The initial rapid photo-polymerization of resin-modified glass ionomers should make the on going acid base reaction less susceptible to any deleterious effects of water.  Objective: This study evaluated the effect of maturation time and storage conditions on the fracture toughness of resin-modified glass ionomer (RMGI, Fuji II LC, GC America). Methods: Triangular molds (4x4x4x8 mm) were used to make 60 specimens, which were randomly divided into 5 groups. Light curing at 650 mW.cm^-2 was performed 1 minute after mixing through a silicon-coated glass slide for 40 seconds, with an additional 40s light-cure from the bottom after removal from the mold. Fracture toughness (K_QIvM) was determined according to the methods of Ruse et al. (J Biomed Mater Res 1996;31:457-463) with a crosshead speed of 0.3 mm/min. Group 1 was tested 4 minutes from the beginning of mixing, groups 2-5 were tested after being stored in 100%RH for either 1 or 3 days before transferring to artificial saliva or oil (Inland 45, Inland Vacuum Industries) for a total of 1 week storage at 37°C. Differences in fracture toughness were tested with ANOVA and Tukey (HSD) post-hoc at 95% confidence level. Results:

	Storage Conditions		Mean (SD)	Homogenous
Group	Initial	Final	(MPa m1/2)	Subsets
1	4 min bench	NA	0.37(0.05)	1
2	1d 100% RH	6d saliva	0.89(0.18)	2        3
3	1d 100% RH	6d oil	0.73(0.11)	2
4	3d 100% RH	4d saliva	0.98(0.20)	3
5	3d 100% RH	4d oil	0.82(0.35)	2        3

Conclusions: The photo-polymerization reaction may account for roughly half of the fracture toughness of RMGI based on immediate K_QIvM data.  Under the conditions of this study the exclusion of extrinsic water (oil) demonstrated no significant different effect on K_QIvM.