Cyclic fatigue tests on non-anatomic specimens of dental ceramic materials: A scoping review

Current prospect of dental zirconia recycling: A scoping review

PURPOSE

This scoping review aimed to identify and discuss the recyclability of dental zirconia residuals along with the factors influencing the properties of dental recycled zirconia.

STUDY SELECTION

In vitro studies written in both English and Chinese that evaluated the recyclability of dental zirconia residue were selected. A literature search was conducted in Medline, PubMed, Web of Science, and Cochrane Library databases with no restrictions on the publication year.

RESULTS

Sixteen studies were included in this review. Residual dental zirconia can be recycled. Cold isostatic pressing improved the density of recycled zirconia compared with uniaxial pressing. Recycled zirconia powder with fine particles enhanced the performance of recycled zirconia compared with coarse particles. A presintering temperature of 950 to 1100°C was suitable for preparing recycled zirconia presintered bodies. Recycled zirconia may require a higher sintering temperature (≥1500°C) compared with commercially available zirconia. Additionally, recycled zirconia powder can enhance the properties of polymethylmethacrylate as a reinforcing filler and can be combined with alumina powder to obtain an alumina/zirconia composite material.

CONCLUSIONS

Despite a limited body of literature, the recycling and reutilization of dental zirconia residuals are feasible and reliable, and can reduce economic and environmental losses. Nevertheless, several factors influence the recycling effect, including the particle size, molding techniques, and sintering parameters. In addition, recycled zirconia powder can act as a reinforcing filler, potentially enhancing the mechanical properties of other materials. To realize commercially viable recycled zirconia materials, further studies are essential to comprehensively explore and understand their recycling performance.

Is dentin analogue material a viable substitute for human dentin in fatigue behavior studies?

This study aimed to compare the fatigue performance of a lithium disilicate ceramic cemented on different substrates (human dentin and glass fiber-reinforced epoxy resin - GFRER), treated with different types of conditioning (CTR - without surface conditioning; HF5 - 5% hydrofluoric acid; HF10 - 10% hydrofluoric acid; H3PO4 - phosphoric acid 37%; SAND - sandblasting with aluminum oxide). The occlusal surface of human molars (DENT group) (n = 15) was ground for dentin exposure and the root portion was cut, then the dentin slice (2.0 mm thick) was conditioned with 37% phosphoric acid and a dual-curing dental adhesive was applied. The GFRER in a round-rod format was cut into discs (Ø = 10 mm, 2.0 mm thick). Lithium disilicate glass ceramic blocks (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein) were shaped into a cylinder format and cut, resulting in 90 discs (Ø = 10 mm, 1.5 mm thick). The substrate materials of each group were etched according to the groups and the ceramic was etched with 5% hydrofluoric acid for 30 s. A silane coupling agent was applied over the cementation surface in ceramic and GFRER surfaces and a dual cement was used for cementation (ceramic/GFRER or dentin). The disc/disc sets were submitted to thermocycling (25,000 cycles + storage for 6 months), and then tested in step-wise accelerated cyclic fatigue test. The failure pattern and topography were analyzed and the roughness and contact angle were measured before and after surface treatment. The DENT group presented the lowest load to failure values and number of cycles to failure in fatigue (637.33 N; 118.333), showing no statistical similarity with any of the other tested groups (p < 0.05). The topographic analysis showed that all proposed surface treatments modified the substrate surface when compared to the CTR group. All of the fractographical inspections demonstrated failure by radial crack. Considering the roughness analysis, the post-etched DENT group showed similar roughness to all groups of GFRER materials with their surface treated, except for SAND, which showed greater roughness and statistically different from the other groups. The DENT group (49.5) showed statistically different post-conditioning contact angle values from the HF10 group (96.5) and similar to the other groups. The glass fiber-reinforced epoxy resin was not able to simulate the results presented by the human dentin substrate when cemented to lithium disilicate regarding fatigue failure load and number of cycles for failure, regardless of the surface treatment. Lithium disilicate cemented on dentin analogue overestimates the load values for fatigue failure.

Advanced lithium disilicate: A comparative evaluation of translucency and fatigue failure load to other ceramics for monolithic restorations

The aim of this in vitro study was to evaluate the surface roughness, translucency, fatigue failure load (FFL), and number of cycles for fatigue failure (CFF) of a recently released lithia-based material called advanced lithium disilicate and three other ceramics indicated for monolithic restorations. First, ALD (advanced lithium disilicate, CEREC Tessera, Dentsply Sirona), LD (lithium disilicate, IPS e. max CAD, Ivoclar), LS (lithium silicate-disilicate, Suprinity, Vita Zahnfabrik), and 4Y-PSZ (Yttria-stabilized zirconia, IPS e.max ZirCAD MT, Ivoclar) discs (n = 15, Ø = 10 mm and thickness = 1.0 mm) were fabricated from CAD/CAM blocks/discs, A2 shade. The discs were sintered/crystallized and subsequently analyzed by a rugosimeter (Mitutoyo SJ-410) to determine Ra and Rz surface roughness parameters. Next, they were evaluated to determine the translucency parameter (TP) using a bench-top spectrophotometer (SP60, EX-Rite). The discs were subsequently cemented to glass fiber epoxy resin discs, and the specimens were tested under cyclic loading (Step-test), immersed in distilled water at a frequency of 20 Hz, with an initial cyclic load of 200 N for 5,000 cycles and increments of 50 N every 10,000 cycles until failure. Fatigue failure load (FFL) and number of cycles for fatigue failure (CFF) were recorded for subsequent Kaplan Meier analysis, with post-hoc Mantel-Cox and Weibull analysis (α = 0.05). Complementary fractographic, topographic and energy dispersive spectroscopy analyses (EDS) were performed. 4Y-PSZ showed higher survival (p < 0.05), with higher FFL and CFF (1077 N; 180,333 cycles), followed by LD (980 N; 161,000 cycles), LS (937 N; 152,333 cycles) and ALD (910 N; 147,000 cycles). No differences were observed between the tested groups regarding Weibull modulus. ALD presented TP (28.14) equal to DL (28.27) and higher than LS (25.51). All lithia-based materials had higher translucency than 4Y-PSZ (TP = 8.62) (p < 0.05). ALD appears to have a similar elemental composition to LD and LSD for oxygen and silicon. ALD and LSD have a similar zirconium content. Fractures originated on the cemented surface of the ceramic discs. Lithia-based ceramics showed lower surface roughness, with ALD (Ra = 0.04 μm; Rz = 0.66 μm) showing the lowest values (p < 0.05). Despite showing lower FFL when compared to LD and 4Y-PSZ, ALD has compatible translucency and mechanical fatigue performance with its indication for fabricating monolithic, anterior and posterior adhesively cemented single-unit restorations. However, further studies are needed to substantiate its clinical performance.

Stress-strain and fatigue life numerical evaluation of two different dental implants considering isotropic and anisotropic human jaw

Dental prostheses are currently a valid solution for replacing potential missing tooth or edentulism clinical condition. Nevertheless, the oral cavity is a dynamic and complex system: occlusal loads, external agents, or other unpleasant events can impact on implants functionality and stability causing a future revision surgery. One of the failure origins is certainly the dynamic loading originated from daily oral activities like eating, chewing, and so on. The aim of this paper was to evaluate, by a numerical analysis based on Finite Elements Method (FEM), and to discuss in a comparative way, firstly, the stress-strain of two different adopted dental implants and, subsequently, their fatigue life according to common standard of calculations. For this investigation, the jawbone was modeled accounting for either isotropic or anisotropic behavior. It was composed of cortical and cancellous regions, considering it completely osseointegrated with the implants. The impact of implants' fixture design, loading conditions, and their effect on the mandible bone was finally investigated, on the basis of the achieved numerical results. Lastly, the life cycle of the investigated implants was estimated according to the well-established theories of Goodman, Soderberg, and Gerber by exploiting the outcomes obtained by the numerical simulations, providing interesting conclusions useful in the dental practice.

How does the occlusal contact region influence the mechanical fatigue performance and fracture region of monolithic lithium disilicate ceramic crowns?

OBJECTIVE

To characterize the effect of the occlusal contact region on the mechanical fatigue performance and on the fracture region of monolithic lithium disilicate ceramic crowns.

MATERIALS AND METHODS

Monolithic lithium disilicate ceramic crowns were machined in a CAD/CAM system and adhesively luted onto glass-fiber reinforced epoxy resin preparations with resin cement. The crowns were divided into three groups (n = 16) according to load application region (cusp tip: restricted to cusp tips; cusp plane: restricted to cuspal inclined plane; or mixed: associating tip cusp and cuspal inclined plane). The specimens were submitted to a cyclic fatigue test (initial load: 200 N; step-size: 100 N; cycles/step: 20,000; loading frequency: 20 Hz; load applicator: 6 mm or 40 mm diameter stainless steel) until observing cracks (1st outcome) and fracture (2nd outcome). The data were analyzed by the Kaplan-Meier + Mantel-Cox post-hoc tests for both outcomes (cracks and fracture). Finite element analysis (FEA), occlusal contact region, contact radii measurements, and fractographic analyzes were performed.

RESULTS

The mixed group presented worse fatigue mechanical behavior (550 N / 85.000 cycles) compared to the cuspal inclined plane group (656 N / 111,250 cycles) (p < 0.05) for the first crack outcome, while the cusp tip group was similar to both groups (588 N / 97,500 cycles) (p > 0.05). The mixed group had the worst fatigue behavior (1413 N / 253,029 cycles) in relation to the other groups (Cusp tip: 1644 N / 293,312 cycles; Cuspal inclined plane: 1631 N / 295,174 cycles) considering the crown fracture outcome (p < 0.05). FEA showed higher tensile stress concentration areas just below the load application region. In addition, loading on the cuspal inclined plane induced a higher tensile stress concentration in the groove region. The most prevalent type of crown fracture was the wall fracture. Groove fracture was observed in 50% of the loading specimens exclusively on the cuspal inclined plane.

CONCLUSION

Load application on distinct occlusal contact regions affects the stress distribution pattern and consequently the mechanical fatigue performance and fracture region of the monolithic lithium disilicate ceramic crowns. A combination of loading at distinct regions is recommended to promote better evaluation of the fatigue behavior of a restored set.

Ceramic surface conditioning, resin cement viscosity, and aging relationships affect the load-bearing capacity under fatigue of bonded glass-ceramics

This study aimed to evaluate the influence of ceramic surface treatments, resin cement viscosities, and storage regimens on the fatigue performance of bonded glass-ceramics (lithium disilicate, LD; feldspathic, FEL). Ceramic discs (Ø = 10 mm; thickness = 1.5 mm) were allocated into eight groups per ceramic (n = 15), considering three factors: "ceramic surface treatment" in two levels - 5% hydrofluoric acid etching and silane-based coupling agent application (HF), or self-etching ceramic primer (E&P); "resin cement viscosity" in two levels - in high or low viscosity; and "storage regimen" in two levels - baseline, 24 h to 5 days; or aging, 180 days + 25,000 thermal cycles. Adhesive luting was performed onto glass fiber-reinforced epoxy resin discs (Ø = 10 mm; thickness = 2 mm) and the bonded assemblies were subjected to cyclic fatigue tests: initial load = 200 N; step-size = 25 N (FEL) and 50 N (LD); 10,000 cycles/step; 20 Hz. Scanning electron microscopy (SEM) inspections were performed. Regarding the LD ceramic, the fatigue behavior was reduced after aging for HF_HIGH and E&P_LOW conditions, while stable performance was observed for HF_LOW and E&P_HIGH. Regarding the FEL results, aging negatively affected HF_HIGH, E&P_HIGH, and E&P_LOW, being that only the HF_LOW condition presented a stable behavior. The failure initiated from defects on the etched surface of the ceramics, where the cross-sectional analysis commonly revealed unfilled areas. Long-term aging might induce a decrease in mechanical behavior. The 'ceramic microstructure/surface conditioning/resin cement viscosity relationships' modulate the fatigue performance of lithium disilicate and feldspathic glass-ceramics.

Effect of the region of the CAD/CAM block on the flexural strength and structural reliability of restorative materials

The aim of the study was to evaluate whether the region (internal or external) of the CAD/CAM block influences the mechanical performance of restorative materials. Blocks of different CAD/CAM materials (Composites: KAV - Katana Avencia; LULT - LULT Ultimate; Ceramics: ENAM - Vita Enamic; NICE - N!CE; EMP - IPS Empress CAD; VMII - Vita Mark II; EMAX - IPS e.max CAD) were selected, and direct resin composite blocks (APX - Clearfil AP-X; FSUP - Filtek Supreme) were built using the incremental technique on a mold from one of the CAD/CAM blocks. All blocks were sectioned into bar-shaped specimens (1 × 1 × 14 mm), which were separated into two groups according to the region of the block (inside or outside). 3-point bending tests were performed following ISO 6872:2015 and flexural strength data were analyzed using one-way ANOVA and Bonferroni post-hoc tests (p < 0.05). Weibull module and characteristic strength were also calculated. In general, the mean flexural strength values were not statistically different (p > 0.05) between the inside and outside regions of each material, except for LULT (inside > outside) and EMAX (outside > inside). Comparing the materials, a similar strength (only discrete variations) was observed for inside, outside or merged (inside and outside data assembled together) regions: EMAX > KAV = LULT > NICE = APX > EMP = ENAM = VMII ≥ FSUP (merged condition). Characteristic strength at 63.2% failure probability and at 5% failure probability generally corroborates such observations. There were no statistical differences for Weibull module data (inside, outside, or merged), except for KAV and NICE (outside > inside). Thus, the region from where the restoration is milled within the CAD/CAM block generally does not influence the mechanical performance hereof (flexural strength, failure probability, risk of premature failures, and mechanical reliability) of the material, except for LULT and EMAX.

A brief review on fatigue test of ceramic and some related matters in Dentistry

The characteristics of dental ceramics have been extensively studied over the years to provide highly qualified materials for use in prosthetic restorations. The ability to adhere to dental substrates, outstanding aesthetics (translucency, color, and substrate masking ability) and improved mechanical properties provide these materials with optical features and high strength to withstand masticatory stimuli. Different classifications are adopted, and it is generally considered that glass-ceramics have better optical characteristics due to the high glass content, and polycrystalline ceramics have superior strength favored by their densified and organized crystals, hampering crack growth. This knowledge was largely built-up during years of scientific research through different testing methodologies, but mainly employing static loads. It is important to not only take into account the intensity of loads that these materials will be exposed to, but also the effect of the intermittence of cyclic load application leading to mechanical fatigue and the influence of factors related to the crack origin and their propagation under this condition. Furthermore, the bonding surface of ceramic restorations requires surface treatments that improve the bond strength to luting agents; however, these treatments require caution because of their potential to produce defects and affect the structural behavior. Moreover, ceramic restorations often require internal adjustments for proper seating or external adjustments for fitting the occlusal contact with the antagonist. In this sense, finishing/polishing protocols may alter the defect population, as luting agents may also interact by filling in the superficial defects on the restoration intaglio surface. Thus, the balance among all these factors will define the performance of a restorative setup, as well as the posterior exposure to the humid environment and the masticatory stimuli (cyclical loading), which may favor developing slow and subcritical growth of cracks in ceramic materials and the degradation of the bond interface. Therefore, it is essential that the concepts which explain the fatigue mechanism are understood, as well as the crack propagation and failure patterns of restorative ceramic materials.

Which dentine analogue material can replace human dentine for crown fatigue test?

OBJECTIVE

To seek dentine analogue materials in combined experimental, analytical, and numerical approaches on the mechanical properties and fatigue behaviours that could replace human dentine in a crown fatigue laboratory test.

METHODS

A woven glass fibre-filled epoxy (NEMA grade G10; G10) and a glass fibre-reinforced polyamide-nylon (30% glass fibre reinforced polyamide-nylon 6,6; RPN) were investigated and compared with human dentine (HD). Flexural strength and elastic modulus (n = 10) were tested on beam-shaped specimens via three-point bending, while indentation hardness (n = 3) was tested after fracture. Abutment substrates of G10, RPN and HD were prepared and resin-bonded with monolithic lithium disilicate crowns (n = 10), then subjected to wet cyclic loading in a step-stress manner (500 N initial load, 100 N step size, 100,000 cycles per step, 20 Hz frequency). Data were statistically analysed using Kruskal-Wallis one-way ANOVA followed by post-hoc comparisons (α = 0.05). Survival probability estimation was performed by Mantel-Cox Log-Rank test with 95% confidence intervals. The fatigue failure load (FFL) and the number of cycles until failure (NCF) were evaluated with Weibull statistics. Finite Element Models of the fatigue test were established for stress distribution analysis and lifetime prediction. Fractographic observations were qualitatively analysed.

RESULTS

The flexural strength of HD (164.27 ± 14.24 MPa), G10 (116.48 ± 5.93 MPa), and RPN (86.73 ± 3.56 MPa) were significantly different (p < 0.001), while no significant difference was observed in their flexural moduli (p = 0.377) and the indentation hardness between HD and RPN (p = 0.749). The wet cyclic fatigue test revealed comparable mean FFL and NCF of G10 and RPN to HD (p = 0.237 and 0.294, respectively) and similar survival probabilities for the three groups (p = 0.055). However, RPN promotes higher stability and lower deviation of fatigue test results than G10 in Weibull analysis and FEA.

SIGNIFICANCE

Even though dentine analogue materials might exhibit similar elastic properties and fatigue performance to human dentine, different reliabilities of fatigue on crown-dentine analogues were shown. RPN seems to be a better substrate that could provide higher reliability and predictability of laboratory study results.

Adhesive application after ceramic surface treatment is detrimental to load-bearing capacity under fatigue of a lithium disilicate glass-ceramic

OBJECTIVE

To evaluate whether an adhesive application after surface treatment on a lithium disilicate ceramic (LD) has an influence on its load-bearing capacity under fatigue.

METHODS

LD discs (Ø= 10 mm; thickness= 1 mm) were allocated into 8 groups (n= 15), considering 3 factors: "ceramic surface treatment" - HF: hydrofluoric acid + universal primer application; or MEP: single-component ceramic primer; "adhesive application" - with or without; and "aging protocol" - baseline: 24 h to 7 days; or aging: 180 days of storage + 25,000 thermal cycles. The LD discs were adhesively bonded to glass fiber-reinforced epoxy resin discs (Ø= 10 mm; thickness= 2 mm) and stored according to the condition and each group. Cyclic fatigue testing (initial load= 100 N; step size= 100 N until600 N and after step size= 25 N to failure; 10,000 cycles/step; 20 Hz frequency) was performed. Fractographic and adhesive interface analyzes were also performed. The collected data were then analyzed by Kaplan Meier and Mantel-Cox tests and One-way ANOVA.

RESULTS

The adhesive application in the baseline condition had no influence on the load-bearing capacity under fatigue when the HF surface treatment was performed, however, adhesive application for the MEP treatment led to worse results than without it. The adhesive application in the aged condition showed worse fatigue outcomes for both treatments. All specimens presented radial cracks. MEP treatment followed by adhesive application presented the thickest luting layer.

CONCLUSION

The adhesive application after surface treatments of a lithium disilicate glass-ceramic is detrimental to its load-bearing capacity under fatigue when adhesively luted onto a supporting substrate.

Mechanical characterization of a multi-layered zirconia: Flexural strength, hardness, and fracture toughness of the different layers

This study compared the flexural strength under monotonic (static - sσ) and cyclic load application (fatigue - fσ), hardness (H) and fracture toughness (K_IC) of different layers of a multi-layered zirconia (IPS e.max ZirCAD MT Multi, Ivoclar). Each layer was sectioned, classified into three groups according to yttria content (4-YSZ, 4/5-YSZ and 5-YSZ), and shaped on samples for flexural strength and fracture toughness tests (bars: 1.0 × 1.0 × 11 mm); and Vickers hardness test (plates: 1.5 × 4.0 × 5.0 mm). Flexural strength under monotonic load application (sσ; n = 10) was obtained through two different devices (three-point-bending and ball-in-hole device) and fatigue flexural strength (fσ; n = 15; initial load: 10 N; step-size: 5 N; 10,000 cycles/step) was assessed using a ball-in-hole device under cyclic load application. Vickers hardness test (n = 5), fracture toughness test (n = 10), and additional analyzes (Finite Element Analysis - FEA, Energy-dispersive X-ray spectroscopy - EDS and Scanning Electron Microscopy - SEM) were also performed. No differences were found between the different devices in the monotonic flexural strength test, and FEA showed similar tensile stress distribution for the two devices. 4-YSZ showed higher values of flexural strength under monotonic and cyclic load application modes (sσ = 1114.73 MPa; fσ = 798.84 MPa), and fracture toughness (K_IC = 3.90 MPa√m). 4/5-YSZ had an intermediate sσ; however, fσ was similar to 5-YSZ (404.00-429.36 MPa) and K_IC similar to 4-YSZ (K_IC = 3.66 MPa√m). No statistical differences were found for hardness and Weibull modulus for fatigue flexural strength data. The amount of yttria in the layer compositions increased from 4-YSZ to 5-YSZ, and larger zirconia crystals were observed in the topographic images of 5-YSZ. Failures in the flexural strength and toughness tests started from the face subjected to tensile stress. Different layers of the multi-layered zirconia blank presented distinct mechanical properties. 4-YSZ (cervical layer) presented the highest flexural strength under monotonic and cyclic loads (fatigue), and higher fracture toughness even similar to the transition layer (4/5-YSZ). Hardness was similar between the layers. The ball-in-hole device performed similarly to the three-point bending device and can be used as an alternative to the traditional method.