Journal of Dentistry 30 (2002) 371–382
www.elsevier.com/locate/jdent
Single-step adhesives are permeable membranes
Franklin R. Taya,*, David H. Pashleyb, Byoung I. Suhc, Ricardo M. Carvalhod, Anut Itthagaruna
a
Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road,
Hong Kong, SAR, People’s Republic of China
b
Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129, USA
c
Bisco, Inc, 1100 West Irving Park Road, Schaumburg, IL 60193, USA
d
Department of Operative Dentistry, Bauru School of Dentistry, University of São Paulo, São Paulo, Brazil
Revised 5 October 2002; accepted 17 October 2002
Abstract
Objectives. This study tested the hypotheses that micro-tensile bond strengths of all currently available single-step adhesives to dentine are
adversely affected by delayed activation of a light-cured composite, and that such a phenomenon only occurs in the presence of water from
the substrate side of the bonded interface.
Methods. In experiment I, a control three-step adhesive (All-Bond 2, Bisco) and six single-step adhesives (One-Up Bond F, Tokuyama;
Etch&Prime 3.0, Degussa; Xeno CF Bond, Sankin; AQ Bond, Sun Medical; Reactmer Bond, Shofu and Prompt L-Pop, 3M ESPE) were
bonded to sound, hydrated dentine. A microfilled composite was placed over the cured adhesive and was either light-activated immediately,
or after leaving the composite in the dark for 20 min. In experiment II, three single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ
Bond) were similarly bonded to completely dehydrated dentine using the same delayed light-activation protocol. In experiment III, a piece of
processed composite was used as the bonding substrate for the same three single-step adhesives. The microfilled composite was applied to the
cured adhesives using the same immediate and delayed light-activation protocols. Bonded specimens were sectioned for micro-tensile bond
strength evaluation. Fractographic analysis of the specimens was performed using SEM. Stained, undemineralised sections of unstressed,
bonded specimens were also examined by TEM.
Results. When bonded to hydrated dentine, delayed light-activation had no effect on the control three-step adhesive, but significantly
lowered the bond strengths of all the single-step adhesives ( p , 0.05). This adverse effect of delayed light-activation was not observed in the
three single-step adhesives that were bonded to either dehydrated dentine or processed composite. Morphological manifestations of delayed
light-activation of composite in the hydrated dentine bonding substrate were exclusively located along the composite – adhesive interface,
and were present as large voids, resin globules and honeycomb structures that formed partitions around a myriad of small blisters along the
fractured interfaces.
Conclusion. These features resembled of the ‘overwet phenomenon’ that was previously reported along the dentine – adhesive interfaces of
some acetone-based three-step adhesives. The cured adhesive layer in single-step adhesives may act as semi-permeable membranes that
allow water diffusion from the bonded hydrated dentine to the intermixed zone between the adhesive and the uncured composite. Osmotic
blistering of water droplets along the surface of the cured adhesive layer and emulsion polymerisation of immiscible resin components
probably account for the compromised bond strength in single-step adhesives after delayed activation of light-cured composites.
q 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Single-step adhesive; Delayed light-activation; Light-cured composite; Semi-permeable membrane; Osmotic blistering
1. Introduction
Dentine adhesives are currently available as three-step,
two-step and single-step systems depending on how the three
cardinal steps of etching, priming and bonding to tooth
* Corresponding author. Tel.: þ 86-852-28590251; fax: þ 86-85223933201.
E-mail address: [email protected] (F.R. Tay).
substrates are accomplished or simplified [1]. Two-step
systems are sub-divided into the self-priming adhesives that
require a separate etching step, and the self-etching primers
that require an additional bonding step [2]. The recently
introduced all-in-one adhesives further combined these three
bonding procedures into a single-step application. Irrespective of their packaging designs, these systems are supplied
as two-component assemblies to maintain adequate shelflives. They are mixed together immediately before use, and
0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 6 4 - 7
372
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
the mixture of hydrophilic and hydrophobic resin components is then applied to the tooth substrate. Of the six
commercially available all-in-one adhesives, Prompt L-Pop
(3M ESPE, Seefeld, Germany), Etch&Prime 3.0 (Degussa
AG, Hanau, Germany), AQ Bond (Sun Medical, Kyoto,
Japan) are unfilled versions, whereas One-Up Bond F
(Tokuyama, Tokyo, Japan), Reactmer Bond (Shofu, Kyoto,
Japan), and Xeno CF Bond (Sankin, Tokyo, Japan) are filled
versions that contain fluoride-releasing glass fillers or
monomers.
For a long time, dentists have assumed that resin
composites bond well to dentine adhesives, and that the
weak links occur between the adhesives and dentine [3 – 9].
However, this may not be entirely true with the advent of
contemporary two-step, and single-step adhesives. We
previously showed that some two-step, self-priming
adhesives are incompatible with chemical-cured composites, and that the decreases in bond strength are inversely
proportional to the acidity of these single-bottle systems
[10] Single-step adhesives are even more acidic in nature by
virtue of their self-etching capabilities. There is evidence to
suggest that they may not be compatible even with hybrid
light-cured hybrid composites that are placed on top of these
cured adhesives for too long before light-activation [11] In
that study, micro-tensile bond strengths of two single-step
adhesives were found to decrease exponentially with the
time-delay in light-activation of the composite.
The objectives of this study were to further examine the
hypotheses that all currently available single-step adhesives
to dentine are adversely affected by delayed activation of a
light-cured, microfilled composite, and that such a phenomenon only occurs in the presence of water from the substrate
side of the bonded interface. Specimens bonded to hydrated
and dehydrated dentine, as well as processed composites
using these adhesives were evaluated using the microtensile bond testing technique. Specimens that were stressed
to failure were examined using scanning electron
microscopy (SEM). In addition, unstressed, intact resin –
dentine interfaces that were bonded with delayed lightactivation of the resin composite were examined with
transmission electron microscopy (TEM). The null hypotheses that were tested are: (1) prolonged contact of lightcured resin composite to cured single-step adhesives before
light-activation does not result in compromised bond
strengths to sound, hydrated dentine, and (2) the presence
or absence of water on the substrate side of the bonded
interface of single-step adhesives does not affect the results
of delayed activation of a light-cured composite.
2. Materials and methods
2.1. Tooth preparation for bond strength evaluation
Thirty-four caries-free, human third molars were stored
in 0.5% chloramine T at 4 8C, and used within 1 month
following extraction. The occlusal enamel of each tooth was
first removed using a slow-speed saw equipped with a
diamond-impregnated disk (Isomet, Buehler Ltd, Lake
Bluff, IL, USA) under water lubrication. A 180-grit silicon
carbide paper was used under running water to create a
smear layer on the dentine surface. Bonding was subsequently performed on the occlusal surfaces of deep,
coronal dentine.
2.1.1. Experiment I: effect of delayed activation of
a light-cured composite on the mTBSs of single-step
adhesives to hydrated dentine
Twenty-eight teeth were used in this part of the study.
They were randomly divided into seven groups of four teeth
each. All the teeth in Experiment I were bonded in their
normal hydrated status, as they were retrieved from the
storage medium.
All-Bond 2 (Bisco Inc, Schamburg, IL, USA; Group
AB), a three-step adhesive system, was used as the control
group. The six light-cured, single-step adhesives, One-Up
Bond F (Group OU), Etch&Prime 3.0 (Group EP), Xeno CF
Bond (Group CF), AQ Bond (Group AQ; also marketed in
North America as Touch&Bond by Parkell Inc, Farmingdale, NY, USA), Reactmer Bond (Group RB) and Prompt LPop (Group PL) constituted the other six groups. A
microfilled resin composite that contains pre-polymerised,
TMPT organic fillers (Metafil CX, Sun Medical Co. Ltd,
Shiga, Japan; also marketed in North America as Epic –
TMPT by Parkell, Inc.) was used as the bonding composite
in all groups. This microfilled composite does not contain
inorganic glass fillers and facilitates subsequent TEM
preparation of bonded, unstressed specimens. The general
composition and batch numbers of these adhesives and resin
composite are listed in Table 1. The application techniques
of these adhesives are also outlined in Table 2. During the
application of the primer mixture of the control three-step
adhesive and the single-step adhesive mixtures, care was
taken to ensure that the dentine surfaces were adequately
covered by resin after evaporation of the solvents. In the
event that matte dentine was encountered, additional coats
were applied to produce shiny surfaces prior to lightactivation of the adhesives.
Each adhesive group was further divided into two
subgroups of two teeth each, based upon the time of
contact of the resin composite with the cured adhesive
layer prior to light-activation. In subgroup X-0 (where
X ¼ adhesive group designation), a 1 mm layer of the
microfilled composite was applied to each bonded
dentine surface and light-activated immediately for 40 s,
using a halogen light-curing unit (Variable Intensity
Polymerizer, Bisco) with the curing intensity set at
500 mW cm22. Additional composite layers were subsequently added to the first composite layer, in 1 mm
increments and light-activated separately, until a 5 mm
thick core buildup was achieved. In subgroup X-20,
the first layer of composite that was in contact with
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
373
Table 1
Composition of the adhesives and composite used in the study
Material
Three-step
adhesive
Single-step
adhesive
Resin composite
Brand name
Type
Composition
Lot number
All-bond 2
Unfilled
Etchant. 32% phosphoric acid
gel, xanthum gum thickener
Primer A. NTG-GMA, acetone,
ethanol, water
Primer B. BPDM, photoinitiator, acetone
D/E bonding resin. Bis-GMA, UDMA, HEMA
Water, MMA, HEMA, coumarin dye,
methacryloyloxyalkyl acid phosphate,
methacryloxyundecane dicarboxylic acid
(MAC-10), multifuctional methacrylic
monomer, fluoroaluminosilicate glass,
photoinitiator (aryl borate catalyst)
Universal: water, ethanol, HEMA, stabilisers
0100001442
(Bisco Inc. Schaumburg,
IL, USA)
One-up bond F (Tokuyama
Corp. Tokyo, Japan)
Filled
Etch&Prime 3.0 (Degussa
AG Hanau, Germany)
Unfilled
Xeno CF Bond (Sankin Tokyo,
Japan)
Filled
AQ Bonda (Sun Medical
Co. Ltd Shiga, Japan)
Unfilled
Reactmer bond (Shofu Inc.
Kyoto, Japan)
Filled
Prompt L-Pop (3M ESPE
Seefeld, Germany)
Unfilled
Metafil CXb (Sun Medical Co.
Ltd Japan)
Microfilled
Catalyst: pyrophosphate, HEMA,
photoinitiators, stabilisers
Water, ethanol, HEMA,
methacryloxyethylpyrophosphate,
fluoride-releasing phosphazene monomer,
UDMA, micro-filler, photoinitiator
Base liquid. Water, acetone, 4-META,
HEMA, MMA, UDMA, photoinitiator
AQ sponge. Polyurethane foam,
Initiator ( p-TSNa)
Reactmer Bond A. Water, acetone, F-PRG fillers,
FASG fillers, Initiators (TMBA, p-TSNa)
Reactmer bond B. 4-AET, 4-AETA, HEMA,
UDMA, photoinitiator
Water, stabiliser, parabenes, methacrylated
phosphoric acid esters, fluoride complex,
photoinitiator (BAPO)
Dimethacrylates such as UDMA (34 wt%
organic TMPT filler (40 wt%) micro silica
(26 wt%) photoinitiator (with aromatic
tertiary amine) pigments, stabiliser
0000011967
0000011968
0000007876
23074510902
204
Universal: 350-04
Catalyst: 350-33
VK5
040002
FW0062849
TL2
Abbreviations: 4-AET, 4-acryloxyethyltrimellitic acid; 4-AETA, 4-acryloxyethyltrimellitic anhydride; 4-META, 4-methacryloxyethyltrimellitic
anhydride; BAPO, bis-acyl phosphine oxide; Bis-GMA, bisphenol A diglycidyl ether dimethacrylate; BPDM, biphenyl dimethacrylate; F-PRG, full-reaction
type pre-reacted glass ionomer filler; FASG, fluoroaluminosilicate glass; HEMA, 2-hydroxylethyl methacrylate; MMA, methyl methacrylate; NTG-GMA, Ntolylglycine-glycidyl methacrylate; p-TSNa, p-toluenesulfinic acid sodium salt; TMBA, trimethyl barbituric acid; TMPT, trimethylolpropane-trimethacrylate;
UDMA, urethane dimethacrylate.
a
Also marketed in North America as Touch&Bond by Parkell, Inc., Farmingdale, NY, USA.
b
Also marketed in North America as Epic-TMPT by Parkell, Inc.
the cured adhesive layer was left in the dark for 20 min
before light-activation. Subsequent core buildup followed
the procedures described in the previous subgroup. The
choice of 20 min was based on our previous study that
such a period of delayed light-activation could result in
null bond strength in some single-step adhesives [11].
We realised that such a lengthy delayed period was far
removed from clinical practice, as clinicians are unlikely
to leave a composite unactivated for more than 2 –3 min.
However, it was the purpose of this study to investigate
the effects of prolonged contact of unpolymerised
composites on the surfaces of single-step adhesives that
were bonded to hydrated dentine.
2.1.2. Experiment II: effect of delayed activation of
a light-cured composite on the mTBSs of single-step
adhesives to completely dehydrated dentine
Three of six single-step adhesives (Etch&Prime 3.0,
Xeno CF Bond and AQ Bond) were randomly selected
for this experiment. The two teeth that were used for
each adhesive were first dehydrated in an ascending
ethanol series (70, 80, 95%, three changes in 100%) for
2 h each, following the TEM preparation protocol by Tay
et al. [12]. After the third change, the teeth were left to
completely dehydrate in absolute ethanol for an
additional 48 h. This produced a bonding dentine
substrate that is completely devoid of moisture, but
374
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Table 2
Application techniques of the control three-step and the single-step adhesives
Adhesive systems
Dentine conditioning
Priming
Bonding
All-Bond 2
Uni-etch for 15 s, rinse and kept slightly moist
Mixed All-Bond
2 primer A and B,
air-drylight-activate
for 20 s
Apply D/E bonding
resin,gently thinned
down,light-activate
for 10 s
One-Up Bond F
Mix equal droplets of the bonding agents A (clear liquid) and B (bright
yellow liquid) for until a pink, homogenous liquid mixture was obtained
Apply to tooth substrates and left undisturbed for 20 s, no rinsing
Without further air-drying, light-activate for 10 s
Mix equal amount of the universal and catalyst
Apply to tooth substrates and allow acting for 30 s, no rinsing
Briefly air-dry, light-activate for 10 s
Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s
Mix equal amount of the universal and catalyst
Apply to tooth substrates and allow acting for at least 20 s, no rinsing
Briefly air-dry, light-activate for 10 s
Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s
Dispense one drop of AQ liquid into dispensing well containing one piece
of AQ sponge
Apply the adhesive-coated AQ sponge to dentine substrates for 20 s, no
rinsing
Air-dry 3– 5 s, re-apply adhesive, air-dry 5–10 s, light-activate for 10 s
using only halogen light-curing unit
Mix equal droplets of reactmer bond bonding agent A (white liquid) and the
B (amber liquid) for 5 s
Apply to tooth substrates and allow acting for 20 s, no rinsing
Briefly air-dry, light-activate for 20 s
Activate blister pack by emptying the liquid out of the red blister into the
yellow blister
The activated mixture was applied to tooth substrates with agitation for
15 s, no rinsing
Briefly air-dry, light-activate for 10 s using only halogen light-curing unit
Etch&Prime 3.0
Xeno CF Bond
AQ Bond
Reactmer Bond
Prompt L-Pop
without reducing its buffering capacity for the acidic
single-step adhesives.
In subgroup D-X-20 (where X ¼ adhesive group designation), the completely dehydrated dentine bonding surfaces were similarly treated with one of the three single-step
adhesives. The delayed light-activation protocol of the resin
composite followed that previously described for subgroup
X-20.
2.1.3. Experiment III: effect of delayed activation of
a light-cured composite on the mTBSs of single-step
adhesives to processed composites
To verify that any deterioration in mTBS during delayed
light-activation is caused solely by the diffusion of water
from the bonded substrate through the cured adhesive layer,
Experiment I was repeated for the three single-step
adhesives, Etch&Prime 3.0, Xeno CF Bond and AQ Bond,
using light- and heat-processed Metafil CX as the bonding
substrate.
Five-millimeter thick layers of Metafil CX were
dispensed into 4 £ 2 cm flat Teflon moulds (Electron
Microscopy Sciences, Fort Washington, PA, USA). The
moulds containing the uncured composite were placed
inside an experimental composite inlay processing
chamber (Nitro-Therma-Lite; Bisco, Inc.) and light-activated under a pressurised nitrogen atmosphere maintained
at 551.6 kPa (i.e. 80 psi) for one complete cycle at 125 8C
for 20 min. The bonding surface of each processed
composite block was ground with 180-grit SiC paper and
further sandblasted with 50 mm alumina for 10 s. These
blocks were sonicated in distilled water, air-dried, and
bonded using the three single-step adhesives in the manner
previously described.
In the immediate light-activation subgroup C-X-0 (where
C ¼ adhesive group designation), Metafil CX was applied
and light-activated incrementally as in subgroup X-0.
Similarly, the delayed light-activation subgroup C-X-20,
the first 1 mm layer of Metafil CX was placed over the cured
adhesive and left in the dark for 20 min before lightactivation.
2.2. mTBS evaluation
After storage in distilled water at 37 8C for 24 h,
bonded teeth from Experiment I and II were sectioned
occluso-gingivally into serial slabs using an Isomet saw
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
under water lubrication. One slab from each bonded
tooth was not subjected to tensile testing and reserved for
TEM examination. The other slabs tooth were sectioned
into 0.9 £ 0.9 mm2 composite – dentine beams, according
to the technique for the ‘non-trimming’ version of the
micro-tensile test reported by Shono et al. [13]. Each
group of bonded teeth yielded 20– 22 beams for bond
strength evaluation. Beams with premature failure during
sectioning were assigned a null bond strength value and
were included in the compilation of the mean tensile
bond strength.
The bonded composite blocks from Experiment III were
similarly aged in distilled water at 37 8C for 24 h and then
sectioned into 0.9 mm slabs. Due to the high incidence of
cohesive failures in resin composites initially observed
using the Shono’s technique, each slab was hand-trimmed
into 0.9 £ 0.9 mm2 dumbbell-shaped specimens according
to the version of micro-tensile bond testing reported by Sano
et al. [14]. Fifteen specimens were produced for each of the
six experimental subgroups.
Specimens were fixed to a Bencor Multi-T device
(Danville Engineering, San Ramon, CA, USA), using
Zapit cyanoacrylate (Dental Ventures of America, Corona,
CA, USA) and tested to failure under tension in a universal
testing machine (Model 4440; Instron Inc, Canton, MA,
USA) at a crosshead speed of 1 mm/min.
2.3. Statistical analysis
For each adhesive, the bond strength data obtained for
the corresponding subgroups were statistically analysed
with either Mann – Whitney Rank Sum tests or Kruskal –
Wallis one-way ANOVA on ranks, using SigmaStat Version
2.03 (SPSS, Chicago, IL, USA). Statistical significance was
set in advance at the 0.05 probability level. In adhesive
groups that involved additional subgroups from Experiments II and III, multiple comparisons were done with
Dunn’s test at a ¼ 0.05.
2.4. SEM fractographic analysis
Six fractured composite – dentine beams from Experiment I and II of each adhesive that were representative of
the mean bond strengths of the corresponding subgroups
were prepared for SEM examination. Both the dentine and
composite sides of the fractured beams were air-dried. They
were not dehydrated using methods that involve passing the
specimens through organic solvents [15], to avoid the
possibility of extracting uncured monomers or partially
polymerised oligomers from the fractured interfaces.
Completely air-dried specimens were secured to brass
stubs using Zapit. They were sputter-coated with gold/
palladium and examined using a scanning electron microscope (Cambridge Stereoscan 440, Cambridge, United
Kingdom) operating at 12 kV. Failures were classified as:
(a) adhesive failure, if the fracture site was maintained
375
entirely within the adhesive; (b) mixed failure, if the fracture
site continued from the adhesive into either the composite or
dentine; and (c) substrate failure, if the fracture occurred
exclusively within the composite or dentine. In addition,
abnormalities along the fractured interfaces that were
associated with delayed light-activation of the resin
composites were also recorded.
2.5. TEM examination of unstressed, bonded
dentine –composite interfaces
TEM was only performed to provide additional information on the abnormal features associated with delayed
light-activation of resin composites on adhesive-coated,
hydrated dentine, and to validate that the previous SEM
observations were not artefacts produced during tensile
stress, or by contamination of the fractured interfaces. The
two remaining composite –dentine slabs from subgroup X20 (Experiment I) of each adhesive were used for TEM
examination. Undemineralised sections of the bonded
specimens, 90 – 120 nm thick, were prepared according to
the TEM protocol described in Tay et al. [12]. These
undemineralised sections were collected using single slot,
carbon- and formvar-coated copper grids (Electron
Microscopy Sciences, Fort Washington, PA, USA). They
were double-stained with uranyl acetate for 10 min and
Reynold’s lead citrate for an additional 5 min. Examination
was performed using a TEM (Philips EM208S, Eindhoven,
The Netherlands) operating at 100 kV. Digitised images
were recorded using the charge couple device (CCD)
camera (Bioscan, Model 792, Gatan Inc, Pleasanton, CA,
USA) attached to the microscope.
3. Results
mTBS data of the three-step, control adhesive and the six
single-step adhesives bonded to hydrated dentine (Experiment I) are summarised in Table 3 and also graphically
shown in Fig. 1. Delayed light-activation has no effect on
the control adhesive ( p . 0.05). In contrast, delayed
light-activation significantly reduced the mTBS of all the
single-step adhesives ( p , 0.05).
For the three single-step adhesives tested in Experiments
II and III, delayed light-activation has no effect when these
adhesives were bonded to completely dehydrated dentine
( p . 0.05; Table 3). Similarly, no deterioration in mTBSs
was observed in these single-step adhesives when delayed
light-activation was used with the processed composite as
the bonding substrate ( p . 0.05; Table 3).
Fig. 2A shows a representative mixed failure mode that
was observed in AB-0 and AB-20 subgroups. No abnormalities were detected along the fractured interfaces.
Similarly, mixed failure was the predominant failure mode
in the corresponding immediate light-activation subgroups
c
Control three-step adhesive.
Values are means ^ standard deviation in MPa. For each adhesive (i.e. across a row), subgroups labelled with the same lower case superscript are not statistically significant ( p . 0.05).
Number of beams tested that were without premature failure. Beams with premature failure during specimen preparation were included as zero bond strengths in the calculation of mean bond strength.
a
b
NA
NA
38.8 ^ 7.0a[15/15]
47.3 ^ 15.3a[15/15]
32.4 ^ 9.6a[15/15]
NA
NA
NA
NA
35.0 ^ 9.5a[15/15]
48.3 ^ 11.9a[15/15]
38.2 ^ 10.9a[15/15]
NA
NA
All-Bond 2 (AB)a
One-Up Bond F(OU)
Etch&Prime 3.0(EP)
Xeno CF Bond(CF)
AQ Bond(AQ)
Reactmer bond(RB)
Prompt L-Pop(PL)
48.4 ^ 11.7ab[22/22]c
42.9 ^ 7.9a[22/22]
37.0 ^ 9.3a[20/20]
34.5 ^ 8.3a[21/21]
26.8 ^ 6.1a[23/23]
25.8 ^ 3.6a[21/21]
24.1 ^ 5.2a[22/22]
50.9 ^ 9.6a[20/20]
10.1 ^ 4.4b[20/20]
0.6 ^ 1.7b[3/21]
10.9 ^ 5.1b[22/22]
9.3 ^ 3.2b[22/22]
9.7 ^ 5.0a[21/21]
2.2 ^ 2.7b[13/22]
NA
NA
37.4 ^ 6.5a[20/20]
35.2 ^ 5.7a[21/21]
26.7 ^ 5.3a[20/20]
NA
NA
Delayedlight-activation (C-X-20)
Immediatelight-activation (C-X-0)
Delayed light-activation on
completely dehydrateddentine (D-X-20)
Immediate light-activation
onhydrateddentin (X-0)
Delayed light-activation
on hydrated dentine (X-20)
Experiment II
Experiment I
Deep coronal dentine
Group designations(X)
Table 3
mTBS of the control three-step adhesive and the single-step adhesives examined in the study
Experiment III
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Processed composite
376
(i.e. X-0) of each single-step adhesive (Fig. 2B). Pure
adhesive failures were rare in these subgroups.
SEM examination of fractured specimens and TEM
examination of unstressed specimens from the delayed
light-activation subgroups of the six single-step adhesives
bonded to hydrated dentine (i.e. X-20) revealed that
the dentine –adhesive interfaces were intact and that
abnormal morphological features occurred solely along
the composite – adhesive interfaces. They could be
classified into voids (Fig. 3), resin globules (Fig. 4)
and honeycomb structures that formed partitions around
a myriad of small blisters along the fractured interfaces
(Figs. 5 and 6). All unstressed TEM specimens that
appeared intact by visual inspection after laboratory
processing were found to have separated on TEM
examination, with the spaces infiltrated by the embedding
epoxy resin.
4. Discussion
As delayed light-activation of resin composite only
adversely affected the bond strengths of hydrated dentine
bonded with single-step adhesives but not the control threestep adhesive (Experiment I), we have to reject the first null
hypothesis. This abnormal phenomenon occurred in all
the single-step adhesives available, irrespective of their
resin composition and initiator systems (Table 1).
The phenomenon also occurred regardless of whether a
hybrid composite with ion-leachable glass fillers [11] or a
microfilled composite was used for the restoration. Based on
the results of Experiment I alone, it was tempting to attribute
the phenomenon to some kind of adverse interactions that
occurred between uncured resin monomers from the
adhesive and the resin composite (i.e. the intermixed zone
Fig. 1. Delayed activation of light-cured composites adversely affects the
bonding of all commercially available single-step adhesives to sound
dentine. In contrast, the micro-tensile bond strength of a three-step adhesive
system is not affected by delayed light-activation.
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 2. Control specimens: (A) low magnification SEM micrograph of the
dentine side of a fractured beam in the All-Bond 2 delayed light-activation
subgroup (AB-20). A mixed failure mode was observed involving dentine
(D), adhesive primer (P), bonding resin (B) and resin composite (C). The
fracture interfaces were free of voids; (B) SEM micrograph of a mixed
failure mode that was representative of those that occurred in the immediate
light-activation subgroups of the six single-step adhesives. The one shown
here was taken from the fractured composite side of a fractured beam in the
Etch&Prime 3.0 immediate light-activation subgroup (EP-0). Minimal
voids could be found within the fractured composite (C). A: fractured
adhesive; D: fractured dentine. Asterisk: area in which collagen fibrils
could be identified at high magnification.
(IZ)). Reaction of isocyanate groups of UDMA-containing
composites with water, for example, could generate carbon
dioxide that might contribute to the honeycomb structure.
Likewise, condensation polymerisation involving dicarboxylic acids and diols could result in the liberation of water
377
Fig. 3. Adverse effect of delayed light-activation of a resin composite that
was applied over cured single-step adhesives. I. Voids (arrows): (A) SEM
micrograph of the dentine side of a fractured beam in the Reactmer Bond
delayed light-activation subgroup (RB-20). Numerous large voids up to
50 mm in diameter could be observed along the interface between the
fractured composite (C) and fractured adhesive (A). D: exposed dentine; (B)
SEM micrograph from the dentine side of a fractured beam in the AQ Bond
delayed light-activation subgroup (AQ-20). An intermediate zone along the
fractured composite adhesive interface could be clearly observed, in which
the bases of numerous large voids were present. D: exposed dentine.
as a by-product, which in turn could be trapped within the
adhesive – composite interfaces. However, these chemical
reactions could not be expected at the ambient temperature
in which our experiments were conducted.
The observed results that adverse reactions did not occur
when delayed light-activation was performed using processed
composite as bonding substrates (Experiment III) corroborated
378
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 4. Adverse effect of delayed light-activation. II. Resin globules (arrowheads): (A) SEM micrograph of the region labelled with an asterisk in Fig. 3A (RB20). Numerous resin globules were observed within the fractured adhesive resin (A): (B) SEM micrograph of the region labelled with an asterisk in Fig. 3B
(AQ-20). Numerous resin globules could be seen on the surface of the adhesive layer (A); (C) TEM micrograph of the fractured dentine side of a One-Up Bond
F (OU-20) specimen. Failure occurred along the adhesive–composite interface, which was subsequently infiltrated by epoxy resin (E). Numerous electronlucent resin globules were observed along the surface of IZ. A: adhesive containing fluoroaluminosilicate glass fillers (G). H: hybrid layer; U: undemineralized
dentine; (D). TEM micrograph of the fractured composite side of a Prompt L-Pop (PL-20) specimen, showing the presence of numerous resin globules within
the microfilled composite. P: pre-polymerised TMPT fillers; M: composite resin matrix. Partitions (pointer) within the resin globules were probably contributed
by the Prompt L-Pop adhesive. E: epoxy resin.
that the phenomenon observed was not caused by inherent
resin–initiator incompatibility that was previously reported to
occur between acidic resin monomers and chemical-cured
composites that utilise binary redox initiator systems [10,
16–18]. As water is an indispensable component in single-step
adhesives (Table 1), the results further suggested that the
phenomenon was not caused by retention of incompletely
evaporated water within the oxygen inhibition layer of the
adhesives [19,20]. The results also focused our attention to
the fact that the underlying cause of this abnormal phenomenon
is likely to be environmental in nature. Our conjecture was
eventually substantiated by the results of Experiment II,
affirming that water derived from the bonding substrate is
responsible for the decline in bond strength of the single-step
adhesives after delayed light-activation of resin composites. We
thus have to reject the second null hypothesis.
The morphological manifestation of resin globules and
blisters along the composite –adhesive interface in delayed
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
379
Fig. 5. Adverse effect of delayed light-activation. III. SEM of the honeycomb structures: (A) Fractured composite side of a specimen beam in the Reactmer
Bond delayed light-activation subgroups (RB-20); (B) High magnification of B (RB-20) showing the presence of a honeycomb resin structure that formed the
partitions of a myriad of blisters around along the fractured interface. Bar ¼ 1 mm; (C) High magnification of the honeycomb structure from the dentine side of
a fractured beam in the delayed light-activation subgroup of an unfilled single-step adhesive, AQ Bond (AQ-20).
light-activation is reminiscent of the characteristic features
of the ‘overwet phenomenon’ that was observed along the
adhesive – dentine interface of some acetone-based
adhesives when a wet bonding technique was used [21,
22]. The ‘overwet phenomenon’ that was seen in total-etch,
three-step and two-step adhesives may also be caused by
transudation of dentinal fluid, from tubules that are rendered
patent after acid-conditioning, in vital teeth that exhibit
positive pulpal pressures [23]. However, the single-step
adhesives employed in this study are self-etching in nature
and do not require the removal of smear plugs. Moreover,
the experiments on hydrated dentine in this study were not
performed under dentine perfusion [24,25], and none of the
previously reported TEM features of the overwet phenomenon could be seen along the dentine – adhesive interfaces,
which remained intact in all specimens examined. Thus, the
only explanation for the overwet phenomenon along the
adhesive – composite interface that we can think of is that
the cured single-step adhesive layers act as permeable
membranes that permit water to diffuse from the substrate
side to the IZ.
It has been shown that water is capable of passing
through diffusion barriers provided by organic resin
coatings [26,27]. Most single-step adhesives contain
380
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 6. Adverse effect of delayed light-activation. III. TEM of the honeycomb structures (pointers): (A) AQ Bond subgroup (AQ-20). Finger-like projections
(pointers) corresponded with the honeycomb structure that formed partitions around blisters along an IZ between the composite (C) and the adhesive (A). P:
TMPT fillers in the composite; H: hybrid layer; U: undemineralised dentine. E: epoxy resin; (B) the fractured honeycomb structure from the composite side of
the Reactmer Bond subgroup (RB-20). The IZ contained fluoroaluminosilicate glass fillers (G) from the filled adhesive. Numerous globular resin phases
(arrowheads) were identified. C: composite; E: epoxy resin; (C) The dentine side of the same specimen in Fig. 5B (subgroup RB-20). A: filled adhesive layer
that contained fluoroaluminosilicate glass fillers (G) and fumed silica (arrow). IZ: intermixed zone. Arrowhead: resin globular phases within the IZ. E: epoxy
resin.
hydroxyethyl methacrylate (HEMA) which can polymerise in the presence of water to form ‘microporous’
hydrogel with pore sizes ranging from 10 to 100 nm
[28]. Differential water movement across the cured
adhesive layer may occur in the presence of increased
concentrations of dissolved inorganic ions, uncured,
water-soluble, hydrophilic resin monomers, or dissolved
collagen/proteoglycans components within the oxygen
inhibition layer of the cured adhesive. This concentration
difference may establish an osmotic pressure gradient,
causing water movement from a region of low solute
concentration (i.e. dentinal tubules in hydrated dentine
substrate) to a region of high solute concentration (i.e. IZ
along the adhesive – composite interface) [29]. This, in
turn, may result in osmotic blistering of microscopic
water droplets along the uncured IZ [29,30].
Blister initiation and growth via osmosis is common in
resin-coated metal systems, being one of the first signs for
corrosion that eventually causes delamination of the coating.
In conventional osmotic blistering, interfacial contamination
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
in the form of trapped ions between the metal and resin
surface results in the coating acting as a semi-permeable
membrane, allowing water transport from the environment
into the interface. Blisters are initiated in weak spots along
the adhesive interface. With time, blisters may enlarge to a
point where adjacent blisters coalesce. The initial osmotic
pressure buildup within the blisters is high, but gradually
diminishes as the concentration of the impurities is being
diluted [30,31]. What we observed in delayed lightactivation of resin composites may be comparable to
conventional osmotic blistering. However, unlike conventional osmotic blistering, the osmotic gradient results in
water movement from the dentine substrate into an
unpolymerised interface, producing an immiscible blend of
hydrophobic and hydrophilic monomers. Emulsion polymerisation of the hydrophobic resins in water (oil-in-water type
emulsion) [32] and the diluted hydrophilic adhesive resins
within the hydrophobic composite (water-in-oil type emulsion) [32] eventually resulted in the formation of different
types of resin globules within the interface of a single
specimen (Fig. 4). Resin polymerisation around the osmotic
blisters produces in the honeycomb resin structures observed
in Figs. 5 and 6. Similar to our previous use of a hydrophobic
impression material to take impressions of dentinal fluid
transudates from acid-etched dentine, [23] the honeycomb
structures that were formed as the relatively hydrophobic
resin composite polymerised are essentially negative
impressions of the water droplets that extruded out of cured
adhesive interfaces. The larger voids observed in Fig. 3 may
also be accounted for by the coalescence of the smaller water
blisters. The high osmotic pressure build up within these
blisters may also explain why most of the TEM specimens
were spontaneously fractured during laboratory processing.
We previously reported that micro-tensile bond strengths
of single-step adhesives decreased exponentially with time
[11]. Using two mathematical models based on Fickian
diffusion and osmotic theory to predict blister growth in
resin coating systems, Pommersheim and Nguyen showed
that irrespective of whether impurities were uniformly
distributed or present over concentrated spots along the
interface, the changes in blister radii, surface areas, heights,
volumes and osmotic pressures all varied directly as a
fractional power of time [33]. The implication of these
mathematical models in the present context is that the
overwet phenomenon caused by osmotic blistering along
the uncured adhesive – composite interface is negligible in
immediate light-activation, wherein the adhesive oxygen
from the adhesive inhibition layer is quickly absorbed into
the composite layer [34], trapping these ‘impurities’ at least
semi-permanently within the interface as the resin polymerises. This probably explained why delayed light-activation has no adverse effect on the micro-tensile bond
strength of the control three-step adhesive, or when a cured
single-step adhesives were covered with a layer of lightcured bonding resin prior to the application of the composite
[11]. It must be remembered, however, that even under these
381
circumstances, the adhesive layer still function as a semipermeable membrane. Inherent weaknesses such as airvoids within the resin – dentine interfaces cannot be totally
eliminated even with the utmost care during adhesive
application [35,36]. This implies that with time, water may
accumulate within these inherent weak spots via osmosis,
with the buildup of osmotic pressure helping to enlarge
these flaws so that blisters can form. If the composite is
light-activated immediately, such weak spots may no longer
reside solely along the adhesive –composite interface. This
provides an alternative explanation to the leaching of
hydrophilic resin components in interpreting why bond
strength decreased on ageing of adhesives that contain
polymerised hydrophilic resin components [6,7]. This
hypothesis has to be further substantiated.
In conclusion, clinicians should be aware of the potential
drop in bond strength on prolonged contact of single-step
adhesives with light-cure composites before light-activation. Although clinicians are unlikely to leave a lightcured composite unactivated for more than 2 – 3 min,
multiple direct or indirect restorations should be lightactivated individually and as soon as the composite is
applied. The use of these acidic adhesives to treat dentin
surfaces before luting indirect restorations or posts with
dual-cure resin composite cements should be avoided.
Although the light-activated reaction can be initiated soon
after luting, some clinicians may take longer to remove
excess cement before light-activation. Moreover, the
chemical reaction lasts over hours and may be significantly
compromised. It is possible also that the recently introduced
light-activation techniques that attempt to reduce the effect
of polymerisation shrinkage stress may lead to compromised results with the use of these single-step adhesives
[37]. This is an immediate concern that requires further
research, as dentists and researchers have usually assumed
that bond failures occur between adhesives and dentine and
not between adhesives and composites. Permeability of
single-step adhesives to water may also hasten the rate of
water sorption and leaching of resin components [38],
challenging the durability of resin – dentine bonds produced
by these adhesives. This is a subject of considerable concern
and should be investigated further.
Acknowledgements
We thank Amy Wong and W.S. Lee of the Electron
Microscopy Unit, the University of Hong Kong for technical
assistance. The AQ Bond and Metafil CX used in this study
were supplied by Sun Medical Co. Ltd. The One-Up Bond F
was supplied by Tokyyama Corp. The Reactmer Bond was
also sponsored by Shofu Inc. This study was supported, in
part, by CRC grant 1020 3278 24993 08004 323 01 from
The University of Hong Kong, Hong Kong SAR, China, by
grant CNPq #300481/95-0 from The University of São
Paulo, Brazil, and by grant DE 06427 from the National
382
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Institute of Dental and Craniofacial Research, USA. The
authors are grateful to Michelle Barnes for secretarial
support.
[19]
[20]
References
[1] Kugel G, Ferrari M. The science of bonding: from first to sixth
generation. The Journal of the American Dental Association 2000;
131(Suppl.):20S–5S.
[2] Tanumiharja M, Burrow MF, Tyas MJ. Microtensile bond strengths of
seven dentin adhesive systems. Dental Materials 2000;16:180–7.
[3] Kiyomura M. Bonding strength to bovine dentin with 4-META/
MMA-TBB resin—long-term stability and influence of water. Journal
of the Japanese Society for Dental Materials and Devices 1987;6:
860–72.
[4] Wang T, Nakabayashi N. Effect of 2-(methacryloxy)-ethyl phenyl
hydrogen phosphate on adhesion to dentin. Journal of Dental Research
1991;70:59 –66.
[5] Eick JD, Robinson SJ, Byerley TJ, Chappell RP, Spencer P,
Chappelow CC. Scanning transmission electron microscopy/energydispersive spectroscopy analysis of the dentin adhesive interface using
a labeled 2-hydroxyethylmethacrylate analogue. Journal of Dental
Research 1995;74:1246– 52.
[6] Sano H, Yoshikawa T, Pereira PN, Kanemura N, Mongami M,
Tagami J, Pashley DH. Long-term durability of dentin bonds made
with a self-etching primer, in vivo. Journal of Dental Research 1999;
78:906–11.
[7] Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. In vivo
degradation of resin–dentin bonds in humans over 1 to 3 years.
Journal of Dental Research 2000;79:1385–91.
[8] Spencer P, Wang Y, Walker MP, Wielicka DM, Swafford JR.
Interfacial chemistry of the dentin/adhesive bond. Journal of Dental
Research 2000;79:1458– 63.
[9] Walker MP, Wang Y, Swafford J, Evans A, Spencer P. Influence of
additional acid etch treatment on resin cement dentin infiltration.
Journal of Prosthodontics 2000;9:77–81.
[10] Sanares AME, Itthagarun A, King NM, Tay FR, Pashley DH. Adverse
surface interactions between one-bottle light-cured adhesives and
chemical-cured composites. Dental Materials 2001;17:542–56.
[11] Tay FR, King NM, Suh BI, Pashley DH. Effect of delayed activation
of light-cured resin composites on bonding of all-in-one adhesives.
Journal of Adhesive Dentistry 2001;3:207–25.
[12] Tay FR, Moulding KM, Pashley DH. Distribution of nanofillers from
a simplified-step adhesive in acid-conditioned dentin. Journal of
Adhesive Dentistry 1999;2:103–17.
[13] Shono Y, Ogawa T, Terashita M, Carvalho RM, Pashley EL, Pashley
DH. Regional measurement of resin– dentin bonding as an array.
Journal of Dental Research 1999;78:699–705.
[14] Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho R,
Pashley DH. Relationship between surface area for adhesion and
tensile bond strength—evaluation of a micro-tensile bond test. Dental
Materials 1994;10:236–40.
[15] Perdigão J, Lambrechts P, Van Meerbeek B, Vanherle G, Lopes AL.
Field emission SEM comparison of four postfixation drying
techniques for human dentin. Journal of Biomedical Materials
Research 1995;29:1111– 20.
[16] Nakamura M. Adhesive self-curing acrylic resin. Composition of 4META bonding agent. Japanese Journal of Dental Materials 1985;4:
672–91.
[17] Yamauchi J, Yamada K, Shibatani K. (inventors). Kuraray Company
Limited, assignee. Adhesive compositions for the hard tissues of the
human body. United States Patent 4,182,035; January 8 1990.
[18] Ikemura K, Endo T. Effect on adhesion of new polymerization initiator
systems comprising 5-monosubstituted barbituric acids, aromatic
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
sulphonate amides, and tert-butyl peroxymaleic acid in dental adhesive
resin. Journal of Applied Polymer Science 1999;72:1655 –68.
Pashley EL, Zhang Y, Lockwood PE, Rueggeberg FA, Pashley DH.
Effects of HEMA on water evaporation from water–HEMA mixtures.
Dental Materials 1998;14:6–10.
Paul SJ, Leach M, Rueggeberg FA, Pashley DH. Effect of water
content on the physical properties of model dentine primer and
bonding resins. Journal of Dentistry 1999;27:209 –14.
Tay FR, Gwinnett AJ, Wei SHY. The overwet phenomenon: a
scanning electron microscopic study of surface moisture in the acidconditioned, resin–dentin interface. American Journal of Dentistry
1996;9:109–14.
Tay FR, Gwinnett AJ, Wei SHY. Micromorphological spectrum from
overdrying to overwetting acid-conditioned dentin in water-free,
acetone-based, single-bottle primer/adhesives. Dental Materials 1996;
12:236–44.
Itthagarun A, Tay FR. Self-contamination of deep dentin by dentin
fluid. American Journal of Dentistry 2000;13:195 –200.
Pereira PNR, Sano H, Ogata M, Zheng L, Nakajima M, Tagami J,
Pashley DH. Effect of region and dentine perfusion on bond strengths
of resin-modified glass ionomer cements. Journal of Dentistry 2000;
28:347–54.
Escribano N, Del-Nero O, de la Macorra JC. Sealing and dentin bond
strength of adhesive systems in selected areas of perfused teeth.
Dental Materials 2001;17:149–55.
Funke W. Toward a unified view of the mechanism responsible for
paint defects by metallic corrosion. Industrial and Engineering
Chemistry Products Research and Development 1984;24:343–7.
Nguyen T, Bentz D, Byrd E. Method for measuring water diffusion in
a coating applied to a substrate. Journal of Coating Technology 1995;
67:37–46.
Chirila TV, Chen YC, Griffin BJ, Constable IJ. Hydrophilic sponges
based on 2 hydroxyethyl methacrylate I: effect of monomer mixture
composition on the pore size. Polymer International 1993;32:221 –32.
van der Meer-Lerk LA, Heertjes PM. Blistering of varnish films on
substrates induced by salts. Journal of Oil and Colour Chemists
Association 1975;58:79–84.
van der Meer-Lerk LA, Heertjes PM. Mathematical model of growth
of blisters in varnish films on different substrates. Journal of Oil and
Colour Chemists Association 1979;62:256–63.
Nguyen T, Hubbard J, Pomersheim JM. Unified model for the
degradation of organic coatings on steel in a neutral electrolyte.
Journal of Coating Technology 1996;68:45 –56.
Myers D. Surfactant science and technology, 2nd ed. New York: VCH
Publishers, Inc; 1992. p. 209–45.
Pomersheim JM, Nguyen T. Prediction of blistering in coating
systems. In: Bierwagen GP, editor. Organic coatings for corrosion
control. Proceedings of the American Chemical Society Symposium
Series No. 689, Washington, DC: American Chemical Society
publisher; 1998. p. 137 –50.
Rueggeberg FA, Margeson DH. The effect of oxygen inhibition on an
unfilled/filled composite system. Journal of Dental Research 1990;69:
1652–8.
Pashley DH, Sano H, Cuicchi B, Yoshiyama M, Carvalho RM.
Adhesion testing of dentin bonding agents: a review. Dental Materials
1995;11:117–25.
Eick JD, Gwinnett AJ, Pashley DH, Robinson SJ. Current concepts on
adhesion to dentin. Critical Reviews in Oral Biology and Medicine 1997;
8:306–35.
Kanca J, Suh BI. Pulse activation: reducing resin-based composite
contraction stresses at the enamel cavosurface margins. American
Journal of Dentistry 1999;12:107–12.
Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. Resin–
tooth adhesive interfaces after long-term function. American Journal
of Dentistry 2001;14:211–25.