Transient thermal stresses developed during speed sintering of 3 mol% yttria-stabilized tetragonal zirconia polycrystals

Fatigue performance of 4YSZ: Effect of finishing/polishing protocol and surface treatments of the bonding surface

The study aims to evaluate the effect of finishing and polishing protocol after CAD/CAM grinding and surface treatments of bonding surface on the topographical characteristics and fatigue behavior of 4YSZ adhesively luted to an epoxy resin substrate. Ninety 4YSZ discs (IPS e.max ZirCAD MT, Ivoclar AG) (Ø = 10 mm, thickness = 1 mm) were obtained, submitted to in-lab simulation with a CAD/CAM system bur, and randomly allocated into six groups (n = 15) based on 'Surface condition' (with or without finishing and polishing) and 'Surface treatments' (no treatment, air abrasion with 45 μm aluminum oxide and glaze spray application). The surface roughness of all samples was measured, and the discs were luted with resin cement onto an epoxy resin substrate to be tested under cyclic fatigue (step-stress approach: 20 Hz, 5,000 cycles for piston accommodation in the first step, followed by 10,000 cycles per step with a step size of 100 N, starting at 100 N until failure). The results indicated that surface roughness (Ra and Rz) decreased in the groups with polishing and finishing protocol, with no significant differences between surface treatments in groups without finishing and polishing. The air abrasion groups (whit and without finishing and polishing) showed superior fatigue performance, comparable to no treatment with polishing and finishing. While the groups no treatment without finishing and polishing, treated with glaze spray without finishing and polishing and treated with glaze spray with finishing and polishing exhibited inferior fatigue behavior. In conclusion, while the finishing and polishing protocol can minimize defects and influence the fatigue behavior of 4YSZ, its effect is not significant when combined with surface treatments. The surface treatment, particularly air abrasion, plays a more decisive role in improving fatigue performance, proving to be the most effective approach.

Balancing strength and translucency: The role of microstructure in additive and subtractive dental zirconia

OBJECTIVES

To elucidate the effect of microstructure on the strength and translucency of dental zirconia fabricated using additive (AM) and subtractive manufacturing (SM) technologies.

METHODS

Twelve 3Y-TZP discs were fabricated using AM with two print orientations (0°: group AM0; 90°: group AM90; n = 6), and six via CAD/CAM machining (group SM). Density, composition, roughness, translucency parameter (TP), and biaxial flexural strength (σ) were evaluated. Fractographic analysis was conducted and defect size estimated. Based on the preliminary σ results (n = 6), the optimal print orientation was identified. Nine additional specimens were prepared for each of the AM90 and SM groups for Weibull σ analysis (n = 15). Differences in Weibull modulus were assessed via non-overlapping 95 % confidence intervals. An one-way ANOVA followed by Tukey's post-hoc test and an independent samples t-test were used (α = 0.05).

RESULTS

The relative density was consistent across all groups (>99 %). The tetragonal and cubic phases were comparable among groups, with proportions exceeding 82 wt% and 17 wt%, respectively. Group SM exhibited significantly higher roughness (1.18 µm) than AM0 (0.71 µm) and AM90 (0.51 µm). Group SM exhibited the highest TP values, while groups AM0 and AM90 had statistically similar TP values. AM0 showed the lowest σ value (411.60 ± 73.99 MPa) and larger defects. Groups AM90 and SM (n = 15) possessed comparable σ values (969.85 ± 123.13 MPa and 989.72 ± 107.78 MPa, respectively) (p = 0.6417) and Weibull moduli (9.17 and 10.62, respectively).

SIGNIFICANCE

SM zirconia showed higher translucency and roughness, while defects reduced translucency in AM zirconia. Flexural strength was lower for AM0 due to larger defects, whereas AM90 matched SM strength.

In vitro comparison of physical characteristics of milled versus printed zirconia discs

PURPOSE

The purpose of this study was to compare the dimensional accuracy, translucency, and biaxial flexural strength of milled zirconia (MZ) versus 3D-printed zirconia (PZ) discs.

MATERIALS & METHODS

A circular disc measuring 14.0 mm in diameter and 1.20 mm in thickness was designed using computer-aided design (CAD) software. The resulting standard tessellation language (STL) file was used both as a control and to fabricate 36 zirconia (3Y-TZP) disc specimens (n = 36): 18 were milled (group MZ) and 18 were 3D-printed (group PZ). The diameter and thickness of each disc were measured using a digital caliper. Translucency was evaluated using a calibrated dental colorimeter. The flexural strength was determined using the piston-on-three-ball biaxial flexure test. All measurements were done by one blinded examiner. The statistical significance level was set to α = 0.05.

RESULTS

The MZ discs had significantly more accurate dimensions than the PZ discs in both diameter and thickness when compared to the control CAD software-designed disc. The MZ discs exhibited significantly higher translucency (translucency parameter (TP) = 16.95 ±0.36 vs. 9.24 ±1.98) and biaxial flexural strength (996.16 ±137.37 MPa vs. 845.75 ±266.16 MPa) than the PZ discs. Finally, MZ possessed a significantly higher Weibull modulus relative to PZ.

CONCLUSIONS

The results showed that the milled specimens achieved better dimensional accuracy and were more translucent, stronger, and less prone to failure than printed specimens.

Current speed sintering and high-speed sintering protocols compromise the translucency but not strength of yttria-stabilized zirconia

OBJECTIVES

To investigate the impacts of speed and high-speed sintering on the densification, microstructure, phase composition, translucency, and flexural strength of yttria-stabilized zirconia (YSZ).

METHODS

A total of 162 disc-shaped specimens (n = 18) were cold-isostatically pressed from 3YSZ (Zpex), 4YSZ (Zpex 4), and 5YSZ (Zpex Smile) powders (Tosoh Corporation) and sintered according to the following protocols: conventional (control, ∼12 h), speed (∼28 min for 3YSZ; ∼60 min for 4YSZ and 5YSZ), and high-speed (∼18 min) sintering. Dimensions of zirconia specimens after sintering and polishing (1-μm diamond grit finish) were Ø13.75 × 1 mm. Density, microstructure, phase content, translucency parameter, and biaxial flexural strength were evaluated using Archimedes', SEM, XRD, spectrophotometric, and piston-on-3-ball methods, respectively. Data were analyzed with either one-way ANOVA and Tukey's test or Kruskal-Wallis with Dunn's test (α = 0.05).

RESULTS

For all YSZ compositions, conventional sintering yielded the highest density followed by speed then high-speed sintering. All sintering protocols resulted in similar strength values; however, speed and high-speed sintering protocols afforded significantly lower translucency relative to conventional sintering. XRD analysis revealed similar spectra for YSZs sintered by various protocols. The speed sintered specimens had the smallest grain size whereas the high-speed sintered 5YSZ possessed the largest grain size among all groups. SEM examination of all YSZ compositions revealed that the average pore size was an order of magnitude smaller than the average grain size.

SIGNIFICANCE

Speed and high-speed sintering of YSZs yield similar strength but diminished density and translucency relative to their conventionally sintered counterparts.

Zirconia restoration types, properties, tooth preparation design, and bonding. A narrative review

OBJECTIVE

The purpose of this review was to provide dental professionals with information regarding the various types of zirconia restorations, their mechanical and optical properties, tooth preparation design, and bonding protocol in an effort to enhance the longevity and durability of zirconia restorations.

OVERVIEW

The yttria content of zirconia ceramics determines their classification. The mechanical and optical properties of each type are discussed, with an emphasis on the effect of yttria concentration on the properties of zirconia. The processing and sintering methods are also discussed as they have a direct impact on the properties of zirconia. The design of tooth preparation, specifically occlusal reduction, varies depending on the type of zirconia used in each case. Finally, a protocol for zirconia restoration bonding is described to ensure optimal bonding to the tooth structure.

CONCLUSION

Not all zirconia restorations are the same. The selection of zirconia type based on yttria concentration, processing and sintering methods, tooth preparation design, and adherence to the bonding protocol are all critical to the success and longevity of zirconia restorations.

CLINICAL SIGNIFICANCE

Zirconia restorations are the most commonly used indirect restorative material. The selection of the most appropriate zirconia type based on its yttria content, which determines its strength and translucency, is critical to the success and the longevity of the restoration. Tooth preparation design also influences the strength and translucency of the zirconia. Air-borne particle abrasion, followed by a ceramic primer and resin cement, can ensure a durable bond to the tooth structure.

Incomplete Polymerization of Dual-Cured Resin Cement Due to Attenuated Light through Zirconia Induces Inflammatory Responses

Zirconia restorations are becoming increasingly common. However, zirconia reduces the polymerization of dual-cured resin cement owing to light attenuation, resulting in residual resin monomers. This study investigated the effects of dual-cured resin cement, with incomplete polymerization owing to attenuated light through zirconia, on the inflammatory response in vitro. The dual-cured resin cement (SA Luting Multi, Kuraray) was light-irradiated through zirconia with three thickness diameters (1.0, 1.5, and 2.0 mm). The light transmittance and the degree of conversion (DC) of the resin cement significantly decreased with increasing zirconia thickness. The dual-cured resin cement in 1.5 mm and 2.0 mm zirconia and no-irradiation groups showed significantly higher amounts of hydroxyethylmethacrylate and triethyleneglycol dimethacrylate elution and upregulated gene expression of proinflammatory cytokines IL-1β and IL-6 from human gingival fibroblasts (hGFs) and TNFα from human monocytic cells, compared with that of the 0 mm group. Dual-cured resin cement with lower DC enhanced intracellular reactive oxygen species (ROS) levels and activated mitogen-activated protein (MAP) kinases in hGFs and monocytic cells. This study suggests that dual-cured resin cement with incomplete polymerization induces inflammatory responses in hGFs and monocytic cells by intracellular ROS generation and MAP kinase activation.