The crushing truth about glass ionomer restoratives: exposing the standard of the standard.

OBJECTIVES The compressive fracture strength (CFS) test is the only strength test for glass ionomers (GIs) in ISO 9917-1: 2003. The CFS test was the subject of much controversy in 1990 and has been challenged over its appropriateness and reproducibility and the study aimed to revisit the suitability of the CFS test for GIs. METHODS Groups of 20 (four batches of n=5) cylinders (6.0±0.1mm height, 4.0±0.1mm diameter) of three encapsulated GIs were prepared for CFS testing using two mechanical mixing regimes and two operators. The CFS data for each GI restorative were pooled, three-, two- and one-way analyses of variance (ANOVAs) were conducted (p=0.05) for operator, mixing regime and batch to assess reliability. The data was also analysed according to ISO 9917-1: 2003. RESULTS The three-way ANOVAs showed a significant interaction of operator×mixing regime×batch (p<0.017) for two of the three encapsulated GIs. However, no significant effects of operator×mixing regime (p>0.042), operator×batch (p>0.332), mixing regime×batch (p>0.056), operator (p>0.094), mixing regime (p>0.118) or batch (p>0.054) were evident. When examined in batches of five (or ten where appropriate) as specified in ISO 9917-1: 2003, inter- and intra-operator variability were evident. CONCLUSIONS The use of batch-censoring in accordance with ISO 9917-1: 2003 is unsafe when the data scatter reflects a homogenous flaw distribution as it misidentifies operative variability. Despite demonstrating that the CFS test can be performed reliably, the validity of the CFS test for GIs remains under scrutiny.