Glass-Ionomer Restoratives – What Test Provides the Best Performance Indicator?

Objectives: The compressive fracture strength is the only mechanical property investigated in ISO 9917-1 for assessing glass-ionomer (GI) peformance. Therefore it is difficult to predict how the materials will perform when placed by dental practitioners in clinical practice due to the complex stress patterns generated under testing. The aim was to investigate the fracture strength and reliability of the strength data in compression, bi-axial flexure and three-point flexure to assess the most reliable method of strength determination.

Methods: The encapsulated GI restoratives (Ketac™ Molar Aplicap™, Fuji IXGP Fast Capsule and ChemFlex™ in Caps) were mechanically mixed in either a Capmix™ or Rotomix™ mixing machine. The 24 h mean compressive fracture strengths (6mm height, 4mm diameter), bi-axial flexure strength (12mm diameter, 1.5mm thickness) and three-point flexure strength (25mm length, 2mm thickness, 2mm width) were determined for groups of 20 specimens. The data was analysed using a one-way ANOVA and Tukey's test comparisons at a 95% significance level (P<0.05) and Weibull statistics. Results: The method of mechanical mixing (Capmix™ or Rotomix™) was not a factor in the mean strength achieved regardless of the testing method. The mean GI compressive fracture strengths (115-137MPa) of the encapsulated GI restoratives were significantly increased (P<0.05) compared with the mean bi-axial (54-72MPa) and three-point flexure strengths (18-41MPa). However, the bi-axial fracture strength data was indicative of a single defect mechanism with R2-values of >0.95 being produced.

Conclusion: Bi-axial flexure strength testing offers a methodology that produces a pure tensile stress state rather than the complex stress states the compression and three-point flexure methodologies. Bi-axial flexure strength testing methodology therefore afford reproducible data which would increase its usefulness as a GI performance indicator and inclusion in ISO 9917-1.