In vivo ageing of zirconia dental ceramics - Part II: Highly-translucent and rapid-sintered 3Y-TZP

Signs of in-vivo aging of zirconia from explanted dental implants with porous coating after several years in function

The present study investigated the in vivo aging of yttria-stabilized zirconia (YSZ) oral implants (ZiUnite®) removed after 37 to 181 months. These implants featured a porous zirconia surface to enhance osseointegration. They were placed in prospective clinical investigations and had to be explanted due to peri‑implant bone breakdown. Since no single clinical/non-clinical parameter or combination of parameters were found to influence the bone loss, we have assessed the extent of the in vivo aging, known as low temperature degradation (LTD), on five explants as a possible critical influencing parameter. This research represents, to the best of our knowledge, the first report on low-temperature degradation-induced tetragonal-to-monoclinic (t-m) transformation in clinically applied YSZ oral implants after up to 15 years. Using scanning electron microscopy (SEM) and focused ion beam (FIB)-SEM tomography, the analysis revealed significant surface t-m transformation, pronounced micro-cracking of the porous coating, and degradation. This extensive transformation and resultant volume expansion may have negatively impacted the bone-to-implant interface, contributing to clinical failure. Additionally, differently produced YSZ implants can exhibit varied behaviors, even with identical raw materials. The presented type of assessment of susceptibility to in vivo aging of oral implants could inspire the study protocol for other medical device systems. STATEMENT OF SIGNIFICANCE: This study is the first to document substantial t-m transformation of clinically used YSZ implants after up to 15 years in the oral environment. Extensive t-m transformation in the porous coatings at body temperature occurred rapidly, leading to microcracking and potential loss of cohesion with the implant bulk. The porous, cracked zirconia coatings were linked to brittle fractures in vivo and it is hypothesized that these changes possibly contributed to bone loss, loss of osseointegration and subsequent implant failures. The findings of such transformation underscore the potential clinical risks of these zirconia coatings leading to the mentioned changes. The present assessment method could serve as a valuable protocol model for investigating the in vivo aging susceptibility of other medical devices.

Adhesion of Streptococcus mutans on highly translucent zirconia: Influence of surface properties and polyelectrolyte multilayer coatings

STATEMENT OF PROBLEM

Low-pressure airborne-particle abrasion has been used to improve the adhesion of zirconia to resin cement. However, whether a polyelectrolyte multilayer can be used to reduce bacterial adhesion to abraded zirconia is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate whether polyelectrolyte multilayers added to airborne-particle abraded zirconia can minimize biofilm development.

MATERIAL AND METHODS

Commercially available zirconia powders with yttria content between 3 and 5 mol% were isostatically pressed into Ø20-mm disks and sintered at 1450 °C for 2 hours (n=8). Untreated specimens were compared with airborne-particle abraded ones. Specimens with 3 mol% yttria were further coated with polyelectrolyte multilayers (n=4). The surfaces were characterized by measuring the roughness, hydrophobicity, and surface charge using profilometry, atomic force microscopy, tensiometry, and electrokinetic analyzer, respectively. The extent of bacterial adhesion was determined using spectrophotometry and scanning electron microscopy. Data were analyzed with a single-factor ANOVA and F-test for variance (α=.05).

RESULTS

The airborne-particle abrasion of zirconia increased the surface roughness, which led to the pronounced adhesion of Streptococcus mutans. However, polyelectrolyte multilayer coatings made of chitosan and pol(yacrylic acid) reduced the extent of bacterial adhesion, especially in as-sintered specimens, with 70% fewer adhered bacteria than airborne-particle abraded specimens. The effect of polyelectrolyte multilayer coating on the airborne-particle abraded series was greatest with the poly(acrylic acid)-terminating specimens, with 50% fewer adhered bacteria than the uncoated ones.

CONCLUSIONS

Airborne-particle abraded zirconia specimens coated with biocompatible polyelectrolyte multilayer coatings with a negatively charged terminating layer were associated with a 50% reduction in bacteria adhesion compared with uncoated specimens.

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.

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.

Influence of shading on zirconia's phase transformation and flexural strength after artificial aging

OBJECTIVES

The aim of this study was to determine the phase transformation and the influence of aging on the flexural strength of different colored zirconia. The effect of hydrothermal aging in an autoclave was compared with the effect of mechanical stress by simulating chewing.

METHODS

High-strength zirconia (3Y-TZP) was investigated in three different colors: uncolored, A3, and D3. Disc-shaped specimens (N = 3) were analyzed by X-ray diffraction (XRD), and flexural strength was determined on bar-shaped specimens (N = 15) in a 4-point bending test before and after performing two different aging protocols: aging in an autoclave (134 °C, 70 h) and aging in a chewing simulator (5 kg load, 1.2 million cycles). During autoclave aging, the fraction of monoclinic phase on the surface was determined every 5 h. Once this exceeded 25 vol%, aging of the bar specimens was stopped.

RESULTS

While in the unstained group the mean value of the proportion of monoclinic phase already exceeded 25 vol% after 30 h in the autoclave, this was the case in both stained groups only after 70 h. No measurable phase transformation could be detected after chewing simulation. Only color A3 showed a statistically significant (p ≤ 0.05) decrease in flexural strength after aging in the chewing simulator.

SIGNIFICANCE

The colored zirconia showed a higher resistance to phase transformation through hydrothermal aging. The metal oxides in the staining solutions are assumed to hinder the phase transformation in the zirconia. Therefore, the significant reduction in the stained zirconia after chewing simulation is particularly interesting.

Stability of fatigued and aged ZTA compared to 3Y-TZP and Al2O3 ceramic systems

To evaluate the effect of fatigue and aging on the crystalline content and reliability of a zirconia-toughened-alumina (ZTA) composite compared to its individual counterpart materials (3Y-TZP and Al₂O₃). Thirty-six disc-shaped specimens per group were obtained to comply with ISO 6872:2015. Crystalline content, microstructure and reliability of experimental groups were evaluated in four stages: 1) immediate; 2) aged; 3) fatigued; 4) aged + fatigue. Aging was performed in autoclave and Step-Stress-Accelerated-Life-Testing (SSALT) was performed using three stress profiles. Weibull statistics were used to determine Weibull parameters and life-expectancy. A significant increase in monoclinic phase in 3Y-TZP was observed after aging (19.31%), fatigue (17.88%) and aging + fatigue (55.81%), while ZTA evidenced minimal variation among all conditions (<5.69%). 3Y-TZP presented higher reliability than ZTA at 300 and 500 MPa, and ZTA outperformed Al₂O₃ at the same stress missions. None of the ceramics yielded acceptable reliability at 800 MPa. A higher characteristic strength was observed for 3Y-TZP, followed by ZTA and Al₂O₃. While after aging ZTA and Al₂O₃ remained stable, 3Y-TZP exhibited a significant increase in the characteristic stress. Aging did not affect the reliability of ZTA and Al₂O₃. 3Y-TZP demonstrated an increase in monoclinic content and characteristic strength after aging.

The Effect of Y3+ Addition on Morphology, Structure, and Electrical Properties of Yttria-Stabilized Tetragonal Zirconia Dental Materials

Zirconia (ZrO₂), a widely known material with an extensive range of applications, is especially suitable for dental applications. This kind of dental material is produced in the form of blocks or discs (mostly discs—depending on CAD/CAM machines) by cold isostatic pressing (CIP). Such discs are subsequently milled by CAM/CAD technology into a desirable form. Due to the application of CIP, the resulting discs consist of different yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) powders, additives and pigments. The diverse composition of the discs (e.g., different Y³⁺ content) governs material properties, such as hardness, toughness and translucence. The aim of this work was to investigate the impact of Y³⁺ addition on the grains and grain boundaries, on the ZrO₂ phases lattice parameter values and on the electrical equivalent circuit parameters of the prepared Y-TZP samples. The disc-shaped samples were prepared by using CAM/CAD technology. It was observed that the grain size and the grain density were increased by Y³⁺ addition. The sample with the lowest Y³⁺ content was characterized by the highest portion of the tetragonal phase, whilst the disc with the highest Y³⁺ addition consisted mainly of the cubic phase. It was also observed that at the higher Y³⁺ ion concentrations, these ions mainly incorporated the tetragonal phase. Furthermore, conductivity investigations showed that the resistivity of the grains in the samples with the higher Y³⁺ concentrations was decreased as these ions were mainly trapped in the grain boundary. On the other hand, the Y³⁺ trapping increased the capacitance of the grain boundary.

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.