Glazed samples presented higher surface roughness and higher microhardness compared to the polished samples before and after thermocycling.

Influence of Conventional Versus Digital Workflow on Marginal Fit and Fracture Resistance of Different Pressable Occlusal Veneers After Thermomechanical Fatigue Loading

Objectives: The aim of the study was to evaluate the marginal fit and fracture resistance after thermomechanical fatigue of two types of glass ceramic occlusal veneers constructed using two techniques for wax pattern. Methods: Thirty-two non-carious maxillary molars were used in this study. Teeth were prepared to receive all ceramic occlusal veneers. Prepared teeth were randomly divided into two equal groups according to the fabrication material of the occlusal veneer: Group I (e-max): pressable lithium disilicate glass ceramics. Group II (Celtra): pressable zirconia-reinforced lithium silicate glass ceramics. Each group was further subdivided into two equal subgroups according to the technique of construction of the wax pattern: Subgroup A (CAD/CAM): CAD/CAM technique. Subgroup B (Conventional): Conventional technique. Wax patterns were sprued, invested and heat pressed to produce the two types of ceramic occlusal veneers. All occlusal veneers were cemented to their corresponding teeth using adhesive resin cement. Thermomechanical aging was performed on all the samples. Vertical marginal gap distance measurement and fracture resistance testing were done. Results: The results of this study revealed that e.max (Conventional) recorded the highest vertical marginal gap mean values followed by Celtra (Conventional) then Celtra (CAD/CAM) while e.max (CAD/CAM) recorded the lowest mean values. It was found that e.max (conventional) recorded the highest fracture resistance mean values followed by Celtra (conventional) then e.max processed by (CAD/CAM) while Celtra processed by (CAD/CAM) recorded the lowest mean values. Conclusions: CAD/CAM constructed wax pattern presents a viable alternative option to conventional wax pattern. Celtra Press material revealed promising results.

Effect of surface finishing protocols on color and translucency of zirconia reinforced lithium silicate glass-ceramic after thermo-mechanical aging and different staining solutions

Objectives: To investigate the color stability and translucency of zirconia-reinforced lithium silicate glass-ceramic (ZLS) prepared with different surface finishing procedures after thermo-mechanical aging and immersion in different beverages. Sixty samples were fabricated from heat-pressed and milled ZLS. All samples were finished and divided into three groups (n=20), according to the finishing methods; (CD) polished and fired, (CD) glazed and fired, and (CP) glazed and power fired. The specimens were subjected to aging using chewing simulator integrated with thermocycling protocol. Then, each group was divided into 4 equal subgroups (n = 5) according to exposure to immersion medium (coffee, tea, Coca-Cola and distilled water). Color and translucency parameter (TP) were measured according to CIELAB color space using spectrophotometer at baseline and after subjecting the specimens to aging and different staining solutions. Changes in color (ΔE) and TP (ΔTP) were calculated. The data was analyzed using the analysis of variance (ANOVA) and Tukey’s post hoc test (p < 0.05). Aging and staining significantly affected the baseline color and decreased TP of all specimens. Generally, regarding the tested groups, CD polished showed the highest ΔE and ΔTP (p <0.05). Coffee had the most significant effect on ΔE (p < 0.05). For each staining solution subgroup, polished CD had the highest ΔE. Conclusions: Aging and staining beverages had a significant effect on color stability and decreased translucency of all surface finished ZLS. Polished celtra duo showed the highest ΔE and ΔTP. Coffee had the most significant effect on the color change of all surface finished ZLS.

In Vitro Study of Comparative Evaluation of Marginal and Internal Fit between Heat-Pressed and CAD-CAM Monolithic Glass-Ceramic Restorations after Thermal Aging.

The accuracy of newly developed ceramic materials is still being studied. Marginal and internal adaptation are known factors that have an essential impact on the long term success of dental restorations. The aim of this in vitro study was to evaluate the marginal and internal fit of heat-pressed and milled monolithic glass-ceramic restorations based on their ceramic type, processing technique, and in vitro thermocycling. Thirty-two crowns were studied and divided into four groups (n = 8), according to the ceramic material (feldspathic glass-ceramic (F) and zirconia reinforced lithium silicate glass-ceramic (ZLS)) and to their technological obtaining processes (milling (M) and heat-pressing (P)). A typodont preparation was scanned with a D2000 3D scanner to obtain identical 32 resin 3D-printed abutment teeth. Marginal and internal gaps were measured using the silicone replica technique under 40× magnification. The crowns were further cemented and thermally aged for 10,000 cycles After cementation and thermocycling of the samples, marginal and internal gaps were assessed using micro-CT (micro-computed tomography)) analysis. Data were statistically analyzed using statistical tests. Significant differences were found before and after cementation and thermocycling among the tested materials (p < 0.05). Related to technological processing, significant differences were seen in the marginal area between FP and FM (p < 0.05) Significant differences were also found in the axial and occlusal areas between the ZLSP and ZLSM. Thermocycling and cementation did not have a significant effect on the tested materials (p < 0.05). The technological processes influenced the marginal and internal fit of the crowns in favor of the CAD/CAM (computer aided design/computer aided manufacturing)technologies. Thermal aging had little effect on marginal adaptability; it increased the values for all the tested samples in a small way, but the values remained in their clinically acceptable range for all of the crowns.

Load effect on the mechanical behaviour of zirconia-reinforced lithium silicate glass ceramics

The mechanical behaviour of pre-crystallized and crystallized zirconia-reinforced lithium silicate glass ceramics (ZLS) using micro-indentation techniques is reported. Vickers micro-indentations were conducted on these materials at the load of 0.245 N–9.8 N and 4.9 N–9.8 N to determine the load effect on the hardness and fracture toughness, respectively. The hardness increased with the load by 78.2% (ANOVA, p < 0.05) and 163% (ANOVA, p < 0.05) on pre-crystallized and crystallized ZLS, respectively, indicating the occurrence of the reverse indentation size effect (RISE) on both materials. The fracture toughness increased with the load by 20% and 12.3% on pre-crystallized and crystallized ZLS, respectively. Hardness data of both materials were analyzed using the Meyer's, indentation-induced cracking (IIC), proportional specimen resistance (PSR), modified PSR, Hays–Kendall and elastic plastic deformation (EPD) models. The Meyer's and IIC models described the RISE behaviour of both materials satisfactorily. The PSR, Hays–Kendall and EPD models provided consistent load-independent hardnesses while the modified PSR model gave erratic load-independent hardnesses for both materials. The continuum model provided intrinsic compressive yield strengths of both materials which were consistent with those obtained from the EPD model and the Tabor relation, implying the EPD model as the most successful. The brittleness index predicted better machinability for crystallized ZLS than pre-crystallized ZLS. Finally, the variations of hardness and fracture toughness with load might account for the different micro-material removal mechanisms in the sharp abrasive machining of these materials.

Cementation of Tooth-colored Restorations - A Survey among Dentists in Germany.

Purpose: To survey dentists in Germany regarding their preferred cementation regimen for fixed dental prostheses (FDPs) fabricated from various restorative materials. Materials and Methods: An online survey was developed for completion by dentists in Germany between 08/2019 and 02/2020. The questionnaire gathered information about the cementation regimen (conventional, including the acid-base cements zinc-oxide phosphate or any glass-ionomer cement; self-adhesive; adhesive; don’t know) that the participant would preferably select for the insertion of single- or multi-unit FDPs fabricated from various restorative materials. Data were also collected on demographic and personal characteristics. Results: A total of 721 dentists completed the survey, and the data from 688 questionnaires were included in the analysis. Conventional cementation was the option of choice for restorations fabricated from alloy/porcelain-fusedto- metal among 91.0% of the participants and for restorations fabricated from polycrystalline ceramics among 42.9% of the participants. Adhesive cementation was preferred for restorations fabricated from feldspathic/leucitereinforced glass ceramics (77.9%), lithium-disilicate glass ceramics (67.5%), zirconia-reinforced lithium-silicate glass ceramics (42.0%), and CAD/CAM resin composites (63.8%). Uncertainties regarding the appropriate cementation protocol for restorations fabricated from CAD/CAM resin composites were reported by 15% of the participants. More than 20% of the participants selected a technique that is less or not at all recommended for the cementation of restorations fabricated from zirconia-reinforced lithium-silicate glass ceramic and CAD/CAM resin composites. Conclusion: The results of the survey suggest that dentists in Germany select the cementation regime depending on the restorative material. Some of the participating dentists selected cementation regimens that are less or not at all recommended for restorations fabricated from zirconia-reinforced lithium-silicate glass ceramic and CAD/CAM resin composites, or were not sure about the appropriate cementation protocol.

Effect of Three Commonly Consumed Beverages on Surface Roughness of Polished and Glazed Zirconia-Reinforced Lithium Silicate Glass Ceramics

Objectives: This study aimed to assess the effect of three commonly consumed beverages on surface roughness of polished and glazed zirconia-reinforced lithium silicate (ZLS) glass ceramics.&#x0D; Materials and Methods: In this experimental study, 104 rectangular specimens were cut from Vita Suprinity blocks with 2 mm thickness. After ultrasonic cleaning and firing of the specimens, they were finished and polished in two groups. Specimens in the first group were polished using a 2-step polishing kit while the second group specimens were glazed and heated in a porcelain firing oven according to the manufacturer’s instructions. Baseline surface roughness was measured using a profilometer. Specimens in each group were then randomly divided into 4 subgroups (n=13) for immersion in artificial saliva (control group), cola, orange juice and black tea. Surface roughness was measured again and data were analyzed using two-way ANOVA.&#x0D; Results: The highest and the lowest mean Ra were found in orange juice and saliva subgroups, respectively in both glazed and polished groups. The Ra values of both polished and glazed groups significantly increased after immersion in orange juice and cola (P&lt;0.05). The polished surfaces showed insignificantly higher surface roughness compared with glazed surfaces (P&gt;0.05).&#x0D; Conclusion: Orange juice and cola significantly increased the surface roughness of both polished and glazed ZLS ceramics. Type of surface finishing (polishing versus glazing) had no significant effect on the surface roughness of specimens following immersion in different beverages.

Effect of Three Commonly Consumed Beverages on Surface Roughness of Polished and Glazed Zirconia-Reinforced Lithium Silicate Glass Ceramics.

Objectives:This study aimed to assess the effect of three commonly consumed beverages on surface roughness of polished and glazed zirconia-reinforced lithium silicate (ZLS) glass ceramics.Materials and Methods:In this experimental study, 104 rectangular specimens were cut from Vita Suprinity blocks with 2 mm thickness. After ultrasonic cleaning and firing of the specimens, they were finished and polished in two groups. Specimens in the first group were polished using a 2-step polishing kit while the second group specimens were glazed and heated in a porcelain firing oven according to the manufacturer’s instructions. Baseline surface roughness was measured using a profilometer. Specimens in each group were then randomly divided into 4 subgroups (n=13) for immersion in artificial saliva (control group), cola, orange juice and black tea. Surface roughness was measured again and data were analyzed using two-way ANOVA.Results:The highest and the lowest mean Ra were found in orange juice and saliva subgroups, respectively in both glazed and polished groups. The Ra values of both polished and glazed groups significantly increased after immersion in orange juice and cola (P<0.05). The polished surfaces showed insignificantly higher surface roughness compared with glazed surfaces (P>0.05).Conclusion:Orange juice and cola significantly increased the surface roughness of both polished and glazed ZLS ceramics. Type of surface finishing (polishing versus glazing) had no significant effect on the surface roughness of specimens following immersion in different beverages.

The Effects of Surface Conditioning and Aging on the Bond Strength Between Composite Cement and Zirconia-reinforced Lithium-Silicate Glass-Ceramics.

To determine the effects of ceramic-surface conditioning and aging on the bond strength between composite cement and zirconia-reinforced lithium-silicate glass-ceramics (ZLS) under simulated clinical conditions. ZLS disks (Celtra Duo, Dentsply Sirona, n = 110 test group n = 10, diameter: 8.3 mm, height: 3.4 mm) were assigned to four surface-conditioning groups: (I) 30 s of ~5% hydrofluoric-acid etching (Vita Ceramics Etch, Vita; HF), silanization (Calibra Silane; SIL); (II) successive contamination with saliva and silicone (CONT), HF, SIL; (III) CONT, tribochemical silicatization (CoJet), SIL; (IV) HF, SIL, application and light polymerization of an adhesive (Prime&Bond Active), CONT, reapplication and light polymerization of the adhesive. The ZLS disks were bonded to composite-resin cylinders in acrylic tubes (inner diameter: 3.3 mm) using self-adhesive composite cement (Calibra Universal). The tensile-bond strength (TBS) was measured after both 24 h and 6 months of water storage (WS). Additional aging protocols were tested for group I (3-day WS; 30-day WS including 7500 thermocycles between 6.5 and 60°C; 150-day WS including 37,500 thermocycles). After 24 h, the mean TBS ranged between 21 MPa (group III) and 30-35 MPa (remaining groups). With the exception of 3-day WS, TBS was statistically significantly reduced by aging. The greatest reduction was observed for silicatized specimens (group III, mean TBS after aging: 9.8 MPa). Both ceramic surface conditioning and aging had a statistically significant effect on the bond strength between composite cement and ZLS. A treatment protocol based on tribochemical silicatization cannot be recommended for the adhesive cementation of ZLS.

Precision of different fatigue methods for predicting glass-ceramic failure

This study aimed to characterize the fatigue behavior using two fatigue methods, boundary and staircase, and to predict the probability of failure (Pf) of zirconia-reinforced lithium silicate glass-ceramic (ZLS). Bar-shaped specimens of ZLS (18 ×4 ×1.2 mm) were fabricated. Thirty specimens were subjected to a three-point flexural strength test using a universal testing machine with 0.5 mm/min crosshead speed, in 37 °C distilled water. Flexural strength data were analyzed with Weibull statistics. Eighty-six bars were subjected to cyclic fatigue using boundary and staircase methods. Fatigue tests were performed in a pneumatic cycling machine (2 Hz, 37 °C distilled water) for 10 ³ and 104 cycles. Fatigue data were analyzed using an inverse power law relationship and log normal-lifetime distribution. Fracture toughness (KIc) was determined using V-notched specimens (18 ×4 ×3 mm) and the short beam toughness method (n = 7). Vickers hardness (VH) was evaluated (4.9 N, 20 s). Fractographic and EDS analyses were also performed. ZLS showed a characteristic strength of 197 MPa, Weibull modulus of 4, VH of 6.67 GPa and KIc of 1.93 MPa m1/2. After 103 cycles, for both methods, there was a degradation of 78% of the initial strength. There was no significant degradation when the number of cycles increased from 103 to 104. Both methods resulted in similar Pf and precision at 40 MPa (~50% Pf). Yet, staircase shows good accuracy and precision in predicting the stress amplitude for a Pf near 50%; while boundary is also effective for Pf lower than 50%. The fatigue methods evaluated show similar accuracy and precision for predicting the Pf of a glass-ceramic when simulations were made in the range of stress levels and lifetimes used in the fatigue tests.

Impact of crystallization firing process on the microstructure and flexural strength of zirconia-reinforced lithium silicate glass-ceramics

ObjectivesThe aim of this study was to characterize the microstructure of two zirconia-reinforced lithium silicate (ZLS) glass-ceramics and evaluate their mechanical properties before and after the crystallization firing process (CFP). MethodsField emission-scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed for microstructural characterization. To evaluate the pattern of crystallization and the molecular composition of ZLS glass-ceramics, was used X-ray diffraction (XRD). Vickers hardness, fracture toughness by the indentation method, and biaxial flexural strength were also measured. One hundred and forty ceramic discs were produced (diameter=12mm; thickness=1.2mm) and allocated among four groups (n=30): Sfir, Sunf-ZLS Vita Suprinity; and Cfir and Cunf-ZLS Celtra Duo; fired and unfired, respectively. Statistical analysis was performed and Weibull failure probabilities were calculated. ResultsCfir showed the highest characteristic strength (251.25MPa) and hardness (693.333±10.85GPa). Conversely, Sunf presented the lowest characteristic strength (106.95MPa) and significantly lowest hardness (597.533±33.97GPa). According to Weibull analysis, Sunf had the highest structural reliability (m=7.07), while Sfir presented the lowest (m=5.38). The CFP was necessary to crystallize zirconia in the partially crystallized ZLS glass-ceramics. Sfir had a lower percentage of crystallized zirconia than did Cfir. Fractographic analyses showed that all failures initiated from an inherent critical defect in Sunf and from processing defects in the remaining groups. SignificanceThe CFP had a direct influence on the flexural strength and microstructural characteristics of both ZLS materials.

Fatigue failure load of two resin-bonded zirconia-reinforced lithium silicate glass-ceramics: Effect of ceramic thickness

ObjectivesTo evaluate the effect of ceramic thickness on the fatigue failure load of two zirconia-reinforced lithium silicate (ZLS) glass-ceramics, adhesively cemented to a dentin analogue material. MethodsDisc-shaped specimens were allocated into 8 groups (n=25) considering two study factors: ZLS ceramic type (Vita Suprinity — VS; and Celtra Duo — CD), and ceramic thickness (1.0; 1.5; 2.0; and 2.5mm). A trilayer assembly (ϕ=10mm; thickness=3.5mm) was designed to mimic a bonded monolithic restoration. The ceramic discs were etched, silanized and luted (Variolink N) into a dentin analogue material. Fatigue failure load was determined using the Staircase method (100,000 cycles at 20Hz; initial fatigue load ∼60% of the mean monotonic load-to-failure; step size ∼5% of the initial fatigue load). A stainless-steel piston (ϕ=40mm) applied the load into the center of the specimens submerged in water. Fractographic analysis and Finite Element Analysis (FEA) were also performed. ResultsThe ceramic thickness influenced the fatigue failure load for both ZLS materials: Suprinity (716N up to 1119N); Celtra (404N up to 1126N). FEA showed that decreasing ceramic thickness led to higher stress concentration on the cementing interface. SignificanceDifferent ZLS glass-ceramic thicknesses influenced the fatigue failure load of the bonded system (i.e. the thicker the glass ceramic is, the higher the fatigue failure load will be). Different microstructures of the ZLS glass-ceramics might affect the fatigue behavior. FEA showed that the thicker the glass ceramic is, the lower the stress concentration at the tensile surface will be.

Effect of surface treatments and coffee thermocycling on the color and translucency of CAD-CAM monolithic glass-ceramic

Statement of problemThe effects of surface treatments and coffee thermocycling on the color and relative translucency of a recently introduced computer-aided design and computer-aided manufacturing (CAD-CAM) monolithic glass-ceramic are unknown. PurposeThe purpose of this in vitro study was to evaluate the effect of coffee thermocycling on the color and relative translucency parameter (RTP) of CAD-CAM monolithic glass-ceramics after different surface treatments. Material and methodsSpecimens (1.5-mm-thick) were sectioned from zirconia-reinforced lithium silicate glass-ceramic (ZLS) (n=18) and lithium disilicate glass-ceramic (LDS) blocks (n=18). Two different types of surface treatments (glazing or polishing) were applied to the specimens. The specimens were subjected to 5000 thermocycles in a coffee solution. The color coordinates of specimens were measured before and after coffee thermocycling by using a spectroradiometer, and color differences and relative translucency values were calculated by using CIEDE2000 color difference and RTP formulas. ANOVA was used to analyze the color difference and relative translucency values by using maximum likelihood estimation and the Satterthwaite degrees of freedom methods. Any significant interaction between surface subgroups was further analyzed by using the Tukey-Kramer adjustment (α=.05). ResultsMaterial type had a significant effect on color difference (P=.018). All color difference values of all materials were smaller than the clinical acceptability threshold (<1.8 units). For relative translucency, material (P<.001) and coffee thermocycling had a significant effect (P=.014), and an interaction was found between the surface treatments and materials (P<.001). The Tukey-Kramer test revealed significant differences between glazed and polished subgroups of LDS material, except for ZLS-glazed and ZLS-polished subgroups. ConclusionsDifferent surface treatments of CAD-CAM monolithic zirconia-reinforced lithium silicate and lithium disilicate glass-ceramics resulted in clinically acceptable color changes after coffee thermocycling. The color changes in all groups, except for LDS-polished, were not perceivable. Lithium disilicate was more translucent than zirconia-reinforced lithium silicate before and after coffee thermocycling. Coffee thermocycling decreased the translucency of both of the materials. Different surface treatments affected the translucency of only lithium disilicate for tested thickness.

FLEXURAL STRENGTH AND ADHESION OF ZIRCONIA-REINFORCED LITHIUM SILICATE GLASS CERAMIC TO RESIN CEMENT AFTER THERMO-MECHANICAL LOAD CYCLING

Purpose: The aims of this study were to evaluate the flexural strength and adhesion of zirconia-reinforced lithium silicate glass ceramic with those of lithium disilicate after applying a combined thermo-mechanical load cycling. Materials and Methods: Zirconia-reinforced lithium silicate (vita suprinity; SV) and lithium disilicate glass ceramics (IPS e.max CAD; EM) were the materials used in this study. Specimens with the required dimensions for microtensile bond strength and flexural strength test were prepared and finished. The specimens were divided into two groups: control group that was stored in distilled water at 370C for 24 h (without thermo-mechanical load cycling) and test group (with thermo-mechanical load cycling). A total of 3,500 thermal cycles and 250,000 mechanical loading cycles were performed. Microtensile bond strength and flexural strength of each material were evaluated before and after thermo-mechanical load cycling. Moreover, additional micromorphological analysis such as those involving scanning electron microscopy and surface roughness of the etched ceramic bonded surfaces were performed for qualitative analysis. Data were analyzed using independent t-tests between the two glass ceramic groups, and paired t-test was used to detect significance within each group with and without thermo-mechanical load cycling.Results: The effect of thermo-mechanical load cycling on microtensile bond strength was statistically significant for both types of glass ceramics (EM and SV) (P<.001) and the most frequent failure type was mode I (adhesive failure between cement and ceramic type) with pronounced effect on EM (66.67%) than on SV (40%). Regarding thermo-mechanical load cycling effect on flexural strength, there was statically significant difference (P<.001) only for EM.Conclusions: SV glass ceramic showed higher bonding values than EM glass ceramic only after thermo-mechanical load cycling. However, it provides higher flexural strength values either before or after thermo-mechanical load cycling than EM glass ceramic.