Flexural fatigue behavior of machinable and light-activated hybrid composites for esthetic restorations

The purpose of this study was to determine the flexural strength and flexural fatigue strength of a machinable composite (GN-I) and three hybrid composites (Artglass, Estenia, and Gradia). Specimens (2 x 2 x 25 mm) were polymerized in a laboratory photo-curing unit and then immersed in water at 37 degrees C for 24 h, 6 months, and 1 year. After each immersion period, flexural strengths (4-PFS) were measured at a cross-head speed of 1 mm/min in 4-point bending (span = 20 mm; distance between inside loading points = 10 mm). The cyclic fatigue test was performed at 2.0 Hz on a fatigue tester. The staircase method was employed for flexural fatigue strength (FFS) using a 5000 cyclic load limitation, 5-MPa stress increment, and 20 specimens for each material. Both 4-PFS and FFS of the machinable composite after all immersion periods were significantly greater than those of the three hybrid composites. The FFS results followed the same pattern as those of the 4-PFS. The Estenia material exhibited the highest 4-PFS and FFS after 24 h of immersion among three hybrid composites, whereas after 1 year of immersion, all three composites showed almost identical 4-PFS and FFS statistically. A strong correlation was observed between 4-PFS and FFS of the machinable composite and three hybrid resin composites.