Luís Felipe Guilardi

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

This study aimed to evaluate the effect of etching time and hydrofluoric acid (HF) concentration on the fatigue failure load and surface characteristics of zirconia-reinforced lithium silicate glass (ZLS) ceramic cemented to a dentin-like, fiber reinforced epoxy resin. Ceramic (Suprinity, VITA) (1.0mm thick) and epoxy resin (2.5mm thick) discs (10mm diameter) were produced. The bonding surface of the ceramic samples was nonetched (control group), or etched for 30, 60 or 90s by 5% or 10% HF. The epoxy resin discs were etched by 10% HF for 30s followed by the application of an adhesive material (Single Bond Universal, 3M ESPE). Pairs of ceramic/epoxy resin discs were cemented with a dual cure resin cement. The fatigue failure load was determined by the staircase method (500,000 cycles at 20Hz; initial load = 925N; step size = 45N). In 10% HF the etching time was shown to influence the fatigue failure load, which increased as the etching time increased (30s < 60s < 90s), and in 5% HF the fatigue failure load was not shown to be affected by the etching time; the lowest fatigue failure loads were produced in the control group without ceramic etching followed by 10% HF acid etching for 30s. Topography analysis showed variations based on the etching protocols. All fractures (radial cracks) were shown to originate from defects at the ceramic surface on the cementing interface. For fatigue loading improvements of ZLS ceramic, 10% HF acid etching for 90s and silanization of the ceramic surface is recommended.

CAD/CAM machining Vs pre-sintering in-lab fabrication techniques of Y-TZP ceramic specimens: Effects on their mechanical fatigue behavior

This study evaluated the effects of different pre-sintering fabrication processing techniques of Y-TZP ceramic (CAD/CAM Vs. in-lab), considering surface characteristics and mechanical performance outcomes. Pre-sintered discs of Y-TZP ceramic (IPS e.max ZirCAD, Ivoclar Vivadent) were produced using different pre-sintering fabrication processing techniques: Machined- milling with a CAD/CAM system; Polished- fabrication using a cutting device followed by polishing (600 and 1200 SiC papers); Xfine- fabrication using a cutting machine followed by grinding with extra-fine diamond bur (grit size 30 μm); Fine- fabrication using a cutting machine followed by grinding with fine diamond bur (grit size 46 μm); SiC- fabrication using a cutting machine followed by grinding with 220 SiC paper. Afterwards, the discs were sintered and submitted to roughness (n=35), surface topography (n=2), phase transformation (n=2), biaxial flexural strength (n=20), and biaxial flexural fatigue strength (fatigue limit) (n=15) analyses. No monoclinic-phase content was observed in all processing techniques. It can be observed that obtaining a surface with similar characteristics to CAD/CAM milling is essential for the observation of similar mechanical performance. On this sense, grinding with fine diamond bur before sintering (Fine group) was the best mimic protocol in comparison to the CAD/CAM milling.

Fatigue strength of yttria-stabilized zirconia polycrystals: Effects of grinding, polishing, glazing, and heat treatment

This study aimed to evaluate and compare the effect of different surface post-processing treatments (polishing, heat treatment, glazing, polishing + heat treatment and polishing + glazing) on the superficial characteristics (micromorphology and roughness), phase transformation and fatigue strength of a Y-TZP ceramic ground with diamond bur. Discs of Y-TZP ceramic were manufactured (ISO:6872-2015; final dimensions of 15mm in diameter and 1.2 ± 0.2mm in thickness) and randomly allocated according to the surface condition: Ctrl - as-sintered; Gr - ground with coarse diamond bur; Gr+HT - ground and subjected to the heat treatment; Gr+Pol - ground and polished; Gr+Pol+HT - ground, polished and heat treated; Gr+Gl - ground and glazed; Gr+Pol+Gl - ground, polished and glazed. The following analyses were performed: roughness (n = 25), surface topography (n = 2), phase transformation (n = 2) and fatigue strength by staircase method (n = 20). All treatments influenced to some extent the surface characteristics of Y-TZP, being that polishing reduced the surface roughness, the m-phase content and improved the fatigue strength; glazing led to the lowest roughness values (Ra and Rz), although it showed the worst fatigue strength; heat treatment showed limited effect on surface roughness, led to complete reversion of the existing m-phase content to t-phase, without enhancing fatigue performance. Thus, a polishing protocol after clinic adjustment (grinding) of monolithic restorations based on polycrystalline zirconia material is mandatory for surface characteristics and fatigue performance improvements.

Mechanical reliability, fatigue strength and survival analysis of new polycrystalline translucent zirconia ceramics for monolithic restorations

This study characterized the mechanical properties (static and under fatigue), the crystalline microstructure (monoclinic - m, tetragonal - t and cubic - c phase contents) and the surface topography of three yttrium-stabilized zirconia (YSZ) materials with different translucent properties, before and after aging in an autoclave (low temperature degradation). Disc-shaped specimens were produced from second generation (Katana ML/HT - high-translucent) and third generations (Katana STML - super-translucent and UTML - ultra-translucent) YSZ ceramics (Kuraray Noritake Dental Inc.), following ISO 6872-2015 guidelines for biaxial flexural strength testing (final dimensions: 15 mm in diameter and 1.2 ± 0.2 mm in thickness), and then subjected to the respective tests and analyses. ML was mainly composed of tetragonal crystals, while STML and UTML presented cubic content. Aging increased the monoclinic content for ML and did not affect STML and UTML. Topographical analysis highlights different grain sizes on the ceramic surface (UTML > STML > ML) and aging had no effect on this outcome. Weibull analysis showed the highest characteristic strength for ML both before and after aging, and statistically similar Weibull moduli for all groups. ML material also obtained the highest survival rates (ML > STML > UTML) for both fatigue strength and number of cycles to failure. All fractures originated from surface defects on the tensile side. Third generation zirconia (Katana STML and UTML) are fully stabilized materials (with tetragonal and cubic crystals), being totally inert to the autoclave aging, and presented lower mechanical properties than the second-generation zirconia (Katana ML - metastable zirconia).

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

OBJECTIVES To 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. METHODS Disc-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. RESULTS The 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. SIGNIFICANCE Different 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.

Hydrofluoric acid concentrations: Effect on the cyclic load-to-failure of machined lithium disilicate restorations

OBJECTIVES To evaluate the effects of the etching with different hydrofluoric acid (HF) concentrations on the cyclic load-to-failure (CLf) of machined lithium disilicate crowns cemented to dentin analogue material. METHODS Pairs of dentin analogue prosthetic preparations and lithium disilicate ceramic crowns with simplified and standardized designs were machined (n=18). The preparations were etched with 10% HF (60s), followed by primer application. The intaglio surface of the ceramic crowns was treated as follows: non-etched (control, CTRL); or etched for 20s with different HF concentrations - 3% (HF3), or 5% (HF5), or 10% (HF10). A silane coating was then applied onto the treated ceramic surfaces, and they were adhesively cemented to the preparations. To perform the fatigue tests (staircase approach), a hemispheric stainless-steel piston (Ø=40mm) applied cyclic loads in the center of the crowns under water (initial load: 720N; step-size: 70N; cycles: 500,000; frequency: 20Hz). Additionally, topographic, fractographic, and fractal analyses were carried out. The fatigue data were analyzed using the Dixon and Mood method. RESULTS Although the topographic and fractal analyses depicted the action of HF etching altering the superficial complexity and topography, the preponderant topography pattern was established by machining on CAD/CAM. All groups showed similar CLf (in N) (CTRL=805.00±91.23; HF3=781.25±29.87; HF5=755.00±154.49; HF10=833.75±100.74). SIGNIFICANCE Etching with different HF acid concentrations did not promote a deleterious effect on the cyclic load-to-failure of machined lithium disilicate crowns.

Effect of grinding and aging on subcritical crack growth of a Y-TZP ceramic

This study aimed to investigate slow crack growth (SCG) behavior of a zirconia ceramic after grinding and simulated aging with low-temperature degradation (LTD). Complementary analysis of hardness, surface topography, crystalline phase transformation, and roughness were also measured. Disc-shaped specimens (15 mm Ø × 1.2 mm thick, n = 42) of a full-contour Y-TZP ceramic (Zirlux FC, Amherst) were manufactured according to ISO:6872-2008, and then divided into: Ctrl - as-sintered condition; Ctrl LTD - as-sintered after aging in autoclave (134°C, 2 bar, 20 h); G - ground with coarse diamond bur (grit size 181 μm); G LTD - ground and aged. The SCG parameters were measured by a dynamic biaxial flexural test, which determines the tensile stress versus stress rate under four different rates: 100, 10, 1 and 0.1 MPa/s. LTD led to m-phase content increase, as well as grinding (m-phase content: Ctrl - 0%; G - 12.3%; G LTD - 59.9%; Ctrl LTD - 81%). Surface topography and roughness analyses showed that grinding created an irregular surface (increased roughness) and aging did not promote any relevant surface change. There was no statistical difference on surface hardness among different conditions. The control group presented the lowest strength values in all tested rates. Regarding SCG, ground conditions were less susceptible to SCG, delaying its occurrence. Aging (LTD) caused an increase in SCG susceptibility for the as-sintered condition (i.e. G < G LTD < Ctrl < Ctrl LTD).

Mechanical behavior of yttria-stabilized tetragonal zirconia polycrystal: Effects of different aging regimens

This study aimed to characterize and compare the effect of different aging regimens on surface characteristic (topography and roughness), structural stability (phase transformation) and mechanical performance (Weibull analysis) of a Y-TZP ceramic. Discs (15 × 1.2 mm; VITA In-Ceram YZ) were prepared according to ISO 6872-2015 for biaxial flexural strength testing and randomly assigned into five groups (n = 30): as-sintered, no aging treatment (CTRL); 20 h in autoclave at 134°C, 2 bar pressure (AUT); intermittent mechanical loading at 20 Hz/106 load pulses (MechLoad); AUT followed by MechLoad (AUT+MechLoad); and storage in distilled water at 37°C, for 1 year (STO). The following analyses were performed: roughness (n = 30), surface topography (n = 2), phase transformation (n = 2) and biaxial flexure strength (n = 30). Phase transformation (increase of m-phase content) was shown to be a spontaneous, unavoidable and time-dependent process, occurring even under ambient conditions (dry storage after 1 year = 6.0% increase), and is considerably accelerated in the presence of moisture (STO = 17.6%; AUT= 63.1%; and AUT+MechLoad = 59.9%). For roughness parameters, only Ra was affected by aging, and the highest values were observed for AUT+MechLoad (0.25 ± 0.07 µm). For Weibull analysis, structural reliability (Weibull moduli) and characteristic strength were not impaired after aging, and some aging conditions led to increased values (highest weibull moduli in AUT, and highest characteristic strength in STO). Phase transformation proves to be a time-dependent spontaneous mechanism that is accelerated in the presence of different stimuli. However, none of the aging regimens had a negative effect on the characteristic strength and structural reliability of Y-TZP ceramic.

Mechanical performance of Y-TZP monolithic ceramic after grinding and aging: Survival estimates and fatigue strength

This study aims to evaluate the flexural fatigue strength and survival estimates of an Y-TZP monolithic ceramic after grinding and aging. Ceramic discs (1.2 mm thickness × 15.0 mm diameter - ISO, 6872, 2015) of zirconia (Zirlux FC2 - Ardent; Ivoclar Vivadent) were made and randomly allocated into 6 groups (n = 10), according to grinding and aging factors: Ctrl - as-sintered; Ctrl Sto - as-sintered and dry stored at room temperature for 2 years; Ctrl Aut Sto - as-sintered, submitted to autoclaved aging (134 °C, 2 bar, 20 h) and then dry stored for 2 years; and similar conditions for ground samples (Ground; Ground Sto; Ground Aut Sto). Grinding was performed with diamond burs (#3101G, KG Sorensen) coupled to a contra-angle torque multiplier attached to a low speed motor under constant irrigation. Fatigue testing followed a step-stress approach. Data from strength and number of cycles until fracture were recorded and analyzed through Kaplan-Meier and Mantel-Cox tests. Both grinding and aging increase monoclinic phase content. The topography was altered by grinding but not by aging procedures. Grinding did not alter the fatigue strength (Ctrl = Ground), while aging increase it only for ground groups (Ground Sto, Ground Aut Sto). Aged conditions (Ctrl Sto; Ground Sto; Ctrl Aut Sto; Ground Aut Sto) showed increased survival probabilities for both flexural fatigue strength and cycles for failure. Therefore, despite promoting monoclinic phase increase, aging and grinding did not deleteriously affect the fatigue behavior of Y-TZP ceramics.

The effect of extended glaze firing on the flexural fatigue strength of hard-machined ceramics

Statement of problem. It is unclear whether an extended glaze firing could improve the long‐term mechanical performance of densely sintered CAD‐CAM ceramics. Purpose. The purpose of this in vitro study was to analyze the effect of an extended glaze firing on the flexural fatigue strength (FFS) of densely sintered milled (hard‐machined) leucite‐based (LEU) and lithium disilicate‐based (DIS) ceramics. Material and methods. Disks were machined from ceramic blocks and divided into 6 groups (n=20) according to the material, LEU or DIS, and to the applied glaze firing: manufacturer‐recommended glaze (G group), extended glaze (EG group), and control/no firing (C group). The surface roughness of the disks was measured before and after firing by using a contact profilometer, and data were compared by paired sample tests. Specimens were submitted to fatigue by using the staircase test design in water (piston‐on‐3 balls; 500 000 cycles, 20 Hz, and sinusoidal loading). Mean (±SD) FFS values were then calculated and analyzed by using 1‐way analysis of variance and post hoc Tukey test (&agr;=.05). Results. Surface roughness did not change after the firing (P>.05). The highest FFS value in both ceramics was obtained after EG firing (LEU‐EG=80.52 ±6.3 MPa; DIS‐EG=147.25 ±10.5 MPa), which was statistically superior to G firing (LEU‐G=73 ±6.8 MPa, P=.003; DIS‐G=134.34 ±15.6 MPa; P=.023) and C group (LEU‐C=61.94 ±6.3 MPa; P<.001; DIS‐C=134.13 ±17.3 MPa; P=.023). Conclusions. EG firing optimized the biaxial flexural fatigue strength of hard‐machined leucite and lithium disilicate ceramics compared with conventional glaze firing.