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

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the specimens that survived specimen processing
with an explicit note of the number of pre-testing
failures.
Study design
All specimens were randomly divided into nine groups
(3 adhesives!3 experimental groups, Fig. 1) and
subjected to a bonding treatment strictly according
to the respective manufacturer’s instructions
(Table 1). After adhesive procedures, all teeth were
stored in water for 24 h at 37 8C. For each adhesive,
three teeth were subjected to 20,000 thermal cycles
(group 1: thermo-cycling/cavity), i.e. the restored
cavity was changed between two water baths of 5 and
55 8C with a dwell time of 30 s at each temperature
extreme (Thermocycler, Willytec, Munich,
Germany). From six other teeth per adhesive, four
mTBS specimens (per tooth) were prepared. Two of
these specimens were also subjected to the same
thermo-cycling regimen (group 2: thermo-cycling/
stick). The other half of these specimens were stored
for 20 days, the time needed for the thermo-cycling
procedure, in 100% humidity to serve as control
(group 3: control).
Statistical analysis
The results were analyzed at a significance level of
0.05 using a two-way ANOVA and post hoc Tukey–
Kramer multiple comparisons. All statistics were
performed using the statistical software package
(StatSoft, Tulsa, OK, USA).
Failure analysis
The mode of failure was determined light-microscopically
at a magnification of 50! using a
stereomicroscope, and recorded as either ‘failure
within dentin’, ‘interfacial failure’ or ‘failure
within resin’.
From each group, representative mTBS-specimens
were processed for field-emission gun scanning
electron microscopy (Feg-SEM, Philips XL30,
Eindhoven, The Netherlands) using common electron
microscopic specimen processing techniques
including fixation, dehydration, chemical drying,
and gold-sputter coating [12].
Results
The mean mTBS, SDs, the number of pre-testing
failures (ptf) and the total number of specimens (n)
are summarized per adhesive and experimental
condition in Table 2, and graphically presented in
box-whisker plots in Fig. 2. Thermo-cycling of
neither the mTBS specimens, nor the restored
cavities decreased the bond strength of the
adhesives tested.
Pre-testing failures were only recorded for the
one-step self-etch adhesive (iBOND). All pre-testing
failures occurred during specimen processing
(mostly during preparation of the sticks with the
diamond saw). No additional pre-testing failures
were produced by thermo-cycling. Because of the
high number of pre-testing failures, the data of
iBOND were excluded from the statistical analysis,
as too few valid data were available to perform an
adequate analysis and thus to draw a valid
conclusion regarding degradation of this adhesive.
The two-way ANOVA analysis disclosed no significant
difference in mTBS between OptiBond FL
and Clearfil Protect Bond (pZ0.321), nor between
the different experimental conditions (control,
thermo-cycling/cavity and thermo-cycling/stick;
pZ0.111). The bonding effectiveness of the onestep
self-etch adhesive tested, iBOND, was however
already compromised at baseline, given the high
number of pre-testing failures (Table 2).
For none of the adhesives, were morphological
changes induced by thermo-cycling observed using
light-microscopy (Table 3) or Feg-SEM (Fig. 3–5) of
the fracture surfaces. For the one-step self-etch
adhesive, most specimens failed within the resin
(Table 3; Fig. 5). Especially in the areas that
fractured very close (a few mm) to the interface,
the resin appeared very porous, and at higher
magnifications many porosities could be noticed.
The porosity amount and density was clearly higher
in the area near to the interface with dentin, but
also in the adhesive resin itself, some larger
porosities could be observed (Fig. 5).
Table 2 mTBS to dentin.
mTBS (SD)
ptf/n
Control no
thermocycling
Thermo-cycling (20,000
cycles)
Cavity Stick
OptiBond FL 20.0 (3.6) 27.0 (11.5) 18.3 (9.8)
0/11 0/11 0/13
Protect
Bond
J. De Munck et al.
23.8 (8.3) 24.7 (9.9) 23.1 (7.5)
0/11 0/14 0/12
iBOND 14.7 (11.9) 12.1 (4.9) 12.6 (3.8)
6/9 8/12 11/20
mTBS, micro-tensile bond strength, value in MPa; ptf, pretesting
failure; n, total number of specimens; SD, standard
deviation.