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.