Morphology and Mechanical Properties of 3Y-TZP Nanofiber Mats

The mats of yttria-stabilized tetragonal zirconia nanofibers were prepared using electrospinning. The effect of calcination temperature in the range of 600–1200 °C on their microstructure, phase composition and mechanical properties was investigated. Phase composition of the nanofibers did not change in all ranges of the calcination temperatures, while the average grain size increased from 8 to 39 nm. Nanoindentation testing of the mats showed a decrease in the hysteresis loop energy in samples with higher calcination temperature. Hardness and the elastic modulus measured with the indentation technique were the highest for the mats calcined at 900 °C.

Mechanical properties enhancement of self-cured PMMA reinforced with zirconia and boron nitride nanopowders for high-performance dental materials

We report on the mechanical properties regarding self-cured acrylic polymethyl methacrylate (PMMA) reinforced with hexagonal boron nitride (h-BN) and stabilized zirconia (8Y ZrO₂) nanopowders. The nanocomposites were prepared by using both manual and ultrasonic mixing techniques. The fabricated specimens were subjected to micro indentation, bending strength, and modulus of elasticity measurements. A fully complete polymerization process under liquid monomer was provided by ultrasonic mixing as evidenced by Fourier transform infrared (FTIR) measurement. Independently of the nanopowder used, the hardness, bending strength, and modulus of elasticity of the formed nanocomposites highly increase in values with the increase of the filler concentrations. Higher bending strengths and modulus of elasticity of the nanocomposite were recorded when using h-BN nanopowder fillers whereas hardness increases when using 8Y ZrO₂ nanopowder. Results showed that with respect to the unloaded specimens made by manual mixing, ultrasonic mixing of PMMA with a 5 wt% h-BN increased the flexural strength (FS) and the modulus of elasticity or Young's modulus (YM) values to about 550% and 240%, respectively. However, a similar concentration of 8Y ZrO₂ increased the Vickers Hardness numbers (VH) to about 400%. This may suggest that PMMA loaded with a combination of h-BN and 8Y ZrO₂ nanopowders may lead to nanocomposites with outstanding mechanical performance.

Influence of ZrO2 Addition on Structural and Biological Activity of Phosphate Glasses for Bone Regeneration

Zirconium doped calcium phosphate-based bioglasses are the most prominent bioactive materials for bone and dental repair and regeneration implants. In the present study, a 8ZnO–22Na₂O–(24 − x)CaO–46P₂O₅–xZrO₂ (0.1 ≤ x ≤ 0.7, all are in mol%) bioglass system was synthesized by the conventional melt-quenching process at 1100 °C. The glass-forming ability and thermal stability of the glasses were determined by measuring the glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m), using differential thermal analysis (DTA). The biological activity of the prepared samples was identified by analyzing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy dispersive spectra (SEM-EDS), before and after immersion in simulated body fluid (SBF) for various intervals of 0, 1 and 5 days, along with the magnitude of pH and the degradation of glasses also evaluated. The obtained results revealed that the glass-forming ability and thermal stability of glasses increased with the increase in zirconia mol%. The XRD, FTIR, and SEM-EDS data confirmed a thin hydroxyapatite (HAp) layer over the sample surface after incubation in SBF for 1 and 5 days. Furthermore, the development of layer found to be increased with the increase of incubation time. The degradation of the glasses in SBF increased with incubation time and decreased gradually with the increase content of ZrO₂ mol% in the host glass matrix. A sudden rise in initial pH values of residual SBF for 1 day owing to ion leaching and increase of Ca²⁺ and PO₄³⁻ ions and then decreased. These findings confirmed the suitability of choosing material for bone-related applications.

Review of the Modern Dental Ceramic Restorative Materials for Esthetic Dentistry in the Minimally Invasive Age

Material selection is one of the most important decisions to be made by clinicians. Proper material selection can affect the long-term function, longevity, and esthetics of restorations. There are a large number of restorative materials available, which has increased the complexity of the decision-making process. Improper material selection can lead to failures in the outcome. This article is designed to provide the practitioner with up-to-date practical information on ceramic restorative materials and techniques in a clear, evidence-based, and unbiased manner. It also provides decision-making guides to help the practitioner determine the best ceramic material for various clinical scenarios.

Effect of zirconia-mullite incorporated biphasic calcium phosphate/biopolymer composite scaffolds for bone tissue engineering

New bioactive scaffolds with improved mechanical properties, biocompatibility and providing structural support for bone tissue are being developed for use in the treatment of bone defects. In this study, we have synthesized bioactive scaffolds consisting of biphasic calcium phosphate (BCP) and zirconia-Mullite (2ZrO₂·[3Al₂O₃ ·2 SiO₂] (ZAS)) (BCPZAS) combined with polymers matrix of polycaprolactone (PCL)-alginate (Alg)-chitosan (Chi) (Chi/Alg-PCL) (BCPZAS@Chi/Alg-PCL). The composite material scaffolds were prepared by a blending technique. The microstructure, mechanical, bioactivity and in vitro biological properties with different ratios of BCP to ZAS of 1:0, 3:1, 1:1, 1:3 and 0:1 wt% in polymer matrix were analyzed. Microstructure analysis showed a successful incorporation of the BCPZAS particles with an even distribution of them within the polymer matrix. The mechanical properties were found to gradually decrease with increasing the ratio of ZAS particles in the scaffolds. The highest compressive strength was 42.96 ± 1.01MPa for the 3:1 wt% BCP to ZAS mixing. Bioactivity test, the BCPZAS@Chi/Alg-PCL composite could induce apatite formation in simulate body fluid (SBF). In-vitro experiment using UMR-106 osteoblast-like cells on BCPZAS@Chi/Alg-PCL composite scaffold showed that there is cell attachment to the scaffolds with proliferation. These experimental results demonstrate that the BCPZAS@Chi/Alg-PCL composite especially for the BCP:ZAS at 3:1 wt% could be utilized as a scaffold for bone tissue engineering applications.

Effect of surface treatment on the retention of zirconia crowns to tooth structure after aging

OBJECTIVE

To evaluate the effect of different surface treatments on the retention between zirconia crowns and tooth structure after in vitro aging.

MATERIALS AND METHODS

Human third molars (n = 44) received crown preparations and CAD/CAM zirconia crowns were manufactured. Specimens were divided into two groups: no aging, control (NAC, n = 11) or aging (A, n = 33). NAC were bonded with universal adhesive (UA). Aged specimens were divided into three subgroups (n = 11) according to surface treatment: Control: no abrasion + UA; Alumina: alumina abrasion + UA; Silica: tribochemical silica coating + UA. The crowns were cemented with dual-cure resin cement. Specimens in group A were aged by a combination of mechanical, thermal, and pH cycling. Retention strength values were obtained by tensile tests and results were analyzed by one-way analysis of variance (ANOVA) and Tukey test (P < .05).

RESULTS

Aging decreased the retention strength in control specimens (P < .001). Surface treatment improved the retention strength of aged specimens (P < .001), with similar results between alumina and tribochemical silica coating.

CONCLUSION

The chemical interaction between the universal bonding system and zirconia's surface was not sufficient to withstand artificial aging. Tribochemical silica coating did not promote additional retention in comparison to alumina blasting.

CLINICAL SIGNIFICANCE

The long-term retention of translucent zirconia crowns to tooth structure using phosphate-based materials is improved by means of mechanical surface treatments such as alumina blasting and tribochemical silica coating.

Effect of ceramic material type on the fracture load of inlay-retained and full-coverage fixed dental prostheses

Objective: Ceramic inlay-retained fixed partial denture (IRFPD) is a conservative prosthetic option but the mechanical durability of new high strength zirconia reinforced glass ceramic FPDs is not investigated. The purpose of this study was to compare fracture load of 3-unit ceramic FPDs. Materials and methods: Extracted premolars and molars (N = 64) were used to create three test groups (IRFPDs) and one control group (full coverage FPD) (n = 8). The teeth were embedded in PMMA resin with a mesiodistal distance of 6 mm. Premolars had a distal and molars had a mesial inlay preparation (width: 3 mm; height: 4 mm) in the test groups. IRFPDs were made from a zirconia reinforced lithium silicate (VS) or a monolithic zirconia. Zirconia IRFPDs received 2 types of surface treatments: sandblasting (Zr-IRFPD) or internal coating with feldspathic porcelain (ZrC-IRFPD). Control group was made from monolithic zirconia with the same connector size and zirconia surfaces were sandblasted (Zr-FPD). All restorations were cemented using a resin luting cement. After 5000 thermo-cycles, fracture load values (N) were determined with a universal testing machine at a crosshead speed of 0.75 mm/min. Data were analyzed using 1-way ANOVA and Tukey`s post hoc test (p ˂ .05). Result: Fracture load (mean ± SD) of Zr-FPDs, Zr-IRFPDs and ZrC-IRFPDs were 672 ± 183, 672 ± 123 and 638 ± 59, respectively, being not statistically different (p > .05). VS-IRFPD exhibited statically lower values (391 ± 136). The predominant mode of failure was fracture at the connector area in all groups. Conclusion: The fracture load of 3-unit IRFPD was significantly affected by types of ceramics but the retainer design and surface treatment in Zr groups did not show a significant effect.

Ceramic Materials and Technologies Applied to Digital Works in Implant-Supported Restorative Dentistry

Computer-aided design and manufacturing technology has been closely associated with implant-supported restoration. The digital system employed for prosthodontic restorations comprises data acquisition, processing, and manufacturing using subtractive or additive methods. As digital implantology has developed, optical scanning, computer-based digital algorithms, fabricating techniques, and numerical control skills have all rapidly improved in terms of their accuracy, which has resulted in the development of new ceramic materials with advanced esthetics and durability for clinical application. This study reviews the application of digital technology in implant-supported dental restoration and explores two globally utilized ceramic restorative materials: Yttria-stabilized tetragonal zirconia polycrystalline and lithium disilicate glass ceramics.

In-vitro polishing of CAD/CAM ceramic restorations: An evaluation with SEM and confocal profilometry

AIM

The objective of this In-vitro investigation was to analyze and compare the surface after polishing of 63 all-ceramic restorations fabricated out of monolithic zirconium dioxide (ZIR), lithium disilicate (LS) or feldspathic ceramic (FC) under standardized laboratory conditions with different protocols. The primary outcome was defined as the roughness (R_a/S_a) of different ceramic surfaces after distinctive polishing procedures.

MATERIAL AND METHODS

The study set-up consisted of three main groups: ZIR, LS, FC (20 crowns each), every group divided into two sub-groups (10 crowns each) depending on the polishing method. The untouched glazed surface of one crown per material served as a control. Every crown displayed a defined supra-contact at the palatal cusp which was removed with a fine grain (38-45 μm) diamond bur. Surface polishing was carried out with either a two-step system (one kit for zirconium dioxide (ZIR₂), another kit for lithium disilicate (LS₂) and feldspatic ceramic (FC₂)), or a three-step system (ZIR₃, LS₃, FC₃) under standardized conditions. Roughness parameters (R_a and S_a) were measured by means of confocal profilometry. Specimens were also visually inspected with scanning electron microscopy (SEM). Statistical analyses were performed using SPSS Statistics software.

RESULTS

Visual examination of the specimens using SEM showed several inhomogeneities on the glazed surface of the control samples, i.e. pores and particles. On every test sample, the grinding curves of the diamond bur were still recognizable. Polishing revealed similar median R_a 0.491 μm (ZIR₂) and 0.434 μm (LS₂) after two-step polishing (p = 0.754), and 0.311 μm (ZIR₃) and 0.208 μm (LS₃) after three-step polishing (p = 0.917). Surface roughness in group FC measured 0.889 μm (FC₂) after the two-step polishing process and 0.903 μm (FC₃) following three-step surface refinement. No significant difference was detectable between surface roughness of glazed controls compared to either polished surfaces with two-step or three-step treatment within one material. ZIR and LS presented significantly lower median roughness R_a after two-step and three-step procedures than test samples of FC, measured subsequent to either of the polishing methods (p = 0.016, p = 0.009).

CONCLUSION

The surface roughness of ZIR, LS and FC crowns after the use of chairside polishing kits was comparable with the roughness measured before occlusal adjustment. A two-step procedure showed as good results as a three-step process. A smoother surface was obtained for ZIR and LS compared to FC with both polishing protocols.

Preparation, Microstructure, Mechanical Properties and Biocompatibility of Ta-Coated 3Y-TZP Ceramic Deposited by a Plasma Surface Alloying Technique

A Ta coating has been successfully fabricated on the surface of zirconia polycrystals ceramic (3 mol% yttria, 3Y-TZP) by a plasma surface alloying technique. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that a α-Ta coating with a continuous and compact surface morphology which consisted of a deposited layer with a thickness of 390 nm and a diffusion layer with a thickness of 200 nm covered the 3Y-TZP. Due to the effect of inhabitation the t→m transformation by the deposited Ta coating, the biaxial flexural strength caused by the phase transformation during hydrothermal aging is reduced e.g. p < 0.05 after 20 h and/or 100 h. In addition, the Ta coating shows non-cytotoxicity and improved proliferation ability of osteoblasts.

Suitability of 3D printed pieces of nanocrystalline zirconia for dental applications

OBJECTIVES

The main goal of this work is to evaluate the suitability of nanostructured zirconia pieces obtained by robocasting additive manufacturing (AM), for dental applications.

METHODS

The density, crystalline structure, morphology/porosity, surface roughness, hardness, toughness, wettability and biocompatibility of the produced samples were compared with those of samples obtained by conventional subtractive manufacturing (SM) of a similar commercial zirconia material. Chewing simulation studies were carried out against dental human cusps in artificial saliva. The wear of the material was quantified and the wear mechanisms investigated, as well as the influence of glaze coating.

RESULTS

AM samples, that revealed to be biocompatible, are slightly less dense and more porous than SM samples, showing lower hardness, toughness and wettability than SM samples. After chewing tests, no wear was found both on AM and SM samples. However, the dental wear was significantly lower when AM samples were used as counterbody. Concerning the glazed samples, both coated surfaces and dental cusps suffered wear, being the cusps' wear higher than that found for unglazed samples. More, cusps tested against AM coated samples suffered less wear comparatively to those opposed to SM coated samples.

SIGNIFICANCE

Overall, the results presented in this paper show that AM processed nanostructured zirconia can be used in dental restorations, with important advantages from the point of view of processing and tribological performance. Moreover, the option for glaze finishing should be carefully considered both in SM and AM processed specimens.

Designed single-phase ZrO2 nanocrystals obtained by solvothermal syntheses

Crystal growth pathways controlled by the acidity, type and concentration of the capping agent lead to different nanostructures and crystalline phases.

Effect of Implant Platform Connection and Abutment Material on Removal Torque and Implant Hexagon Plastic Deformation

Objectives  The aim of this study was to evaluate the plastic deformation of the hexagonal connection, and the removal torque of the implant-abutment joint of two dental implants combined with internal or external hexagonal connection implants after mechanical cycling.

Materials and Methods  Twenty-four dental implants were used in the study. Half of the implants had internal hexagonal connections (IH; Titamax II Plus) and the other half had external hexagonal connections (EH; Titamax Ti Ex). Four groups of two types of dental implant abutments (titanium: Ti, UCLA II Plus and zirconia: Zr, fabricated by CAD/CAM; n = 6) were investigated. The abutments received a metallic crown and the settings were submitted to mechanical cycling (MC; 10 ⁶ cycles, axial load, 120N). The connection surface area was measured by scanning electron microscope (SEM) images. The removal torque was evaluated and the plastic deformation of the hexagonal surface of the implant was measured by comparing the images before and after MC.

Statistical Analysis  Paired- t test was used to analyze the data statistically at a significance level of α = 0.05.

Results  The torque values decreased for all groups after MC, and the hexagonal surface area decreased due to plastic deformation for IH and EH associated with Zr abutments.

Conclusions  Zirconia abutments showed the worst plastic deformation of the implant connection surface and torque loosening when associated with IH implant.

"CAD-on" Interfaces - Fracture Mechanics Characterization

PURPOSE

To apply fracture mechanics methodology to determine the interfacial fracture toughness of the interfaces present in "CAD-on" crowns consisting of CAD/CAM milled lithium disilicate veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal framework.

MATERIALS AND METHODS

The notchless triangular prism specimen fracture toughness test was used to determine interfacial fracture toughness. Four groups, each consisting of (6 × 6 × 6 × 12) mm prisms (n = 22), were produced. Half-size [(6 × 6 × 6 × 6) mm] specimens of IPS e.max CAD and IPS e.max ZirCAD were approximated under vibration with Crystal Connect fusing glass and sintered according to manufacturer's guidelines to obtain the following three interfaces: (1) e.max CAD/Crystal Connect/e.max CAD (Group I); (2) Zir CAD/Crystal Connect/Zir CAD (Group II); and (3) Zir CAD/Crystal Connect/e.max CAD (Group III). For Group IV (control, based on the "press-on" veneering technique), half-size [(6 × 6 × 6 × 6) mm] IPS e.max ZirCAD prisms were coated with ZirLiner and pressed with IPS e.max ZirPress ingots to obtain (6 × 6 × 6 × 12) mm prisms. All specimens were tested using a computer controlled material testing machine. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α = 0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured surfaces were characterized under a scanning electron microscope.

RESULTS

All experimental groups demonstrated a cohesive mode of failure in the fusing glass layer. The number and size of defects appeared to correlate with the variability of fracture toughness values. There were no significant differences between the fracture toughness of the "CAD-on" interfaces (p = 0.052). The results suggested that the fracture toughness of Crystal Connect limited the interfacial fracture toughness values. The "CAD-on" fracture toughness value (Group III) was significantly greater than that of the ZirPress "press-on" control (Group IV) (p < 0.001).

CONCLUSION

The "CAD-on" process results in stronger bonding between veneer and framework, compared to conventional veneering. The clinical use of "CAD-on" crowns could therefore be advocated. The selection of any restorative material requires a thorough analysis of advantages, limitations and results from clinical studies to inform the clinical decision in a case-by-case approach.

Microstructure and Phase Composition of Yttria-Stabilized Zirconia Nanofibers Prepared by High-Temperature Calcination of Electrospun Zirconium Acetylacetonate/Yttrium Nitrate/Polyacrylonitrile Fibers

For the first time, dense nanofibers of yttria-stabilized tetragonal zirconia with diameter of ca. 140 nm were prepared by calcination of electrospun zirconium acetylacetonate/yttrium nitrate/polyacrylonitrile fibers at 1100–1300 °C. Ceramic filaments were characterized by scanning electron microscopy, X-ray diffractometry, and nitrogen adsorption. With a rise in the calcination temperature from 1100 to 1300 °C, the fine-grain structure of the nanofibers transformed to coarse-grain ones with the grain size equal to the fiber diameter. It was revealed that fully tetragonal nanofibrous zirconia may be obtained at Y2O3 concentrations in the range of 2–3 mol% at all used calcination temperatures. The addition of 2–3 mol% yttria to zirconia inhibited ZrO2 grain growth, preventing nanofibers’ destruction at high calcination temperatures. Synthesized well-sintered, non-porous, yttria-stabilized tetragonal zirconia nanofibers can be considered as a promising material for composites’ reinforcement, including composites with ceramic matrix.

Impact of different pretreatments and aging procedures on the flexural strength and phase structure of zirconia ceramics

OBJECTIVE

To test the impact of zirconia pretreatment and aging on flexural strength and phase structure.

METHODS

For flexural strength measurements, 180 3Y-TZP_0.25 specimens were fabricated and pretreated: (i) air-abraded (105-μm alumina, 0.25MPa), (ii) air-abraded (50-μm alumina, 0.25MPa), (iii) air-abraded (30-μm silica-coated alumina, 0.28MPa) (iv) non-pretreated. Each pretreated group (n=15) was aged: (a) hydrothermal (134°C, 0.23MPa, 2h) (b) in a mastication simulator (1,200,000×, 5/55°C) and (c) not aged. The fractured specimens were stored dry for 5 years (23°C) for analysis of phase transformation. Additionally, specimens were fabricated from 3Y-TZP_0.25 (n=12) and 3Y-TZP_0.05 (n=8), pretreated (i, ii, iii, iv), and hydrothermally aged. Each air-abrasion method was alternated using 0.05, 0.25 and 0.4MPa pressure. The phase transformation was examined by Raman spectroscopy and surface topography by scanning electron microscope. Data were analyzed using univariate ANOVA with the Scheffé post hoc test and partial-eta-squared (ƞ_p²) (α=0.05).

RESULTS

The highest impact on flexural strength was exerted by the pretreatment (η_P²=0.261, p<0.001), followed by interactions between pretreatment and aging (η_P²=0.077, p=0.033). Non-pretreated and non-aged specimens showed the lowest monoclinic percentage. Hydrothermal aging and 5 years of storage at room temperature increased the monolithic percentage of 3Y-TZP_0.25. The highest phase transformation was observed in groups air-abraded with 105-μm alumina particles. Increasing pressure during the air-abrading process increased the content of the monoclinic phase in zirconia surfaces.

SIGNIFICANCE

Air-abrasion with 30-μm silica-coated alumina powder can be recommended for pretreatment of 3Y-TZP_0.25 and 3Y-TZP_0.05. For air-abrasion using alumina powder lower pressure should be used.

Comparative Effectiveness of Multiple Laser Scanning and Conventional Techniques on Zirconia Shear Bond Strength

This study aimed to compare the impact of different laser scanning with that of conventional methods on zirconia surface treatment through evaluation of shear bond strength (SBS) values. One hundred and thirty-two sintered zirconia cubic-samples were prepared and randomly divided into six study groups: milling control (without surface treatment); grinding; sandblasting; and three-times, four-times, and five-times laser scanning groups. The treatment process for the first three groups was performed before the zirconia coating, while the last three groups were treated after zirconia coating with veneer slurry through a spraying technique. In the current study, the surface roughness Ra, contact angle measurement, phase transformation, topography and interfaces, SBS in unaged and aged conditions, and fracture mode patterns of zirconia cores were investigated. The results were analyzed using laser confocal scanning microscopy, drop analyzer, X-ray diffractometry (XRD), scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), universal testing machine and stereomicroscope. The results indicated that three-times laser scanned specimens presented higher Ra values than the other studied groups. The minimum contact angle value was detected in the mentioned group, while the control group presented the highest value. The XRD showed phase transformation from tetragonal to monoclinic t–m following grinding and sandblasting. However, the laser scanned specimens and the control group preserved the structural integrity of the zirconia core, presenting the tetragonal phase only. The highest SBS values were recorded in specimens treated with three-times laser scanning in the unaged and aged conditions. A mixed fracture was a common fracture pattern among the studied groups. The results confirmed that SBS could be optimized through three-times laser scanning and it provided better adhesion between zirconia and the veneer ceramic material. Multiple scanning processes of more than three times are not recommended for zirconia surface treatment.

Collective substitutions of selective rare earths (Yb3+, Dy3+, Tb3+, Gd3+, Eu3+, Nd3+) in ZrO2: an exciting prospect for biomedical applications

The study aims to understand the significance of collective rare earth (RE3+) substitutions in ZrO2 structures for biomedical applications. The RE3+ ions namely Yb3+, Dy3+, Tb3+, Gd3+, Eu3+, and Nd3+ were selected and their concentrations were adjusted to obtain three different combinations. The influence of RE3+ on the crystal structure of ZrO2 alongside the absorption, luminescence, mechanical, magnetic, computed tomography (CT), magnetic resonance imaging (MRI) properties was explored. The concomitant effect of the average ionic size and RE3+ concentration determines the crystallization behavior of ZrO2 at elevated temperatures. The collective RE3+ substitutions exhibit both up-conversion and down-conversion emissions with their respective excitation at 793 and 350 nm. Nevertheless, increment in the concentration of RE3+ is found to be detrimental to the mechanical stability of ZrO2. The collective characteristics of multiple RE3+ demonstrate the potential of the investigated system in multimodal imaging applications. The unique luminescence characteristics of Eu3+ and Tb3+ are promising for fluorescence imaging while the presence of Dy3+, Tb3+, Gd3+ and Nd3+ unveils a paramagnetic response required for MRI. In addition, Dy3+ and Yb3+ contribute to the high X-ray absorption coefficient values suitable for X-ray CT imaging.

Additive manufacturing of ceramics for dental applications: A review

OBJECTIVE

The main goal of this review is to provide a detailed and comprehensive description of the published work from the past decade regarding AM of ceramic materials with possible applications in dentistry. The main printable materials and most common technologies are also addressed, underlining their advantages and main drawbacks.

METHODS

Online databases (Web of knowledge, Science Direct, PubMed) were consulted on this topic. Published work from 2008 to 2018 was collected, analyzed and the relevant papers were selected for inclusion on this review.

RESULTS

Ceramic materials are broadly used in dentistry to restore/replace damaged or missing teeth, due to their biocompatibility, chemical stability and mechanical and aesthetic properties. However, there are several unmet challenges regarding their processing and performance. Due to their brittleness nature, a very tight control of the manufacturing process is needed to obtain dental pieces with adequate mechanical properties. Additive manufacturing (AM) is an emerging technology that constitutes an interesting and viable manufacturing alternative to the conventional subtractive methods. AM enables the production of customized complex 3D parts in a more sustainable and less expensive way. AM of ceramics can be achieved with an extensive variety of methods.

SIGNIFICANCE

There is no perfect technology for all materials/applications, capable alone of fulfilling all the specificities and necessities of every patient. Although very promising, AM of ceramic dental materials remains understudied and further work is required to make it a widespread technology in dentistry.

Structure, luminescence, mechanical and in vitro behavior of zirconia toughened alumina due to terbium substitutions

The significance of Tb³⁺ inclusions at the zirconia toughened alumina (ZTA) structure was explored. The influence of Tb³⁺ content at the crystal structures of ZrO₂ and Al₂O₃ and the resultant optical, mechanical, magnetic and cytotoxicity properties were deliberated. The critical role of Tb³⁺ to attain a structurally stable ZTA until 1500 °C is ensured. Depending on the Tb³⁺ content, either tetragonal zirconia (t-ZrO₂) or cubic zirconia (c-ZrO₂) structures were stabilized while the propensity of Tb³⁺ reaction with Al₂O₃ to yield TbAlO₃ is transpired only after exceeding the occupancy limit in ZrO₂. The green emission and paramagnetic features are imparted by the Tb³⁺ inclusions at the ZTA structure. Dense and pore free microstructures with a direct impact on the improved mechanical features of ZTA is empowered by the presence of Tb³⁺. Further, the results from MTT assay and live/dead cell staining ensured the negligence of Tb³⁺ contained ZTA systems to induce toxicity.

Enhanced Cell Response to Zirconia Surface Immobilized with Type I Collagen

Zirconia (ZrO₂) dental implants provide good biocompatibility, have good corrosion resistance, and have a color that is similar to that of natural teeth. Unfortunately, ZrO₂ is a bioinert material and therefore achieves osseointegration difficultly. In this study, we sought to enhance osseointegration by producing rough ZrO₂ surfaces that contain hydroxyl groups (designated ZSA) through the use of sandblasting in conjunction with alkaline treatment. We immobilized type I collagen on ZSA surfaces using the natural cross-linker, procyanidin. Our results further showed that surfaces produced in ZSA-P/C featured more and steadier type I collagen than surfaces produced in ZSA-C. The ZSA-P/C also presented superior cell responses in terms of adhesion, proliferation, and mineralization of human bone marrow mesenchymal stem cells. The enhanced cell responses in the ZSA-P/C were induced through the prolonged activation of focal adhesion kinase, AKT (the phosphoinositide 3-kinase pathway), and p38 (the mitogen-activated protein kinase pathway). The simple and novel approach to immobilize type I collagen on roughened ZrO₂ surfaces presented in this article can likely benefit dental implant applications.

Response of pre-crystallized CAD/CAM zirconia-reinforced lithium silicate glass ceramic to cyclic nanoindentation

This paper reports on a cyclic nanoindentation approach of a pre-crystallized zirconia-reinforced lithium silicate glass ceramic (ZLS) to understand its material behavior associated with its machinability in dental CAD/CAM diamond abrasive milling. The material response to the cyclic nanoindentation using a Berkovich diamond tip was quantitatively determined in terms of the indentation contact hardness, elastic modulus, elasticity and plasticity for each loading cycle at peak loads of 2.5-10 mN. The cyclic load-displacement curves at different load levels indicate discrete discontinuities and hysteresis loops, which might have arisen from viscoelasticity behavior. Material properties degraded with an increasing number of loading cycles due to the mechanical softening which may facilitate machining in dental CAD/CAM milling. Elastic and plastic displacements and indentation energies revealed the pre-crystallized ZLS experiences predominantly elastic deformation and thus has a high capacity to retain its structure and shape. Furthermore, elastic energy dominated cyclic loading led to pseudoelasticity due to plastic strain accumulation. In situ scanning probe microscopy (SPM) images of cyclic indentation imprints reveal the fracture-free plastic deformation of the pre-crystallized ZLS under cyclic nanoindentation conditions. The outcomes of this study provide the mechanics model of diamond milling of the pre-crystallized ZLS due to the cyclic loading nature of dental CAD/CAM abrasive processing.

Structural, charge density and bond length variations in c-Y2O3 influenced by progressive cerium additions

Cerium occupancy induces expansion of the cubic Y₂O₃ unit cell and yields a dense microstructure alongside enhanced mechanical properties.

A low cost minimally invasive adhesive alternative for maxillary central incisor replacement

OBJECTIVE

The beneficial mechanical properties of zirconia, together with the adhesive techniques, provide a wide range of clinical applications, including the construction of thin structures and minimally invasive adhesive restorations.

CLINICAL CONSIDERATIONS

A periodontal disease-related missing maxillary central incisor, in a pregnant patient with chronic periodontal disease in the first trimester of pregnancy, and with type II diabetes treated with resin-bonded fixed partial denture using the missing tooth cemented to a zirconia framework and then bonded to the abutment teeth.

CONCLUSIONS

According to the systemic conditions presented in this case report, using a low-cost technology and the missing tooth it was possible to obtain an easier and satisfactory esthetic and functional result.

CLINICAL SIGNIFICANCE

The mechanical properties of zirconia made it possible to treat a pregnant patient with chronic periodontal disease in the first trimester of pregnancy, and with type II diabetes with a thin zirconia framework and minimally invasive adhesive restorations.

Comparison of Cytomorphometry and Early Cell Response of Human Gingival Fibroblast (HGFs) between Zirconium and New Zirconia-Reinforced Lithium Silicate Ceramics (ZLS)

New zirconia-reinforced lithium silicate ceramics (ZLS) could be a viable alternative to zirconium (Y-TZP) in the manufacture of implantological abutments-especially in aesthetic cases-due to its good mechanical, optical, and biocompatibility properties. Although there are several studies on the ZLS mechanical properties, there are no studies regarding proliferation, spreading, or cytomorphometry. We designed the present study which compares the surface, cellular proliferation, and cellular morphology between Y-TZP (Vita YZ® T [Vita Zahnfabrik (Postfach, Germany)]) and ZLS (Celtra® Duo [Degudent (Hanau-Wolfgang, Germany)]). The surface characterization was performed with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and optical profilometry. Human gingival fibroblasts (HGFs) were subsequently cultured on both materials and early cellular response and cell morphology were compared through nuclear and cytoskeletal measurement parameters using confocal microscopy. The results showed greater proliferation and spreading on the surface of Y-TZP. This could indicate that Y-TZP continues to be a gold standard in terms of transgingival implant material: Nevertheless, more in vitro and in vivo research is necessary to confirm the results obtained in this study.

Electrospun ZrO2 nanofibers: precursor controlled mesopore ordering and evolution of garland-like nanocrystal arrays

We observed that the hydrolysis-condensation reaction of precursors makes a significant difference in constructing ordered mesopores in electrospun ZrO2 nanofibers. Transmission-SAXS studies confirm the generation of uniform clusters of size ∼1.44 nm in the ZrOCl2·8H2O (inorganic salt) derived sol due to its relatively slow hydrolysis-condensation process. These initial -Zr-O-Zr- clusters acted as building blocks to form uniform 3D ordered cubic (Pm3[combining macron]m) mesopores in the presence of Pluronic F127 surfactant. In contrast, the commonly used Zr-alkoxide (zirconium n-propoxide) precursor, which is highly hydrolysable even after the use of a controlling agent, generates larger clusters with broad size distributions due to the uncontrolled hydrolysis-condensation of alkoxy groups. Accordingly, in the presence of F127, the alkoxide derived sol yielded disordered mesopores in the resultant fibers. XRD under dynamic heating conditions (up to 900 °C) and the corresponding TEM studies of the ZrOCl2·8H2O derived nanofibers confirmed the retention of mesopores even in the extremely thin nanofibers (diameter ∼15-25 nm) after the amorphous to crystal phase transformation (cubic/tetragonal). An interesting morphological transformation has been observed in the nanofibers at 900 °C where the fibers have been uniformly segmented by distinct single nanocrystals (width ∼15-65 nm) with mesopores. Further heat-treatment at 1100 °C made these segmented nanofibers nonporous, and a garland-like appearance with monoclinic nanocrystal arrays was formed.

Zirconia stimulates ECM-remodeling as a prerequisite to pre-osteoblast adhesion/proliferation by possible interference with cellular anchorage

The biological response to zirconia (ZrO₂) is not completely understood, which prompted us to address its effect on pre-osteoblastic cells in both direct and indirect manner. Our results showed that zirconia triggers important intracellular signaling mainly by governing survival signals which leads to cell adhesion and proliferation by modulating signaling cascade responsible for dynamic cytoskeleton rearrangement, as observed by fluorescence microscopy. The phosphorylations of Focal Adhesion Kinase (FAK) and Rac1 decreased in response to ZrO₂ enriched medium. This corroborates the result of the crystal violet assay, which indicated a significant decrease of pre-osteoblast adhesion in responding to ZrO₂ enriched medium. However, we credit this decrease on pre-osteoblast adhesion to the need to govern intracellular repertory of intracellular pathways involved with cell cycle progression, because we found a significant up-phosphorylation of Mitogen-Activated Protein Kinase (MAPK)-p38 and Cyclin-dependent kinase 2 (CDK2), while p15 (a cell cycle suppressor) decreased. Importantly, Protein phosphatase 2 A (PP2A) activity decreased, guaranteeing the significant up-phosphorylation of MAPK -p38 in response to ZrO₂ enriched medium. Complementarily, there was a regulation of Matrix Metalloproteinases (MMPs) in response to Zirconia and this remodeling could affect cell phenotype by interfering on cell anchorage. Altogether, our results show a repertory of signaling molecules, which suggests that ECM remodel as a pre-requisite to pre-osteoblast phenotype by affecting their anchoring in responding to zirconia.

The Role of Nanomechanics in Healthcare

Nanomechanics has played a vital role in pushing our capability to detect, probe, and manipulate the biological species, such as proteins, cells, and tissues, paving way to a deeper knowledge and superior strategies for healthcare. Nanomechanical characterization techniques, such as atomic force microscopy, nanoindentation, nanotribology, optical tweezers, and other hybrid techniques have been utilized to understand the mechanics and kinetics of biospecies. Investigation of the mechanics of cells and tissues has provided critical information about mechanical characteristics of host body environments. This information has been utilized for developing biomimetic materials and structures for tissue engineering and artificial implants. This review summarizes nanomechanical characterization techniques and their potential applications in healthcare research. The principles and examples of label-free detection of cancers and myocardial infarction by nanomechanical cantilevers are discussed. The vital importance of nanomechanics in regenerative medicine is highlighted from the perspective of material selection and design for developing biocompatible scaffolds. This review interconnects the advancements made in fundamental materials science research and biomedical technology, and therefore provides scientific insight that is of common interest to the researchers working in different disciplines of healthcare science and technology.

Probing the combined additions of Ca2+ and PO43− in the stabilized ZrO2 polymorph: structural, morphological and mechanical analysis

Selective uptake of Ca²⁺ at c-ZrO₂ lattice: additions beyond critical limit lead to β-Ca₃(PO₄)₂ crystallization in the presence of PO₄³⁻.

Fracture Resistance of Monolithic Glass-Ceramics Versus Bilayered Zirconia-Based Restorations

PURPOSE

To compare the fracture resistance of monolithic reinforced glass-ceramic restorations with bilayer zirconia-based restorations.

MATERIALS AND METHODS

Fifteen ceramic crowns were fabricated on epoxy dies duplicated from a stainless steel master die. They were divided into 3 equal groups (n = 5) according to the type of ceramic material used: group I, monolithic lithium disilicate (IPS e.max CAD), group V, monolithic zirconia-reinforced lithium silicate (Vita Suprinity), and group B (bilayered zirconia substructure with veneering ceramic). All specimens were cemented on epoxy dies with a self-adhesive resin cement (Rely X Unicem), subjected to a chewing simulator, and then loaded until fracture in a universal testing machine. The results were tabulated and statistically analyzed using one-way ANOVA to compare among the 3 materials. The Bonferroni post hoc test was used for pairwise comparisons when the ANOVA test was significant.

RESULTS

Zirconia-reinforced lithium silicate (Vita Suprinity) crowns showed the highest statistically significant (p < 0.05) mean fracture resistance values (1742.9 ± 102.7 N), followed by lithium disilicate (IPS e.max CAD) (1565.2 ± 89.7 N). Bilayered zirconia-based crowns showed the lowest statistically significantly mean fracture resistance values (1267.8 ± 86.1 N).

CONCLUSIONS

Monolithic reinforced glass-ceramics (lithium disilicate and zirconia-reinforced lithium silicate) have better fracture resistance than bilayered zirconia-based ceramics.

CLINICAL IMPLICATIONS

The use of monolithic reinforced ceramic restorations (lithium disilicate and zirconia-reinforced lithium silicate) is preferred to bilayered zirconia-based restorations to avoid chipping of the ceramic veneer.

Micro-Raman Vibrational Identification of 10-MDP Bond to Zirconia and Shear Bond Strength Analysis

So far, there is no report regarding the micro-Raman vibrational fingerprint of the bonds between 10-methacryloyloxy-decyl dihydrogen phosphate (10-MDP) and zirconia ceramics. Thus, the aim of this study was to identify the Raman vibrational peaks related to the bonds of 10-MDP with zirconia, as well as the influence on microshear bond strength. Micro-Raman spectroscopy was employed to assess the vibrational peak of 10-MDP binding to zirconia. Microshear bond strength of the dual-cure resin cement to zirconia with the presence of 10-MDP in composition of experimental ceramic primer and self-adhesive resin cement was also surveyed. Statistical analysis was performed by one-way ANOVA and Tukey's test (p < 0.05). Peaks at 1545 cm⁻¹ and 1562 cm⁻¹ were found to refer to zirconia binding with 10-MDP. The presence of 10-MDP in both experimental ceramic primer and self-adhesive resin cement improved microshear bond strength to zirconia ceramic. It can be concluded that the nondestructive method of micro-Raman spectroscopy was able to characterize chemical bonds of 10-MDP with zirconia, which improves the bond strengths of resin cement.

A CAD/CAM Zirconium Bar as a Bonded Mandibular Fixed Retainer: A Novel Approach with Two-Year Follow-Up

Stainless steel alloys containing 8% to 12% nickel and 17% to 22% chromium are generally used in orthodontic appliances. A major concern has been the performance of alloys in the environment in which they are intended to function in the oral cavity. Biodegradation and metal release increase the risk of hypersensitivity and cytotoxicity. This case report describes for the first time a CAD/CAM zirconium bar as a bonded mandibular fixed retainer with 2-year follow-up in a patient who is subjected to long-term treatment with fixed orthodontic appliance and suspected to have metal hypersensitivity as shown by the considerable increase of nickel and chromium concentrations in a sample of patient's unstimulated saliva. The CAD/CAM design included a 1.8 mm thickness bar on the lingual surface of lower teeth from canine to canine with occlusal rests on mesial side of first premolars. For better retention, a thin layer of feldspathic ceramic was added to the inner surface of the bar and cemented with two dual-cured cement types. The patient's complaint subsided 6 weeks after cementation. Clinical evaluation appeared to give good functional value where the marginal fit of digitized CAD/CAM design and glazed surface offered an enhanced approach of fixed retention.

Stabilization of a t-ZrO2 polymorph in a glassy SiO2 matrix at elevated temperatures accomplished by ceria additions

Cerium additions into the SiO₂–ZrO₂ binary system delay SiO₂ crystallization at elevated temperatures with Ce⁴⁺ occupancy at the t-ZrO₂ lattice for minor additions whereas its excess additions crystallize as discrete CeO₂.