Comparison of subcritical growth parameters of a Y-TZP obtained via cyclic or dynamic fatigue tests

OBJECTIVE

The objective of this study was to 1) compare the stress corrosion coefficient (n) of a Y-TZP obtained by two fatigue tests: cyclic and dynamic and 2) evaluate the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and slow crack growth.

METHODS

A total of 145 Y-TZP specimens were produced in accordance with the manufacturer's instructions. These specimens, measuring 4.0 × 3.0 × 25.0 mm, were used for dynamic (n = 70) and cyclic fatigue tests (n = 75). The specimens were obtained from CAD/CAM blocks, sectioned, and sintered in a furnace at 1530 °C with a heating rate of 25 °C/min. They were tested in their "as-sintered" form without any additional surface treatment. The fatigue tests were conducted using a four-point bending to obtain the slow crack growth parameters (n). The cyclic fatigue test was also conducted in two frequencies (2 and 10 Hz), using stress levels between 350 and 600 MPa. Data from these tests were analyzed using ASTM C 1368-00 formulas and Weibull statistics. Scanning electron microscope (SEM) was used for fracture surface analysis to identify the origin of the fracture. Critical defect size was measured and used, along with flexural strength values, to estimate fracture toughness. Dynamic fatigue test data were used to obtain subcritical crack growth (SCG) parameters and perform Weibull statistical analysis. The cyclic fatigue data were used in the General Log-linear Model equation using the ALTA PRO software. Data were analyzed using one-way ANOVA followed by Tukey post-hoc tests and Student's t-test at a significance level of p ≤ 0.05.

RESULTS

In the dynamic fatigue test, the values obtained for σfo and n were 667 and 54, respectively. This parameter indicates how the strength of the material diminishes over time due to internal cracks. The Weibull parameters obtained from the same test results were m = 7.9, σ0 = 968, 9 and σ5% = 767, which indicates the reliability of the material. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (η) were 1.93 × 106 and 40,768, respectively. The n values obtained by cyclic fatigue were 48 and 40 at frequencies of 2 and 10 Hz, respectively. There was no effect of the frequency, the stress level or the interaction of the two in the Y-TZP lifetime, when analysed by General Log Linear Model.

SIGNIFICANCE

the n values obtained by cyclic and dynamic fatigue tests showed no statistically significant difference and the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and subcritical growth were not observed in the tested specimens.

Screening of hydrogel-based scaffolds for dental pulp regeneration-A systematic review

OBJECTIVE

The aim of this systematic review was to evaluate the most appropriate hydrogel scaffold type (natural, synthetic or hybrid) to be applied with stem cells for dental pulp regeneration. The findings should help clinicians make an informed choice about the appropriate scaffold to be applied for this approach.

DESIGN

Three electronic databases were searched (Medline, Web of Science and Scopus). The review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA).

RESULTS

From 4990 potentially relevant studies initially identified, 18 papers fulfilled the eligibility criteria and were considered for this review. Natural scaffolds were applied in most studies. Collagen was the most studied scaffold. In 5 of 10 studies, only growth factors were added to the constructs. Even without growth factors, these scaffolds containing stem cells were able to support the formation of dentin. The synthetic scaffolds were the least studied. Only 4 studies were selected, and in 3 of them, the same scaffold (Puramatrix) was evaluated. Puramatrix by itself was unable to form dental pulp when dental pulp stem cells were not present. Synthetic and hybrid hydrogels were unable to attract stem cells from the host. The presence of growth factors in these constructs seems to be of relevance since dental pulp tissue formation was achieved only when the hybrid scaffold was applied with growth factors.

CONCLUSION

All types of hydrogel-based scaffolds, when containing mesenchymal stem cells, are able to form connective tissue with different degrees of similarity to dental pulp. However, current data is too heterogeneous to compare and identify the advantages of any specific scaffold.

Efeito do Processamento Cerâmico e Adição de Nanotubos de TiO2 na Confiabilidade da Y-TZP: Análises Fractográfica e de Weibull

O objetivo deste estudo foi avaliar a influencia do processamento e adição de diferentes concentrações de nanotubos TiO2 (TiO2 nt) (0, 1, 2 e 5% em volume) em Y-TZP experimental quanto à confiabilidade, comparada à Y-TZP comercial (e.max ZirCad, Ivoclar/Vivadent). Todas as etapas da confecção das Y-TZPs experimentais foram controladas: mistura do pó, secagem, granulação, conformação e sinterização. Espécimes (12 ø x 1,1mm) foram obtidos para cada grupo (n28). A Y-TZP comercial foi preparada para se obter o mesmos formatos e dimensões (pós sinterização) dos espécimes experimentais. Todos espécimes foram submetidos ao teste de flexão biaxial (RFB) até a fratura, de acordo com as normas da ISO 6872, 2016. Dados de RFB foram submetidos     aos testes de ANOVA e Tukey (a0,05). Os dados de RFB também foram utilizados para o cálculo do módulo de Weibull (m) e resistência característica (o) de cada grupo (Weibull 7++, Reliasoft, Tucson, USA). Cinco espécimes de cada grupo foram analisados em Microscopia Eletrônica de Varredura (MEV) para análise fractográfica. RFB: Y-TZP comercial apresentou os maiores valores (p<0,05), diferindo dos grupos de Y-TZP experimentais. m e o: os grupos de Y-TZP experimentais obtiveram os maiores valores em comparação à Y-TZP comercial, sendo que a Y-TZP 0% de TiO2 nt obteve os maiores valores, diferindo das concentrações 1, 2 e 5%. A análise fractográfica mostrou padrões de fratura similares entre os grupos. O controle do processamento da Y-TZP aumentou a confiabilidade populacional do material.Palavras-chave: Cerâmica. Nanotubos. Engenharia Biomédica.

Effect of Nd:YAG and CO2 Laser Irradiation on Prevention of Enamel Demineralization in Orthodontics: In Vitro Study

OBJECTIVE

The aim of this study was to investigate Nd:YAG and CO₂ laser effects in the prevention of demineralization in deeper layers of enamel via successive acid challenge cycles.

BACKGROUND DATA

Lasers are promising in the prevention of enamel demineralization around the orthodontic brackets; however, there are very few studies that evaluate if the effects of treatment could be extended after successive acid challenge cycles due to permanent enamel structural alterations.

MATERIALS AND METHODS

Human enamel samples were divided into five groups (n = 12): G1-application of 1.23% acidulated fluoride phosphate gel (AFP, control); G2-Nd:YAG laser irradiation (0.6 W, 84.9 J/cm², 10 Hz, 110 μs, contact mode); G3-Nd:YAG laser irradiation associated with AFP; G4-CO₂ laser irradiation (0.5 W, 28.6 J/cm², 50 Hz, 5 μs, and 10 mm focal distance); and G5-CO₂ laser irradiation associated with AFP. The samples were submitted to successive acid challenge cycles. Quantitative light-induced fluorescence and scanning electron microscopy were used to assess enamel demineralization. The data were statistically compared (α = 5%).

RESULTS

G1: 50.87 ± 4.57; G2: 47.72 ± 2.87; G3: 50.96 ± 4.01; G4: 28.21 ± 2.19; and G5: 30.13 ± 6.38. The CO₂ laser groups had significantly lower mineral losses than those observed in all other groups after successive acid challenge cycles.

CONCLUSIONS

Only the CO₂ laser (10.6 μm) irradiation prevents enamel demineralization around the orthodontic brackets even after exposure to successive acid challenges. The CO₂ laser at 10.6 μm showed a deeper effect in enamel regarding caries prevention.

Effect of water storage and surface treatment on the microshear bond strength of different luting cements to zirconia

This study sought to determine the microshear bond strength of 4 luting cements to zirconia. Zirconia substrates were divided randomly into 16 groups (n = 12), according to the luting cement, surface treatment, and storage time. Matrices were placed onto each zirconia substrate, filled with the cements, and photocured for 40 seconds. Microshear bond strength was determined after storage periods of 48 hours and 75 days; the results were analyzed by ANOVA and Tukey's test (α = 0.05). Surface treatment increased the bond strength to zirconia for 3 of the 4 cements tested. Even after a relatively long period of water storage, the results indicated that surface treatment did not significantly decrease the bond strength values for the same 3 cements.