Effect of different coloring liquids on the flexural strength of multilayered zirconia

Effect of metal elements in coloring liquids used in the infiltration method on the physical properties of zirconia

OBJECTIVES

This study aimed to clarify the effect of metal elements in the coloring liquids used in the infiltration method on the physical properties of zirconia.

METHODS

Two types of zirconia discs 5Y-PSZ (SHOFU Disc ZR Lucent FA, SHOFU, Kyoto, Japan) were used: with monolayer shades from W2 to W3 (Pearl White) and 5Y-PSZ with multilayer shades from A3 to A4 (L). Five kinds of coloring liquid were used to infiltrate into semi-sintered Pearl White (T-glass [CT], A4 [CA], White-Opaque [CW], Gingiva 1 [CG], and Blue X [CB]). In addition, uncolored Pearl White set to as the control (C). These specimens were analyzed using a three-point bending test (3PBT), and the fracture surface after the test was analyzed by scanning electron microscopy, elemental analysis, and crystal structure analysis. In addition, from the polished surface part of the after the 3PBT specimens, the elemental composition was analyzed using X-ray fluorescence spectroscopy (XRF).

RESULTS

The flexural strength of CB and CG were lower than that of C (p < 0.05). XRF results showed that the Erbium (Er) content of CG was significantly greater than that of C (p < 0.05). CB exhibited a significantly higher Yttrium (Y) content compared with C (p < 0.05), and numerous pores were observed in the micrographs of the fracture surface of CG and CB.

SIGNIFICANCE

In zirconia, where the content of Y and Er was significantly increased by infiltration with a coloring liquid, pores were observed between the zirconia crystals, and the mechanical strength decreased.

Effect of glass infiltration and modified cooling rates on color characteristics alteration of monochrome and multilayer high yttrium oxide containing zirconia

Background

Sintering technique impacted color of zirconia. This study examined the effect of glass infiltration and altering cooling rate on color alteration of monochrome (Mo) and multilayer (Mu) 5 mol% yttria-partially stabilized zirconia (5Y-PSZ).

Material and Methods

180 specimens (width, length, thickness = 10, 20, 2 mm) were prepared from Mo and Mu (comprising cervical (C) and incisal (I) zone) 5Y-PSZ shade VITA-A2. Unintentionally categorized samples (n=15/group) were sintered with traditional (T) versus glass infiltrated (G) technique and cooled down with slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). CIE-L*a*b* color characteristics were determined for translucency parameter (TP), contrast ratio (CR), opalescence parameter (OP), and color difference (∆Ediff). Microstructures were investigated with SEM and XRD. ANOVA and Tamhane's comparisons were determined for significant differences (p<0.05).

Results

TP and OP were significantly higher for Mo than MuC and MuI, but no significant difference in CR among them. Comparable ΔEdiff between Mo and MuC were indicated, but both were significantly lesser than MuL. Glass infiltration and raising cooling rate significantly decreased TP and OP, whereas increased CR and ΔEdiff, which amplified color alteration.

Conclusions

Glass infiltration sintering and modified cooling rate significantly altered color parameters of 5Y-PSZ. Monochrome demonstrated higher translucency and opalescence than multilayer, possibly due to colorant additives. Glass infiltration decreased translucency and opalescence due to different refractive indices. Increased cooling rate resulted in decreasing translucency and opalescence due to smaller grain size and t→m transformation. Nevertheless, altered sintering and cooling rates still rendered an acceptable color alteration. Key words:Cooling rate, glass infiltration, optical characteristics, translucency.

Color Characteristics of High Yttrium Oxide-Doped Monochrome and Multilayer Partially Stabilized Zirconia upon Different Sintering Parameters

OBJECTIVES

 Sintering influences the optical properties of zirconia. This study examined the effect of altering sintering temperature and time of monochrome (Mo) and multilayer (Mu) 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) on color characteristics.

MATERIALS AND METHODS

 Three hundred specimens (width × length × thickness = 10 × 20 × 2 mm) were prepared from Mo and Mu (with cervical [C], middle [M], and incisal [I] region) 5Y-PSZ and randomly sintered at decreasing (Td: 1,450°C), regular (Tr: 1,500°C), and increasing (TI: 1,550°C) sintering temperature, with extremely short (He: 10 minutes), ultrashort (Hu: 15 minutes), short (Hs: 30 minutes), and regular (Hr: 135 minutes) sintering time (n = 15/group). Color appearance (EW ), translucency parameter (TP), contrast ratio (CR), opalescence parameter (OP), and color appearance difference (∆E diff) were evaluated in the CIE L*a*b* system. Microstructures were evaluated by scanning electron microscope (SEM) and X-ray diffractometer (XRD).

STATISTICAL ANALYSIS

 Analysis of variance (ANOVA) and Bonferroni comparisons were determined for significant differences (p < 0.05).

RESULTS

 Significant differences in color parameters upon zirconia type, sintering temperature, and sintering time, and their interactions were indicated (p < 0.05). Increasing sintering temperature and extended sintered time resulted in larger grain, reduced tetragonal-to-monoclinic phase transformation, and significantly increased the TP and OP, but decreased the CR and ∆E diff (p < 0.05). Decreasing sintering temperature and time led to clinically unacceptable color appearance.

CONCLUSION

 Mo was found to be more translucent than Mu. To achieve the most favorable optical properties, increasing sintering temperature and extending sintering time are recommended. Decreasing sintering temperature is not suggested. However, shortened sintering time is feasible, but it needs sintering with increasing sintering temperature to achieve a promising color appearance.

Fracture characteristics and translucency of multilayer monolithic zirconia crowns of various thicknesses

OBJECTIVES

Multilayer monolithic zirconia (M-Zr) crowns can be engineered to achieve gradational translucency and color intensity. However, this modification may compromise the mechanical strength, raising concerns regarding the ability of M-Zr crowns to withstand occlusal stresses. The effects of M-Zr crown thickness on translucency and ability to endure occlusal forces were investigated at different tooth positions (incisors, premolars, and molars). The objective was to determine the minimal thickness of M-Zr crowns used in tooth preparation to meet aesthetic and functional demands.

METHODS

M-Zr samples (Vita A1) with four thicknesses (0.5, 1.0, 1.5, and 2.0 mm) were prepared and subjected to translucency testing using a digital colorimeter by 3-third and 9-square division methods. Crown-shaped M-Zr samples with three thicknesses (1.0, 1.5, and 2.0 mm) and three tooth positions (incisor, premolar, and molar) were digitally designed, and 2.0 mm metal abutments were fabricated. The samples were bonded to the abutments; their fracture characteristics were evaluated using a universal testing machine, and their fracture surfaces examined using an optical microscope. Statistical analyses included the Shapiro-Wilk test, Pearson correlation, and one-way and two-way ANOVA with a post hoc Tukey HSD test (α = 0.05).

RESULTS

Color analysis results revealed a significant negative correlation between thickness and translucency (r < -0.96, P < 0.01), with the highest values in the incisal region. Cross-sectional profiles confirmed the uniform thickness and morphology of the digitally designed M-Zr crowns. The results of fracture strength analysis showed position-dependent variability, a strong positive correlation with thickness (r > 0.96, P < 0.01), and fracture strengths consistently exceeding 1200 N across all tooth positions. Fracture patterns indicated that thinner crowns at the incisors and molars were more prone to cracking, whereas those at the premolars demonstrated significantly higher strength (4872.51 N, P < 0.05), only with crack or even no fracture occurring at 2.0 mm.

CONCLUSIONS

Thickness significantly influenced both the translucency and fracture strength of M-Zr, with the tooth position playing an additional role, albeit to a lesser extent. Although thinner crowns exhibited lower strength at each tooth position, even at a thickness of 1.0 mm, fracture strength exceeding 1200 N was maintained, surpassing the typical occlusal forces. Thus, it can be asserted that M-Zr crowns with a minimum thickness of 1.0 mm can meet both aesthetic and functional requirements.

Optical Characteristics of Monochrome and Multilayer Fully Stabilized Zirconia Upon Sintered Cooling Speed

OBJECTIVES

 Firing protocols influence optical properties of dental ceramics. Effects of varying cooling rates of monochrome and multilayer 5 mol% yttria-stabilized tetragonal polycrystalline (5YTZP) on optical properties are subjected for investigation.

MATERIALS AND METHODS

 Ninety specimens (width, length, thickness = 10 × 20 × 2 mm) were prepared from monochrome (Mo: Cercon xt) and multilayer (Mu: Cercon xt ML with cervical (C) and incisal (I) zoning) 5YTZP. Specimens were sintered and randomly treated with three cooling rates (n = 15/group): slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). Color appearance (∆EW), color appearance difference (∆E diff), translucency parameter (TP), contrast ratio (CR), and opalescence parameter (OP) were evaluated in CIEL*a*b* (Commission International de I'Eclairage) system. ∆E diff was achieved from the coordinate difference of specimen to VITA classic shade A2. Microstructures and compositions were evaluated by scanning electron microscope and energy dispersive spectroscopy. Monoclinic (m), tetragonal (t), and cubic (c) phases were investigated with X-ray diffraction.

STATISTICAL ANALYSIS

 An analysis of variance and Bonferroni multiple comparisons were determined for significant differences (p < 0.05).

RESULTS

 ΔEW of MoF was highest (66.04 ± 1.86), while MuN-I was lowest (62.60 ± 0.86). TP and OP of MoS were highest at 2.85 ± 0.11, and 2.25 ± 0.10, while MuF-I was lowest at 2.16 ± 0.10 and 1.60 ± 0.12. CR of MuF-I was highest (0.948 ± 0.005), while MoS was lowest (0.936 ± 0.005). ΔEdiff of MoF was highest (3.83), while MuN-I was lowest (0.93). Limited grain growth and m-phase composition were indicated upon fast cooling. There were significant differences for all color parameters due to varied materials, cooling rates, and their interactions (p < 0.05) except for interaction in ∆EW and OP.

CONCLUSIONS

 Translucency of monochrome and multilayer 5YTZP were different, possibly due to colorant additives. Incisal layer of multilayer 5YTZP was perfectly matched with VITA shade. Increasing cooling speed resulted in smaller grain size, t-m transformation, and finally lower translucency and opalescence. Therefore, to achieve most favorable optical properties, slow cooling rate is recommended.

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 low-temperature degradation treatment on hardness, color, and translucency of single layers of multilayered zirconia

STATEMENT OF PROBLEM

Evaluating the effect of low-temperature degradation (LTD) on multilayered zirconia blocks is important for the long-term life of zirconia restorations.

PURPOSE

The purpose of this in vitro study was to analyze the changes in color, translucency, and hardness of a single layer of multilayered zirconia after an LTD treatment.

MATERIAL AND METHODS

Ninety multilayered 10×10×1.0-mm zirconia specimens (3M Lava Esthetic) were designed using Auto CAD 2006 (Autodesk) and divided into 6 groups (n=15) based on LTD (no treatment, LTD treatment) and the inherent layer type (incisal, transition, and body). The hardness of each specimen was measured using a microVickers hardness tester. The CIELab color coordinates were measured using a spectrophotometer, and the data were used to calculate the color difference and translucency (TP). Data were analyzed using 2-way ANOVA and the Scheffé test (α=.05).

RESULTS

LTD treatment did not alter the hardness of multilayered zirconia (P=.572). A significant difference was found in the hardness between the layers of multilayered zirconia (P<.001). LTD treatment did not alter the b∗ or L∗ values (P=.773, P=.701) but did change the a∗ values (P<.001). On LTD treatment, the a∗ value was found to decrease. The L∗, a∗, and b∗ values differed based on the respective layer within the material (P<.001). As layers progress from the incisal to the body, the L∗ value decreased and the a∗ and b∗ values increased. In all groups, the color difference values were highest in the following order: the incisal and transition layers < the transition and body layers < the incisal and body layers. On LTD treatment, the color difference between the layers decreased. The LTD treatment did not affect the TP value (P=.208), but the TP value for each layer showed a significant difference (P<.001).

CONCLUSIONS

LTD treatment did not affect the optical properties or hardness of the multilayered zirconia. The hardness of the multilayered zirconia increased from the incisal to the body layer. In multilayered zirconia, the brightness decreased from the incisal to the body layer, and red and yellow color changes were observed. The TP of multilayered zirconia increased from the incisal to the body layer.

State of the Art of Different Zirconia Materials and Their Indications According to Evidence-Based Clinical Performance: A Narrative Review

The aim of this study was to perform a narrative review to identify the modifications applied to the chemical structure of third- and fourth-generation zirconia ceramics and to determine the influence of these changes on the mechanical and optical properties. A bibliographical search using relevant keywords was conducted in the PubMed^® and EBSCO databases. The abstracts and full texts of the resulting articles were reviewed for final inclusion. Fifty-four articles were included in this review. The analyzed topics were: (1) the composition of first- and second-generation zirconia materials (Y-TZP), (2) the behavior of the studied generations in relation to mechanical and optical properties, and (3) the modifications that were carried out on third-generation (5Y-TZP) and fourth-generation (4Y-TZP) zirconia materials. However, studies focusing on these specific characteristics in third- and fourth-generation zirconia materials are scarce. The review shows that there is a lack of sufficient knowledge about the chemical modifications of zirconia in the new generations.

Effect of thickness of externally characterized stains on optical properties of high-translucency zirconia

OBJECTIVES

There is insufficient information about the effects of the thickness of externally characterized stains or glazes on CIELAB color parameters and the translucency parameter (TP) of 5 mol% yttria partially stabilized zirconia (5Y-PSZ). The present study evaluated the effect of a thickness reduction of the externally characterized or glazed layer on its CIE2000 color parameters and TP₀₀ of 5Y-PSZ.

MATERIALS AND METHODS

Twenty-eight specimens were sectioned from 5Y-PSZ blanks. Three brands of powdered stains (namely VI, SH, and IV) and a glaze (GL) were used to extrinsically stain the specimen surfaces (n = 7 per group). After measuring the final thickness and color, specimens of each group were carefully ground and finally polished. The reduced thickness was carefully controlled in decrements of 0.02 mm on the characterized stains or glazed layers at each reduction cycle until the surface of the un-stained zirconia was exposed. The CIELAB color parameters of specimens were measured using a spectrophotometer. Subsequently, differences in lightness (∆L'), chroma (∆C'), and hue (∆H'), and TP₀₀ values and color differences (ΔE₀₀) were calculated using the CIEDE2000 formula to determine differences between the specimen and the un-stained zirconia at each reduction cycle. A paired-samples t-test, two-way mixed-design analysis of variance, and Pearson correlation coefficient were used to analyze the data (α = 0.05).

RESULTS

There were no significant differences in lightness, chroma, hue, or TP₀₀ values before or after glazing or after each reduction cycle of the glazed layer. Mean ΔE₀₀ values ranged between 20.94 and 33.55 after applying the externally characterized stains. With a decreased thickness of the characterized stain layer, there were observed that significant decreases in ∆L', ∆H', ∆C', and ΔE₀₀, and significant increases in TP₀₀.

CONCLUSIONS

Increasing the thickness of externally characterized stains decreased the lightness and caused the alternation of chroma and hue, whereas these phenomena were not observed in the glazed group. There was a slight change of CIE2000 color parameters and TP₀₀ when the thickness of the characterized layer exceeded 0.06 mm.

CLINICAL RELEVANCE

The application and thickness of externally characterized stains can sensitively affect CIE2000 color parameters and translucency of 5Y-PSZ.

Effect of Different Shading Techniques on the Color of Zirconia Ceramic Restoration (An In vivo Study)

BACKGROUND: The color of all ceramic restorations has a significant effect on the final appearance of the restoration. AIM: This study aimed to evaluate the effect of different shading techniques on the color stability of monolithic zirconia crowns cemented on the upper anterior teeth. METHODS: In this study, there were nine patients. Each patient has undergone a reduction for the upper six anterior teeth. Fifty-four zirconia jacket crowns were constructed using CAD/CAM technique. According to the shading technique, the crowns were categorized into two groups (pre-shaded and liquid shaded), where the upper right reduced teeth received pre-shaded zirconia crowns while the upper left reduced teeth received liquid-shaded zirconia crowns according to the selected shade. The color was assessed using easy shade after cementation and 1 year follow-up period. Data were collected, tabulated, and statistically analyzed. RESULTS: Color change (ΔE) of the cemented pre-shaded zirconia crowns on the upper right central incisors, lateral incisors, and canines was 2.37, 2.84, and 1.92, respectively, while color change (ΔE) of the cemented liquid-shaded zirconia crowns on the upper left central incisors, lateral incisors, and canines was 2.52, 2.88, and 2.77, respectively. Color change of both pre-shaded and liquid-shaded zirconia crowns after 1 year follow-up was within the clinically acceptable range 1>E>3.7, and there was no significant difference between them. CONCLUSIONS: The aging and technique of shading had no significant effect on the color of zirconia ceramic.

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.

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.

Effect of toothbrushing on the optical properties and surface roughness of extrinsically stained high-translucency zirconia

OBJECTIVES

The effect of toothbrushing on the color, relative translucency, and surface roughness of extrinsically stained zirconia dental restorations is still unclear. The purpose of the present study was to evaluate the optical properties and surface textures of extrinsically stained or glazed partially stabilized zirconia with 5 mol% yttria (5Y-PSZ) after toothbrushing by determining changes in the color, translucency parameter (TP), and surface roughness.

MATERIALS AND METHODS

Specimens (2 mm thick; n = 50) were sectioned from 5Y-PSZ blanks. Three brands of powdered stains (namely VI, SH, IV) and a glaze (GL) were used to extrinsically stain the specimen surfaces (n = 10 per group). The specimens were subjected to 10,000-50,000 simulated toothbrushing strokes using a toothpaste slurry (Ora2 PREMIUM Stain clear paste). The color parameters were measured before staining or glazing and before and after simulated brushing. The color changes and TP values can be calculated before and after simulated brushing. The surface roughness was also evaluated.

RESULTS

Significant changes were found in the shade and TP values of 5Y-PSZ after extrinsic staining, but no significant changes were found after toothbrushing, irrespective of the staining brand (p > 0.05). The surface roughness of the characterized 5Y-PSZ specimens decreased after toothbrushing, whereas that of the non-characterized 5Y-PSZ group increased.

CONCLUSIONS

Toothbrushing markedly affects the surface roughness of extrinsically stained or glazed 5Y-PSZ specimens, whereas no impacts on the color or translucency were observed.

CLINICAL RELEVANCE

For esthetic reasons, clinicians and dental technicians prefer to apply extrinsic stains to zirconia restorations, which profoundly affects its appearance. The limited influence of toothbrushing on color and translucency of 5Y-PSZ is promising for clinical applications.

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.