Strength and aging resistance of monolithic zirconia: an update to current knowledge

Advances in zirconia-based dental materials: Properties, classification, applications, and future prospects

OBJECTIVES

Zirconia (ZrO2) ceramics are widely used in dental restorations due to their superior mechanical properties, durability, and ever-improving translucency. This review aims to explore the properties, classification, applications, and recent advancements of zirconia-based dental materials, highlighting their potential to revolutionize dental restoration techniques.

STUDY SELECTION, DATA AND SOURCES

The most recent literature available in scientific databases (PubMed and Web of Science) reporting advances of zirconia-based materials within the dental field is thoroughly examined and summarized, covering the major keywords "dental zirconia, classification, aesthetic, LTD, applications, manufacturing, surface treatments".

CONCLUSIONS

An exhaustive overview of the properties, classifications, and applications of dental zirconia was presented, alongside an exploration of future prospects and potential advances. This review highlighted the importance of addressing challenges such as low-temperature degradation resistance and optimizing the balance between mechanical strength and translucency. Also, innovative approaches to improve the performances of zirconia as dental material was discussed.

CLINICAL SIGNIFICANCE

This review provides a better understanding of zirconia-based dental biomaterials for dentists, helping them to make better choice when choosing a specific material to fabricate the restorations or to place the implant. Moreover, new generations of zirconia are still expected to make progress on key issues such as the long-term applications in dental materials while maintaining both damage resistance and aesthetic appeal, defining the directions for future research.

Evaluation of fit and accuracy of single crowns fabricated from self-glazed zirconia compared with milled zirconia

STATEMENT OF PROBLEM

Self-glazed zirconia is a novel dental material fabricated with a combination of subtractive and additive manufacturing techniques. However, how the marginal and internal fit of these restorations compare with conventionally fabricated zirconia is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the marginal and internal fit and internal accuracy of single crowns made from self-glazed zirconia compared with those made from milled zirconia.

MATERIAL AND METHODS

A metal die was scanned, and a crown was designed using a computer-aided-design program. The computer-aided-manufacturing milling method (conventional technique) was used to produce 10 zirconia crowns, and additive manufacturing was used to produce 10 self-glazed zirconia crowns. Internal and marginal fit measurements were made by using the silicone replica method; marginal fit was also measured by using the direct-view technique. An independent t test was performed to compare both types of crowns and test the null hypothesis (α=.05).

RESULTS

For the occlusal, internal, and total mean distances, significant differences were found between self-glazed and milled zirconia crowns (P<.001). With the direct-view technique, a significant difference (P=.004) between the 2 techniques was found in the marginal area. The accuracy for the axial and occlusal areas combined for self-glazed and milled zirconia was 27 μm and 49 μm, respectively.

CONCLUSIONS

Both techniques produced crowns with clinically acceptable marginal and internal fit. The self-glazed zirconia crowns had higher accuracy of internal fit than milled zirconia crowns.

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.

Biological Properties of 3D-Printed Zirconia Implants with p-Cell Structures

Research on 3-dimensional (3D) printed porous zirconia-based dental implants is still in its infancy. This study aimed to evaluate the biological responses of novel zirconia implants with p-cell structures fabricated by 3D printing. The solid zirconia samples exhibited comparable density, 3-point flexural strength, and accelerated aging properties compared to specimens prepared previously by conventional methods. Cell-based experiments showed that the p-cell structure promoted cell proliferation, adhesion, and osteogenesis-related protein expression. Mechanical tests showed that both p-cell and control implants could withstand a torque of 35 Ncm without breaking. The mean maximum breaking loads of p-cell and control implants were 1,222.429 ± 115.591 N and 1,903.857 ± 250.673 N, respectively, which were much higher than the human physiological chewing force and human mean maximum occlusal force. An animal experiment showed that the bone trabeculae around the implants were significantly thicker, more numerous, and denser in the p-cell group than in the control group. This work could provide promising guidance for further exploring 3D printing techniques for porous zirconia bionic implants in dentistry.

Optimized 3D printed zirconia-reinforced leucite with antibacterial coating for dental applications

OBJECTIVES

This study aims to produce by robocasting leucite/zirconia pieces with suitable mechanical and tribological performance, convenient aesthetics, and antibacterial properties to be used in dental crown replacement.

METHODS

Leucite pastes reinforced with 12.5%, 25%, and 37.5% wt. ZrO2 nanoparticles were prepared and used to print samples that after sintering were characterized in terms of density, shrinkage, morphology, porosity, mechanical and tribological properties and translucency. A coating of silver diamine fluoride (SDF) and potassium iodide (KI) was applied over the most promising material. The material's antibacterial activity and cytotoxicity were assessed.

RESULTS

It was found that the increase of ZrO2 reinforcement up to 25% enhanced both microhardness and fracture toughness of the sintered composite. However, for a superior content of ZrO2, the increase of the porosity negatively affected the mechanical behaviour of the composite. Moreover, the composite with 25% ZrO2 exhibited neglectable wear in chewing simulator tests and induced the lowest wear on the antagonist dental cusps. Although this composite exhibited lower translucency than human teeth, it was three times higher than the ZrO2 glazed material. Coating this composite material with SDF+KI conferred antibacterial properties without inducing cytotoxicity.

SIGNIFICANCE

Robocasting of leucite reinforced with 25% ZrO2 led to best results. The obtained material revealed superior optical properties and tribomechanical behaviour compared to glazed ZrO2 (that is a common option in dental practice). Moreover, the application of SDF+KI coating impaired S. aureus proliferation, which anticipates its potential benefit for preventing pathogenic bacterial complications associated with prosthetic crown placement.

Effect of low-temperature degradation on the fatigue performance of dental strength-gradient multilayered zirconia restorations

OBJECTIVES

Fatigue and low-temperature degradation (LTD) are the main factors contributing to zirconia restoration failure. This study evaluated the effect of LTD on the fatigue performance of the novel "strength & shade-gradient" multilayered zirconia restorations.

METHODS

Discs (15 mm × 1.2 mm) of each yttria content layer from a newly developed strength-gradient multilayered zirconia were fabricated and under accelerated aging in an autoclave at 134℃ for 0 h, 32 h, and 64 h. Then, the phase transformation, microstructure, and mechanical properties after LTD were assessed. In addition, the crown samples, including the multi-Zir, 3Y-Zir, and 5Y-Zir were fabricated, and their monotonic and fatigue load before and after LTD, percentage of fatigue degradation (Sd) and the fracture morphology were investigated. Statistical analyses were performed using paired samples t-test (α' = α/3 = 0.017), one-way ANOVA and Weibull analysis.

RESULTS

After LTD, the phase transformation, surface roughness, depth of transformed zone, and residual stress were increased and inversely associated with the yttria content. The indentation elastic modulus and hardness after LTD decreased; however, there was no significant difference between the different yttria content layers. The monotonic and fatigue load of multi-Zir restorations decreased, but their Weibull modulus increased, and Sd decreased, similar to 3Y-Zir. The crack origin was associated with the cervical region.

CONCLUSION

These results show that although LTD reduces the absolute fatigue strength of strength-gradient multilayered zirconia restorations, it also reduces the effect of cyclic fatigue itself on the strength of zirconia (relative to monotonic strength), which might be due to the increase of residual stress.

CLINICAL SIGNIFICANCE

The novel "strength & shade-gradient" multilayered zirconia restorations show a promising performance during in vitro LTD and fatigue test and their reliability to some extent is comparable to 3Y-Zir. Yet, further in vivo longitudinal studies are warranted to confirm their precise performance.

Bio-inspired nacre-like zirconia/PMMA composites for chairside CAD/CAM dental restorations

OBJECTIVES

To introduce a versatile fabrication process to fabricate zirconia/PMMA composites for chairside CAD/CAM dental restorations. These zirconia composites have nacre-like lamellar microstructures, competent and tooth-matched mechanical properties, as well as crack resistance behaviours.

METHODS

Bi-directional freeze casting was used to fabricate ceramic green bodies with highly aligned lamellar structure. Pressure was then applied to control the ceramic volume fraction. PMMA was infiltrated into the ceramic scaffold. Mechanical tests including 3-point bending, Vickers hardness, and fracture toughness were performed on the composites. The machinability of the composites was also characterised.

RESULTS

Two types of nacre-like zirconia/PMMA composites, i.e., 3Y-YZP/PMMA and 5Y-PSZ/PMMA composites were fabricated. The microstructure created was similar to the 'brick and mortar' structure of nacre. Excellent flexural strength (up to 400 MPa and 290 MPa for 3Y-TZP/PMMA and 5Y-PSZ/PMMA composite, respectively), tuneable hardness and elastic modulus within the range similar to enamel, along with improved crack-resistance behaviour were demonstrated on both zirconia composites. In addition, both zirconia/PMMA composites showed acceptable machinability, being easy to mill, as would be required to produce a dental crown.

SIGNIFICANCE

Nacre-like zirconia/PMMA composites therefore exhibit the potential for use in the production of chairside CAD/CAM dental restorations.

Bibliometric analysis of zirconia publications between 1980 and 2021: Global productivity and publication trends

Purpose With an increase in patients' aesthetic demands and advancements in dental technologies, tooth-colored materials have grown in popularity. This study aimed to statistically analyze the scientific output of zirconia.Methods Articles published between 1980 and 2021 were downloaded from the Web of Science database and analyzed using various statistical/bibliometric methods. Correlations were evaluated using the Spearman's coefficient. Time-series forecasting was used to predict the number of articles in the coming years.Results Of the 18773 recordings, 16703 (88.9%) were articles. China contributed the most to the literature (n=3345, 20%). The Chinese Academy of Sciences was the most active institution (n=666). Furthermore, Ceramics International was the journal that published the most articles (N=611). The Journal of Catalysis was the journal with the highest average number of citations per article (average number of citations, 81.4). A high level of significant correlation was found between the number of articles produced by different countries on zirconia and gross domestic product (r=0.742, P<0.001).Conclusions It is expected that zirconia research will continue to increase parallel with the increase in aesthetic expectations. Recent trends include dental implants, resin cement, surface roughness, shear bond strength, monolithic zirconia, osseointegration, flexural strength, aging, geochemistry, zircon U-Pb dating, detrital zircon, adhesion, computer-aided design-computer-aided manufacturing, bond strength, adsorption, titanium, spark plasma sintering, corrosion, SEM, zirconium dioxide, surface modification, XRD, finite-element analysis, and yttria-stabilized zirconia. Clinicians and scientists interested in zirconia can refer to this comprehensive article as a useful resource for the relevant global and multidisciplinary outcomes.

Effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia fixed partial dentures: An in vitro study

STATEMENT OF PROBLEM

Strength-gradient zirconia combining 3 zirconia formulations with different flexural strengths has been reported to have outstanding mechanical properties. However, data concerning the effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia fixed partial dentures (FPDs) are sparse.

PURPOSE

The purpose of this in vitro study was to test the effect of different sintering protocols on the fracture strength of 3-unit monolithic gradient zirconia FPDs.

MATERIAL AND METHODS

Two custom-made stainless-steel master dies were designed to replicate a mandibular right second premolar and second molar prepared to receive a 3-unit monolithic zirconia FPD. Thirty monolithic zirconia FPDs were milled from gradient zirconia blanks and allocated to 3 groups (n=10) according to the sintering protocols: high-speed sintering, speed sintering, and conventional sintering. The FPDs were cemented onto the corresponding dies with traditional glass ionomer cement. All FPDs were cyclic loaded (600 000 cycles/49 N/1.7 Hz) in a mastication simulator. Fracture load measurements for each FPD were determined by using a universal testing machine. Scanning electron microscopy (SEM) at ×80 magnification was used to examine a fractured FPD from each group. A representative specimen from each group was examined with SEM at ×30 000 magnification to determine the grain size. One-way ANOVA, pair-wise Tukey honestly significant difference (HSD), and Pearson correlation tests were used for statistical analysis of the data (α=.05).

RESULTS

The high-speed sintered FPDs recorded the highest statistically significant fracture load mean ±standard deviation value (2526 ±300 N), followed by the speed sintered FPDs (2136 ±127 N), while the lowest statistically significant fracture load mean value was recorded with the conventionally sintered FPDs (1361 ±181 N) (P<.001). In addition, the mean ±standard deviation grain size values were 488 ±272 nm for the high-speed sintered specimen, 578 ±409 nm for the speed sintered specimen, and 832 ±551 nm for the conventionally sintered specimen (P<.001). A significant negative correlation was found between fracture strength and grain size among the 3 groups.

CONCLUSIONS

The fracture strength of 3-unit monolithic gradient zirconia FPDs sintered by using a high-speed protocol was significantly higher than that of speed and conventionally sintered FPDs (P<.001). The high-speed sintering protocol reduced the mean grain size of gradient zirconia FPDs compared with that of both speed and conventional sintering protocols.

A comparative study of mechanical properties of yttria stabilized zirconia monolithic and bilayer configuration for dental application

Zirconia multilayer presents promising prospects, but there is scarce information about its microstructural and mechanical characterization. Therefore, this study sought to produce them in-house and to perform their characterization by comprising four groups of specimens to assess the biaxial flexural strength, microhardness, fracture toughness, phase characterization and quantification, fractography, and microstructural features. Weibull analysis was performed to determine the Weibull modulus and characteristic strength. The results showed that bilayers 3YSZ and 5YSZ presented intermediate mechanical properties when compared to 3YSZ and 5YSZ monolithic controls (680 MPa, 464 MPa, 885 MPa, 594 MPa, respectively). Fractographic analysis revealed that the failure origin was not at the interface in the bilayer groups, but residual stress was present between the layers. Hardness and fracture toughness were not affected by the interface.

The effect of low-temperature degradation and building directions on the mechanical properties of additive-manufactured zirconia

This study aimed to investigate the effect of low-temperature degradation (LTD) on the mechanical properties of additive-manufactured zirconia. In addition, the mechanical properties of additive-manufactured were compared with those before aging under similar experimental conditions. This study prepared stereolithography apparatus fabricated zirconia specimens with flexural strength, modulus of elasticity, Vickers hardness, and fracture toughness. The specimen position data were set as parallel (0°), diagonal (45°), and perpendicular (90°) to the direction of the building. The LTD condition was 5 h under 134ºC and 0.2 MPa in an autoclave. It was found that the 0° direction differed significantly from all other conditions before and after aging, and the highest flexural strength was obtained when the additive specimen was manufactured perpendicular to the building direction. However, the results indicate that there is a negligible effect of aging on the mechanical properties of additive-manufactured zirconia.

Development of TiO2/ZrO2 Multi-Material Obtained from Ceramic Pastes for Material Extrusion

The application of additive manufacturing method such as material extrusion (MEX) allows the successful fabrication of ceramic products, including multi-ceramic products. Promising materials in this research area are TiO2 and ZrO2 ceramics, which can be used in electrical and electronic engineering. The aim of this work is to investigate the possibility of fabricating TiO2/ZrO2 multi-materials from ceramic pastes that can be used in the MEX. In this work, defects, chemical and phase composition, and microhardness were analyzed in multi-ceramic samples after sintering. Multi-ceramic TiO2/ZrO2 samples after the sintering process without interlayer could not be fabricated due to a too large difference in shrinkage between TiO2 and ZrO2. The samples with one and three interlayers also have defects, but they are less significant and can be fabricated. The average hardness for the TiO2 zone was 636.7 HV and for the ZrO2 zone was 1101 HV. In the TiO2 zone, only TiO2 phase in rutile is observed, while in the interlayer zones, in addition to rutile, ZrO2 and ZrTiO4 are also present, as is a small amount of Y2O3. In the zone ZrO2, only the ZrO2 phase is observed. The chemical analysis revealed that the interlayers comprise sintered ZrO2 granules enveloped by TiO2, ZrO2, and ZrTiO4.

Effects of Plasma Treatment on the Strength of Bonding to Ceramic Surfaces in Orthodontics-A Comprehensive Review

Over the past several decades, orthodontic treatment has been increasingly sought out by adults, many of whom have undergone restorative dental procedures that cover enamel. Because the characteristics of restorative materials differ from those of enamel, typical bonding techniques do not yield excellent restoration-bracket bonding strengths. Plasma treatment is an emerging surface treatment that could potentially improve bonding properties. The purpose of this paper is to evaluate currently available studies assessing the effect of plasma treatment on the shear bond strength (SBS) and failure mode of resin cement/composite on the surface of ceramic materials. PubMed and Google Scholar databases were searched for relevant studies, which were categorized by restorative material and plasma treatment types that were evaluated. It was determined that cold atmospheric plasma (CAP) treatment using helium and H2O gas was effective at raising the SBS of feldspathic porcelain to a bonding agent, while CAP treatment using helium gas might also be a potential treatment method for zirconia and other types of ceramics. More importantly, CAP treatment using helium has the potential for being carried out chairside due to its non-toxicity, low temperature, and short treatment time. However, because all the studies were conducted in vitro and not tested in an orthodontic setting, further research must be conducted to ascertain the effectiveness of specific plasma treatments in comparison to current orthodontic bonding treatments in vivo.

Research on the low-temperature degradation of dental zirconia ceramics fabricated by stereolithography

STATEMENT OF PROBLEM

Stereolithography is a promising method of fabricating zirconia ceramics with high strength and accuracy. However, studies of the aging effects on zirconia ceramics fabricated by this technique are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the aging effects on the crystalline content, microstructure, and mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) printed by stereolithography apparatus (SLA) and digital light processing (DLP) compared with those of zirconia milled by computer numerical control (CNC).

MATERIAL AND METHODS

Bar-shaped specimens were fabricated after layer-by-layer printing, debinding, and sintering by SLA and DLP. Specimens milled and sintered by CNC were used as controls (n=24/material). The specimens were divided into 12 groups (n=6) and aged (0/5/10/15 hours, 134 °C, 0.2 MPa), after which the crystalline content, microstructure, and mechanical properties were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and 3-point bend tests. The flexural strength and monoclinic (m) phase content were statistically evaluated (α=.05).

RESULTS

The XRD results showed that an m peak was not detected for any of the tested materials before aging. The m-phase content was the highest for SLA (5/10/15 hours: 19.64%/34.76%/41.88%), followed by DLP (5/10/15 hours: 9.62%/21.76%/28.43%) and CNC (5/10/15 hours: 2.29%/7.77%/7.66%). The SEM images showed zirconia grain fragments for DLP and grain pullout for SLA, while surface defects were not obvious for CNC. Within the materials, the flexural strength was the highest for SLA after aging for 5 hours (1010.3 MPa), followed by 10 hours (913.06 MPa) and 15 hours and 0 hours, which exhibited no difference (0/15 hours: 776.71/814.28 MPa) (P<.001). The flexural strength for CNC and DLP did not significantly change after aging for 5 hours, 10 hours, and 15 hours (P>.05). The flexural strength for CNC was always more than 1200 MPa, and that for DLP was approximately 800 MPa before and after aging.

CONCLUSIONS

Although the m-phase content for SLA and DLP increased with the aging time, the mechanical properties did not significantly decrease, indicating the stability of both materials.

Effect of CAD/CAM Position and Thickness of Ultra-Translucent Multilayered Zirconia on Color Aspects

Background

Ultra-translucent multilayered zirconia restorations fabricated using computer-aided design and computer-aided manufacturing (CAD/CAM) technology have recently gained popularity. Their esthetic appeal is crucially dependent on the color accuracy, influenced by prosthesis thickness and multilayer composition due to CAD/CAM milling positions. This study comprehensively investigated how these two factors impacted color accuracy, thereby enhancing our understanding of color outcomes.

Materials and Methods

One hundred monolithic multilayer zirconia specimens with 10 × 10 mm square shape were milled in four different positions and five different thicknesses (1.0, 1.5, 2.0, 2.5, and 3.0 mm). The specimens were placed on an A3 shade resin composite substrate, and CIELAB values (L ∗, a ∗, and b ∗) were measured using a spectrophotometer. Delta E (ΔE) values were calculated to quantify the color differences between the specimens and the A3 VITA classical shade tab and compared with the perceptibility and acceptability thresholds of ΔE = 1.2 and 2.7, respectively. Pearson correlation, two-way ANOVA, and Tukey multiple comparisons (α = 0.05) were performed.

Results

The proportion of the dentin layer was positively correlated with the a ∗ and b ∗ values, while specimen thickness was positively correlated with the a ∗ value and negatively correlated with the L ∗ and b ∗ values. Significant difference in ΔE value due to different CAD/CAM positions was not observed within the same specimen thickness. Perceptible color differences were observed in specimens with thicknesses greater than 1 mm, while specimens with 1 mm thickness fell within the clinically acceptable range. Highest ΔE value was found in the specimen with 1 mm thickness.

Conclusions

Different compositions of multilayers in the final restoration due to different CAD/CAM positions do not significantly affect the color appearance of ultra-translucent multilayer zirconia, with color only influenced by specimen thickness.

Accuracy and margin quality of advanced 3D-printed monolithic zirconia crowns

STATEMENT OF PROBLEM

Nanoparticle jetting (NPJ) is a novel ceramic 3D-printing technology with high printing accuracy. However, studies reporting the accuracy of zirconia crowns manufactured by NPJ and comparing them with conventional zirconia crowns are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate and compare the trueness, crown fit, and margin quality of monolithic zirconia crowns manufactured by NPJ with those milled by a computer numerical control system.

MATERIAL AND METHODS

A gypsum left mandibular first molar was prepared and scanned with an intraoral scanner (TRIOS 4). Three types of monolithic crowns were manufactured through 3D printing and subtractive manufacturing (SM): NPJ (3D printing), VITA (milling), UPCERA (milling). The crowns were scanned, and the dimensional deviation (trueness) was evaluated and compared by using a software program. The triple scan method was used to measure crown fit and uniform index through precise alignment in the software program, and margin quality was also observed with an optical microscope. The data were analyzed with 1-way analysis of variance and the Tukey post hoc test (α=.05).

RESULTS

The NPJ group reported better trueness of all crown and axial surfaces compared with the other SM group (P<.001), but marginal trueness (P=.601), intaglio surface (P=.596), and occlusal surface (P=.641) were statistically similar compared with the Vita milled group. All 3 groups reported clinically acceptable crown fit and uniformity with statistically similar values (P>.05). The NPJ group had more crowns judged to have flawless margin quality compared with the milled groups.

CONCLUSIONS

All 3 manufacturing methods can fabricate zirconia crowns with a clinically acceptable crown fit. The NPJ system could be used to manufacture monolithic zirconia crowns with better margin quality and proximal surface trueness than milled crowns.

Effect of dynamic loading on fracture resistance of gradient zirconia fixed partial denture frameworks

STATEMENT OF PROBLEM

The new strength-gradient zirconia composed of 3-mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 5-mol% yttria-stabilized tetragonal zirconia polycrystal (5Y-TZP) has been claimed to have superior mechanical properties. However, data on the fracture resistance of 3-unit gradient 5Y-TZP and 3Y-TZP fixed partial denture frameworks are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of dynamic loading on the fracture resistance of gradient zirconia fixed partial denture frameworks.

MATERIAL AND METHODS

Two standardized stainless-steel master dies were designed to simulate a mandibular left second premolar and second molar prepared to receive zirconia frameworks. The frameworks were designed with a 0.6-mm uniform wall thickness. The mesiodistal width of the connectors was 3 ±0.02 mm, and the occlusogingival height was 3 ±0.02 mm. Forty zirconia frameworks were fabricated and divided into 2 groups according to the tested materials (n=20): 3Y-TZP and gradient 5Y-TZP and 3Y-TZP. The frameworks were cemented onto their corresponding dies with a conventional glass ionomer cement. Half of the cemented frameworks in each group underwent 600 000 cycles of dynamic loading in a mastication simulator (n=10). The other half was fractured without dynamic loading (n=10). Fracture resistance measurements (N) for each framework were recorded by using a universal testing machine at a crosshead speed of 1 mm/min. A fractured framework from each group was examined by using a scanning electron microscope (SEM) at ×100 magnification. The data obtained were statistically analyzed by using 2-way ANOVA, the pairwise Tukey honestly significant difference (HSD), and simple main effect tests to detect the difference between group mean values (α=.05).

RESULTS

The mean ±standard deviation of fracture load value before dynamic loading was 1919 ±193 N for the 3Y-TZP group and 908 ±99 N for the gradient 5Y-TZP and 3Y-TZP group. In addition, the mean fracture load value after dynamic loading was 1418 ±163 N for the 3Y-TZP group and 716 ±85 N for the gradient 5Y-TZP and 3Y-TZP group. The interaction between the effects of the zirconia material and dynamic loading on the fracture resistance was statistically significant (P=.002). The 3Y-TZP group had a statistically significant, higher fracture load mean value the gradient 5Y-TZP and 3Y-TZP group before and after dynamic loading (P<.001).

CONCLUSIONS

The fracture resistance of 3Y-TZP frameworks was significantly higher than that of gradient 5Y-TZP and 3Y-TZP frameworks before and after dynamic loading. Dynamic loading significantly reduced the fracture resistance of 3Y-TZP and gradient 5Y-TZP and 3Y-TZP frameworks.

Current classification of zirconia in dentistry: an updated review

Zirconia, a crystalline oxide of zirconium, holds good mechanical, optical, and biological properties. The metal-free restorations, mostly consisting of all-ceramic/zirconia restorations, are becoming popular restorative materials in restorative and prosthetic dentistry choices for aesthetic and biological reasons. Dental zirconia has increased over the past years producing wide varieties of zirconia for prosthetic restorations in dentistry. At present, literature is lacking on the recent zirconia biomaterials in dentistry. Currently, no article has the latest information on the various zirconia biomaterials in dentistry. Hence, the aim of this article is to present an overview of recent dental zirconia biomaterials and tends to classify the recent zirconia biomaterials in dentistry. This article is useful for dentists, dental technicians, prosthodontists, academicians, and researchers in the field of dental zirconia.

Effect of Hydrothermal Degradation on Flexural Fatigue Strength of Various Cubic-Containing Translucent Zirconia

OBJECTIVE

 The aim of this study is to investigate the fatigue and hydrothermal degradation behavior among the cubic-containing translucent yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP).

MATERIALS AND METHODS

 Four groups of commercial Y-TZP (T, ST, XT, and P), containing different amount of cubic crystal, were examined. Artificial aging was accomplished by autoclaving at 122°C under 2 bar pressure for 8 hours. Fatigue simulation parameters were set using an axial 50 N load, a total of 240,000 cycles. Flexural fatigue strength was evaluated.

STATISTICAL ANALYSIS

 Two-way analysis of variance with Tukey's honestly significant difference test was used to determine the difference in fatigue strength values between various type of zirconia materials within control and aging with a significant level of 5%. Weibull analysis was conducted on the fatigue strength data.

RESULTS

 All groups showed the mean flexural fatigue strength had no significant difference in hydrothermally aged specimens compared with specimens without aging (p = 0.154). Group T showed significantly higher flexural fatigue strength followed by group ST, XT, and P. Group XT and P showed no significant difference in strength value (p > 0.05). Group T demonstrated a lower probability of failure than group ST, XT, and P whether with or without aging. Weibull modulus in group T and ST with aging condition was increased and higher than all the experimental groups.

CONCLUSION

 Cubic-containing zirconia materials (group ST, XT, and P) exhibited lower flexural fatigue strength than non-cubic 3Y-TZP zirconia (group T). However, various cubic-containing translucent zirconia was assumed to have high resistance to hydrothermal degradation.

Fracture strength and energy-dispersive spectroscopy analysis of 3-unit fixed partial dentures fabricated from different monolithic zirconia materials

STATEMENT OF PROBLEM

Mastication forces in different regions affect the survival of multiunit posterior restorations. The fracture strength of 3-unit posterior monolithic zirconia fixed partial dentures (FPDs) and their fracture patterns require investigation.

PURPOSE

The purpose of this in vitro study was to evaluate and compare the fracture strength and fracture pattern of 3-unit posterior FPDs fabricated from different monolithic zirconia materials.

MATERIAL AND METHODS

Thirty 3-unit FPDs were fabricated from BruxZir, FireZr, and Upcera (n=10 per group). Energy-dispersive spectroscopy analysis was performed on 2 selected specimens from each group. All specimens were exposed to a mastication simulator for 1.2×10⁶ cycles and then monotonically loaded to fracture at a crosshead speed of 1 mm/min. The surfaces of a selected fractured specimen were examined at magnifications of ×25 and ×500 with scanning electron microscopy. Conformity to normal distribution was evaluated with the Shapiro-Wilk test. One-way analysis of variance was used to compare the normally distributed initial crack formation load F initial (F_i) and catastrophic failure strength F maximum (F_m) means by group. Weibull statistics were calculated by using the maximum likelihood estimation method. The chi-square test was used to compare shape and scale parameters (α=.05).

RESULTS

The mean F_m values were fail1878.9 N for Upcera, 2177.8 N for BruxZir, and 2229.4 N for FireZr. Upcera and BruxZir showed statistically significant differences for the F_m mean values (P=.039). The differences between the fracture type distributions according to the groups were statistically similar (P>.05). For F_i, Upcera presented the highest Weibull modulus value (2.199), FireZr had the lowest (1.594), while for F_m, BruxZir had the highest Weibull modulus value (9.267) and FireZr the lowest (6.572).

CONCLUSIONS

Using the zirconia materials BruxZir, FireZr, and Upcera resulted in high F_m values after aging procedures. With all materials, the fractures were most commonly found in the connector areas in the tested FPDs.

In vitro fracture and fatigue resistance of monolithic zirconia crowns fabricated by stereolithography

STATEMENT OF PROBLEM

Stereolithography has been used to print zirconia ceramic crowns with acceptable dimensional accuracy and fracture force. However, studies that compared the fatigue resistance of zirconia crowns fabricated by stereolithography are lacking.

PURPOSE

The purpose of this in vitro study was to compare the fracture and fatigue resistance of monolithic zirconia crowns printed by stereolithography apparatus (SLA) and digital light processing (DLP) with those of zirconia crowns milled by computer numerical control (CNC).

MATERIAL AND METHODS

A total of 120 crowns were fabricated (n=40/material) and underwent 0, 10⁴, 10⁵, or 10⁶ dynamic loading cycles of 30 to 300 N in artificial saliva, followed by a static fracture loading test (n=10). After fracture, 1 crown from each group was selected for fractography analysis by scanning electron microscopy (SEM). Data were statistically analyzed through 2-way ANOVA and post hoc analysis for multiple comparisons (α=.05).

RESULTS

The 2-way ANOVA results showed that the mean ±standard deviation force at fracture was the highest for CNC (before fatigue loading: 5154 ±568 N, 10⁴: 5735 ±1231 N, 10⁵: 5523 ±797 N, and 10⁶: 6007 ±1258 N), followed by DLP (before fatigue loading: 3381 ±612 N, 10⁴: 4046 ±1146 N, 10⁵: 2929 ±559 N, and 10⁶: 3223 ±739 N), and the lowest for SLA (before fatigue loading: 2956 ±598 N, 10⁴: 2757 ±421 N, 10⁵: 3326 ±391 N, and 10⁶: 3103 ±246 N) (P<.01). The fracture force of the crowns was not significantly affected by the number of fatigue cycles (P>.05). Fractography analysis showed that the number of arrest lines increased for crowns of all 3 materials. SEM images also showed the steps of SLA and DLP from their layer-by-layer printing and small cracks between layers of SLA after 10⁶ loading cycles.

CONCLUSIONS

The fracture force of monolithic zirconia crowns milled by CNC was significantly higher than that of zirconia crowns printed by stereolithography. Zirconia crowns printed by SLA and DLP could withstand typical clinical conditions, and their fracture and fatigue resistance exceeded the clinically estimated average occlusal forces.

Microstructure, composition, and flexural strength of different layers within zirconia materials with strength gradient

OBJECTIVES

The aim of this study was to compare composition, microstructure, and mechanical strength of current multilayer zirconia blanks.

METHODS

Bar shaped specimens were made from several layers of multilayer zirconia blanks (Cercon ht ML, Dentsply Sirona, US; Katana Zirconia YML, Kuraray, J;SHOFU Disk ZR Lucent Supra, Shofu, J; priti multidisc ZrO₂ Multi Translucent, Pritidenta, D; IPS e.max ZirCAD Prime, Ivoclar Vivadent, FL). Flexural strength was determined in a three-point bending test on extra-thin bars. X-ray diffraction (XRD) with Rietveld refinement was used to assess crystal structure and scanning electron microscopy (SEM) imaging to visualize the microstructure of each material and layer.

RESULTS

Mean flexural strength varied between 467.5 ± 97.5 MPa (top layer, IPS e.max ZirCAD Prime) and 898.0 ± 188.5 MPa (bottom layer, Cercon ht ML) with significant (p ≤ 0.055) differences between the individual layers. XRD indicated 5Y-TZP for enamel-layers, 3Y-TZP for dentine-layers, individual mixtures of 3Y-TZP, 4Y-TZP, or 5 Y-TZP for intermediate layers. SEM analysis showed grain sizes between approx. 0.15 and 4 µm. Grain size tended to decrease from top to bottom layers.

SIGNIFICANCE

The investigated blanks differ predominantly in the intermediate layers. In addition to dimensioning of restorations, the milling position in the blanks must also be taken into account when using multilayer zirconia as restorative material.

A biocompatible silicon nitride dental implant material prepared by digital light processing technology

For decades, titanium has been the preferred material for dental implant fabrication. However, metallic ions and particles can cause hypersensitivity and aseptic loosening. The growing demand for metal-free dental restorations has also promoted the development of ceramic-based dental implants, such as silicon nitride. In this study, silicon nitride (Si₃N₄) dental implants were fabricated for biological engineering by photosensitive resin based digital light processing (DLP) technology, comparable to conventionally produced Si₃N₄ ceramics. The flexural strength was (770 ± 35) MPa by the three-point bending method, and the fracture toughness was (13.3 ± 1.1) MPa · m^1/2 by the unilateral pre-cracked beam method. The elastic modulus measured by the bending method was (236 ± 10) GPa. To confirm whether the prepared Si₃N₄ ceramics possessed good biocompatibility, in vitro biological experiments were performed with the fibroblast cell line L-929, and preferable cell proliferation and apoptosis were observed at the initial stages. Hemolysis test, oral mucous membrane irritation test, and acute systemic toxicity test (oral route) further confirmed that the Si₃N₄ ceramics did not exhibit hemolysis reaction, oral mucosal stimulation, or systemic toxicity. The findings indicate that Si₃N₄ dental implant restorations with personalized structures prepared by DLP technology have good mechanical properties and biocompatibility, which has great application potential in the future.

Selective and High-Capacity Binding of Immunoglobulin G to Zirconia Nanoparticles Modified with Phosphate Groups

High-performance and cost-effective purification is necessary for the development of antibody drugs. This study found that nanoparticles of zirconia modified with phosphate groups selectively adsorb immunoglobulin G (IgG) antibodies against serum proteins with high adsorption capacity. The IgG antibodies collected from the zirconia nanoparticle surfaces retain their molecular conformation. Importantly, zirconia nanoparticles have the highest affinity for human IgG antibodies among tested mammalian IgG antibodies. The affinity for human IgG subclasses is in the order IgG3 > IgG1 > IgG2, which contrasts with a conventional ligand (Protein A) that has a lower affinity for IgG3. Because zirconia nanoparticles are chemically and mechanically stable, they can be utilized for the purification of antibody drugs not only in batch methods but also in chromatography as a process upstream or downstream of Protein A chromatography and even as an alternative process.

Trueness and precision of milled and 3D printed root-analogue implants: A comparative in vitro study

OBJECTIVES

The present study aimed to evaluate the accuracy (trueness and precision) of titanium and zirconia multi-rooted root analogue implants (RAIs) manufactured by milling and 3D-printing.

METHODS

A multi-rooted RAI was designed based on a mandibular second molar segmented from cone-beam computed tomography (CBCT). The manufactured RAIs were divided into four groups: 3D-printed titanium (PT) and 3D-printed zirconia (PZ) (n=10 each), as well as milled titanium (MT) and milled zirconia (MZ) (n=5 each). The specimens were scanned with a high-precision scanner, and the scanned data were imported into 3D-measurement software to evaluate the precision and trueness of each group. Root mean square (RMS) deviations were measured and statistically analysed (One-way ANOVA, Tukey's, p≤0.05).

RESULTS

PZ showed the highest precision with RMS value of 21±6 µm. Nevertheless, there was no statistically significant difference in precision among the other groups. Regarding trueness, MZ showed the highest trueness with RMS value of 66±3 µm, whereas MT showed the lowest trueness result. Inspection sections showed that MT had significantly high RMS deviation in the furcation area (612±64 µm), whereas PZ showed significantly high RMS deviation at the apical area (197±17 µm).

CONCLUSIONS

The manufacturing process significantly influenced the RAI accuracy. PZ exhibited the highest precision, whereas MZ exhibited the highest trueness, followed by PT. Finally, our results suggest that 3D-printing can reproduce concave surfaces and less accessible areas better than milling.

CLINICAL SIGNIFICANCE

Milled and 3D-printed RAIs showed promising results in terms of precision and trueness. However, further clinical research is needed to advocate their use as immediate implants. Additionally, the inherent volumetric changes of the various materials during manufacturing should be considered.

Recent Advances on 3D-Printed Zirconia-Based Dental Materials: A Review

Zirconia-based materials are widely used in dentistry due to their biocompatibility and suitable mechanical and tribological behavior. Although commonly processed by subtractive manufacturing (SM), alternative techniques are being explored to reduce material waste, energy consumption and production time. 3D printing has received increasing interest for this purpose. This systematic review intends to gather information on the state of the art of additive manufacturing (AM) of zirconia-based materials for dental applications. As far as the authors know, this is the first time that a comparative analysis of these materials' properties has been performed. It was performed following the PRISMA guidelines and using PubMed, Scopus and Web of Science databases to select studies that met the defined criteria without restrictions on publication year. Stereolithography (SLA) and digital light processing (DLP) were the techniques most focused on in the literature and the ones that led to most promising outcomes. However, other techniques, such as robocasting (RC) and material jetting (MJ), have also led to good results. In all cases, the main concerns are centered on dimensional accuracy, resolution, and insufficient mechanical strength of the pieces. Despite the struggles inherent to the different 3D printing techniques, the commitment to adapt materials, procedures and workflows to these digital technologies is remarkable. Overall, the research on this topic can be seen as a disruptive technological progress with a wide range of application possibilities.

Effect of Simulated Mastication on Structural Stability of Prosthetic Zirconia Material after Thermocycling Aging

Recent trends to improve the aesthetic properties-tooth-like color and translucency-of ceramic dental crowns have led to the development of yttria-stabilized zirconia (Y-TZP) materials with higher stabilizer content. These 5Y-TZP materials contain more cubic or t' phase, which boosts translucency. The tradeoff as a consequence of a less transformable tetragonal phase is a significant reduction of strength and toughness compared to the standard 3Y-TZP composition. This study aims at determining the durability of such 5Y-TZP crowns under lab conditions simulating the conditions in the oral cavity during mastication and consumption of different nutrients. The test included up to 10,000 thermal cycles from 5 °C to 55 °C "from ice cream to coffee" and a chewing simulation representing 5 years of use applying typical loads. The investigation of the stress-affected zone at the surface indicates only a very moderate phase transformation from tetragonal to monoclinic after different varieties of testing cycles. The surface showed no indication of crack formation after testing. It can, therefore, be assumed that over the simulated period, dental crowns of 5Y-TZP are not prone to fatigue failure.

Aging resistance of highly translucent zirconia ceramics with rapid sintering

PURPOSE

Rapid sintering technology has become one of the most direct methods for shortening the manufacturing time of zirconia restorations. This study aimed to explore the aging resistance of rapid-sintered 5 mol% yttria-partially-stabilized zirconia (5Y-PSZ).

METHODS

Specimens were made from two types of 5Y-PSZ material and subjected to rapid sintering (RS) and conventional sintering (CS). After in vitro aging for 5 h, morphology observation, grain size measurement, and phase composition analysis were performed. The mechanical properties were evaluated by biaxial, three-point flexural tests, and the Vickers microhardness test. Results were analyzed by 3-way ANOVA.

RESULTS

Both the RS group and the CS group had a dense microstructure. The tested zirconia ceramics had different grain sizes, which were affected by the interaction between the sintering method and aging. Both groups revealed the same characteristic peaks of the cubic phase after aging. Regardless of the sintering method used, there was no significant difference in the mechanical properties of the tested zirconia before and after aging.

CONCLUSION

The rapid-sintered 5Y-PSZ materials had a microstructure, phase composition and mechanical properties similar to those of conventional sintered materials. The characteristics of the materials prepared using the two sintering methods did not change significantly after aging.

Mechanical Properties of Translucent Zirconia: An In Vitro Study

Background: The introduction of translucent zirconia has improved mimetics: nevertheless, a reduction in the mechanical performance was registered. The study aim was to investigate the mechanical characteristics of a high-translucent zirconia used for monolithic restorations before and after the aging process compared to a low-translucent zirconia. Methods: A total of 23 specimens were used in the present study. Group A (n = 10) was made of a high-translucent Y-TZP; group B (n = 7) was made of a low-translucent Y-TZP and finally group C (n = 6) was an aged high-translucent Y-TZP. Flexural strength, fracture toughness, brittleness, microcrack’s propagation and grain size were analyzed. Results: The Vickers hardness was: 1483 ± 187 MPa (group C); 1102 ± 392 MPa (group A); 1284 ± 32 MPa (group B). The flexural strength was: 440 (±96.2) MPa (group C); 427 (±59.5) MPa (group A); 805 (±198.4) MPa (group B). The fracture toughness was: 5.1 (±0.7) MPa.m1/2 (group C); 4.9 (±0.9) MPa.m1/2 (group A); 8.9 (±1.1) MPa.m1/2 (group B). The brittleness was: 295 (±42.8) (group C), 230.9 (±46.4) (group A) and 144.9 (±20.3) (group B). The grain size was: 2.75 (±1.2) µm2 (group A); 0.16 (±0.05) µm2 (group B); 3.04 (±1.1) µm2 (group C). Conclusions: The significant reduction in the mechanical properties of high-translucent zirconia, compared to the traditional one, suggests their use in the anterior/lateral area (up to premolars).

Performance of high-translucent zirconia CAD/CAM fixed dental prostheses using a digital workflow: A clinical study up to 6 years

Background/purpose

Zirconia has recently become a popular material for fixed restorations. The purpose of this study was to use a digital workflow to fabricate monolithic zirconia fixed dental prostheses (FDPs) and assess the connection between variable connector sizes compared to their clinical performance.

Materials and methods

Clinicians evaluated monolithic zirconia FDPs in 58 patients. After definitive impressions were made, stone casts were obtained. The stone casts were scanned to a standard triangle language (STL) file. A digital wax up was fabricated, and corresponding provisional restorations were milled. Final FDPs were fabricated from a high-translucent zirconia material. During digital fabrication, the connector area of each FDP was recorded while meticulous attention was paid to ensure that the connector cross-sectional area was ≥9 mm² for the 3-unit restorations (pontic to retainer) and ≥12 mm² for the 4-unit restorations (pontic to pontic). Biological an technical outcomes of the FDPs were performed at 1 week, 6 months and then annually for 6 consecutive years.

Results

A total of 23 men and 35 women received a total of 63 full-contour zirconia FDPs in the posterior regions and were observed for a time period ranging between 50 and 70 months. No decementation occurred and no caries were detected during the observation period, however signs of gingivitis were detected in 4 patients. The dimension of the connector areas was 12 mm² in the two broken 4-unit FDPs.

Conclusion

The results of this study suggest that the use of digital scanning and milling to fabricate monolithic zirconia FDPs of posterior regions may be an acceptable alternative restorative approach to traditional metal-ceramic restorations.

Effect of thermocycling on the mechanical properties, inorganic particle release and low temperature degradation of glazed high translucent monolithic 3Y-TZP dental restorations

The influence of thermocycling on the surface deterioration of glazed monolithic high translucent 3Y-TZP dental restorations is still unclear. The purpose of this study therefore was to evaluate low temperature degradation (LTD), elemental release and surface degradation pattern after five years of simulated clinical time. A total of 123 specimens were prepared from second-generation high translucent 3Y-TZP as per ISO 6872:2015 standards (3 mm × 4 mm × 30 mm). They were classified as per glazing and thermocycling protocol; group CPT, DGT and IGT. Glaze materials were applied on one surface of the specimen and subjected to a thermocycling in artificial saliva, four-point bending test, inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD). Flexural strength, characteristic strength and Weibull modulus values were derived from four-point bending test. Descriptive fractographic analysis of surfaces was conducted to observe the surface degradation characteristics and point of failure. Control/no glaze with thermocycling (CPT = 621.5, 1σ = 117.0 MPa) presented higher flexural strength values compared to glaze I with thermocycling (442.4, 1σ = 45.4 MPa) and glaze II with thermocycling (534.3, 1σ = 46.3 MPa). Characteristic strength from Weibull analysis also observed higher values (669.2 MPa) for the control specimens. XRD analysis showed that monoclinic volume fraction (Vm = 11.0, 1σ = 0.7%) was highest in control specimens. Fractographic analysis suggested that there was no correlation between the point of failure initiation and mean flexural strength values. Glazing protected the high translucent 3Y-TZP surface against LTD during thermocycling but negatively impacted on the flexural strength.

Effects of argon plasma and aging on the mechanical properties and phase transformation of 3Y-TZP zirconia

To evaluate the flexural strength (FS) and flexural modulus (FM) of a commercial 3Y-TZ0P ceramic after artificial aging and either without or with two application times of non-thermal plasma treatments (NTP). In addition, changes in crystalline phase transformation and surface nano-topography after NTP application, during different aging periods, were evaluated. Ninety 3Y-TZP bars (45x4x3 mm) were made for FS and FM testing, and assigned to nine groups (n=10): no NTP/no aging (Control); no NTP/4h aging; no NTP/30h aging; 10s NTP/no aging; 10s NTP/4h aging; 10s NTP/30h aging; 60s NTP/no aging; 60s NTP/4h aging and 60s NTP/30h aging. Artificial accelerated aging was simulated using an autoclave (134º C at 2 bar) for up to 30h. FS and FM were assessed using a universal testing machine and data analyzed using a ANOVA and Tukey test (α=0.05). The volume change in zirconia monoclinic phase (MPV) was evaluated using X-ray diffraction and surface nano-topography was assessed using atomic force microscopy (baseline until 30h-aging). NTP application did not influence the FS and FM of zirconia. Compared to the Control (no NTP/no aging), the FS of zirconia samples treated for 30 hours in autoclave (“no NTP/30h aging” group) increased. Artificial aging for 30 hours significantly increased the FM of zirconia, regardless of NTP application. MPV tended to increase following the increase in aging time, which might result in the surface irregularities observed at 30h-aging. NTP did not alter the zirconia properties tested, but 30h-aging can change the zirconia FS, FM and MPV.

Effect of Speed Sintering on Low Temperature Degradation and Biaxial Flexural Strength of 5Y-TZP Zirconia

Translucent zirconia is becoming the material of choice for the esthetic restorative material. We aimed to evaluate the surface structure, phase determination, translucency, and flexural strength of 5Y-TZP Zirconia (Katana STML Block and Disc) between the regular sintering and the speed sintering with and without low-temperature degradation (LTD). A total of 60 zirconia discs (30 per group; regular sintering and speed sintering) were used in this study. A CAM machine was used to mill cylinders out of the zirconia blanks and then cut into smaller discs. For the speed sintering, the zirconia blocks were milled into smaller discs. The zirconia discs were subjected to regular and speed sintering with and without LTD. Scanning electron microscopy was used to characterize the zirconia specimens and the zirconia grain size. Furthermore, the zirconia specimens were analyzed for elemental analysis using energy dispersive spectroscopy and phase identification using X-ray diffraction. The zirconia specimens were subjected to translucency measurements and biaxial flexural strength testing. The results of the zirconia specimens were compared among the groups. Statistical analysis was completed using SPSS version 20.0 to detect the statistically significant differences (p value = 0.05). A one-way ANOVA with multiple comparisons was performed using Scheffe analysis among the groups. The speed sintering presented smaller grain sizes. The zirconia specimens with and without LTD in regular and speed sintering presented a similar surface structure. Regular sintering showed more translucency compared to speed sintering. Multiple comparisons of the translucency parameter were a significant difference (p value < 0.05) between the various groups except for the comparison between speed sintering and speed sintering LTD. The regular sintering showed bigger gain sizes and slightly more translucency compared to speed sintering. The speed sintering showed higher biaxial flexural strengths compared to regular sintering. This shows that speed sintering can be considered a suitable method of sintering zirconia.

Comparison of Testing Designs for Flexural Strength of 3Y-TZP and 5Y-PSZ Considering Different Surface Treatment

The aim of this study was to analyze the influence of different surface treatments and the corresponding surface roughness on the ball-on-three-balls test and piston-on-three-balls test for measuring flexural strength 3Y-TZP and 5Y-PSZ. Additionally, the influence of cutting the material into the specimens when pre-sintered or fully sintered was analyzed. A total of 120 specimens for each material group, 3Y-TZP zirconia (Katana HT, Kuraray) and the 4 different layers of multilayered 5Y-PSZ zirconia (Katana UTML, Kuraray), were produced. The used material was cut into half of the specimens in a fully sintered stage, the other half was cut when pre-sintered. Each subgroup was divided into 3 different surface treatment groups being air abraded with 50 µm alumina particles at 1 bar pressure, ground with 600 SiC paper or polished up to 1 µm with a polycristalline diamond suspension. These were then analyzed by X-ray diffraction (XRD) (N = 3) and tested for flexural strength using the ball-on-three-balls test (N = 10) or piston-on-three-balls test (N = 10). For 3Y-TZP groups different surface roughness did not result in statistically significant differences in flexural strength but cutting the specimens in fully sintered state significantly reduced flexural strength of 1133 ± 109 to 741 ± 81 MPa. For 5Y-PSZ groups air abrasion of the specimens with alumina particles significantly reduced the flexural strength of 562 ± 68 MPa to 358 ± 58 MPa. Cutting and surface treatment in pre-sintered or fully sintered state had no significant influence. Flexural strength testing with the ball-on-three-balls test resulted in about 20% higher strengths compared to the piston-on-three-balls test. Results of both tests showed the same tendencies when compared.

Physical Properties and Color Stainability by Coffee and Red Wine of Opaque and High Translucency Zirconia Dental Ceramics after Hydrothermal Degradation

The objective was to evaluate the bending strength, phase transformation, roughness, and color stainability by coffee and red wine of opaque and high translucency yttria-stabilized zirconia before and after hydrothermal degradation in saline solution or oral mouthwash. Presintered zirconia blocks with medium opacity (ZrO₂-3 mol. % of Y₂O₃) designed as ZrOp and high translucency zirconia (5.2 mol. % of Y₂O₃) designed as ZrTrans were used. Specimens (n = 80/group) were cut and sintered at 1500°C for 2 h. The specimens were hydrothermally degraded in an autoclave (134°C–1.8 kg/cm²) for 20 h in saline solution (0.5 g/L) and oral mouthwash solution (0.02% sodium fluoride, without alcohol and with 21.6% alcohol). After hydrothermal degradation, the samples were immersed in containers with coffee or red wine for 14 days to determine their color stainability. The results showed that the ZrOp had a higher bending strength than the ZrTrans before and after hydrothermal aging. In ZrOp and ZrTrans, the roughness increased after hydrothermal degradation. ZrOp samples had a higher Ra roughness than the ZrTrans samples. Roughness did not change after immersion in coffee or red wine. The X-ray diffraction (XRD) results showed that ZrOp samples underwent a tetragonal to monoclinic phase transformation, while ZrTrans samples were unchanged. Both ZrOp and ZrTrans samples changed color after immersion in coffee and red wine.

Mechanical properties-translucency-microstructure relationships in commercial monolayer and multilayer monolithic zirconia ceramics

OBJECTIVES

To evaluate the phase composition, microstructure, optical properties and mechanical properties of eight commercially available multilayer and monolayer monolithic dental zirconias.

METHODS

Five commercial 3Y-TZP (GC ST, GC HT [GC, Tokyo Japan]; Katana ML, Katana HT [Kuraray Noritake] and Lava Plus [3M Oral Care]) and three Y-PSZ (Katana STML, Katana UTML [Kuraray Noritake]; GC UHT [GC, Tokyo Japan]) zirconia ceramic grades were cut in plate-shaped specimens, sintered according to the manufacturer's instructions and mirror polished. The zirconia chemical composition was determined using X-ray fluorescence (XRF), phase composition was characterized using X-ray diffraction (XRD), while the grain size was measured using scanning electron microscopy (SEM). The translucency Parameter (TP) and Contrast Ratio (CR) were measured with a spectrophotometer (n = 10/group). The indentation fracture toughness (n = 10), Vickers hardness (n = 10) and biaxial strength (n = 20) of the sintered ceramics were assessed. The stress distribution during biaxial testing was assessed by Finite element analysis (FEA). Statistical analysis involved one-way ANOVA and post-hoc Tukey's HSD test and Pearson correlation test (α = 0.05).

RESULTS

FEA showed that the stress distribution in plate shape specimens was the same as for disks, rationalizing the use of plates for biaxial strength testing. As expected, higher quantities of Y₂O₃ were related to a higher cubic ZrO₂ phase content and lower tetragonality t-ZrO₂, which improved translucency but diminished flexural strength and toughness. While there was no significant correlation between grain size and other material properties, addition of pigments to the zirconia grade statistically negatively affected hardness.

CONCLUSION

Even though an improvement in strength and translucency could be recorded for the last Y-TZP generation, future research still needs to strive for combined improvement of optical properties and mechanical reliability of zirconia ceramics.

Fracture Resistance of Sintered Monolithic Zirconia Dioxide in Different Thermal Units

The purpose of this study was to compare the fracture strength of monolithic zirconium dioxide subjected to a sintering process in two different furnaces: InFire HTC Speed and CEREC SpeedFire. Methods: Forty restorations were designed and machined using a computer aided design / computer aidded machine (CAD/CAM) system. The restorations were randomly assigned to two groups of 20 samples each, Group 1 for the SpeedFire furnace (fast sintering) and Group 2 for the InFire furnace (slow sintering). Each of the crowns was subjected to a maximum compression load recorded in Newtons (N) and a displacement control with a speed of 1 mm/min. Results: Group 1 presented an average of 1222.8 N and a standard deviation of 136.91 N. Group 2 obtained a mean of 1068.5 N and a standard deviation of 316.39 N. Conclusions: There were no significant differences between the two groups, and the mechanical strength of the material was not affected, which would imply a saving of clinical and laboratory time when performing rapid sintering on monolithic translucent zirconium dioxide restorations. However, rapidly sintered restorations have limited reliability.

Multilayer Super-Translucent Zirconia for Chairside Fabrication of a Monolithic Posterior Crown

This case report describes the chairside fabrication of a monolithic posterior crown using a multilayer super-translucent zirconia material. According to the manufacturer’s information, the newly introduced multilayer zirconia (4-YTZP) offers a unique combination of fracture strength (>850 MPa with speed-sintering) and improved optical properties, thus allowing a reduced minimum material thickness and optional temporary luting. By using up-to-date components of the CEREC system, including superfast dry-milling and a speed-sintering process, the fabrication of a monolithic zirconia crown is possible within an acceptable timeframe for the chairside workflow (60-75 min). The usage of a multilayer super-translucent material allows for the individualization of the restoration, typically in a single combined stain and glaze firing. However, it should be noted that clinical data for this type of restoration are still sparse. Therefore, manufacturer recommendations regarding indication, preparation, and cementation must be followed very strictly.

Surface Roughness and Bond Strength of Resin Composite to Additively Manufactured Zirconia with Different Porosities

PURPOSE

To investigate the bond strength of resin cement to additively manufactured (AM) zirconia with different porosities when compared to milled zirconia.

MATERIALS AND METHODS

A 12 × 5 mm disk virtual design file was used to fabricate a total of 48 disks divided into 4 groups: 3 groups were AM with different porosities including 0%-porosity (AMZ0 group), 20%-porosity (AMZ20 group), and 40%-porosity (AMZ40 group), and 1 milled zirconia (control or CNCZ group). The dimensions of all specimens were measured using a digital caliper. A 3D- confocal laser scanner was used to analyze surface morphology and measure the surface roughness (Sa), followed by SEM analysis. Tensile bond strength of composite resin cement to specimens was measured before and after aging procedures using a universal testing machine (n = 10). Failure modes were evaluated under a light microscope. Volumetric change data was analyzed using one-way ANOVA, and two-way ANOVA was used to compare bond strength values (α = 0.05).

RESULTS

There was a significant difference in volumetric changes among the groups. The CNCZ group showed the least changes in diameter 0.027 ± 0.029 mm and thickness 0.030 ± 0.012 mm and AM zirconia with 40% porosity showed the most volumetric changes in diameter 5.237 ± 0.023 mm. ANOVA test indicated an overall significant difference in surface roughness across all groups (F = 242.6, p < 0.001). The CNCZ group showed the highest mean Sa of 1.649 ± 0.240 µm, followed by AMZ40 group with Sa of 0.830 ± 0.063 µm, AMZ20 group with Sa of 0.780 ± 0.070 µm, and the AMZ0 group with Sa of 0.612 ± 0.063 µm. Two-way ANOVA showed significant difference in bond strength between the CNCZ group 12.109 ± 3.223 MPa and the AMZ0 group 8.629 ± 0.914 MPa, with significant pretest failures in specimens with porosities. Thermal cycling methods reduced the bond strength non-significantly in CNCZ group with no effect in the AMZ0 group.

CONCLUSION

Milled zirconia had a higher surface roughness and bond strength to composite resin cement than AM zirconia, and porosities in AM zirconia decreased the bond strength with significant pretest failures.

Wear Behavior of Different Generations of Zirconia: Present Literature

Objective. The wear behavior of the novel zirconia generation is less well understood and may be affected by compositional modifications compared to the conventional zirconia. Materials and Methods. Combinations of keywords such as “zirconia,” “high translucent,” and “wear” were searched in PubMed and Google Scholar databases up to May 2021. The total of 23 relevant articles was selected according to inclusion criteria. Results. Reports show comparable wear resistance of translucent zirconia to the conventional zirconia despite an increased cubic phase content and lower mean flexural strength. A meticulously polished surface creates the lowest surface roughness, producing favorable zirconia wear resistance and antagonist wear compared to a glazed surface. In comparison to other ceramic materials, zirconia produces the least wear on an enamel antagonist and almost undetectable wear when opposed by zirconia. Wear when paired against resin materials yields a favorable outcome, whereas wear behavior against a metal antagonist varies with the surface hardness of the metal. Conclusions. All zirconia generations are considered wear-friendly to all types of antagonists. Nonetheless, comparative studies on antagonist wear opposing zirconia of different compositions are still limited and further investigation is required.

Sintering distortion of monolithic zirconia in 4-unit fixed partial denture: Effect of layered structure and vertical milling area

Layered-type zirconia disks, which offer color gradation from enamel to cervical shade, have been employed in recent years to replicate the shades of natural teeth. The layered structure is effective at replicating colors and has helped popularize monolithic zirconia restorations. However, the sintering shrinkage of zirconia is very large; thus, controlling the sintering distortion is very important. Thus, the objective of this study was to determine the influence exerted by the layered structure of the zirconia disk and the vertical milling area on the sintering distortion. An experimental fixed partial denture (FPD) was designed based on a 4-unit monolithic zirconia FPD. A single-composition (SC)-type disk with no shade and a single-composition-layered (SCL)-type disk with shade gradation were selected for this study. In particular, three milling areas, the top end of the disk (area I), vertical center (area II), and bottom end of the disk (area III), were investigated. Moreover, the sintering distortions generated by the experimental FPDs were measured. Results showed that sintering distortion in 4-unit monolithic zirconia FPDs occurred in all SC and SCL areas. Additionally, the sintering distortions were affected by the layered structure of the zirconia disks, the degree of which depended on the milling area (area I > area II > area III). Thus, when fabricating dental prosthesis using SCL zirconia disks, the milling area must be selected considering both the color adjustment and sintering distortion.

Effects of Hydrothermal Treatment on the Phase Transformation, Surface Roughness, and Mechanical Properties of Monolithic Translucent Zirconia

OBJECTIVES

This study aimed to investigate the effects of hydrothermal treatment on four types of monolithic, translucent, yttria-stabilized, tetragonal zirconia polycrystals (Y-TZPs).

METHODS AND MATERIALS

Two commercially available Y-TZP brands-SuperfectZir High Translucency (Aidite Technology Co, China) and Katana HT (Kuraray Noritake Dental, Japan) were assessed. For each brand of Y-TZP, materials of four coloring types, including noncolored (NC), colored by staining (CS), precolored (PC), and multilayered (ML) specimens were investigated after hydrothermal aging in an autoclave at 134°C/0.2 MPa for 0 (control group), 5, 10, and 20 hours. The tetragonal-to-monoclinic phase transformation, surface roughness, flexural strength, and structural reliability (Weibull analysis) were measured and statistically analyzed (α=0.05). The subsurface microstructure was analyzed with scanning electron microscopy.

RESULTS

The group ML exhibited the lowest flexural strength and Weibull characteristic strength among the four coloring types (p<0.05). Slight increases in the monoclinic phase volume, flexural strength, and Weibull characteristic strength were observed after hydrothermal aging (pall<0.05). Regardless of coloring type, no significant effects of aging on the Weibull modulus or surface roughness were found for the tested materials. Compared with the Katana HT cross-sections, the SuperfectZir High Translucency cross-sections exhibited a similar but thicker transformation zone.

CONCLUSIONS

The coloring procedure and material type were found to affect the mechanical properties and aging resistance of translucent monolithic Y-TZP ceramics. Regardless of the aging time, the surface roughness of the tested Y-TZP ceramics remained unchanged.

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.

Influence of firing temperature and duration on the hardness of dental zirconia for optimum selection of sintering conditions

OBJECTIVE

In order to optimize the properties of dental zirconia, the sintering process involves firing zirconia to elevated temperatures for an extended time that can take several hours. The aim of this study is to investigate the effect of firing temperature and firing duration on the hardness of dental zirconia to indicate the optimum sintering conditions.

METHODS

Thirty-six zirconia specimens in shape of bars were randomly assigned to nine groups. The zirconia specimen groups were sintered using a sintering furnace with different firing temperatures (900°C, 1200°C, and 1800°C) and firing durations (6, 9, and 12 h). A total of 108 hardness measurements were conducted for all specimens (12 hardness readings per group). For each of the specimen groups, micro Vickers hardness test was performed using a load of 1 Kgf (9.807 N) and the Vickers hardness number was computed. Statistical analysis using Kruskal-Wallis test was conducted to examine the significant differences on Vickers hardness number HV among the specimen groups according to the firing parameters with 0.005 p-value used as an indicator.

RESULTS

Results suggest that there is an association between the increase in the hardness number and the increase in firing duration at a given firing temperature. The results also indicate that there is an association between the increase in the hardness number and increase in firing temperature at a given firing duration.

CONCLUSIONS

The greatest rate of hardness increase with time is associated with groups of firing temperature 1200°C. The highest rate of hardness increase with temperature happened during the first 6 h of sintering process. On the other hand, there is no significant increase in the hardness number when increasing the firing temperature beyond 1200°C.

Influence different polishing systems on roughness and colour stability of chairside CAD/CAM blocks with laminate veneer thickness

OBJECTIVE

The aim of this study was to evaluate the effect of different polishing systems on the colour stability of different laminate veneer blocks for chair-side CAD-CAM after ultraviolet (UV) ageing.

METHODS

About 240 sample (13 × 13 × 0.7) were prepared from four different CAD-CAM blocks IPS e.max CAD (IP), IPS Empress CAD (IE), Mark II (M) and Lava Ultimate (LU)) with A1 shade. Each group was divided into five subgroups (n = 12) according to the control (C) and four different polishing system: Sof-Lex (S), Edenta (E), Identoflex (I) and Zircon Brite (Z). Surface roughnesses (Ra) were measured by a profilometer before and after polishing. The baseline colour values were recorded according to the CIELab system. The colour coordinates (L*, a* and b*) of the samples were measured before and after UV ageing, and colour differences (ΔE₀₀) were calculated by using the CIEDE2000 colour difference formula. A two-way ANOVA and Tukey test methods were used to analyse the data (α = 0.05).

RESULTS

A significant interaction was observed between CAD-CAM blocks and polishing system (p < 0.001). The higher Ra and colour difference was calculated in LU in the Z group after polished and the lower Ra and colour difference was calculated in the IP in the S group after polished. For the IP group, the glazing procedure showed higher Ra and ΔE values with Z group than polished S system, whereas there is no statistical differences.

CONCLUSION

It is concluded that polishing with the Sof-Lex system improves significantly smoothness and colour stability. However, there is no success in using the Zircon Brite polishing system.

Influence of different combinations of CAD-CAM crown and customized abutment materials on the force absorption capacity in implant supported restorations - In vitro study

OBJECTIVES

To evaluate the force absorption capacity of implant supported restorations utilizing different CAD-CAM materials for the fabrication of crowns and customized abutments.

METHODS

80 titanium inserts were scanned to design customized abutments and crowns. The specimens were divided into four groups (n = 20/material): (Z): zirconia, (P): PEEK, (V): VITA Enamic, and (E): IPS e.max. Each group was subdivided into two subgroups according to customized abutment material: (Z) zirconia, and (P) for PEEK. For the assessment of force absorption, all specimens were loaded in a universal testing machine, applied loads curves were collected from the machine's software, and resulting loads curves were collected from forcemeter below the assembly. The slopes of all curves were analyzed using Two-way multivariate analysis of variance with pairwise comparisons using Tukey Post Hoc test (p < 0.05).

RESULTS

The curve progression of the applied and resulting forces varied among the investigated materials for each specimen. For zirconia abutments, ZZ showed the highest slope values of the applied and resulting force curves, followed by EZ, VZ, and PZ demonstrating statistically significant differences (P < .001). As for PEEK abutments, ZP and EP showed the least slope values, followed by PP then VP demonstrating statistically significant differences (P < .001). For Zirconia and e.max crowns, using PEEK abutments significantly increased slope loss. As for PEEK and Vita Enamic crowns changing abutment material did not significantly affect slope loss.

SIGNIFICANCE

Combining rigid crown materials with less rigid abutments might enhance their force absorption capacity. However, with less rigid crown materials a stiff substructure might be mandatory to preserve their force absorption behavior.

Surface Roughness and Translucency of Various Translucent Zirconia Ceramics after Hydrothermal Aging

OBJECTIVE

 This study investigated the effect of hydrothermal aging on surface roughness and translucency of various translucent zirconia materials.

MATERIALS AND METHODS

 Four types of zirconia were tested. Group 1 was translucent zirconia with no cubic structure. Group 2, 3, and 4 included cubic-containing zirconia with different amounts of cubic structures (less than 30%, 30-50%, and more than 50%, respectively). Each group contained 15 disk-shape specimens with dimensions of 15 mm in diameter and 1 mm in thickness. As-sintered surface roughness, translucency parameter, and contrast ratio were evaluated in the two different sessions, before and after aging.

STATISTICAL ANALYSIS

 Two-way repeated measures ANOVA with Bonferroni test was used to analyze statistically significant difference in those tested parameters. Phase structure before and after aging was analyzed by X-ray diffraction analysis (XRD).

RESULTS

 Groups 1 and 2 showed significant increased surface roughness after aging while groups 3 and 4 showed no alteration of surface. There was no effect of aging on translucency in all groups. After aging, group 1 and 2 presented monoclinic structure (16.63 and 5.01%, respectively).

CONCLUSION

 Hydrothermal aging caused phase transformation and increasing surface roughness in group 1 and 2 but did not affect translucency in all groups.

Monolithic zirconia crowns: effect of thickness reduction on fatigue behavior and failure load

PURPOSE

The objective of this study was to evaluate the effect of thickness reduction and fatigue on the failure load of monolithic zirconia crowns.

MATERIALS AND METHODS

140 CAD-CAM fabricated crowns (3Y-TZP, inCorisTZI, Dentsply-Sirona) with different ceramic thicknesses (2.0, 1.5, 1.0, 0.8, 0.5 mm, respectively, named G2, G1.5, G1, G0.8, and G0.5) were investigated. Dies of a mandibular first molar were made of composite resin. The zirconia crowns were luted with a resin composite cement (RelyX Unicem 2 Automix, 3M ESPE). Half of the specimens (n = 14 per group) were mouth-motion-fatigued (1.2 million cycles, 1.6 Hz, 200 N/ 5 – 55℃, groups named G2-F, G1.5-F, G1-F, G0.8-F, and G0.5-F). Single-load to failure was performed using a universal testing-machine. Fracture modes were analyzed. Data were statistically analyzed using a Weibull 2-parameter distribution (90% CI) to determine the characteristic strength and Weibull modulus differences among the groups.

RESULTS

Three crowns (21%) of G0.8 and five crowns (36%) of G0.5 showed cracks after fatigue. Characteristic strength was the highest for G2, followed by G1.5. Intermediate values were observed for G1 and G1-F, followed by significantly lower values for G0.8, G0.8-F, and G0.5, and the lowest for G0.5-F. Weibull modulus was the lowest for G0.8, intermediate for G0.8-F and G0.5, and significantly higher for the remaining groups. Fatigue only affected G0.5-F.

CONCLUSION

Reduced crown thickness lead to reduced characteristic strength, even under failure loads that exceed physiological chewing forces. Fatigue significantly reduced the failure load of 0.5 mm monolithic 3Y-TZP crowns.