Dental Materials (2005) 21, 397–412
www.intl.elsevierhealth.com/journals/dema
Self-etch vs etch-and-rinse adhesives: effect
of thermo-mechanical fatigue loading
on marginal quality of bonded resin
composite restorations
Roland Frankenbergera,*, Franklin R. Tayb
a
Dental Clinic I Operative Dentistry and Periodontology, University of Erlangen-Nuremberg,
Glückstraße 11, D-91054 Erlangen, Germany
b
Paediatric Dentistry and Orthodontics, Faculty of Dentistry, University of Hong Kong,
34 Hospital Road, Hong Kong SAR,China
Received 19 January 2004; received in revised form 13 May 2004; accepted 13 July 2004
KEYWORDS
Dentine;
Enamel;
Adhesives;
Resin composites;
Fatigue
Summary Objective. This study evaluated the marginal integrity of dentine
adhesives bonded to enamel and dentine, before and after thermo-mechanical
loading (TML).
Methods. MO cavities with proximal boxes beneath the CEJ were prepared in
extracted human third molars. Direct resin composite restorations (Tetric Ceram)
were bonded with 3-step etch-and-rinse (Syntac Classic, Solobond Plus, OptiBond
FL), 2-step etch-and-rinse (Admira Bond, Single Bond), 2-step self-etch (AdheSE,
Clearfil SE Bond), and 1-step self-etch (all-in-one) adhesives (Adper Prompt, Xeno III,
iBond). Marginal gaps were analyzed using SEM of epoxy resin replicas. Bonded
interfaces before TML were examined with TEM to identify pre-existing attributes for
subsequent marginal disintegration.
Results. In enamel, high percentages of gap-free margins were initially identified
for all adhesives. After TML, etch-and-rinse adhesives exhibited significantly higher
percentages of gap-free margins (w90%) compared with two-step self-etch (w75%)
and all-in-one (w55%) adhesives (p!0.05). iBond did not completely etch through
the enamel smear layer. In dentine, 89–100% gap-free margins were initially
observed. After TML, there were no statistical differences among etch-and-rinse
(62–70%) and two-step self-etch (62–63%) adhesives (pO0.05). The all-in-one
adhesives exhibited significantly less gap-free margins (!40%) in dentine
(p!0.05), with iBond showing the worst marginal integrity (15%). The presence of
pre-existing water channels within the adhesives probably expedited water sorption
when restorations were under functional stresses.
Conclusion. Enamel bonding was more effective with phosphoric acid-etching.
Etch-and-rinse and 2-step self-etch adhesives showed promising marginal adaptation
* Corresponding author. Fax: C49 9131 853 3603.
E-mail address: frankbg@dent.uni-erlangen.de (R. Frankenberger).
0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2004.07.005
398
R. Frankenberger, F.R. Tay
to dentine and may have a better clinical prognosis than the all-in-one bonding
approach.
Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
Durable adhesion of dental materials to tooth
substrates is indispensable for clinical success
with tooth-coloured restorative materials that
shrink on polymerisation [1–3]. When direct resin
composites are bonded to tooth structures using
dentine adhesives, the initial and residual polymerisation stresses that are present along the cavity
walls may result in gap formation, leakage, recurrent caries and pulpal irritation [4–6]. The detrimental effect of marginal gap formation cannot be
offset even with the use of fluoride-releasing
adhesives or restorative materials that prevent
demineralization along cavity margins [7]. Thus,
only hermetic sealing of restorations guarantees
clinical success [8,9].
Phosphoric acid etching of enamel produces an
increase in bonding surface area for micromechanical retention that has been shown to be clinically
reliable [1,8]. High retention rates and excellent
marginal seal have been reported in clinical
techniques that involve bonding to phosphoric
acid-etched enamel, as exemplified with the use
of pit-and-fissure sealants, resin composite fillings,
or ceramic inlays [1,2]. New approaches to bonding
to enamel and dentine without phosphoric acid pretreatment have been introduced by the manufacturers of self-etch adhesives [9–12]. Two-step
self-etch adhesives were characterised by separate
chemical formulations for priming and bonding,
utilising a self-etching, hydrophilic primer that is
followed by the application of a comparatively
more hydrophobic bonding agent [13–15].
The recently introduced 1-step self-etch (all-inone) adhesives contain two liquids which are
applied to tooth substrates after mixing [16–18].
The rationale for using two separate bottles for
these adhesives is to isolate the potentially hydrolytically unstable acidic resin monomers from the
water that is present for ionization of these
monomers. The latest approach in simplifying
bonding to enamel and dentine are the adhesives
iBond (Heraeus Kulzer, Dormagen, Germany) and
Brush & Bond (Sun Medical Inc., Shiga, Japan), in
which all the adhesive components for etching,
priming and bonding are supplied in a single bottle.
The manufacturers claim that reliable results can
be consistently achieved without the concern for
the hydrolytic degradation of the acidic resin
monomer 4-methacryloxyethyltrimellitic acid
anhydride (4-META) in these adhesives. It has
previously been shown that although mild selfetching adhesive systems are effective for bonding
to bur-cut enamel, only minimal etching effects are
achieved on uncut enamel and their use on the
latter is not recommended [11,12,19,20].
Compared with adhesion to enamel, dentine is a
completely different bonding substrate. Due to its
tubular structure, the problem of dentinal fluid
transudation, and the presence of the smear layer,
bonding to dentine was introduced considerably
later [21,22]. The main advantage arising from the
use of dentine adhesives is their ability to reduce
post-operative sensitivity and to support enamel
margins of the restoration [1,2]. Dentine bonding is
technique sensitive [23,24]. Therefore, one major
goal for manufacturers in the development of new
adhesives is to simplify their application procedures.
The first step was to reduce conventional 3-step,
etch-and-rinse adhesives to 2-step adhesives that
combine the primers and bonding agents into a single
solution [25]. Faster bonding and easier handling
made these single-bottle adhesives very popular
among dental practitioners all over the world [1].
Reports of post-operative sensitivity with the
etch-and-rinse technique [26], irrespective of the
3-step or 2-step approach, have popularized the use
of self-etch adhesive systems that are non-rinsing in
nature and preserve the integrity of smear plugs
within the dentinal tubules. Whereas 2-step selfetch systems have been reported to give reliable
results, the all-in-one adhesives exhibited potential
shortcomings in their incompatibility with autocuring resin composites and permeability to water
movement after polymerisation [3,9,17,18].
The objective of the present study was to
compare different approaches for bonding to
enamel and dentine by use of a Class II fatigue
loading design. Evaluation of marginal adaptation
to enamel and dentine was performed with examination of epoxy resin replicas of the restorative
margins under scanning electron microscopy (SEM)
[25]. The null hypothesis tested was that there are
no differences in the marginal integrity of either
enamel or dentine margins in Class II cavities that
were bonded with the different classes of dentine
adhesives.
Self-etch vs etch-and-rinse
Materials and methods
Specimen selection and involved materials
One hundred and thirty intact, non-carious, unrestored human third molars, extracted for therapeutic
reasons, were stored in an aqueous solution of 0.5%
chloramine T at 4 8C for up to 30 days. The teeth
were debrided of residual plaque and calculus, and
examined to ensure that they were free of defects
under a light microscope at x20 magnification.
Standardised class II cavity preparations (OD,
4 mm in width bucco-lingually, 2 mm in depth at
the bottom of the proximal box) with proximal
margins located 1–2 mm below the cementoenamel
junction were performed. The cavities were cut
using coarse diamond burs under profuse water
cooling (80 mm diamond, Two-Striperw Prep-Set,
Premier, St Paul, USA), and finished with a 25 mm
finishing diamond (one pair of diamonds per four
cavities). Inner angles of the cavities were rounded
and the margins were not bevelled to deliver
comparable results to previous experiments [25].
The cavities were restored by use of different
adhesives (Table 1).
Morphological investigation
Morphological examination was performed on
Class II restorations that were not subjected to
TML, with the objective of identifying pre-existing
features within the resin-bonding substrate interfaces that may account for the differences exhibited by the adhesives in response to functional
stresses.
Three teeth were used in each adhesive group for
examination of the etching effect and bonding to
cut enamel. After cavity preparation, one tooth was
sectioned mesio-distally into two halves using a
slow-speed saw (Isomet, Buehler Ltd, Lake Bluff, IL,
USA) under water-cooling. Each half was conditioned with either phosphoric acid or the all-inadhesive, but without further restoration with the
resin composite. The adhesive was rinsed off with
acetone in order to compare the etching effect of
phosphoric acid vs the self-etch adhesives on cut
enamel. The specimens were air-dried, mounted on
aluminum stubs, sputter-coated with gold, and
examined with a SEM (Cambridge Stereoscan 360,
Cambridge, United Kingdom) operating at 20 kV.
The other two teeth from each group were bonded
with the respective adhesive and then restored with
a microfilled composite with pre-polymerized fillers
(EPIC-TMPT, Parkell Inc., Farmingdale, NY, USA) to
facilitate ultramicrotomy for subsequent examination using transmission electron microscopy
399
(TEM). The restored teeth were also in distilled
water at 37 8C for 21 days. A 1-mm thick, mesiodistal slab was sectioned from each Class II
restoration using the slow speed saw under water
cooling. These slabs were dehydrated in an ascending ethanol series and embedded in epoxy resin.
Undemineralized 90–110 nm thick sections containing the resin-enamel interfaces were prepared
according to the TEM preparation protocol
described by Lai et al. [27]. The sections were
collected on single-slot, carbon- and formvarcoated copper grids and were first examined
unstained, using a transmission electron microscope (Philips EM208S, Eindhoven, The Netherlands) operating at 80 kV. After the initial TEM
examination, the sections were further demineralized as reported by Hannig et al. [20]. The
demineralized sections were rinsed with distilled
water, double-stained with uranyl acetate and lead
citrate for TEM re-examination.
An additional 1-mm thick slab was prepared
from each Class II restoration for examination of
the extent of nanoleakage along the resin-dentine
interfaces. The slabs were immersed in 50 wt%
ammoniacal silver nitrate for 24 h, according to
the TEM silver tracer technique reported by Tay
and Pashley [3,28]. Following the reduction of
the diamine silver ions into metallic silver grains,
the slabs were dehydrated in an ascending
ethanol series and embedded in epoxy resin in
preparation for ultramicrotomy. 90–100 nm thick
sections containing the resin-dentine interfaces
were prepared as described previously for enamel
and examined with the TEM without further
staining.
Marginal quality investigation
The prepared cavities (nZ8) were treated with
different classes of dentine adhesives according
to the manufacturers’ instructions (Table 1).
Eight teeth were randomly selected for each
adhesive. The dentine adhesives and resin composite were polymerised with a Translux CL lightcuring unit (Heraeus Kulzer, Dormagen,
Germany). The intensity of the light was checked
periodically with a radiometer (Demetron
Research Corp, Danbury, CT, USA) to ensure
that 400 mW/cm2 was always delivered during
the experiments. The adhesive was polymerized
for 40 s prior to application of the resin composite in all cases. The resin composite Tetric
Ceram (Ivoclar Vivadent, Schaan, Liechtenstein;
shade A2; batch no. E00194) was used for all
experimental restorations.
Each cavity preparation was surrounded with a
metal matrix band, bonded with the respective
400
R. Frankenberger, F.R. Tay
Table 1 Chemical compositions, batch numbers, dentine pre-treatment, bonding procedures, and manufacturers
of the dentine adhesives tested.
Adhesive
Components
Batch #
Composition
Application
protocol
Manufacturer
Syntac Classic
(3-step etchand-rinse)
Etchant
F51814
36% phosphoric acid
Ivoclar
Vivadent
Primer
E52572
Maleic acid, TEGDMA, water,
acetone
Adhesive
(2. Primer)
E08386
PEGDMA, glutaraldehyde,
water
Heliobond
E10061
BisGMA, TEGDMA, UDMA
Etchant
26608
36% phosphoric acid
Primer
29503
Adhesive
29503
Water, acetone, maleic acid,
acid-functionalized methacrylates, fluorides
Acetone, dimethacrylate,
hydroxymethacrylate
Etch enamel
and dentine
for 15 s, rinse,
dry.
Apply Primer,
leave undisturbed for
15 s, air-dry.
Apply
Adhesive,
leave undisturbed for
10 s, air-dry.
Apply Bond,
air-thin, lightcure.
Etch for 15 s,
rinse, dry
gently.
Apply and airthin.
Etchant
707583
37.5% phosphoric acid
Primer
709322
Adhesive
709126
Etchant
26608
HEMA, GPDM, MMEP, ethanol, water, initiators
Bis-GMA, HEMA, GPDM, barium-aluminum borosilicate
glass, disodium hexafluorosilicate, silica
36% phosphoric acid
PrimerCBond
25509
Etchant
7EJ
PrimerCBond
1FW
Primer
E35881
Solobond Plus
(3-step etchand-rinse)
OptiBond FL
(3-step etchand-rinse)
Admira Bond
(2-step etchand-rinse)
Single Bond
(2-step etchand-rinse)
AdheSE
(2-step selfetch)
Acetone, bonding ormocer,
dimethacrylates, functionalizing methacrylates,
Initiators (camphorquinone,
amine), stabilizer (BHT)
35% phosphoric acid
BisGMA, HEMA, dimethacrylates, polyalkenoic acid
copolymer, initiator, 3-8%
water, ethanol
Dimethacrylate, phosphonic
acid acrylate, water, stabilisers
Apply, air-thin
and lightcure.
Etch for 15 s,
rinse, dry
gently.
Scrub for 30 s,
dry.
Apply, air-thin
and lightcure.
Schaan
Principality of
Liechtenstein
Voco, Cuxhaven, Germany
Kerr, Orange,
CA, USA
Etch for 15 s,
rinse, dry
gently.
Scrub for 30 s,
air-thin, lightcure.
Voco
Etch for 15 s,
rinse, dry
gently.
Scrub for 30 s,
air-thin, lightcure.
3M ESPE, Seefeld, Germany
Apply Primer,
leave undisturbed for
30 s, air-dry.
Ivoclar Vivadent
(continued on next page)
Self-etch vs etch-and-rinse
Table 1
401
(continued)
Adhesive
Clearfil SE
Bond (2-step
self-etch)
Adper Prompt
L-Pop (1-step
self-etch;
two-component system)
Xeno III (1step selfetch; twocomponent
system)
iBond (1-step
self-etch;
single-component system)
Components
Batch #
Composition
Application
protocol
Bond
E35881
Self-etching
primer
00191A
Adhesive
00186A
Apply Bond,
air-thin, lightcure.
Apply Primer,
leave undisturbed for 20 s
Apply Bond,
light-cure
Blister A
132629
Dimethacrylate, HEMA,
silica, initiators and stabilisers
HEMA, hydrophilic dimethacrylate, 10-MDP, toluidine,
camphorquinone, water
Silanated silica, BisGMA,
HEMA, hydrophilic dimethacrylate, 10-MDP, toluidine,
camphorquionone
Methacrylated phosphates,
photoinitiator, stabiliser
Blister B
132629
Liquid A
0209000043
Liquid B
0209000043
Liquid
0100121
Water, complexed fluorides,
stabiliser
HEMA, purified water, ethanol, 2,6-Di-tert-butyl-p
hydroxy toluene, nanofiller
Pyro-EMA, PEM-F, UDMA,
BHT, Camphorquinone, EPD
Acetone, 4-methacryloxyethyltrimellitic anhydride,
glutaraldehyde
adhesives, and restored incrementally with the
resin composite in layers up to 2 mm thick. The
increments were separately light-cured for 40 s
each with the light source in contact with the
coronal edge of the matrix band. After removal of
the matrix band, the restorations were light-cured
from their buccal and lingual aspects for an
additional 20 s on each side. Prior to the finishing
process, visible overhangs were removed using a
posterior scaler (A8 S204S, Hu-Friedy, Leimen,
Germany). Proximal margins were finished
with flexible disks (SofLex Pop-on, 3M ESPE, St
Paul, USA).
After storage in distilled water at 37 8C for 21
days, impressions (Provil Novo, Heraeus Kulzer,
Hanau, Germany) of the teeth were taken and a first
Manufacturer
Kuraray Medical Inc.,
Tokyo, Japan
Mix blisters A
and B, scrub
continuously
for 15 s and
re-apply until
glossy surface
appears.
3M Espe
Mix liquids A
and B, apply
and leave
undisturbed
for 20 s, airthin, lightcure.
DeTrey Dentsply, Konstanz,
Germany
Apply sufficient layers of
adhesive (3-5
coats), airthin, lightcure.
Heraeus Kulzer, Dormagen, Germany
set of epoxy resin replicas (Alpha Die, Schuetz
Dental, Rosbach, Germany) was made for SEM
evaluation.
Functional loading of class II cavities
Thermo-mechanical loading of specimens was then
performed in an artificial oral environment (‘QuaQuasimodo’ chewing simulator, University of Erlangen, Germany). Two specimens were arranged in
one simulator chamber in proximal contact, similar
to the oral situation with the two restored
marginal ridges in a normal intercuspation
(Fig. 1). The two adjacent lateral ridges were
occluded against a steatite (a multi-component
semi-porous crystalline ceramic material)
402
R. Frankenberger, F.R. Tay
Figure 1 A photograph of the chewing simulator employed in the study. The insert at the right illustrates
schematically the alignment of two specimens in one chamber of the chewing simulator.
antagonist (6 mm in diameter) for 100,000
cycles at 50 N at a frequency of 0.5 Hz. The
specimens were simultaneously subjected to
2500 thermal cycles betweenC5 8C andC55 8C
by filling the chambers with water in each
temperature for 30 s. The mechanical action
and the water temperature within the
chewing chambers were checked periodically to
ensure a reliable thermo-mechanical loading (TML)
effect.
Figure 2 SEM micrographs (1:200) of epoxy resin replicas reproduced from impressions of Class II cavities bonded with
different dentine adhesives. RC: resin composite; D: dentine. A. Replica of Solobond Plus with ‘gap-free margin’ after 21
days of water storage, prior to thermo-mechanical loading. B. Gap formation (pointer) between adhesive and dentine,
observed after thermo-mechanical loading (iBond). C. ‘Marginal irregularity’ between resin composite and dentine,
observed after thermo-mechanical loading. The replica shows fluid transudation through dentine and the adhesive
layer, as visible by blisters (Single Bond).
Self-etch vs etch-and-rinse
Analysis of marginal quality
After the completion of the 100,000 mechanical
loading and the 2500 thermal cycles, impressions
of the teeth were made again and another set of
replica was made for each restoration. The
replicas were mounted on aluminum stubs, sputter-coated with gold and examined under a SEM
(Leitz ISI 50, Akashi, Tokyo, Japan) as before at
x200 magnification.
SEM examination was performed by one operator
having experience with quantitative margin analysis
who was blinded to the restorative procedures. The
marginal integrity between resin composite and
dentine was expressed as a percentage of the entire
margin length in enamel and dentine. Marginal
qualities were classified according to the criteria
‘continuous margin’ (Fig. 2A), ‘gap/irregularity’
(Fig. 2B and C) and ‘not judgeable/artifact’. Afterwards the percentage ‘continuous margin’ in
relation to the individual judgeable margin was
calculated as marginal integrity.
Statistical appraisal
Statistical analysis was performed using SPSS/PCC,
Version 10 (SPSS Inc., Chicago, IL, USA) for Windows.
As the majority of groups in each of the two
investigations (i.e. enamel or dentine marginal
integrity) did not exhibit normal data distribution
(Kolmogorov–Smirnov test), non-parametric tests
403
were used (Wilcoxon matched-pairs signed-ranks
test, Mann–Whitney-U test) for pairwise comparisons at the 95% significance level.
Results
Morphological investigation
Bonding to cut enamel
SEM and TEM micrographs of the bonding of
representative adhesive to cut enamel are shown
in Figs. 3–6. iBond, a mild all-in-one adhesive, did
not completely dissolve the enamel smear layer
(Fig. 3A). The enamel hybrid layer comprised
mostly of the smear layer, with incomplete and
minimal etching of the underlying prismatic enamel
(Fig. 3B). The enamel smear layer was completely
dissolved in Xeno III, producing a mild surface
etching effect of the enamel crystallites but with no
differential etching of the enamel prisms (Fig. 4A).
This all-in-one adhesive created a 1.5–3 mm thick
hybrid layer in the underlying prismatic enamel
(Fig. 4B), that consisted of predominantly intercrystallite infiltration (Fig. 4C) and with no resin tag
formation.
Mild differential etching of the enamel prisms
could be observed with the use of Adper Prompt
(Fig. 5A), creating 5–8 mm thick enamel hybrid layers
that consisted predominantly of intercrystallite
Figure 3 A. SEM micrograph of cut enamel from a Class II cavity that was etched with iBond (the adhesive was not
cured and was rinsed off with ethanol to illustrate the etching effect). The bulk of the enamel smear layer (ES) was
retained. In the partially exposed prismatic enamel (P), prism boundaries could barely be identified (open arrowheads).
B. Corresponding TEM micrograph (unstained, undemineralised) of cut enamel bonded with iBond. The hybrid layer
consisted of a surface enamel smear layer (ES) and a thin zone of prismatic enamel (P). In some areas, the one-step selfetch adhesive was not aggressive enough to etch through the enamel smear layer (open arrow). S: empty space from
which the unbonded enamel has separated from the enamel hybrid layer during ultramicrotomy.
404
R. Frankenberger, F.R. Tay
Figure 4 A. SEM micrograph depicting the etching effect of Xeno III on cut enamel. The smear layer was completely
dissolved, exposing the etched prismatic enamel. There was very little differential etching of the enamel prisms and the
prism boundaries could vaguely be recognised (black arrowheads). B. Unstained, undemineralised TEM micrograph of
cut enamel bonded with Xeno III. The enamel smear layer was completely dissolved and a 1–3 mm thick enamel hybrid
layer (H) could be observed. The interprismatic boundary (open arrowheads) of an enamel prism could be seen.
Nanofiller clusters (pointer) were present within the adhesive (A). S: empty space. C. The corresponding stained,
demineralised TEM micrograph showing the entrapment of enamel proteins within the hybrid layer (H). A: adhesive;
S: empty space.
infiltration, as the amount of differential etching
was insufficient to produce frank resin tags (Fig. 5B).
However, the adhesive was weak (Fig. 5B) and
frequently separated from the surface of the hybrid
layer, generating gaps between the adhesive layer
and the latter (Fig. 5C). On the contrary, differential
etching of enamel prisms could be clearly identified
with phosphoric acid-etching (Fig. 6A), creating
8–10 mm thick hybrid layers in 3-step etch-and-rinse
adhesives such as Syntac Classic that consisted of
both intercrystallite infiltration (Fig. 6B) and resin
tag formation (Fig. 6C).
Bonding to dentine
Phosphoric acid etching of dentine created
5–6 mm thick hybrid layers in 2-step etch-andrinse adhesives such as Single Bond (Fig. 7A) and
Admira Bond (Fig. 7C). Regions of incomplete
resin infiltration could be identified in these
adhesives, as indicated by the reticular patterns
of nanoleakage within the hybrid layers. This
feature was also observed in all the etch-andrinse adhesives examined (not shown). Two
additional modes of nanoleakage [35] could also
be identified within the adhesive layer. For
Self-etch vs etch-and-rinse
405
Figure 5 A. SEM micrograph showing that the more aggressive one-step self-etch adhesive Adper Prompt produced a
mild differential etching of the enamel prisms in cut enamel. B. Unstained, undemineralised TEM micrograph showing a
5–8 mm thick enamel hybrid layer. The adhesive layer (A) was partially torn (open arrow), entrapping loose apatite
crystallites that were dislodged during sectioning. S: empty space. C. The corresponding stained, demineralised TEM
micrograph showing separation of the adhesive layer (open arrows), creating a gap (G) that was probably infiltrated by
the embedding epoxy resin. H: hybrid layer that contains entrapped enamel proteins. S: empty space.
example, in Single Bond, silver-filled water
channels (water trees [3]) were seen from the
surface of the hybrid layer into the polyalkenoic
acid copolymer component of the adhesive
(Fig. 7B). Isolated silver grains could be observed
throughout the hybrid layer and adhesive layer of
Admira Bond (Fig. 7D).
An example of the bonding of a 2-step self-etch
adhesive to dentine is illustrated with AdheSE in
Fig. 8A and B. AdheSE completely dissolved
the dentine smear layer and created a 2 mm thick
hybrid layer within the intertubular dentine, with
some nanoleakage observed within the hybrid layer.
The adhesive layer was completely devoid of water
trees. Similar features were observed in Clearfil SE
Bond (not shown). By contrast, both water trees and
isolated silver grains were extensively observed
within the adhesive layer of the all-in-one adhesive
iBond (Fig. 8C and D). These two modes of
nanoleakage were also observed in the other two
all-in-one adhesives, Xeno III and Adper Prompt (not
shown).
Marginal quality investigation
Marginal quality in enamel
Results for marginal quality in enamel are presented in Table 2. Prior to TML, high percentages of
gap-free margins were found. iBond with 89% gapfree margins exhibited significantly more gaps/
406
R. Frankenberger, F.R. Tay
Figure 6 A. SEM micrograph illustrating the highly varied differential etching created in cut enamel with the use of
phosphoric acid for 15 s. B. Unstained, undemineralised TEM micrograph of cut enamel bonded with Syntac classic. The
apatite crystallites along the surface of the 8–10 mm thick hybrid layer (H) appeared very loose, creating channels for
intercrystallite resin retention. The unbonded enamel was fragile and separated from the hybrid layer during sectioning,
resulting in an empty space (S). Open arrowheads: interprismatic boundary. A: adhesive. C. The corresponding stained,
demineralised TEM micrograph, showing a region with extensive differential etching, creating resin tags (pointer) for
retention. A: adhesive; H: hybrid layer; S: empty space.
irregularities than Syntac Classic, Solobond Plus,
OptiBond FL, Admira Bond, Single Bond, and Clearfil
SE Bond (p!0.05; Mann–Whitney U-test).
Comparing the results before and after TML, all
adhesive systems showed a significant loss of gapfree margins (p!0.05; Wilcoxon matched-pairs
signed-ranks test). After TML, etch-and-rinse
adhesives performed significantly better than
self-etch adhesives (p!0.05; Mann–Whitney Utest). Among the self-etch systems, the 2-step
adhesives with separate bonding agents (AdheSE,
Clearfil SE Bond) exhibited better marginal quality
than the all-in-one adhesives; however, the
differences were not statistically significant for
clearfil SE Bond (pO0.05; Mann–Whitney U-test).
Only iBond showed a significantly smaller percentage of gap-free margins than the other adhesives
(p!0.05; Mann–Whitney U-test).
Marginal quality in dentine
Results for marginal adaptation in dentine are
displayed in Table 3. Similar to enamel, high
percentages of gap-free margins were found in
dentine before TML. iBond with 88% gap-free
margins, exhibited significantly more gaps/irregularities than Syntac Classic, Solobond Plus, OptiBond FL, AdheSE, and Clearfil SE Bond (p!0.05;
Mann–Whitney U-test).
Self-etch vs etch-and-rinse
407
Figure 7 TEM micrographs of dentine bonded with representative etch-and-rinse adhesives prior to thermomechanical loading. Unstained, undemineraliszed sections were obtained after immersion of the bonded specimens in
ammoniacal silver nitrate. C: composite; A: adhesive; H: hybrid layer; D: dentine. A. Low magnification view of the
resin-dentine interface in Single Bond. Nanoleakage, in the form of silver deposits (arrows), could be observed within
the hybrid layer and within the polyalkenoic acid copolymer component of the adhesive (P). B. Silver deposits within the
polyalkenoic acid copolymer component were not restricted to the surface of the dentine hybrid layer. In this high
magnification view of Single Bond, silver-stained water channels (arrow) could be observed in the polyalkenoic acid
copolymer (P) that was adjacent to the resin composite. C. A low magnification view of the resin-dentine interface in
Admira Bond. D. A high magnification view of the hybrid layer in Admira Bond, showing the presence of heavy reticular
patterns of silver deposits (arrow) as well as isolated silver grains (open arrowheads) within the resin-dentine interface.
Comparing the results before and after TML, all
adhesive systems showed a significant decline in the
percentages of gap-free margins (p!0.05; Wilcoxon matched-pairs signed-ranks test). After
TML, etch-and-rinse adhesives and 2-step selfetch adhesives performed significantly better than
the all-in-one systems (p!0.05; Mann–Whitney
U-test). Among the all-in-one adhesives, iBond
showed significantly less gap-free margins (p!
0.05; Mann–Whitney U-test).
Discussion
In this study, we investigated the marginal quality
of four classes of dentine adhesives under simulated
clinical conditions with the use of a chewing
simulator. Understandably, clinical trials remain
the gold standard in evaluating the performance of
dental materials. However, one has also to take into
consideration that the products under investigation
may become obsolete by the time useful clinical
408
R. Frankenberger, F.R. Tay
Figure 8 TEM micrographs of dentine bonded with representative self-etch adhesives prior to thermo-mechanical
loading. C: composite; A: adhesive; H and between open arrows: hybrid layer; D: dentine. A. A low magnification view of
the resin-dentine interface in the two-step self-etch adhesive AdheSE. B. A high magnification view of AdheSE, showing
the presence of comparatively minimal nanoleakage within the hybrid layer. Some isolated silver grains were associated
with the nanofiller clusters. C. A low magnification view of the resin-dentine interface in iBond, showing the presence of
many silver-filled water channels (water trees—pointer) and isolated silver grains (open arrowhead) throughout the
adhesive layer. D. A high magnification view of iBond, showing the presence of extensive nanoleakage within the hybrid
layer. Primary water trees (open arrowheads) could be seen extending vertically from the surface of the hybrid layer,
giving rise to silver-filled water blisters (asterisk) with the adhesive layer. Secondary water trees (pointer) extended
circumferentially from these water blisters.
data are collected. This is further complicated by
the time lag between obtaining the clinical results
and having them published in peer-reviewed journals. Thus, preclinical screening via laboratory tests
is still an important tool for the evaluation of
dentine adhesives [29]. Bond strength tests are
commonly carried out with quasistatic load until
fracture. However, failure of clinical restorations
due to high loads is exceptional [5,29]. More often,
the materials or interfaces fail after repeated subcatastrophic loading, with stresses that are too
small to provoke spontaneous failures during their
initial applications [29]. Thus, the most
frequent
observation
is
gap
formation
between the resin composite and tooth substrates.
These gaps may result from either insufficient
compensation for the initial high polymerization
shrinkage stresses that occur prior to occlusal
loading, or from the lower, repeated stresses
which are below the maximum stress the
adhesive
restoration
could
resist
[25].
Therefore, fatigue tests provide a better understanding of the in vivo behaviour of dentine
adhesives [25,29].
Self-etch vs etch-and-rinse
409
Table 2 Results of the SEM analysis of enamel
margins before and after thermo-mechanical loading
(TML).
Adhesive
Gap-free margins in enamel [%]
(SA)
Prior to TML
After TML
Syntac classic
Solobond Plus
OptiBond FL
Admira Bond
Single Bond
AdheSE
Clearfil SE Bond
Adper Prompt
Xeno III
iBond
100A
100A
100A
99.3 (2.1)A
98.9 (2.2)A
92.1 (6.1)AB
96.3 (5.9)A
91.2 (9.9)AB
94.7 (7.3)AB
89.0 (9.7)B
92.9
94.3
94.7
90.6
94.5
80.0
71.3
56.2
54.6
54.0
(7.4)A
(6.7)A
(5.3)A
(9.3)A
(7.2)A
(10.0)B
(14.1)BC
(14.8)C
(18.1)C
(14.3)C
Same letters within one column indicate no statistically
significant difference (pO0.05, Mann–Whitney-U test).
The results of this study clearly indicated that
conventional phosphoric acid-etching remains
the most reliable mode of pre-treatment in obtaining a durable and more fatigue-resistant enamel
bond [20,30,31]. Although most of the self-etch
adhesives bonded well to cut enamel prior to
functional and thermal stresses, they were significantly less effective after fatigue testing. It is
known from earlier reports that the micromorphological interaction of etch-and-rinse adhesives
extends deeper into enamel. On the other hand,
it is also known that self-etching systems provide a
network of intercrystallite retention leading to a
large surface for bonding [1,12,20]. This kind of
enamel bond produced by these self-etching systems proved to be able of initially compensating for
Table 3 Results of the SEM analysis of dentine
margins before and after thermo-mechanical loading
(TML).
Adhesive
Gap-free margins in dentine [%]
(SA)
Prior to TML
After TML
Syntac Classic
Solobond Plus
OptiBond FL
Admira Bond
Single Bond
AdheSE
Clearfil SE Bond
Adper Prompt
Xeno III
iBond
100A
97.9 (3.3)A
100A
98.9 (2.0)A
96.5 (5.5)AB
98.9 (3.2)A
100A
94.7 (7.8)AB
95.2 (6.9)AB
87.7 (10.4)B
69.8
70.3
65.6
64.2
62.5
62.8
62.9
34.0
38.1
15.0
(14.0)A
(19.7)A
(14.0)A
(22.5)A
(16.1)A
(21.0)A
(16.8)A
(15.7)B
(13.5)B
(11.2)C
Same letters within one column indicate no statistically
significant difference (pO0.05, Mann–Whitney-U test).
polymerisation shrinkage stresses. However, marginal quality of these interfaces seems to be lower
when compared to the etch-and-rinse adhesives.
Although aggressive self-etch adhesives like Adper
Prompt creates hybrid layers that approach the
thickness of those created by adhesive systems that
utilise phosphoric acid-etching, it may be the lack
of frank resin tags in the self-etch systems that is
responsible for their compromised marginal quality. Although a flat hybrid layer that relies solely
on intercrystallite retention is mechanically retentive, in-plane crack propagation may occur more
easily in the presence of stress raisers such as
minute air voids that are trapped along the resinenamel interface. On the contrary, the incorporation of resin tags provides a three-dimensional
grasp of the etched enamel. This may deter crack
propagation via crack branching or deflection that
consume fracture energy, thereby improving the
fracture toughness of the interface and increasing
its resistance to fatigue stresses. A similar retardation in crack propagation at the bone-cement
interface has been observed with the penetration of
polymethyl methacrylate bone cement spikes or
‘posts’ into cancellous bone [32,33]. For iBond, the
inability to etch completely through the enamel
smear layer produced thin, incomplete hybridisation of the subsurface prismatic enamel. This may
additionally account for the decline in marginal
integrity after TML in this mild self-etch adhesive,
with the weak link possibly occurring between the
hybridised enamel smear layer and the underlying
unetched prismatic enamel.
There were also significant differences between
2-step self-etch systems and the all-in-one
adhesives in their ability to withstand stresses
generated via fatigue testing. The difference
between 2-step and all-in-one self-etching systems
should not be related to different ways of interacting
with enamel. It is probably attributed to the fact
that the all-in-one adhesives are more susceptible to
water sorption. In the absence of a coupling
hydrophobic bonding agent, they behave as permeable membranes after polymerization [17,18].
This may expedite water sorption between the
partially demineralized enamel and the restorative
material, plasticizing and eventually weakening the
bonded enamel interfaces.
The present study also demonstrated pronounced differences among the adhesives in their
bonding performance on dentine, with the general
trend that conventional systems with separate
primers and bonding agents perform better than
simplified systems that combine the functions of
priming and bonding, irrespective of the etch-andrinse or the self-etch approach. The results of
410
the functional cavity test clearly showed that all
adhesives performed very well initially in their
capacity to compensate for shrinkage stresses
generated during the polymerisation of the resin
composite. This is reflected by the high percentages
of gap-free margins after setting of the resin
composite. Incorrect handling or chemical incompatibility between adhesive and resin composite of
different manufacturers can be ruled out, as shown
by the successful initial results. However, it is
disenchanting to observe that after 100,000 loading
cycles, the fatigue phenomenon has a profound
influence on the bonded dentine margins, resulting
in up to 85% gap formation over time.
The most important property of dentine
adhesives relating to marginal quality seems to be
the presence of a hydrophobic bonding agent [25].
All the adhesive systems that utilize separate
bonding resins exhibited promising results that are
independent of a phosphoric acid-etching step.
Exceptions were the 2-step etch-and-rinse
adhesives Admira Bond and Single Bond. In particular, replicas of the Single Bond specimens exhibited
water transudation from the dentine aspects of the
specimens, as demonstrated by the occurrence of
blisters within the adhesive and swelling of the
adhesive layer itself. This may be perceived as the
first sign of water sorption that is precipitated by
the permeability of this adhesive, with the release
of the water during impression taking.
Although nanoleakage was universally observed
in dentine hybrid layers of all the etch-and-rinse
and self-etch adhesives, the adhesive layer in Single
Bond is characterised by the presence of water
trees [3] in the polyalkenoic acid copolymer
component (Fig. 7A and B). Although these water
trees were absent in Admira Bond, isolated silver
grains could be detected throughout the entire
adhesive layer (Fig. 7C and D). These features were
not observed in the 2-step self-etch adhesive
AdheSE (Fig. 8A and B), but were present in
abundance in the all-in-one adhesives such as
iBond (Fig. 8C and D). It has been suggested
recently that the water trees represent channels
containing free, unbound water that are trapped
within the adhesive, while the isolated silver grains
represent bound water that are attached via
hydrogen bonding to the ionic or hydrophilic
domains within the adhesive [34]. Thus the water
trees and the isolated silver grains are the morphologic manifestation of sites within the adhesive
wherein water sorption and movement is likely to
occur. Whereas the water trees provide the venues
for rapid capillary fluid flow through the adhesive
[35], the isolated water grains represent sites
within the adhesive in which ions and small
R. Frankenberger, F.R. Tay
molecules can jump from nanopore to nanopore
via the process of ion hopping [36,37], being the
molecular mechanism for diffusion of water through
the adhesive [38].
In the absence of a more hydrophobic coating in
the simplified adhesive systems, rapid water sorption via the water trees and isolated silver grains can
occur via the hydrophilic and permeable adhesive
layer. Compared with the 2-step etch-and-rinse
adhesives, water sorption is likely to be more
prominent in all-in-one adhesives due to the
incorporation of high concentration of ionic (acidic)
resin monomers. This may account for the severe
compromise in marginal integrity when dentine
bonded with the all-in-one adhesives were subjected to TML. Although it is known that compression stresses result in a decrease, while tensile
stresses result in an increase in the rate of water
sorption through polymerized resins [39,40], it is
possible that the release of compression stresses
during each loading cycle creates a partial vacuum
that actively promotes water transport via capillary
fluid flow through the water trees that pre-existed
prior to TML. It is intriguing that water sorption is
rapid enough to cause a severe degradation of the
marginal integrity during the period in which TML
was performed. By measuring the electric impedance of hydrophobic vs hydrophilic resin films, it
was shown that 20 mm thick hydrophobic resin films
gave an initial, high impedance value of 1–5!
109 ohms to electric current flow and did not change
over time. Conversely, 20 mm thick hydrophilic resin
films gave an initial impedance of 7!106 ohms
(almost 1000-fold less impedance), that rapidly fell
to 1!103 ohms after four days (i.e. another 1000fold decrease in electric impedance in four days)
[David Pashley—personal communication]. These
data illustrated the extremely rapid rate in which
water sorption can occur in the very hydrophilic allin-one adhesives. As all dentine adhesives contain
hydrophilic resin components to a variable extent, it
is not surprising therefore to see a decline in
marginal integrity for all adhesives, although such
a phenomenon was exacerbated in the all-in-one
adhesives. A recent long-term water storage study
also showed that simplified 2-step etch-and-rinse
adhesives performed worse compared to 3-step
etch-and-rinse systems [30]. From a morphologic
perspective, it has also been shown that the
propensity for the occurrence of isolated silver
grains and water trees increased when exposed
bonded dentine specimens were aged in water for 12
months [41]. Further studies should be performed to
correlate the morphologic expression of these two
modes of silver deposition and the changes in
electric impedance of bonded dentine with and
Self-etch vs etch-and-rinse
without TML, in order to confirm the hypothesis that
water sorption is expedited during TML.
We have to reject the null hypothesis that are no
differences in the marginal integrity of either
enamel or dentine margins in Class II cavities that
were bonded with the different classes of dentine
adhesives. The enamel and dentine bonding fatigue
performances reported in this study are in agreement with the conclusion of a recently published
TML study [42], in that simplified adhesive systems
cannot be recommended for unrestricted clinical
use in Class II restorations. With respect to the use
of self-etch adhesives, 2-step self-etch adhesives
exhibited potentially more promising results than
the all-in-one bonding approach.
Conclusions
All adhesives under investigation exhibited a certain amount of deterioration relating to marginal
quality in enamel and dentine.
Regarding bonding efficacy to enamel and dentine, conventional 3-step etch-and-rinse adhesives
are still not surpassed by the newer simplified
adhesive systems.
For self-etch adhesives, the all-in-one bonding
approach was less effective than 2-step self-etch
systems that encompass the use of separate
hydrophobic bonding resins, especially in bonding
to dentine.
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