fracture toughness of nanohybrid and hybrid composites stored wet and dry up to 60 days

statement  of  problem:  patients’ demand  for  tooth-colored  restoratives  in the posterior region is increasing. clinicians use universal nanohybrid resin composites for both anterior and posterior regions. there are few published reports comparing fracture toughness of nonohybrids and that of hybrid composite stored wet and dry. objectives: to investigate the fracture toughness of three nanohybrids compared to that of a hybrid resin composite stored dry or wet up to 60 days, using four- point bending test. materials and methods: four resin composites were used: three nanohybrids; filtek supreme (3m), ice (sdi), tph3 (dentsply) and one hybrid filtek p60 (3m). for each material, 40 rectangular notched beam specimens were prepared with dimensions of 30 mm × 5mm × 2mm. the specimens were randomly divided into 4 groups (n = 10) and stored at 37ºc either in distilled water or dry for 1 and 60 days. the specimens were placed on the four-point test jig and subjected to force (n) using universal testing machine loaded at a crosshead speed of 0.5mm/min and maximum load at specimen failure was recorded and k ic  was calculated. results: three-way anova showed a significant interaction between all the factors (all p < .0001). except for tph3, all tested materials showed significantly higher k ic  when stored dry than stored wet ( p < 0.05). after 1 day of dry storage, ice showed the highest k ic  (2.04 ± 0.32) followed by filtek p60 and the lowest was for filtek supreme (1.39 ± 0.13). the effect of time on fracture toughness was material dependent. conclusions: wet storage adversely affected the fracture toughness of almost all materials. keeping the restoration dry in the mouth may increase their fracture toughness. therefore, using a coating agent on the surface of restoration may protect them from early water uptake and increase their strength during a time period.