Fracture toughness of glass ionomers measured with two different methods

Glass Ionomer Cements (GICs) are brittle materials with low fracture toughness and strength. Therefore, understanding the fracture toughness is an important parameter to know, due to GICs being promoted for load-bearing restorations. Also, little is known about the effects of artificial saliva (AS) on fracture toughness of GICs. This project aimed to study the effects of storage and compare two different fracture toughness test methods, namely: Compact-Tension test and 4-point bending test. Samples were made from a Zinc-reinforced-GIC (ChemFil), High viscosity bulk-fill-GIC (HV) with and without LC-coating (EQUIA), Resin-Modified GIC (RM-GIC) (Riva-LC), HV-RMGIC (Riva-HV-LC) and flowable bulk-fill Giomer (Beautifil-Bulk Flowable) using a custom-made mould. Specimens were stored in either AS or distilled-water (DW) for 7 or 30 days. After storage, specimens were tested for fracture toughness. Results were analysed with Analysis of variance (ANOVA), Mann-Whitney and Weibull statistics. ANOVA showed a significant difference (P < 0.05) between methods, however the same trend among materials was observed in both methods. Some materials showed significantly higher values for samples stored in DW and others revealed the same trend but were not significant. Significantly higher (P < 0.05) mean fracture toughness values were observed for materials stored for 30 days. Since both fracture toughness test methods revealed the same fracture toughness strength order among materials, either test appears appropriate. GICs maturation led to a difference between 7 and 30-day results. The AS used had a high ionic strength compared to DW, which may have led to increased solubility of GICs and consequently to the reduced fracture toughness values. The results revealed that coating the GIC has a protective effect against storage media.