Silver nitrate was selected in this study, because it has
been accepted as a suitable method for measuring both
microleakage and nanoleakage. The silver ion is very
small (0.059 nm-diameter) when compared to the size of
a typical bacterium (0.5-1.0 µm). This small size, and ate agent to detect nanoporosities within the
hybrid layer.23
Clinical failure of resin composite restorations
due to disruption of the bonded interface between
composite and dentin remains a frequent occurrence.1 Such interfacial defects may develop as a consequence of long-term thermal and mechanical
stresses or during the restorative procedure itself,
due to stresses generated by composite polymerization shrinkage. It has been reported that marginal gaps increase as cavity design changes from a
V-shaped to a box-shaped configuration. The term
“cavity configuration factor” (C-factor) has been
used to describe the difference in cavity design.A
high C-factor, which indicates a high number of
bonded to unbonded composite surfaces, corresponds to a high stress value.26 Hence, saucershaped, shallow cavities were employed in this
study to reduce composite contraction stress. Also,
all samples in this study were exposed to the same
thermal stress through thermocycling. However,
gap formation was observed at the
composite/dentin interface mostly in cavities
restored with Filtek Flow and Admira Flow. The
gaps found in the current study were considered
true gaps, because, by using silver nitrate staining,
the artifactual gaps created upon high vacuum
dehydration during specimen preparation for SEM
can easily be differentiated from true gaps by the
absence of silver staining along the gap border.28
In the current study, one bonding agent was used
with the four restorative materials in order to study
the leakage pathway of the resin composites and to
exclude variation of the different bonding systems
to eliminate marginal leakage. Nevertheless, the
different materials showed different leakage pathways. Filtek Flow and Admira Flow showed typical
microleakage manifested as gap formation at the
tooth/restoration interface. This indicates polymerization shrinkage, which results in resin pulling
away from dentin, leading to gap formation. On the
other hand, Grandio Flow showed similar behavior
to the hybrid restorative Z250, where gaps were
rarely seen. Instead, the last two materials showed
the typical leakage pathway that was described by
Sano and others8 and termed as nanoleakage.
The different behavior of the four materials could
be attributed to the differing chemistry of these
materials. Volumetric shrinkage experienced by
composite is determined by several factors, including its filler volume.15 Flowable composites present
higher volumetric shrinkage than hybrid composites due to their reduced filler content and
increased resin matrix. Filtek Flow has 47% filler
by volume and Admira flow has 50% filler by vol