A discriminatory mechanical testing performance indicator protocol for hand-mixed glass-ionomer restoratives

ObjectivesTo identify a reproducible and discriminatory mechanical testing methodology to act as a performance indicator for hand-mixed glass-ionomer (GI) restoratives.MethodsGroups of 20 (five batches of four) cylinders (6.0 ± 0.1 mm height, 4.0 ± 0.1 mm diameter) for compressive fracture strength (CFS) and compressive modulus (CM) testing, bars (25.0 ± 0.1 mm length, 2.0 ± 0.1 mm width, 2.0 ± 0.1 mm thickness) for three-point flexure strength (TFS) and tensile flexural modulus (TFM) testing, discs (13.0 ± 0.1 mm diameter, 1.0 ± 0.1 mm thickness and 10.0 ± 0.1 mm diameter, 3.10 ± 0.03 mm thickness) for biaxial flexure strength (BFS) and Hertzian indentation (HI) testing, respectively, were prepared using a hand-mixed GI restorative manipulated with 100–20% (in 10% increments) of the manufacturers recommended powder content for a constant weight of liquid. Data were statistically analyzed at p < 0.05, the coefficient of variation (CoV) was assessed for the four tests at each powder:liquid mixing ratio investigated (n = 9) and a Weibull analysis performed on the CFS, TFS and BFS data to assess the reliability of the data sets. The failure mode and fracture origin of the HI specimens was assessed by fractography.ResultsFor the hand-mixed GI restorative, a progressive reduction in the powder content (by 10% for a constant weight of liquid) resulted in a progressive linear deterioration (p < 0.001) in the CFS (R2 = 0.957), CM (R2 = 0.961) and TFM (R2 = 0.982) data. However, no linear deterioration (p > 0.05) was identified for the TFS (R2 = 0.572), BFS (R2 = 0.81) and HI (R2 = 0.234). The CoV and Weibull data identified distinct regions – three for the CFS and TFS data and two for the BFS data sets, within the range of powder:liquid mixing ratios investigated. Fractographic analysis of HI specimens revealed a transition in failure mode from bottom-initiated radial cracking to top-initiated cone cracking on reducing the powder content for a constant weight of liquid.SignificanceThe CFS test is the only discriminatory performance indicator for hand-mixed GIs from amongst the four mechanical testing approaches (CFS, TFS, BFS and HI) investigated. The CM and TFM represent an intrinsic material property independent of specimen dimensions and may be used as an adjunct to a mechanical testing approach when investigating hand-mixed GIs.