A Narrative Review on Polycrystalline Ceramics for Dental Applications and Proposed Update of a Classification System

Novel bilayered zirconia systems using recycled 3Y-TZP for dental applications

OBJECTIVE

To synthesize bilayer zirconia systems based on commercial or recycled 3Y-TZP obtained from non-milled remnants and to compare their optical and mechanical properties before and after aging.

METHODS

Bilayer zirconia samples were fabricated using either recycled 3Y-TZP (3Y-R/4Y and 3Y-R/5Y) or commercial powders (3Y/4Y and 3Y/5Y). Microstructure and phase composition were analyzed using ScanningElectronMicroscopy (SEM) and X-Ray Diffraction (XRD). Optical and mechanical properties were assessed via reflectance and biaxial flexural strength tests (BFS), followed by fractographic analysis. Optical properties and BFS data were analyzed using two-way ANOVA and Tukey test, and Weibull statistics, respectively.

RESULTS

Recycled powder exhibited particle sizes < 2.07μm. SEM micrographs depicted dense surfaces with largest grains in the 5Y, followed by recycled-3Y, 4Y, and commercial-3Y. XRD analysis revealed tetragonal peaks in commercial and recycled 3Y-TZPs, and tetragonal and cubic phases in the 4Y and 5Y surfaces. Aging induced significant phase transformation in 4Y (∼40 %), commercial- (58 %) and recycled-3Y (53 %), with no effect in 5Y surfaces. Commercial bilayers exhibited higher translucency and strength (∼1130 MPa) compared to recycled bilayers (∼935 MPa), with no significant differences within commercial, nor within recycled groups. Aging decreased contrast ratio for recycled groups and increased the strength of all groups. While all groups presented high reliability up to 500MPa, commercial bilayers outperformed recycled systems at 800-MPa.

SIGNIFICANCE

The synthesis of bilayered systems using recycled-3Y was successful, resulting in high reliability in missions up to 500MPa. Bilayers based on commercial powder demonstrated superior translucency, strength, and reliability at 800MPa compared to their recycled counterparts.

Experimental bilayer zirconia systems after aging: Mechanical, optical, and microstructural characterization

OBJECTIVES

To characterize two experimental zirconia bilayer materials compared to their monolithic controls, before and after hydrothermal aging.

METHODS

Commercial zirconia powders were utilized to fabricate two bilayer materials: 3Y-TZP+ 5Y-PSZ (3Y+5Y/BI) and 4Y-PSZ+ 5Y-PSZ (4Y+5Y/BI), alongside control groups 3Y-TZP (3Y/C), 4Y-PSZ (4Y/C), and 5Y-PSZ (5Y/C). Compacted specimens were sintered (1550 °C- 2 h, 3 °C/min), and half of them underwent hydrothermal aging (134 °C-20h, 2.2 bar). Characterizations were performed through scanning-electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, reflectance tests and biaxial flexural strength test (ISO:6872). Weibull statistics were applied to determine the characteristic strength and Weibull modulus. Grain size and optical properties were analyzed using two-way ANOVA followed by the Tukey test.

RESULTS

Degradation regions and monoclinic phase were observed at aged 3Y-TZP and 4Y-PSZ surfaces. Significant differences were observed in the evaluation of optical properties between the bilayer and control groups. The bilayer materials presented intermediate characteristic strength values compared to their controls and aging significantly increased the strength of some groups.

SIGNIFICANCE

Experimental bilayer materials presented lower mechanical properties than monolithic controls, 3Y/C and 4Y/C. Hydrothermal aging increased the characteristic strength of bilayered and monolithic controls, except for 5Y-PSZ. Both experimental bilayer systems, as well as monolithic controls, met the ISO 6872:2015 requirements for single-unit crowns (100 MPa), 3-unit fixed dental prostheses (FDPs) up to premolars (300 MPa), and 3-unit FDPs involving molars (500 MPa). However, for FDPs with four or more units, only monolithic 3Y-TZP and 4Y-PSZ, and bilayered 3Y+5Y met the required minimum flexural strength (≥800 MPa).

Two-step sintering suppresses grain growth and improves flexural strength of dental zirconia

OBJECTIVES

This study aimed to elucidate the effect of various two-step sintering (TSS) protocols on the physical, mechanical, and optical properties of partially stabilized zirconia with different yttria dopant concentrations (Y-PSZ).

METHODS

Disc-shaped specimens were obtained from most widely used commercial dental zirconia powders of various Y contents (Tosoh Corp.) by uniaxial pressing followed by cold-isostatic pressing. Densification was carried out using TSS protocols with varying temperatures for both sintering steps. Relative density (ρRel), microstructure, and phase content were analyzed. Biaxial flexural strength (σ) and translucency parameter (TP) were evaluated.

RESULTS

The TSS results were compared with optimized conventional sintering (CS) results from a previous study for the same Y-PSZ compositions. TSS 3Y-PSZ and 4Y-PSZ reached similar ρRel to those of their CS counterparts, whereas 5Y-PSZ failed to achieve that regardless of TSS protocol. TSS yielded less cubic phase compared to CS, especially for 3Y-PSZ and for higher temperatures. TSS suppressed the grain growth throughout the temperature range investigated, promoting smaller grains than CS (p < 0.05). The TP values for TSS Y-PSZ were lower than those of CS (p ≤ 0.0001), except for 3Y-PSZ. The σ values for TSS Y-PSZ were significantly higher than those of CS (p ≤ 0.0002).

SIGNIFICANCE

TSS increased strength without significantly jeopardizing the optical properties of various Y-PSZ compositions relative to their CS counterparts. This alternative sintering method appears to be a promising technique for controlling grain growth while eliminating porosities in dental Y-PSZ ceramics, thus potentially enhancing the clinical longevity of zirconia restorations.

Effect of metal elements in coloring liquids used in the infiltration method on the physical properties of zirconia

OBJECTIVES

This study aimed to clarify the effect of metal elements in the coloring liquids used in the infiltration method on the physical properties of zirconia.

METHODS

Two types of zirconia discs 5Y-PSZ (SHOFU Disc ZR Lucent FA, SHOFU, Kyoto, Japan) were used: with monolayer shades from W2 to W3 (Pearl White) and 5Y-PSZ with multilayer shades from A3 to A4 (L). Five kinds of coloring liquid were used to infiltrate into semi-sintered Pearl White (T-glass [CT], A4 [CA], White-Opaque [CW], Gingiva 1 [CG], and Blue X [CB]). In addition, uncolored Pearl White set to as the control (C). These specimens were analyzed using a three-point bending test (3PBT), and the fracture surface after the test was analyzed by scanning electron microscopy, elemental analysis, and crystal structure analysis. In addition, from the polished surface part of the after the 3PBT specimens, the elemental composition was analyzed using X-ray fluorescence spectroscopy (XRF).

RESULTS

The flexural strength of CB and CG were lower than that of C (p < 0.05). XRF results showed that the Erbium (Er) content of CG was significantly greater than that of C (p < 0.05). CB exhibited a significantly higher Yttrium (Y) content compared with C (p < 0.05), and numerous pores were observed in the micrographs of the fracture surface of CG and CB.

SIGNIFICANCE

In zirconia, where the content of Y and Er was significantly increased by infiltration with a coloring liquid, pores were observed between the zirconia crystals, and the mechanical strength decreased.

Is zirconia surface etching a viable alternative to airborne particle abrasion? A systematic review and meta-analysis of in vitro studies

OBJECTIVES

This systematic review aimed to determine the effectiveness of various etching surface treatments on zirconia bond strength with the following research question: "Can zirconia etching serve as a viable alternative to airborne particle abrasion (APA) for achieving reliable bonding?".

DATA

In vitro studies comparing APA, performed with either conventional or silica-coated aluminum oxide (Al2O3) particles, with various etching protocols in terms of bonding performance were included. The risk of bias of the included studies was assessed using the QUIN's tool for in vitro studies. Meta-analyses were performed using RevMan; random-effects models were applied, and heterogeneity was tested using the I2 index. The significance level was set at p < 0.05.

SOURCES

A comprehensive literature search was conducted across electronic databases, including Clarivate Analytics' Web of Science, Cochrane Library, EMBASE, PubMed, Scopus and ProQuest.

STUDY SELECTION

Fifty-four relevant articles were included in this systematic review. According to the QUIN's tool, 7 studies were rated as "high risk of bias", 46 studies were rated as "medium risk", and 1 study was rated as "low risk". Nineteen studies were used for meta-analyses. Mostly, APA demonstrated significantly higher bond strength compared to various etching protocols (p < 0.05). However, no statistical difference was found between APA and high concentrations (40-48 %) of hydrofluoric acid (HF) in terms of immediate- and medium-term bond strength to resin composite (p > 0.05). On enamel, an experimental hot etching solution performed significantly better than APA in short-term follow-up (p < 0.05). A novel multi-acid solution exhibited significantly higher immediate shear bond strength to resin cement than APA (p < 0.05). Variable heterogeneity, ranging from low to high, was observed.

CONCLUSIONS

APA remains the surface treatment with the strongest evidence in the literature and it is usually more efficacious than zirconia etching. However, highly concentrated HF and an experimental hot etching solution have demonstrated similar or significantly higher bond strength values over time compared to APA, depending on the adhesive substrate. A recently introduced multi-acid solution (Zircos-E) needs to be further explored, especially in regards to long-term bond durability.

CLINICAL SIGNIFICANCE

This systematic review provides a comprehensive analysis of the existing in vitro evidence on the potential of zirconia etching and the bond durability of resin-based materials after artificial aging. Selecting appropriate surface treatment protocols is crucial for achieving optimal clinical outcomes.

Amelogenesis imperfecta: Analysis of the genetic basis and treatment with a digital workflow: A clinical report

A 20-year-old woman with type I hypoplastic amelogenesis imperfecta (AI) experiencing esthetic concerns and hypersensitivity is presented. The treatment was aimed at both functional and esthetic aspects, focusing on complete mouth rehabilitation using a fully digital workflow. In an interdisciplinary approach, a multigene panel analysis was performed. Digital intraoral scans and a face scan facilitated precise virtual planning, guiding minimally invasive preparations. Long-term interim restorations milled from high-performance polycarbonate preceded definitive restorations in translucent zirconia. This approach resulted in a time- and cost-efficient treatment, helped the patient to understand her disease by highlighting the pivotal role of genetics, and demonstrated the success of collaborative dental interventions. A strict recall program will be most important for the long-term success of this patient with AI and a pathogenic frameshift variant in the AMELX gene.

A Comprehensive Review of Techniques for Enhancing Zirconia Bond Strength: Current Approaches and Emerging Innovations

The increasing use of zirconia in dental restorations necessitates a comprehensive understanding of effective bonding techniques to ensure long-term clinical success. Zirconia's unique chemical composition presents challenges in achieving a durable bond as it lacks the glass phase necessary for traditional etching and silanization processes. This review evaluates current methods and emerging innovations for enhancing zirconia bond strength to resin cements. Our findings emphasize the importance of mechanical surface treatments such as air-particle abrasion and tribochemical silica-coating, which significantly improve micromechanical retention. Laser irradiation, while less commonly used, also shows promise in enhancing bond strength without compromising zirconia's structural integrity; 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) primers emerged as critical agents in forming stable P-O-Zr bonds, particularly when used with resin cements containing 10-MDP. However, variations in formulation and application methods impact their overall efficacy. Resin cement demonstrated superior bond strength compared to conventional cement, but clinical outcomes varied, highlighting the importance of cement-primer compatibility and strict procedural adherence. Emerging technologies such as polymer-infiltrated ceramic networks (PICNs) and additive manufacturing (AM) of zirconia offer potential for future advancements, although they require further research to address mechanical and aesthetic challenges. In conclusion, while established methods such as sandblasting and the use of MDP primers remain reliable, ongoing research into novel materials and techniques continues to offer opportunities for enhancing zirconia bonding. Clinicians must balance effectiveness, procedural complexity, and clinical practicality when selecting the most appropriate bonding protocols for zirconia restorations.

Effect of calcination on minimally processed recycled zirconia powder derived from milling waste

OBJECTIVE

To assess the influence of calcination process on the properties of minimally processed recycled 3Y-TZP, and to compare it with its commercial counterpart.

METHODS

Non-milled 3Y-TZP waste was collected, fragmented and ball-milled to a granulometric < 5 µm. Half of the recycled powder was calcined at 900 °C. Recycled 3Y-TZP disks were uniaxially pressed and sintered to create two recycled groups: 1) Calcined and 2) Non-calcined to be compared with a commercial CAD/CAM milled 3Y-TZP. The microstructure of experimental groups was assessed through density (n = 6), scanning electron microscopy (n = 3) and energy-dispersive X-ray spectroscopy (n = 3); and the crystalline content was evaluated through X-ray diffraction (XRD) (n = 3). Optical and mechanical properties were investigated through reflectance tests (n = 10), and Vickers hardness, fracture toughness (n = 5), and biaxial flexural strength tests (n = 16), respectively. Fractographic analysis was performed to identify fracture origin and crack propagation. Statistical analyses were performed through ANOVA followed by Tukey´s test, and by Weibull statistics.

RESULTS

Particle size distribution of recycled powder revealed an average diameter of ∼1.60 µm. The relative density of all experimental groups was > 98.15 % and XRD analysis exhibited a predominance of tetragonal-phase in both recycled groups, which were similar to the crystallographic pattern of the control group. Cross-section micrographs presented flaws on the non-calcined group, and a more homogeneous microstructure for the calcined and commercial groups. Commercial samples showed lower contrast-ratio and higher translucency-parameter than the recycled groups, where non-calcined presented higher translucency-parameter and lower contrast-ratio than its calcined counterpart. The commercial group presented higher fracture toughness and characteristic strength than the recycled groups. Moreover, the calcined group exhibited higher hardness, characteristic strength, and probability of survival at higher loads than the non-calcined group. Fractographic analysis depicted the presence of microstructural flaws in the non-calcined group, which may have acted as stress-raisers and led to failures at lower flexural strengths values.

SIGNIFICANCE

The calcination process improved the microstructure, optical, and mechanical properties of the recycled 3Y-TZP.

Cytokines profile in gingival crevicular fluid of subjects wearing fixed dental prostheses: a systematic review and meta-analysis

BACKGROUND

Fixed dental prostheses (FDP) can affect the production of inflammatory cytokines causing damage to periodontal tissues. A systematic review and meta-analysis was carried out with the following two objectives: (1) to determine the prevalence and function of the different inflammatory cytokines present in gingival crevicular fluid (GCF) of teeth with metal-ceramic (M/C) and all-ceramic (A-Cs) prostheses, and (2) to analyze and compare the levels of inflammatory cytokines in GCF of teeth with M/C and A-Cs prostheses.

METHODS

The protocol followed PRISMA and Cochrane guidelines and was registered in the OSF:10.17605/OSF.IO/RBHJU. A digital search was conducted in the databases PubMed/MEDLINE, Cochrane Library, Dentistry & Oral Sciences Source, Scopus, Web of Science, ScienceDirect, and Google Scholar, from July 15th, 2000 to March 1st, 2024. Study quality was assessed using the JBI tool for cross-sectional and longitudinal studies. A meta-analysis was performed using a random-effects model to evaluate the concentration of IL-1β in GCF of teeth with FDP of M/C and A-Cs.

RESULTS

The search strategy provided a total of 8,172 articles, of which 14 investigations met the inclusion criteria. The total number of patients studied was 468 of whom 53% were women and the rest (47%) were men. The ages of the patients ranged from 19 to 73 years, with a mean age ± standard deviation (SD) of 38,5 ± 12,8 years. A total of 843 fixed dental prostheses were studied, of which 407 (48,27%) were M/C prostheses and 410 (48,63%) were A-Cs prostheses. We found that the levels of IL-1β, IL-1α, PGE2, NKA, CGRP, and CX3CL1 were increased in teeth with M/C prostheses compared to teeth with A-Cs prostheses. Meta-analysis revealed that there are no significant differences between IL-1β levels in GCF in teeth with M/C prostheses compared to teeth with A-Cs prostheses (SMD = 13.89 pg/ml (CI = -14.29-42.08), p =  > 0.05).

CONCLUSIONS

A trend toward increased levels of inflammatory cytokines was found in GCF of teeth with M/C prostheses compared to teeth with A-Cs prostheses.

Materials and Techniques in Dentistry, Oral Surgery and Orthodontics

Modern dentistry encompasses a broad spectrum of disciplines-restorative dentistry, endodontics, prosthodontics, periodontics periodontology, aesthetic dentistry, paediatric dentistry orthodontics, and oral hygiene [...].

Advances in ceramics for tooth repair: From bench to chairside

OBJECTIVES

To give a comprehensive review of advancement in dental ceramics, fabrication methods, and the challenges associated with clinical application.

DATA, SOURCES AND STUDY SELECTION

Researches on chemical composition, biomechanical behaviors, optical properties, bonding strategies and fabrication methods were included. The search of articles was independently conducted by two authors in the PubMed, Scopus, Medline and Web of Science.

CONCLUSIONS

Dental ceramics have shown significant advancements in terms of esthetics and function. However, improving fracture toughness without compromising optical properties remains a challenge. Repairing fractured zirconia or glass-matrix ceramic prostheses with the same material is difficult due to the sintering process. Developing innovative bonding techniques that provide strong and long-lasting bonding strength between ceramics and tooth structures poses a recurring obstacle.

CLINICAL SIGNIFICANCE

Despite the emergence of dental ceramics and fabrication techniques, certain limitations such as susceptibility to brittleness and fracture still exist. Therefore, the current review provided valuable information around the advanced dental ceramics in tooth repair. The laboratory test data and the clinical outcome are also presented in details, aiming to guide clinicians in making informed decisions regarding ceramic restorations.

Development of ZTA (80% Al2O3/20% ZrO2) pre-sintered blocks for milling in CAD/CAM systems

The present work aims to develop a production method of pre-sintered zirconia-toughened-alumina (ZTA) composite blocks for machining in a computer-aided design and computer-aided manufacturing (CAD-CAM) system. The ZTA composite comprised of 80% Al2O3 and 20% ZrO2 was synthesized, uniaxially and isostatically pressed to generate machinable CAD-CAM blocks. Fourteen green-body blocks were prepared and pre-sintered at 1000 °C. After cooling and holder gluing, a stereolithography (STL) file was designed and uploaded to manufacture disk-shaped specimens projected to comply with ISO 6872:2015. Seventy specimens were produced through machining of the blocks, samples were sintered at 1600 °C and two-sided polished. Half of the samples were subjected to accelerated autoclave hydrothermal aging (20h at 134 °C and 2.2 bar). Immediate and aged samples were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Optical and mechanical properties were assessed by reflectance tests and by biaxial flexural strength test, Vickers indentation and fracture toughness, respectively. Samples produced by machining presented high density and smooth surfaces at SEM evaluation with few microstructural defects. XRD evaluation depicted characteristic peaks of alpha alumina and tetragonal zirconia and autoclave aging had no effect on the crystalline spectra of the composite. Optical and mechanical evaluations demonstrated a high masking ability for the composite and a characteristic strength of 464 MPa and Weibull modulus of 17, with no significant alterations after aging. The milled composite exhibited a hardness of 17.61 GPa and fracture toughness of 5.63 MPa m1/2, which remained unaltered after aging. The synthesis of ZTA blocks for CAD-CAM was successful and allowed for the milling of disk-shaped specimens using the grinding method of the CAD-CAM system. ZTA composite properties were unaffected by hydrothermal autoclave aging and present a promising alternative for the manufacture of infrastructures of fixed dental prostheses.

Fracture Resistance of 3D-Printed Occlusal Veneers Made from 3Y-TZP Zirconia

The aim of this paper was to evaluate the fracture resistance of 3D-printed zirconia occlusal veneers (OVs) of different thicknesses and supported by different abutment materials. Materials and Methods: The standard OV of a natural molar was prepared and digitized using a laboratory 3D scanner. The resulting digital tooth abutment was milled either using cobalt-chromium (CoCr) or a fiber-reinforced composite (FRC). All the abutments were digitized and standardized OVs (30° tilt of all the cusps) designed with 0.4 mm, 0.6 mm, or 0.8 mm wall thicknesses. The OVs were fabricated using either the Programill PM7 milling device (Ivoclar Vivadent, PM) or one of two 3D zirconia printers, Cerafab 7500 (Lithoz, LC) or Zipro-D (AON, ZD). The ZD samples were only tested on CoCr abutments. The completed OVs were luted to their abutments and subjected to artificial aging, consisting of thermocycling and chewing simulation before fracture testing with a steel sphere (d = 8 mm) as an antagonist with three contact points on the occlusal OV surface. Besides the total fracture resistance Fu,tot, the lowest contact force Fu,cont leading to the local fracture of a cusp was of interest. The possible effects of the factors fabrication approach, wall thickness, and abutment material were evaluated using ANOVA (α = 0.05; SPSS Ver.28). Results: The total fracture resistance/contact forces leading to failure ranged from Fu,tot = 416 ± 83 N/Fu,cont = 140 ± 22 N for the 0.4 mm OVs fabricated using LC placed on the FRC abutments to Fu,tot = 3309 ± 394 N (ZD)/Fu,cont = 1206 ± 184 N (PM) for the 0.8 mm thick OVs on the CoCr abutments. All the factors (the fabrication approach, abutment material, and OV wall thickness) had an independent effect on Fu,tot as well as Fu,cont (p < 0.032). In pairwise comparisons for Fu,tot of the OVs luted to the CoCr abutments, the ZD samples statistically outperformed the LC- and PM-fabricated teeth irrespective of the thickness (p < 0.001). Conclusions: Within the limitations of this study, the printed occlusal veneers exhibited comparable fracture resistances to those of the milled variants. However, more resilient abutments (FRC as a simulation of dentine) as well as a thinner wall thickness led to reduced OV fracture resistance, suggesting that 0.4 mm thick zirconia OVs should not be unreservedly used in every clinical situation.

Impact of ZrO2 Content on the Formation of Sr-Enriched Phosphates in Al2O3/ZrO2 Nanocomposites for Bone Tissue Engineering

This study investigates the profound impact of the ZrO2 inclusion volume on the characteristics of Al2O3/ZrO2 nanocomposites, particularly influencing the formation of calcium phosphates on the surface. This research, aimed at advancing tissue engineering, prepared nanocomposites with 5, 10, and 15 vol% ZrO2, subjecting them to chemical surface treatment for enhanced calcium phosphate deposition sites. Biomimetic coating with Sr-enriched simulated body fluid (SBF) further enhanced the bioactivity of nanocomposites. While the ZrO2 concentration heightened the oxygen availability on nanocomposite surfaces, the quantity of Sr-containing phosphate was comparatively less influenced than the formation of calcium phosphate phases. Notably, the coated nanocomposites exhibited a high cell viability and no toxicity, signifying their potential in bone tissue engineering. Overall, these findings contribute to the development of regenerative biomaterials, holding promise for enhancing bone regeneration therapies.