Polymerization Shrinkage Stress and Biaxial Flexural Strength of Bulk-fill Composites

Objectives: The objectives of the study were to investigate the polymerization shrinkage stress (PS) and influence of depth on biaxial flexural strength (FS) and modulus (MO) of bulk-fill composites.
Methods: The bulk-fill composites tested were: Surefil SDR Flow (S); Tetric EvoCeram Bulk-fill (T); Filtek Bulk-fill (F); EverX Posterior (E). A regular composite was used as control: Herculite Classic (H). The PS (n=5) was determined by insertion of the composite (height = 0.8 mm) between polymethylmethacrylate rods (diameter = 4 mm) attached to a universal testing machine with an extensometer. The force development was monitored for 10 minutes from the beginning of light-activation and the shrinkage stress was calculated (MPa). To the FS and MO tests (n=10), the materials were placed in eight overlapped molds (6 mm diameter x 0.5 mm thickness), separated by polyester stripes. The sets with 4 mm depth were light-activated and the discs were submitted to FS and MO test in a universal testing machine (1.27 mm/min).
Results: The results were analyzed by two-way ANOVA and Tukey`s test (p<0.05). The PS of bulk-fill composites ranged from 4.3 to 2.6 MPa (Graphic 1). E presented the highest PS values, while T the lowest one. Both materials were statistically different from the other groups. The FS test (Table 1) did not showed significant statistical difference in the eight depths evaluated to the groups S, T, F and E. To the H group, higher FS values were observed in the 2 first millimeters when compared to the last layer. Distinct values of MO were found between the composites, except for F and T.
Conclusions: Most of bulk-fill composites have similar PS to the control group, only T showed lower shrinkage stress. The depth does not affect the FS of bulk-fill materials, which suggest appropriate polymerization in the full extent of these materials.