Improving the standard of the standard for glass ionomers: an alternative to the compressive fracture strength test for consideration?

OBJECTIVES Three strength tests (compressive, three point flexure and biaxial) were performed on three glass ionomer (GI) restoratives to assess the most appropriate methodology in terms of validity and reliability. The influence of mixing induced variability on the data sets generated were eliminated by using encapsulated GIs. METHODS Specimen groups of 40 (eight batches of n=5) cylinders (6.0±0.1mm height, 4.0±0.1mm diameter) for compressive fracture testing, bars (25.0±0.1mm length, 2.0±0.1mm height, 2.0±0.1mm width) for three point flexure testing and discs (13.0±0.1mm diameter, 1.0±0.1mm thickness) for biaxial flexure testing were randomly prepared by an operator. The strength data sets for each GI restorative were pooled and one-way analyses of variance (ANOVAs) were conducted to compare between GI restoratives (p=0.05). The coefficient of variation (CoV) values for each test were pooled and a one-way ANOVA was conducted to test for differences between the reliability of the three tests. RESULTS For the GI restoratives, the one-way ANOVA showed significant differences when tested in compression (p=0.001) but not when tested in three point (p=0.271) or biaxial (p=0.134) flexure. The pooled CoV values showed no significant difference between the three strength tests (p=0.632). CONCLUSIONS The compressive fracture strength test specified for GIs in the International Organisation for Standardisation (ISO 9917-1: 2003) should be replaced and should no longer be advocated for the predictive performance modelling of GI restoratives.