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

Dental Materials (2005) 21, 999–1007
Micro-tensile bond strength of adhesives bonded to
class-I cavity-bottom dentin after thermo-cycling
Jan De Munck, Kirsten Van Landuyt, Eduardo Coutinho, André Poitevin,
Marleen Peumans, Paul Lambrechts, Bart Van Meerbeek*
Leuven BIOMAT Research Cluster, Department of Conservative Dentistry, School of Dentistry, Oral
Pathology and Maxillo-Facial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, 3000 Leuven,
Belgium
Received 8 June 2004; received in revised form 6 October 2004; accepted 19 November 2004
KEYWORDS
Thermo-cycling;
Adhesion;
Dental adhesive;
Enamel;
Dentin;
Bond strength
www.intl.elsevierhealth.com/journals/dema
Summary A widely used artificial aging methodology is thermo-cycling. The ISO
TR 11450 standard (1994) recommends 500 cycles in water between 5 and 55 8C.
Recent literature revealed that more cycles are needed to mimic long-term bonding
effectiveness. Furthermore, the artificial aging effect induced by thermo-cycling is
not clearly established. Two underlying mechanisms can be advanced: (1) hot water
may accelerate hydrolysis and elution of interface components and (2) repetitive
contraction/expansion stress can be generated.
Objectives: The purpose of this study was to evaluate the relative contribution of both
chemical (hydrolysis and elution of interface components) and mechanical (repetitive
contraction/expansion stress) degradation pathways on the thermo-cycling-induced
artificial aging of dentin–adhesive interfaces at the bottom of class-I cavities.
Methods: The micro-tensile bond strength (mTBS) of contemporary adhesives
(a three-step etch and rinse, a two-step and a one-step self-etch adhesive) bonded
to class-I cavity-bottom dentin was determined after 20,000 cycles as well as after 20
days of water storage (control). Restored class-I cavities (repetitive contraction/expansion
stress) as well as prepared micro-specimens (diffusion-dependent
hydrolysis and elution) were subjected to the thermo-cycling regimen.
Results: Thermo-cycling did not enhance chemical or mechanical degradation of the
bonds produced by a two-step self-etch and a three-step etch and rinse adhesive to
dentin. The one-step self-etch adhesive tested was, however, not able to withstand
polymerization shrinkage stress, nor thermo-cycling, when applied in class-I cavities.
Significance: Thermo-cycling results in combined contraction/expansion stress and
accelerated chemical degradation. However, the relative contribution of each is
strongly dependent on the specific test set-up and the adhesive used.
Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: C32 16 33 75 87; fax: C32 16 33 27 52.
E-mail address: bart.vanmeerbeek@med.kuleuven.ac.be (B. Van Meerbeek).
0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2004.11.005

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,

Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling 1001
Table 1 Adhesives used.
Adhesive Composition Application
OptiBond FL Etchant: 37.5% phosphoric acid,
Apply the etchant for 15 s; rinse for 15 s; gently
(Kerr, Orange, silica thickener [301194]
air dry for 5 s; scrub the surface for 15 s with
CA, USA) Primer: HEMA, GPDM, PAMM, ethanol, water, primer; apply a thin coat of bonding agent and
photoinitiator [212652]
Bond: TEGDMA, UDMA, GPDM, HEMA, bis-GMA,
filler, photoinitiator [301335]
light cure for 30 s
Protect Bond Primer: MDP, MDPB, HEMA, initiator, water Apply the primer for 20 s using a rubbing
(Kuraray, [ABB-002]
motion; gently air dry; apply the bonding
Osaka, Japan) Bond: MDP, HEMA, dimethacrylates, colloidal
SiO2, surface treated NaF, initiator [ABP-001]
agent; light cure for 10 s
iBOND (Heraeus Adhesive: UDMA, 4-MET, gluteraldehyde, Apply in three consecutive times and rub for
Kulzer, Hanau, acetone, water, stabilizer, photoinitiator 30 s; gentle air dry until adhesive moves no
Germany) [010028]
more; thoroughly air dry for 5 s; light cure
for 20 s
Bis-GMA, bisphenol-glycidyl methacrylate; GPDM, glycerol phosphate dimethacrylate; HEMA, 2-hydroxyethylmethacrylate; MDP,
10-methacryloyloxydecyl dihydrogen phosphate; MDPB, 12-methacryloyloxydodecylpyridinium bromide; PAMM, phthalic acid
monoethyl methacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate; 4-MET, 4-methacryloyloxyethyl
trimellitic acid.
USA). Light-curing was performed using a highpower
LED curing device (L.E.Demetron 1, Demetron/Kerr,
Danbury, CT, USA).
Then, the restored cavities were sectioned
perpendicular to the adhesive–tooth interface
using an Isomet diamond saw (Isomet 1000,
Buehler Ltd, Lake Bluff, IL, USA) to obtain
rectangular sticks (1.8!1.8 mm wide; 8–9 mm
long). Out of each tooth, four sticks were
sectioned from the central cavity floor (Fig. 1).
They were mounted in the pin-chuck of the
MicroSpecimen Former and trimmed at the biomaterial–tooth
interface to a cylindrical hour-glass
Figure 1 Schematic study design.
shape with a bonding surface of about 1 mm 2 using
a fine cylindrical diamond bur (835KREF, Komet,
Lemgo, Germany) in a high-speed handpiece under
air/water spray coolant. Specimens were then fixed
to Ciucchi’s jig with cyanoacrylate glue (Model
Repair II Blue, Sankin Kogyo, Tochigi, Japan)
and stressed at a crosshead speed of 1 mm/min
until failure in a LRX testing device (LRX, Lloyd,
Hampshire, UK) using a load cell of 100 N. The mTBS
was expressed in MPa, as derived from dividing
the imposed force (N) at the time of fracture by the
bond area (mm 2 ). When specimens failed before
actual testing, the mTBS was determined from

1002
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.

Micro-tensile bond strength of adhesives bonded to class-I cavity-bottom dentin after thermo-cycling 1003
Figure 2 mTBS after thermo-cycling. The box represents the spreading of the data between the first and third quartile.
The central vertical line represents the median. The whiskers denote the range of variance and outliers are represented
by a dot.
Discussion
The hypothesis that thermo-cycling of restored
occlusal class-I cavities (repetitive contraction/
expansion stress) as well as of mTBS sticks (diffusion-dependent
hydrolysis and elution) does not
decrease mTBS was confirmed, as for none of the
adhesives a decreased mTBS was recorded after
thermo-cycling (20,000 cycles). Because of the
specific study design, in which thermo-cycled as
well as non-thermo-cycled sticks originated from
the same teeth (Fig. 1), an additional paired
analysis was carried out to compare the control
and thermo-cycling/stick group. This analysis is
more powerful than the standard ANOVA analysis,
because the variable ‘tooth’ was statistically
excluded. Also using this more powerful analysis,
no significant effect of thermo-cycling was
recorded for OptiBond FL (pZ0.68), nor for Clearfil
Protect Bond (pZ0.8624).
Storage of small mTBS specimens in water for
relatively short periods (3 months and longer) can
significantly reduce the mTBS [13]. Given the long
time needed to implement the thermo-cycling
Table 3 Failure analysis under the light microscope.
Experimental group Interfacial Mixed failure
failure
a
Failure in Total (n)
resin
OptiBond FL Control 1 5 5 11
Thermo-cycling Cavity 2 5 4 11
Stick 4 7 2 13
Protect
Bond
Control 0 6 5 11
Thermo-cycling Stick 1 5 8 14
Cavity 0 8 4 12
1 3
Thermo-cycling Stick 0 3 b
1 4
Cavity 1 b
5 b
3 9
iBOND Control 0 2 b
a Mixed failure, interfacial failure and failure within resin.
b Feg-SEM evaluation revealed that some interfacially failed areas, actually failed within resin (Fig. 5).

1004
J. De Munck et al.
Figure 3 Feg-SEM of OptiBond FL. (a) Photomicrograph of the fractured surface of a control specimen (stored as mTBS
stick in water for 20 days) at the dentin side. The specimen failed at the interface (I) and within the adhesive resin (Ar).
(b) Composite counterpart of (a). (c) Photomicrograph of the fractured surface of a thermo-cycling/cavity specimen at
the dentin side. The specimen failed at the interface (I) and within the adhesive resin (Ar). (d) Higher magnification of
the composite counterpart of (c). The specimen failed at the top of the hybrid layer (Hy), with the heads of the resin tags
still attached to the adhesive resin. (e) Photomicrograph of the fractured surface (dentin side) of a thermo-cycling/stick
specimen. The specimen failed at the interface (I) and within the adhesive resin (Ar). (f) Higher magnification of (e) at
an area that failed near the interface. The specimen actually failed at the top of the hybrid layer (Hy).
Figure 4 Feg-SEM of Clearfil Protect Bond. (a) Photomicrograph of the fractured surface of a control specimen (stored
as mTBS stick in water for 20 days) at the dentin side. The specimen failed mainly near the interface (I), apart from a
small part that failed within the adhesive resin (Ar). (b) Higher magnification of the composite counterpart of (a). The
specimen failed within the hybrid layer (Hy). Some resin flashes (Ar) remained attached to the dentin side of the beam.
(c) Photomicrograph of the fractured surface (dentin side) of a thermo-cycling/cavity specimen. The specimen failed at
the interface (I) and within the adhesive resin (Ar). (d) Higher magnification of the composite counterpart of (c). The
specimen failed within the hybrid layer (Hy). (e) Photomicrograph of the fractured surface (dentin side) of a thermocycling/stick
specimen. The specimen failed at the interface (I) and within the adhesive resin (Ar). (f) Higher
magnification of (e) at an area that failed near the interface. Again the specimen failed within the hybrid layer (Hy). No
morphological differences with the control group could be observed.

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

1006
etch and rinse control (Table 2; Fig. 2). Given the nonchanged
mTBS and fracture surface ultra-morphology
(Fig. 4), this adhesive also resisted to the thermocycling
regimen very well. This is consistent with in
vitro research, in which its predecessor Clearfil SE
(very similar in composition to Clearfil Protect Bond,
apart from the antibacterial monomer added to the
latter) performed very well. This two-step self-etch
adhesive resisted to 1 year in vivo functioning [26,27],
up to 30,000 thermo-cycles in a ‘shear-bond’ strength
test [5], and combined thermal and occlusal loading
[9]. Long-term water storage of prepared mTBSbeams
on the other hand, decreased the bond
strength to dentin [16,23]; 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 adhesive performed very well [28,29].
iBOND was not able to produce a strong bond to
dentin at the bottom of an occlusal class-I cavity
(Table 2). For the control and the thermo-cycling/
stick group, all pre-testing failures occurred after
24 h during further specimen preparation. Because
of this low bonding effectiveness at baseline and
the low number of remaining specimens, no
conclusion can be drawn regarding degradation of
the resultant adhesive–tooth bond. Nonetheless,
the bond strength of this adhesive is too low to
resist the polymerization shrinkage of the restorative
composite in a class-I cavity. Analysis of the
fracture planes revealed that in all groups porosities
were observed in the adhesive resin near the
interface. This certainly must have weakened the
bond and is to a large extent responsible for the low
bonding effectiveness recorded. Similar porosities
were observed by Tay et al. [30,31]. These
porosities may be due to residual solvent (H2O)
that was not adequately removed because of
inefficient drying in a narrow cavity. Alternatively,
these porosities may also be caused by an osmotic
driven water uptake from dentin and/or the
environment, as these one-step self-etch adhesives
can act as semi-permeable membranes [30]. The
large amount and density, as seen in this study, may
be due to the storage in water for 20 days that
allowed this water uptake to take place to its full
extent. The bonding effectiveness after 24 h would,
however, not have been that different, as all pretesting
failures occurred during specimen preparation
1 day after adhesive procedures. The most
plausible explanation that follows out of recent
research [32,33] is that these porosities represent
water droplets that separated from the monomers
that no longer remained dissolved in water upon
evaporation of acetone.
In conclusion, thermo-cycling did not result in an
enhanced chemical or mechanical degradation of
the bonds to dentin produced by a two-step selfetch
and a three-step etch and rinse adhesive. The
bonding effectiveness of the one-step self-etch
adhesive tested was, however, too low to withstand
polymerization shrinkage stress, as produced in an
occlusal class-I cavity.
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