Translucency of Zirconia Ceramics before and after Artificial Aging

Translucency of recent zirconia materials and material-related variables affecting their translucency: a systematic review and meta-analysis

BACKGROUND

Recent forms of translucent zirconia material have been developed, offering a wide range of options and varieties for enhancing aesthetics, making it a preferred choice in the field of prosthetic dentistry. However, there is insufficient understanding regarding the recent types of zirconia materials and their optical behavior. Understanding the variables that influence the translucency of zirconia and identifying strategies to enhance its esthetics are crucial.

PURPOSE

The current systemic review highlights a comprehensive understanding of different zirconia generations in relation to their optical characteristics and evaluates material-related variables affecting their translucency.

METHODS

The present review studied in-vitro studies that evaluated the optical characteristics of different yttria content of yttria stabilized materials. The topics explored were: (1) the different zirconia material generations and their optical behavior; (2) material-related factors that affect their translucency. The research was restricted to online publication in the English language from July 1, 2010, to July 31, 2023, using PubMed, Scopus, and Science Direct resources. The search key terms and their combinations were "zirconia," "translucent zirconia," "cubic zirconia," "highly translucent zirconia," "yttria partially stabilized zirconia," "monolithic zirconia," "translucency," "optical properties," and "light transmission."

RESULTS

The data obtained from fifty-three studies addressed the optical characteristics of various zirconia generations. They reported that changing yttria content had a significant impact on translucency. Different kinds of zirconia ceramics of the same generation have varying translucencies. Achieving optimum aesthetics with monolithic zirconia is challenging due to factors related to material aspects such as the presence of additives, point defects, microstructure, thickness, phase distribution, and sintering conditions.

CONCLUSIONS

Newly developed monolithic dental zirconia ceramics have improved aesthetics and translucency. However, additional research is necessary to evaluate their performance and long-term durability.

TRIAL REGISTRATION

This systematic review was registered in PROSPERO, under number CRD42023474482.

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.

The effect of phase contents on the properties of yttria stabilized zirconia dental materials fabricated by stereolithography-based additive manufacturing

The aim is to investigate the impact of phase contents on mechanical properties, translucency, and aging stability of additively manufactured yttria partially stabilized zirconia ceramics. For that purpose, we evaluated two PSZ materials. The first type was prepared utilizing commercially available 5 mol% yttria-stabilized zirconia(5Y-PSZ), while the second type, denoted as 3Y+8Y-PSZ ceramics, was fabricated by blending 3 mol% and 8 mol% yttria-stabilized zirconia powders. Compared to 5Y-PSZ (39.90 wt% tetragonal phases and c/a2 = 1.0141), 3Y+8Y-PSZ is characterized by a greater abundance of tetragonal phases (47.68 wt%), which display higher tetragonality (c/a2 = 1.0165) and lower yttrium oxide content (2.25mol%). As a result, the 3Y+8Y-PSZ demonstrates elevated strength (816.52 MPa) and toughness (4.32 MPa m1/2), accompanied by reduced translucency(CR:0.47) and it exhibits greater susceptibility to aging. The phase contents, yttrium oxide content, and lattice parameters in the tetragonal phase play a crucial role in determining the mechanical properties, translucency, and aging stability of PSZ ceramics.

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.

Optical effect of rapid sintering protocols on different types of zirconia

STATEMENT OF PROBLEM

Rapid sintering protocols are available for the fabrication of zirconia restorations, but whether rapid sintering influences color or translucency is unclear.

PURPOSE

The purpose of this in vitro study was to investigate the effect of different rapid sintering protocols on the color and translucency of cubic and tetragonal zirconias.

MATERIAL AND METHODS

Sixty disk-shaped specimens of 1-mm-thick cubic (DD CubeX2) and tetragonal (DD Bio ZX2) zirconia were investigated. Specimens of each type of zirconia were divided into three groups: conventional, speed, and superspeed sintering protocols. The conventional group of each zirconia type served as the control for calculating color differences. Translucency for each group was assessed by the translucency parameter and contrast ratio. Two-way analysis of variance was used for statistical analysis of the data (α=.05).

RESULTS

The translucency of cubic and tetragonal zirconia decreased after speed and superspeed sintering (P<.001). Superspeed sintering resulted in a greater color change than speed sintering (P<.001).

CONCLUSIONS

Rapid sintering protocols produced a significant effect on the color and translucency of cubic and tetragonal zirconias.

The Influence of Polishing and Artificial Aging on BioMed Amber® Resin's Mechanical Properties

Currently, 3D print is becoming more common in all branches of medicine, including dentistry. Some novel resins, such as BioMed Amber (Formlabs), are used and incorporated to more advanced techniques. The aims of the study were to check whether or not polishing and/or artificial aging influences the properties of the 3D-printed resin. A total of 240 specimens of BioMed Resin were printed. Two shapes (rectangular and dumbbell) were prepared. Of each shape, 120 specimens were divided into four groups each (with no influence, after polishing only, after artificial aging only, and after both polishing and artificial aging). Artificial aging took place in water at the temperature of 37 °C for 90 days. For testing, the universal testing machine (Z10-X700, AML Instruments, Lincoln, UK) was used. The axial compression was performed with the speed of 1mm/min. The tensile modulus was measured with the constant speed of 5 mm/min. The highest resistance to compression and tensile test were observed in the specimens that were neither polished nor aged (0.88 ± 0.03 and 2.88 ± 0.26, respectively). The lowest resistance to compression was observed in the specimens that were not polished, but aged (0.70 ± 0.02). The lowest results of the tensile test were observed when specimens were both polished and aged (2.05 ± 0.28). Both polishing and artificial aging weakened the mechanical properties of the BioMed Amber resin. The compressive modulus changed much with or without polishing. The tensile modulus differed in specimens that were either polished or aged. The application of both did not change the properties when compared to the polished or aged probes only.

A simple method for quick evaluation of the anterior tooth ratio: an observational study

Background

An ideal relationship of anterior teeth is closely related to postoperative function, stability, and aesthetics. Therefore, it is necessary to estimate the proportion of anterior teeth when communicating with patients about possible treatment plans and outcomes. This study aimed to establish a simple method for assessing the proportion of anterior teeth and to identify the standard ratio value to provide references for clinical work.

Methods

Five hundred fourteen patients were divided into derivation, standard, and validation datasets. We first deduced our novel simplified anterior tooth ratio (SATR) by finding the key teeth with the derivation datasets, then established standard values by measuring the standard models, and finally validated the diagnostic performance of SATR. Independent sample t-test was used to select key teeth. Pearson’s correlation analysis and linear regression analysis was used to test and verify the correlation between SATR and the anterior Bolton ratio. Chi-square test and diagnostic test were used to verify the diagnostic results using SATR. P values of < 0.05 were considered statistically significant.

Results

Patients with an abnormal anterior Bolton ratio were more likely to have variations in the maxillary and mandibular lateral incisors. Therefore, the ratio of maxillary and mandibular lateral incisors was chosen as a simple way to assess the anterior tooth ratio and was defined as SATR (simplified anterior tooth ratio). A positive correlation was observed between SATR and anterior Bolton ratio (r = 0.702, p < 0.001), with the linear regression equation as follows: y = 0.503 + 0.328x, x = SATR, y = anterior Bolton ratio. The standard value of SATR was established (85.69% ± 3.57%) and proven reliable in clinical practice.

Conclusions

The ratio of maxillary and mandibular lateral incisors can be used to estimate the anterior tooth ratio, which showed high reliability and efficiency.

Novel protein-repellent and antibacterial polymethyl methacrylate dental resin in water-aging for 6 months

Background

The present study aimed to develop a novel protein-repellent and antibacterial polymethyl methacrylate (PMMA) dental resin with 2-methacryloyloxyethyl phosphorylcholine (MPC) and quaternary ammonium dimethylaminohexadecyl methacrylate (DMAHDM), and to investigate the effects of water-aging for 6 months on the mechanical properties, protein adsorption, and antibacterial activity of the dental resin.

Methods

Four groups were tested: PMMA control; PMMA + 3% MPC; PMMA + 1.5% DMAHDM; and PMMA + 3% MPC + 1.5% DMADDM in acrylic resin powder. Specimens were water-aged for 1 d, 3 months, and 6 months at 37 ℃. Their mechanical properties were then measured using a three-point flexure test. Protein adsorption was measured using a micro bicinchoninic acid (BCA) method. A human saliva microcosm model was used to inoculate bacteria on water-aged specimens and to investigate the live/dead staining, metabolic activity of biofilms, and colony-forming units (CFUs).

Results

The flexural strength and elastic modulus showed a significant loss after 6 months of water-ageing for the PMMA control (mean ± SD; n = 10); in contrast, the new protein repellent and antibacterial PMMA resin showed no strength loss. The PMMA–MPC–DMAHDM-containing resin imparted a strong antibacterial effect by greatly reducing biofilm viability and metabolic activity. The biofilm CFU count was reduced by about two orders of magnitude (p < 0.05) compared with that of the PMMA resin control. The protein adsorption was 20% that of a commercial composite (p < 0.05). Furthermore, the PMMA–MPC–DMAHDM-containing resin exhibited a long-term antibacterial performance, with no significant difference between 1 d, 3 months and 6 months (p > 0.05).

Conclusions

The flexural strength and elastic modulus of the PMMA–MPC–DMAHDM-containing resin were superior to those of the PMMA control after 6 months of water-ageing. The novel PMMA resin incorporating MPC and DMAHDM exhibited potent and lasting protein-repellent and antibacterial properties.

The Effect of Smoking and Brushing on the Color Stability and Stainability of Different CAD/CAM Restorative Materials

This study aimed to investigate and compare the color stability and stainability of computer-aided design/computer-aided manufacturing (CAD/CAM) restorative materials in their glazed (G) and polished (P) state when exposed to cigarette smoke, as well as after brushing. Three CAD/CAM restorative materials were investigated: lithium disilicate CAD (LD), zirconia (Zr), and Telio PMMA CAD (PMMA), according to their surface finishing and assignment to cigarette smoking exposure or soaking in the saliva (control) group. The color change (∆E) was calculated before and after the intervention performed for all specimens, using the L*a*b values to quantitatively assess the shade differences. Statistical analysis was performed using one-way repeated measures ANOVA and Bonferroni multiple comparison analysis (α = 0.05). The surface finishing did not influence the materials’ stainability. Color change was noted after smoking, LD and Zr-G and Zr-P had a comparable color change (p > 0.05), while PMMA presented lower ∆E values (p < 0.05). After brushing, all specimens had a significant color change that was high for LD-G and LD-P, and Zr-G, compared with Zr-P and PMMA (p < 0.05). In conclusion, the exposure to cigarette smoke showed that LD, Zr, and PMMA are all susceptible to staining, but brushing decreases surface staining.

Effect of different surface treatments on the bonding potential and physical and mechanical properties of ultratranslucent zirconia

STATEMENT OF PROBLEM

Studies to determine a suitable surface treatment that improves bonding without compromising the strength or translucency of ultratranslucent zirconia are scarce.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different surface treatments on translucency, surface topography, phase transformation, biaxial flexural strength, bond strength, and durability.

MATERIAL AND METHODS

A total of 169 ultratranslucent zirconia disks were randomly divided into 4 groups according to the surface treatment applied: airborne-particle abrasion (APA) (n=46), tribochemical silica airborne-particle abrasion (TS) (n=46), nonthermal oxygen plasma (NTP) (n=46), and no treatment, control (C) (n=31). The translucency parameter (ΔTP00) was evaluated with a spectrophotometer (n=15), phase transformation was assessed with an X-ray diffractometer (n=5), surface topography was evaluated with a scanning electron microscope (SEM) (n=3), and biaxial flexural strength (BFS) was tested with a universal testing machine (n=15). For the microshear bond strength (μSBS) test, 40 composite resin specimens were attached to 8 disks and tested with a universal testing machine; 20 specimens were tested after 24 hours, and 20 specimens after hydrothermal aging in a thermocycler (TC). The data were analyzed with ANOVA followed by the Tukey post hoc test (α=.05). Weibull analysis was performed for the flexural strength and μSBS results.

RESULTS

Significant ΔTP00 differences were found among all groups and were highest for the APA group (7.33) and lowest for the NTP group (4.79). The NTP group had a significantly higher monoclinic weight fraction value (4.54%) than other groups. The NTP group had significantly higher BFS (581.31) than other groups, while the APA group showed significantly lower values than other groups (340.43). The APA group had significantly higher μSBS values after 24 hours (13.51 MPa) and after TC (13.68 MPa) than the other groups (P<.05).

CONCLUSIONS

Although APA and TS are effective techniques for improving resin-zirconia bonding, they result in significantly higher deterioration of translucency and strength. NTP significantly improved the BFS of zirconia; however, it showed lower bond strength values than other methods.

An In Vitro Study to Evaluate the Effect of Artificial Aging on Translucency, Contrast Ratio, and Color of Zirconia Dental Ceramic at Different Sintering Levels

Increasing demands for aesthetically pleasing dental restorations have promoted the use of materials that display superior optical properties. Zirconia-based all-ceramic systems have good optical properties, thus providing the desired esthetics for dental restorations. The altered oral conditions impact the physical properties of these materials. Multiple studies have been conducted to evaluate the effect of aging on the mechanical properties of computerized-aided design and computerized-aided manufacturing (CAD/CAM)-based zirconia; however, there is a scarcity of literature discussing the effect of aging on change in translucency, contrast ratio, and color. Therefore, this research aimed to evaluate the effect of accelerated artificial aging on translucency parameter (TP), contrast ratio (CR), and color of CAD/CAM zirconia at different sintering temperatures. Twenty-eight rectangular-shaped specimens were obtained by CAD/CAM milling of zirconia blank. Sintering of the samples was carried out at four different temperatures 1350 °C, 1400 °C, 1450 °C, and 1500 °C, respectively. Thermocycling of the samples was performed in a thermocycler. TP, CR, and color evaluation of all the samples was done again using a Spectrophotometer. Data acquired were statistically evaluated by one-way ANOVA and Post Hoc test. The highest amount of change in TP was recorded for specimens sintered at 1400 °C (1.86), whereas the least change was attributed to 1350 °C (0.51). The highest change in CR was observed in specimens sintered at 1500 °C (0.0971), and the least change in CR was observed at 1450 °C (0.0086). The highest ΔE values were recorded for 1400 °C against both white (1.86) and black background (2.17), whereas the least change was attributed to 1350 °C against white (0.86) and against a black background (1.41). The changes in TP were significant, but CR did not show any significant change. ΔE values for all the experimental groups were significant, but were below the clinically perceptible range. An increase in sintering temperature increased the translucency.

Effect of Veneering and Hydrothermal Aging on the Translucency of Newly Introduced Extra Translucent and High Translucent Zirconia with Different Thicknesses

Purpose

The purpose of this study was to evaluate the effect of veneering and aging on the translucency of newly introduced extra and high translucent zirconia with different thickness.

Materials and Methods

One hundred forty disk-shaped specimens were fabricated from two translucent zirconia blocks (VITA YZ XT and VITA YZ HT), and they are milled with CAD/CAM system. Then, specimens were divided into nonveneered (XT, HT) and veneered groups (XTV, HTV). Nonveneered groups were prepared with four different thicknesses (0.5-1-1.5-2 mm). Veneered groups were divided into three subgroups (n = 10) for veneering with base dentin ceramic with thicknesses of 0.5 + 0.5, 0.5 + 1, and 0.5 + 1.5 mm. A spectrophotometer was used to calculate the translucency parameter (TP) and contrast ratio (CR) of all specimens before and after aging. Statistical analysis was performed using 3-way ANOVA and Tukey HSD tests (p < 0.05).

Results

TP values were significantly affected by thickness of specimens (p < 0.001). VITA YZ XT was significantly found more translucent than VITA YZ HT. The highest translucency was observed in the XT-0.5 mm group. There is no significant difference between translucency of the veneered and nonveneered groups in the same thickness for XT. On the contrary, veneering significantly affected translucency of HT. TP values significantly decreased after aging for all groups. After aging, translucency value difference before and after aging was the highest in the XT-0.5 mm group whereas the HTV-2 mm group showed the lowest difference after aging. TP decreased significantly as thickness of specimen increases regardless of the material type. Extra translucent and nonveneered zirconia groups are more prone to hydrothermal aging.

Conclusions

The translucency parameter of zirconia ceramics was significantly influenced by both material type and veneering. Also, extra translucent and nonveneered zirconia groups are more susceptible to hydrothermal aging.

Influence of the chemical composition of monolithic zirconia on its optical and mechanical properties. Systematic review and meta-regression

PURPOSE

This systematic review set out to investigate the influence of chemical composition and specimen thickness of monolithic zirconia on its optical and mechanical properties. Meta-analysis and meta-regression analyzed the effects of variations in percentages of yttrium, aluminum, and specimen thickness of monolithic zirconia.

STUDY SELECTION

The review followed recommendations put forward in the PRISMA checklist. An electronic search for relevant articles published up to October 2019 was conducted in the Pubmed, Cochrane, Scopus, Scielo, and Web of Science databases, with no language limits and articles published in the last 10 years. From 167 relevant articles; applying inclusion criteria based on the review's PICO question, 26 articles were selected for qualitative synthesis (systematic review) and 24 for quantitative synthesis (meta-analysis). Experimental in vitro studies published were selected and their quality was assessed using the modified Consort scale for in vitro studies of dental materials.

RESULTS

The variables yttrium, aluminum and thickness were analyzed in random effects models, observing high heterogeneity ( > 75%), and finding statistically significant influences on the properties of monolithic zirconia (p<0.05).

CONCLUSIONS

Within the review's limitations, it may be concluded that variations in the percentage of yttrium and aluminum influence the optical and mechanical properties of monolithic zirconia, making it more or less esthetic and resistant in relation to each variable. The clinical implications of these findings can help select the most appropriate type of zirconia to meet the different clinical needs when restoring different regions (posterior or anterior).

Effect of sintering and aging processes on the mechanical and optical properties of translucent zirconia

STATEMENT OF PROBLEM

Sintering holding times and aging conditions may affect the optical, mechanical, and structural properties of polychromatic multitranslucent zirconia. However, a consensus on the ideal sintering condition for this material is lacking.

PURPOSE

The purpose of this in vitro study was to investigate the effect of different sintering holding and autoclave aging times on the flexural strength (FS), grain size, and translucency parameter (TP₀₀) of a translucent zirconia.

MATERIAL AND METHODS

Sixty bar-shaped and 60 rectangular-shaped specimens were prepared from computer-aided design and computer-aided manufacturing (CAD-CAM) zirconia blocks and split into 2 groups to be sintered at 1550 °C for different holding times (regular: 2 hours, prolonged: 5 hours). The specimens were then divided into 3 subgroups and subjected to aging processes (control, aging for 60 minutes, aging for 120 minutes). FS values were obtained by using a 3-point bend test and Weibull analysis was conducted. Grain size evaluations were performed by using scanning electron microscope (SEM) observations. TP₀₀ was measured with a spectrophotometer. Statistical evaluation included the Kolmogorov-Smirnov, 2-way ANOVA, and Tukey HSD tests (α=.05).

RESULTS

The sintering procedure, aging, and their interaction significantly influenced the TP₀₀ (P<.05). FS values and grain sizes were affected by aging only (P<.001). Both aging times resulted in reduced FS and increased grain sizes. Prolonged sintering in combination with 120 minutes of aging negatively affect the TP₀₀ scores (P<.05).

CONCLUSIONS

FS values were similar under regular and prolonged sintering. Prolonged sintering led to a decrease in translucency after 120 minutes of aging. Steam autoclave aging can affect the optical and mechanical properties of translucent zirconia.

Influence of Additive Firing on the Surface Characteristics, Streptococcus mutans Viability and Optical Properties of Zirconia

The purpose of this study was to observe whether the repetitive firing of dental zirconia caused changes in surface characteristics, S. mutans viability, and optical properties of zirconia. Dental zirconia blocks were sintered and randomly distributed into seven experimental groups: F0–F6. Except for F0, which only went through sintering, the additive firing was performed in order for F1–F6. Surface roughness, contact angle, S. mutans viability by fluorescence, and translucency parameter were measured. They were all highest after sintering (F0) and decreased after additive firings (F1–F6). The additive firing of zirconia after sintering decreased surface roughness, contact angle, S. mutans viability, and translucency. The number of firings after the first firing was not found to be critical in surface characteristics, S. mutans viability, and optical property. Changes in surface characteristics might have led to a decrease in S. mutans viability, while the change of translucency was not clinically significant. This implies that additive firing may prevent secondary caries under zirconia restorations, not compromising esthetic appearance.

Effects of Aging on the Color and Translucency of Monolithic Translucent Y-TZP Ceramics: A Systematic Review and Meta-Analysis of In Vitro Studies

Background

Monolithic restorations made of translucent yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have become popular over the past few decades. However, whether aging affects the color and translucency of monolithic translucent Y-TZP is unclear.

Objective

The aim of this systematic review and meta-analysis of in vitro studies was to evaluate the effects of aging on the color and translucency of monolithic translucent Y-TZP ceramics.

Materials and Methods

This systematic review/meta-analysis was reported according to the PRISMA statement and registered in the OSF registries (https://osf.io/5qjmu). Four databases including Medline via the PubMed, Embase, and Web of Science databases and the Cochrane Library were searched using no publication year and language limits. The last search was executed on November 20, 2020. In vitro studies comparing the translucency and/or color of monolithic translucent Y-TZP ceramics before and after simulated aging were selected. Meta-analyses were performed using Review Manager software (version 5.3, Cochrane Collaboration, Oxford, UK) with random-effects models at a significance level of 0.05. A risk-of-bias assessment was also performed for the included studies.

Results

Of the 188 potentially relevant studies, 13 were included in the systematic review. The hydrothermal aging duration ranged from 1 to 100 h at relatively similar temperatures (~134°C). In the general meta-analyses, the aged Y-TZP ceramics exhibited similar translucency parameter (TP), L^∗, and b^∗ values compared with the nonaged controls (P = .73, P = .49, and P = .62, respectively). Moreover, there was a significant difference between the aged and nonaged Y-TZP ceramics in the a^∗ value (P = .03; MD = −0.26; 95% CI = −0.51 to − 0.02), favoring the nonaged Y-TZP ceramics. The subgroup analyses showed that the duration of aging contributed to changes in the translucency and color of the Y-TZP ceramics.

Conclusions

The optical properties of monolithic translucent Y-TZP ceramics were stable after hydrothermal aging at 134°C and 0.2 MPa for ≤20 h. Moreover, clinically unacceptable changes in the translucency and color of monolithic translucent Y-TZP ceramics were found after hydrothermal aging for >20 h.

Effect of in vitro aging and acidic storage on color, translucency, and contrast ratio of monolithic zirconia and lithium disilicate ceramics

STATEMENT OF PROBLEM

How the translucency and color of ceramic restorations are affected by surface changes from the corrosive environment in the oral cavity and aging of materials is unclear.

PURPOSE

The purpose of this in vitro study was to compare the impact of acidic exposure and aging on the color and translucency of monolithic zirconia and lithium disilicate ceramics.

MATERIAL AND METHODS

Twenty computer-aided design and computer-aided manufacturing (CAD-CAM) monolithic zirconia specimens and 20 pressed lithium disilicate specimens were fabricated. Half of the specimens of each group were aged in an autoclave (7.5 hours, 134 °C, 0.2 MPa), and the rest were immersed in hydrochloric acid to simulate the acidic conditions in the oral cavity from gastric reflux. The color coordinates L∗, a∗, and b∗ were measured with an ultraviolet spectrophotometer before and after aging or acidic storage. The translucency parameters and contrast ratios were calculated, and the CIEDE2000 color difference formula was used to determine color differences before and after each treatment. ANOVA and ANCOVA test models were used for data analysis (α=.05), while differences of color parameters in respect to acceptability and perceptibility thresholds were evaluated with the 1-sample t test (α=.05).

RESULTS

Lithium disilicate presented a significantly higher translucency parameter and lower contrast ratio at baseline compared with monolithic zirconia specimens (P<.001). Acidic storage significantly impacted all parameters compared with aging, especially for the lithium disilicate group. Color differences were above the acceptability ΔΕ₀₀ threshold for lithium disilicate after acidic storage (P=.001) and below for monolithic zirconia after acidic storage (P=.003).

CONCLUSIONS

The performance of lithium disilicate was slightly inferior compared with that of monolithic zirconia specimens, as they presented significant and clinically observable differences for the translucency parameter and ΔΕ₀₀ after acidic storage and aging.

Microstructural, mechanical, and optical characterization of an experimental aging-resistant zirconia-toughened alumina (ZTA) composite

OBJECTIVE

To evaluate the effect of aging on the microstructural, mechanical, and optical properties of an experimental zirconia-toughened alumina composite with 80%Al₂O₃ and 20%ZrO₂ (ZTA Zpex) compared to a translucent zirconia (Zpex) and Alumina.

METHODS

Disc-shaped specimens were obtained by uniaxial and isostatic pressing the synthesized powders (n = 70/material). After sintering and polishing, half of the specimens underwent aging (20 h, 134 °C, 2.2 bar). Crystalline content and microstructure were evaluated using X-ray diffraction and scanning electron microscopy, respectively. Specimens underwent biaxial flexural strength testing to determine the characteristic stress, Weibull modulus, and reliability. Translucency parameter (TP) and Contrast ratio (CR) were calculated to characterize optical properties.

RESULTS

ZTA Zpex demonstrated a compact surface with a uniform dispersion of zirconia particles within the alumina matrix, and typical alumina and zirconia crystalline content. ZTA Zpex and alumina exhibited higher CR and lower TP than Zpex. ZTA Zpex and Zpex showed significantly higher characteristic stress relative to alumina. While aging did not affect optical and mechanical properties of ZTA Zpex and alumina, Zpex demonstrated a significant increase in translucency, as well as a in characteristic stress. Alumina reliability was significantly lower than others at 300 MPa, ZTA Zpex and Zpex reliability decreased at 800 MPa, except for aged Zpex.

SIGNIFICANCE

While aging did not affect the mechanical nor the optical properties of ZTA Zpex and alumina, it did alter both properties of Zpex. The results encourage further investigations to engineer ZTA as a framework material for long span fixed dental prostheses specially where darkened substrates, such as titanium implant abutments or endodontically treated teeth, demand masking.

Hydrothermal degradation methods affect the properties and phase transformation depth of translucent zirconia

OBJECTIVES

To characterize the optical and mechanical properties of a commercial and in-house translucent Y-TZP before and after aging in autoclave or hydrothermal reactor.

METHODS

In-house experimental discs were obtained through uniaxial and isostatic pressing a translucent Y-TZP powder and sintering at 1,550 °C/1 h. Commercial discs were milled from pre-sintered blocks fabricated with the same powder through uniaxial and isostatic pressing and sintering. Discs were allocated into three groups according to aging condition: immediate, aged via autoclave, or reactor (134 °C, 20 h, 2.2 bar). Crystalline content and microstructure were evaluated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Residual compressive stress (CS) was determined by Raman spectroscopy. Optical properties were determined by the contrast ratio (CR) and translucency parameter (TP) using reflectance data. Mechanical properties were assessed by Vickers hardness, fracture toughness and biaxial flexural strength tests.

RESULTS

XRD and SEM revealed a typical Y-TZP crystalline content, chiefly tetragonal phase, and a dense crystalline matrix for both processing protocols. Reactor aging triggered a more pronounced t-m transformation relative to autoclave. In-house and commercial Y-TZPs demonstrated similar CR and TP, with reactor aging significantly increasing their translucency. Similarly, reactor aging influenced Vickers hardness and fracture toughness. In-house processed Y-TZP clearly demonstrated the presence of CS, whereas commercial Y-TZP showed no presence of CS. Non-aged in-house Y-TZP resulted in significantly lower characteristic strength relative to commercial Y-TZP. While aging protocols significantly increased the characteristic strength of in-house Y-TZP, reactor significantly decreased commercial Y-TZP characteristic strength. Both Y-TZP processing protocols demonstrated high reliability at high-stress missions, with no detrimental effect of aging.

CONCLUSIONS

Laboratory aging methodology significantly influenced optical and mechanical properties of a commercial and in-house translucent Y-TZP.

Aging resistant ZTA composite for dental applications: Microstructural, optical and mechanical characterization

OBJECTIVE

To synthesize a zirconia toughened alumina (ZTA) composite with 70% alumina reinforced by 30% zirconia for dental applications and to characterize its microstructure and optical properties for comparison with the isolated counterpart materials and a first-generation 3Y-TZP.

METHODS

Disc-shaped specimens were divided in four groups (n = 70/material): (1) 3YSB-E (first generation 3Y-TZP), (2) Zpex (second generation 3Y-TZP), (3) alumina, and (4) ZTA-Zpex 70/30. After synthesis, ceramic powders were pressed, and green-body samples sintered following a predetermined protocol. Specimens were polished to obtain a mirror surface finish. Apparent density was measured by Archimedes principle. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to characterize the crystalline content and microstructure. Reflectance tests were performed to determine the contrast-ratio (CR) and translucency-parameter (TP). Mechanical properties were assessed by biaxial flexural strength (BFS) test. All analyses were conducted before and after artificial aging (20 h, 134 °C, 2.2 bar). Optical parameters were evaluated through repeated-measures analysis of variance and Tukey tests (p < 0.05). BFS data were analyzed using Weibull statistics (95% CI).

RESULTS

High density values (95-99%) were found for all ceramic materials and SEM images exhibited a dense microstructure. While XRD patterns revealed the preservation of crystalline content in the ZTA composite, an increase in the monoclinic peak was observed for pure zirconias after aging. Significantly higher CR and lower TP values were observed for the ZTA composite, followed by alumina, 3YSB-E, and Zpex. The highest characteristic stress was recorded for 3YSB-E, followed by intermediate values between ZTA and Zpex, and the lowest for alumina. Aging affected the optical and mechanical properties of both zirconias, while remained stable for ZTA composite and alumina.

SIGNIFICANCE

The synthesis of experimental 70-30% ZTA composite was successful and its relevance for dental applications relies on its higher masking ability, aging resistance, and strength similar to zirconia.

Translucency of Monolithic Zirconia after Hydrothermal Aging: A Review of In Vitro Studies

PURPOSE

Recent modifications in composition, structure, and fabrication methods have led to monolithic zirconia ceramics of superior translucency. However, during aging, intrinsic microstructural features, such as grain boundaries and pores, may affect light scattering and consequently the translucency of zirconia. The aim of this study was to systematically review if hydrothermal aging affects the translucency of monolithic zirconia.

MATERIALS AND METHODS

An electronic search in Medline and Scopus was conducted to identify the effect of hydrothermal aging on the translucency of zirconia. The search was limited to English-language publications and in vitro studies. The following search terms were used alone or in combination: "monolithic zirconia," "full contour zirconia," "Y-TZP zirconia," "tetragonal zirconia," "cubic zirconia," "aging," "hydrothermal aging," "steam autoclave," "translucency," "translucency parameter," and "contrast ratio." From the titles found after searching the electronic databases, only 10 articles met the inclusion criteria and were included in this review. The translucency parameter or total transmittance was used to extract data from the studies.

RESULTS

Aging reduced the translucency of monolithic zirconia in most of the studies; however, the differences varied according to the brand tested and the microstructure and thickness of the specimens. The thinner specimens presented higher translucency alterations. Although in all studies a change in translucency was recorded, its mean values suggest that the differences are within the acceptability threshold in most cases, independent of the color system used to calculate translucent parameter, so they are clinically undetectable. For longer aging times, beyond the perspective clinical life of the restorations, the change in translucency is higher than the acceptability threshold.

CONCLUSIONS

There is a controversy regarding how aging affects the translucency of monolithic zirconia ceramics; however, this may not be clinically important, as in most cases, changes (either decrease or increase) in translucency were within the acceptability thresholds.

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.

The effects of artificial aging on high translucent zirconia

Zirconia is known for its high strength but lacking translucency. Recently, a new type of high translucent zirconia, 5 mol% yttria partially stabilized zirconia (5Y-PSZ), with a larger fraction of cubic zirconia phase has become commercially available. However, the resistance to aging of these commercially available zirconia materials is not yet fully established.

Purpose: The aim of the present study was to analyze the effects of artificial aging on surface roughness, transparency, phase transformation and biaxial flexural strength of two 5Y-PSZ products, DD cubeX² and Prettau Anterior.

Materials and methods: The artificial aging was performed in an autoclave under 2 bars of pressure at 134 °C for 10 hours, which is estimated to correspond to 30–40 years in vivo. Artificial aging for 10 hours had no significant effect on surface roughness, transparency, or phase transformation for either of the tested materials.

Results: DD cubeX² had higher mean flexural strength than Prettau Anterior both before and after artificial aging for 10 hours (p < .05). DD cubeX² showed, however, a significant reduction in flexural strength after artificial aging (p < .05), whereas Prettau Anterior showed a slight increase in flexural strength after artificial aging but not at a significant level.

Conclusion: Within the limitation of the present study, both DD cubeX² and Prettau Anterior seems to be relatively resistant to aging. However, a wider range of measured flexural strength indicated that Prettau Anterior probably is a less stable material than DD cubeX², which also means that the flexural strength of DD cubeX² could be more predictable.

Monolithic Zirconia: An Update to Current Knowledge. Optical Properties, Wear, and Clinical Performance

The purpose of this paper was to update the knowledge concerning the wear, translucency, as well as clinical performance of monolithic zirconia ceramics, aiming at highlighting their advantages and weaknesses through data presented in recent literature. New ultra-translucent and multicolor monolithic zirconia ceramics present considerably improved aesthetics and translucency, which, according to the literature reviewed, is similar to those of the more translucent lithium disilicate ceramics. A profound advantage is their high strength at thin geometries preserving their mechanical integrity. Based on the reviewed articles, monolithic zirconia ceramics cause minimal wear of antagonists, especially if appropriately polished, although no evidence still exists regarding the ultra-translucent compositions. Concerning the survival of monolithic zirconia restorations, the present review demonstrates the findings of the existing short-term studies, which reveal promising results after evaluating their performance for up to 5 or 7 years. Although a significant increase in translucency has been achieved, new translucent monolithic zirconia ceramics have to be further evaluated both in vitro and in vivo for their long-term potential to preserve their outstanding properties. Due to limited studies evaluating the wear properties of ultra-translucent material, no sound conclusions can be made, whereas well-designed clinical studies are urgently needed to enlighten issues of prognosis and long-term survival.