Failure behavior of glass ionomer cement under Hertzian indentation

Mechanical test relevance—A personal perspective on some methods and requirements

Mechanical tests of various kinds are widely used in dental research to study the behaviour of its materials. Unfortunately, despite often long history, the relevance of a test or its outcome is hard to discern. But even for tests that have a more apparent appropriateness many details that ensure good accuracy and reproducibility appear not to be appreciated or understood in that there is little evidence in the published literature that the necessary steps and precautions have been taken. The present purpose to is to examine a number of these aspects in the context of a selection of tests to illustrate the care and attention that are essential for sound results.

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.

Clinical performance of laminate and non-laminate resin composite restorations: a systematic review

BACKGROUND

A systematic review was undertaken to determine the clinical outcomes of resin modified glass-ionomer cement or glass-ionomer cement-resin composite (RMGIC/GIC-RC) laminate restorations and flowable resin composite (FRC)-lined RC restorations compared to that of non-laminate RC restorations.

METHODS

Electronic databases were searched and filtered for relevant papers by assessing titles, abstracts and full-text articles. Randomized controlled clinical trials (RCTs) were included, comparing the clinical performance of RMGIC/GIC-RC laminate restorations and FRC-lined restorations with RC restorations as the control. The articles were categorized and critically appraised. Raw data were used for a fixed effects meta-analysis.

RESULTS

Thirteen articles were included in the review. Five evaluated FRC-lined restorations, and eight studies evaluated RMGIC/GIC-RC laminate restorations, comparing with non-laminate RC restorations. Three of eight RMGIC/GIC-RC laminate restorations assessed only postoperative sensitivity. A meta-analysis could only be conducted in three studies with the FRC-lined restorations as the intervention. The meta-analysis found no significant difference in clinical failures between FRC-lined RC restorations and RC restorations with no lining (p > 0.05).

CONCLUSIONS

Based on current clinical evidence, a FRC lining is no more advantageous than RC restorations with no FRC lining. More long-term RCTs are required, particularly for evaluating RMGIC/GIC-RC laminate restorations.

Cimento de ionômero de vidro: revisão de literatura

Introdução: Em âmbito odontológico ocorrem ações preventivas na tentativa de se evitar a instalação de cáries, doença que tem prevalência aumentada na população e que constitui um problema de Saúde Pública. Alguns recursos são empregados para tal, como: realização de diagnóstico precoce e a opção por tratamentos conservadores de mínima intervenção. O cimento de ionômero de vidro (CIV), advindo das suas características benéficas que vão de encontro às tendências atuais, mostra-se intimamente relacionado aos preceitos da Odontologia Preventiva e Minimamente Invasiva e as novas técnicas conservadoras preconizadas. Objetivo: O objetivo do presente artigo foi por intermédio da realização de um estudo de revisão de literatura, averiguar as características do CIV que detém papel de destaque no perfil da Odontologia Minimamente Invasiva. Resultados: O cirurgião dentista deve conscientizar-se da classificação, conforme sua composição e natureza físico-química: ionômeros convencionais; ionômeros reforçados por metais; de alta viscosidade e vários tipos de ionômeros de vidro modificados por resina para que escolha corretamente o CIV que será utilizado em suas intervenções clínicas, o que deve ocorrer baseado nas propriedades do material e em sua indicação clínica. Conclusão: Concluiu-se que a efetivação de técnicas preventivas com o CIV na assistência à saúde pública, tendem a minimizar os tratamentos curativos, valorizando concomitantemente os procedimentos odontológicos de baixa complexidade realizados na Atenção Básica, evitando os encaminhamentos para tratamento de casos de maior complexidade a nível da atenção secundária e terciária, economizando recursos.

Hertzian indentation testing of glass-ionomer restoratives: A reliable and clinically relevant testing approach

OBJECTIVES

To investigate the load to failure of encapsulated posterior glass-ionomer (GI) restoratives tested under Hertzian indentation and to explore the validity and reproducibility of the test results achieved for consideration of inclusion as an ISO testing protocol.

METHODS

Groups of 20 disc-shaped specimens (10.0±0.1mm diameter, 3.10±0.03mm thickness) were prepared (in batches of four) from three encapsulated posterior GI restoratives. Discs were tested while resting freely on a dentine analogue material at 24h under Hertzian indentation at a rate of 1mm/min delivered through a 20mm diameter hard steel ball. The failure mode and fracture origin of the GI specimens was assessed by fractography. Statistical analyses of the load to failure data were conducted using SPSS software (p<0.05) with the normality and homogeneity of variance of the load to failure data assessed using the Shapiro-Wilk and Levene's test, respectively. Data was also analysed using regression analyses to identify trends within the load to failure data sets.

RESULTS

The load to failure data for the GI restorative groups investigated were normally distributed (p>0.05), homogenous (p>0.05) and not significantly influenced by batch (p>0.780) or specimen number (p>0.447) although significant differences (p<0.05) between the GI restorative materials were evident. Fractographic analysis identified smooth fracture surfaces parallel to the loading axis where the failure mode was bottom initiated radial cracking. The mean coefficient of variation (CoV) for the GI restorative load to failure data sets achieved using Hertzian indentation testing was 7%.

SIGNIFICANCE

The failure mode and fracture origin of the GI restoratives tested using Hertzian indentation is representative of the clinical situation in vivo. The reliability of the load to failure data sets produced were improved compared with routinely employed mechanical testing approaches suggesting the possibility of inclusion as an ISO testing protocol.

Development of strength in dental silver amalgam

OBJECTIVES

To characterize the development of strength during the setting process of dental silver amalgam in the context of 'early strength' measurements for standards compliance testing in relation to patient instructions, and demonstrate the applicability of the Hertzian 'ball on disc' method.

MATERIALS AND METHODS

Sixteen dental silver amalgam products were tested using the 'ball on disc' protocol at 1, 2, 3, 4 and 24h after setting at 37°C in air. The mixed materials were packed into a tapered steel disc mold (10mm diameter, 3mm thick) resting on a glass surface, slightly overfilled and carved level with a sharp edge, then ejected at ∼10min and placed immediately into an incubator at 37°C. Testing was in Hertzian mode, using a 20mm steel ball, with the specimen resting on a disc of glass-filled polyamide (E=10GPa) at a cross-head speed of 0.2mm/min on a universal testing machine (E3000, Instron). The load at first crack was recorded, as was the number of radial cracks produced.

RESULTS

Radial cracking into 2-5 pieces, in a clinically-relevant (non-explosive) mode was observed in all cases. Considerable variation in setting rate between products, as indicated by the development of load at failure with time, was found. The distribution of normalized failure load values overall was lognormal (Weibull was excluded). The RMS coefficient of variation overall was 12.4%.

SIGNIFICANCE

The ball-on-disc test provides a facile, relevant measure of the strength of dental silver amalgam, and is viable as a standards compliance test. Early strength testing at a minimum of 2h is suggested.