Effect of coloring with various metal oxides on the microstructure, color, and flexural strength of 3Y-TZP

The influence of hygroscopic expansion of resin supporting dies on the fracture resistance of ceramic restorations during thermal cycling

OBJECTIVES

To evaluate the hygroscopic expansion characterization of resin composite dies during thermal cycling, and their influence on the fracture resistance of dental ceramic materials as well as the effect of pre-immersion on these measurements.

METHODS

Disc-shaped specimens (φ = 15.0 mm, h = 1.2 mm) and anatomical crown dies of four resin composites (epoxy, Z350, P60, G10) were fabricated. Disc-shaped samples were continuously soaked in distilled water and the volume expansion was measured at different time point by Archimedes method. Disc-shaped samples were pre-immersed for 0, 7, or 30 days, elastic modulus and hardness were measured using Nanoindentation test; thermal cycling (TC) test was performed (5 °C-55 °C, 104 cycles), and volume expansion during TC was measured. Four kinds of resin die with pre-immersion for 0, 7, or 30 days were cemented to 5Y-Z crown, or epoxy dies without pre-immersion were cemented to 5Y-Z, 3Y-Z and lithium disilicate glass (LDG) crowns, and load-to-failure testing was performed before and after TC. Finite element analysis (FEA) and fractography analysis were also conducted.

RESULTS

The hygroscopic expansion was in the order: epoxy > Z350 > P60 > G10. Except for G10, the other three resin composites exhibited different degrees of hygroscopic expansion during TC. Only the elastic modulus and hardness of epoxy decreased after water storage. However, only the fracture loads of 5Y-Z and LDG crowns supported by epoxy dies were significantly decreased after TC. FEA showed a stress concentration at the cervical region of the crown after volume expansion of the die, leading to the increase of the peak stress at the crown during loading.

SIGNIFICANCE

Only the hygroscopic expansion of epoxy dies caused by TC led to the decrease in the fracture resistance of the 5Y-Z and LDG crown, which may be related to the decrease in the elastic modulus of the epoxy die and the tensile stress caused by it.

Effect of the coloring liquid shade and dipping time on the color, transparency, and flexural strength of monolithic zirconia

STATEMENT OF PROBLEM

The impact of different coloring liquid shades and dipping times on the color, transparency, and flexural strength of monolithic zirconia ceramics is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effects of different coloring liquid shades (A2, 3M2, and 5M2) and dipping times (no dipping, 30 seconds, 60 seconds, and 90 seconds) on the color difference (ΔE00), relative translucency parameter (ΔRTP00), and 3-point flexural strength (σ) of monolithic zirconia ceramics.

MATERIAL AND METHODS

Yttria-stabilized zirconia (3Y-TZP, 3 mol%) was cut into Ø16×1.2-mm plates (n=10) and 25×4×1.2-mm bars (n=15), which were colored using 3 shades of coloring liquid at 4 dipping times. Color coordinates were measured on a gray background by using a digital spectrophotometer with an integrating sphere attachment. The color and translucency differences were evaluated using 50:50% perceptibility (PT00 and TPT00) and acceptability (AT00 and TAT00) thresholds. The 3-point flexural strengths of the bar-shaped specimens were measured using a universal testing machine and analyzed using the Weibull distribution to calculate the Weibull modulus (m) and characteristic fracture strength (σ0). The data were analyzed with the 2-way ANOVA, Kruskal-Wallis, and LSD post hoc tests (α=.05).

RESULTS

Both shade and dipping time significantly affected the color and translucency of monolithic zirconia (P<.001). The ΔE00 was above the PT00 for all groups, with only 3M2-90 and A2-60 being below the AT00. The main cause of color differences was the difference in lightness. Only A2 showed ΔRTP00 below the TPT00 (A2-30 (ΔRTP00=0.26), A2-60 (ΔRTP00=0.29), and A2-90 (ΔRTP00=0.46)). All experimental groups showed translucency differences below TAT00. In addition, only the dipping time had a significant effect on the flexural strength of zirconia (P<.001).

CONCLUSIONS

The optical properties of monolithic zirconia ceramics were affected by the shade and dipping time of the coloring liquid. The mismatch in lightness was the main reason for the color difference. The dipping time affects the flexural strength of monolithic zirconia, whereas the shade of the coloring liquid did not seem to influence flexural strength.

Recent advances in dental zirconia: 15 years of material and processing evolution

OBJECTIVES

The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

METHODS

A literature search was performed in May/2023 using Web of Science Core Collection with the term "dental zirconia". The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

RESULTS

The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

SIGNIFICANCE

The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore imperative for dental clinicians and laboratory technicians to get the needed knowledge on zirconia material science, to follow manufacturers' instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Effects of Coloring Liquid Dipping Time and Surface Finishing Procedures on the Optical Properties of Monolithic Zirconia

BACKGROUND

Monolithic zirconia restorations can be produced from self-colored or uncolored blocks. The coloring procedure of uncolored blocks and surface treatments may affect the appearance of the restoration.

AIM

The purpose of this study is to evaluate the effect of liquid coloring and surface treatments on optical properties of translucent monolithic zirconia.

METHODS

All specimens were cut from zirconia blocks with a thickness of 1 mm. Specimens were dipped in the coloring liquids for 3 min, 5 min, and 7 min and then divided into groups of 10 specimens for surface treatments. Glaze and mechanical polishing were applied, and the color of the specimens was measured under the D65 lighting condition with a spectrophotometer device. Color values obtained from the CIE Lab formula and the translucency parameter (TP), opalescence parameter (OP), and contrast ratio (CR) were calculated. The normality of the data was confirmed with the Shapiro-Wilk test. A three-way analysis of variance (ANOVA) was used to assess the effect of dipping time, liquid shade, and surface treatments.

RESULTS

The effect of liquid shade and dipping time on the TP, OP, and CR parameters were statistically significant, and the TP values were reduced with the application of coloring liquid. Mechanical polishing groups had higher OP values and lower TP values than glazed groups.

CONCLUSION

In cases where high translucency is required, it may be advisable to use self-colored blocks and apply glaze as a surface treatment to achieve the desired optical properties.

Effect of low-temperature degradation and sintering protocols on the color of monolithic zirconia crowns with different yttria contents

This study investigated the effects of low-temperature degradation (LTD) on the L*, a*, and b* values of highly translucent zirconia crowns. Four types of zirconia disks with different yttria contents (IPS e.max ZirCAD LT, IPS e.max ZirCAD MT, IPS e.max ZirCAD MT Multi, IPS e.max ZirCAD Prime, Ivoclar) and two shades (A2 and BL) were used. A crown was manufactured using four types of zirconia and LTD treated. Color measurements were performed, and the color difference (ΔE00) before and after LTD was calculated. The microstructure was determined through X-ray fluorescence and X-ray diffractometry. Highly translucent zirconia crowns showed greater changes in the a* and b* values than in the L* value after LTD, regardless of the shade. The Multi2 crowns exhibited a discernible color change due to the LTD treatment. The X-ray fluorescence results did not reveal any apparent change in the microstructure between sintering programs for all zirconia specimens.

Optical properties of dental zirconia, bovine dentin, and enamel-dentin structures

OBJECTIVE

To evaluate the optical properties and the relative translucency parameter of Ceramill ZI White (3Y-TZP) and Ceramill Zolid FX White (5Y-PSZ) zirconia ceramic systems and compare them with those of the bovine dentin and enamel/dentin structures.

MATERIALS AND METHODS

3Y-TZP and 5Y-PSZ zirconia ceramic systems were evaluated. A 0.5-mm-thick 3Y-TZP (3Y-NC.5), 0.5-mm-thick (5Y-NC.5), and 1.4-mm-thick (5Y-C.14) were used. A 0.5-mm-thick dentin specimens and 1.4-mm-thick enamel/dentin specimens (n = 5) were obtained from anterior bovine maxillary teeth. Scattering, absorption, transmittance, and albedo coefficient were calculated using Kubelka-Munk's model. Data were statistically analyzed using Kruskal-Wallis and Mann-Whitney tests (p < 0.001), and goodness-of-fit coefficient (GFC). Relative translucency parameter differences were evaluated using translucency thresholds.

RESULTS

Reflectance, scattering, absorption, and transmittance properties were wavelength dependent. Good matches (GFC ≥ 0.999) in spectral reflectance were observed between 0.5-mm-thick dentin and 1.4-mm-thick enamel/dentin, and 3Y-NC.5 and 5Y-NC.5. Scattering was the main optical extinction process during light interaction with zirconia and dental structures, as indicated by albedo coefficient. Translucency differences were acceptable only for 3Y-NC.5 and the dentin structure, and 5Y-C.14 and the enamel/dentin structure.

CONCLUSIONS

Optical properties of 3Y-TZP and 5Y-PSZ dental zirconia differed from each other and from bovine dental structures. Nevertheless, 3Y-TZP showed similar relative translucency parameter to bovine dentin.

CLINICAL SIGNIFICANCE

To achieve the best esthetic results in restorative dentistry, it is crucial for clinicians to know about the optical properties of 3Y-TZP and 5Y-PSZ and to be able to compare these properties with those of dental structures.

High-Strength Organic-Inorganic Composites with Superior Thermal Insulation and Acoustic Attenuation

We demonstrate facile fabrication of highly filled, lightweight organic-inorganic composites comprising polyurethanes covalently linked with naturally occurring clinoptilolite microparticles. These polyurethane/clinoptilolite (PUC) composites are shown to mitigate particle aggregation usually observed in composites with high particle loadings and possess enhanced thermal insulation and acoustic attenuation compared with conventionally employed materials (e.g., drywall and gypsum). In addition to these functional properties, the PUC composites also possess flexural strengths and strain capacities comparable to and higher than ordinary Portland cement (OPC), respectively, while being ∼1.5× lighter than OPC. The porosity, density, and mechanical and functional properties of these composites are tuned by systematically varying their composition (diisocyanate, polyurethane, and inorganic contents) and the nature of the organic (reactivity and source of polyol) components. The fabrication process involves mild curing conditions and uses commonly available reagents (naturally occurring aluminosilicate particles, polyols, and diisocyanate), thereby making the process scalable. Finally, the composite properties are shown to be independent of the polyol source (virgin or recycled), underlining the generality of this approach for the scalable utilization of recycled polyols.

Effect of artificial aging on optical properties and crystalline structure of high-translucency zirconia

PURPOSE

To investigate the effect of different in vitro aging protocols on the optical properties and crystalline structure of high-translucency (HT) zirconia.

MATERIALS AND METHODS

Thirty-six specimens of HT and extra-high translucency (XT) zirconia were divided into three groups: control (CO)-no treatment; hydrothermal aging (HA)-autoclave aging for 12.5 h at 134°C, 2 bar; clinically related aging (CRA)-aging in the chewing simulator for 1.2 million cycles, followed by 50,000 thermocycles (5-55°C) and immersion in HCl (pH 1.2) for 15 h. Optical properties, crystalline structure, and surface roughness were analyzed and compared using analysis of variance (5% significance level).

RESULTS

There was no statistically significant effect of aging on translucency (p = 0.10), but CRA promoted the development of a high contrast ratio (p = 0.03). Aging did not cause significant color changes for HT (p = 0.65) or XT (p = 0.36). The proportion of monoclinic crystals increased to 40% for HT-zirconia after HA and 5% after CRA. No monoclinic crystals were detected for XT groups. There was no effect of aging on surface roughness (p = 0.77).

CONCLUSIONS

Although hydrothermal aging has been widely used to verify zirconia crystalline stability, it did not generate an effect similar to clinically related aging on the optical properties and crystalline structure of zirconia. HA affected the crystalline structure of HT-zirconia, and CRA compromised the optical properties of XT zirconia.

Evaluation of Shade Reproducibility and Mechanical Properties of Preshaded and Manually Shaded Monolithic Zirconia

OBJECTIVES

This study aimed to investigate the effect of manual coloring of blank monolithic zirconia blocks of four materials on their shade reproducibility, flexural strength, and surface roughness as compared with preshaded blocks of the same material.

METHODS AND MATERIALS

Eight disc-shaped specimens (diameter=12mm; height=2mm) and eight bar-shaped specimens (25×2×2 mm) were milled from the shaded and non-shaded blocks of the four zirconia systems: Ceramill, Kerox, White Peaks, and Dental Direkt. In total, 128 specimens were fabricated. The non-shaded specimens of each system were coloured according to A2 shade. The surface roughness (Ra) was measured using a contact profilometer. Color difference was calculated according to the CIEDE2000 formula. The bar-shaped specimens were loaded to fracture following three-point bend testing at 1 mm/minute crosshead speed. Data were analysed following one-way analysis of variance (ANOVA), and the Tukey post-hoc test (p=0.05).

RESULTS

Manual-coloring affected the surface roughness, color difference, and flexural strength of three materials in comparison to their preshaded blocks (p<0.05). White Peaks was not affected (p>0.05). Ra values ranged from 1.1-1.9; three materials were not affected (p>0.05). Kerox preshaded blocks were the smoothest (p<0.05) but became the roughest once colored (p<0.05). White Peaks and Dental Direkt materials had preshaded blocks matching the A2 guide-tab (p>0.05). DE was perceptible and unacceptable for the preshaded and manually shaded blocks of all materials tested (ΔE>1.8). While manual coloring reduced flexural strength of Ceramill and Dental Direkt (p<0.05), flexural strength was retained by Kerox and White Peaks (p>0.05).

CONCLUSIONS

White Peaks was not affected by the coloring technique. Coloring had negative effects on Ceramill flexural strength and color reproducibility.

Residual stress associated with crystalline phase transformation of 3-6 mol% yttria-stabilized zirconia ceramics induced by mechanical surface treatments

Monolithic dental prostheses made of 3-6 mol% yttria-stabilized zirconia (3-6YSZ) have gained popularity owing to their improved material properties and semi-automated fabrication processes. In this study, we aimed to evaluate the influence of mechanical surface treatments, such as polishing, grinding, and sandblasting, on the residual stress of 3-6YSZ used for monolithic prostheses in association with crystalline phase transformation. Plate specimens were prepared from five dental zirconia blocks: Aadva Zirconia ST (3YSZ), Aadva Zirconia NT (6YSZ), Katana HT (4YSZ), Katana STML (5YSZ), and Katana UTML (6YSZ). The specimens were either polished using 1, 3, or 9 μm diamond suspensions, ground using 15, 35, or 55 μm diamond discs, or sandblasted at 0.2, 0.3, or 0.4 MPa. The residual stress, crystalline phase, and hardness were analyzed using the cosα method, X-ray diffraction (XRD), and Vickers hardness test, respectively. Additionally, we analyzed the residual stress on the surfaces of monolithic zirconia crowns (MZCs) made of 4YSZ, 5YSZ, and 6YSZ, which were processed using clinically relevant procedures, including manual grinding, followed by polishing using a dental electric motor on the external surface, and sandblasting on the internal surface. Residual stress analysis demonstrated that grinding and sandblasting, particularly the latter, resulted in the generation of compressive residual stress on the surfaces of the plate specimens. XRD revealed that the ground and sandblasted specimens contained a larger amount of the rhombohedral phase than that of the polished specimens, which may be a cause of the residual stress. Sandblasting significantly increased the Vickers hardness compared to polishing, which may possibly be due to the generation of compressive residual stress. In the case of MZCs, compressive residual stress was detected not only on the sandblasted surface, but also on the polished surface. The difference in the residual stress between the plate and crown specimens may be related to the force applied during the automated and manual grinding and polishing procedures. Further studies are required to elucidate the effects of the compressive residual stress on the clinical performance of MZCs.

Influence of coloring techniques on the surface characteristics and color stability of a monolithic zirconia ceramic

STATEMENT OF PROBLEM

The color of monolithic zirconia restorations is obtained by presintering or postsintering coloring techniques. However, studies on the differences in surface characteristics and their influence on color stability are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the influence of shading and staining techniques for a zirconia ceramic on the surface characteristics and colorimetric parameters (color difference, translucency, and whiteness index) after exposure to coffee or red wine and then polishing.

MATERIAL AND METHODS

Ceramic disks (N=30; Ø10×1mm) were allocated into 3 groups: preshaded-shaded by the manufacturer (IPS e.max ZirCAD MT, shade A2); manually shaded-unshaded zirconia (IPS e.max ZirCAD MT, bleaching shade-BL) colored by the brushing technique, before sintering; stained-unshaded zirconia (IPS e.max ZirCAD MT BL) colored by the staining technique, after sintering. Spectrophotometric color assessments ensured the same initial perceived color (Vita Classical A2) for specimens included in the study (∆E00<1.77 acceptability threshold). Surface characteristics were assessed by scanning electron microscopy and atomic force microscopy. The specimens were immersed in coffee (n=5) or red wine (n=5) for 12 and 24 days and subsequently polished. The data were statistically and descriptively analyzed for color differences (∆E00), translucency parameters (TP00), and whiteness indexes for dentistry (WID), considering acceptability and perceptibility thresholds.

RESULTS

The shaded groups found an irregular ceramic surface with uniformly sized zirconia crystals. The stained group found a glass-covered smoother surface. Significant alterations in color parameters (∆E00, TP00, WID) were observed with immersion in pigmenting beverages (P<.001) both after 12 days, and after 24 days. The shaded specimens had greater color alterations after immersion but benefited from the polishing procedure, which reduced color differences below an acceptable threshold in comparison with the baseline. Stained specimens had lower color alterations after immersion, but the polishing protocol was detrimental as it whitened the ceramic by subsurface exposure.

CONCLUSIONS

The coloring technique influences the surface characteristics of zirconia ceramic and also the color parameters after exposure to colored beverages and polishing.

Effect of dynamic loading on fracture resistance of gradient zirconia fixed partial denture frameworks

STATEMENT OF PROBLEM

The new strength-gradient zirconia composed of 3-mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 5-mol% yttria-stabilized tetragonal zirconia polycrystal (5Y-TZP) has been claimed to have superior mechanical properties. However, data on the fracture resistance of 3-unit gradient 5Y-TZP and 3Y-TZP fixed partial denture frameworks are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of dynamic loading on the fracture resistance of gradient zirconia fixed partial denture frameworks.

MATERIAL AND METHODS

Two standardized stainless-steel master dies were designed to simulate a mandibular left second premolar and second molar prepared to receive zirconia frameworks. The frameworks were designed with a 0.6-mm uniform wall thickness. The mesiodistal width of the connectors was 3 ±0.02 mm, and the occlusogingival height was 3 ±0.02 mm. Forty zirconia frameworks were fabricated and divided into 2 groups according to the tested materials (n=20): 3Y-TZP and gradient 5Y-TZP and 3Y-TZP. The frameworks were cemented onto their corresponding dies with a conventional glass ionomer cement. Half of the cemented frameworks in each group underwent 600 000 cycles of dynamic loading in a mastication simulator (n=10). The other half was fractured without dynamic loading (n=10). Fracture resistance measurements (N) for each framework were recorded by using a universal testing machine at a crosshead speed of 1 mm/min. A fractured framework from each group was examined by using a scanning electron microscope (SEM) at ×100 magnification. The data obtained were statistically analyzed by using 2-way ANOVA, the pairwise Tukey honestly significant difference (HSD), and simple main effect tests to detect the difference between group mean values (α=.05).

RESULTS

The mean ±standard deviation of fracture load value before dynamic loading was 1919 ±193 N for the 3Y-TZP group and 908 ±99 N for the gradient 5Y-TZP and 3Y-TZP group. In addition, the mean fracture load value after dynamic loading was 1418 ±163 N for the 3Y-TZP group and 716 ±85 N for the gradient 5Y-TZP and 3Y-TZP group. The interaction between the effects of the zirconia material and dynamic loading on the fracture resistance was statistically significant (P=.002). The 3Y-TZP group had a statistically significant, higher fracture load mean value the gradient 5Y-TZP and 3Y-TZP group before and after dynamic loading (P<.001).

CONCLUSIONS

The fracture resistance of 3Y-TZP frameworks was significantly higher than that of gradient 5Y-TZP and 3Y-TZP frameworks before and after dynamic loading. Dynamic loading significantly reduced the fracture resistance of 3Y-TZP and gradient 5Y-TZP and 3Y-TZP frameworks.

Shear Bond Strength of Colored and Sandblasted Zirconia to Ceramic Veneers Fabricated by the Pressing and Layering Techniques: An In Vitro Study

Introduction

Porcelain-veneered zirconia (PVZ) restorations are increasingly used due to their optimal esthetics and high strength. However, chipping of porcelain limits the application of PVZ restorations. The aim of this study was to assess the shear bond strength (SBS) of colored and sandblasted zirconia to ceramic veneers fabricated by the pressing and layering techniques.

Materials and Methods

Sixty cubic zirconia specimens (10 × 10 × 2 mm) were assigned to three groups according to their surface treatment: (I) control, (II) sandblasting with 50 μm alumina particles (S), and (III) coloring (C). Each group was subsequently divided into two subgroups according to the porcelain-veneering technique: (I) layering (L) and (II) pressing (P). The specimens underwent 10,000 thermal cycles between 5 and 55°C, and their biaxial SBS was measured in an electromechanical universal test machine (0.5 mm/min with 2.5 kN load cell). The failure mode was also assessed under a stereomicroscope. Three samples were randomly selected from each subgroup (n = 18) for examination of zirconia-phase transformation by X-ray diffraction (XRD). Two-way and one-way ANOVA followed by the post hoc Tukey test were used to analyze statistical differences among the groups and subgroups (α = 0.05).

Results

The sandblasted zirconia with press porcelain (SP) subgroup showed the highest (24.40 ± 8.16 MPa) and the colored zirconia with press porcelain (CP) subgroup showed the lowest (13.76 ±3.62 MPa) SBS. All failures were cohesive. Rate of phase transformation in layered porcelain was significantly lower than that in pressed porcelain (P < 0.01).

Conclusion

The sandblasted group showed the highest and the colored group showed the lowest SBS; the layered group showed higher SBS than the pressed group.

Fabrication of color-graded feldspathic dental prosthetics for aesthetic and restorative dentistry

OBJECTIVE

Feasibility investigation of natural teeth shades replication on dental prosthetics fabricated via functionally graded additive manufacturing (FGAM) using combination of feldspathic porcelain (FP) and yttrium aluminum garnet cerium (Y₃Al₅O_12:Ce, YAG:Ce) as a promising esthetic restoration option.

METHODS

Color-graded feldspathic crown fabrication parameter through FGAM method was comprehensively examined from the slurry rheology, cure depth, debinding to sintering temperature. Effect of light absorbent also checked towards overcuring reaction during UV exposure by the shape comparison. Lastly, the flexural bending strength measured following ISO 6872:2015 to assure the applicability. Applying the studied parameter, natural teeth shades then imitated and investigated by alteration of FP and FP + 0.1 wt% YAG:Ce (Y-FP). Generated color across the structure captured through mobile camera, interpreted through the CIELAB coordinate and the gradation confirmed by the color differences (ΔE₀₀) calculated using CIEDE2000 formula.

RESULT

Parameter study indicated that 70 wt% of FP slurry with 3 wt% dispersant and 0.2 wt% light absorbent is favored. It produces excellent flowability in our FGAM system with less overcuring justified by edge margin reduction from 95.65° to 90.00° after UV exposure on rectangle shapes masking. The obtain structure also offers adequate flexural bending strength of 106.26 MPa (FP) and 101.36 MPa (Y-FP) after sintering at 780 °C. This validated the materials as class 2 dental prosthetics citing ISO 6872:2015. Color gradation was verified by the yellow b* value reduction (14.8 to -3.33) as it shifted from cervical to incisal area while ΔE₀₀ further affirmed the differences from each segment in comparison with the FP and Y-FP.

SIGNIFICANCE

Color gradation was successfully replicated by FP and YAG:Ce composition shift via FGAM technique. This result highlights the potential of FGAM as an alternative for fabricating dental prosthetics with high efficiency and improved esthetic appeal.

The effects of heating rate and sintering time on the biaxial flexural strength of monolithic zirconia ceramics

The strength of zirconia ceramic materials used in restorations is dependent upon sintering. Varying sintering protocols may affect the biaxial flexural strength of zirconia materials. This in vitro study was conducted to investigate the effects of sintering parameters on the biaxial flexural strength of monolithic zirconia. Two different monoblock zirconia ceramics were used. Following coloration, samples of both types of ceramics were divided into groups according to whether or not biaxial flexural strength testing was performed directly after sintering or following thermocycling. Biaxial flexural strength data was analysed with a Shapiro Wilk normality test, followed by 1-way ANOVA, Tukey post hoc tests for inter-group comparisons, and paired samples t-tests for intra-group comparisons. A significant difference was found between the biaxial flexural strengths of Zircon X and Upcera ceramics before thermocycling (p<0.05). In both Zircon X and Upcera ceramic groups, the thermocycling process created a significant difference in the biaxial flexural strength values of the ceramic samples in Group 6 (p<0.05) which had the slowest heating rate and longest holding time. The zirconia ceramics have higher BFS at higher heating rates either before or after thermocycling. The holding time has significant effects on thermocycling and flexural strength. The zirconia achieved its optimum strength when it sintered at longer time regardless of heating rates.

Translucency and mechanical behavior of partially stabilized monolithic zirconia after staining, finishing procedures and artificial aging

Partially stabilized zirconia (5Y-PSZ) has been widely used to manufacture indirect monolithic restorations, and the effect of finishing procedures on the optical and mechanical properties of these materials are still unclear. The purpose of this study was to evaluate the effect of staining, polishing and glazing on surface roughness, crystalline phase content, microhardness, fracture toughness, dynamic elastic modulus, three-point flexural strength, strain distribution, color (∆E₀₀/∆L/∆a/∆b), and translucency before and after artificial accelerated aging (water spray and ultraviolet) of 5Y-PSZ. Bar-shaped and rectangle-shaped specimens of the 5Y-PSZ were prepared and divided into six groups, according to finishing procedure: GC (control), GS (staining), GG (glazing), GSG (staining and glazing), GP (polishing), GSP (staining and polishing). There was a significant difference between groups for surface roughness (p < 0.05), dynamic elastic modulus (p = 0.007), microhardness (p =  < 0.05), ∆E₀₀ (p = 0.010), and ∆a (p = 0.008). GC presented higher cubic phase content, and the stained groups (GS, GSG and GSP) presented higher monoclinic content. The different finishing procedures affected roughness, dynamic elastic modulus, microhardness, and color of 5Y-PSZ; polishing being the finish that provides minors changes to the 5Y- PSZ. Accelerated artificial aging caused color change, regardless of finishing procedure used.

Pre-sintering pigmentation techniques do not affect the fatigue behavior of adhesively luted 4YSZ restorations

This study aims to characterize the effect of shading techniques on the fatigue behavior of a 4YSZ ceramic (4 mol% yttrium stabilized zirconia) adhesively bonded to a dentin analogue (fiber-reinforced epoxy resin). 4YSZ ceramic discs (IPS e.max ZirCAD, Ø = 10 mm and 1 mm of thickness) were allocated according to the factor 'shading technique' into 4 groups: Brush- unshaded ceramic disc (IPS e.max ZirCAD BL) pigmented at the pre-sintered stage with pigment solution applied manually using a round liner brush; Immersion- unshaded ceramic disc pigmented through immersion in the solution for 1 s on only one side of the ceramic disc; Manufacturer- specimens already shaded by the manufacturer (IPS e.max ZirCAD MT A2 - Manufacturer group); Control- a control condition with absence of pigment (i.e. non-pigmented specimens). The specimens were sintered and a spectrophotometer (SP60, EX- Rite) was used to ensure that the same perceived color (i.e. pigment saturation) was achieved in the different shading strategies (Manufacturer, Brush or Immersion groups). To do so, the color differences (ΔE00) were calculated using the CIEDE 2000 equation; and an ΔE00 of up to 1.77 was considered as an acceptability threshold. Dentin analogue discs were obtained (Ø = 10 mm and 2.5 mm of thickness) and randomly allocated into pairs with the 4YSZ ceramic discs. Next, the pairs were adhesively bonded using a resin cement (Multilink N). The bonded assemblies (n = 15) were tested for fatigue using the step-stress test method (frequency of 20 Hz; 10,000 cycles per step, initial load 200 N; step-size of 100 N, up to 700 N; and after, step-size of 50 N, until specimen failure/fracture or radial cracks). Fatigue failure load (FFL) and number of cycles for failure (CFF) were recorded for statistical analysis. Fractographic features were accessed, and complementary roughness, topography, grain size and phase content analyses were performed. No statistical differences were observed in the fatigue behavior among the non-shaded condition (Control group - 880 N) and the shaded specimens (Manufacturer - 887 N, Brush - 820 N, and Immersion - 850 N groups; p > 0.05). However, the use of a brush shading technique induced slightly inferior fatigue mechanical behavior of the restorative set compared to the specimens already shaded by the manufacturer (p = 0.027). No differences in Weibull modulus were observed among the tested groups. The specimens pigmented by the brush technique demonstrated a rougher surface, with statistically higher Rz values, in addition to a larger grain size in comparison to all other conditions (p< 0.05). No m-phase content was identified (only t and c phases were detected). Thus, the shading techniques used to provide a Vita classic A2 shade does not negatively affect the mechanical fatigue properties of a bonded 4YSZ ceramic. However, the brush technique has detrimental effect on the fatigue behavior compared to when the ceramic was already provided in a shaded format by its manufacturer.

Effect of pigmentation techniques on the fatigue mechanical behavior of a translucent zirconia for monolithic restorations

This study aims to characterize the effect of pre- and/or post-sintering pigmentation techniques on the fatigue behavior of a 4YSZ ceramic (4 mol% yttrium stabilized zirconia). First, 4YSZ ceramic discs (IPS e.max ZirCAD, 15 mm diameter, and 1.2 mm thickness) were obtained and allocated according to the 'pigmentation technique' factor into 5 groups: Ctrl - unshaded ceramic disc (IPS e.max ZirCAD BL); Manuf - ceramic discs shaded by the manufacturer at block formation (IPS e.max ZirCAD MT A2); Brush - unshaded ceramic disc pigmented in the pre-sintered stage using a brush; Stain - unshaded ceramic disc pigmented in the post-sintering stage using a stain glaze (shade A2); Brush + Stain - combination of Brush and Stain techniques. The specimens were sintered and analyzed by a spectrophotometer (VITA Easyshade) for color differences (ΔE₀₀ calculated according to CIEDE, 2000) to ensure that they present the same perceived color (ΔE₀₀ ≤ 1.77, acceptability threshold). The specimens (n = 15) were submitted to biaxial flexural fatigue testing using a cyclic fatigue method (frequency of 20 Hz; 10,000 cycles per step; initial stress of 200 MPa; and step-size of 25 MPa) until specimen fracture. Fatigue strength (FS) and number of cycles until failure (CFF) were recorded with statistical purposes. Fractographic, complementary topography, elemental and roughness analyses were performed. The Ctrl shown the highest FS, CFF, and survival rates compared to the other conditions (p < 0.05), being only statistically similar to Brush. Stain showed the lowest FS, CFF and survival rates (p < 0.05). The Manuf and Brush + Stain shown intermediary performance. In regards of Weibull moduli, Brush shown the lowest values (lowest structural reliability), for both FS and CFF, being only statistically similar to Manuf. Topography analysis shown a uniform size of zirconia crystals in Ctrl, Manuf and Brush. In addition, a smooth glassy surface with some spots of impregnated pigment was observed in the groups where glaze was applied (Stain and Brush + Stain). This finding was corroborated by elemental analysis. Despite that, Stain and Brush + Stain presented the roughest surface (p < 0.05). In summary, the pigmentation techniques used to provide a Vita classical A2 color shade have a detrimental effect on the mechanical fatigue properties of a 4YSZ ceramic, except for the technique performed with the brush in the block pre-sintering stage. Despite this, the brush technique presents great variability in mechanical performance, resulting in lower structural reliability.

Optimization of self-colored dental zirconia block parameters for an effective machining performance using response surface methodology based artificial bee colony

OBJECTIVES

Self-colored dental zirconia blocks eliminate the drawbacks of the coloring process of traditional blocks and ensure disposal time and cost in dental prosthesis laboratories. Like traditional dental zirconia frameworks, self-colored dental zirconia frameworks are produced by grinding pre-sintered zirconia blocks via manual or CAD/CAM operating devices. The process parameters such as powder type, pre-sintering temperature and doping amount of coloring powder affect the machining performance of blocks through microhardness, bulk density and shrinkage values (diameter and thickness) during fabrication. In this study, we propose the response surface methodology (RSM) based artificial bee colony (ABC) algorithm to adjust the suitable combination of process parameters on the machining performance for self-colored dental zirconia blocks.

METHODS

The optimum machining performance of the self-colored dental zirconia blocks is investigated using a multi-objective model named as RSM integrated ABC algorithm. Sensitivity analysis is utilized to identify the effects of objectives on the responses considering the different importance weights of each objective.

RESULTS

After obtaining the optimized parameters of pre-sintering temperature= 1066.546 ℃, doping amount of coloring powder= 10.638 (wt%) and powder type=TZP-1 by multi-objective ABC algorithm, the machining performance-related properties are found out Hυ1= 44.918 MPa, bulk density= 3.098 g/cm³, and shrinkage (%) (diameter)= 0.197, shrinkage (thickness)= 0.254.

SIGNIFICANCE

Process parameters that directly affect the machining performance of the self-colored dental zirconia blocks were investigated with a multi-objective method for the first time in the literature. The results provide a base for further research on optimizing the self-colored dental zirconia block parameters for an effective machining performance.

Mechanical properties-translucency-microstructure relationships in commercial monolayer and multilayer monolithic zirconia ceramics

OBJECTIVES

To evaluate the phase composition, microstructure, optical properties and mechanical properties of eight commercially available multilayer and monolayer monolithic dental zirconias.

METHODS

Five commercial 3Y-TZP (GC ST, GC HT [GC, Tokyo Japan]; Katana ML, Katana HT [Kuraray Noritake] and Lava Plus [3M Oral Care]) and three Y-PSZ (Katana STML, Katana UTML [Kuraray Noritake]; GC UHT [GC, Tokyo Japan]) zirconia ceramic grades were cut in plate-shaped specimens, sintered according to the manufacturer's instructions and mirror polished. The zirconia chemical composition was determined using X-ray fluorescence (XRF), phase composition was characterized using X-ray diffraction (XRD), while the grain size was measured using scanning electron microscopy (SEM). The translucency Parameter (TP) and Contrast Ratio (CR) were measured with a spectrophotometer (n = 10/group). The indentation fracture toughness (n = 10), Vickers hardness (n = 10) and biaxial strength (n = 20) of the sintered ceramics were assessed. The stress distribution during biaxial testing was assessed by Finite element analysis (FEA). Statistical analysis involved one-way ANOVA and post-hoc Tukey's HSD test and Pearson correlation test (α = 0.05).

RESULTS

FEA showed that the stress distribution in plate shape specimens was the same as for disks, rationalizing the use of plates for biaxial strength testing. As expected, higher quantities of Y₂O₃ were related to a higher cubic ZrO₂ phase content and lower tetragonality t-ZrO₂, which improved translucency but diminished flexural strength and toughness. While there was no significant correlation between grain size and other material properties, addition of pigments to the zirconia grade statistically negatively affected hardness.

CONCLUSION

Even though an improvement in strength and translucency could be recorded for the last Y-TZP generation, future research still needs to strive for combined improvement of optical properties and mechanical reliability of zirconia ceramics.

Fracture Resistance and Fracture Behaviour of Monolithic Multi-Layered Translucent Zirconia Fixed Dental Prostheses with Different Placing Strategies of Connector: An in vitro Study

Purpose

To evaluate the effect of different placing strategies performed in the connector area on fracture resistance and fracture behaviour of monolithic multi-layered translucent zirconia fixed dental prostheses (FDPs).

Materials and Methods

Thirty 3-unit monolithic FDPs were produced and divided into three groups (n = 10) based on the different strategies for placing the connector area of FDPs in multi-layered zirconia blank with varying contents of yttria ranging from 4 to 5 mol%. The groups were as follows: FDPs with connectors placed in dentin layer with 4 mol% yttria content, FDPs with connectors placed in gradient layer, and FDPs with connectors placed in translucent layer with 5 mol% yttria content. A final group (n = 10) of conventional monolithic zirconia with a monolayer of yttria content (4 mol%) has been used as a control group. The specimens were artificially aged using thermocycling and pre-loading procedures and subsequently loaded to fracture using a universal testing machine. Fracture loads and fracture behaviour were analyzed using one-way ANOVA and Fisher’s exact tests and statistically evaluated (p ≤ 0.05).

Results

There were no significant differences in fracture loads among the groups based on the placing strategies of the connector area of the FDPs in the multi-layered translucent zirconia blank (p > 0.05). There was no significant difference in fracture loads between monolithic multi-layered translucent zirconia and conventional monolithic translucent zirconia materials (p > 0.05). Fracture behaviour of FDPs with connector area placed in translucent layer differed significantly compared to FDPs with connector area placed in dentin layer and FDPs in control group (p = 0.004).

Conclusion

The placing strategies of the connector used in the computer aided design and manufacturing procedures do not considerably affect fracture resistance of monolithic FDPs made of multi-layered translucent zirconia. Monolithic FDPs made of multi-layered translucent zirconia show comparable strength to FDPs made of conventional translucent zirconia, but with different fracture behaviour.

Effect of grinding on the optical properties of monolithic zirconia

OBJECTIVES

The purpose of this study was to analyze the color infiltration in monolithic zirconia after grinding, which plays an essential role in the color stability of restorations after occlusal adjustment.

METHODS

One hundred and eight white zirconia plates (36 Dentsply Sirona-Cercon high translucency [C], 36 Zirkonzahn-Prettau [P], 36 Zirkonzahn-Prettau anterior [Pa]) were stained by infiltration with target colors Vita A2, A3.5, and A4. In a standardized experimental setup, all plates were grinded in nine successive steps (from baseline to 500 μm), and color was measured with a spectroradiometer at each grinding step. Color differences ΔE₀₀ between initial color and after each grinding step were calculated. The data was analyzed using regression, Kruskal-Wallis test, inverse prediction, and simultaneous 95%-confidence intervals.

RESULTS

Grinding had a significant effect on color stability across all zirconia types and target colors (p < 0.001). At each cut level and target color, ΔE₀₀ means for groups C, P, and Pa were statistically different (p < 0.05). Among the three zirconia types, Pa had the lowest ΔE₀₀ mean score for cut levels 20-100 μm for A2 and between 50 and 300 μm for A3.5 and A4. For all other cut levels, differences between Pa and P were not significant (α = 0.05). C had the largest mean ΔE₀₀ except for A4 until 100 μm, where it was second best.

CONCLUSION

Zirconia type had a significant effect on the color infiltration depth across all target colors. Pa showed the best color stability until 150 μm material reduction, whereas C presented significantly less color stability than the other two zirconia materials.

CLINICAL SIGNIFICANCE

Color stability is important for minimizing the risk of restoration failure due to unacceptable color changes after grinding. Pa is less susceptible to color change and has an additional buffer of 60 μm until exceeding the color acceptability threshold. When higher flexural strength is needed, P is to be preferred.

Effects of Hydrothermal Treatment on the Phase Transformation, Surface Roughness, and Mechanical Properties of Monolithic Translucent Zirconia

OBJECTIVES

This study aimed to investigate the effects of hydrothermal treatment on four types of monolithic, translucent, yttria-stabilized, tetragonal zirconia polycrystals (Y-TZPs).

METHODS AND MATERIALS

Two commercially available Y-TZP brands-SuperfectZir High Translucency (Aidite Technology Co, China) and Katana HT (Kuraray Noritake Dental, Japan) were assessed. For each brand of Y-TZP, materials of four coloring types, including noncolored (NC), colored by staining (CS), precolored (PC), and multilayered (ML) specimens were investigated after hydrothermal aging in an autoclave at 134°C/0.2 MPa for 0 (control group), 5, 10, and 20 hours. The tetragonal-to-monoclinic phase transformation, surface roughness, flexural strength, and structural reliability (Weibull analysis) were measured and statistically analyzed (α=0.05). The subsurface microstructure was analyzed with scanning electron microscopy.

RESULTS

The group ML exhibited the lowest flexural strength and Weibull characteristic strength among the four coloring types (p<0.05). Slight increases in the monoclinic phase volume, flexural strength, and Weibull characteristic strength were observed after hydrothermal aging (pall<0.05). Regardless of coloring type, no significant effects of aging on the Weibull modulus or surface roughness were found for the tested materials. Compared with the Katana HT cross-sections, the SuperfectZir High Translucency cross-sections exhibited a similar but thicker transformation zone.

CONCLUSIONS

The coloring procedure and material type were found to affect the mechanical properties and aging resistance of translucent monolithic Y-TZP ceramics. Regardless of the aging time, the surface roughness of the tested Y-TZP ceramics remained unchanged.

The effect of thermocyclic aging on color stability of high translucency monolithic lithium disilicate and zirconia ceramics luted with different resin cements: an in vitro study

Background

High-translucency monolithic zirconia were developed to combine the esthetics of all ceramic restorations with the strength properties of zirconia. The purpose of this study was to compare the color stability of high-translucency monolithic zirconia ceramics with lithium disilicate luted using light-cure versus dual-cure resin cements following thermocyclic aging.

Methods

Forty specimens, each composed of 10 × 10 × 1 mm ceramic slice luted to dentin surface of an extracted tooth, were prepared and assigned into four groups (n = 10) as follows; LiDi/LC: lithium disilicate luted by light-cure resin cement; LiDi/DC: lithium disilicate luted by dual-cure resin cement; Zr/LC: zirconia luted by light-cure resin cement; and Zr/DC: zirconia luted by dual-cure resin cement. Color analysis of the specimens was performed before and after 3000 thermal cycles by means of spectrophotometry. The CIE L*a*b* values of the specimens were measured, and data were analyzed statistically at a significance value of p < 0.05.

Results

Thermocycling resulted in a significant change in color coordinates of specimens with an overall ΔE = 3.59 ± 1.60, but there was no statistically significant difference in the color change value among all tested groups (P = 0.756).

Conclusions

At 1 mm restoration thickness, the color stability of high-translucency monolithic lithium disilicate and zirconia ceramics were not significantly different irrespective of the cement type used.

Clinical implication Understanding the difference in color stability of dental ceramics may help in determining long-term esthetic result.

Effect of external staining on the optical properties and surface roughness of monolithic zirconia of different thicknesses

STATEMENT OF PROBLEM

The effect of the number of external staining applications on the color, translucency, and surface roughness of monolithic zirconia is not known.

PURPOSE

The purpose of this in vitro study was to investigate the effect of glazing and repeated external staining on the color, translucency, and surface roughness of 2 types of monolithic zirconia with 2 different thicknesses.

MATERIAL AND METHODS

Thirty-two 10×10-mm monolithic zirconia blocks, of which 16 were super high translucent and 16 were high translucent, were milled. The thickness of half the blocks was 0.7 mm, and the others were 1.2-mm thick. Four specimens in each subgroup were colored with value stain, and the other half with yellow stain. Staining was repeated twice for each specimen. The color and translucency of each specimen were measured before overglazing as the baseline, after overglazing, and after each staining process by using a spectrophotometer. The surface roughness (Ra) was measured after overglazing, the first staining, and the second staining. Data were analyzed by using repeated measures ANOVA and the independent t test (α=.05).

RESULTS

The number of coloring, type of coloring, and thickness changed the optical properties of monolithic zirconia. a∗, b∗, L∗ Value and ΔE were affected by the stage of firing (glazing, first staining, and second staining) (P<.001). ΔE Increased after the first and second staining in each type and thickness of monolithic zirconia and each type of stain. The amount of ΔE change was more for the first staining stage. The color difference (ΔE) after overglazing ranged from 3.08 to 3.75. The highest changes in color were related to the first staining with yellow colorant of super-high-translucent monolithic zirconia (cubeX²) with 0.7-mm thickness (ΔE=17.62). The surface roughness of the specimens was affected by the number of colorings (P<.001). In all groups, the first staining increased the surface roughness, and the second staining decreased the surface roughness. The type of monolithic zirconia (P=.756), thickness (P=.735), and type of stain (P=.239) had no significant effect on the surface roughness.

CONCLUSIONS

The first stage of staining with both stains caused more color change in all types and thicknesses of monolithic zirconia. Translucency increased after glazing and decreased after the first and second staining. The first staining increased the surface roughness, and the second decreased the surface roughness.

Effect of grinding adjustments on the color of monolithic zirconia

STATEMENT OF PROBLEM

Monolithic zirconia restorations have become popular because of their excellent mechanical properties and acceptable esthetics. While the biomechanical properties of zirconia have been investigated, research into their esthetic properties is sparse. Zirconia can be colored by infiltration before sintering, although how occlusal adjustment can affect restoration color is unclear.

PURPOSE

The purpose of this in vitro study was to analyze the color of differently characterized monolithic white and precolored zirconia specimens after standardized grinding.

MATERIAL AND METHODS

White and precolored monolithic zirconia plates (Cercon ht) (n=36) were stained by infiltration with target colors Vita A2, A3.5, and A4. In a standardized experimental arrangement, all plates were ground in 9 steps, and the color was measured with a spectroradiometer at each step. Color differences (ΔE₀₀) were analyzed by using regression analysis, the Kruskal-Wallis test, and the inverse prediction with confidence intervals (α=.05).

RESULTS

Mechanical material removal had a significant effect on the color stability (P<.05) of both white and precolored monolithic zirconia. For each grinding step and each target color, the precolored groups had a significantly lower ΔE₀₀ mean than the white groups (P<.05), except at the 20-μm step for groups W-A4 and P-A4 (P>.05). For target color A2, the difference was significant with P<.001 at all 9 grinding steps. For target color A3.5, the difference was significant with P=.003 at grinding level 20 μm, and P<.001 for all other grinding steps. For target color A4, the difference was not significant with P=.603 at grinding level 20 μm, whereas the difference was significant with P=.007 at grinding level 50 μm, and with P<.001 for all other grinding steps.

CONCLUSIONS

Precolored zirconia had less color change after grinding than white zirconia. This study established grinding depths for white and precolored zirconia corresponding to color perceptibility and acceptability thresholds.

Effects of sintering time on translucency and color of translucent zirconia ceramics

OBJECTIVE

To investigate the effect of sintering time on translucency and color of translucent zirconia ceramics.

MATERIALS AND METHODS

Sixty translucent zirconia (5Y-TZP) specimens (12 × 1.6 mm, Katana (KAT) and NexxZr (NEX)) were divided into six groups (n = 10) according to sintering time; 1, 2 (specified by manufacturer-control), and 3 hs. Color differences (ΔEab) within same material when sintered using different times and between materials when same sintering time was used were calculated, as well as translucency parameters (TP) after the use of each sintering time. Data for ΔEab and TP were analyzed with two-way ANOVAs, pairwise comparisons, and Fisher's LSD Test (Post-hoc).

RESULTS

When 1 or 3 h were used, color difference from the color of specimens when manufacturer-specified time (2 h) was used was similar for both KAT and NEX. ΔEab values of NEX were significantly higher than KAT when sintered for 1 or 3 h (P < 0.001). TP increased for KAT when sintered for 1 h, and was higher than NEX. TP for NEX increased when sintered for 3 h (P < 0.001), but was not significantly different than that of KAT.

CONCLUSIONS

Effect of 1 or 3-h sintering on color was similar for both zirconias. For NEX, this effect resulted in greater differences from the color of specimens when manufacturer-specified time was used. Sintering for 1 h increased the translucency of KAT.

CLINICAL SIGNIFICANCE

When high translucency is desired, clinicians may prefer 1-h sintering for KAT zirconia. When lower translucency is desired, 3-h sintering for NEX can be used. However, for NEX, the color after 1- or 3-h sintering was different than the color obtained with manufacturer-specified time, and this difference may affect the final color of restoration compared to the final color when manufacturer-specified time is used.

Effect of mouth rinses on optical properties of CAD-CAM materials used for laminate veneers and crowns

OBJECTIVE

The purpose of this study was to investigate the effect of mouth rinses on the color and translucency of three computer-aided design and computer-aided manufacturing (CAD-CAM) restorative materials in laminate veneer and crown thicknesses.

METHODS

Specimens from two different 5Y-TZP zirconia (InCoris TZI (IT), and Zirkonzahn (ZH)) and lithium disilicate (IPS e.max CAD [IC]) in two different thicknesses (0.7 mm for laminate veneer, and 1.5 mm for crown) were sectioned. All specimens were colored with an A2-shade liquid, and the baseline color values were recorded according to the CIELab system with a spectrophotometer. Each group was divided into two subgroups (n = 15) according to the immersion solution: two different mouth rinses, KL (Klorhex), and LI (Listerine, cool mint) for 180 hours. The color coordinates (L*, a*, b*) of the specimens were measured before and after immersion in a mouth rinse, and TP and ΔE₀₀ color differences were calculated by using the CIEDE2000 color difference formula. A 3-way ANOVA, Bonferroni test, and 1-sample t tests were used to analyze the data (α = 0.05).

RESULTS

The 3-way ANOVA revealed a significant interaction of material, thickness, and mouth rinse for translucency parameter and color difference (ΔE₀₀ ) data (p < 0.001). TP decreased for both zirconia materials in laminate veneer thickness when immersed in LI mouth rinse (p < 0.05). No difference was found among the TP mean values of three materials in crown thickness after immersed in mouth rinses (p > 0.826). Both zirconia materials immersed in LI showed greater discoloration than after immersed in KL (p < 0.05). A significant difference was found in color change values among three materials for the laminate veneer thickness after immersed in LI (p < 0.001). However, all color difference values were within the clinical acceptability threshold, except for when ZH in laminate veneer thickness was immersed in LI.

CONCLUSIONS

The color change of ZH zirconia with LI mouth rinse in laminate veneer thickness was high. For both zirconia ceramics, translucency decreased and the color was less stable in laminate veneer thickness after immersed in LI compared to the crown thickness.

CLINICAL SIGNIFICANCE

The results of this in vitro study suggest that long-term use of alcohol-containing mouth rinse may alter the optical properties of tested CAD-CAM materials in tested laminate veneer thickness. For color stability with the long term use of tested mouth rinses, lithium disilicate may be preferred for both types of restorations.

Influence of coloring liquid immersion on flexural strength, Vickers hardness, and color of zirconia

STATEMENT OF PROBLEM

Zirconia is a widely used restorative material, yet its white color does not meet esthetic requirements and coloring is needed. However, the effects of different durations of coloring have not been thoroughly investigated.

PURPOSE

The purpose of this in vitro study was to evaluate the effects of coloring liquid immersion of different durations on the flexural strength, Vickers hardness, and color of zirconia.

MATERIAL AND METHODS

Fifty bar-shaped and 60 disk-shaped zirconia specimens were milled from nonshaded (Copran Zri) and preshaded (Copran Zri Precoloured Medium) disks by using a computer-aided design and computer-aided manufacturing (CAD-CAM) system. Preshaded specimens were assigned to the group PS and received no further immersion. Nonshaded specimens were divided into 4 subgroups as per immersion time in coloring liquid (Copran Color A2 Shade): control (C; no immersion), G45 (45 seconds of immersion), G90 (90 seconds of immersion), and G135 (135 seconds of immersion). Bar-shaped specimens were subjected to a 3-point flexural strength test by using a universal testing machine (Lloyd LRX), following the International Organization for Standardization (ISO) 6872:2015 specification. Color coordinates of disk-shaped specimens were measured with a spectrophotometer (VITA Easyshade Advance 4.0) over a gray background. Color differences were calculated by using both CIE76 and CIEDE2000 formulas, followed by a comparison of obtained values with perceptibility (CIE76: 3.7, CIEDE2000: 0.8) and acceptability (CIE76: 6.8, CIEDE2000: 1.8) thresholds of each formula. Subsequently, Vickers hardness measurements were conducted. The data were statistically analyzed by using the Shapiro-Wilk test, 1-way ANOVA, and the Tukey HSD test (α=.05).

RESULTS

Immersing zirconia specimens in coloring liquid decreased the flexural strength and hardness values, as group C presented the highest values for both parameters. In addition, prolonged duration had a negative effect on mechanical properties. The flexural strength of groups PS and G45 (P=.993) and groups G90 and G135 (P=.999) was statistically similar. For Vickers hardness, group G135 presented the lowest values with the difference between groups G90 and G135 being statistically similar (P=.061). Color differences among groups varied from 2.49 to 27.33 for ΔE∗, whereas ΔE₀₀ values ranged from 0.58 to 8.26. Compared with group C, ΔE∗ values of all shaded specimens exceeded the acceptability threshold of 6.8. The color difference between each group was higher than the acceptability threshold values of both ΔE∗ and ΔE₀₀ except for groups G45 and PS.

CONCLUSIONS

Colored zirconia specimens had lower flexural strength and Vickers hardness values than nonshaded specimens, regardless of the immersion time or coloring technique.

Mechanical properties, aging stability and translucency of speed-sintered zirconia for chairside restorations

OBJECTIVE

To evaluate the performance of zirconia ceramics sintered in a speed sintering induction furnace by comprehensive understanding of their optical and mechanical properties, microstructure, phase composition and aging stability, in comparison to ceramics sintered in a conventional furnace.

METHODS

Speed sintered (SS) Katana STML_SS (Kuraray Noritake) (total thermal cycle/sintering time/dwell temperature: 30min/16min/1560°C) and CEREC Zirconia (CEREC Zr_SS) (Dentsply Sirona) (15min/2min/1578°C) were compared to conventionally sintered (CS) Katana STML_CS (6.8h/2h/1550°C) and inCoris TZI_CS (4h/2h/1510°C). The translucency parameter (TP) and contrast ratio (CR) were measured with a spectrophotometer. The chemical composition of the materials was determined by XRF and phase composition was characterized using XRD. Hydrothermal aging behavior was evaluated by measuring the tetragonal-to-monoclinic ZrO₂ phase transformation after accelerated hydrothermal aging in steam at 134°C. The indentation fracture toughness, Vickers hardness and biaxial strength of the sintered ceramics were assessed.

RESULTS

Speed and conventionally sintered zirconia revealed similar density, microstructure, average strength and hydrothermal aging stability. Both Katana STML_SS/CS 5Y-PSZ ceramics were characterized with a higher content of cubic phase (≈53wt%), which resulted in a higher amount of Y₂O₃ in the remaining tetragonal ZrO₂ phases compared to the 3Y-TZP CEREC Zr_SS and inCoris TZI_CS (8 and 20wt%, respectively). The sintering program did not affect the hydrothermal aging behavior of Katana STML_SS and CEREC Zr_SS. TP of Katana STML_SS (TP≈32) was not affected by speed sintering, while the translucency of CEREC Zr_SS (TP=14) was significantly reduced. Hardness, fracture toughness and Weibull characteristic strength of Katana STML_SS and CEREC Zr_SS also reached the optimal level, but speed sintering substantially lowered their mechanical reliability.

SIGNIFICANCE

Speed sintering of 3Y-TZP and 5Y-PSZ in a speed sintering induction oven appeared suitable for clinical applications. However, further studies should focus on improving of translucency and mechanical reliability of the speed-sintered zirconia ceramics.

Translucency, flexural strength, fracture toughness, fracture load of 3-unit FDPs, Martens hardness parameter and grain size of 3Y-TZP materials

OBJECTIVE

This investigation tested pre-shaded 3Y-TZP materials on optical, mechanical and structural properties and calculated correlations between these properties.

METHODS

Seven A2-shaded 3Y-TZP zirconia materials were investigated on translucency (T) via UV-vis-spectrophotometer, fracture load of 3-unit FDPs (FL), biaxial flexural strength (FS), Chevron-Notch Beam (CNB), fracture toughness (K_IC) and Martens parameter (hardness: HM and indentation modulus: E_IT). FL, FS and K_IC were measured in a universal testing machine. The grain size was evaluated by scanning electron microscopy (SEM). Data was analyzed using one-way ANOVA followed by post hoc Scheffé, Kruskal-Wallis-, Mann-Whitney-U- and Pearson-test (p<0.05).

RESULTS

For translucency, negative correlations were found with results of facture load (R=-0.444, p<0.001) and K_IC (R=-0.503, p<0.001). While a positive correlation was found between translucency and flexural strength (R=0.238, p=0.019), between fracture load and E_IT (R=0.227, p<0.029), between fracture load and K_IC (R=0.362, p<0.001) as well as between fracture load and the grain size (R=0.598, p=0.007). While the grain size positively correlated with E_IT (R=0.534, p=0.017) as well as E_IT with HM (R=0.720, p<0.001).

SIGNIFICANCE

Despite of being based on the same raw material, tested zirconia materials significantly differed regarding optical, mechanical (except biaxial flexural strength and Martens hardness) and structural properties. Materials with highest optical properties were those with lowest mechanical properties (CER, COP).

Impact of hydrothermal aging on the light transmittance and flexural strength of colored yttria-stabilized zirconia materials of different formulations

STATEMENT OF PROBLEM

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) materials of different formulations (3Y-TZP, 4Y-TZP, and 5Y-TZP) can be colored by using color liquids. However, practically and clinically relevant factors such as modifications of sintering protocols and hydrothermal aging might affect the extent of light transmittance and flexural strength of zirconia materials of different formulations; studies on these outcomes, however, are lacking.

PURPOSE

The purpose of this in vitro study was to test the impact of hydrothermal aging on the light transmittance and flexural strength of colored zirconia materials compared with a lithium-disilicate (LiSi₂) ceramic.

MATERIAL AND METHODS

A total of 210 specimens were prepared from 3Y-TZP_0.25 (n=30), 3Y-TZP_0.05 (n=30), 5Y-TZP (n=30), 4Y-TZP (n=60), _pre4Y-TZP (preshaded, n=30), and LiSi₂ (n=30). All specimens, except for _pre4Y-TZP and LiSi₂, were manually colored, predried, and either conventionally sintered at 1450 °C (3Y-TZP_0.25, 3Y-TZP_0.05, 5Y-TZP, and half of 4Y-TZP) or high-speed sintered at 1580 °C (other half of 4Y-TZP and _pre4Y-TZP). Light transmittance was measured initially and after 2, 5, 10, 20, 40, and 160 hours of hydrothermal aging (134 °C, 0.2 MPa). Biaxial flexural strength was tested initially and after 160 hours of hydrothermal aging (n=15). The Kolmogorov-Smirnov test, multivariate analysis, and 1-way ANOVA with the Tukey HSD post hoc test, the t test, and linear mixed models were calculated (α=.05).

RESULTS

LiSi₂ showed the highest translucency, followed by 5Y-TZP, 4Y-TZP, _pre4Y-TZP_speed, 3Y-TZP_0.05, and 3Y-TZP_0.25. 4Y-TZP_speed was most opaque and matte. The decrease in translucency related to aging hours was higher for LiS₂ and conventional sintered zirconia materials than for 4Y-TZP_speed and _pre4Y-TZP_speed. Initially, 3Y-TZP_0.25 had the highest flexural strength, followed by 3Y-TZP_0.05, 4Y-TZP, and _pre4Y-TZP_speed. _pre4Y-TZP_speed was comparable with 4Y-TZP_speed but significantly higher than 5Y-TZP. LiSi₂ had the lowest biaxial flexural strength. Hydrothermal aging increased biaxial flexural strength for 3Y-TZP_0.25 and 3Y-TZP_0.05 (P<.001) but decreased it for 5Y-TZP (P=.005) and _pre4Y-TZP_speed (P<.001). After aging, 4Y-TZP_speed showed comparable values of flexural strength with 4Y-TZP (P=.06) and higher values than _pre4Y-TZP_speed after aging (P=.019).

CONCLUSIONS

Manually colored, conventionally sintered 4Y-TZP was resistant to hydrothermal aging regarding flexural strength. High-speed sintering inhibited color development for manually colored 4Y-TZP but did not affect the resistance to hydrothermal aging. The findings were reversed for industrially preshaded 4Y-TZP.

Strength and aging resistance of monolithic zirconia: an update to current knowledge

New zirconia compositions with optimized esthetic properties have emerged due to the fast-growing technology in zirconia manufacturing. However, the large variety of commercial products and synthesis routes, make impossible to include all of them under the general term of “monolithic zirconia ceramics”. Ultra- or high translucent monolithic formulations contain 3–8 mol% yttria, which results in materials with completely different structure, optical and mechanical properties. The purpose of this study was to provide an update to the current knowledge concerning monolithic zirconia and to review factors related to strength and aging resistance. Factors such as composition, coloring procedures, sintering method and temperature, may affect both strength and aging resistance to a more or less extend. A significant reduction of mechanical properties has been correlated to high translucent zirconia formualtions while regarding aging resistance, the findings are contradictory, necessitating more and thorough investigation. Despite the obvious advantages of contemporary monolithic zirconia ceramics, further scientific evidence is required that will eventually lead to the appropriate laboratory and clinical guidelines for their use. Until then, a safe suggestion should be to utilize high-strength partially-stabilized zirconia for posterior or long span restorations and fully-stabilized ultra-translucent zirconia for anterior single crowns and short span fixed partial dentures.

Effect of extrinsic pigmentation and surface treatments on biaxial flexure strength after cyclic loading of a translucent ZrO2 ceramic

OBJECTIVE

To evaluate the influence of extrinsic pigmentation on the biaxial flexural strength and surface topographic of translucent Y-TZP (InCoris TZI - Sirona - USA) subjected to several surface treatments.

METHODS

Sintered zirconia discs-shaped specimens (n=120) (ø:12mm; thickness:1.2mm; ISO 6872) were prepared and divided (n=15) according to various factors: "extrinsic pigmentation" (n: without; p: with) and "surface treatments" (C: control - as sintered; A: abraded with silica-coated alumina particles (30μm); G: glazed with a thin film of low-fusing porcelain glaze; GH: glazed and etched with 10% hydrofluoridric acid for 60s. Mechanical cycling (1.2×106 cycles, 200N, 3.8Hz) and flexural strength test (1mm/min - 1000kg cell) were performed. Two-way ANOVA and Tukey's were used for statistical test (α=0.05). Weibull analysis was used to evaluate the strength reliability. Samples were analyzed via (1) an optical profilometer to determine the surface roughness (Ra); (2) an X-ray diffraction (XRD) to evaluate phase transformations; and (3) a SEM equipped with an energy dispersive X-ray spectroscopy (EDX) to elucidate morphological properties and chemical compositions.

RESULTS

Regardless of the surface treatment (p=0.5459) (Cn: 560.16MPa; Gn: 573.36MPa; An: 643.51MPa; GHn: 542.94MPa; Cp: 628.04MPa; Gp: 641.90MPa; Ap: 554.47MPa; GHp :602.84MPa) and extrinsic pigmentation (p=0.1280) there was no difference in the flexural strength among the experimental groups. According to the XRD analysis, phase transformations occurred in the An group (t→m) and in Ap group (t→c). Surface roughness was affected by surface treatments (An - p=0.001) and extrinsic pigmentation (Gp - p=0.001).

SIGNIFICANCE

The biaxial flexural strength of the tested samples was not affected neither by surface treatments nor by pigmentation, although it can cause phase transformation and promote surface roughness.

Influence of shading technique on mechanical fatigue performance and optical properties of a 4Y-TZP ceramic for monolithic restorations

This study aimed to evaluate the effect of shading procedures on the fatigue performance and optical properties of an yttria-stabilized tetragonal zirconia polycrystal ceramic (4Y-TZP - stabilized by 4 mol% Y₂O₃). Disc-shaped specimens were produced at pre-sintered stage and randomly allocated into 5 groups (n=15) considering the shading technique: Ctrl (no pigmentation); Brush 1× and Brush 3× (brushing on 1 or 3 applications, respectively); Immer 2min and Immer 4min (immersion for 2 or 4 min, respectively). Following each pigment application, the specimens were dried in accordance with the manufacturers' guidelines (15 min in a stove at 70 °C) and sintered in a specific furnace. Biaxial flexural fatigue test followed a step-stress approach (initial strength: 200 MPa; step-size: 25 MPa; 10.000 cycles/step; frequency: 20 Hz). Optical measurements were conducted in a spectrophotometer following the CIEDE2000 parameters. Roughness, topography, crystalline grain size, crystalline phase content and fractography analysis were also performed. The flexural fatigue strength (FFS) and the number of cycles to fracture (CFF) were statistically affected by pigmentation techniques (mean FFS in MPa/mean CFF): Ctrl (446.7/105,619) > Brush 1 × (436.7/102,854) ≥ Brush 3 × (405.0/89,962) ≥ Immer 2 min (395.0/85,103) ≥ Immer 4 min (383.3/81,382), although the exposure intensity (increase in number of applications or in application time) to the pigment had no effect (Brush 1 × = Brush 3 × ; Immer 2 min = Immer 4 min). Regarding optical properties, shading techniques similarly affected translucency and opalescence parameters, regardless of the exposure intensity to the pigment (Brush 1 × = Brush 3 × = Immer 2 min = Immer 4 min > Ctrl), whereas the exposure intensity to the pigment only promoted relevant alterations in the color change parameter (ΔE₀₀) for the brushing technique (Brush 3 × > Brush 1 × ). No difference was observed in crystalline phase content, topographic pattern or roughness, although an increase in the zirconia grain size could be detected. Thus, the shading technique might affect the fatigue behavior and promote optical changes in a 4Y-TZP ceramic.

The influence of altering sintering protocols on the optical and mechanical properties of zirconia: A review

OBJECTIVE

As a result of advancements in chairside technology and speed sintering techniques and increased esthetic demands of patients, efforts have been made to produce monolithic zirconia restorations that are highly translucent, strong, and dense. While methods for processing zirconia are well known, there is a tendency to modify the process parameters with the aim of decreasing the overall processing time and, in particular, the sintering time. This review provides clinicians with scientific evidence of the effects of altering sintering parameters used for dental zirconia on its microstructure, phase transformation, and mechanical and optical properties.

MATERIALS AND METHODS

A systematic search of Embase and Medline using Boolean operators was performed to locate relevant articles.

RESULTS

Eleven articles were selected for this review. The following characteristics of monolithic zirconia have been confirmed to be affected by alterations in sintering: the microstructure, mechanical properties, optical properties, wear behavior, and low thermal degradation.

CONCLUSIONS

The alteration of sintering parameters has been found to alter the grain size, wear behavior, and translucency of zirconia. There is a lack of clinical studies that investigate the influence of altering sintering parameters or methods on the clinical performance of monolithic zirconia restorations.

CLINICAL SIGNIFICANCE

Alteration of sintering parameters alters the microstructural, mechanical, and optical properties of zirconia. This will consequently impact the clinical performance of zirconia prostheses. Future clinical investigations are encouraged to support these in vitro findings.

New multi-layered zirconias: Composition, microstructure and translucency

OBJECTIVES

To fully realize the range of indication and clinical advantages of the new multi-layered zirconias, a comprehensive understanding of their chemical composition, microstructure, low temperature degradation (LTD) resistance, and translucency properties is paramount.

METHODS

A zirconia system (Katana, Kuraray Noritake), including 3 distinct grades of multi-layered zirconias, was selected for study: Ultra Translucent Multi-layered zirconia (UTML), Super Translucent Multi-layered zirconia (STML), and Multi-layered zirconia (ML). For different materials and their individual layers, the chemical composition, zirconia phase fractions, and microstructure were determined by X-ray fluorescence (XRF), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). Also, their resistance to LTD and translucency properties were characterized.

RESULTS

Our findings revealed no major differences amongst layers, but the 3 materials were very distinct-UTML: 5Y-PSZ (5 mol% yttria-partially-stabilized zirconia) with ˜75 wt% cubic content and a 4.05 (±0.85) μm average grain size, STML: 4Y-PSZ with ˜65 wt% cubic content and a 2.81 (±0.17) μm average grain size, and ML: 3Y-PSZ with <50 wt% cubic content and a 0.63 (±0.03) μm average grain size. After water aging at 120 °C for 12 h, greater monoclinic content was found in ML. UTML and STML did not show detectable monoclinic phase. The translucency was similar among layers, and also between UTML and STML, which were superior to ML.

SIGNIFICANCE

For each multi-layered zirconia grades, the layers are only differed in pigment types and contents, which yield remarkably natural shade gradients. Also, despite the significant compositional difference between STML and UTML, both materials showed similar translucencies.

Evaluation of the color and translucency of glass-infiltrated zirconia based on the concept of functionally graded materials

STATEMENT OF PROBLEM

Infiltrated zirconia has promising mechanical properties. However, information about its optical behavior is scarce.

PURPOSE

The purpose of this in vitro study was to evaluate the color and translucency of zirconia submitted to infiltration and aging.

MATERIAL AND METHODS

Sixty zirconia disks were machined. Ten disks received no treatment (NT group), 10 disks were immersed in a coloring liquid (A2 group), and 10 disks were immersed in a fluorescent liquid (F group). The other 30 disks were submitted to the same treatments plus glass infiltration (NT+I, A2+I, and F+I groups). The coordinates L*, a*, and b* and the Y tristimulus values were obtained to calculate the color (ΔE₀₀), lightness, chroma, and hue differences; the translucency parameter (TP); and the contrast ratio (CR) associated with the specimens. After aging in an autoclave for 4 hours (T1), new measurements were made. Two- and 3-way ANOVAs were used to analyze color differences, TP, and CR. The lightness, chroma, and hue differences were evaluated by a repeated measures ANOVA. Multiple comparisons were made with the Tukey honestly significant difference (HSD) test (α=.05).

RESULTS

The greatest color differences were observed in the A2+I group (11.23 ΔE₀₀) (P<.001). Aging affected the chroma of the colored groups (P=.013 and P=.001) but did not affect their translucency (P=.347 for TP and P=.132 for CR). The greatest TP values were found in the NT and NT+I groups (2.54 and 2.34, respectively), whereas the CR was equal to or close to 1 in all groups.

CONCLUSIONS

Color differences were observed in the glass-infiltrated groups. The TP and CR were affected by infiltration. Aging did not influence the optical behavior of the specimens.

Evaluation of mold-enclosed shear bond strength between zirconia core and porcelain veneer

This study aimed to evaluate the mold-enclosed shear bond strength (ME-SBS) of zirconia to veneering porcelain with different surface treatments. Colored or uncolored zirconia coupons were either highly polished or airborne-particle abraded. The specimens were divided into groups with/without application of liner. Veneering porcelain was fired into an alumina ring mold on the zirconia coupons. The assembled specimens were subjected to the ME-SBS test. The mean ME-SBS for groups ranged from 7-10 MPa with no significant difference (p>0.05). A three-way ANOVA showed that coloring and surface roughening of the zirconia specimen had no significant influence on the ME-SBS value, but liner application exhibited a significant effect with a minor decrease in the MESBS (p=0.049). Surface treatments (coloring, airborne-particle abrasion, and liner application) were found to not cause a significant increase to the zirconia-porcelain bond strength. The application of zirconia liner had a slight negative influence on the ME-SBS results.

Fracture load of colored and non-colored high translucent zirconia three-unit fixed dental prosthesis frameworks

Aim: The use of colored translucent zirconia may enable restorations of a more natural tooth-like appearance than previous opaque white zirconia. The shift from non-colored to colored zirconia may however entail a risk of reduced strength. The aim of the present study was to compare fracture load and fracture mode of fixed dental prostheses frameworks made of colored translucent zirconia to that of non-colored controls.

Methods: A total of forty three-unit FDP frameworks were manufactured from two different high translucent zirconia materials (Zenostar, Wieland Dental, and DD cubeX2, Dental Direkt). Each group contained two subgroups, one colored and one non-colored. Coloring was performed before final sintering using two different infiltration techniques. All FDPs underwent an artificial aging process in the form of heat treatment, thermocycling and preloading whereafter the specimens were subjected to load until fracture. Fracture load and mode was registered.

Results: For one of the zirconia materials, Zenostar, the non-colored frameworks showed significantly higher fracture loads (p < .0001) compared to its colored counterpart. No significant difference (p > .05) was found between colored and non-colored frameworks in the other zirconia material, DD cubeX2. All FDPs fractured through the connector. Some fractures ran through the mesial and some through the distal side of the connector but there were no significant differences in fracture mode between groups.

Conclusion: Coloring before sintering of high-translucent zirconia may decrease the fracture load of FDP frameworks for certain materials and techniques. Fracture mode however, does not appear to be affected.

Impact of laboratory treatment with coloring and fluorescent liquids on the optical properties of zirconia before and after accelerated aging

STATEMENT OF PROBLEM

Laboratory procedures, such as dipping in coloring and fluorescent liquids, can be used to improve the optical properties of zirconia. However, information is lacking on the effect of these liquids.

PURPOSE

The purpose of this in vitro study was to evaluate the color differences and degree of fluorescence of zirconia (3Y-TZP) treated with coloring and fluorescent liquids before and after an accelerated aging protocol.

MATERIAL AND METHODS

Forty disk-shaped specimens of 3Y-TZP were fabricated by milling and separated according to the laboratory treatment performed: white zirconia (control group); zirconia treated with coloring liquid (A2 group); zirconia treated with fluorescent liquid (fluorescent group); and zirconia treated with both liquids (A2 fluorescent group). The L*a*b* coordinates before aging (T₀) were obtained with a spectrophotometer, and the degree of fluorescence was measured. The disks were subjected to accelerated aging for 1 hour (T₁) and 5 hours (T₂). Measurements were made before and after each time interval. Color differences (ΔE₀₀) were calculated using the CIEDE2000 formula and analyzed by 2-way ANOVA. Lightness (ΔL'), chroma (ΔC'), and hue differences (ΔH') were analyzed by multivariate ANOVA. Degrees of fluorescence were obtained as percentages and were analyzed by 2-way ANOVA. Multiple comparisons were performed by the Tukey HSD test (α=.05).

RESULTS

Color differences were observed when 3Y-TZP disks were treated with coloring (7.91 ΔE₀₀), with fluorescent liquid (5.81 ΔE₀₀), and with both liquids (5.52 ΔE₀₀). Accelerated aging resulted in color differences in the T₂ A2 group (6.74 ΔE₀₀) and at both times evaluated in the fluorescent group (T₁=8.59 ΔE₀₀ and T₂=8.47 ΔE₀₀) (P<.001). In the A2 fluorescent group, the degree of fluorescence was not influenced significantly (P>.05). The use of fluorescent liquid influenced the degree of fluorescence in the fluorescent group (T₀=20%).

CONCLUSIONS

Significant differences in color, lightness, chroma, and hue were achieved in all tested groups before and after aging. The degree of fluorescence was statistically different only in the fluorescent group and was not influenced by accelerated aging.

Relative Translucency of a Multilayered Ultratranslucent Zirconia Material

AIM

The aim of this study was to compare the translucency parameter (TP) of ultratranslucent multilayered (UTML) zirconia according to thickness and layer level.

MATERIALS AND METHODS

Rectangles of UTML zirconia with four layers [dentin layer (DEL), first transitional layer (FTL), second transitional layer (STL), and enamel layer (ENL)] and four different thicknesses (0.4, 0.6, 0.8, and 1 mm) were milled from blanks. Digital images were taken in a dark studio against white and black backgrounds under simulated daylight illumination and international commission on illumination (CIE) Lab* color values recorded using Photoshop Creative Cloud software. The TP was computed and compared according to thickness and layer level using analysis of variance (ANOVA) followed by Bonferroni post hoc analysis for multiple comparisons. Significance was set at p < 0.05.

RESULTS

In each thickness, TP values were similar between any two layers. The significant effect of thickness on the TP was observed only in the first two layers. In the DEL, translucency was significantly greater at 0.4 mm than all other thicknesses. In the FTL, differences were significant between 0.4 and 0.8 mm and between 0.4 and 1 mm.

CONCLUSION

The investigated zirconia does not seem to show gradational changes in relative translucency from dentin to enamel levels regardless of the thickness used. Thickness affected the TP only in the first two layers with better translu-cency at 0.4 mm.

CLINICAL SIGNIFICANCE

Since relative translucency does not seem to be significantly different between layers, clinicians can modify the apicocoronal positioning of the UTML layers within the restoration according to the desired Chroma without any implications on the clinically perceived translucency. While the thickness of 0.4 mm may be suggested for anterior esthetic veneers because of its higher translucency, the other thicknesses of 0.6 to 1 mm can be used to mask colored abutments in full contour restorations.

Evaluation of color stability of preshaded and liquid-shaded monolithic zirconia

STATEMENT OF PROBLEM

The effect of thermal aging on the color stability of monolithic zirconia has not been thoroughly investigated.

PURPOSE

The purpose of this in vitro study was to evaluate the color stability before and after thermocycling of monolithic zirconia specimens, which were either preshaded or characterized in the laboratory through the assessment of color parameters L*, a*, and b*.

MATERIAL AND METHODS

A total of 80 specimens (10×10×15 mm) were prepared from zirconia disks (BruxZir) and divided into 2 groups. The first group consisted of preshaded specimens from disks 100, 200, 300, and 400, and the second group consisted of white specimens characterized before the sintering stage with coloring liquids in shades A2, B2, C2, and D2. Ten specimens of each shade were used. A double-beam ultraviolet-visible light recording spectrophotometer was used to assess the color parameters L*, a*, and b* before and after thermocycling. The color difference ΔE was calculated based on ΔΕ=[(ΔL*)²+(Δa*)²+(Δb*)²]^½. The reliability of the examiner was investigated by MedCalc v12.5 software, and the tests of normality, homogeneity of variances, and 1-way ANOVA statistical analysis of variance were performed by IBM Statistics SPSS v20.0 software (α=.05).

RESULTS

No statistically significant ΔΕ (P>.05) was observed among the different groups after thermocycling. All ΔΕ values were below the limit of the 3.7 that an untrained observer can perceive. Only for the specimens in shades 200 and 400 was ΔΕ above 1, revealing a visible color difference, which, however, was still clinically acceptable. The L*, a*, and b* values for both liquid-shaded and preshaded zirconia specimens were not significantly affected by thermocycling. A higher color stability for laboratory-characterized compared with preshaded monolithic zirconia was observed irrespective of the color shade.

CONCLUSIONS

Within the limitations of this in vitro study, monolithic zirconia specimens of any shade, either preshaded or characterized in the laboratory, can be considered as color-stable materials after thermocycling.