Fracture Toughness of Restorative Glass-ionomer Cement with Nanoparticles

Objectives:To determine if the fracture toughness (K_IC) of restorative grade glass ionomer cement is increased by the addition of polyhedral oligomeric silsesquioxane (POSS) particles (Hybrid Plastics). Conventional glass ionomer cements have low fracture toughness values limiting their clinical use. POSS particles are inorganic silicon-oxygen molecules with external functional groups grafted, with the potential to interact chemically with glass-ionomer cements and influence physical properties at the molecular level. Methods: A commercially-available conventional restorative glass-ionomer cement was used in this study (Fuji IX, GC International). Six groups were tested: control (C) no POSS particles added, then 0.3, 0.5, 1, 2 and 5% (weight) particles were added to the cement liquid and mixed with an ultrasonic probe for 5 minutes, prior to hand-mixing with the cement powder. The cements were moulded in glass cylinders and stored in paraffin at 37^oC for 14 days. On day 7, the specimens were prepared for chevron notch short-rod specimens, prior to testing to failure in a mechanical testing machine (Lloyds LRX Tensile Tester) at a cross-head speed of 0.004 mm/s. Fracture toughness (K_IC) was calculated using Barker's method (1977). The results were subjected to ANOVA (p<0.05) and, if appropriate, Kruskal Wallis post-hoc tests. Results: Mean (K_IC+SD) values MN/m^3/2 for each group were: C 0.33 (0.03), 0.3% 0.35 (0.05), 0.5% 0.36 (0.06), 1% 0.34 (0.04), 2% 0.31 (0.05) 5% 0.28 (0.04). Analysis of Variance indicated that the fracture toughness of the 5% group was significantly lower than the 0.5% group. There were no significant differencews between all other groups. Conclusions: The significant reduction in fracture toughness by addition of 5% POSS may be due to agglomeration of particles at this relatively high concentration.