The
aim was to evaluate the effect of short glass-fiber/filler particles
proportion on fracture toughness (FT) and flexural strength (FS) of an
experimental flowable fiber-reinforced composite (Exp-SFRC) with two methacrylate
resin formulations. In addition, we wanted to investigate how the
fracture-behavior of composite restorations affected by FT values of
SFRC-substructure.

Exp-SFRC was prepared by mixing 50 wt% of dimethacrylate based resin matrix (bisGMA or UDMA based) to 50 wt% of various weight fractions of glass-fiber/particulate filler (0:50, 10:40, 20:30, 30:20, 40:10, 50:0 wt%, respectively). FT and FS were determined for each experimental material following standards. Specimens (n = 8) were dry stored (37 °C for 2 days) before they were tested. Four groups of posterior composite crowns (n = 6)
composed of different Exp-SFRCs as substructure and surface layer of
commercial particulate filler composite were fabricated. Crowns were
statically loaded until fracture. Failure modes were visually examined.
The results were statistically analysed using ANOVA followed by post hoc
Tukey’s test.

ANOVA revealed that ratio of glass-fiber/particulate filler had significant effect (p < 0.05) on tested mechanical properties of the Exp-SFRC with both monomer systems. Exp-SFRC (50 wt%) had significantly higher FT (2.6 MPam^1/2) and FS (175.5 MPa) (p < 0.05) compared to non-reinforced material (1.3 MPam^1/2, 123 MPa).
Failure mode analysis of crown restorations revealed that FT value of
the substructure directly influenced the failure mode.

This study shows that short glass-fibers can significantly reinforce flowable composite resin and the FT value of SFRC-substructure has prior importance, as it influences the crack arresting mechanism.