Purpose: By combining the
increased toughness of a resin composite reinforced with discontinuous fibers
and the surface wear resistance of a particulate filler composite (PFC), a
bilayered composite technique was recently introduced. This study aimed to evaluate the effect of the
thickness of the overlaying PFC placed over a fiber-reinforced composite (FRC)
core, on the
fracture-behavior of direct crown restorations.

Methods: Six groups of posterior crown restorations (n=8/group)
were fabricated having a discontinuous FRC-core (everX Flow) and a layer of surface PFC (Essentia U) with various thicknesses (0.5, 1.0, 1.5, 2.0 mm), with the remaining
thickness of the bilayered being 6 mm.  Control
groups were only made of PFC or FRC materials. Restorations were statically
loaded until fracture. Failure-modes were
visually examined. Data were analyzed using ANOVA (p = 0.05) and
regression analysis.

Results: The regression analysis showed that by decreasing
the thickness of PFC layer, the load bearing capacity of restorations increased
linearly (R²=0.7909). ANOVA revealed that crown restorations made
only from everX Flow composite had significantly higher load-bearing capacities
(3990 ±331 N) (p<0.05) among all the groups tested. With regard to the
failure-mode analysis, crowns that had a FRC core material of everX Flow
revealed delamination of the PFC surface composite from the core. Crowns which
were made only of PFC i.e. with no fiber
reinforcement, showed a crushing-like fracture pattern.

Conclusions: Restorations
combining a thick FRC-core and a thin surface layer of PFC (0.5-1 mm),
displayed promising performance related to fracture-behavior and load-bearing
capacity.