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Bonding effectiveness of two contemporary self-etchadhesives to enamel and dentin
Atsushi Mine a, Jan De Munck a, Marcio V. Cardoso a, Kirsten L. Van Landuyt a,Andre Poitevin a, Takuo Kuboki b, Yasuhiro Yoshida c, Kazuomi Suzuki c,Paul Lambrechts a, Bart Van Meerbeek a,*a Leuven BIOMAT Research Cluster, Department of Conservative Dentistry, School of Dentistry, Oral Pathology and Maxillo-Facial Surgery,
Catholic University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, BelgiumbDepartment of Oral and Maxillofacial Rehabilitation, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical
Science, Okayama, JapancDepartment of Biomaterials, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Science, Okayama, Japan
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3
a r t i c l e i n f o
Article history:
Received 26 April 2009
Received in revised form
23 June 2009
Accepted 30 June 2009
Keywords:
Adhesion
Strong self-etch adhesive
Mild self-etch adhesive
Bond strength
TEM
Dentin
Enamel
a b s t r a c t
Objectives: Among contemporary adhesives, self-etch adhesives have been adopted by
general practitioners for routine adhesive restorative purposes, mainly because of their
ease of use. However, many versions that differ for their clinical application procedure, pH,
number of components, etc., are currently available on the market. The purpose of this
study was to determine the bonding effectiveness of two new self-etch adhesives (Adper
Easy Bond and Adper ScotchBond SE, 3M ESPE) to enamel and dentin using a micro-tensile
bond strength (mTBS) protocol and to characterise the interfacial ultra-structure at enamel
and dentin using transmission electron microscopy (TEM).
Methods: The adhesives were applied onto coronal human enamel and dentin surfaces and
built up with the micro-hybrid resin composite Z100 (3M ESPE). The ‘gold-standard’ two-step
self-etch adhesive Clearfil SE Bond (Kuraray) served as control. Specimens were sectioned to
sticks and trimmed at the interface to a cylindrical hour-glass shape (‘trimmed’ micro-
specimens). Non-demineralized and demineralized TEM sections through the adhesive-
dentin/enamel interface were prepared by ultra-microtomy.
Results: The mTBS of the two self-etch adhesives to enamel was statistically significantly
lower than that of the control. To dentin, the mTBS of Adper Easy Bond was significantly
lower than that of Adper ScotchBond SE and the control. TEM showed a tight interface to
enamel for all three self-etch adhesives. A relatively thick, completely demineralized and
acid-resistant hybrid layer was formed at dentin by Adper ScotchBond SE, whereas the
interaction of Adper Easy Bond was much shallower, and comparable to that of so-called
‘ultra-mild’ self-etch adhesives. Some degree of spot- and cluster-like nano-leakage was
observed for both adhesives, but did not differ in extent and form from that observed for the
control.
Conclusions: Although the new two self-etch adhesives revealed a tight interaction at both
enamel and dentin, their bond strength to both tooth tissues was generally lower than that
of the control adhesive. Nevertheless, their bonding effectiveness appears in line with other
simplified self-etch adhesives.
# 2009 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +32 16 33 75 87; fax: +32 16 33 27 52.
avai lable at www.sc iencedi rec t .com
journal homepage: www.intl.elsevierhealth.com/journals/jden
E-mail address: [email protected] (B. Van Meerbeek).
0300-5712/$ – see front matter # 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2009.06.020
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3 873
1. Introduction
Among contemporary adhesives, self-etch adhesives have
become popular, especially because of their ease-of-use and
faster application procedure.1 Two new self-etch adhesives,
marketed as Adper Easy Bond (3M ESPE, Seefeld, Germany) and
Adper ScotchBond SE (3M ESPE), were developed. Adper
ScotchBond SE is a two-step system with a pH below 1, and
thus can be categorized as a ‘strong’ self-etch adhesive. Adper
Easy Bond is a one-step adhesive with a relatively high pH (2.4),
and thus can be categorized as an ‘ultra-mild’ self-etch
adhesive.
Adper Easy Bond is a typical one-step self-etch adhesive
that makes use of phosphoric acid ester methacrylates as
functional monomers. Its key advantage is the ‘easy’, fast and
straightforward application procedure. With regard to its
clinical application procedure, Adper ScotchBond SE is rather
unique because of the color change during the successive
application of the two liquids; the first pink solution helps the
practitioner to control the application of the solution; the
second yellow color appears when the second liquid is
adequately mixed with the solution and thus the etching
process is successfully initiated. The first pink liquid is in fact a
HEMA-water solution and has no etching capacity. Self-
etching is only initiated as soon as the yellow acidic monomer
solution is applied and mixed with the HEMA-water solution.
Keeping water separate from the acidic functional monomers
is advantageous with regard to shelf-life, while the HEMA
present in the pink solution enables the monomers to readily
dissolve in water.
The purpose of this study was to determine the bonding
effectiveness of these two new adhesives to enamel and
dentin using a micro-tensile bond strength protocol and to
characterize the interfacial ultra-structure at both enamel and
dentin using transmission electron microscopy (TEM). Com-
parison was made to a ‘mild’ self-etch adhesive Clearfil SE
Bond (Kuraray, Okayama, Japan) that performed excellent in
vitro2–5 and in vivo,6–8 and therefore can be considered as ‘gold-
standard’ among the self-etch adhesives. The hypotheses
tested in this study were (1) that the two new self-etch
Fig. 1 – Schematic illustrating the study design. (a) Human third
dentin surfaces. (b) The adhesive was applied and a resin comp
rectangular composite-tooth sticks were prepared using the au
adhesive-tooth interface was trimmed into a cylindrical hour-g
Iowa) and the specimens were stressed in tensile until failure.
adhesives bond to enamel and dentin as effective as the
control adhesive, and (2) that their bonding mechanism to
enamel and dentin, as characterized by TEM interfacial
examination, resembles that of the control adhesive.
2. Materials and methods
2.1. mTBS
2.1.1. Specimen preparationThe experimental set-up is schematically presented in Fig. 1.
Forty-eight non-carious human third molars (gathered
following informed consent approved by the Commission
for Medical Ethics of the Catholic University of Leuven) were
stored in 0.5% chloramine solution at 4 8C and used within 1
month after extraction. The teeth were randomly divided into
6 groups (enamel and dentin specimens for the three
adhesives: Adper Easy Bond (experimental precursor
EXL683 was used), Adper ScotchBond SE (experimental
precursor EXL678A/EXL671C was used) and the control
Clearfil SE Bond). For the enamel specimens, lingual and
buccal enamel was flattened using a high-speed medium-grit
(100 mm) diamond bur (842, Komet, Lemgo, Germany)
mounted in the MicroSpecimen Former (The University of
Iowa, Iowa, IA, USA). For the dentin specimens, the occlusal
two-third of the crown was removed using a slow-speed
diamond saw (Isomet 1000, Buehler, lake Bluff, IL, USA) and a
standard smear layer was produced by removing a thin layer
of the surface using a high-speed medium-grit (100 mm)
diamond bur mounted in the MicroSpecimen Former. All
dentin surfaces were carefully verified for absence of enamel
and/or pulp tissue using a stereo-microscope (Wild M5A,
Heerbrugg, Switzerland).
The adhesives were then applied according to manufac-
turer’s instructions (Table 1) and the surface was built up with
a micro-hybrid resin composite Z100 (3M ESPE) in 3–4 layers to
a height of 5–6 mm. Light-curing was performed using an
Optilux 500 (Demetron/Kerr, Danbury, CT, USA) device with a
light output not less than 600 mW/cm2. After bonding
molars were used to prepare standardized enamel and
osite build-up was made. (c) After 24-h water storage,
tomatic precision water-cooled diamond saw. (d) The
lass shape using the MicroSpecimen Former (University of
Table 1 – Adhesives used.
Adhesive Composition Application
Adper Easy Bond (Adper Easy One) HEMA, Bis-GMA, water, phosphoric
acid-methacryloxy-hexylesters, ethanol,
silane-treated silica, HDDMA, copolymer
of acrylic and itaconic acid, DMAEMA,
phosphine oxide, CQ
(1) Dispense one drop of the
adhesive into a dappen dish
and apply liberally with an
applicator for 15 s using rubbing motion.
Lot. P-0275, pH: 2.4*
(2) Gently air-blow until the liquid
does not move anymore.
3M ESPE, Seefeld, Germany (3) Light-cure for 10 s.
Adper ScotchBond SE (Adper SE Plus) Liquid A: water, HEMA,
polyethylene–polypropylene
glycol, Rose Bengal dye
(1) Dispense 1 drop of Liquid A into
a dappen dish and apply so that a
continuous red-colored layer is
obtained on the surface.
Liquid A: Lot. MFG135L-002-AB, pH: <1*
Liquid B: surface modified zirconium
dioxide, Di-HEMA phosphate, TEGDMA,
TMPTMA, diurethane dimethacrylate,
6-methacryloxyexacryloxyphosphate,
ethyl 4-dimethyl aminobenzoate,
DL-camphorquinone
(2) Dispense 1 drop of Liquid B into
second mix well and scrub into the
entire wetted surface of the bonding
area. The red color will disappear
quickly. Continue scrubbing with
moderate pressure for 20 s.
Liquid B: Lot. MFG135L001-AC-2,
(3) Air dry thoroughly for 10 s, the
adhesive should remain in place and
shiny in appearance.
3M ESPE, Seefeld, Germany (4) Re-coat brush with Liquid B,
and apply second coat to the entire
bonding surface. Lightly air dry to obtain
a thin adhesive layer.
(5) Light-cure for 10 s.
Clearfil SE Bond (Clearrfil Mega Bond) Primer: 10-MDP, HEMA, hydrophilic
dimethacrylate, photo-initiator, water
(1) Apply the primer for 20 s.
Primer: Lot. 00709A, pH: 2.0* Bond: 10-MDP, Bis-GMA, HEMA, hydrophobic
dimethacrylate, photo-initiator,
silanated colloidal silica
(2) Gently air-blow.
Bond: Lot. 01014A(3) Apply the bond and light-cure for 10 s.
Kuraray, Okayama, Japan
HEMA, 2-hydroxyethyl methacrylate; Bis-GMA, bisphenol a diglycidyl ether dimethacrylate; HDDMA, 1,6-hexanediol dimethacrylate;
DMAEMA, dimethylaminoethyl methacrylate; CQ, camphorquinone; Di-HEMA phosphate, di-2-hydroxyethyl meyhacryl hydrogenphosphate;
TEGDMA, triethylene glycol dimethacrylate; TMPTMA, trimethylopropane trimethacrylate; 10-MDP, 10-methacryloyoxydecyl dihydrogen
phosphate. *Information as received from manufacturer.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3874
procedures, specimens were stored for 24 h in tap water at
37 8C. The teeth were then sectioned perpendicular to the
bonding surface using a automatic precision water-cooled
diamond saw (Accutom-50, Struers A/S, Ballerup, Denmark) to
obtain rectangular sticks (1.8 mm � 1.8 mm wide; 8–9 mm
long). The 4 central specimens were mounted in the pin-chuck
of the MicroSpecimen Former and trimmed at the adhesive-
dentin/enamel interface to a cylindrical hour-glass shape with
a bonding surface of about 1 mm2 using a fine cylindrical
diamond bur (835KREF, Komet, Lemgo, Germany) mounted in
a high-speed handpiece under continuous air/water spray
coolant.
2.1.2. mTBS testingSpecimens were fixed to Ciucci’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 (mm2). When specimens failed before actual testing,
a bond strength of 0 MPa was included in the calculation of the
mean mTBS. The actual number of pre-testing failures (ptfs)
was explicitly noted as well.
2.1.3. Failure analysisThe mode of failure was determined light-microscopically at
a magnification of 50� using a stereo-microscope (wild M5A,
Heerbrugg, Switzerland), and recorded as either ‘cohesive
failure in dentin’, ‘adhesive failure’, ‘mixed adhesive’ or
‘cohesive failure in resin’. Representative samples
were processed for field-emission-gun scanning electron
microscopy (Feg-SEM; Philips XL30, Eindhoven, The
Netherlands), using common SEM specimen-preparation
procedures.9
2.1.4. Statistical analysisThe mTBSs were subjected to a one-way analysis of variance
(ANOVA) and a post hoc multi-comparisons test (Tukey–
Kramer test) at a significance level of 0.05 to differentiate the
bonding performance of the three adhesives tested. This
analysis was performed separately for the mean mTBS of the
enamel and dentin groups.
2.2. Transmission electron microscopy (TEM)
Two enamel and dentin surfaces were prepared for each group
in the same way as for the mTBS testing. The specimens were
processed for TEM according to the procedure described in
Ta
ble
2–
Mic
ro-t
en
sile
bo
nd
stre
ng
thto
en
am
el
an
dd
en
tin
(MP
a).
En
am
el
(8te
eth
)D
en
tin
(8te
eth
)
Ad
per
Ea
syB
on
dA
dp
er
Sco
tch
Bo
nd
SE
Cle
arfi
lS
EB
on
dA
dp
erE
asy
Bo
nd
Ad
per
Sco
tch
Bo
nd
SE
Cle
arfi
lS
EB
on
d
12
ptf
6p
tf0
ptf
3p
tf0
ptf
0p
tf
ptf
=0
MP
a8.0
MP
a(�
9.3
)A,n
=32
13.6
MP
a(�
11.1
)A,n
=32
27.6
MP
a(�
10.5
)B,n
=32
25.1
MP
a(�
11.9
)a,n
=32
36.3
MP
a(�
12.1
)b,n
=32
38.1
MP
a(�
11.0
)b,n
=32
ptf
ex
clu
ded
12.7
MP
a(�
8.8
)A,n
=20
16.7
MP
a(�
9.9
)A,n
=26
27.6
MP
a(�
10.5
)B,n
=32
27.7
MP
a(�
9.0
)a,n
=29
36.3
MP
a(�
12.1
)b,n
=32
38.1
MP
a(�
11.0
)b,n
=32
ptf
=th
elo
west
mea
sure
d8.8
MP
a(�
8.6
)A,n
=32
14.5
MP
a(�
10.0
)A,n
=32
27.6
MP
a(�
10.5
)B,n
=32
26.2
MP
a(�
9.8
)a,n
=32
36.3
MP
a(�
12.1
)b,n
=32
38.1
MP
a(�
11.0
)b,n
=32
Mea
n(S
.D.=
sta
nd
ard
dev
iati
on
);p
tf=
pre
-test
ing
fail
ure
;n
=to
tal
nu
mb
er
of
speci
men
s.M
ea
ns
wit
hth
esa
me
sup
ers
crip
tle
tters
(A,
Bo
ra
,b
)a
ren
ot
sign
ifica
ntl
yd
iffe
ren
t(p>
0.0
5).
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3 875
detail by Van Meerbeek et al.10 Non-demineralized and Lab-
demineralized ultra-thin sections were cut (Ultracut UCT,
Leica, Vienna, Austria) and examined unstained and positively
stained (5% uranyl acetate for 12 min/saturated lead citrate for
13 min) using TEM (EM 900, Zeiss, Oberkochen, Germany). In
order to reveal potential infiltration incompleteness within
the formed hybrid layer, additional specimens immersed in a
50 wt% ammoniacal silver-nitrate solution were prepared
according to a nano-leakage-detection protocol previously
described by Tay et al.11
3. Results
3.1. mTBS
The mean mTBS, standard deviation, total number of speci-
mens and number of pre-testing failures are summarized per
group in Table 2 and Fig. 2. One-way ANOVA revealed a
significant difference in mTBS between the different adhesives
when bonded to enamel (p < 0.0001) as well as to dentin
(p < 0.0001). When bonded to enamel, both self-etch adhe-
sives presented a lower mTBS as compared to that of the
control. The control adhesive was the only adhesive, for which
no pre-testing failures were recorded. No statistically sig-
nificant difference was recorded between the two adhesives.
When bonded to dentin, Adper Easy Bond presented a
significantly lower mTBS when compared to that of both other
adhesives. No significant difference in mTBS was observed
between Adper ScotchBond SE and the control.
3.2. Failure analysis
The failure analysis data are graphically presented in Fig. 3.
Most fractured surfaces failed partially at the interface and
partially within the resin, by which they were categorized as
‘mixed’ failures. When Adper Easy Bond was bonded to
enamel, most specimens failed adhesively at the interface,
as evaluated using the stereo-microscope. Detailed Feg-SEM
analysis however revealed that even these specimens failed
partially within the adhesive resin (Fig. 4). Most surfaces
categorized as ‘interfacial’ failure actually failed within the
hybrid layer or within the adhesive resin adjacent to
the hybrid layer. Close examination of the fractured
surfaces also revealed that the scratches produced by the
diamond bur were still filled with adhesive resin. Crack
propagation occurred thus more in a single plane, rather
than following the relatively irregular resin–dentin interface
(Fig. 5).
3.3. TEM
TEM evaluation of the interface showed a tight interaction free
of voids for all adhesives. At enamel, a tight interface was
obtained for both self-etch adhesives, with distinct micro-tags
observable for the Adper ScotchBond SE. The interaction of
Adper Easy Bond was much shallower with almost no micro-
tags observable. Immersion of specimens in AgNO3 did not
reveal significant traces of Ag along the enamel–adhesive
interface for all the adhesives (Fig. 6).
Fig. 2 – Mean mTBS to enamel and dentin in MPa. The boxes represent the spreading of the data between the first and third
quartile. The central line represents the median. The whiskers denote the 5th and 95th percentiles and dots represent
minimum and maximum values. Groups connected with a horizontal line are statistically significant different (Tukey–
Kramer test, p < 0.05). Each pair of graphs shows the data distribution (left the mTBS to enamel, and right the mTBS to dentin)
when either a ptf was considered as 0 MPa (top), a ptf was excluded (middle), or a ptf received the lowest measured mTBS
within the respective group (bottom).
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3876
At dentin, a relatively thin hybrid layer of a few hundreds
nanometer, which still contained residual hydroxyapatite,
was formed by Adper Easy Bond (Fig. 7). Also, no clear resin
tags were formed. The adhesive resin of Adper Easy Bond is
filled with silica filler and resulted in a film thickness of about
5 mm. Adper ScotchBond SE presented with a 2 mm thick
distinct hybrid layer, wherein nearly all hydroxyapatite was
dissolved (Fig. 8). In contrast to Adper Easy Bond, Adper
ScotchBond SE presented with distinct resin tags. Adper
ScotchBond SE is filled with fine ZrO2 nanofiller, resulting in a
substantially thicker film thickness of about 10 mm, as
compared to Adper Easy Bond.
Silver-nitrate infiltration showed a varying pattern of both
spot- and cluster-like appearance of nano-leakage for both
Fig. 3 – Failure analysis. The predominant occurrence of mixed failures was evident in all groups. Most areas tended to fail
cohesively in resin and adhesively between the bonding layer and the substrate. Especially when Adper Easy Bond was
bonded to enamel, specimens tended to fail adhesively.
Fig. 4 – Feg-SEM fractography of enamel mTBS specimens. SEM photomicrographs of fractured surfaces of Adper Easy Bond (a
and b), Adper ScotchBond SE (c) and Clearfil SE Bond (d). (a) Adper Easy Bond (enamel side): despite mostly interfacial failures
were observed under the stereo-microscope (see also image at low magnification in the insert), resin remnants could be
observed on almost all fracture surfaces of Adper Easy Bond. Scratch marks remaining from smear-layer preparation
confirmed that the failure occurred at/near the interface. (b) Higher magnification of the composite side: even though the
specimen failed at the interface, some enamel structure could still be observed. (c) Adper ScotchBond SE (composite side): most
fractured surfaces of this group were recorded as ‘mixed’, but the areas that fractured at the interface itself revealed similar
surface characteristics as for Adper Easy Bond. (d) Clearfil SE Bond (composite side): like for Adper ScotchBond SE, most
fractures were categorized as ‘mixed’. For this adhesive, we observed more enamel remnants at the composite side of the
fractured specimens. A: Adhesive failure. Ca: Cohesive failure in adhesive. Ce: Cohesive failure in enamel. Ar: Adhesive resin.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3 877
Fig. 5 – Feg-SEM fractography of dentin mTBS specimens. SEM photomicrographs of fractured surfaces of Adper Easy Bond (a
and b), Adper ScotchBond SE (c) and Clearfil SE Bond (d). (a) Adper Easy Bond (dentin side): the specimen failed mainly at the
interface. Some part of the fracture occurred within the resin composite; this was associated with some large air bubbles
(arrow). Note that at all borders the specimen failed at the resin–dentin interface, which can easily be confirmed by the
scratch marks remaining from smear-layer preparation. As the fracture most likely initiated at this location, this area must
be considered the weakest link of the interface complex. (b) Higher magnification of (a): failure occurred at the interface, but
dentin remained covered with remnants of the hybrid layer and resin-impregnated smear debris. (c) Adper ScotchBond SE
(dentin side): resin tags were shown to have extended into the opened dentinal tubules. Intertubular dentin remained
covered with remnants of the hybrid layer and adhesive resin. (d) Clearfil SE Bond (dentin side): the fracture occurred near
the bottom of the interface, exposing the typical dentin anatomy with inter- and peritubular dentin. A: Adhesive failure. Ar:
Adhesive resin. Ca: Cohesive failure in adhesive. Cc: Cohesive failure in composite. Dt: Dentinal tubule. Pd: Peritubular
dentin. Rt: resin tag. Sp: Smear plug.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3878
adhesives. Nano-leakage appearance varied widely with the
region, but was in general similar for the two adhesives in both
form and extent (Fig. 9).
4. Discussion
In this study, the bonding effectiveness of two new self-etch
adhesives, namely Adper Easy Bond and Adper ScotchBond SE,
was assessed and compared to that of the mild two-step self-
etch adhesive Clearfil SE Bond, that can be considered as ‘gold-
standard’ among the self-etch adhesives.2–8 The mTBS of the
control adhesive to enamel as well as to dentin was higher
than that of both self-etch adhesives. Only when Adper
ScotchBond SE was bonded to dentin, the mTBS was not
significantly different from that of the control. Also ultra-
morphological interfacial analysis of both adhesives revealed
significant differences between all three adhesives. Therefore,
both the hypotheses that the two new self-etch adhesives
bond to enamel and dentin as effective as the control
adhesive, and that their respective bonding mechanism to
enamel and dentin resembles that of the control adhesive
must be rejected.
The control adhesive Clearfil SE Bond was the only
adhesive, of which the micro-specimens did not fail during
specimen preparation, leading to so-called ‘pre-testing’
failures. Adper Easy Bond, on the other hand, revealed not
only a much lower bond strength to both enamel and dentin,
but also a lot of pre-testing failures. Adper ScotchBond SE
performed equally well as the control adhesive when bonded
to dentin, but less favourably when bonded to enamel, as
then some pre-testing failures were recorded as well. The
Fig. 6 – TEM photomicrographs of both self-etch adhesives bonded to enamel. (a) A tight interface was obtained for Adper
Easy Bond, despite that micro-mechanical interlocking was limited to the present roughness of the enamel surface. (b)
Higher magnification of (a). (c) A tight interface was obtained by Adper ScotchBond SE, and exhibited macro- as well as
micro-tags. A homogenous distribution of the zirconium dioxide nanofiller within the adhesive resin was observed. (d)
Higher magnification of (c).
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3 879
mTBS test has been shown to have many advantages over
conventional macro-shear/tensile bond strength testing, of
which the most important are the economical of teeth (with
multiple micro-specimens prepared from one tooth), the
better control of regional differences (e.g. peripheral versus
central dentin), the better stress distribution at the true
interface (avoiding cohesive failure in tooth substrate or
composite), etc.12,13 However, several mTBS-specimen pre-
paration protocols have been used worldwide, one being
more technique-sensitive than others.14–17 The mTBS test can
better discriminate between adhesives with a lower mTBS,
often being associated with a higher incidence of pre-testing
failures. However, correct interpretation of pre-testing fail-
ures with regard to the calculation of the average mTBS is
currently a matter of debate at scientific meetings. We have
therefore provided a table and graph, in which (1) a ptf was
considered as 0 MPa, which actually penalizes the adhesive
too severely (as there was a certain bond strength), (2) a ptf
was excluded from the average mTBS calculation, and (3) a ptf
received the lowest mTBS measured within the respective
group (Table 2, Fig. 2). Although the box plots changed clearly
(depending on how the ptf’s were taken into account), the
statistical analysis did not reveal new information. Nowa-
days, specific measures are taken to reduce the incidence of
ptf’s by better supporting the slices with for instance gypsum
or alginate during sectioning.
After the mTBS test, the resultant surface morphology of the
fractured surface is the representation of the failure that
occurred during the tensile loading. These failures occur in
complex patterns, especially for very heterogeneous inter-
faces like adhesive–enamel/dentin interfaces. Most failures
were categorized as ‘mixed’ and for some fractured surfaces
that were categorized as ‘interface failure’ by light-microscopy
(Fig. 3), SEM revealed that the actual failure occurred within
the adhesive resin immediately adjacent to the interface.
Especially when Adper Easy Bond was bonded to enamel,
several of these cases were observed (Figs. 3 and 4). Also the
‘strong’ one-step self-etch adhesive Adper Prompt L-pop
revealed in a previous study a similar tendency to fail within
the adhesive resin immediately adjacent to the hybrid layer.18
A bond strength, certainly when measured using a mTBS
protocol, represents the strength of the whole adhesive-tooth
assembly, also including the strength of the adhesive resin
itself. A lower cohesive strength of the adhesive resin of the
adhesives might explain such failures within the adhesive
resin, and thus to some extent their lower bond strength
measured. A strong correlation was indeed found between the
resin–dentin bond strength and the mechanical properties of
Fig. 7 – TEM photomicrographs of the interface of Adper Easy Bond bonded to dentin. (a) Non-demineralized, unstained
section, showing the silica-filled adhesive resin with a film thickness of about 5 mm. (b) Higher magnification of the
interface between the adhesive resin and the unaffected dentin. Even at this 30 000T (original) magnification, it is very
difficult to distinguish a distinct hybrid layer. Hydroxyapatite crystals are scattered throughout the interaction zone
(arrows). It is not clear if this area represents a true hybrid layer or rather a resin-infiltrated smear layer. (c) Non-
demineralized, UA/LC stained section. The shallow submicron interaction becomes evident after staining. (d)
Demineralized, UA/LC stained section, disclosing a clearly electron dense hybrid layer with a thickness of a few hundreds
of nanometer.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3880
the cured resin for self-etch adhesives.19 In this respect, the
control adhesive that provides a separate hydrophobic resin is
known for its excellent mechanical properties.20,21 In case the
adhesives failed at the interface, failure actually occurred
along the top of the hybrid layer and the smear layer, so that
the dentin anatomy with its difference in peri- and inter-
tubular dentin remained hidden (Fig. 5). The control adhesive,
on the other hand, failed more at the bottom of the hybrid
layer, or even below the hybrid layer within the unaffected
dentin.
For both new adhesives, a tight interface to enamel was
observed by TEM (Fig. 6). For the stronger two-step self-etch
adhesive (Adper ScotchBond SE), distinct micro-tags were
formed (Fig. 6d). The interaction of the milder one-step self-
etch adhesive (Adper Easy Bond) was much shallower. This
difference in interfacial ultra-structure at enamel must in the
first place be attributed to the difference in acidity between
both self-etch adhesives, with the two-step adhesive having a
much lower pH (pH < 1 for Adper ScotchBond SE versus
pH = 2.4 for Adper Easy Bond), by which it etches enamel much
deeper than the one-step adhesive. In a clinical context, the
‘milder’ adhesive will probably be more sensitive to substrate
variability; dense smear layers, a biofilm, or any other form of
surface alteration or contamination may prevent direct
contact of the adhesive with ‘intact’ tooth tissue, and can
thus hamper the bonding effectiveness to enamel (as well as to
dentin).
When Adper Easy Bond was bonded to dentin, a relatively
thin interaction layer was observed. The interaction zone of a
few hundreds of nanometers thickness, as observed in Fig. 7d,
is thought to be composed of resin-impregnated smear along
with hybridization of the deeper underlying dentin. Conse-
quently, this micro-mechanical bonding mechanism of Adper
Easy Bond is highly dependent on the way the surface is
prepared. So will, for example, rougher diamond burs result in
thicker and more compact smear layers and thus lower bond
strengths.22,23 Clinically, however, one rather prefers an
adhesive that bonds to any kind of surface, irrespective the
cavity-preparation method employed. Apart from micro-
mechanical interaction, chemical interaction of the functional
Fig. 8 – TEM photomicrographs of Adper ScotchBond SE bonded to dentin. (a) Non-demineralized, unstained section,
revealing that the adhesive is filled with the much finer ZrO2 nanofiller, reaching a substantially thicker film thickness. The
relatively low-pH adhesive has dissolved hydroxyapatite up to a depth of about 2 mm. The white areas represent
(unstained) collagen, with the grey area in between representing resin that infiltrated in the exposed 3D collagen network.
The dentin tubules were opened, having enabled the formation of resin tags and even some micro-tags in lateral tubule
branches (arrows). (b) Non-demineralized, unstained section, detailing the hybrid layer, and showing that the transition of
the hybrid layer to unaffected dentin is not abrupt, with gradually more hydroxyapatite remaining near the bottom of the
hybrid layer. The nanofiller appeared small enough to have penetrated into the collagen network, at least to a certain
extent. (c) Non-demineralized, UA/LC stained section, showing the loosely organized network of collagen fibrils with a
typical shagged-carpet appearance, resulting from ‘massaging’ the adhesive into the exposed collagen. (d) Demineralized,
UA/LC stained section, disclosing a densely stained hybrid layer as proof of acid resistance.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3 881
monomers (included in the adhesive) with hydroxyapatite
that remains at the surface could also contribute to the
bonding effectiveness of these ultra-mild adhesives.24 Little
information is, however, known about the chemical interac-
tion potential of the phosphoric acid-methacryloxy-hexyles-
ters (as included in this adhesive) with hydroxyapatite, nor
about the stability of the resultant salt in an aqueous
environment.25 The interaction of ‘raw’ monomers in a high
purity with hydroxyapatite (and dentin) should therefore be
investigated according to a protocol used by Yoshida et al.26
When Adper ScotchBond SE was bonded to dentin, the
interaction appeared more intense. Any smear debris pro-
duced by the medium-grit diamond bur was dissolved and a
distinct hybrid layer with a thickness of about 2 mm was
formed. This must be attributed to the higher acidity of the
self-etch primer (pH < 1), by which this adhesive should be
categorized as a ‘strong’ self-etch adhesive.27 However, other
strong self-etch adhesives, such as Adper Prompt L-Pop (3M
ESPE), presented with an interfacial ultra-structure that
resembles that of etch&rinse adhesives: a completely demi-
neralized hybrid layer of about 3–5 mm thickness. For Adper
ScotchBond SE tested, a hybrid layer thickness of about 2 mm
was formed (Fig. 8), while at the bottom of the hybrid layer still
some hydroxyapatite remained (Fig. 8a and b). This suggests a
slightly milder interaction than that of typical ‘strong’ self-
etch adhesives, more alike that of the ‘intermediately strong’
self-etch adhesives.27,28
Silver-nitrate impregnation of resin–dentin interfaces
disclosed dense deposits of silver throughout the whole
hybrid layer of both adhesives (Fig. 9). These were more
prevalent in the bottom half of the hybrid layer, but did not
differ in extent and form in comparison to those observed with
the Clearfil SE Bond control adhesive.29 This consistent pattern
of silver deposition resembles the reticular nano-leakage
Fig. 9 – TEM photomicrographs of AgNO3-immersed adhesive-enamel/dentin interfaces. (a) A tight interface of Adper Easy
Bond to enamel was obtained, revealing only very few spots of silver deposition (arrows). (b) Adper ScotchBond SE also
formed a tight interface to enamel with only very few spots of silver deposition (arrows). (c) Adper Easy Bond revealed
regionally substantially varying nano-leakage patterns with a typical clustered silver deposition within the dentin tubules
and its orifice, while a very tiny spot-like appearance was disclosed at regions more remote from dentin tubules. (d) Adper
ScotchBond SE revealed a varying pattern of both spot- and cluster-like appearance of nano-leakage at the dentin–adhesive
interface.
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 8 7 2 – 8 8 3882
mode observed previously by Tay et al.11,30 Besides incomplete
resin infiltration, such regions of silver deposition may also
represent sites of retained water, especially in case of these
HEMA-containing adhesives that tend to absorb water more
easily.31
Adper ScotchBond SE can be categorized as a two-step
self-etch system,1 but differs from other two-step systems,
as the first self-etching step involves the application of two
solutions. First, Liquid A, a HEMA-water solution (and not a
self-etching primer), is applied, to which a pH indicator (dye)
was added (Table 1). Second, this solution is mixed in situ
with Liquid B to activate the self-etching process. After air-
drying in order to remove all excess water, Liquid B is re-
applied as the actual adhesive resin. Despite that the bond
strength to enamel of Adper ScotchBond SE was significantly
lower than that of the control, the overall bonding effec-
tiveness was in line with other recently marketed (one-step)
adhesives that were tested following a similar study
protocol.28 So, the unique advantages of this adhesive comes
with some sacrifice in bonding performance, but the balance
could be positive if the outcome of future clinical studies is
favourable.
Acknowledgements
We thank Prof. Johan Billen (Laboratory of Entomology,
Department Biology, Catholic University of Leuven) for
extensive technical assistance (TEM). Dr. Mine has been
granted a scholarship of the Government of Flanders to
conduct research at the Leuven BIOMAT Research Cluster of
the Catholic University of Leuven. This study was supported in
part by 3M ESPE.
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