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Research Article
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Conversion of one-step to two-step self-etch adhesives
for improved efficacy and extended application
NIGEL M. KING, BDS, MS, PHD, FRANKLIN R. TAY, BDSC, FADM, PHD, DAVID H. PASHLEY, DDS, FADM, PHD,
MASANORI HASHIMOTO, DDS, PHD, SHUICHI ITO, DDS, PHD, WILLIAM W. BRACKETT, DDS, MSD,
FRANKLIN GARCÍA-GODOY, DDS, MSD & MICHELLE SUNICO, DDS, MDS
ABSTRACT: Purpose: One-step self-etch adhesives have restricted use due to their acid-base incompatibility with autocured composites and their behavior as permeable membranes after polymerization. This study examined the feasibility
of their conversion to two-step self-etch adhesives via the adjunctive use of a non-solvented, relatively hydrophobic
resin coating. Materials and Methods: iBond (Heraeus Kulzer), Xeno III (Dentsply DeTrey) and Adper Prompt (3M
ESPE) were used either in multiple coats, or in a single coat followed by the use of a layer of Scotchbond Multi-Purpose
Plus bond resin (3M ESPE) for coupling to light- and auto-cured composites. Four types of experiments were
performed. Bonded specimens were examined with TEM after immersion in an ammoniacal silver nitrate tracer. Fluid
flow measurements of iBond were conducted using the two application protocols to compare the permeability of the
bonded dentin with the original smear layer. Permeability of vital dentin bonded with both application protocols were
compared for the transudation of dentinal fluid across the bonded dentin. Microtensile bond strengths of dentin bonded
with the two protocols were examined for their compatibility with an auto-cured composite. Results: The results of the
four experiments were complementary. iBond and Xeno III exhibited “apparent incompatibility” to auto-cured
composites that resulted from their inherent permeability. This was confirmed by the presence of dentinal fluid
transudate on the adhesive surfaces when they were bonded to vital dentin. Conversely, Adper Prompt exhibited “true
incompatibility” to auto-cured composites that was caused by adverse acid-base interaction, masking the inherent
permeability of this adhesive. “True” and “apparent” incompatibility issues were eliminated upon their conversion to
two-step self-etch adhesives. (Am J Dent 2004; 17:000-000)
CLINICAL SIGNIFICANCE: One-step self-etch adhesives may be improved by using the first adhesive coat as a dentin
primer, followed by the use of a non-solvented resin coating as the coupling resin. In the future, manufacturers should
consider providing a bottle of non-solvented resin in these adhesive kits as an application option.
: Dr. Franklin Tay, Pediatric Dentistry and Orthodontics, The University of Hong Kong, Prince Philip Dental
Hospital, 34 Hospital Road, Hong Kong SAR, China. E-: [email protected]
Introduction
The development of dentin adhesives has reached a
point where a genuine technological breakthrough cannot be
expected without the input of paradigms from other
scientific disciplines.1,2 Under the influence of customeroriented strategic marketing3,4, research chemists have been
forced to alter existing bonding strategies and formulate
adhesives that are simpler, speedier and more user-friendly.5
However, compromises have to be made when dentin
adhesives are formulated with a reduced number of bonding
steps.6-8
An immediate consequence of adhesive simplification is
a sacrifice of the universality of the multi-bottle
adhesives6,7, with most of the simplified versions capable of
bonding only to light-cured composites.9,10 Although the
adhesion of auto-cured and dual-cured composites may be
improved with the adjunctive use of ternary catalysts that
offset the acid-base incompatibility between acidic
methacrylate monomers and tertiary amines11,12, the bonding
efficacy of both total-etch and self-etch simplified adhesives
to auto/dual-cured composites/resin cements are hampered
by the intrinsic permeability of these simplified systems to
water that results from their increase in hydrophilicty.8,13-17
A more far-reaching consequence is that heterogeneously
distributed over the adhesive interface and the hybrid
layer18,19 are liable to water sorption and subsequent
reduction in mechanical properties. Although these
adhesives may initially bond reasonably well to dentin,20,21
subsequent water uptake and binding via hydrogen bonding
to these polar sites on these resins can result in the
plasticizing of the hydrophilic polymers22,23, resulting in a
reduction of bond durability.24,25
In conventional 3-step total-etch adhesives, etched and
rinsed dentin surfaces are primed with solvented hydrophilic
monomers. The primed surfaces are then covered with
uniform layers of hydrophobic dimethacrylates that are free
of volatile solvents.
They produce tough, relatively
impermeable surface seals that do not absorb water or
permit much water permeation.26,27 In 2-step total-etch
adhesives, the hydrophilic primer-like monomers and
hydrophobic dimethacrylates that are dissolved in various
solvents, render the mixture more like a hydrophilic primer
than a hydrophobic sealing film. Although these simplified
adhesives are often recommended to be applied in two
coats, with the first coat simulating the primer, and the
subsequent coat simulating the coupling resin of a 3-step
system, they produced at best, a 40% reduction in hydraulic
conductance across the bonded dentin28. This was roughly
half of the reduction in hydraulic conductance achieved with
the retention of the original smear layer on cut dentin.29 The
lack of a hermetic seal associated with the permeability of
the 2-step total-etch adhesives may be visualized clinically
in the form of droplets of dentinal fluid transudate that
appeared along the surface of the polymerized adhesives.8,30
Depending upon the promptness with which an overlying
hydrophobic composite is cured, these fluid droplets may be
trapped by both light-cured and auto-cured composites, and
subsequently act as stress raisers during function.14
As the acidity of self-etch adhesives is increased with
the incorporation of higher concentration of hydrophilic and
acidic monomers, the problems that are associated with
acid-base incompatibility and water permeability become
even more acute. These concerns are alleviated in 2-step
self-etch adhesives that utilize non-solvented resin coatings,
making them compatible with auto-cured composites.16 The
permeability of one of these adhesives was also reported to
be comparable with the 2-step total-etch adhesives.28
However, when manufacturers produce 1-step self-etch
adhesives that combine acidic, hydrophilic and hydrophobic
monomers, organic solvent and water into either two-bottle
sets or single-bottles, none of these adhesives bond
satisfactorily with auto-cured composites, even with the
incorporation of different types of ternary catalysts in these
adhesives.15,31 Such highly hydrophilic polymers function as
permeable membranes that permit the diffusion of water
molecules from dentin across the adhesive layer.31 The
retention of unbound water, either from residual water that
is incompletely evaporated from the adhesive, or from the
underlying dentin as a result of the high osmolality of the
hydrophilic adhesive mixture,32 creates water-filled
channels within the adhesive. These channels can be
visualized after silver impregnation,33 and have been termed
“water-trees”34 to connote their resemblance with similar
water channels that are formed by water degradation of
insulation polymer in electrical transmission cables.35 The
existence of water trees permits rapid transport of unbound
water across the adhesive interface, apart from the ionhopping of bound water along the polar domains of these
adhesives.36 The polar domains can be visualized as isolated
tiny silver grains when these adhesives were immersed in
ammoniacal silver nitrate as a tracer.37 These two forms of
nanoleakage within the adhesive provide the morphologic
correlates that account for the water permeability observed
in this class of adhesives.37
Recent studies showed that the bond strengths of
several 1-step self-etch adhesives to dentin can be improved
using multiple coats of the adhesives.38-40 Although such a
protocol is implemented as a means to increase the bulk of
the adhesive layer to prevent direct contact of the composite
with the hybridized dentin, the same protocol may be
perceived as using the first coat as a dentin primer, and
subsequent coats as coupling resins for the resin composites.
Based on such an interpretation, it is anticipated that the
technique may be further improved by using one coat of a
hydrophilic 1-step self-etch adhesive, followed by the
application of a more hydrophobic, non-solvented resin, to
simulate the bonding protocol in 2-step self-etch adhesives.
We speculate that by substituting the subsequent coats of 1step self-etch adhesives with a non-solvented resin, the
amount of hydrophilic and acidic resin components would
be reduced in the bonded dentin interfaces, rendering them
less permeable to water movement. As the acidic monomers
from the 1-step adhesives are covered by the non-solvented,
non-acidic resin coating, this would also extend their use to
include the bonding of auto-cured or dual-cured composites
and resin cements. Thus, the objective of this study was to
test the null hypothesis that no difference exist between the
1-step adhesives and their simulated conversion to 2-step
self-etch adhesives, in their capacity to reduce water
permeability across bonded dentin and in their susceptibility
to acid-base incompatibility reactions during the coupling of
auto-cured composites to the bonded dentin.
Materials and methods
Experimental design - Three 1-step self-etch adhesives,
iBonda, Xeno IIIb and Adper Promptc were investigated in
this study. These adhesives were used, either in their
designated technique as 1-step self-etch adhesives, or in
conjunction with a non-solvented, comparatively
hydrophobic resin taken from a 3-step total-etch adhesive
(bonding resin from Scotchbond Multi-Purpose Plusc; SMP)
to simulate the bonding protocol of contemporary 2-step
self-etch adhesives.
As a 1-step self-etch adhesive, iBond was applied in
three consecutive coats to the bonding substrates. After a
dwelling period of 30 s, excess adhesive was blown away
and the volatile solvents were evaporated with a gentle air
stream for 5 s and then light-cured for 20 s. For Xeno III,
liquids A and B were dispensed and mixed thoroughly for 5
s. As the manufacturer’s indication of the application of
“generous amounts” is ambiguous41, three consecutive coats
of this adhesive mixture were also employed to achieve
more consistent bonding results with this adhesive.40 The
mixed adhesive was left undisturbed for 20 s, air-dried for 5
s and then light-cured for 20 s. For Adper Prompt, the
adhesive was mixed and dispensed from the blister pack.
The adhesive mixture was applied with agitation for 15 s,
air-dried and light-cured for 10 s. As recently recommended
by the manufacturer, a second coat of adhesive was further
applied, air-thinned and light-cured.
To simulate the application protocol of a 2-step self-etch
adhesive, each of these three adhesives was employed as a
self-etching primer by utilizing one respective coat of the
adhesive to the bonding substrates, using the same
dwelling/agitation time as described. After solvent
evaporation and light-activation, a coat of the non-solvented
resin (SMP) was then placed over the “cured” adhesive, airthinned and then light-cured for an additional 10 s.
Four different types of experiments were performed, to
generate complementary information. They included:
transmission electron microscopy (TEM) of dentin bonded
in vitro, fluid conductance of dentin bonded in vitro,
scanning electron microscopy (SEM) of resin replicas of
vital dentin bonded in vivo, and finally, microtensile bond
strength evaluation of dentin bonded in vitro.
TEM examination – Bonding to dentin was performed on
the occlusal surfaces of deep coronal dentin from extracted,
human third molars. They were stored in a 1% chloramine T
aqueous solution at 4°C and used within one month
following extraction. The occlusal enamel and half of the
dentin were removed using a slow-speed saw (Isometd)
under water cooling. The tooth surfaces were polished with
180-grit silicon carbide (SiC) papers to create bonding
surfaces in mid-coronal dentin with thick, clinically relevant
smear layers.42
1. Delayed coupling of a light-cured composite to dentin
This part of the experiment was performed to simulate
the scenario of composite adaptation, sculpturing or shade
blending prior to light-activation. Two teeth with the
exposed occlusal dentin were bonded by using the
respective adhesives in their designated function as 1-step
self-etch adhesives. A 2-mm thick layer of light-cured
microfilled resin composite containing pre-polymerized
fillers (EPIC-TMPTe, also known as Metafil CXf outside
USA) was placed over the cured adhesive for 60 s under
ambient light before light-activation.
2. Coupling of an auto-cured composite to two different
bonding substrates
These experiments were performed with the knowledge
that the three 1-step self-etch adhesives are not
recommended by the manufacturers for use with auto-cured
composites, and under the assumption that incompatibility
of these adhesives with auto-cured composites should be
independent of the bonding substrates. Three separate
experiments were performed.
2a. Bonding of the 1-step self-etch adhesives to resin
composite as a bonding substrate
The rationale for this experiment was that if true acidbase incompatibility exists between the cured adhesives and
the tertiary amine accelerator from the auto-cured
composite, this should be more severe when the acidic resin
monomers in the 1-step self-etch adhesives are not buffered
by the tooth substrates. To test this hypothesis, 2-mm thick
composite wafers were first prepared using a different
microfilled composite (Durafilla). The respective 1-step selfetch adhesive was applied in 2-3 coats as previously
described. Then an experimental auto-cured compositef that
has the same resin and filler composition as EPIC-TMPT,
was hand-mixed and placed on top of the cured adhesives.
The experimental composite is a generous gift from Dr.
Takashi Yamamoto of Sun Medical Co. Ltd., to
complement the light-cured commercial version for use in
ultramicrotomy.13-15 This created an adhesive layer that was
sandwiched between two different resin composites, a lightcured composite on one side, and an auto-cured composite
on the other. Two bonded specimens were examined for
each adhesive.
2b. Bonding of the 1-step self-etch adhesives to dentin
The above experiment was repeated by substituting the
Durafill wafer with dentin as the bonding substrate. Two
bonded specimens were examined for each adhesive.
2c. Bonding of the simulated 2-step self-etch adhesives to
dentin
Experiment 2b was repeated but using only one coat of
each respective adhesive, followed by one coat of the nonsolvented resin (SMP), in the manner described previously.
Two bonded specimens were examined for each adhesive.
After storage in distilled water at 37oC for 24 h, the
bonded specimens were prepared for TEM tracer
examination by immersing 1-mm thick slabs in 50 wt%
ammoniacal silver nitrate (pH=9.5), according to the
technique described by Tay et al.37 Undemineralized, epoxy
resin-embedded, 90-100 nm thick sections were prepared
and examined without further staining, using a TEM
(Philips EM208S, Eindhoven, The Netherlands) operated at
80 kV.
In vitro fluid flow measurements - An in vitro fluid-transport
model was used to measure the fluid flow through dentin,
following the protocol for hydraulic conductance evaluation
reported by Pashley et al.43 Because of the large number of
teeth required, only one of the three adhesives (iBond) was
examined. Forty-eight dentin disks were first prepared from
human third molars by sectioning the teeth perpendicular to
their longitudinal axes from the mid-coronal crowns using
the Isomet saw under water. Each surface was ground with
SiC paper under running water for 30 s. Disks were 0.3 mm
thick as measured to the nearest 0.01 millimeter using a
digital micrometerg. The pulp side of each dentin disk was
acid-conditioned with 35% H3PO4 for 15 s to remove the
smear layer, leaving the smear layer on the upper surface
intact. After rinsing with water, the disk was placed in a
split-chamber device. The test area of each dentin disk was
limited by identical rubber “O” rings, giving a surface area
of 0.283 cm2 (Fig.1). Fluid flow was measured using an
automated apparatus (Flodech) incorporating a capillary
glass tube (0.7 mm inside diameter). An infrared beam was
passed through one side of the tube, and a photosensitive
diode positioned on the opposite side of the tube to detect
any movement of an air bubble. The rate of fluid flow (µL
cm-2 min-1) was calculated as Jv/At, where: Jv = fluid flow
in µL, A = resin surface area in cm2, and t = time in min.
Eight dentin disks were used for each adhesive.
Fig.1. Schematic representation of the setup for in vitro fluid flow
measurement.
The convective fluid flow across the smear layercovered dentin of each disk was first measured under a film
of water, using a physiological hydrostatic pressure44 of 20
cm H2O for 10 min. Each adhesive was then applied to the
smear layer-covered dentin, either as a 1-step self-etch
adhesive, or as a simulated 2-step self-etch adhesive, using
the protocols described in the previous sections. Bonding
was performed at 0 cm H2O. After light-curing, the bonded
dentin specimens in the split chamber devices were stored
without pressure (0 cm H2O) for 1 hr. The pulpal pressure
was then raised to 20 cm H2O while the specimens were
stored in a 37°C water bath. After 24 hrs of water storage,
the fluid conductance was re-measured for 10 min at 20 cm
H2O. The bonded dentin was stored without pressure for 1
hr. Concentrated calcium chloride (CaCl2) solution (4.8
moles/L) was then placed over the adhesive side of each
split chamber device, to create an osmotic gradient that
allowed water to be drawn across the polymerized adhesive
layer.45 Fluid flow induced by the osmotic gradient was
measured at 0 cm of H2O pressure. One-way ANOVA and
Fisher’s PLSD test were used to compare fluid movement
for each of the six experimental groups (N=8), with the
statistical significance set in advance at α = 0.05.
In vivo dentinal fluid transudation - Eighteen vital posterior
teeth that require crown preparation for fixed prosthodontics
were selected, with informed consent of the subjects
obtained under an in vivo protocol reviewed and approved
by an ethical committee from the University of Philippines.
Crown preparations were performed under local analgesia
(Lidocaine 2% with epinephrine 1:200,000), with the
exposed dentin sealed with one of the three adhesives prior
to impression taking as a means to preserve the health of the
pulpodentinal complex.46,47 Similar to the other sections, the
adhesives were employed either as 1-step self-etch
adhesives with the application of 2-3 coats, or in a single
priming coat that was followed by the application of a coat
of SMP resin to simulate the bonding procedures employed
in contemporary 2-step self-etch adhesives.
After bonding, the oxygen-inhibition layer was gently
removed with a cotton pellet soaked in 50% ethanol. A low
viscosity polyvinyl siloxane impression material (Affinis
LightBodyi) with an intraoral setting time of 3.5 min was
used for taking impressions of these crown preparations.
After the research impressions were taken, working
impressions were then produced for the construction of the
fixed prostheses. The research impressions were
ultrasonically cleaned in distilled water, degassed for 48
hours, and then poured up with epoxy resin (TAAB 812j).
The resin replicas were then mounted on brass stubs,
sputter-coated with gold/palladium and examined with a
SEM (Stereoscan 360k) operating at 20 kV. Micrographs
were recorded from the region of each epoxy resin cast in
which the most profuse transudation of dentinal fluid was
observed.
Microtensile bond strength evaluation - This part of the
study was performed with the limited objectives in
determining if the three 1-step self-etch adhesives were not
compatible with auto-cured composites when they were
bonded to dentin, and if the bond strengths were
significantly different when these 1-step self-etch adhesives
were used in the conjunction with the non-solvented resin
(SMP) to simulate a 2-step self-etch bonding approach.
Bonding was performed on mid-coronal dentin, with three
teeth being used for each of the two experimental groups
(1step vs 2-step) of each adhesive. An auto-cured composite
(BisFil 2Bk) was applied in bulk for coupling to the bonded
dentin specimens.
After storing in water at 37ºC for 24 hr, the built-up
teeth were sectioned into 0.9x0.9 mm beams with the slow
speed saw under water cooling, according to the technique
for the "non-trimming" version of the microtensile test
described by Pashley et al.48 The exact dimension of each
beam was individually measured using a pair of digital
calipers (Model CD-6BSm). Eight beams were obtained per
tooth, producing 24 beams for bond strength evaluation
during the respective periods of each adhesive kit. Beams
prepared were attached to a testing apparatus with a
cyanoacrylate adhesive (Zapitn). The beams were stressed
to failure under tension using a universal testing machine
(Model 4440o) at a crosshead speed of 1 mm per min, to
obtain the tensile bond strength, calculated in MPa. To
differentiate whether there were difference between the 1step and 2-step bonding protocol, bond strength results of
the two experimental groups of each adhesive were
statistically analyzed using the Mann Whitney Rank Sum
test at α = 0.05.
Results
The effect of a 60 s delay in activation of the light-cured
composite to dentin bonded with the one-step self-etch
adhesives is shown in Fig.2. Irrespective of the adhesive
examined, a line of fine blisters, partially filled with silver
deposits, could be identified at discrete locations along the
adhesive-composite interface (Fig.2A). When these blisters
formed a continuous line, partial (Fig.2A) or total
detachment (Fig.2C) of the interface occurred during
specimen sectioning, leaving behind traces of the resin
composite that were in contact with the underlying
adhesive. Silver-filled water trees in the adhesive that were
often seen directly beneath the water blisters (Figs.2B and
2C).
TEM micrographs of iBond and Xeno III showed that
their claimed incompatibility to auto-cure composite was
only “apparent” in nature, being dependent upon the
bonding substrates. When resin composite was employed as
the bonding substrate, both iBond (Fig.3A) and Xeno III
(Fig.4A) applied in three consecutive coats before lightcuring exhibited excellent coupling with the auto-cured
composite, with minimal (not shown) or no water trees
present within the adhesive layer. However, when dentin
was used as the bonding substrate, larger water blisters,
similar to those observed with the coupling of light-cured
composite, were observed along the adhesive-composite
interface (Figs.3B and 4B). The presence of these blisters as
stress-raisers also resulted in detachment of the adhesivecomposite interfaces. In addition, water trees were
frequently detected within the adhesives when they were
bonded to dentin. By contrast, excellent coupling of the
auto-cured composite was achieved when one coat of these
adhesives was used in conjunction with one coat of nonsolvented resin (SMP) for bonding to dentin (Figs.3C and
4C). The thicknesses of three consecutive coats of these
adhesives were similar to one coat of these adhesives plus
one coat of non-solvented resin (ca. 12-20 µm).
Unlike the other two 1-step self-etch adhesives, Adper
Prompt demonstrated true acid-base incompatibility that is
independent of the bonding substrate employed. Decoupling
of the auto-cured composite occurred when either a resin
composite (Fig.5A) or dentin (Fig.5B) was used as the
bonding substrate. When an additional coat of nonsolvented resin (SMP) was used in conjunction with Adper
Prompt to simulate a 2-step bonding protocol, the autocured composite coupled well to the surface of the nonsolvented resin when dentin was used as the bonding
substrate (Fig.5C).
Table 1 summarizes the results of fluid filtration across
the smear layer-covered dentin before and after bonding
with iBond. At a simulated pulpal pressure of 20 cm H2O,
there was a slight outward movement of water across the
smear layer-covered dentin. Under the same simulated
Fig.2 TEM micrographs illustrating the entrapment of fine water blisters
(arrows) along the adhesive-composite interface when a light-cured
composite with pre-polymerized fillers (LC) was placed immediately on
top of the polymerized 1-step self-etch adhesives (A), left undisturbed, and
light-activated after 60 s. a. A low magnification view of the entire resindentin interface in iBond. The presence of these fine water blisters as stress
raisers resulted in the partial separation (asterisk) of the composite during
ultramicrotomy. Pointer: water trees within the adhesive; Between open
arrows: hybrid layer containing some nanoleakage; D: dentin. b. A high
magnification view of the adhesive-composite interface in Xeno III,
showing the presence of water trees (pointer) beneath the silver-filled
blisters. Nanofiller clusters (open arrowhead) were present within the
adhesive. c. A high magnification view of the adhesive-composite interface
in Adper Prompt, showing a similar connection of the water trees (pointer)
with the silver-filled blisters. The bulk of the composite was detached
during sectioning (asterisk) and only a thin layer was retained along the
adhesive-composite interface.
physiological pulpal pressure, dentin disks bonded with
three consecutive coats of iBond exhibited a comparable
fluid flow rate that was not significantly different from that
of the original smear layer-covered dentin (P>0.05). The
addition of 4.8 moles/L CaCl2 solution at zero hydrostatic
pressure to the adhesive side of the bonded dentin disks
induced a significant increase in outward fluid movement
across the permeable adhesive (P<0.05). The permeability
the bonded interface at 20 cm hydrostatic pressure was
significantly reduced (P<0.05) when the three coats of
iBond (1-step bonding protocol) was replaced with one coat
of iBond plus one coat of non-solvented resin (SMP) to
simulate a 2-step bonding protocol. The further reduction in
osmotic fluid flow (P<0.05) in the presence of CaCl2 at 0
cm hydrostatic pressure indicated that an excellent seal of
the dentin was achieved with the simulated 2-step bonding
protocol.
SEM of resin replicas of vital dentin bonded with three
coats of iBond revealed the transudation of dentinal fluid
droplets across the bonded dentin (Fig.6A). Generally, more
dentinal fluid droplets could be observed from regions that
were adjacent to the pulp horns in deep dentin (Fig.6A).
Fluid transudation across similar regions of dentin bonded
with Xeno III or Adper Prompt was comparatively more
profuse (Fig.6B). Conversely, no transudation of dentin
fluid was observed when one coat of the respective adhesive
was used in conjunction with an adjunctive coat of nosolvented resin (SMP) to simulate a 2-step bonding protocol
(Fig.6C).
The microtensile bond strength results in Table II
illustrate how the application of the three self-etch adhesives
may be extended to include the coupling of auto-cured
composites by their conversion from 1-step to 2-step selfetch adhesives. Both iBond and Xeno III exhibited
significantly high bond strengths (P<0.05) when three coats
of these adhesives were replaced by using one coat of the
adhesive as primer, followed by the use of an additional
coat of non-solvented resin (SMP) to simulate the
application protocol of a 2-step self-etch adhesive. Adper
Prompt exhibited null bond strength to dentin when coupled
in the excellent coupling of the auto-cured composite (AC) to dentin. Open
arrowheads: interaction zone between the adhesive and the non-solvented
resin. The thickness of this combination (ca. 15-20 µm) was similar to that
achieved with three coats of adhesive (Fig. 2B). Between open arrows:
hybrid layer.
with the auto-cured composite. Conversely, significantly
higher bond strength was achieved with the use of the nonsolvented resin (SMP) to simulate a 2-step self-etching
technique.
Discussion
Fig.3 TEM micrographs illustrating the substrate-dependent nature (i.e.
apparent incompatibility) of the acclaimed incompatability of iBond to
auto-cured composite and how this may be improved by converting the
adhesive from a 1-step to a 2-step self-etch adhesive. A: adhesive; D:
dentin. a. Excellent coupling of iBond (3 consecutive coats) was observed
between a light-cured microfilled composite (LC) and an auto-cured
composite with pre-polymerized fillers (AC). Water trees were absent from
the entire adhesive interface. This indicated that the acclaimed
incompatibility is not caused by acid-base reaction between the adhesive
and auto-cured composite. b. Decoupling of the auto-cured composite (AC)
to dentin bonded with 3 coats of iBond. The appearance of water trees
(pointer) in the adhesive suggested that these water channels were derived
from water from the underlying dentin. Similar to the use of a light-cured
composite in Fig.1, coupling with the auto-cured composite resulted in the
formation of larger blisters (pointer), causing partial detachment of the
adhesive-composite interface during sectioning. The more extensive blister
formation was probably caused by the longer period (setting time 3.5 min)
in which the setting auto-cured composite was in contact the adhesive. This
permitted more water movement across the polymerized adhesive. Between
open arrow: hybrid layer. c. When the three coats of iBond was replaced by
one coat of iBond (cured individually) adhesive (A) as a primer and one
coat of non-solvented, SMP resin (R), absence of the water blisters resulted
Light-cured composites do not exhibit acid-base
incompatibility with acidic monomers from dentin
adhesives, as the tertiary amines employed as
photoaccelerators are less nucleophilic than those utilized as
redox initiators in auto-cured composites.12 Thus, the small
water blisters that were present along the adhesivecomposite interface in Fig.1 could only be caused by the
permeability of the three 1-step self-etch adhesives to water.
Normally, light-cured composites are immediately lightactivated when they are placed over the surface of the cured
adhesives. Under such a condition, the adhesive-composite
interfaces produced by these adhesives are intact and water
blisters are absent (Tay, unpublished results). Although the
occurrence of water blisters as early as after a 60 s delay in
the light-activation process may not induce immediate
concerns to the use of these adhesives, they may
subsequently act as stress-raisers during function. Thus,
clinicians have to be careful not to manipulate the uncured
composite for too long, particularly when bonding is
performed on deep vital dentin.
Both iBond and Xeno III did not exhibit acid-base
incompatibility with the use of the auto-cured composite
when composite wafers were employed as bonding
substrates. The absence of acid-base incompatibility was
also reflected by the high bond strengths obtained for these
1-step self-etch adhesives when they were coupled to the
composite wafers using an auto-cured composite.49 The
appearance of water blisters that were similar to those
Fig.4 TEM micrographs illustrating the substrate-dependent nature (i.e.
apparent incompatibility) of the acclaimed incompatability of Xeno III to
auto-cured composite and how this may be improved by converting the
adhesive from a 1-step to a 2-step self-etch adhesive. A: adhesive; D:
dentin. a. Coupling of a light-cured microfilled composite (LC) to the autocured composite with pre-polymerized fillers (AC) using three consecutive
coats of Xeno III. b. Decoupling (asterisk) along the adhesive-auto-cured
composite (AC) interface that was caused by the formation of large water
blisters (arrow) when the bonding substrate was replaced by dentin. Water
trees (pointer) could be seen beneath the water blisters). Between open
arrows: hybrid layer. c. Τhe combination one coat of Xeno III adhesive as
the primer and one coat of non-solvented SMP resin (R) resulted in
coupling of the auto-cured composite (AC) to dentin. A similar thickness
(ca. 12−20 µm) was achieved with the adhesive-resin combination. Open
arrowheads: hybrid layer.
observed in Fig. 1 showed that the incompatibility of these
two adhesives to auto-cured composites was only apparent
in nature, being dependent upon the inherent permeability of
resins to water from the underlying dentin. The larger
blisters observed with the use of the auto-cured composite
may be explained by the increase in contact time of the
auto-cured composite with the adhesive interface before
polymerization. This permitted a greater amount of water
movement from the underlying dentin.
The apparent incompatibility that is attributed to the
permeability of 1-step self-etch adhesives to water was also
confirmed with the in vitro fluid flow measurements and the
in vivo demonstration of dentinal fluid transudation. As no
composite was used in these two experiments, their results
validated the TEM observation that water blisters could
occur irrespective of the use of light-cured or auto-cured
composites. The additional use of a concentrated solution of
CaCl2 at zero hydrostatic pressure in the in vitro fluid flow
experiment further indicated that water could be osmotically
drawn across the polymerized adhesive when an osmotic
gradient was present between the surface of the adhesive
bonded to hydrated dentin. One may argue that the results of
the in vitro fluid flow study were invalid as water was used
instead of dentinal fluid, which contains plasma proteins.
However, the in vivo results clearly indicated that
permeability of these adhesives is a clinically-relevant issue,
even if the use of solvented adhesives coagulated plasma
proteins in dentinal fluid, as was previously demonstrated
using bovine serum.50 In particular, iBond contains
glutaraldehyde that is supposed to coagulate plasma
proteins51 and form partitions within the dentinal tubules to
reduce the dentinal fluid flow.52 From the in vivo results, it
is apparent that coagulation of the plasma proteins was
insufficient to completely eliminate fluid flow across the
polymerized adhesive.
Unlike iBond and Xeno III, Adper Prompt demonstrated
a true incompatibility to auto-cured composites that was
independent of the type of substrates employed for bonding
(Figs.5A and 5B). The true acid-base incompatibility tends
to mask the apparent incompatibility that is associated with
Fig.5 TEM micrographs illustrating the substrate-independent nature (i.e.
true incompatibility) between Adper Prompt and auto-cured composite, and
how this may also be improved, as in the case of “apparent
incompatibility”, by converting the adhesive from a 1-step to a 2-step selfetch adhesive. A: adhesive; D: dentin. a. There was no coupling of the
auto-cured composite (AC) to the light-cured composite (LC) when two
coats of individually light-cured Adper Prompt was employed as the
adhesive. The auto-cured composite was probably incompletely
polymerized along the adhesive-composite interface, resulting in a thick
discontinuous band of silver deposits (between open arrowheads) and a gap
(asterisk) that was infiltrated with the laboratory epoxy resin. Water trees
(pointer) could be seen in the adhesive. b. When the light-cured composite
was replaced by dentin as the bonding substrate, there was also no coupling
of the auto-cured composite (completely detached). A band of silver (S)
was present on top of the adhesive, and was trapped by a layer of
incompletely polymerized laboratory epoxy resin (E; note: similar acidamine reaction). Water trees (pointer) were present in the second coat of
adhesive. H: hybrid layer. c. The use of an additional coat of non-solvented
resin (R) over the adhesive (A) simulated the conversion of Adper Prompt
into a 2-step self-etch adhesive, making it compatible with the auto-cured
composite (AC). H: hybrid layer.
this 1-step self-etch adhesive (Fig.2C), which was better
revealed in the in vivo fluid flow part of the study (Fig.6A).
The differences between true and apparent incompatibility
for the three adhesives could also be seen from their
microtensile bond strengths to auto-cured composites, When
bonded to dentin, Adper Prompt exhibited null bond
strength, with premature failures occurring during the
creation of beams by sectioning. Conversely, iBond and
Xeno III exhibited no premature failure during sectioning,
although low bond strengths were obtained. It appears that
depending on the presence of true or apparent
incompatibility, different adhesive incompatibility profiles49
should exist for the 1-step self-etch adhesives that are
commercially available. In the future, manufacturers should
be aware of the inherent permeability of their products to
water by conducting appropriate in vitro testing, similar to
what is currently being performed with bond strength and
microleakage, before these adhesives are marketed. They
should also consider including adhesive incompatibility
profiles in the instruction manuals and MSDS literature,
instead of simply informing clinicians that these adhesives
can only be used with light-cured composites.
Irrespective of whether true or apparent incompatibility
prevails in 1-step self-etch adhesives, the associated
problems could be eliminated by their conversion to 2-step
self-etch adhesives, using a coat of non-solvented resin
(SMP) to replace the subsequent coats of adhesives. By
applying a layer of neutral, non-solvated resin on top of the
acidic monomers that were present in the oxygen-inhibition
layer of Adper Prompt, the adverse reaction of the acidic
monomer with the tertiary amine is eliminated, thereby
permitting coupling of auto or dual-cure composites to
Adper Prompt treated dentin.53 The reduction in water
permeability that occurred by substituting the subsequent
coats of highly hydrophilic adhesive with the non-solvented
resin (SMP) is not so easily appreciated. One may assert
that these subsequent coats of adhesive perform the similar
function of coupling resin composites to the underlying
primed dentin in the way that a non-solvented resin coat
would have achieved, particularly when these subsequently
Table 1 Fluid filtration across dentin after bonding with iBond
Fluid flow (µL cm-2min-1)
Test
condition
Smear layercovered dentin
(20 cm H2O)
Resin-bonded
dentin
(20 cm H2O)
Immersion in
4.8 M CaCl2*
(0 cm H2O)
iBond 3 coats
0.237 ± 0.139 B
0.264± 0.127 B
2.504 ± 0.368 A
iBond 1 coat
+
0.069 ± 0.052 C
0.006 ± 0.011 D
0.248 ± 0.114 B
SMP 1 coat
Values are means ± standard deviation. Groups with the same letter
superscript are not significantly different using Fisher’s PLSD test (P>0.05;
n=8 for each group)
*Concentrated calcium chloride solution was used to create an osmotic
gradient, allowing water to be drawn across the polymerized adhesive layer
SMP: Scotchbond Multi-Purpose Plus bonding resin
Table II
Microtensile bond strengths of the coupling of 1-step selfetch adhesives to dentin with the use of an auto-cured composite (a
procedure not recommended by manufacturers), and after their
conversion into 2-step self-etch adhesives with the use of a nonsolvented, comparatively more hydrophobic resin coating
Application
Adhesive
Bonding
steps
Actual procedure
Microtensile
bond strength
(MPa)
3
consecutive
8.7±4.6 a
coats
iBond
1 coat (individually
Simulated 2light-cured)
53.5±19.0 b
step
+
1 coat of SMP resin
3
1-step
consecutive
11.9±4.8 a
coats
Xeno III
1 coat (individually
Simulated 2light-cured)
50.2±9.9 b
step
+
1 coat of SMP resin
2 coats,
1-step
0.0±0.0 a
individually
light-cured
Adper
1 coat (individually
Prompt
Simulated 2light-cured)
46.5±8.8 b
step
+
1 coat of SMP resin
The two groups in each adhesive were analyzed using Mann Whitney Rank
Sum test. Different superscripts indicated significant difference at P<0.05.
1-step
Fig.6 SEM micrographs taken from epoxy resin replicas of polyvinyl
siloxane impressions of vital crown preparations that were bonded with the
adhesives with or without an additional coat of non-solvented SMP resin. a.
Dentinal fluid transudates that appeared as fluid droplets over the surface
of vital dentin after the application of three coats of iBond (with the
inhibition layer removed prior to impression taking). b. After the
application of two coats of Adper Prompt c. When one coat of these
simplified self-etch adhesives was used in conjunction with a coat of nonsolvented resin, fluid transudation was completely eliminated.
coats are light-cured individually. Using FTIR-MIR
(Fourier transform infrared spectroscopy-multiple internal
reflection) to investigate the permeation of water through
multiple layers of organic resin coatings, Nguyen et al.54
observed that increasing the number of coats of a
hydrophilic epoxy resin from one to three coats only
affected the time required (i.e. time-lag) for water to move
from the outside to the inside of the coatings, but did not
reduce the equilibrium intensity of the water OH stretching
band (i.e. the amount of water that passed through the
coatings). In terms of diffusion kinetics, the diffusion
coefficients that govern the process of either Fickian or nonFickian diffusion remain largely unaltered.55,56 It is only
when a polymer exhibits a notable change in diffusion
characteristics (as in the case of a non-solvented, relative
hydrophobic resin coating SMP), can the permeability of the
adhesive interface be substantially reduced (Table 1). From
a clinical perspective, the simulated conversion of the 1-step
to 2-step self-etch adhesives did not result in a substantial
increase in the thickness of the adhesive interfaces (Fig. 2C,
3C and 4C), so that the technique may be used for the
bonding of indirect restorations. It may be an interesting
exercise for the time-conscious clinician to calculate how
much time may be saved by using three coats of 1-step selfetch adhesive vs one-coat of the adhesive and one coat of
non-solvented resin. As non-solvented coupling resins have
existed as long as the history of the multi-step total-etch
adhesives53, they are neither technologically difficult to
manufacture, nor expensive to include in 1-step self-etch
adhesive kits. This would provide the clinicians an option to
select the best mode of delivery of the adhesives based on
their practicing philosophies.
Within the limits of the study, we have to reject the null
hypothesis and accept that there are differences when 1-step
self-etch adhesives are converted using a non-solvented
resin coating to simulate 2-step self-etch adhesives.
Although these advantages may not be readily apparent with
the use of light-cured composites, a reduction in the amount
of hydrophilic and acidic resin monomers along the
adhesive interface may reduce water sorption and the
hydrolysis of incompletely polymerized adhesive
components. The improved efficacy and extended
application with the conversion of 1-step to 2-step self-etch
adhesives parallel what has been popularized as the “resin
coating technique” when 1-step self-etch resin cements such
as Panavia Fp or Linkmaxq are used for bonding to
dentin.57,58 As these systems already contain ternary
catalysts to render them compatible with the acidic primers
included in the kits, the same non-solvented SMP resin has
been shown to be effective in reducing the permeability of
the primed dentin prior to the placement of the resin
cements.17 The marketing of 1-step self-etch adhesives
represents an innovative response to the clinicians’ desire
for operational efficiency. However, it may be worth
pointing out that optimizing speed and efficiency should be
accomplished without tradeoffs in product quality or
reliability.59 In the pursuit of the “future generation of
dentin adhesives”, it is prudent for designers of these
adhesives to appreciate that the physiological principles that
govern dentin permeability are as valid today as when they
were first established.29,60
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
Heraeus Kulzer, Hanau, Germany.
Dentsply DeTrey, Konstanz, Germany.
3M ESPE, St. Paul., MN, USA.
Buehler, Lake Bluff, IL, USA.
Parkell Inc., Farmingdale, NY, USA.
Sun Medical Co. Ltd., Moriyama Shiga, Japan.
Sylvae Ultra-Cal 2, Fomuler Inc., Newton, MA, USA.
DeMarco Engineering, Geneva, Switzerland.
Colténe AG, Altstätten, Switzerland.
TAAB Laboratories, Aldermaston, United Kingdom.
Cambridge Scientific Instrument Co., Cambridge, United Kingdom.
Bisco Inc., Schaumburg, IL, USA.
Mitutoyo, Tokyo, Japan.
Dental Ventures of America, Inc., Corona, CA, USA.
Instron Inc., Canton, MA, USA.
Kuraray Medical Inc., Tokyo, Japan.
GC Corp. Tokyo, Japan.
Acknowledgments: We thank Amy Wong, Electron Microscope Unit, The
University of Hong Kong for technical assistance. This study was
supported by grant 20003755/90800/08004/400/01, Faculty of Dentistry,
the University of Hong Kong, by grants DE 014911 and DE 015306 from
the National Institute of Dental and Craniofacial Research. The authors are
grateful to Michelle Barnes and Zinnia Pang for secretarial support.
Dr. King is Professor in Pediatric Dentistry & Orthodontics, University of
Hong Kong. Dr. Tay is Visiting Professor in the Department of Restorative
Dentistry and Dental Materials, University of Siena, Italy, and Honorary
Assistant Professor in Pediatric Dentistry & Orthodontics, University of
Hong Kong. Dr. Pashley is Regent’s Professor in the Department of Oral
Biology and Maxillofacial Pathology, School of Dentistry, Medical College
of Georgia, Augusta, GA, USA. Dr. Hashimoto is Instructor, Division of
Pediatric Dentistry, Hokkaido University, Graduate School of Dental
Medicine, Hokkaido, Japan. Dr. Ito is Instructor, Department of Operative
Dentistry & Endodontology, School of Dentistry, Health Sciences
University of Hokkaido, Hokkaido, Japan. Dr. Brackett is Associate
Professor, Department of Oral Rehabilitation, School of Dentistry, Medical
College of Georgia, Augusta, Georgia, USA. Dr. García-Godoy is
Professor and Head in Clinical Research Center, College of Dental
Medicine, Nova Southeastern University, Fort Lauderdale, Florida, USA.
Dr, Sunico is Senior Lecturer, Operative Dentistry, University of the
Philippines, College of Dentistry, Manila, Philippines
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