Crushing truth about glass-ionomers: Exposing the standard of the standard

Objectives: The compressive fracture strength (CFS) test is the only strength property for glass-ionomers (GIs) specified for inclusion in ISO 9917-1: 2003. The CFS test was the subject of much controversy in the dental literature in 1990 and has been challenged over its appropriateness and reproducibility. Despite the strong evidence for the removal of CFS from the standard – the CFS test remains. The aim of the study was to revisit the suitability of the CFS testing methodology for GIs. Methods: The CFS of groups of 20 cylindrical specimens (6.0 ± 0.1 mm height, 4.0 ± 0.1 mm diameter) for three encapsulated GIs, manipulated using two mechanical mixing regimes, by two operators were examined. The CFS data for each GI restorative were pooled and three- and two-way analyses of variance (ANOVAs) were conducted at a significance value of p=0.05 where the factors (each at two levels) were operator, mixing regime and batch. Results: The three-way ANOVAs showed a significant interaction of operator × mixing regime × batch (p<0.017) for two of the three materials. However, there were no significant effects of operator × mixing regime (p>0.042), operator × batch (p>0.332) or mixing regime × batch (p>0.056). Also no significant differences between operators (p>0.094), mixing regime (p>0.118) or batch (p>0.054). Two-way ANOVAs identified no significant effect of operator × batch (p>0.369), operator (p>0.122) or batch (p>0.061) for all GI restoratives. When the 20 specimens were examined in batches of five (or ten where appropriate) as specified in ISO 9917-1: 2003 and its predecessor ISO DP 9917, inter- and intra-operator variability were evident. Conclusions: The inter- and intra-operator variability reported seriously undermine the validity of the CFS testing methodology as an international standard and an alternative means of evaluating GIs should be adopted by the ISO panel.