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

1000
Introduction
Thermo-cycling is a widely used artificial aging
methodology. The ISO TR 11450 standard [1]
indicates that a thermo-cycling regimen comprising
500 cycles in water between 5 and 55 8C is an
appropriate artificial aging test. A recent literature
review [2] concluded that 10,000 cycles corresponds
approximately to 1 year of in vivo functioning,
rendering 500 cycles, as proposed by the ISO
standard, very minimal to mimic long-term bonding
effectiveness.
The artificial aging effect induced by thermocycling
can be two-fold: (1) hot water may accelerate
hydrolysis of non-protected collagen and
extract poorly polymerized resin oligomers [3–5];
(2) due to the higher thermal contraction/expansion
coefficient of the restorative material (as
compared to that of tooth tissue) repetitive
contraction/expansion stresses are generated at
the tooth–biomaterial interface. This may result in
cracks that propagate along bonded interfaces, and
once a gap is created, changing gap dimensions can
cause in- and outflow of pathogenic fluids, a process
known as ‘percolation’ [2,6]. In the light of the first
aging effect (diffusion-dependent hydrolysis and
elution), thermo-cycling should be applied to
micro-specimens, of which the interface is directly
exposed to the changing temperature environment.
Then, degradation of the adhesive–tooth interface
is most severe, clinically corresponding to the most
vulnerable restoration margins. Aging of mTBSspecimens
might also be more appropriate than
aging of the larger ‘shear-bond’-strength specimens,
because the surrounding composite and
tooth tissue may thermally protect the adhesive–
tooth interface. If the second aging effect (repetitive
contraction/expansion stress) is focused upon,
thermo-cycling should be applied to specimens in
which stress similar to that occurring clinically can
be generated. In vivo stress will be generated if the
ratio of bonded to unbonded surfaces or the
‘C-factor’ is high [7]. Clinically, about the highest
C-factor is generated in narrow occlusal class-I
cavities. In this case, thermo-cycling of the whole
tooth including the high C-factor restoration will
result in the highest possible stress imposed at the
interface.
It is, however, not clear whether or not thermocycling
affects the bond strength to dentin. A
recent meta-analysis [8], concerning data published
between 1992 and 1996, concluded that
thermo-cycling has no significant effect on bond
strength. Most studies included in the meta-analysis
were carried out following the ISO standard of 500
cycles (mean number of cycles in the studies
analyzed was 630). This number of cycles was
probably too low to obtain an aging effect [2,8,9].
Also specimen geometry has often not been taken
into account. In most studies of this review,
relatively large composite cylinders bonded to flat
surfaces were thermo-cycled, prior to being pulled
apart following a shear or tensile bond strength
test protocol [8]. As a result, a large part of the
interface must have been thermally protected
by surrounding dentin [10] and composite [11]
(which are known to be good thermal insulators).
Because of the low C-factor of a flat restored
surface (about 1/6), little repetitive expansion/
contraction stress must have been generated at the
interface. Both reasons might explain why thermocycling
did not affect bonding effectiveness in those
studies.
Therefore, the purpose of this study was to
evaluate the relative contribution of diffusiondependent
chemical degradation and repetitive
contraction/expansion stress on the thermocycling-induced
degradation of dentin–adhesive
interfaces at the bottom of occlusal class-I cavities.
The hypothesis tested was that thermo-cycling of
restored class-I cavities (repetitive contraction/expansion
stress), as well as of mTBS-sticks (diffusiondependent
hydrolysis and elution) does not
decrease bonding effectiveness.
Materials and methods
Specimen preparation
J. De Munck et al.
For this study, non-carious human third molars
(gathered following informed consent approved by
the Commission for Medical Ethics of the Catholic
University of Leuven), stored in 0.5% chloramine
solution at 4 8C were used within 1 month after
extraction. First, all teeth were mounted in gypsum
blocks in order to ease manipulation. A standard
box-type class-I cavity (4.5!4.5 mm) was then
prepared at the occlusal crown center with the
pulpal floor ending at mid-coronal dentin, using a
high-speed hand piece with a cylindrical mediumgrit
(100 mm) diamond bur (842; Komet, Lemgo,
Germany) mounted in a MicroSpecimen Former
(University of Iowa, Iowa City, IA, USA). Next, the
cavities were subjected to a bonding treatment
(Table 1) using either a three-step etch and rinse
adhesive (OptiBond FL), a two-step self-etch
adhesive (Clearfil Protect Bond) or a one-step
self-etch adhesive (iBOND). Subsequently, the
cavities were filled in three horizontal layers with
a resin-composite (Z100, 3M ESPE, St Paul, MN,