Effect of thickness and surface modifications on flexural strength of monolithic zirconia

Prosthetic rehabilitation of a completely edentulous maxilla with a sectioned implant-supported screw-retained prosthesis in a multiamputee patient with microstomia: A clinical report

Providing fixed implant-supported prostheses for edentulous patients with microstomia or limited mouth opening is challenging. The clinical treatment of a patient who was a multiple amputee with microstomia and a complete loss of lips elasticity is described. A custom half-sectioned prosthesis was fabricated to resolve numerous difficulties. A screw-retained complete implant-supported prosthesis was developed from a monolithic zirconia framework supported by 4 implants and with digital scanning and computer-aided design and computer-aided manufacturing of the prosthesis. The protocol achieved a complete screw-retained implant-supported prosthesis and offered a solution for a patient with microstomia and limited mouth opening, leading to improved quality of life.

Effect of preparation designs and CAD-CAM materials on step-stress fatigue survival of premolar partial coverage restorations: An in-vitro study with fractographic analysis

OBJECTIVES

The growing trend of minimally invasive approaches has encouraged the application of partial coverage designs in restorative dentistry. This study aimed to evaluate the cyclic fatigue performance of different CAD-CAM materials used in various partial coverage designs for premolar restorations.

METHODS

A freshly extracted upper premolar was prepared using a high-speed handpiece to create mesio-occluso-distal (MOD) cavities with standardized dimensions. Three preparation designs were investigated: onlay, overlay, and vonlay. CAD software was used to create digital models of the restorations, which were milled into one of four CAD-CAM materials: Zr - zirconia polycrystals (4Y-PSZ); EX - lithium disilicate glass-ceramic; VS - zirconia-reinforced lithium silicate glass-ceramic; GB - nanohybrid composite. Partial coverage restorations were bonded to tooth replicas and subjected to step-stress cyclic fatigue testing using a chewing simulator device (n = 10). The number of cycles-to-failure and fatigue fracture resistance (N) were recorded and analysed with Kaplan-Meier survival statistics and Weibull analysis, respectively. Fracture surfaces were analysed using fractography.

RESULTS

Zr exhibited significantly higher number of cycles-to-failure and greater characteristic fracture load (L0) than the other materials, for onlay and overlay designs. For vonlay design, Zr, VS and EX had similar and greater number of cycles-to-failure than GB. When comparing different designs for the same material, Zr had lower number of cycles-to-failure for vonlay in comparison to onlay and overlay, while the other materials had similar values among the different designs. Weibull modulus was similar among experimental groups.

SIGNIFICANCE

The findings of this study emphasize the critical role of material selection and preparation design in optimizing the mechanical performance and clinical success of partial coverage restorations. 4Y-PSZ is recommended for high-stress areas due to its superior fatigue and fracture resistance. Overlay designs provide better fatigue resistance, making them more suitable for clinical scenarios requiring higher durability. Understanding the fatigue performance of different materials and designs aids in making informed decisions to enhance the longevity and success of partial coverage restorations in premolars.

Evaluation of Physical Changes and Bond Properties of Monolithic Zirconia Following Surface Treatment with Alumina and Synthetic Diamond Particles: A Comparative X-ray Diffraction Analysis

AIM

The aim of this study was to compare and evaluate the phase transformation and effect on the bond strength and fracture toughness of monolithic zirconia after surface treatment with alumina and synthetic diamond particles.

MATERIALS AND METHODS

Forty samples of monolithic sintered zirconia discs (Y-TZP) were divided into two groups of 20 samples each. Group A - air abrasion with alumina particles (n = 20); group B - air abrasion with synthetic diamond particles (n = 20). Pretreatment phase and posttreatment of each zirconia sample from group A and group B were evaluated using an X-ray diffraction machine. The surface roughness of each zirconia sample was evaluated using a profilometer. Composite discs were fabricated and bonded to the air-abraded surface of each zirconia sample from group A and group B using a dual-cured resin cement, respectively. These samples were mounted in an acrylic block to determine the bond strength of zirconia with resin cement using a universal testing machine. This was followed by a fracture toughness test of the samples using a Vickers indentation hardness tester. The results were subjected to statistical analysis using a t-test, and relevant statistical conclusions were drawn.

RESULTS

The mean ± SD of monoclinic content in group A (alumina particles) and group B (synthetic diamond particles) was 0.82 ± 0.010% and 0.76 ± 0.015%, respectively. The mean ± SD of surface roughness in group A (alumina particles) and group B (synthetic diamond particles) was 0.507 ± 0.106 and 0.513 ± 0.116 µm, respectively, and the mean ± SD of bond strength in group A (alumina particles) and group B (synthetic diamond particles) was 6.11 ± 1.47 and 6.49 ± 0.83 MPa, respectively. The mean ± SD of fracture toughness in group A (alumina particles) and group B (synthetic diamond particles) was 2.63 ± 0.46 0.5 and 5.70 ± 1.03 MPam0.5, respectively. p < 0.05 was considered statistically significant.

CONCLUSION

The distribution of mean monoclinic content was significantly higher in zirconia samples abraded by alumina (Group A) as compared to synthetic diamond particles (Group B). The mean surface roughness and bond strength results were statistically insignificant between both groups. The distribution of mean fracture toughness was significantly higher in group B compared to group A.

CLINICAL SIGNIFICANCE

Synthetic diamond particles for air abrasion of Y-TZP can be a promising alternative to alumina as they cause minimal changes in the structural integrity without compromising the bond strength. How to cite this article: Kazi AI, Dugal R, Madanshetty P, et al. Evaluation of Physical Changes and Bond Properties of Monolithic Zirconia Following Surface Treatment with Alumina and Synthetic Diamond Particles: A Comparative X-ray Diffraction Analysis. J Contemp Dent Pract 2024;25(12):1110-1117.

Effect of the Intaglio Surface Treatment and Thickness of Different Types of Yttria-Stabilized Tetragonal Zirconia Polycrystalline Materials on the Flexural Strength: In-Vitro Study

BACKGROUND

Surface treatment of the intaglio surface of zirconia is important for bonding. However, it could affect the strength of the materials. The purpose of this study is to compare the effect of laser, etching, and air abrasion surface treatment methods to a control group on the flexural strength of three zirconia materials with two different thicknesses. (1) Methods: A total of 120 disks were divided into three groups according to the type of zirconia and the ceramic thickness. Then, according to the surface treatment method, the groups were divided into four subdivisions. The change in the microstructure of the ceramic material was investigated through Scanning Electron Microscope (EVO LS10, Carl Zeiss SMT Ltd. Oberkochen, Germany). Phase identification was performed using an X-ray diffraction device (XRD; Ultimate IV X-ray Diffractometer, Rigaku Inc., Tokyo, Japan). The flexural strength was assessed with a biaxial flexural strength test in a universal testing machine. Data were analyzed using SPSS Software (SPSS version 26.0.Armonk, NY: IBM Corp). A three-way ANOVA and a post hoc Dunnett T3 test were employed to evaluate the effect of the yttria concentration, thickness, and surface treatment on the flexural strength of zirconia (α = 0.05). (2) Results: At 0.8 mm thickness, air abrasion significantly increased the flexural strength of 3Y-TZP (1130.6 ± 171.3 MPa) and 4Y-TZP (872 ± 108.6 MPa). However, air abrasion significantly decreased the flexural strength of 5Y-TZP materials (373 ± 46.8 MPa). Laser irradiation significantly decreased the flexural strength of 5Y-TZP (347 ± 50.3 MPa), while etching significantly decreased the flexural strength of both 3Y-TZP (530 ± 48.8) and 4Y-TZP (457.1 ± 57.3). When the thickness increased to 1 mm, air abrasion continued to significantly decrease the flexural strength of 5Y-TZP materials. (3) Conclusions: There was a negative effect of surface treatment on the flexural strength at 0.8 mm thickness rather than at 1 mm thickness. Air abrasion enhances the flexural strength of 3Y-TZP and 4Y-TZP materials but significantly reduces the flexural strength of 5Y-TZP materials. Zircos-E etching and Er:YAG surface treatment methods did not significantly reduce the flexural strength of 5Y-TZP materials at 1 mm thickness and can be recommended as an alternative surface treatment for 5Y-TZP materials.

Impact of Different Surface Treatments on Shear Bond Strength between Two Zirconia Ceramics and a Composite Material

The purpose of this study was to compare the surface changes and shear bond strength between a resin composite and two zirconia ceramics subjected to sandblasting and forming gas (5% H2 in N2) plasma surface treatment. Two types of zirconia ceramic specimens (3Y-TZP and (Y,Nb)-TZP) were divided into groups based on the following surface treatment methods: polishing (Control), sandblasting (SB), sandblasting and plasma (SB-P), and plasma treatment (P). Subsequently, chemical surface modification was performed using Clearfil SE Bond (Kuraray, Tokyo, Japan), and the Filtek Z-250 (3M, Maplewood, MN, USA) resin composite was applied. Shear bond strengths (SBS) and surface characteristics were determined. Plasma treatment was effective in increasing the wettability. For SBS, there were significant differences among the groups, and the (Y,Nb)-TZP and SB-P groups showed the highest bond strength. Similarly, for the 3Y-TZP specimens, the shear bond strength increased with both plasma and sandblasting treatments, although no statistically significant change was observed. In the P group, both (Y,Nb)-TZP and 3Y-TZP showed a significant decrease in shear bond strength with the resin composite compared to the control group.

Flexural Strength of Different Monolithic Computer-Assisted Design and Computer-Assisted Manufacturing Ceramic Materials Upon Accelerated Aging

OBJECTIVES

 The durability of ceramic is crucial, which is probably influenced by aging. This study evaluated the effect of aging on flexural strength of different ceramics.

MATERIALS AND METHODS

 One-hundred twenty ceramic discs (Ø 12 mm, 1.5 mm thickness) were prepared from zirconia-reinforced lithium silicate (ZLS, C), lithium disilicate (LS2, E), precolored yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP, Ip), and customized color Y-TZP (Ic). Samples were randomly divided into two groups for accelerated aging (A) between 5 and 55°C water baths, 30-second immersing time each, for 10,000 cycles, and nonaged group (N), serving as control. Biaxial flexural strength (σ) was evaluated utilizing the piston-on-three-balls at 0.5 mm/min speed. Analysis of variance and Tukey comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σo). Microstructures were evaluated with scanning electron microscopy and X-ray diffraction (XRD).

RESULTS

 The highest σ and σo were seen for IcN, followed by IcA, IpN, IpA, EN, CA, CN, and EA, respectively. CN showed the highest m, while EA showed the lowest m. Significant differences of σ for each ceramic were indicated (p < 0.05). Aging caused a significant difference in σ (p < 0.05). XRD showed t→m phase transformation of Ip and Ic after aging.

CONCLUSION

 Aging affected strength of ceramics. Comparable strength between LS2 and ZLS was evidenced, but both were less strength than Y-TZP either aging or non-aging. Comparable strength between precolored Y-TZP and customized color Y-TZP was indicated. Better resisting aging deterioration of Y-TZP than LS2 and ZLS is suggested for fabrication restorative reconstruction.

In Vitro Evaluation of Speed Sintering and Glazing Effects on the Flexural Strength and Microstructure of Highly Translucent Multilayered 5 mol% Yttria-Stabilized Zirconia

This study aimed to investigate the impact of speed sintering and glazing on the flexural strength and microstructure of multilayered 5 mol% yttria-stabilized (5Y-) zirconia, which remains unknown. Bar-shaped specimens (N = 600) were fabricated from 5Y-zirconia (FX; Ceramill Zolid FX ML, ST; Katana STML) by cutting, polishing, sintering (conventional and speed sintering), and then glazing. A flexural strength test (n = 30/group), field emission scanning electron microscopy (FE-SEM) observation (n = 2/group), and an X-ray diffraction (XRD) study with Rietveld refinement (n = 1/group) were performed. The flexural strength was analyzed using three-way ANOVA and a post hoc Scheffé test. The grain size was analyzed using the Kruskal-Wallis H test and Bonferroni-Dunn post hoc test. Flexural strength slightly decreased in the nonglazed FX after speed sintering (p < 0.05). Glazing with and without glazing paste did not affect flexural strength at both sintering speeds (p > 0.05). Speed sintering and glazing minimally changed the Weibull modulus and phase fraction, and did not affect grain size (p > 0.05). ST had a larger grain size and lower tetragonal phase content than FX and had a lower flexural strength than FX in most groups (p < 0.05). Overall, the multilayered 5Y-zirconia is considered suitable for dental application using speed sintering and glazing.

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.

A parametrical finite element analysis for functionally graded material overlay restoration

The main cause of failure in bonded ceramic restorations is material fracture due to excessive stress concentration at the base of the prosthesis. The design of restorative functionally graded materials (FGM) could represent a major advance in dissipating mechanical stresses during occlusal contacts. The aim of this paper is to carry out a complete factorial design of finite element analyses to optimize a multilayer FGM introduced at the bottom of an overlay prosthesis. The number and thickness of layers vary within a spectrum compatible with ceramic shaping processes whereas Young's moduli variations are set in the range of dental tissues. For a 1.5-mm thick prosthesis, the optimal FGM configuration appears to be a 5 layers of 0.2 mm thickness with a linear distribution of Young's modulus from 30 to 70 GPa. This configuration was implemented in a 3D model of a restored tooth with realistic geometry to validate the proof-of-concept.

Comparative Analysis of the Structural Weights of Fixed Prostheses of Zirconium Dioxide, Metal Ceramic, PMMA and 3DPP Printing Resin-Mechanical Implications

The aim of this study was to determine the mechanical implications of four-unit fixed dental prostheses (FDPs) made of (1) monolithic zirconium dioxide (ZR O2), (2) polymethylmethacrylate (PMMA), (3) metal ceramic (PFM) and (4) impression resin (3DPP).

METHODS

Four groups were studied with eight samples for each material (n: 32). Each structure was weighed, subjected to compressive tests and analyzed using 3D FEA.

RESULTS

PMMA presented the lowest structural weight (1.33 g), followed by 3DPP (1.98 g), ZR O2 (6.34 g) and PFM (6.44 g). In fracture tests, PMMA presented a compressive strength of 2104.73 N and a tension of 351.752 MPa; followed by PFM, with a strength of 1361.48 N and a tension of 227.521 MPa; ZR O2, with a strength of 1107.63 N and a tension of 185.098 MPa; and 3DPP, with a strength of 1000.88 N and a tension of 143.916 MPa. According to 3D FEA, 3DPP presented the lowest degree of deformation (0.001 mm), followed by PFM (0.011 mm), ZR O2 (0.168 mm) and PMMA (1.035 mm).

CONCLUSIONS

The weights of the materials did not have a direct influence on the mean values obtained for strength, stress or strain. Since the performance was related to the tension and forces supported by the structures in critical zones, the importance of considering design factors is clear. In vitro and 3D FEA assays allowed us to simulate different scenarios for the mechanical properties of certain materials before evaluating them clinically. Thus, they can generate predictions that would allow for the design of a better research methodology in future clinical trials.

Effect of abutment types and resin cements on the esthetics of implant-supported restorations

PURPOSE

The aim of the study was to evaluate the optical properties of new generation (3Y-TZP) monolithic zirconia (MZ) with different abutment types and resin cement shades.

MATERIALS AND METHODS

A1/LT MZ specimens were prepared (10 × 12 × 1 mm, N = 30) and divided into 3 groups according to cement shades as transparent (Tr), yellow (Y) and opaque (O). Abutment specimens were obtained from 4 different materials including zirconia (Group Z), hybrid (Group H), titanium (Group T) and anodized yellow titanium (Group AT). MZ and abutment specimens were then cemented. L^*, a^*, and b^* parameters were obtained from MZ, MZ + abutment, and MZ + abutment + cement. ΔE₀₀₁^* (between MZ and MZ + abutment), ΔE₀₀₂^* (between MZ and MZ + abutment + cement) and ΔE₀₀₃^* (between MZ + abutment and MZ + abutment + cement) values were calculated. Statistical analyses included 2-way ANOVA, Bonferroni, and Paired Sample t-Tests (P < .05).

RESULTS

Abutment types and resin cements had significant effect on L^*, a^*, b^*, ΔE₀₀₁^*, ΔE₀₀₂^*, and ΔE₀₀₃^* values (P < .001). Without cementation, whereas zirconia abutment resulted in the least discoloration (ΔE₀₀₁^* = 0.68), titanium abutment caused the most discoloration (ΔE₀₀₁^* = 4.99). The least ΔE₀₀₂^* = 0.68 value was seen using zirconia abutment after cementation with yellow shaded cement. Opaque shaded cement caused the most color change (ΔE₀₀₃^* = 5.24). Cement application increased the L^* values in all groups.

CONCLUSION

The least color change with/without cement was observed in crown configurations created with zirconia abutments. Zirconia and hybrid abutments produced significantly lower ΔE₀₀₂^* and ΔE₀₀₃^* values in combination with yellow shaded cement. The usage of opaque shaded cement in titanium/anodized titanium groups may enable the clinically unacceptable ΔE₀₀^* value to reach the acceptable level.

Effect of different surface treatments on the biaxial flexural strength of zirconia ceramics

STATEMENT OF PROBLEM

Different surface treatments have been applied to zirconia restorations in clinical practice to increase the bond strength between zirconia and cement, but their effect on flexural strength is unknown.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different surface treatments before and after sintering on the flexural strength of zirconia.

MATERIAL AND METHODS

Sixty disk-shaped specimens with an initial diameter of 18.6 ±0.1 mm and thickness of 2 ±0.1 mm were prepared from preshaded presintered 3Y-TZP blocks. The specimens were randomly divided into 3 groups (n=20) according to surface treatments (Group Laser, Group APA, Group Rocatec), and the groups were then divided into 2 subgroups (n=10) according to surface treatment before and after sintering. The phase compositions of the groups were examined by using an X-ray diffractometer (XRD) with 3 randomly selected specimens from each group. Biaxial flexural strength testing was conducted using a universal testing machine to examine the flexural strength of the zirconia specimens. Two-way ANOVA and post hoc least significant difference tests were performed (α=.05).

RESULTS

According to the XRD analysis, no monoclinic phases were determined on the surface of the presintered laser-treated specimens, but tetragonal phases were observed on the surface of the postsintered specimens. Surface treatment type and application stage (presintering to postsintering) have a significant effect on the biaxial flexural strength of the specimens (P<.05). The lowest biaxial flexural values were observed in the Laser group, and postsintered specimens showed higher biaxial flexural strength than presintered specimens (P<.05).

CONCLUSIONS

Postsintered specimens showed higher monoclinic content than presintered specimens. Laser-treated specimens showed the lowest biaxial flexural strength for both presintered and postsintered specimens.

Indirect restorative systems-A narrative review

OBJECTIVE

The background and clinical understanding of the properties of currently available indirect restorative systems and fabrication methods is, along with manufacturer and evidence-based literature, an important starting point to guide the clinical selection of materials for tooth and/or implant supported reconstructions. Therefore, this review explores most indirect restorative systems available in the market, especially all-ceramic, along with aspects of manufacturing process, clinical survival rates, and esthetic outcomes.

OVERVIEW

Progressive incorporation of new technologies in the dental field and advancements in materials science have enabled the development/improvement of indirect restorative systems and treatment concepts in oral rehabilitation, resulting in reliable and predictable workflows and successful esthetic and functional outcomes. Indirect restorative systems have evolved from metal ceramics and polymers to glass ceramics, polycrystalline ceramics, and resin-matrix ceramics, aiming to improve not only biological and mechanical properties, but especially the optical properties and esthetic quality of the reconstructions, in attempt to mimic natural teeth.

CONCLUSIONS

Based on several clinical research, materials, and patient-related parameters, a decision tree for the selection of indirect restorative materials was suggested to guide clinicians in the rehabilitation process.

CLINICAL SIGNIFICANCE

The pace of materials development is faster than that of clinical research aimed to support their use. Since no single material provides an ideal solution to every case, professionals must continuously seek information from well designed, long-term clinical trials in order to incorporate or not new materials and technological advancements.

The effect of nonthermal argon plasma surface treatment on the fracture resistance of monolithic zirconia restorations containing tetragonal and cubic grains

PURPOSE

The aim of this study was to investigate the effect of nonthermal argon plasma (NP) surface treatment on the fracture resistance of monolithic zirconia restorations with different microstructures.

METHODS

Twenty restorations were prepared from each of two tetragonal and two cubic zirconia materials (80 restorations in total). The restorations were then divided into two subgroups (n = 10) for each material according to the surface treatment applied: air abrasion or NP. The surface topography of the treated groups was examined using a scanning electron microscope. All restorations were fixed to metal dies with resin cement, subjected to thermal cycling, and then underwent fracture resistance testing with a universal testing device. Two-way ANOVA and Bonferroni tests were used for statistical analysis of the data (α = 0.05).

RESULTS

The type of surface treatment and the type of zirconia material were shown to significantly affect the fracture resistance of the restorations. The air-abraded groups showed significantly higher fracture resistance (N) than the NP groups (P < 0.001).

CONCLUSION

The results of this study suggest that air abrasion surface treatment has a more favorable effect on the fracture resistance of tetragonal and cubic zirconia restorations than NP surface treatment.

Biaxial Flexural Strength of Different Monolithic Zirconia upon Post-Sintering Processes

OBJECTIVE

 Different post-sintering processes are expected to be a reason for alteration in the strength of zirconia. This study evaluated the effect of post-sintering processes on the flexural strength of different types of monolithic zirconia.

MATERIALS AND METHODS

 A total of 120 classical- (Cz) and high-translucent (Hz) monolithic zirconia discs (1.2 mm thickness and 14 mm in Ø) were prepared, sintered, and randomly divided into four groups to be surface-treated with (1) as-glazed (AG); (2) finished and polished (FP); (3) finished, polished, and overglazed (FPOG); and (4) finished, polished, and heat-treated (FPHT) technique (n = 15). Biaxial flexural strength (σ) was determined on a piston-on-three ball in a universal testing machine at a speed of 0.5 mm/min.

STATISTICAL ANALYSIS

 Analysis of variance, and post hoc Bonferroni multiple comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σ₀). The microstructures were examined with a scanning electron microscope and X-ray diffraction.

RESULTS

 The mean ± standard deviation value of σ (MPa), m, and σ₀ were 1,626.43 ± 184.38, 9.51, and 1,709.79 for CzAG; 1,734.98 ± 136.15, 12.83, and 1,799.17 for CzFP; 1,636.92 ± 130.11, 14.66, and 1,697.63 for CzFPOG; and 1,590.78 ± 161.74, 10.13, and 1,663.82 for CzFPHT; 643.30 ± 118.59, 5.59, and 695.55 for HzAG; 671.52 ± 96.77, 3.28, and 782.61 for HzFP; 556.33 ± 122.85, 4.76, and 607.01 for HzFPOG; and 598.36 ± 57.96, 11.22, and 624.89 for HzFPHT. The σ was significantly affected by the post-sintering process and type of zirconia (p < 0.05), but not by their interactions (p > 0.05). The Cz indicated a significantly higher σ than Hz. The FP process significantly enhanced σ more than other treatment procedures.

CONCLUSION

 Post-sintering processes enabled an alteration in σ of zirconia. FP enhanced σ, while FPOG and FPHT resulted in a reduction of σ. Glazing tends to induce defects at the glazing interface, while heat treatment induces a phase change to tetragonal, both resulted in reducing σ. Finishing and polishing for both Cz and Hz monolithic zirconia is recommended, while overglazed or heat-treated is not suggested.

Fracture Strength of Monolithic Zirconia Crowns with Modified Vertical Preparation: A Comparative In Vitro Study

Objective  The aim of this study was to evaluate the influence of different marginal designs (deep chamfer, vertical, and modified vertical with reverse shoulder) on the fracture strength and failure modes of monolithic zirconia crowns.

Materials and Methods  Thirty sound human maxillary first premolar teeth with comparable size were used in this study. The teeth were divided randomly into three groups according to the preparation design ( n = 10): (1) group A: teeth prepared with a deep chamfer finish line; (2) group B: teeth prepared with vertical preparation; and (3) group C: teeth prepared with modified vertical preparation, where a reverse shoulder of 1 mm was placed on the buccal surface at the junction of middle and occlusal thirds. All samples were scanned by using an intraoral scanner (CEREC Omnicam, Sirona, Germany), and then the crowns were designed by using Sirona InLab 20.0 software and milled with a 5-axis machine. Each crown was then cemented on its respective tooth with self-adhesive resin cement by using a custom-made cementation device. A single load to failure test was used to assess the fracture load of each crown by using a computerized universal testing machine that automatically recorded the fracture load of each sample in Newton (N).

Statistical Analysis  The data were analyzed statistically by using one-way analysis of variance test and Bonferroni test at a level of significance of 0.05.

Results  The highest mean of fracture load was recorded by chamfer (2,969.8 N), which followed by modified vertical (2,899.3 N) and the lowest mean of fracture load was recorded by vertical (2,717.9 N). One-way ANOVA test revealed a significant difference among the three groups. Bonferroni test showed a significant difference between group A and group B, while a nonsignificant difference was revealed between group C with group A and group B.

Conclusion  Within the limitations of this in vitro study, the mean values of fracture strength of monolithic zirconia crowns of all groups were higher than the maximum occlusal forces in the premolar region. The modification of the vertical preparation with a reverse shoulder placed at the buccal surface improved the fracture strength up to the point that it was statistically nonsignificant with the chamfer group.

The Effect of Translucency and Surface Treatment on the Flexural Strength of Aged Monolithic Zirconia

Aims

This in vitro study aimed to evaluate the effect of the degrees of translucency in different types of monolithic zirconia as well as the aging and surface treatment with airborne particle abrasion on the flexural strength of monolithic zirconia.

Materials and Methods

Sixty bar-shaped specimens were fabricated from three different types of presintered monolithic zirconia (n = 20 per group) including low translucent (LT) (DD Bio ZW iso, high strength zirconia, Dental Direkt, Germany), high translucent (HT) (DD Bio ZX² 98, high translucent zirconia, Dental Direkt, Germany), and multilayered system (ML) (DD cubeX2®ML, multilayer, cubic zirconia system, Dental Direkt, Germany). Each monolithic zirconia group was equally subdivided according to be either air-abraded with 110 µm aluminium oxide particles or left untreated (control). After thermocycling, the flexural strength was measured by using a universal testing machine. Two-way ANOVA followed by Tukey's post hoc and independent samples t-test were used for the statistical analyses (P < 0.05).

Results

Surface treatment and types of zirconia were found to have a significant interaction (P = 0.010). Having controlled the effect of surface treatment, the flexural strength of HT and LT zirconia was found to be significantly higher than the ML zirconia system (P ≤ 0.001). Airborne particle abrasion could significantly decrease the flexural strength of monolithic zirconia only in ML zirconia (P = 0.002).

Conclusions

Multilayered zirconia system had the lowest flexural strength among all groups. Moreover, the flexural strength of this system was attenuated by surface treatment with airborne particles abrasion.

Fracture toughness of different monolithic zirconia upon post-sintering processes

Background

Surface treatments are expected to be a reason for alteration in fracture resistance of zirconia. This study evaluated the effect of post-sintering processes on the fracture toughness of different types of monolithic zirconia.

Material and Methods

Classical- (Cz) and high-translucent (Hz) monolithic zirconia discs (1.2 mm thickness, 14 mm in Ø) were prepared, and randomly divided for surface treatments with 1) as-glazed (AG); 2) finished and polished (FP); 3) finished, polished, and overglazed (FPOG); and 4) finished, polished, and heat-treated (FPHT) technique (n=15/group). Fracture toughness (KIC) was determined with indentation fracture toughness method at load 1 kg for AG, FPOG and 10 kg for FP, FPHT with 15 sec dwelling time. Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic toughness (K0). Microstructures were examined with SEM and XRD. ANOVA and multiple comparisons were determined for significant differences (α=0.05).

Results

The mean±sd value of KIC (MPa.m1/2), m, and K0 were 1.60±0.19, 7.27, 1.71 for CzAG; 9.57±0.89, 9.97, 10.96 for CzFP; 1.61±0.15, 10.56, 1.68 for CzFPOG; 6.45±0.31, 20.31, 6.60 for CzFPHT; 1.45±0.13, 10.91, 1.51 for HzAG; 6.58±0.24, 27.00, 6.70 for HzFP; 1.24±0.05, 23.90, 1.27 for HzFPOG; and 5.07±0.16, 30.51, 5.15 for HzFPHT. The KIC was significantly affected by the post-sintering process, type of zirconia (p<0.05). The Cz indicated a significantly higher KIC than Hz. The FP significantly enhanced KIC, while OG was unable to raise KIC. HT reduced KIC due to reverse phase transformation.

Conclusions

Post-sintering processes caused alteration in fracture resistance of zirconia. Fracture toughness was enhanced with FP, but not with either OG or HT process for both Cz and Hz. Surface treatment of zirconia through a finished-polished process is recommended, while glazing and heat-treated are not suggested.

Key words:Fracture toughness, glazing, heat treatment, polishing, post-sintering process, zirconia.

Biaxial Flexural Strength of High-Translucence Monolithic Ceramics upon Various Thicknesses

Introduction

High-translucence ceramics have been used increasingly. This study evaluated the biaxial flexural strength of different ceramics as a result of varying thicknesses.

Materials and Methods

Circular discs with varied thickness of 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 mm were prepared from high-translucence yttria-partially stabilized zirconia (HTY-PSZ); Bruxzir^® Anterior (Bc), and zirconia-reinforced lithium silicate (ZLS) including Celtra^® DUO (Cc) and VITA Suprinity^® (Vc) (n = 15 discs/group). Biaxial flexural strength (σ) was evaluated utilizing piston-on-three-balls in a testing machine at a speed of 0.5 mm/min. A scanning electron microscope (SEM) was used to determine the microscopic structure. ANOVA and multiple comparisons were analyzed for significant differences (a = 0.05).

Results

The mean ± sd value of σ (MPa) for thickness 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 mm was 672.66 ± 107.54, 655.93 ± 93.98, 589.01 ± 63.63, 624.89 ± 87.08, 618.82 ± 83.36, 672.64 ± 84.61, 659.81 ± 122.89, 632.79 ± 92.54, and 657.86 ± 73.17, for Bc; 477.64 ± 88.23, 496.39 ± 86.36, 461.56 ± 57.00, 450.26 ± 86.60, 468.28 ± 83.65, 472.45 ± 53.63, 453.05 ± 72.50, 462.67 ± 47.57, and 535.28 ± 84.33, for Cc; and 500.97 ± 76.36, 506.70 ± 87.76, 557.82 ± 62.78, 543.76 ± 87.29, 507.53 ± 86.09, 502.46 ± 64.75, 557.70 ± 80.91, 527.04 ± 80.78, and 499.88 ± 57.35, for Vc. A significant difference in flexural strength was indicated among groups (p < 0.05). Bc was significantly stronger than Cc and Vc (p < 0.05). Varying thickness did not have a significant influence on strength (p > 0.05). SEM revealed a tight arrangement of crystals for Bc and needle-like crystals diffusing in glass for Vc and Cc.

Conclusion

Flexural strength of ceramics varied among types, but each retained strength equitably with varying thickness. HTY-PSZ was stronger than ZLS, but each was equally strong for thickness in the range of 0.4–2.0 mm.

Effects of thickness of different types of high-translucency monolithic multilayer precolored zirconia on color accuracy: An in vitro study

STATEMENT OF PROBLEM

High-translucency monolithic multilayer precolored zirconia provides acceptable esthetics and eliminates chipping of the veneering porcelain. However, the color is not always consistent with the standard Vita shade guide, and the color saturation may vary with the thickness of the zirconia.

PURPOSE

The purpose of this in vitro study was to characterize the effect of thickness on the color accuracy of high-translucency monolithic multilayer precolored zirconia.

MATERIAL AND METHODS

Plate-shaped (20×20 mm) Vita A2 shade high-translucency monolithic multilayer precolored zirconia specimens of 3 types (SHT Multilayer, AT Multilayer, and 3D Multilayer) in 4 thicknesses (0.5, 1.0, 1.5, and 2.0 mm) were fabricated (N=120, n=10). A spectrophotometer was used to measure the color attributes (CIELab) against gray or A2 substrates to evaluate the color accuracy based on differences in color (ΔE) (versus the Vita shade guide) and chroma. Statistical analysis was performed by using the Pearson correlation, 2-way ANOVA, and post hoc Scheffé test (α=.05).

RESULTS

Against gray substrates, thickness was significantly positively correlated with all color attributes. Against A2 substrates, L∗ values increased with an increase in thickness; however, a∗, b∗, and chroma values remained stable. Zirconia with a thickness of 1.0 mm exhibited the lowest ΔE, regardless of the type, except for AT Multilayer against A2 substrates, where the lowest ΔE was achieved at 0.5 mm. At thicknesses ≥1.0 mm, the ΔE between the 2 substrates was imperceivable.

CONCLUSIONS

Thickness affected the color accuracy of different high-translucency monolithic multilayer precolored zirconia types. It appears that the optimal thickness in terms of color accuracy is 1.0 mm. These results could be used as a reference for the selection and preparation of abutments in clinical applications.

Color and masking properties of translucent monolithic zirconia before and after glazing

Purpose 1. to assess the ability of four brands of translucent monolithic zirconia with different thicknesses to mask discolored substrates; 2. to assess the influence of glazing upon their color coordinates and masking properties.Methods Sixty samples of shade A1 (0.8, 1.5, and 2.0 mm thickness) from: ZirCAD/MT, Katana/HT, Vita YZ/HT, Cercon/HT were fabricated and glazed using a standardized laboratory procedure. CIE L*a*b* parameters were recorded on composite substrates, (IPS Natural Die Material Kit) (ND1=reference; ND2, ND3, ND4, ND5=test backgrounds), before and after glazing. The color changes of zirconia samples induced by glazing were analyzed. Masking properties was calculated as the color difference between CIE L*a*b* parameters of the samples placed on reference and test substrates with CIEDE2000(1:1:1) formula. The effect of material, thickness, substrate, and glazing on the color of monolithic zirconia was analyzed with ANOVA test and the multiple comparisons were analyzed with Tukey HSD tests (α=0.05).Results After glazing, lightness L* significantly increased, while chromatic coordinates a* and b* decreased (p<0.05). Significant differences in the masking properties of the four materials (p<0.001) were found, with IPS e.max ZirCAD/MT having the lowest masking effect (p<0.001). Thickness, substrate and glazing had a significant effect on the color masking properties of monolithic zirconia (p<0.001).Conclusions Masking properties of translucent zirconia were significantly influenced by the materials, the thickness and the color of the substrate; moreover, glazing improved the masking ability for all zirconia samples.

Changes in Crystal Phase, Morphology, and Flexural Strength of As-Sintered Translucent Monolithic Zirconia Ceramic Modified by Femtosecond Laser

Conventional bonding technology suitable for silica-based ceramics is not applicable to zirconia, due to its polycrystalline phase composition, chemical stability, and acid corrosion resistance. The development of an effective treatment to improve its surface roughness and mechanical properties remains an unresolved problem. Therefore, to solve this problem, this in vitro study evaluated the changes in surface morphology and flexural strength of translucent monolithic zirconia surfaces treated with femtosecond laser technology. As-sintered translucent zirconia specimens were subjected to airborne particle abrasion and femtosecond laser treatments, while control group specimens received no treatment. After treatment, the roughness and morphology of the treated zirconia surfaces were examined. The flexural strength and X-ray diffraction of the treated specimens were measured and analyzed. Statistical inferential analysis included one-way analysis of variance at a set significance level of 5%. The surface roughness after femtosecond laser treatment was significantly improved when compared with the control group and the group that received the airborne particle abrasion treatment (p < 0.05). In comparison with the airborne particle abrasion group, the flexural strength of the group that received the femtosecond laser treatment was significantly improved (p < 0.05). The femtosecond laser approach using appropriate parameters enhanced the roughness of the zirconia without reducing its flexural strength; therefore, this approach offers potential for the treatment of zirconia surfaces.

Technical and Biological Complications of Screw-Retained (CAD/CAM) Monolithic and Partial Veneer Zirconia for Fixed Dental Prostheses on Posterior Implants Using a Digital Workflow: A 3-Year Cross-Sectional Retrospective Study

Objective

The introduction of CAD/CAM and the development of zirconia-based restorations have allowed clinicians to use less expensive materials and faster manufacturing procedures. The purpose of the study was to analyze the differences, in terms of mechanical and biological complication, in multiunit zirconia fixed dental prosthesis (FPDs) on posterior implants produced using a digital workflow. Method and Materials. This study was a retrospective investigation, and patients treated with screw-retained monolithic or partial veneer FPDs on dental implants were selected. Periapical radiographs were taken at baseline and at the 3-year follow-up. Complications were recorded and classified as technical and biological ones.

Results

The study population included 25 patients. The occlusal and interproximal corrections were not clinically significant. In the study sample, the survival rate and success rate of the FPDs after 3 years were 100% and 96%, respectively. One implant failed immediately after placement.

Conclusion

Monolithic zirconia FPDs and partial veneer FPDs showed a 100% survival rate, presenting an interesting alternative to metal ceramic restorations. The partial veneer FPDs had a higher technical complication rate than the monolithic FPDs; however, no statistically significant difference was found.

Implant Restorative Options in the Esthetic Zone Anodized versus Zirconia Abutments

Restoring  implants  in  the  esthetic  zone  can  be  regarded  as  one  of  the  more  difficult  procedures  in  dentistry.  The  use  of  customized  abutments  has  become  a  common  practice  to  aid  in  the  development  of  a  proper  emergence  profile  of  restorations.  The  standard  material  over  the  years  has  been  titanium,  however,  the  emergence  of  zirconia  as  a  restorative  option  has  become  popular  when  restoring  with  more  translucent  materials  like  lithium  disilicate.  A  compromise  between  strength  and  esthetics  is  anodizing  the  titanium  abutments  to  have  a  yellow  color  to  help  mask  the  grey  hues  from  the  silver  abutments.  This  case  report  aims  to  provide  readers  with  information  comparing  anodized  and  zirconia  abutments  when  restoring  implants  with  lithium  disilicate  restorations  in  the  esthetic  zone.

Effect of Acid Mixtures on Surface Properties and Biaxial Flexural Strength of As-Sintered and Air-Abraded Zirconia

The aim of this work was to evaluate the effects of application time of an acid mixture solution on the surface roughness, phase transformation, and biaxial flexural strength of 3Y-TZP after sintering or air abrasion. For the biaxial flexural strength measurement, 220 3Y-TZP disk-shaped specimens were prepared after as-sintering or air abrasion. The etching solution comprised a mixture of hydrofluoric acid, sulfuric acid, hydrogen peroxide, methyl alcohol, and purified water. The samples were divided into 11 subgroups according to the etching times (Control, 1, 2, 3, 5, 8, 10, 12, 15, 20, and 30 min). The results showed that acid treatment on both as-sintered and air-abraded 3Y-TZP surfaces increased the surface roughness. However, it had no significant effects on the monoclinic phase or flexural strength of as-sintered zirconia. The monoclinic phase and flexural strength of air-abraded zirconia increased sharply after air abrasion; however, they gradually decreased after acid treatment, to a similar level to the case of the untreated surface. Surface treatment with acid mixture increased the roughness, but the lack of increase of monoclinic phase is thought to be because the loose monoclinic particles remaining on the surface were removed through the etching process.

The effect of preparation taper on the resistance to fracture of monolithic zirconia crowns

OBJECTIVE

Monolithic zirconia crowns have become a viable alternative to conventional layered restorations. The aim of this study was to evaluate whether the taper, and thus wall thickness, of the abutment or pre-defined cement space affect the fracture resistance or fracture mode of monolithic zirconia crowns.

METHODS

A model tooth was prepared with a taper of 15° and a shallow circumferential chamfer preparation (0.5 mm). Two additional models were made based on the master model with a taper of 10° and 30° using computer-aided design software. Twenty monolithic 3rd generation translucent zirconia crowns were produced for each model with pre-defined cement space set to either 30 μm or 60 μm (n = 60). The estimated cement thickness was assessed by the replica method. The cemented crowns were loaded centrally in the occlusal fossa at 0.5 mm/min until fracture. Fractographic analyses were performed on all fractured crowns.

RESULTS

The load at fracture was statistically significant different between the groups (p < 0.05). The crowns with 30° taper fractured at lower loads than those with 10° and 15° taper, regardless of the cement space (p < 0.05). The fracture origin for 47/60 crowns (78%) was in the cervical area, close to the top of the curvature in the mesial or distal crown margin. The remaining fractures started at the internal surface of the occlusal area and propagated cervically.

SIGNIFICANCE

The fracture resistance of the monolithic zirconia crowns was lower for crowns with very large taper compared to 10 and 15° taper even though the crown walls were thicker.

Flexural strength and translucency characterization of aesthetic monolithic zirconia and relevance to clinical indications: A systematic review

OBJECTIVE

Optimizing monolithic zirconia (ZrO2) aesthetically without affecting the unique mechanical properties remains a major ongoing interest. The purpose of this review is to evaluate the improvement of recent monolithic ZrO2 generations to meet aesthetic optimization qualities. Additionally, for how the extent of the former modifications negatively affected their mechanical properties and the impact on their clinical indications.

METHODS

The current literature examines in-vitro studies evaluating both monolithic ZrO2 translucency and mechanical properties. The electronic search was done within these databases: Web of Science, ScienceDirect, Scopus, and PubMed within the period between 2009/10/01 and 2019/10/31. Search results that met eligibility criteria were classified into four groups; one for translucency parameter, two for uniaxial flexural strength and one for biaxial strength.

RESULTS

Articles that remained for comprehensive analysis were fifty-three. They reported a significant impact of composition, microstructure and surface treatment on translucency and flexural strength assessment. Aging was found to be of no concern for zirconia structures with high yttria content. Smooth surface polish was found to significantly enhance strength while coarse grinding and mechanical fatigue was found to do the reverse.

SIGNIFICANCE

Based on analyzing the previous in-vitro studies the following was found: Although significant improvement has occurred with recent monolithic zirconia types with higher yttria content having higher cubic/tetragonal ratio, they are still inferior to glass-ceramics' unique translucency. With improving zirconia aesthetically, some of mechanical performance was sacrificed. Care must be taken when dealing with thin sections of aesthetic zirconia structures especially when used in high bearing stress areas.

Fracture Load of Different Zirconia Types: A Mastication Simulation Study

PURPOSE

To assess the effect of yttria mol% concentration and material thickness on the biaxial fracture load (N) of zirconia with and without mastication simulation.

MATERIALS AND METHODS

Disk-shaped specimens (N = 120) of 3 mol% yttria-partially stabilized zirconia, 3Y-PSZ (Katana High Translucent, Kuraray Noritake), 4 mol% yttria-partially stabilized zirconia, 4Y-PSZ (Katana Super Translucent Multi Layered) and 5 mol% Yttria-partially stabilized zirconia, 5Y-PSZ (Katana Ultra Translucent Multi Layered) were prepared to thicknesses of 0.7 and 1.2 mm. For each thickness, the biaxial fracture load (N) was measured with and without mastication simulation with 1.2 million cycles at a 110-N load and simultaneous thermal cycling at 5°C to 55°C. The data were analyzed by three-way Analysis of Variance (α = 0.05) and Tukey-Kramer adjusted multiple comparison test.

RESULTS

Yttria mol% concentration and material thickness had a statistically significant effect on the mean biaxial fracture load (F = 388.16, p < 0.001 and F = 714.33, p < 0.001 respectively). The mean biaxial fracture load ranged from the highest to the lowest; 3Y-PSZ, 4Y-PSZ, and 5Y-PSZ (p = 0.012). The mean biaxial fracture load of the 1.2 mm thickness groups was significantly higher than 0.7 mm thickness at any given condition (p = 0.002). Not all specimens survived the mastication simulation protocol. Fifty percent of the 0.7-mm-thick 4Y-PSZ specimens, 70% of the 0.7-mm-thick 5Y-PSZ specimens and 20% of 1.2-mm-thick 5Y-PSZ specimens fractured during mastication simulation. Mastication simulation had no statistically significant effect on the biaxial fracture load (F = 1.24, p = 0.239) of the survived specimens.

CONCLUSIONS

Lowering yttria mol% concentration and increasing material thickness significantly increases the fracture load of zirconia. At 0.7 mm thickness, only 3Y-PSZ survived masticatory simulation. A minimum material thickness of 1.2 mm is required for 4Y-PSZ or 5Y-PSZ.

Patient and Operator Centered Outcomes in Implant Dentistry: Comparison between Fully Digital and Conventional Workflow for Single Crown and Three-Unit Fixed-Bridge

BACKGROUND

Scientific information about the effects of implant therapy following a precise workflow and patient and operators' preferences should be considered to choose which implant treatment protocol to use, and to achieve patient's satisfaction and functional results. The aim of this study was to analyze implant rehabilitations with a fully digital workflow and compare this approach with a conventional one.

METHODS

This study comprises 64 patients treated with a fully digital approach and 58 patients treated using a conventional protocol. Patient and operator centered outcomes were assessed through two visual analogue scale (VAS) questionnaires.

RESULTS

The VAS questionnaire demonstrated better results for the digital workflow concerning anxiety, convenience, taste, nausea sensation, pain and breathing difficulties (p < 0.0001). The VAS questionnaire administered to the operators showed better scores for the digital approach in relation to anxiety, convenience, difficulties of the impression procedure and the workflow (p < 0.0001). A significant reduced mean time for the digital workflow as well as a reduced number of required visits were recorded.

CONCLUSION

The analysis of a fully digital and a conventional protocol showed better results according to patient and operators' preferences when a fully digital approach was used.

Effect of different grinding protocols on surface characteristics and fatigue behavior of yttria-stabilized zirconia polycrystalline: An in vitro study

STATEMENT OF PROBLEM

Zirconia frameworks milled by computer-aided design and computer-aided manufacturing (CAD-CAM) often require clinical adjustments. In addition, zirconia prefabricated abutments can also require customization to achieve an adequate emergence profile. However, the influence of grinding adjustment on the surface characteristics and mechanical behavior of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and the best grinding protocol is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different grinding protocols on the surface characteristics, phase transformation, and mechanical behavior of Y-TZP for frameworks and implant abutments.

MATERIAL AND METHODS

Bar-shaped specimens were fabricated according to ISO 6872-2016 and divided into 3 groups: GC (control, untreated), GA (grinding and finishing with medium and fine diamond rotary instruments using high-speed handpiece under constant water cooling), and GB (grinding and finishing with coarse and medium diamond rotary instruments, respectively, using slow-speed handpiece without water cooling). After specimen grinding, the topography and surface roughness were evaluated by using a laser confocal microscope, the Young modulus was measured by the impulse excitation technique, and crystallographic phase transformation was analyzed by X-ray diffraction. Specimens were then submitted to step-stress accelerated life testing (n=18). The surface roughness and Young modulus results were analyzed by 1-way ANOVA and the Tukey honestly significant difference test (α=.05). The data of step-stress accelerated life testing were analyzed by the survival probability considering the number of cycles and force until fracture.

RESULTS

Statistically significant differences were found among groups considering surface roughness (GA>GB>GC) (P<.05) and Young modulus (GB>GA=GC) (P=.003). X-ray diffraction showed that grinding leads to phase transformation, GC showed only tetragonal phase, while GA and GB showed tetragonal and monoclinic phases. No statistically significant difference (P<.05) was found among groups submitted to the same loading profile when the survival probability was compared, but significant difference was found between the light and moderate loading (P=.002) and light and severe loading (P=.014) of GB when different loading profiles in each group were compared.

CONCLUSIONS

Although grinding protocols affected surface characteristics and promoted phase transformation, the mechanical behavior of Y-TZP was not impaired. Therefore, both the grinding protocols tested can be safely used based on the evaluated properties.

Influence of Occlusal Thickness and Radicular Extension on the Fracture Resistance of Premolar Endocrowns from Different All-Ceramic Materials

Endocrowns are primarily recommended in a molar region with a standardized preparation design. The aim of the study was to evaluate the effect of different occlusal preparation depths, pulp chamber-radicular extension, and all-ceramic materials on the fracture resistance of premolar endocrowns. Ninety human premolar teeth were root canal treated, randomly divided into three main groups according to all-ceramic material used for fabrication as Lithium Disilicate (LD) ceramic, Polymer infiltrated ceramic (PIC) and High translucency zirconia (HTZ). They were further subdivided into three subgroups (n = 10) according to preparation design of 2 mm occlusal reduction, 4.5 mm occlusal reduction and 4.5 mm occlusal reduction with 2 mm radicular extension. The endocrowns from respective restorative materials were fabricated, surface conditioned, and cemented with self-adhesive resin cement. All samples were thermocycled for 5000 cycles and subjected to compressive static load at 45° angluation with the cross-head speed of 0.5 mm/minute until the fracture. The mean fracture resistance of LD ceramic at 2 mm, 4.5 mm thickness and radicular extension was 62.55 MPa, 45.80 MPa, 74.27 MPa respectively. The corresponding values for the PIC and HTZ ceramics were 26.30 MPa, 21.65 MPa, 25.66 Mpa and 23.47 MPa, 27.30 MPa, 37.29 MPa respectively. The LD ceramic and greater extension inside the pulp chamber had higher fracture resistance.

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.

Influence of CAD/CAM Fabrication and Sintering Procedures on the Fracture Load of Full-Contour Monolithic Zirconia Crowns as a Function of Material Thickness

OBJECTIVE

The purpose of this in vitro study was to analyze the effect of computer-aided design/computer-aided manufacturing (CAD/CAM) fabrication and sintering procedures on the fracture load of monolithic zirconia crowns with different material thicknesses adhesively seated to methacrylate dies fabricated with stereolithography technology.

METHOD

Monolithic zirconia crowns were fabricated from inCoris TZI C material with a chairside CAD/CAM system (CEREC MCXL) comprising three material thicknesses (0.5/1.0/1.5 mm, n=8 each). Two CAD/CAM fabrication procedures (milling, MI; grinding, GR), two chairside sintering procedures (superspeed, SS; speedfire sintering, SF), and one labside sintering procedure (classic, CL) were evaluated. In total, 144 crowns were fabricated. Restorations were adhesively seated to methacrylate dies fabricated with SLA technology. Thermomechanical cycling (TCML) was performed before fracture testing. Loading forces until fracture were registered and statistically analyzed with one-way analysis of variance (ANOVA), post hoc Scheffé test, and three-way ANOVA (α=0.05).

RESULTS

Test groups showed statistically significant differences (p<0.05). The highest mean value was found for 1.5-mm crowns of group GR_SF with 3678.6 ± 363.9 N. The lowest mean value was found for group 0.5-mm crowns of group MI_SF with 382.4 ± 30.7 N. There was a significant three-way interaction effect between thickness, sintering, and processing [F(4,126)=9.542; p<0.001; three-way ANOVA, significance level α=0.05].

CONCLUSIONS

CAD/CAM fabrication and sintering procedures influence the maximum loading force of monolithic zirconia crowns with different material thicknesses. A material thickness of 0.5 mm should be considered as a critical thickness for monolithic zirconia crown restorations.

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.

Production and characterization of zirconia structures with a porous surface

The aim of this study was to produce zirconia structures with a porous surface by the dip coating technique and assess the mechanical properties of the structures as well as the integrity of the porous layers. Surface porous layers with homogenous and graded porosity were produced over zirconia substrate discs using zirconia powders with different average sizes (d₅₀ = 40 μm; d₅₀ = 70 μm and d₅₀ = 100 μm) and without pore forming fugitive phases. Specimens were inspected using Scanning Electron Microscopy. Bending strength of specimens was obtained from biaxial flexural tests (B3B). Porous layers were successfully produced on zirconia discs substrates and the bending strength of these specimens were ~35% lower than uncoated specimens. Delamination occurred especially in layers with higher thickness and made of bigger particles. Practical application examples were provided in this paper showing the versatility of these porous surfaces in the production of multifunctional surfaces for stronger interfaces.