Evaluation of surface treatments of monolithic zirconia in different sintering stages

Comparing the effect of two polishing systems on surface roughness of feldspathic, lithium disilicate, and translucent zirconia ceramics after orthodontic bracket debonding: An in vitro study

INTRODUCTION

Debonding of orthodontic brackets on ceramic restorations leave a rough surface which should be efficiently polished. In this study the effect of two diamond polishing systems (DPS), namely Optra Fine® (OF) and Diapol Twist® (DT) on surface roughness (SR) of feldspathic (F), lithium disilicate (LD), and translucent zirconia (TZ) ceramics assessed after bracket debonding.

MATERIALS AND METHODS

Ninety disc-shaped specimens fabricated from F, LD, and TZ ceramics and glazed (Gl). Ten glazed specimens of 3 ceramic types served as control (C) groups (n=30). Other 60 specimens were deglazed, and after specific surface preparation according to ceramic type, upper central incisor brackets were bonded. After debonding, the remaining adhesive composite removed with tungsten carbide bur and specimens randomly assigned into 6 groups. The nine experimental groups defined as: Glazed F specimens (C), Deglazed F specimens polished with DT, Deglazed F specimens polished with OF, Glazed LD specimens (C), Deglazed LD specimens polished with DT, Deglazed LD specimens polished with OF, Glazed TZ specimens (C), Deglazed TZ specimens polished with DT, Deglazed TZ specimens polished with OF. The SR assessed quantitatively by profilometry (Ra and Rz parameter) and qualitatively by scanning electron microscopy (SEM). Quantitative data were statistically analysed using Kolmogorov-Smirnov test, two-way ANOVA and Tukey post-hoc test (α=0.05).

RESULTS

No significant difference found in SR of three glazed ceramic types (P=0.293 for Ra and P=0.473 for Rz). There was no significant difference of Ra between polished and glazed groups (P>0.05). However, difference in Rz was significant (P<0.05), and only TZ specimens could reach to the level of smoothness caused by glazing.

CONCLUSIONS

Both diamond polishing systems can efficiently reduce the surface roughness, despite of ceramic type. Optra Fine® system showed a superior performance than Diapol Twist® without significant difference.

LIMITATIONS

In this study only one brand of each ceramic type was investigated. This study was approved by Ethical Review Committee (IR.IAU.KHUISF.REC.1401.156).

Comparative assessment of flexural strength of monolithic zirconia with different thicknesses and two sintering techniques

OBJECTIVES

This study aimed to compare the flexural strength of monolithic zirconia with different thicknesses and two sintering techniques.

MATERIALS AND METHODS

This in vitro, experimental study was conducted on 28 monolithic zirconia discs with 10 mm diameter and 0.5 (n = 14) and 1.2 mm (n = 14) thickness. Each group was divided into two subgroups (n = 7) for fast (60 min) and conventional (120 min) sintering at 1450°C. After sintering, the specimens were thermocycled and their flexural strength was measured by piston-on-3-balls technique in a universal testing machine (0.5 mm/min, 1.2 mm pin diameter). Data were analyzed by the Weibull test, one-way analysis of variance, and Tukey's test (α = .05).

RESULTS

The flexural strength of specimens with 1.2 mm thickness was significantly higher than that of specimens with 0.5 mm thickness (p < .05). The flexural strength of 1.2 mm/120-min group was slightly, but not significantly, higher than that of 1.2 mm/60-min group (p > .05). The flexural strength of 0.5 mm/120-min group was slightly, but not significantly, higher than that of 0.5 mm/60-min group (p > .05).

CONCLUSION

The increase in thickness of monolithic zirconia increases its flexural strength; however, increasing the sintering time appears to have no significant effect on the flexural strength of monolithic 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.

Effect of Sandblasting Parameters and the Type and Hardness of the Material on the Number of Embedded Al2O3 Grains

BACKGROUND

Is abrasive blasting accompanied by the phenomenon of driving abrasive particles into the conditioned material?

METHODS

Three hundred and fifteen cylindrical disks of three types of metal alloy (chromium/cobalt, chromium/nickel, titanium, and sintered zirconium dioxide) were divided into four groups (n = 35) and sandblasted at pressures of 0.2, 0.4, or 0.6 MPa with aluminum oxide (Al₂O₃), grain size 50, 110, or 250 μm. Then, the surface topography was examined using a scanning microscope, and the amount of embedded grain was measured using quantitative metallography. For each group, five samples were randomly selected and subjected to Vickers hardness testing. In the statistical analyses, a three-factor analysis of variance was carried out, considering the type of material, the size of gradation of the abrasive, and the amount of pressure.

RESULTS

The smallest amounts of embedded abrasive (2.62) were observed in the ZrO₂ treatment, and the largest (38.19) occurred in the treatment of the Ti alloy. An increase in the gradation and the pressure were a systematic increase in the amount of embedded grain.

CONCLUSIONS

After abrasive blasting, abrasive particles were found on the surface of the materials. The amount of driven abrasive depends on the hardness of the processed material.

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 speed sintering of monolithic zirconia with different yttria contents on color and crystal phase

This study evaluated the color and microstructure of monolithic zirconia crowns with different yttrium oxide (Y₂ O₃ ) contents treated by conventional or speed sintering. Four types of zirconia ceramics were assessed: two monolayer zirconia, and two multilayer zirconia. The monolithic zirconia crowns were fabricated using a dental computer-aided design/computer-aided manufacturing (CAD/CAM) system and in two shades (A2 and BL). After milling, the zirconia crowns were sintered using either speed sintering or conventional sintering. For each combination of zirconia (4), shade (2), and sintering condition (2), the color parameters were determined at three positions of each of nine crowns using a non-contact dental spectrophotometer. In addition, the zirconia phases in the specimens were quantified using X-ray diffractometry. Significant differences in the ΔE₀₀ values at different measurement positions were observed for the Multi2 crown of the BL shade group. The color difference resulting from conventional and speed sintering programs was not affected by the difference in yttria content of Mono1, Mono2, and Multi1. However, in Multi2, containing 3Y-TZP and 5Y-PSZ, a color change was caused by the use of speed sintering. Therefore, when performing speed sintering with Multi2, it is necessary to select the color in consideration of these results or take measures for staining.

Effect of surface treatments on biaxial flexural strength, fatigue resistance, and fracture toughness of high versus low translucency zirconia

BACKGROUND

Mechanical surface treatments can deteriorate the mechanical properties of zirconia. This study evaluated and compared the biaxial flexural strength, fracture toughness, and fatigue resistance of high translucency (HT) to low translucency (LT) zirconia after various mechanical surface treatments.

METHODS

Four hundred eighty zirconia discs were prepared by milling and sintering two HT (Katana and BruxZir) and LT (Cercon and Lava) zirconia blocks at targeted dimensions of 12 mm diameter × 1.2 mm thickness. Sintered zirconia discs received one of the following surface treatments: low-pressure airborne particle abrasion (APA) using 50 µm alumina particles, grinding using 400 grit silicon carbide paper, while as-sintered specimens served as control. Internal structure and surface roughness were evaluated by scanning electron microscope (SEM) and a non-contact laser profilometer, respectively. Half of the discs were tested for initial biaxial flexural strength, while the rest was subjected to 10⁶ cyclic fatigue loadings, followed by measuring the residual biaxial flexural strength. Fractured surfaces were examined for critical size defects (c) using SEM to calculate the fracture toughness (K_IC). The effect of surface treatments, zirconia type, and cyclic fatigue on the biaxial flexural strength was statistically analyzed using three-way analysis of variance (ANOVA) and Tukey HSD post hoc tests (α = 0.05). Weibull analysis was done to evaluate the reliability of the flexural strength for different materials.

RESULTS

The initial biaxial flexural strength of LT zirconia was significantly higher (p < 0.001) than that of HT zirconia in all groups. While low APA significantly increased the biaxial flexural strength of LT zirconia, no significant change was observed for HT zirconia except for Katana. Surface grinding and cyclic fatigue significantly reduced the flexural strength of all groups. High translucency zirconia reported higher fracture toughness, yet with lower Weibull moduli, compared to LT zirconia.

CONCLUSION

LT zirconia has higher biaxial flexural strength, yet with lower fracture toughness and fatigue resistance, compared to HT zirconia. Low-pressure APA has significantly increased the biaxial flexural strength in all zirconia groups except BruxZir. Grinding was deteriorating to biaxial flexural strength and fracture toughness in all zirconia types. Cyclic fatigue has significantly decreased the biaxial flexural strength and reliability of HT and LT zirconia.

Mechanical behavior and microstructural characterization of different zirconia polycrystals in different thicknesses

PURPOSE

To characterize the microstructure of three yttria partially stabilized zirconia ceramics and to compare their hardness, indentation fracture resistance (IFR), biaxial flexural strength (BFS), and fatigue flexural strength.

MATERIALS AND METHODS

Disc-shaped specimens were obtained from 3Y-TZP (Vita YZ HT), 4Y-PSZ (Vita YZ ST) and 5Y-PSZ (Vita YZ XT), following the ISO 6872/2015 guidelines for BFS testing (final dimensions of 12 mm in diameter, 0.7 and 1.2 ± 0.1 mm in thicknesses). Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and mechanical properties were assessed by Vickers hardness, IFR, quasi-static BFS and fatigue tests.

RESULTS

All ceramics showed similar chemical compositions, but mainly differed in the amount of yttria, which was higher as the amount of cubic phase in the diffractogram (5Y-PSZ > 4Y-PSZ > 3Y-TZP). The 4Y- and 5Y-PSZ specimens showed surface defects under SEM, while 3Y-TZP exhibited greater grain uniformity on the surface. 5Y-PSZ and 3Y-TZP presented the highest hardness values, while 3Y-TZP was higher than 4Y- and 5Y-PSZ with regard to the IFR. The 5Y-PSZ specimen (0.7 and 1.2 mm) showed the worst mechanical performance (fatigue BFS and cycles until failure), while 3Y-TZP and 4Y-PSZ presented statistically similar values, higher than 5Y-PSZ for both thicknesses (0.7 and 1.2 mm). Moreover, 3Y-TZP showed the highest (1.2 mm group) and the lowest (0.7 mm group) degradation percentage, and 5Y-PSZ had higher strength degradation than 4Y-PSZ group.

CONCLUSION

Despite the microstructural differences, 4Y-PSZ and 3Y-TZP had similar fatigue behavior regardless of thickness. 5Y-PSZ had the lowest mechanical performance.

Aging resistance of infiltrated monolithic zirconia compared to noninfiltrated monolithic zirconia: A systematic review of in vitro studies

Aim

The aim of the study is to systematically assess the impact of low-temperature degradation (LTD) simulation in an autoclave on mechanical and microstructural properties of infiltrated monolithic zirconia compared to the noninfiltrated zirconia.

Settings and Design

Systematic review following Preferred Reporting Items for Systematic Reviews and Meta-Analysis 2020 guidelines.

Materials and Methods

An electronic search was done within these databases: PubMed, Scopus, and Web of Science, Science Direct, Embase, Wiley, Google Scholar for articles published between 2000 and March 2021. Search results that met eligibility criteria were categorized into two groups based on properties assessed of infiltrated monolithic zirconia exposed to LTD (also called aging simulation) - (a) mechanical (flexural strength and fracture toughness) and (b) microstructural properties (phase transformation rate and m content).

Statistical Analysis Used

Qualitative analysis.

Results

The search identified 272 preliminary results. After discarding duplicates, and screening of titles, abstracts, and full texts, 10 articles finally met inclusion criteria. Data were collected on author's details and their countries, journal and year of publication, type and percentage of infiltration, aging protocol (duration and temperature), mechanical, and microstructural properties. All the included studies invariably revealed better aging resistance without a change in mechanical properties for infiltrated monolithic zirconia as compared to noninfiltrated species.

Conclusion

Infiltration within monolithic zirconia can reduce degradation and simultaneously maintain their mechanical properties by preventing water entry into grain contours. The final m content was less for infiltrated Zirconium, indicating a lesser phase transformation and better aging resistance.

Other Information

Systematic review protocol registered at PROSPERO CRD42021248153.

Zirconia surface infiltration with low-fusing glass: A surface treatment modality to enhance the bond strength between zirconia and veneering ceramic

Background

The pursuit of esthetics and superior mechanical properties prompted the layering of Yttrium stabilized Zirconia with ceramic material. However, the bioinert nature of zirconia causes the chipping off of this ceramic layer. Selective infiltration etching (SIE) of zirconia provides good bond strength between zirconia and veneering ceramic.

Materials and Methods

One hundred and fifty zirconia specimens of dimensions 5 × 5 × 10 mm were divided into 5 groups. Group 1: Air abrasion with 30 μ Al₂O₃ for 15 s with 0.4 bar pressure. Group 2: SIE and heat-induced maturation (HIM) as demonstrated by Abousheilb. Group 3: Sintered zirconia specimens were taken up for air abrasion followed by SIE/HIM. Group 4: Air abrasion and SIE performed on unsintered specimens followed by heat treatment at 1500°C. Group 5: Air abrasion performed on unsintered zirconia specimens followed by heat treatment at 1500°C followed by SIE/HIM. The samples were then layered with ceramic and subjected to shear bond strength (SBS) analysis.

Results

The mean and standard deviation were calculated for the data. The mean SBS among the groups was compared using ANOVA. The post hoc Bonferroni test was applied to compare between the groups. The mean SBS was highest for Group 5 (47.89 ± 6.53) followed by Group 2 (34.94 ± 3.04), Group 3 (32.56 ± 6.04), Group 1 (29.12 ± 7.37), and Group 4 (27.56 ± 7.54). ANOVA test showed statistically significant differences among the groups (F = 48.86, P = 0.00).

Conclusion

SIE/HIM when combined with sandblasting with appropriate heat treatment demonstrated a significant increase in bond strength. This prolongs the longevity of the restoration, thereby meeting the clinical needs.

Effect of Ti: Sapphire and Nd: YAG Lasers on Shear Bond Strength at the Zirconia - Veneering Ceramic Juncture

Introduction: This study was conducted to assess the impact of nanosecond and femtosecond lasers on shear bond strength at the zirconia - veneering ceramic juncture. Materials and Methods: The first 60 samples of partially sintered zirconia cylindrical discs measuring 7 mm diameter and 4 mm height were milled and sintered. Then they were randomly divided into three groups namely group C (control, n=10), group N (Nd: YAG laser, n=10), and group T (Ti: sapphire laser n=40) which was further divided based on the duration of laser irradiation, into 4 subgroups (n=10 each) which were 30 seconds, 1 minute, 2 minutes and 3 minutes. Surface treatment was done on sintered zirconia discs based on the group. Following the treatment, the discs were ultrasonically cleaned followed by liner application (IPS Emax Zirliner, Ivoclar-Vivadent) and veneer ceramic layering (IPS e.max Ceram, Ivoclar-Vivadent) of 1.5 mm height. Each sample underwent shear stress in the universal test machine on the mounting jig, and bond strength was evaluated. Data were assessed using ANOVA followed by Tukey’s post hoc multiple comparison analyses. Results: According to one-way ANOVA, there was a significant difference in shear bond strength between the groups. Tukey’s post hoc pair wise comparison test showed a significant difference (P value=0.001) in shear bond strength of all pairs except group C and group N. The results of repeated measures, ANOVA (related and dependent groups) and Tukey’s multiple pair wise comparison test showed that there was a significant difference (P value=0.001) in shear bond strength at 30 seconds and all other groups. Conclusion: Ti: sapphire laser irradiation for 30 seconds can be used as potential surface treatment to increase shear bond strength at the zirconia-veneering ceramic juncture.

Impact of changes in sintering temperatures on characteristics of 4YSZ and 5YSZ

OBJECTIVES

The aim of this study was to evaluate the effect of different sintering temperatures on biaxial flexural strength (BFS), dynamic loading, surface hardness, color reproduction, translucency, surface roughness and microstructure of zirconia with 4 mol% yttria (4YSZ) compared to zirconia with 5 mol% yttria (5YSZ).

METHODS

Zirconia discs with 12 mm diameter and 1.2 mm thickness were prepared and divided into three groups (n = 53) according to different sintering temperatures (1400 °C, 1500 °C and 1600 °C). Each group was divided into five subgroups (n = 10) according to the dynamic loading procedure (none, 50%, 65%, 75% and 80%) conducted before the quasi-static BFS test and another subgroup (n = 3) used for X-ray-diffraction (XRD) microstructure analysis. BFS test and dynamic loading were performed with a piston-on-three-ball test. The surface hardness was evaluated according to Vickers. Color reproduction and translucency were measured with a spectrophotometer. A 3D laser scanning microscope was used to determine the surface roughness. Grain size measurements were performed using SEM.

RESULTS

A significant increase in biaxial flexural strength was observed while the sintering temperature decreased. 4YSZ had significantly higher results in biaxial flexural strength than 5YSZ. A decrease in sintering temperature resulted in a significant increase in Vickers hardness. Furthermore, 4YSZ showed significantly better color reproduction with increasing sintering temperature. At higher temperatures (1500 °C and 1600 °C), 4YSZ showed better color reproduction than 5YSZ. Compared to 4YSZ, specimens of 5YSZ exhibited significant higher translucency. Using XRD, a distorted tetragonal phase was detected in addition to regular tetragonal and cubic phases in specimens without any stress and at a low sintering temperature. The grain sizes of both materials increased with an increase in sintering temperature.

CONCLUSION

The sintering temperature has significant effects on the microstructure and thus on the mechanical and optical properties of the evaluated zirconia.

Effect of silica coating and laser treatment on the flexural strength, surface characteristics, and bond strength of a dental zirconia

This study investigated the effect of irradiation with an erbium-doped yttrium aluminium garnet (Er:YAG) laser and coating with silica on the surface characteristics, bond strength, and flexural strength of dental zirconia. Three hundred and forty-three standard zirconia specimens were created, and 49 were assigned to each of seven surface treatment groups: (i) no treatment; Er:YAG laser (80 mJ/2 Hz) with pulse widths of 50 μs (ii), 100 μs (iii), 300 μs (iv), or 600 μs (v); or tribochemical silica coating at the partially sintered stage (vi) or after sintering was complete (vii). All specimens were sintered after the surface treatments, except for the group in which specimens were sintered before treatment. The study outcomes were roughness, surface loss, microshear bond strength (μSBS), and biaxial flexural strength (BFS). Mean roughness and surface loss values were significantly higher in specimens from irradiated groups than in those from silica-coated groups. Regarding μSBS, after aging, specimens from all experimental groups presented very low and similar μSBS values, irrespective of the surface treatment. Silica coating after sintering yielded the highest BFS (1149.5 ± 167.6 MPa), while coating partially sintered specimens with silica resulted a BFS (826.9 ± 60.9 MPa) similar to that of the untreated control group (794.9 ± 101.7 MPa). Laser treatments, irrespective of pulse width used, significantly decreased the BFS. In the group treated with laser at 300 μs pulse width, specimens exhibited the lowest BFS value (514.1 ± 71.5 MPa). Adhesion to zirconia was not stable after aging, regardless of the surface treatment implemented.

Effect of air-abrasion at pre- and/or post-sintered stage and hydrothermal aging on surface roughness, phase transformation, and flexural strength of multilayered monolithic zirconia

This study aimed to evaluate the effect of air-abrasion/sintering order and autoclave aging on the surface roughness (Ra), phase transformation, and biaxial flexural strength (BFS) of monolithic zirconia. A total of 104 monolithic zirconia specimens (Katana ML) were divided into eight groups according to airborne-particle abrasion protocols and hydrothermal aging: control (non-aged: C-, aged: C+), air-abrasion before sintering (BS-, BS+), air-abrasion after sintering (AS-, AS+), and air-abrasion before and after sintering (BAS-, BAS+). A steam autoclave was used for accelerated aging, and Ra values were measured with a surface profilometer. All specimens were analyzed by X-ray diffraction to determine any phase transformation on the zirconia surface. BFS was measured by using the piston-on-three-balls method. Scanning electron microscopy and atomic force microscopy were performed on one specimen per group. BS and BAS groups showed higher Ra values compared with groups C and AS. The aging process significantly increased the monoclinic phase content of all specimens. Lower monoclinic levels were found in AS+ and BAS+ compared with other aged groups. The AS groups exhibited higher flexural strength values relative to control groups, whereas BS groups exhibited significantly lower flexural strength values (p < .05). There was no reduction in flexural strength by using the BAS protocol. Air-abrasion of zirconia at the pre-sintered stage only is not recommended in clinical use because of the remarkable decrease in flexural strength.

Combined application of femtosecond laser and air-abrasion protocols to monolithic zirconia at different sintering stages: Effects on surface roughness and resin bond strength

This study aimed to investigate the effects of femtosecond laser (Fs) and/or air-abrasion protocols on surface roughness (Ra) of zirconia and resin bond strength. Eighty zirconia samples were randomly divided into eight subgroups according to surface treatment protocols: Control (C), Air-abrasion before sintering (ABS), Air-abrasion after sintering (AAS), Air-abrasion before and after sintering (ABS + AAS), Fs laser before sintering (FBS), Fs laser before sintering + air-abrasion after sintering (FBS + AAS), Fs laser after sintering (FAS), and Fs laser after sintering + air-abrasion after sintering (FAS + AAS). Measurements of Ra values were obtained using a surface profilometer. Surface morphological properties were evaluated with scanning electron microscopy (SEM), and crystallographic changes were examined by X-Ray diffractometry (XRD). Self-adhesive resin cement was bonded to zirconia samples, and shear bond strength (SBS) tests were performed. The data were statistically analyzed by one-way ANOVA, followed by Tamhane tests. The control group displayed the lowest Ra and SBS values among all groups. The highest Ra and SBS values were found in the FBS and FBS + AAS groups. Air-abrasion applied before sintering significantly increased the Ra of specimens. FAS, FAS + AAS, and ABS + AAS groups exhibited higher SBS values than AAS and ABS (p < .05). Air-abrasion applied after Fs laser did not produce any significant change in the Ra and SBS compared to Fs laser alone (p > .05). Femtosecond laser application may be a promising method to enhance the surface roughness of zirconia and improve resin bond strength. Air-abrasion at pre- and post-sintered stages may also be a viable surface treatment option.

Does the application of surface treatments in different sintering stages affect flexural strength and optical properties of zirconia?

OBJECTIVE

To investigate the influence of surface treatments conducted in presintering and postsintering stages on flexural strength and optical properties of zirconia.

MATERIALS AND METHODS

Specimens were milled from partially sintered zirconia blocks in different geometries and divided into three main groups as presintered, postsintered, and control groups. Test groups were further divided into three subgroups (n = 10) according to the surface treatments conducted (grinding, Er,Cr:YSGG laser irradiation, air-borne particle abrasion [APA]). Four-point flexural strength (σ) test and Weibull analysis were conducted. Color differences (ΔE₀₀ ) and translucency parameter (TP_ab ) were calculated with a spectrophotometer. Surfaces of specimens were scrutinized under FESEM. Data were statistically analyzed.

RESULTS

Postsintered groups exhibited higher σ values (P < .05). Within all groups, highest and lowest σ values were detected at postsintered and presintered APA groups, respectively (P < .05). All ΔE₀₀ values were above the perceptibility threshold (ΔE₀₀  > 0.8). Higher TP_ab values were obtained and deeper scratches were observed in presintered groups.

CONCLUSIONS

Surface treatments performed at postsintering stage had a favorable effect on the flexural strength of all specimens. Surface treatments performed before sintering increased translucency and caused higher ΔE₀₀ values.

CLINICAL SIGNIFICANCE

Surface treatments performed at different sintering stages can alter mechanical and optical properties of zirconia.

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.

Bonding Strength of Luting Cement to Zirconia-Based Ceramic Under Different Surface Treatments

OBJECTIVES

 The aim of this study was to evaluate the bonding strength of self-adhesive luting cement to zirconia under different surface treatments.

MATERIALS AND METHODS

 Thirty-two zirconia samples were randomly divided into eight experimental groups based on the surface treatment employed (Control: no surface treatment; PMM: wear with diamond bur; JAT: blasting with glass beads; PMA: wear with a medium-roughness milling machine; Primer: primer application on the surface without treatment; PMM +Primer: PMM treatment plus primer application; JAT+Primer: JAT treatment plus primer application; and PMA+Primer: PMA treatment plus primer application). Cement cylinders were built on the ceramic surfaces, and the groups were subdivided according to the storage time employed (i.e., 24 hours or 60 days). After storage, the samples were subjected to microshear testing.

STATISTICAL ANALYSIS

 The Kruskal-Wallis test followed by the Dunn test was employed for comparison between the groups (p < 0.05).

RESULTS

 The PMM group yielded the optimal results and the mean values increased after both storage times following the primer application. The Control, PMA, and JAT groups gave similar results after 24 hours, while the JAT group gave superior results following primer application over this storage time. After 60 days of storage, all groups gave improved results following chemical treatment with a primer.

CONCLUSION

 It was concluded that mechanical preparation using the diamond bur followed by primer application significantly improved the bond strength between the ceramic and the luting cement.

Current status on lithium disilicate and zirconia: a narrative review

Background

The introduction of the new generation of particle-filled and high strength ceramics, hybrid composites and technopolymers in the last decade has offered an extensive palette of dental materials broadening the clinical indications in fixed prosthodontics, in the light of minimally invasive dentistry dictates. Moreover, last years have seen a dramatic increase in the patients’ demand for non-metallic materials, sometimes induced by metal-phobia or alleged allergies. Therefore, the attention of scientific research has been progressively focusing on such materials, particularly on lithium disilicate and zirconia, in order to shed light on properties, indications and limitations of the new protagonists of the prosthetic scene.

Methods

This article is aimed at providing a narrative review regarding the state-of-the-art in the field of these popular ceramic materials, as to their physical-chemical, mechanical and optical properties, as well as to the proper dental applications, by means of scientific literature analysis and with reference to the authors’ clinical experience.

Results

A huge amount of data, sometimes conflicting, is available today. Both in vitro and in vivo studies pointed out the outstanding peculiarities of lithium disilicate and zirconia: unparalleled optical and esthetic properties, together with high biocompatibility, high mechanical resistance, reduced thickness and favorable wear behavior have been increasingly orientating the clinicians’ choice toward such ceramics.

Conclusions

The noticeable properties and versatility make lithium disilicate and zirconia materials of choice for modern prosthetic dentistry, requiring high esthetic and mechanical performances combined with a minimal invasive approach, so that the utilization of such metal-free ceramics has become more and more widespread over time.

Does airborne-particle abrasion before, rather than after, zirconia sintering lead to higher mechanical strength even under aging challenge?

STATEMENT OF PROBLEM

Information concerning the effect of airborne-particle abrasion before zirconia sintering on its strength after aging is lacking.

PURPOSE

The purpose of this in vitro study was to determine the influence of airborne-particle abrasion moment (before or after zirconia sintering), particle size, and aging on the mechanical strength of a yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic.

MATERIAL AND METHODS

Four hundred presintered zirconia disks were allocated to nonabraded (control) and abraded groups with 50 μm or 120 μm Al₂O₃ particles before (50/BS or 120/BS) or after sintering (50/AS or 120/AS). The disks were aged by storing them in distilled water for 24 hours at 37 °C (24 h) (control); mechanical cycling (MC) (1×10⁶ cycles; 2 Hz; 100 N); hydrothermal aging (HA) (134 °C; 0.2 MPa; 20 hours); or by both methods (MC+HA), totaling 20 groups (n=20). The specimens were subjected to biaxial flexural strength (BFS) testing. Fractographic analysis was performed to identify the fracture origin. Tetragonal to monoclinic transformation was determined by X-ray diffraction. The BFS data (MPa) were analyzed by 2-way ANOVA and the Games-Howell post hoc test (α=.05). Weibull statistics was also applied.

RESULTS

The BFS and characteristic strength (σ0) of the 120/AS groups were significantly higher compared with the other abraded groups and statistically similar or higher compared with the respective nonabraded groups. In contrast, the 120/BS/24 h and all the 50/BS groups presented the lowest values. In all groups, the initial defect had its origin on the tensile stress side. Neither airborne-particle abrasion nor aging (except for the 50/AS/MC+HA) reduced the zirconia reliability compared with the respective controls. Abrading the zirconia in its presintered stage or after sintering increased the monoclinic phase content, and the sintering process eliminated this phase.

CONCLUSIONS

Airborne-particle abrasion before zirconia sintering yields an inferior performance compared with that after sintering under aging challenges. The BFS and σ0 were strongly determined by the particle size and airborne-particle abrasion moment, while aging mainly acted in the nonabraded zirconia or when this material was abraded before sintering.

Evaluation of zirconia surface roughness after aluminum oxide airborne-particle abrasion and the erbium-YAG, neodymium-doped YAG, or CO2 lasers: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Veneer chipping and crown decementation are the most frequent failures in restorations using zirconia as an infrastructure. Increasing the roughness of the zirconia surface has been suggested to address this problem.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate yttria-stabilized tetragonal zirconia polycrystal surface roughness, produced with aluminum oxide airborne-particle abrasion and the erbium yttrium aluminum garnet (YAG), neodymium-doped YAG, or CO₂ lasers.

MATERIAL AND METHODS

This study was based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist. The review identified relevant studies through December 2017 with no limit on the publication year in the search databases: Web of Science, Scopus, and MEDLINE via PubMed. The selected studies were submitted to a risk of bias assessment. The means and standard deviations of roughness were evaluated for the meta-analysis using Review Manager software.

RESULTS

The 17 studies that met all inclusion criteria presented a medium risk of bias. All the treatment methods tested were able to create a roughness on the yttria-stabilized tetragonal zirconia polycrystal surface. The I² test values presented a high heterogeneity among the studies.

CONCLUSIONS

The presintered specimens submitted to airborne-particle abrasion had higher surface roughness compared with abrasion after the sintering process. Irradiation with the neodymium-doped YAG and CO₂ lasers was destructive to the zirconia surfaces. The erbium laser used with lower energy intensity appears to be a promising method for surface treatment.

Effect of airborne particle abrasion and sintering order on the surface roughness and shear bond strength between Y-TZP ceramic and resin cement

This study examined the surface roughness (R_a) and shear bond strength (SBS) of Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic after airborne particle abrasion at different pressures and particle sizes, pre- and post-sintering. Ninety specimens, prepared from Y-TZP ceramic blocks (Vita In-Ceram YZ, Vita Zahnfabrik), were divided into nine subgroups: control, and 50 and 110 µm Al₂O₃ airborne particle abrasion at 3 and 4 bar pressure, before and after sintering, respectively. According to the sintering order, before and after surface treatments, R_a values were measured using a profilometer. SBS to Y-TZP was assessed after thermocycling, using self-adhesive resin cement (Rely X U200, 3M ESPE). Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) were performed on one specimen per group. All surface-treated samples were rougher than the controls. ABS50-4 (50 µm Al₂O₃ airborne particle abrasion at 4 bar pressure before sintering), ABS110-3, and ABS110-4 showed the highest R_a values, among all cohorts. The controls displayed lower SBS values than the treated groups (p<0.05), which had statistically similar results to each other. Airborne particle abrasion of pre-sintered Y-TZP, followed by sintering, increased the tetragonal structure contents.

Bond strength of resin cement to zirconia treated in pre-sintered stage

PURPOSE

Aim of this study was to evaluate the tensile bond strength (TBS) between resin cement and zirconia surface treated in different sintering stages.

MATERIALS AND METHODS

Eighty zirconia discs having final dimensions of 12 mm diameter and 3.2 mm thickness were milled then divided into three main groups according to the type of surface treatment performed (group 1: air abrasion using 50 µm Al₂O₃ particles, group 2: silica coating using Rocatec soft, and group 3: a control group receiving no surface treatment). Groups 1 and 2 were divided into two subgroups each according to the stage in which the surface treatment was performed (subgroup A; surface treatment performed in the pre-sintered stage and subgroup B; surface treatment performed in the post-sintered stage). Discs were later bonded to composite core materials using resin cement then tested for TBS either being subjected to 3 days of water storage or 150 days of water storage and 37,500 thermal cycles.

RESULTS

Long-term aging caused a significant decrease in TBS of all subgroups except the subgroup air-abraded in the post-sintered stage. After long-term aging, the group silica coated in the pre-sintered stage showed the significantly lowest TBS compared to all other groups. There was also no significant difference between the subgroups air-abraded and silica coated in the pre-sintered stage. All specimens in the control group debonded during long-term aging.

SIGNIFICANCE

Air-abrading zirconia with Al₂O₃ at a reduced pressure in the pre-sintered stage may result in durable bond strength to resin cement.