Fatigue failure load of zirconia-reinforced lithium silicate glass ceramic cemented to a dentin analogue: Effect of etching time and hydrofluoric acid concentration

Effect of hydrofluoric acid and self-etch ceramic primers on the flexural strength and fatigue resistance of glass ceramics: A systematic review and meta-analysis of in vitro studies

This systematic review evaluated the effect of different hydrofluoric acid (HF) etching regimens and a self-etch ceramic primer (SECP) on the flexural strength (FS) and fatigue failure load (FFL) of glass-ceramic materials.The identification of relevant studies was conducted by two authors in five databases: PubMED, Scopus, Web Of Science, LILACS and Virtual Health Library (BVS) until July 2022 with no year limit. The analysis was conducted in RevMan 5.4.1 Software (Cochrane Collaboration) using Random effect model at 5 %. The risk of bias of the included studies were assessed. From the 5349 articles identified, 34 were included for quantitative analysis. Meta-analysis showed that for predominantly glassy ceramics, etching with HF 5 % had no significant impact on FS, however, HF acid etching with concentrations greater than 5 % negatively impacted FS. For lithium disilicate glass-ceramics (LDGC) HF acid etching, negatively influenced FS, while increasing the FFL. HF etching negatively affected FS of hybrid ceramics. The self-etch ceramic primer and HF acid etching showed a similar impact on FFL and FS. This meta-analysis indicates that the impact of SECP and HF acid etching on the mechanical behavior of glass ceramics is material-dependent.

Mechanical, Adhesive and Surface Properties of a Zirconia-Reinforced Lithium Silicate CAD/CAM Ceramic Exposed to Different Etching Protocols

The aim of this investigation was to evaluate the effect of etching protocols on bond strength, surface roughness, and mechanical properties of a zirconia-reinforced lithium silicate (ZLS) CAD/CAM-ceramic. In total, 100 bars (ISO 6872), 75 plaques, and 25 cubes were cut from ZLS blocks(Vita Suprinity®). The surfaces were standardized, crystallized and divided into five groups: 1. control (no/treatment-C), 2. 5%-Hydrofluoric-acid (HF)/20 s (HF5%20s), 3.HF5%60s, 4.HF10%20s, and 5.HF10%60s. Flexural strength (FS) (three-point bending test, 1 mm/min), roughness (Pa), and micro-shear bond-strength (µSBS) tests were performed. The data were statistically analyzed with one-way ANOVA, Tukey's test (p ˂ 0.05) and Weibull (FS data). C showed higher Pa (1.176 ± 0.370 µm) than HF10%60s (0.627 ± 0.236 µm) and all other groups. Groups C and 20 s showed the most irregular surface patterns. The FS results were not influenced by etching protocols, while the Weibull modulus was, with the 5%HF groups being the most reliable (m: 5.63/6.70), while C and HF10%60s (m: 2.78/2.73) were the least reliable. All fractures originated from surface defects on the treated side of specimens. The 5%HF groups showed higher µSBS (20 s: 21.35 ± 4.70 MPa; 60 s: 23.50 ± 4.27 MPa) than the 10%HF groups (20 s: 14.51 ± 2.47 MPa; 60 s: 16.54 ± 3.12 MPa) and C (6.46 ± 2.71 MPa). The most prevalent failure pattern was "mixed" for etched groups, and "adhesive" for C. Etching protocols affect the evaluated properties by roughening materials' surface and, in some cases, regularizing surface defects. The best overall outcomes were achieved when applying 5%HF.

Is dentin analogue material a viable substitute for human dentin in fatigue behavior studies?

This study aimed to compare the fatigue performance of a lithium disilicate ceramic cemented on different substrates (human dentin and glass fiber-reinforced epoxy resin - GFRER), treated with different types of conditioning (CTR - without surface conditioning; HF5 - 5% hydrofluoric acid; HF10 - 10% hydrofluoric acid; H3PO4 - phosphoric acid 37%; SAND - sandblasting with aluminum oxide). The occlusal surface of human molars (DENT group) (n = 15) was ground for dentin exposure and the root portion was cut, then the dentin slice (2.0 mm thick) was conditioned with 37% phosphoric acid and a dual-curing dental adhesive was applied. The GFRER in a round-rod format was cut into discs (Ø = 10 mm, 2.0 mm thick). Lithium disilicate glass ceramic blocks (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein) were shaped into a cylinder format and cut, resulting in 90 discs (Ø = 10 mm, 1.5 mm thick). The substrate materials of each group were etched according to the groups and the ceramic was etched with 5% hydrofluoric acid for 30 s. A silane coupling agent was applied over the cementation surface in ceramic and GFRER surfaces and a dual cement was used for cementation (ceramic/GFRER or dentin). The disc/disc sets were submitted to thermocycling (25,000 cycles + storage for 6 months), and then tested in step-wise accelerated cyclic fatigue test. The failure pattern and topography were analyzed and the roughness and contact angle were measured before and after surface treatment. The DENT group presented the lowest load to failure values and number of cycles to failure in fatigue (637.33 N; 118.333), showing no statistical similarity with any of the other tested groups (p < 0.05). The topographic analysis showed that all proposed surface treatments modified the substrate surface when compared to the CTR group. All of the fractographical inspections demonstrated failure by radial crack. Considering the roughness analysis, the post-etched DENT group showed similar roughness to all groups of GFRER materials with their surface treated, except for SAND, which showed greater roughness and statistically different from the other groups. The DENT group (49.5) showed statistically different post-conditioning contact angle values from the HF10 group (96.5) and similar to the other groups. The glass fiber-reinforced epoxy resin was not able to simulate the results presented by the human dentin substrate when cemented to lithium disilicate regarding fatigue failure load and number of cycles for failure, regardless of the surface treatment. Lithium disilicate cemented on dentin analogue overestimates the load values for fatigue failure.

Shear bond strength of orthodontic brackets bonded to a new version of zirconium all ceramic restoration: An in vitro comparative study

Objectives

Esthetic restorations such as monolithic zirconia crowns are highly requested for adults nowadays. Bonding orthodontic braces on this type of material became a challenge for orthodontists, because of the special surface treatment needed. This study aims to assess the shear bond strength (SBS) of metal, and ceramic brackets bonded on two types of zirconia ceramics, surface roughness (SR) after different surface treatments for their surfaces, and adhesive remnant index (ARI).

Materials and methods

Brackets' base surface area (BSA) was scanned by an extra-oral scanner, then measured. The doubled labial surface of monolithic zirconia crowns (n = 30) and monolithic high translucent zirconia crowns (n = 30) were prepared and each was divided into three groups (n = 10) depending on surface treatment (hydrofluoric acid etching, no treatment, and rocatec airborne abrasion). Extracted lower central incisors (n = 20) were prepared. Each of them was divided into two subgroups depending on the type of bracket bonded on their surfaces (metal and ceramic). The SR, SBS, and ARI were assessed.

Statistical analysis used

Tests used are independent-samples t-test, Fisher's exact test, One-Way ANOVA, and Kruskal-Wallis test.

Results

The highest SBS and SR were observed in Enamel/Metal and Zirconia/Metal/Rocatec subgroups, respectively.

Conclusion

Adequate bond strength could be obtained with the high translucent zirconia group if bonded with ceramic or metal brackets even if no treatment was used.

Clinical significance

A proportion of simulation was done like practicing inside the dental clinic to reach the best results regarding the adhesion strength of orthodontic brackets.

Comparison of the Micro-Shear Bond Strength of Resin Cements to CAD/CAM Glass Ceramics with Various Surface Treatments

This study aimed to compare the effect of acid etching, sandblasting, or silica coating on the micro-shear bond strength of dual-cured resin cements to computer-aided design and computer-aided manufacturing (CAD/CAM) glass ceramic materials. Feldspathic, lithium disilicate, and zirconia-reinforced CAD/CAM ceramics were divided into four groups: control group (C), no surface treatment; hydrofluoric (HF) group, 5% HF acid-etched; sandblasting (SB) group, abraded with 50 µm aluminum oxide (Al₂O₃) particles; silica-coated (CJ) group, abraded with 30 µm silica-modified Al₂O₃ particles. Roughness values were obtained by using a profilometer. The cements were condensed on the surface-treated specimens and a micro-shear bond test was conducted. The ceramic material (p < 0.001) and surface treatment type (p < 0.001) significantly affected the micro-shear bond strength values. HF acid etching can be recommended for the surface pretreatment of feldspathic, lithium disilicate, and zirconia-reinforced CAD/CAM ceramics. Better bond strengths can be obtained with HF acid etching than with sandblasting and silica coating.

Endocrown restorations in premolars: influence of remaining axial walls of tooth structure and restorative materials on fatigue resistance

OBJECTIVES

To evaluate the effect of the remaining tooth structure and different CAD/CAM materials on the fatigue performance and failure mode of endodontically treated premolars restored with endocrowns.

MATERIALS AND METHODS

Ninety maxillary premolars were endodontically treated and assigned into 6 groups (n = 15) according to the number of remaining axial walls (four, three, and two) and restorative materials (ultra-translucent zirconia 5Y-PSZ [KATANA UTML] and lithium disilicate [IPS e.max-CAD]). The specimens were subjected to cyclic fatigue loading test (initial load 200 N; 20 Hz). An incremental step load of 100 N per 10,000 cycles was applied until failure. The fatigue failure load (FFL) and number of failure cycles (CFFs) data were statistically analyzed with two-way ANOVA and Kaplan-Meier test (α = 0.05). Failed specimens were examined under a stereomicroscope 25 × and failure modes were determined.

RESULTS

FFL and CFF were significantly influenced by restorative material (p < 0.05). 5Y-PSZ endocrowns showed significantly higher FFL when compared with lithium disilicate. The number of remaining walls did not affect the fatigue behavior or failure mode of the specimens. Of the lithium disilicate restorations, 51% had repairable failures, while 95% 5Y-PSZ restorations had non-repairable failures.

CONCLUSIONS

Zirconia endocrowns showed better FFL than lithium disilicate endocrowns, regardless of the number of remaining axis walls. Lithium disilicate and 5Y-PSZ endocrowns showed FFL higher than the normal masticatory loads.

CLINICAL RELEVANCE

Restoring endodontically treated premolars with endocrown could be a promising treatment, regardless of the remaining axial walls. However, precaution should be taken in material selection since it affects the fatigue resistance and failure mode.

A brief review on fatigue test of ceramic and some related matters in Dentistry

The characteristics of dental ceramics have been extensively studied over the years to provide highly qualified materials for use in prosthetic restorations. The ability to adhere to dental substrates, outstanding aesthetics (translucency, color, and substrate masking ability) and improved mechanical properties provide these materials with optical features and high strength to withstand masticatory stimuli. Different classifications are adopted, and it is generally considered that glass-ceramics have better optical characteristics due to the high glass content, and polycrystalline ceramics have superior strength favored by their densified and organized crystals, hampering crack growth. This knowledge was largely built-up during years of scientific research through different testing methodologies, but mainly employing static loads. It is important to not only take into account the intensity of loads that these materials will be exposed to, but also the effect of the intermittence of cyclic load application leading to mechanical fatigue and the influence of factors related to the crack origin and their propagation under this condition. Furthermore, the bonding surface of ceramic restorations requires surface treatments that improve the bond strength to luting agents; however, these treatments require caution because of their potential to produce defects and affect the structural behavior. Moreover, ceramic restorations often require internal adjustments for proper seating or external adjustments for fitting the occlusal contact with the antagonist. In this sense, finishing/polishing protocols may alter the defect population, as luting agents may also interact by filling in the superficial defects on the restoration intaglio surface. Thus, the balance among all these factors will define the performance of a restorative setup, as well as the posterior exposure to the humid environment and the masticatory stimuli (cyclical loading), which may favor developing slow and subcritical growth of cracks in ceramic materials and the degradation of the bond interface. Therefore, it is essential that the concepts which explain the fatigue mechanism are understood, as well as the crack propagation and failure patterns of restorative ceramic materials.

Which dentine analogue material can replace human dentine for crown fatigue test?

OBJECTIVE

To seek dentine analogue materials in combined experimental, analytical, and numerical approaches on the mechanical properties and fatigue behaviours that could replace human dentine in a crown fatigue laboratory test.

METHODS

A woven glass fibre-filled epoxy (NEMA grade G10; G10) and a glass fibre-reinforced polyamide-nylon (30% glass fibre reinforced polyamide-nylon 6,6; RPN) were investigated and compared with human dentine (HD). Flexural strength and elastic modulus (n = 10) were tested on beam-shaped specimens via three-point bending, while indentation hardness (n = 3) was tested after fracture. Abutment substrates of G10, RPN and HD were prepared and resin-bonded with monolithic lithium disilicate crowns (n = 10), then subjected to wet cyclic loading in a step-stress manner (500 N initial load, 100 N step size, 100,000 cycles per step, 20 Hz frequency). Data were statistically analysed using Kruskal-Wallis one-way ANOVA followed by post-hoc comparisons (α = 0.05). Survival probability estimation was performed by Mantel-Cox Log-Rank test with 95% confidence intervals. The fatigue failure load (FFL) and the number of cycles until failure (NCF) were evaluated with Weibull statistics. Finite Element Models of the fatigue test were established for stress distribution analysis and lifetime prediction. Fractographic observations were qualitatively analysed.

RESULTS

The flexural strength of HD (164.27 ± 14.24 MPa), G10 (116.48 ± 5.93 MPa), and RPN (86.73 ± 3.56 MPa) were significantly different (p < 0.001), while no significant difference was observed in their flexural moduli (p = 0.377) and the indentation hardness between HD and RPN (p = 0.749). The wet cyclic fatigue test revealed comparable mean FFL and NCF of G10 and RPN to HD (p = 0.237 and 0.294, respectively) and similar survival probabilities for the three groups (p = 0.055). However, RPN promotes higher stability and lower deviation of fatigue test results than G10 in Weibull analysis and FEA.

SIGNIFICANCE

Even though dentine analogue materials might exhibit similar elastic properties and fatigue performance to human dentine, different reliabilities of fatigue on crown-dentine analogues were shown. RPN seems to be a better substrate that could provide higher reliability and predictability of laboratory study results.

Effects of Particle Abrasion Media and Pressure on Flexural Strength and Bond Strength of Zirconia

OBJECTIVES

To compare the effects of particle abrasion medium and pressure on shear bond strength and biaxial flexural strength of three generations of zirconia (Lava Frame, Lava Plus, and Lava Esthetic) with the goal of optimizing the bond to zirconia.

METHODS

280 discs (14 mm diameter; 1 mm thickness) of each zirconia were milled and sintered. Specimens of each material were randomly distributed into 14 groups (n=20); half were tested for shear bond strength and half were tested for biaxial flexural strength. The specimens were particle abraded on one surface by 2 different media (50 μm alumina particles or 50 μm glass beads) for 10 seconds at three different pressures (15, 30, and 45 psi or 0.1, 0.2, 0.3 MPa). Untreated specimens served as positive control. A tube (1.50 mm diameter) filled with dual cured resin cement (Panavia SA) was placed onto the surface and light cured. Specimens were stored in water (37°C for 24 hours) and shear bond strength was measured in a universal testing machine (Instron). Biaxial flexural strength of each specimen was measured according to ISO 6872. Shear bond strength and biaxial flexural strength were compared individually with a 2-way analysis of variance (ANOVA) for factors surface treatment and zirconia composition.

RESULTS

Significant differences were seen between surface treatments (p<0.01), zirconia composition (p<0.01) and their interaction (p<0.01) for both bond strength and flexural strength. With alumina particle abrasion, higher pressure produced higher bonds for Lava Frame and Lava Plus zirconia while the bond of Lava Esthetic declined with increased pressure. Higher pressure (>0.2 MPa or 30 psi) with alumina decreased biaxial flexural strength with Lava Esthetic zirconia.

CONCLUSIONS

Particle abrasion with alumina produced a significantly better combination of bond strength while maintaining biaxial strength of three zirconia materials than particle abrasion with glass beads. The bond strength also depended upon the pressure of particle abrasion and the generation of zirconia used.

Grinding and polishing of the inner surface of monolithic simplified restorations made of zirconia polycrystals and lithium disilicate glass-ceramic: Effects on the load-bearing capacity under fatigue of the bonded restorations

This study evaluated the effect of grinding and polishing the inner surface of monolithic discs made of zirconia polycrystals (ZR) and lithium disilicate glass-ceramic (LD) on the load-bearing capacity under fatigue of the restorations bonded onto a dentin analogue material (epoxy resin). ZR and LD ceramic discs (10 mm in diameter, 1 mm in thickness) were produced and randomly allocated into 10 groups considering the internal adjustment approach: Ctrl - No adjustment; F - Grinding with fine diamond bur (46 μm); F + Pol - Grinding with fine diamond bur followed by polishing with 2 tips (finisher and polisher); FF - Grinding with extrafine diamond bur (30 μm); FF + Pol - Grinding with extrafine diamond bur followed by polishing. In addition, discs (10 mm in diameter, 2.5 mm in thickness) of fiber reinforced epoxy resin were produced. Afterwards, the intaglio surface of the ZR discs were air-abraded with 45 μm alumina particles for 10 s, the LD and resin epoxy discs were etched with hydrofluoric acid (5%/20 s and 10%/60 s, respectively), and the treated discs were primed as recommended. Each ceramic disc was luted onto the epoxy resin disc with resin cement. Then, the samples were tested under a step-stress fatigue test (20 Hz, 10,000 cycles/step, step-size of 100 N starting at 200 N, and proceeding until failure detection). Fractographic, topographic and surface roughness analysis were also performed. The adjustments (grinding with or without polishing) (ZR: 733-880 N; LD: 1040-1106 N) triggered a detrimental effect on the fatigue behavior in both ceramics compared with the absence of treatment (control group; ZR: 973 N; LD: 1406 N). The polishing step had no effect on fatigue findings. Thus, grinding the inner surface of the tested ceramics should be avoided wherever possible to prevent introducing damage and its detrimental effects on the fatigue behavior.

Evaluation of Marginal/Internal Fit and Fracture Load of Monolithic Zirconia and Zirconia Lithium Silicate (ZLS) CAD/CAM Crown Systems

Fit accuracy and fracture strength of milled monolithic zirconia (Zi) and zirconia-reinforced lithium silicate (ZLS) crowns are important parameters determining the success of these restorations. This study aimed to evaluate and compare the marginal and internal fit of monolithic Zi and ZLS crowns, along with the fracture load, with and without mechanical aging. Thirty-two stone dies acquired from a customized master metal molar die were scanned, and ceramic crowns (16 Zi Ceramill Zolid HT⁺ and 16 ZLS Vita Suprinity) were designed and milled. Absolute marginal discrepancies (AMD), marginal gaps (MG), and internal gaps (IG) of the crowns, in relation to the master metal die, were evaluated using x-ray nanotomography (n = 16). Next, thirty-two metal dies were fabricated based on the master metal die, and crowns (16 Zi; 16 ZLS) cemented and divided into four groups of eight each; eight Zi with mechanical aging (MA), eight Zi without mechanical aging (WMA), eight ZLS (MA), and eight ZLS (WMA). Two groups of crowns (Zi-MA; ZLS-MA) were subjected to 500,000 mechanical cycles (200 ± 50 N, 10 Hz) followed by axial compressive strength testing of all crowns, until failure, and the values were recorded. Independent sample t tests (α = 0.05) revealed no significant differences between Zi and ZLS crowns (p > 0.05); for both internal and marginal gaps, however, there were significant differences in AMD (p < 0.005). Independent samples Mann–Whitney U and Kruskal–Wallis tests revealed significant differences between the two materials, Zi and ZLS, regardless of fatigue loading, and for the individual material groups based on aging (α = 0.05). Multiple comparisons using Bonferroni post-hoc analysis showed significant differences between Zi and ZLS material groups, with or without aging. Within the limitations of this study, the ZLS crown fit was found to be on par with Zi, except for the AMD parameter. As regards fracture resistance, both materials survived the normal range of masticatory forces, but the Zi crowns demonstrated greater resistance to fracture. The monolithic Zi and ZLS crowns seem suitable for clinical application, based on the fit and fracture strength values obtained.

Simple Approach to Medical Grade Alumina and Zirconia Ceramics Surface Alteration via Acid Etching Treatment

In order for bioceramics to be further used in composites and their applications, it is important to change the surface so that the inert material is ready to interact with another material. Medical grade alumina and zirconia ceramic powders have been chemically etched with three selected acidic mixtures. Powder samples were taken for characterization, which was the key to evaluating a successful surface change. Changes in morphology, together with chemical composition, were studied using scanning electron microscopy, phase composition using X-ray diffraction methods, and nitrogen adsorption/desorption isotherms are used to evaluate specific surface area and porosity. The application of HF negatively affected the morphology of the material and caused agglomeration. The most effective modification of ceramic powders was the application of a piranha solution to obtain a new surface and a satisfactory degree of agglomeration. The prepared micro-roughness of the etched ceramic would provide an improved surface of the material either for its next step of incorporation into the selected matrix or to directly aid in the attachment and proliferation of osteoblast cells.

Optical behaviors, surface treatment, adhesion, and clinical indications of zirconia-reinforced lithium silicate (ZLS): A narrative review

OBJECTIVES

The present narrative review was focused on the optical properties, surface treatment, adhesion, and clinical indications of zirconia-reinforced lithium silicate ceramics (ZLS) for Computer-aided design / Computer-aided manufacturing (CAD/CAM) technologies.

DATA/SOURCES

A literature search was performed by 3 calibrated independent researchers on PubMed, Scopus, Embase, Google Scholar, Dynamed, and Open Grey. The criteria for inclusion were: 1) papers addressing at least one of the following variables about ZLS: optical properties, surface treatment, adhesion, and clinical indications; 2) in vitro, in silico, or in vivo studies; 3) case reports; 4) systematic reviews. The exclusion criteria were: 1) animal studies; 2) non-dental studies; 3) studies only focusing on ZLS used in the heat-pressed process.

STUDY SELECTION

98 records among in vitro studies and case reports were included.

CONCLUSIONS

Despite the promising microstructure characteristics of ZLS, increased translucency compared to lithium disilicate ceramics (LS₂) was not proven, but acceptable color changes and stability were reported. Mechanical polishing was the most effective method to reduce surface roughness. Moreover, machinability and handling of ZLS resulted harder than LS₂. Conventional acid etching procedures seemed effective in conditioning ZLS surface, but no protocol has been established yet. Besides, silane-coupling and dual-curing resin cements were recommended.

CLINICAL SIGNIFICANCE

ZLSs can be used for anterior and posterior fixed single-unit CAD/CAM restorations onto both natural teeth and implants, but do not seem to represent a viable treatment option for endocrowns onto posterior teeth or fixed dental prostheses.

Effect of Nd:YAG Laser with/without Graphite Coating on Bonding of Lithium Disilicate Glass-Ceramic to Human Dentin

This study evaluated the effect of different surface treatments on the tensile bond strength between lithium disilicate glass-ceramics, resin cement, and dentin. Fifty truncated cone-shape glass-ceramics were divided into five groups (n = 10): G1, control: 10% hydrofluoric acid (HF); G2, Nd:YAG laser + silane; G3, Sil + Nd:YAG laser; G4, graphite + Nd:YAG laser + Sil; and G5, graphite + Sil + Nd:YAG laser. Fifty human third-molars were cut to cylindrical shape and polished to standardize the bonding surfaces. The glass-ceramic specimens were bonded to dentin with a dual-cured resin cement and stored in distilled water for 24 h at 37ºC. Tensile testing was performed on a universal testing machine (10 Kgf load cell at 1 mm/min) until failure. The bond strength values (mean ± SD) in MPa were G1 (9.4 ± 2.3), G2 (9.7 ± 2.0), G3 (6.7 ± 1.9), G4 (4.6 ± 1.1), and G5 (1.2 ± 0.3). Nd:YAG laser and HF improve the bond strength between lithium disilicate glass-ceramics, resin cement, and dentin. The application of a graphite layer prior to Nd:YAG laser irradiation negatively affects this bonding and presented inferior results.

Effect of prolonged application of single-step self-etching primer and hydrofluoric acid on the surface roughness and shear bond strength of CAD/CAM materials

This in vitro study aimed to assess the influence of different concentrations and durations of hydrofluoric acid (HF) and Monobond Etch & Prime (MEP) etching on the surface roughness (R_a ) of different CAD/CAM materials and on the shear bond strength (SBS) of a self-adhesive resin bonded to the materials. Seventy specimens of hybrid ceramic, leucite-based glass-ceramic, lithium disilicate glass-ceramic, and zirconia-reinforced lithium silicate glass-ceramic were prepared and divided into seven groups according to the surface treatments: Control (C); MEP etching for 60 (MEP₆₀ ) and 120 (MEP₁₂₀ ) s; 5% HF etching for 60 (HF-5%₆₀ ) and 120 (HF-5%₁₂₀ ) s; 9.5% HF etching for 60 (HF-9.5%₆₀ ) and 120 (HF-9.5%₁₂₀ ) s. The R_a was measured using a 3D profilometer. All groups were treated with a universal primer except for the C, MEP₆₀ , and MEP₁₂₀ groups. A self-adhesive resin cement was bonded to all specimens, and the bond strengths were measured using a universal testing machine. All surface treatments increased both R_a and SBS values compared to the control in each material. Neither the duration of surface treatments nor the HF acid concentrations had a statistically significant effect on SBS. Within the limitations of this experimental study, it can be concluded that Monobond Etch & Prime may be a preferable method to achieve high bond strength values.

Fatigue performance of adhesively luted glass or polycrystalline CAD-CAM monolithic crowns

STATEMENT OF PROBLEM

Data comparing the fatigue performance of adhesively luted glass or polycrystalline ceramic systems for computer-aided design and computer-aided manufacturing (CAD-CAM) are scarce.

PURPOSE

The purpose of this in vitro study was to evaluate and compare the fatigue performance of monolithic crowns manufactured from glass or polycrystalline CAD-CAM ceramic systems adhesively luted to a dentin analog.

MATERIALS AND METHODS

Fifty-four pairs of standardized preparations of dentin analog (NEMA Grade G10) and simplified ceramic crowns of 1.5-mm thickness were obtained with 3 ceramic materials: lithium disilicate (LD) glass-ceramic (IPS e.max CAD); zirconia-reinforced lithium silicate (ZLS) glass-ceramic (Vita Suprinity); and translucent yttrium fully stabilized polycrystalline zirconia (Trans YZ) (Prettau Anterior). The simplified crowns (n=15) were adhesively cemented onto the preparations and subjected to step-stress fatigue test (initial load of 400 N, 20 Hz, 10 000 cycles, followed by 100-N increment steps until failure). Collected data (fatigue failure load [FFL] and cycles for failure [CFF]) were submitted to survival analysis with the Kaplan-Meier and Mantel-Cox post hoc tests (α=.05) and to Weibull analysis (Weibull modulus and its respective 95% confidence interval). Failed crowns were submitted to fractography analysis. The surface characteristics of the internal surface (roughness, fractal dimension) of additional crowns were accessed, and the occlusal cement thickness obtained in each luted system was measured.

RESULTS

Trans YZ crowns presented the highest values of FFL, CFF, and survival rates, followed by ZLS and LD (mean FFL: 1740 N>1187 N>987 N; mean CFF: 149 000>92 613>73 667). Weibull modulus and cement thickness were similar for all tested materials. LD presented the roughest internal surface, followed by ZLS (mean Ra: 226 nm>169 nm>93 nm). The LD and ZLS internal surfaces also showed higher fractal dimension, pointing to a more complex surface topography (mean fractal dimension: 2.242=2.238>2.147).

CONCLUSIONS

CAD-CAM monolithic crowns of Trans YZ show the best fatigue performance. In addition, ZLS crowns also showed better performance than LD crowns.

Effect of repeated firings and staining on the mechanical behavior and composition of lithium disilicate

OBJECTIVE

To evaluate the composition, flexural strength and fatigue behaviour of lithium disilicate ceramic (LD) after repeated firings and different staining techniques.

METHODS

LD discs were fabricated and divided according to number of firing cycles and staining technique: CO - control, discs were crystallized (850°C/10min); SC - single-step characterization - crystallization and staining (applied with a thin brush) were performed in a single step with one firing cycle (850°C/10min); and DC - double-step characterization - crystallization firing cycle was performed first (850°C/10min), followed by staining firing cycle (770°C/90s). Specimens were fired two, four or six times (one crystallization firing cycle and one, three or five staining firing cycles), resulting into 9 groups (n=30): COII, COIV, COVI, SCII, SCIV, SCVI, DCII, DCIV and DCVI. The composition of the specimens was investigated (EDS, XRD, Raman spectroscopy), and the biaxial flexural strength (n=10) and staircase tests (n=20, 5×10⁴ cycles, 5Hz) were performed. Data were subjected to one-way ANOVA and Tukey's test (α=0.05).

RESULTS

EDS and XRD revealed amorphous content for stained groups. Biaxial flexural strength was not affected by repeated firings in any group, but stained groups presented lower flexural strength than control groups (p=0.001). The fatigue limit results decreased in all groups compared to flexural strength. SC groups showed similar (SCII and SCIV) or even higher fatigue limits (SCVI) than the control groups, and DC showed the lowest fatigue limit values. SEM and Raman suggested that the interfaces between staining and the LD showed only an overlap for the DC groups, whereas for the SC it was suggested an interaction between the stain and the LD.

SIGNIFICANCE

Repeated firings did not result in decreased lithium disilicate flexural strength.Staining affected flexural strength and also resulted in increased amorphous content in the characterized specimens. Single-step staining resulted in the highest fatigue limit.

Fatigue Failure Load of a Bonded Simplified Monolithic Feldspathic Ceramic: Influence of Hydrofluoric Acid Etching and Thermocycling

Objective:

To evaluate the effect of hydrofluoric acid (HF) etching and thermocycling (Tc) on fatigue failure load of feldspathic ceramic restorations cemented with two resin cements.

Methods:

Disc-shaped feldspathic ceramic (Vitablocs Mark II; Ø=10 mm, 1.0-mm thick) and G10 epoxy resin (Ø=10 mm, 2.5-mm thick) specimens were made and randomly allocated considering three factors: ceramic etching (ie, with vs without 10% HF plus silane application), resin cement (ie, self-adhesive [RelyX U200; U200] or conventional [Multilink Automix; MA]), and Tc (ie, with vs without 5-55°C/12,000 cycles). Adhesive cementation followed each manufacturer's instructions. The fatigue test (n=20) was based on the staircase approach (250,000 cycles; 20 Hz). Contact angle, surface topography, and fractography analysis were also executed. Specific statistical tests were employed for each outcome (α=0.05).

Results:

The interaction of HF and Tc factors decreased the fatigue resistance for both cements (U200 542.63&gt;U200/HF-Tc 495.00; MA 544.47&gt;MA/HF-Tc 506.84). Comparing the cements associated with HF or Tc, there was statistical superiority for MA (U200-Tc 537.37&lt;MA-Tc 561.32; U200/HF 535.79&lt;MA/HF 557.11), and no statistical difference was detected when only cement type or its association with HF-Tc was compared (U200 542.63=MA 544.47; U200/HF-Tc 495.00=MA/HF-Tc 506.84). The fracture always originated from defects at the ceramic-intaglio surface as radial cracks.

Conclusion:

HF etching plus silane agent increased the ceramic surface free energy and its wettability, but it did not provide better results in terms of fatigue resistance compared with silane agent application only. The association of HF etching and aging significantly reduced the fatigue resistance of the material, regardless of the resin cement used.

Mechanical Behavior of Ceramic Monolithic Systems With Different Thicknesses

OBJECTIVES

This study assessed the fully stabilized zirconia (FSZ) Prettau Anterior, the partially stabilized zirconia (PSZ) Prettau, and the lithium disilicate IPS e.max CAD (LD) through microstructural and mechanical characterization and effect of thickness on fracture load of the ceramics.

METHODS AND MATERIALS

Disk-shaped specimens (12 mm diameter and 1.2 mm thickness) were prepared for biaxial flexural strength (BFS) and Weibull statistics (n=30). For the fracture load static test (FLST) and Weibull statistics (n=30), disk-shaped specimens 12 mm in diameter and thicknesses of 0.5 mm, 1 mm, and 1.5 mm were cemented on an epoxy-resin substrate.

RESULTS

BFS (MPa) results were PSZ: 683.0 ± 70.23; FSZ: 438.6 ± 64.1; and LD: 248.6 ± 37.3. One-way analysis of variance (ANOVA) for BFS was significant (p<0.001), and the Tukey post hoc test showed differences among all ceramics. There was difference in characteristic strength, but there was no difference in Weibull modulus. Two-way ANOVA for FLST was significant for ceramic (p<0001), thickness (p<0001), and interaction (p<0001). There was no difference among all ceramics at the 0.5 mm thickness. PSZ had higher values for the 1.0 mm and 1.5 mm thicknesses. LD of 1.5 mm thickness exhibited a higher FLST than FSZ.

CONCLUSIONS

PSZ had the highest BFS, but when cemented on a substrate, all ceramics with 0.5 mm thickness behaved similarly. Despite the lower BFS, LD had a fracture load similar or superior to FSZ when cemented on a substrate.

Newer vs. older CAD/CAM burs: Influence of bur experience on the fatigue behavior of adhesively cemented simplified lithium-disilicate glass-ceramic restorations

This study evaluated the effect of the CAD/CAM burs experience (newer vs older as consequence of the milling sequence) on fatigue failure load (FFL), number of cycles for failure (CFF), and survival rates of lithium disilicate glass-ceramic simplified restorations adhesively cemented to a dentin analogue substrate. Three sets of CAD/CAM burs were used to mill disc-shaped ceramic specimens (1 bur set - 18 milled discs with 10 mm diameter and 1.5 mm thickness), considering the bur experience as a result of the milling sequence to compose the study groups: G1-6 - discs obtained from the 1st to 6th milling of each bur set; G7-12 - specimens from the 7th to 12th milling; G13-18 - discs from the 13th to 18th. Discs of dentin analogue (G10, 10 mm diameter and 2.0 mm thickness) were made to serve as substrate (base material) and randomly assigned into pairs with the respective ceramic discs. Then, the ceramic discs were adhesively cemented onto the dentin analogue substrate, composing a three-layer specimen that mimics a monolithic restoration of a posterior tooth. Specimens were tested under stepwise fatigue approach: frequency = 20 Hz, 5000 cycles at maximum load of 400 N to accommodate the testing assembly, followed by incremental steps of 200 N with initial load ranging from 10 to 1000 N, to a maximum of 20,000 cycles/each step, until the occurrence of failure (radial crack). FFL and CFF were recorded at the end of the testing and subjected to statistical analysis. Supplementary roughness analysis of the milled surface was performed (n = 18) using a contact profilometer. Residual stress after milling and acid etching were accessed via X-ray Diffractometry analysis. FFL and CFF were not affected by increase on bur experience (no statistical differences among groups), despite that, it affected both Ra and Rz parameters (G1-6 had the smoothest surface). The residual stress concentration was negligible (milling did not induce residual stress concentration). It is concluded that the fatigue behavior of adhesively cemented lithium-disilicate glass-ceramic restorations was not influenced by CAD/CAM bur experience (newer vs older as consequence of the milling sequence), and so the residual stress concentration induced by milling was negligible.

CAD-CAM milled versus pressed lithium-disilicate monolithic crowns adhesively cemented after distinct surface treatments: Fatigue performance and ceramic surface characteristics

To evaluate the fatigue failure load (FFL), number of cycles for failure (CFF) and survival probabilities of lithium-disilicate (LD) monolithic crowns manufactured by two processing techniques (pressing vs. CAD/CAM) adhesively cemented to a dentin-analogue material, considering two surface treatments (conventional vs. simplified). Surface characteristics (topography, roughness and fractal dimensions) were also assessed. Forty (40) monolithic crowns were manufactured considering two specific processing techniques for each ceramic system: LD^CAD - CAD/CAM lithium-disilicate (IPS e.max CAD, Ivoclar Vivadent); LD^PRESS - pressed lithium-disilicate (IPS e.max Press, Ivoclar Vivadent). The crowns were adhesively cemented (Multilink Automix System, Ivoclar Vivadent) onto dentin analogue preparations considering two distinct protocols of surface treatments (conventional - hydrofluoric acid etching + silane application [HF+Sil] or simplified - etching with one-step primer (Monobond Etch&Prime, Ivoclar Vivadent) [EP]). The cemented assembly was stored in distilled water at 37 °C for 3 days and fatigue tests were run (step-stress approach: load ranging from 400 to 2000 N, step-size of 100 N, 15,000 cycles/step, 20 Hz). Fractography, surface topography, roughness, and fractal dimension analyses were performed. LD^PRESS[EP] group depicted higher FFL, CFF and survival probabilities in comparison to LD^CAD groups, regardless of the conditioning method. A tendency of higher Weibull modulus (mechanical reliability) was observed when using [EP] for both LD^PRESS and LD^CAD. SEM and AFM analysis showed very distinct initial surface patterns for the distinct processing techniques considered (LD^CAD with higher fractal dimension and lower roughness than LD^PRESS), and both surface treatments distinctly affected these surface characteristics. All failures were radial cracks originating at the ceramic-cement interface. Pressed lithium-disilicate monolithic crowns showed better fatigue performance in comparison to CAD/CAM milled crowns, especially when they were treated with self-etching ceramic primer. The surface treatment with self-etching primer led to similar fatigue performance when compared to hydrofluoric acid plus silane application for the same processing technique, but it tended to provide higher mechanical reliability.

Effect of total occlusal convergence on fit and fracture resistance of zirconia-reinforced lithium silicate crowns

Introduction

Computer-aided design (CAD) and computer-aided manufacturing (CAM) monolithic crowns are gaining momentum. Limited evidence exists about the effect of tooth preparation total occlusal convergence (TOC) on marginal and internal gap distances in addition to load to fracture values.

Aim

The aim of this study was to evaluate, by microcomputed tomography (µCT), the influence of 12° and 20° TOC on marginal and internal adaptation of zirconia-reinforced lithium silicate (ZLS) crowns. Moreover, values of load to fracture with and without initial cyclic loading (CL) were compared.

Material and methods

Forty ZLS crowns were fabricated on dies with TOC of 12° and 20°, 20 crowns for each group. µCT was used to compare marginal and internal adaptation. Each specimen was measured at 140 points distributed on all tooth preparation surfaces for 2D gap distance assessment. 3D gap volume was also evaluated. Crowns were then cemented and divided into two subgroups; the first was stored in distilled water (no cyclic loading [NCL] subgroup), the second was subjected to CL (CL subgroup).

Results

Factorial repeated measures ANOVA followed by Bonferroni post hoc in addition to independent and dependent t-tests were used for statistical analysis. Marginal gap, absolute marginal discrepancy, and occlusal gap measurements showed significant differences between the measurement sites. Static load to fracture values showed significant differences between TOC groups for both NCL (P=0.011) and CL (P=0.025) subgroups.

Conclusion

An increase of TOC from 12° to 20° did not affect marginal and internal adaptation but resulted in higher values of load to fracture of ZLS crowns. CL simulating 1 year of service did not result in fatigue failure.

Fatigue failure load of an adhesively-cemented lithium disilicate glass-ceramic: Conventional ceramic etching vs etch & prime one-step primer

OBJECTIVES

To evaluate the effect of different glass-ceramic surface treatments and aging on the fatigue failure load of a lithium disilicate glass-ceramic adhesively cemented to a dentin analogue material.

METHODS

One hundred and twenty (120) disc-shaped lithium disilicate specimens (Ø=10mm, thickness=1.5mm) were produced and randomly allocated (n=20) into 6 groups, considering 2 study factors: "surface treatment" in 3 levels (SIL-silane application only; HF5+SIL-5% hydrofluoric acid etching and silane application; ME&P-etching with an one-step ceramic primer), and "storage" in 2 levels (baseline-storage for 7 days; aging-storage for 90 days+12,000 thermal cycles). Ceramic discs were adhesively cemented to discs of a dentin analogue material (Ø=10mm, thickness=2.0mm) following the manufacturers' instructions. The fatigue failure load was determined by the staircase approach (250,000 cycles; 20Hz; initial load=1050N [∼70% of mean load-to-failure]; step size=52.5N [5% of initial load]). Micro-morphologic, fractographic, and atomic force microscope analysis were also performed. Fatigue failure load data were evaluated by one-way ANOVA, Bonferroni and t-tests for independent samples.

RESULTS

HF5+SIL presented higher fatigue failure load in both conditions (baseline and aging); ME&P presented intermediary mean values, while the SIL group presented the worst performance. All groups had a statistically significant decrease in the fatigue performance after aging.

SIGNIFICANCE

Hydrofluoric acid followed by silane application showed the best fatigue performance for an adhesively-cemented lithium disilicate ceramic. Aging negatively influenced the fatigue performance for all tested groups.