Micro-tensile bond strength of adhesives bonded to class-I cavity ...

Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling 1005
Figure 5 Feg-SEM of iBOND. (a) Photomicrograph of the fractured surface of a control specimen (stored mTBS
specimen in water for 20 days) at the dentin side. The specimen failed mainly within the adhesive resin (Ar). A small part
failed near the interface (I). (b) Photomicrograph of the fractured surface (dentin side) of a thermo-cycling/cavity
specimen. The specimen failed entirely within the adhesive resin (Ar). A large part failed, however, very near to the
interface (Ar–I) and appeared very porous. (c) Higher magnification of the area marked by the hand-pointer in (b). Many
small porosities can be observed in the resin part close to the dentin interface (Ar–I). Also in the adhesive resin itself,
some porosities can be observed. (d) Photomicrograph of the fractured surface (dentin side) of a thermo-cycling/mTBS
stick specimen. The specimen failed entirely within the adhesive resin (Ar). Again a large part failed near the interface
(Ar–I) and appeared porous. (e) Composite counterpart of (d). Part of the adhesive resin chipped off during processing
(arrow) and disclosed numerous large voids within the adhesive resin. (f) Higher magnification of (e) at an area that
failed near the interface. Many small porosities (0.5–7.5 mm) can be observed.
regimen (20 days), one can speculate that the
degradation of the interface of the thermo-cycling/
stick group is caused by water exposure rather than
by the thermo-cycling itself. To rule out this option,
the mTBS sticks of the control group were also
stored in water for 20 days at 37 8C, this in contrast
to previous studies that had 24-h controls [14,15].
However, degradation caused by 20 days of water
storage should have been minimal, as the bonds
produced by three-step etch and rinse adhesives
and mild two-step self-etch adhesives resisted up to
1-year direct water exposure [16]. This assumption
is substantiated by the fact that for OptiBond FL,
the highest mTBS was recorded in the thermocycling/cavity
group, the only group not directly
exposed to water for 20 days.
The composite used in this study is known for its
high E-modulus (21 GPa) [17]. Applying this composite
in a relatively small class-I cavity, must have
resulted in high polymerization shrinkage stress [7].
By using a high-efficiency, high-power LED curing
device, the polymerization reaction must have
been rather fast, so that also the plastic flow of
composite, which can reduce the shrinkage stress,
must have been limited. As a result, the cavity
model used in this study represents a clinical ‘worst
case scenario’. In this study, only OptiBond FL and
Clearfil Protect Bond were able to withstand the
shrinkage stress in this challenging situation, in
contrast to iBOND that performed unreliably.
From a previous review, it was concluded that
10,000 thermal cycles corresponds to 1 year in vivo
degradation [2]. Therefore, the bonds produced by
OptiBond FL should be durable for at least 2 years of
clinical service. This postulation was supported by
many in vitro studies, in which OptiBond FL
successfully withstood to up to 4 years of water
storage, thermo-cycling and/or mechanical loading
[14,–16,18–20]. Only when miniature (0.4–0.6 mm 2 )
mTBS specimens were aged, was a significant
decrease in mTBS observed for this three-step etch
and rinse adhesive [21,22]. All other types of
adhesive did, however, decrease at least to
the same extent in a similar study [23]. Also in
clinical class-V studies, this three-step etch and
rinse adhesive performed very reliably for up to 5
years of clinical service [24,25].
The bond strengths obtained with the mild twostep
self-etch adhesive Clearfil Protect Bond were
not significantly different from the three-step