Effect of High Irradiance on Fracture Toughness of Bulk-Fill Composites

Objectives: To determine the effect of using a high irradiance and short exposure time on the fracture toughness of bulk-fill resin-based composites (RBCs).
Methods: Three RBCs were tested: Tetric PowerFill (TPF / Ivoclar), Opus Bulk Fill APS (OBF / FGM) and Filtek One Bulk Fill (FOB / Solventum). Sixty single-edge-notched disc specimens (6.5 mm diameter, 2.5 mm thick and 3.5 mm notch) were prepared using a fracture toughness mold (Odeme Dental Research). The RBCs were light-cured (n=10/group) for the exposure time recommended by manufacturers (TPF: 10s, OBF: 30s and FOB: 20s) in “High Power” mode, or in a high-irradiance mode (“3s cure”) using the Bluephase PowerCure (Ivoclar). The wavelength of peak spectral emissions, power output, incident irradiance and radiant exposure on composite surfaces from the curing modes of curing unit were determined using a spectrophotometer (Gigahertz-Optik). The specimens were stored in distilled water at 37°C for 24 h and tested on a universal machine (4411, Instron) at 1.0 mm/min crosshead speed until failure. Data were analyzed by two-way ANOVA and Bonferroni test (α=0.05).
Results: The radiant exposure values (in J/cm²) received by each specimen using the 3s mode was 9.5, while the exposure times for 10, 20 and 30s (in High Power mode) were: 12.4, 24.8 and 37.1, respectively. The highest fracture toughness means (in MPa.m^0.5) were found for FOB (4.22/10s and 3.79/3s), while the lowest one for OBF (2.01/30s and 2.10/3s), regardless the exposure time. TPF produced intermediate results (2.72/10s and 2.70/3s). The exposure times did not affect the fracture toughness results of TPF and OBF, while 3s curing time significantly reduced the fracture toughness for FOB.
Conclusions: The RBCs tested had different fracture toughness values, regardless the exposure time. The use of a high irradiance and short exposure time can reduce fracture toughness depending on the RBC tested.