Australian Dental Journal
The official journal of the Australian Dental Association
Australian Dental Journal 2011; 56:(1 Suppl): 31–44
doi: 10.1111/j.1834-7819.2011.01294.x
Current aspects on bonding effectiveness and stability in
adhesive dentistry
MV Cardoso,* A de Almeida Neves,* A Mine,* E Coutinho,* K Van Landuyt,* J De Munck,*
B Van Meerbeek*
*Catholic University of Leuven, School of Dentistry, Oral Pathology and Maxillo-facial Surgery, Department of Conservative Dentistry, Leuven
BIOMAT Research Cluster, Leuven, Belgium.
ABSTRACT
Improved dental adhesive technology has extensively influenced modern concepts in restorative dentistry. In light of
minimal-invasive dentistry, this new approach promotes a more conservative cavity design, which basically relies on the
effectiveness of current enamel-dentine adhesives. Nowadays, the interaction of adhesives with the dental substrate is based
on two different strategies, commonly described as an etch-and-rinse and a self-etch approach. In an attempt to simplify the
bonding technique, manufacturers have decreased the number of steps necessary for the accomplishment of the bonding
procedure. As a consequence, two-step etch-and-rinse and one-step (self-etch) adhesives were introduced and gained rapid
popularity in the dental market due to their claimed user-friendliness and lower technique sensitivity. However, many
concerns have been raised on the bonding effectiveness of these simplified adhesives, especially in terms of durability,
although this tends to be very material dependent. In order to blend all the adhesive components into one single solution,
one-step adhesives were made more acidic and hydrophilic. Unfortunately, these properties induce a wide variety of
seemingly unrelated problems that may jeopardize the effectiveness and stability of adhesion to the dental substrate. Being
more susceptible to water sorption and thus nanoleakage, these adhesives are more prone to bond degradation and tend to
fail prematurely as compared to their multi-step counterparts. Incidentally, another factor that may interfere with the
bonding effectiveness of adhesives is the technique used for caries removal and cavity preparation. Several tools are on the
market today to effectively remove carious tissue, thereby respecting the current trend of minimum intervention. Despite
their promising performance, such techniques modify the tooth substrate in different aspects, possibly affecting bonding
effectiveness. Altogether, we may conclude that not only the adhesive formulation, but also substrate nature must be taken
into account to achieve a stable bonding interface, rendering the restorative treatment more predictable in terms of clinical
performance. In this review, we analyse the current theoretical and clinical aspects of adhesion to enamel and dentine, and
discuss the diverse possibilities to overcome problems which nowadays still challenge clinicians in their achievement of a
more stable and effective bond to tooth enamel and dentine.
Keywords: Adhesion, bond strength, cavity preparation, caries removal, dental adhesive, dentine, enamel.
INTRODUCTION
Improvements in dental adhesive technology have
extensively influenced modern restorative dentistry.
Nowadays, the surgical approach of ‘extension for
prevention’ proposed by GV Black1 in 1917 is no longer
justifiable, and has been replaced by the concept of
‘minimally-invasive dentistry’.2 This modern approach
focuses on the achievement of a more conservative
cavity design, basically providing sufficient access for
the complete removal of the carious tissue. The
subsequent restorative procedure relies on the bonding
effectiveness of adhesive materials such as resin composites, which do not require the removal of sound
ª 2011 Australian Dental Association
dental structure for additional mechanical retention.
Although these restorations tend to fulfil the main
requirements of a more conservative and aesthetic
treatment, their clinical longevity is still a topical issue,
mainly due to the degradation of the adhesive interface
over time.3 Clinically, the main cause of failure of
composite fillings is related to the occurrence of
marginal leakage, which eventually leads to marginal
discolouration, secondary caries, and subsequent loss of
retention.4,5 This forces dentists to replace composite
restorations in relatively short intervals. In this context,
several aspects should be considered with regard to the
bond strength and durability of adhesion to dental hard
tissues. These include the heterogeneity of tooth
31
MV Cardoso et al.
structure and composition, the hydrophilicity of the
exposed dentine surface, the features of the dental
substrate after cavity preparation6–8 and the characteristics of the adhesive itself, such as its physicochemical
properties and its strategy of interaction with enamel
and dentine.3,9 The present literature review deals with
modern concepts in adhesive dentistry, discussing issues
that still challenge the achievement of an optimal
interaction between adhesives and dental substrates.
For this purpose, a parallel is drawn among the
currently available adhesive strategies and the main
factors that interfere with their interaction with enamel
and dentine.
Interaction with dental hard tissues
The fundamental mechanism of bonding to enamel and
dentine is essentially based on an exchange process in
which minerals removed from the dental hard tissues
are replaced by resin monomers that upon polymerization become micromechanically interlocked in the
created porosities.6,10
Dissolution of superficial dental hard tissue is conventionally achieved by phosphoric-acid etching, one of
the major breakthroughs in adhesive dentistry introduced by Buonocore more than 50 years ago.11 On
enamel, acid-etching selectively dissolves the enamel
rods, creating microporosities which are readily penetrated, even by ordinary hydrophobic bonding agents,
by capillary attraction.12 Upon polymerization, this
micromechanical interlocking of tiny resin tags within
the acid-etched enamel surface still provides the best
achievable bond to the dental substrate.6 It not only
effectively seals the restoration margins in the long
term, but also protects the more vulnerable bond to
dentine against degradation.13
Despite the reliability of adhesion to enamel, bonding
to dentine has been considered more difficult and less
predictable. The main hindrance is the heterogeneous
nature of dentine, with hydroxyapatite deposited on a
mesh of collagen fibres.10 In addition, dentine is
intimately connected with pulpal tissue by means of
numerous fluid-filled tubules, which traverse through
dentine from the pulp to the dentino-enamel junction.
Once under constant outward pressure, this fluid
renders the exposed dentine surface naturally moist
and thus intrinsically hydrophilic.15,16 This hydrophilicity definitely represents one of the major challenges
for the interaction of modern adhesives with dentine. It
has in essence led to the different bond strategies
currently available.
The presence of cutting debris on instrumented
dental surfaces in the form of smear layer and smear
plugs that obstruct the dentine tubules is also a primary
co-factor that may not be underestimated.16,17 The first
bonding protocol that revealed a clinically acceptable
32
outcome involved the complete removal of the smear
layer by a ‘total-etch’ and now better termed ‘etch-andrinse’ approach.6 In order to deal with the organic
collagen mesh exposed following acid-etching dentine,
these ‘adhesive systems’ provide a separate adhesion
promotor or ‘primer’ that contains hydrophilic functional monomers dissolved in an organic solvent.10 In a
third step, a hydrophobic resin is applied and penetrates
the collagen network exposed by the acid-etching
procedure. These multi-step dental adhesives have been
marketed since the early 1990s and can still today be
considered as the ‘gold-standard’ adhesives. On the
other hand, the market-driven demand for simplified
adhesive procedures has rapidly led to the introduction
of the alternative ‘self-etch’ approach. This bond
strategy only dissolves the smear layer (but does not
remove it, as there is no rinse phase) and embeds the
dissolved products within the interfacial transition
zone.18
Despite the major difference in the manner of etching
between etch-and-rinse and self-etch adhesives, the
other fundamental steps for adhesion, namely the
‘priming’ and actual ‘bonding’ phase, can be either
separate or combined.19 Etch-and-rinse adhesives
require either two or three steps depending on whether
the primer and bonding agent are separated or combined in a single bottle. Similarly, self-etch adhesives
can be either one- or two-step systems depending on
whether the self-etching ⁄ primer solution is separated
from the bonding agent or combined with it. The latter
enables a single application procedure of a so-called
‘all-in-one’ adhesive. Figure 1 shows the most recent
and accepted classification of adhesives currently available in the market according to Van Meerbeek et al.19
Etch-and-rinse approach
Etch-and-rinse adhesives are characterized by an initial
etching step, followed by a compulsory rinsing procedure which is responsible for the complete removal of
smear layer and smear plugs. Concurrently, acidetching promotes dentine demineralization over a depth
of 3–5 lm, thereby exposing a scaffold of collagen
fibrils that is nearly totally depleted of hydroxyapatite.20,21 The following step consists of the application
of a primer containing specific monomers with
hydrophilic properties, such as 2-Hydroxy ethyl methacrylate (HEMA), dissolved in organic solvents like
acetone, ethanol or water. While HEMA is responsible
for improving the wettability and promoting the
re-expansion of the collagen network, the solvents are
able to displace water from the dentine surface, thus
preparing the collagen network for the subsequent
adhesive resin infiltration.22,23 In the bonding step, a
solvent-free adhesive resin is applied on the prepared
surface, leading to the penetration of hydrophobic
ª 2011 Australian Dental Association
Bonding effectiveness and stability in adhesive dentistry
Fig 1. Classification of contemporary adhesives according to Van
Meerbeek et al.19 Adhesives within the same classification may vary
considerably in terms of composition and proportional amount of
ingredients. Two-step etch and rinse adhesives are also referred to as
‘one-bottle’ systems, while one-step self-etch adhesives are often
referred to as ‘all-in-one’ systems. Note that each component, either
primer or bonding or even self-etching adhesive can be presented in
two separate bottles that need to be mixed prior to application.
Therefore, one-step self-etch adhesives may be subdivided in one- and
two-component systems.
monomers not only into the interfibrilar spaces of the
collagen network but also into dentine tubules. After
infiltration, these monomers are polymerized in situ,
resulting in the formation of a hybrid layer, which in
combination with the presence of resin tags inside
dentine tubules provides micromechanical retention to
the composite restoration (Fig 2).24
From the traditional three-step etch-and-rinse adhesives, simplified two-step adhesives have been developed
that combine the primer and the adhesive resin into one
single solution. Despite presenting a more friendly
technique, these simplified adhesives tend to perform in
an inferior manner when compared to their three-step
counterparts. By blending primer and bonding components in a single solution, simplified etch-and-rinse
adhesives present a reduced ability to infiltrate the
demineralized dentine substrate, thereby producing
suboptimal hybridization.25 Moreover, the hydrophilic
nature of such adhesives render them more prone to
water sorption and consequently more susceptible to the
effects of hydrolytic degradation. The solvent present in
such adhesives is also more difficult to evaporate,
frequently remaining entrapped within the adhesive
layer after polymerization.26
Classification aside, the infiltration of both two- and
three-step etch-and-rinse adhesives into etched dentine
has been shown to be only partially effective. Therefore, complete replacement of the superficial mineral
content, lost due to acid etching, remains practically
unattainable. As a result, voids are formed that may be
ª 2011 Australian Dental Association
located beneath and ⁄ or within the hybrid layer, leaving
exposed collagen unprotected and thus more susceptible to degradation over time.27,28 Such nanometre-sized
voids can be depicted by infiltration of silver traces
in vitro, which has been referred to as nanoleakage
(Fig 3).29 Although occurring even in the absence of
interfacial gaps, nanoleakage seems to play a negative
role in bonding, especially in terms of durability.30,31 It
is noteworthy, however, that even the so-called goldstandard adhesives seem to be incapable of preventing
nanoleakage,32 despite their satisfactory long-term
clinical performance.33
The technique sensitivity of etch-and-rinse adhesives
is mostly related to the etching step itself and to the
ostensibly antagonistic role of water in the bonding
protocol. It has been widely said that the demineralized
collagen network must be kept loosely arranged during
adhesive procedures in order to allow proper resin
monomer infiltration.34 Actually, a certain amount of
water is crucial to prevent the collagen network from
collapsing,35 while an excessively wet surface may
contribute negatively towards effective bonding to
dentine.36 In ‘over-wet’ conditions, excess water that
is incompletely removed from the dentine surface seems
to cause phase separation between the hydrophobic and
hydrophilic components of the adhesive, resulting in the
formation of blister- and globule-like voids at the resindentine interface.37 Furthermore, excessive moisture on
the adherent substrate is also responsible for a lower
degree of resin monomer conversion, reducing the
mechanical properties of the adhesive layer.38
Adhesive systems containing hydrophilic primers
dissolved in acetone were found to produce higher
bond strengths when acid-conditioned dentine was left
visibly moist prior to bonding, a protocol commonly
referred to as the ‘wet bonding technique’.39 However,
determining how moist the dentine should be left
during the adhesive procedure remains a major concern
and a clinical protocol that is difficult to standardize.
Achieving a ‘window of opportunity’ between over-dry
and over-wet conditions is still a challenge, since not
only extrinsic, but also intrinsic sources of humidity
must be considered when an adhesive procedure is
clinically performed. Self-evidently, over-drying by
which the acid-etched dentine surface is severely
dehydrated should be avoided at any time during the
clinical application procedure. Therefore, the surface
should be gently dried until the etched enamel presents
its white-frosted appearance and dentine loses its shine
and turns dull.34
Interestingly, water-based primers have shown a
potential self-rewetting effect on air-dried dentine.
Their water content seems to be sufficient to rehydrate
or re-wet the air-dried and then collapsed collagen
network, transforming it into a loosely arranged
structure that allows the hydrophilic primer monomers
33
MV Cardoso et al.
Fig 2. Electron photo micrographs illustrating the interface of different adhesives bonded to dentine. (a) SEM image of the two-step etch-and-rinse
adhesive Single Bond (3M ESPE) bonded to dentine. Dentine was demineralized and deproteinized to expose the resin tags formed inside dentine
tubules. Thick hybrid layer (Hy) and long resin tags can be observed as a result of the etching step with a 30–40% phosphoric acid. On the other
hand, short resin tags and a barely visible hybrid layer can be observed in image (b) as a consequence of the interaction of dentine with a ‘mild’ twostep self-etch adhesive (Clearfil SE Bond, Kuraray). (c) TEM image of the interface between the three-step etch-and-rinse adhesive Adper
Scotchbond Multi Purpose (3M ESPE) and dentine. A deeply demineralized hybrid layer of approximately 4 lm can be clearly visualized.
Conversely, the adhesive-dentine interaction showed in image (d) is characterized by a relatively thinner hybrid layer (about 1.0 lm), in which only a
partial demineralization has occurred as a consequence of the mild self-etch approach of Clearfil SE Bond. Note that the collagen fibrils remained
protected by hydroxyapatite, especially at the bottom half of the hybrid layer. A: adhesive; D: dentine.
Fig 3. TEM photomicrograph of the interface between dentine and
the three-step etch-and-rinse adhesive CMF Adhesive System (Saremco, St Galler, Switzerland) after infiltration with silver nitrate. Silver
deposition showed a typical spot-like distribution within the hybrid
layer (Hy) and a localized cluster-like deposition at the bottom of the
hybrid layer (hand-pointer). Even multi-step adhesives have been
reported to present similar types of nano-leakage, by which the
relevance of such phenomena is still unclear.
to interdiffuse.34 Therefore, in contrast with adhesives
containing acetone, water-based adhesives appear to be
less sensitive to variations in the bonding protocol as far
34
as the wetness of the acid-etched dentine is concerned.34
However, it must be emphasized that any remaining
solvent should be evaporated from the dentine surface
by air-drying the applied primer ⁄ adhesive.40 Nonevaporated solvent may jeopardize polymerization of
resin monomers, creating undesirable voids in the
adhesive interface.38 Nevertheless, complete evaporation of the solvent is difficult to achieve, especially in
water-based adhesives. To facilitate the evaporation of
the solvent in water-based adhesives, ethanol and
acetone can be used in conjunction with water as cosolvents, resulting in an azeotropic mixture.9,42 This
implies the formation of hydrogen bonds between
water and ethanol ⁄ acetone molecules, resulting in a
more easily evaporated solvent.9
More recently, a new solvent named tert-butanol has
been introduced for the production of an innovative
two-step etch-and-rinse adhesive, XP Bond (Dentsply,
Konstanz, Germany). The results obtained so far seem
to be promising as this solvent presents an ethanol-like
vapour pressure, with better stability towards chemical
reaction with monomers.42 However, additional research and clinical trials are necessary to confirm the
advantages of tert-butanol-based adhesives.
ª 2011 Australian Dental Association
Bonding effectiveness and stability in adhesive dentistry
Self-etch approach
Differently from their etch-and-rinse counterparts, selfetch adhesives do not require a separate etching step, as
they contain acidic monomers that simultaneously etch
and prime the dental substrate. Due to such acidic
characteristics, self-etch adhesives are able to dissolve
the smear layer and demineralize the underlying
dentine ⁄ enamel.43 Consequently, this approach has
been claimed to be more user-friendly and less
technique-sensitive, thereby resulting in a clinically
reliable performance.33 Nowadays, however, many
concerns have been raised about the bonding effectiveness of self-etch systems, especially in terms of durability, although it may be material dependent.3 Indeed,
some of them have proved to perform satisfactorily,
both clinically and in the laboratory.33
The morphological features of the hybrid layer
produced by self-etch adhesives depend to a great
extent on the ability of their functional monomers to
demineralize the dental substrate. Therefore, according
to their acidity or etching aggressiveness, self-etch
adhesives can be classified as strong (pH £ 1),
intermediate (pH » 1.5) and mild (pH ‡ 2).44 Strong
self-etch adhesives, such as Adper Prompt-L Pop
(3M ESPE), produce rather deep demineralization
effects in both enamel and dentine. The interfacial
ultramorphological features promoted by these adhesives on dental substrates resemble those of etch-andrinse systems, despite the fact that the products
originated from demineralization are not rinsed away.
On the other hand, mild self-etch adhesives demineralize dentine only partially, leaving a substantial
amount of hydroxyapatite crystals around the collagen
fibrils. This remains available for possible additional
chemical interaction (Fig 2d).44 This twofold bonding
mechanism (i.e. micromechanical and chemical adhesion) is believed to be advantageous in terms of
bonding effectiveness and durability.45 The hybrid
layer formed by such adhesives is no deeper than 1 lm
and resin tags are hardly observed (Fig 2b).44 Nevertheless, neither the thickness of hybrid layer, nor the
length of resin tags seem to be important for the
achievement of bonding effectiveness and stability.46,47
A more recent adhesive presents a relatively low
acidity (pH2.7) and, consequently, a considerably
reduced ability to dissolve the smear layer and
demineralize the underlying dentine surface. Being
referred to as an ultra-mild self-etch adhesive, Clearfil
S3 Bond (Kuraray) can only very superficially expose
collagen on dentine, creating a characteristic nanometre-sized hybrid layer, which has been termed a
nano-interdiffusion zone.48 However, the key factor
in this ultra-mild adhesive is related to the presence of
10-MDP in its composition and its consequent ability
ª 2011 Australian Dental Association
to chemically bond to the mineral content of the
partially demineralized dentine.45
The desirable chemical bonding presented by some
self-etch adhesives is related to the presence of specific
functional monomers in their composition, such as 10MDP, 4-META and phenyl-P. These monomers contain
carboxylic and phosphate groups that are able to
ionically bond with calcium in hydroxyapatite.49 However, it has been shown that the chemical bonding
promoted by 10-MDP is not only more effective but
also more stable in an aqueous environment than that
provided by 4-MET and phenyl-P.45,50 Clearfil SE Bond
(Kuraray) is a 10-MDP-based two-step self-etch adhesive and has been proven to yield reliable results in
terms of bonding effectiveness and durability when
compared to other commercially available self-etch
adhesives.33 Other adhesives from the same manufacturer, such as Clearfil S3 Bond, Clearfil Protect Bond,
and Clearfil Liner Bond, also contain the 10-MDP
monomer in their composition.
Generally, the bonding effectiveness of self-etch
adhesives has been attributed to their ability to
demineralize and infiltrate the dentine surface simultaneously to the same depth, theoretically preventing
incomplete penetration of the adhesive into the exposed
collagen network.51 However, traces of nano-leakage
have been reported along the resin ⁄ dentine interfaces
produced by some of these self-etch adhesives, thereby
challenging the original concept of a highly effective
sealing ability.52 It has been suggested that acidic
monomers of some self-etch adhesives (in particular the
simplified one-step versions) are gradually buffered by
the mineral content of the substrate. At this stage, such
weakened monomers are only able to partially etch
dentine.52 As a consequence, zones of partially demineralized but non-infiltrated dentine may be formed
beneath the hybrid layer, defeating the conventional
wisdom that such adhesives do not exhibit discrepancies between the depth of demineralization and the
depth of resin infiltration. This phenomenon, however,
is not common to all self-etch systems and seems to be
material dependent, although affecting both one- and
two-step self-etch adhesives.52
Another factor that may interfere with the bonding
effectiveness of a self-etch adhesive is the type of
smear layer produced on the dental substrate.17,53
Depending on the technique and instrument used for
cavity preparation, the smear layer can vary in
thickness, density and degree of attachment to the
underlying tooth structure (Fig 4).54 As self-etch
adhesives interact with the smear layer rather than
eliminating it, their acidic potential may be buffered
by the mineral content of a thick and dense smear
layer.55 This may hence result in a poor interaction
with the underlying substrate.53 Obviously, the less
35
MV Cardoso et al.
Fig 4. Feg-SEM photomicrographs of fractured dentine (a) and dentine surfaces prepared with different cavity preparation techniques (b–f): (a)
fractured dentine resulted in a rather flat and smear-free surface, exposing the typical structure of a sound dentine tissue; (b) bur-cut dentine shows a
surface entirely covered by a smear layer. The irregularities of the diamond bur produced a scratched dentine surface, with well-defined tops and
valleys. Note that the smear layer is thicker at the tops (left side) as compared to the valleys (right side), where the entrances of the dentine tubules are
barely apparent; (c) diamond tip used in ultrasound produced a relatively regular surface covered by a rather thin smear layer. Not only plugged, but
also opened dentine tubules may be observed; (d) irradiated dentine using an Er:YAG laser (31.5 J ⁄ cm2, 200 mJ, 10 Hz, 100 ls) resulted in a smearfree surface with open dentine tubules and prominent peritubular dentine. Note the irregular and rugged aspect of the surface and its typical
undulating pattern; (e) dentine substrate after chemo-mechanical caries exaction with Carisolv shows open dentine tubules and visible intertubular
dentine with a certain amount of exposed collagen fibrils (caries-affected dentine); (f) sono-abrasion with a diamond tip resulted in a flat surface
covered by a relatively thick smear layer and plugged dentine tubules.
acidic the adhesive, the more the smear layer may
interfere with bonding.17 Therefore, techniques and
instruments that produce thinner and less compact
smear layers should be preferred during cavity preparation. Finishing the cavity walls with an extra-fine
diamond bur results in thinner smear layers, which
consequently may promote a better interaction
between mild and ultra-mild self-etch adhesives and
the dental substrate.17
Regarding adhesion to enamel, it has been reported
that some self-etch adhesives bond reasonably well to
ground enamel, but there are general concerns nowadays about the adhesion of such adhesives to unground
aprismatic enamel where micromechanical retention is
hardly achieved.56,57 Thus, preparing bevelled cavosurface margins is helpful for improving the bonding
effectiveness of self-etch adhesives in cavities whose
margins are placed in enamel.57
Concerns about one-step self-etch adhesives
Attempting to provide faster and thus more userfriendly adhesives, manufacturers have introduced
one-step self-etch adhesives which etch, prime and
bond the dental substrate simultaneously. For this
36
purpose, these adhesives have been made more acidic
and more hydrophilic than their two-step counterparts.
These properties may, however, lead to a wide variety
of seemingly unrelated problems that may jeopardize
the effectiveness and stability of adhesion to a dental
substrate.58 Actually, none of the contemporary all-inone adhesives can compete with the more traditional
multi-step systems in terms of bond effectiveness and
durability.3,44,47,59,60
One of the main disadvantages of one-step self-etch
adhesives is related to their excessive hydrophilicity that
makes the adhesive layer more prone to attract water
from the intrinsically moist substrate.58 Due to such
increased water affinity, these adhesives have been
reported to act as semi-permeable membranes, even
after polymerization, allowing water movement from
the substrate throughout the adhesive layer.61 As a
consequence, small droplets can be found at the
transition between the adhesive layer and the lining
composite, especially when polymerization of the latter
is delayed (Fig 5a). Besides promoting a decrease in
bond strength between composite and substrate,62 such
permeability of the adhesive layer seems to contribute
to the hydrolysis of resin polymers and the consequent
degradation of tooth-resin bond over time.13,63,64
ª 2011 Australian Dental Association
Bonding effectiveness and stability in adhesive dentistry
In this context, the presence and concentration of
HEMA in the adhesive’s composition must be addressed. HEMA is a water-soluble methacrylate monomer frequently present in the composition of dental
adhesives to increase their wettability and hydrophilicity.22,66 Moreover, when incorporated in relatively high
concentration, it improves the miscibility of hydrophobic and hydrophilic components in an adhesive solution.62 However, disadvantages have also been related
to the presence of HEMA, especially when it is
incorporated in high amounts (HEMA-rich adhesives).
Besides the potential allergenic effects of its uncured
monomer,67 HEMA may increase the water sorption of
adhesives, adversely influencing the mechanical properties and stability of the adhesive interface.13,68 These
drawbacks have led to the introduction of HEMA-free
mild one-step self-etch adhesives in an attempt to create
a less hydrophilic adhesive that could promote a more
durable bond to dental substrates.69 However, it must
be considered that the absence of HEMA makes
the adhesive more prone to the occurrence of the
so-called phenomenon of phase separation. In this
process, once ethanol ⁄ acetone starts to evaporate, the
solvent-monomer balance is broken, with water
separating from the other adhesive ingredients.69 When
the adhesive is cured, these water blisters become
entrapped in the adhesive layer (Fig 5b), probably
jeopardizing the bonding effectiveness.62 To prevent
phase-separation droplets from being embedded in the
polymerized adhesive, a strong, prolonged air-blowing
should be performed before polymerization so as to
remove them from the adhesive layer. How successful
this technique can be performed clinically remains a
concern.69 Meanwhile, a controlled amount of HEMA
should be preferred that strikes the right balance
between HEMA-free and HEMA-rich self-etch
adhesives.
One of our most recent clinical Class V studies
evaluates the clinical performance of a HEMA-free one-
step self-etch adhesive (G-Bond, GC) in comparison to
a HEMA-containing adhesive of the same category
(Clearfil S3 Bond, Kuraray). Up to two years of clinical
service, a satisfactory performance was observed for
both adhesives (Fig 6).70 Obviously, longer-term clinical evaluation is needed in respect to bond durability.
Adhesion to carious tissue
The most convenient substrate to test the bonding
performance of dental adhesives in laboratory studies is
sound (‘normal’) dentine. However, this standard
‘laboratory’ dentine differs from the kind of dentine
that clinically remains after carious tissue was removed.
Caries excavation frequently results in a complex
substrate involving areas of caries-infected, cariesaffected, sclerotic, eroded and sound dentine (Fig 7).
Following a more conservative cavity-preparation concept, the current aim of caries excavation is limited to
the removal of the outer highly infected and denaturated dentine,8,71 preserving the inner layer of cariesaffected dentine that still possesses a high potential to
remineralize.72 However, clinically distinguishing between caries-affected and caries-infected dentine is
definitely not straightforward (Fig 8). Depending on
the clinical perception of the operator and the technique
used for caries removal, caries-infected dentine may
remain in the cavity and also needs to be bonded as
effectively as possible. Obviously, such ‘abnormal’
substrates are characterized by structural variations
that may represent a challenge to achieve optimal
adhesion.73
In general, the presence of carious dentine results in a
thicker hybrid layer and lower bond strength.72,74–76
The bonding effectiveness of adhesives to carious
dentine has been reported to be inversely proportional
to the degree of caries progression, with caries-infected
dentine yielding the lowest bond-strength results.72,76
This sort of substrate is characterized by an extremely
Fig 5. Electron micrographs illustrating the occurrence of droplets at the resin-dentine interface. (a) Failure analysis of a specimen of composite
bonded to dentine using the one-step self-etch adhesive Clearfil S3 Bond (Kuraray) after fracture under micro-tensile bond strength testing. The
insert shows the Feg-SEM image of the fracture plan of the composite side of the specimen. These voids have been formed as a consequence of water
flow from the substrate through the semi-permeable adhesive layer, due to osmosis. (b) TEM photomicrograph of the interface between dentine and
the HEMA-free one-step self-etch adhesive G-Bond (GC). Note the presence of droplets entrapped at the bottom of the adhesive layer due to the
phase-separation phenomenon and thus an insufficiently air-blown adhesive.
ª 2011 Australian Dental Association
37
MV Cardoso et al.
Fig 6. Graphs showing the retention rate (%) and overall success rate of 2 one-step self-etch adhesives in Class V restorations up to 2 years of clinical
service. G-Bond is a HEMA-free adhesive while Clearfil S3 Bond does contain HEMA in its composition. Observe the satisfactory performance of
both adhesives after a 2-year follow-up. Overall clinical success includes the evaluation of retention rate, caries recurrence, marginal integrity and
discolouration. Note that the Y-axes start at 80%.
Fig 7. Different residual dentine substrates left for further adhesive procedures after caries removal with a chemo-mechanical method (Carisolv),
ranging from lightly (a), to mixed (b) and darkly stained residual dentine (c).
low cohesive strength, which should be attributed to its
high degree of demineralization and disorganized
collagen matrix. Considering that resin-monomer infiltration is incomplete at caries-infected dentine, a
compromised bonding effectiveness can be expected.72,76 Therefore, caries-infected dentine should
best be removed as much as possible. Occasionally, an
exception can be made to prevent further progression of
the disease in the case of uncooperative patients, while
working on behaviour control and ⁄ or planning other
treatment strategies.
The issue of bonding to caries-affected dentine has
been considered more controversial. Normally, in light
of the concept of minimal-invasive dentistry, one could
bond to caries-infected dentine provided that the cavity
margins are placed in sound dentine or enamel.8,77
Nevertheless, one should not neglect that this modified
substrate may result in a lower bonding effectiveness
and sealing when compared to sound dentine.74,75
These less favourable outcomes are basically related to
important alterations that normally occur in dentine as
a consequence of caries progression. Besides presenting
a significant reduction in its mineral content, cariesaffected dentine undergoes an important loss in the
crystallinity of its remaining mineral phase, as well as
considerable changes in the secondary structure of its
38
organic components.78 This results in a substrate of
inferior mechanical properties74,79 and increased susceptibility to the effects of acid etching.71,72,80,81
Although enabling a thicker hybrid layer to be formed,
this highly permeable substrate also results in a
disproportional penetration of resin monomers. They
are not able to completely fill the voids of such a thick
Fig 8. A typical cavitated occlusal caries lesion seen on a mesio-distal
stereomicroscopic section is delimited by the full black lines.
Transparent blue area corresponds to caries-unaffected dentine
reactions. The different shades of dashed lines indicate possible
morphological endpoints for caries removal, illustrating some of the
variability to which this parameter is exposed.
ª 2011 Australian Dental Association
Bonding effectiveness and stability in adhesive dentistry
mineral-depleted zone,82 rendering the adhesive interface more prone to water degradation over time.80,83
In contrast to the high intertubular dentine permeability mentioned above, the penetration of resin
monomers into the dentine tubules of caries-affected
dentine is usually impaired due to the presence of
acid-resistant mineral casts, like those formed by
b-tricalcium phosphates.74,81,84 Such structures, also
called ‘whitlockite’, are deposited into the dentine
tubules during the early phases of dentine reaction
against further caries progression.80,85 Besides hindering intratubular penetration and, consequently,
resin-tag formation, such enhanced impermeability
impairs also the sideways diffusion of resin monomers
via the dentine tubules to the deepest demineralized
dentine.85 This poorly infiltrated substrate, along with
the low mechanical properties of the caries-affected
dentine, has been pointed out as the most logical
explanation for the lower bond strengths obtained to
such a substrate as compared with that to sound
dentine.72
As described so far, the type of substrate on which
the adhesive is applied strongly influences its bonding
effectiveness. From another standpoint, particular
characteristics of the adhesive selected should also be
considered, such as its specific approach and composition.83 In general terms, there is no ‘ideal’ adhesive to
bond to caries-affected dentine. Alternatively, the
alleged fluoride-releasing properties of some adhesives
such as the Optibond adhesives (Kerr), One-up Bond F
Plus (Tokuyama, Tokyo, Japan) and Clearfil Protect
Bond (Kuraray) could be beneficial for inhibiting
development of secondary caries and for promoting
remineralization of caries-affected dentine.86,87 It is,
however, unclear whether the amount of fluoride
released by such adhesives is sufficient to promote
remineralization and prevent demineralization.
This still remains a controversial issue. Some adhesives
that contain antibacterial monomers have also
been reported to inhibit caries progression underneath
adhesive restorations thanks to their strong bactericidal
activity.88 For example, Clearfil Protect Bond (Kuraray)
incorporates the monomer 12-methacryloyloxydodecylpyridinium bromide (MDPB), and was shown to
possess a significant antibacterial effect in the laboratory, even after polymerization.89 Unfortunately, there
is still a need for further investigation, especially to
provide evidence that such antibacterial property
clinically also results in longer-lasting composite restorations.9 Irrespective of fluoride release and antibacterial properties, the adhesive that performs most favourably on sound dentine will probably also be the best
option to bond to caries-affected dentine. As reported
for normal dentine, etch-and-rinse adhesives tend to
result in higher bond strengths to both sound and
caries-affected dentine than self-etch systems.74,83
ª 2011 Australian Dental Association
Relation between caries-removal techniques and
adhesion
As an alternative to the use of diamond and carbide
rotatary instruments, several techniques for caries
removal and cavity preparation, thereby employing
different concepts and technologies, have been introduced to the market. The one common aspect of all
these techniques is the promotion of a more conservative cavity-preparation approach, aiming to minimally
remove sound tissue. Subsequently, a filling material is
required that can bond to the dental substrate without
the need of additional mechanical retention. Therefore,
the influence of the resulting substrate on the bonding
effectiveness of dental adhesives is an issue that deserves
further attention.
The use of erbium-laser devices for cavity preparation, for example, has gained popularity as a technique
that provides the patient with more comfort during
treatment. The substrate that results from laser ablation
shows a particular undulating surface pattern with
complete absence of surface smear (Figs 4d and 9a).
This could improve the bonding effectiveness of adhesives, especially that of mild and ultra-mild self-etch
adhesives.17 However, despite some minor disagreements in the literature, there is a certain consensus
among researchers that laser-irradiated dentine negatively affects the bonding effectiveness of resin-based
adhesives.7,90–92 These disappointing results are related
not only to the presence of subsurface micro-cracks that
are induced during irradiation (Fig 9b),7,91 but also to
the occurrence of alterations in the chemical composition of dentine and especially its organic matrix.93
Some other cavity-preparation techniques, such as
ultrasono-abrasion, and air-abrasion have also been
reported as alternative techniques for cavity preparation, although their effectiveness in removing carious
tissue remains debateable.8 Both techniques produce a
discrete smear layer but no noticeable damage to the
substrate (Fig 4c), which may result in a more effective
interaction of even mild self-etch adhesives with
dentine, and thus a better bond.6 However, the use of
sono-abrasion seems to less effectively remove surface
smear than ultra-sono abrasion (Figs 4c and d). Obviously, surface smear hardly interferes with the bonding
potential of etch-and-rinse adhesives, since the phosphoric acid readily dissolves and removes all smear
upon rinsing.6
The use of chemo-mechanical excavation has also
received new interest as a less-invasive method for
caries removal. This procedure consists of the use of a
gel based on a mixture of sodium hypochlorite, amino
acids and water (Carisolv, MediTeam Dental, Sävedalem, Sweden). Its working principle is based on
cleavage of exposed and denatured collagen of carious
dentine, thereby facilitating its removal manually using
39
MV Cardoso et al.
an excavator with rounded edges. Using Carisolv, caries
is excavated to the same extent as corresponding to the
auto-fluorescent signature of carious tissue, when
evaluated using confocal microscopy.94 The chemical
composition and microstructure of sound dentine does
not seem to be significantly altered by the use of
Carisolv.95,96 Its chloride is not able to interact with
collagen fibrils that are protected by the mineral content
of sound dentine.97 Therefore, it has been reported that
the use of this technique does not affect the bonding
effectiveness of dental adhesives, provided that the
carious tissue is completely removed (Fig 4e).97,98
With the purpose of providing clinicians a rotating
instrument that could selectively remove carious dentine, polymeric burs have recently been introduced to
the market (SmartPrep, SS-White Burs; Lakewood, NJ,
USA). These burs are made of a polymer (PEKK,
polyether-ketone-ketone) with a particular hardness of
50 KHN, which is higher than the hardness of carious
dentine (0 to 30 KHN), but lower than that of sound
dentine (70 to 90 KHN).99 Therefore, it is not
surprising that both the cutting efficiency and efficacy
of these burs decreases early as soon as sound dentine is
touched. This obviously compromises the effectiveness
of these burs.100 Adhesion to dentine remaining after
the use of these polymeric burs has been reported to be
negatively affected by the fact that residual caries
commonly remains after excavation.99,100 A new version of these burs have recently been marketed (SmartBurs, SS-White Burs; Lakewood, NJ, USA), although
further studies are still necessary. Actually, any kind of
cavity preparation that is not successful in completely
eliminating carious tissue may represent a challenge for
currently available adhesives to bond effectively.
Clinical performance
Despite the importance of laboratory studies attempting to predict the performance of biomaterials, clinical
trials remain the ultimate way to collect scientific
evidence on the clinical effectiveness of a restorative
treatment.101 The popularity of laboratory studies in
the field of adhesive dentistry may in part be ascribed to
the rapid evolution of dental adhesive technology and
the resultant high turnover of adhesive systems. This
often tempts manufacturers to release a successor
product on the market even before its precursor has
been clinically evaluated, at least in the long term. By
carrying out in vivo studies, all possible ageing factors
are concurrently evaluated. Retention, marginal integrity and clinical microleakage are usually the key
parameters recorded to judge upon clinical performance of adhesives.
The best clinical performance with regard to
retention (the most objective criterion to judge
upon clinical effectiveness) has so far been achieved by
glass-ionomers.33,100 Their low annual failure rates can
be ascribed to their unique self-adhesiveness, based on
the twofold micromechanical and chemical bonding
mechanism.19,45 Nevertheless, despite their excellent
clinical performance in terms of retention, glass-ionomers are commonly associated to poorer aesthetic and
mechanical properties when compared to resin-based
restorative materials.103,104 Besides glass-ionomers,
three-step etch-and-rinse adhesives have exhibited a
reasonably good clinical effectiveness.33,105 Some of
these adhesives, such as Optibond FL (Kerr) and its
predecessor Optibond Dual Cure, have been related to
an excellent performance with 94% and 93% of
retention rates being registered after 13 and 12 years
of clinical follow-up, respectively.101,105 The durability
of three-step etch-and-rinse adhesives confirms their
generally superior laboratory results, being currently
considered as ‘gold-standard’ among other adhesive
approaches.101
According to the same standard, mild two-step selfetch adhesives tend to approach three-step etch-andrinse adhesives in terms of low annual failure rates.33 In
Fig 9. Electron photo micrographs illustrating the morphological aspects of dentine after laser irradiation (4.0 W, 20 Hz, 71.4 J ⁄ cm2 and 140 ls)
with an Er,Cr: YSSG laser (Waterlase, Biolase Technology, San Clemente, CA, USA). (a) Feg-SEM photomicrograph of a cross-section of dentine
surface after laser treatment. Note the irregularities produced by laser ablation, resulting in an imbricate patterned surface. (b) Non-demineralized
and non-stained TEM photomicrograph of the interface between Clearfil SE Bond and laser-irradiated dentine (D). Despite the clear interaction
between adhesive (A) and substrate, microcracks (arrows) can be observed at the dentine subsurface as a result of laser ablation. Hy: hybrid layer.
40
ª 2011 Australian Dental Association
Bonding effectiveness and stability in adhesive dentistry
this context, Clearfil SE Bond has been the most tested
adhesive and is frequently employed as a control group
due to its satisfactory performance clinically.101,106 Its
ability to provide a shallow but uniform hybrid layer,
along with its capability to chemically bond to the
dentine substrate seems to play an important role to
resist long-term hydrolytic degradation. Commonly,
the clinical performance of such self-etch adhesives does
not vary substantially from one study to another, which
is indicative of their rather low technique-sensitivity.
Altogether, it is not surprising that such adhesives are
usually related to very low levels of postoperative
sensitivity.107
In general, two-step etch-and-rinse adhesives have
performed less favourably than the conventional
three-step version, sometimes not even meeting the
previous American Dental Associaton full acceptance
guidelines.33,108 Laboratory studies have corroborated
these results, ascribing their poorer performance to
their higher hydrophilicity and reduced hybridization
potential. It is noteworthy that irrespective of the
number of application steps, acetone-based etch-andrinse adhesives have performed less satisfactorily
than their water ⁄ ethanol-based alternatives.33,108 The
above-mentioned high technique-sensitivity of acetonebased adhesives must be the reason for their compromised long-term clinical results.
At this point in time, the clinical performance of onestep adhesives has been shown to be less predictable.
Widely varying retention scores have been recorded,
indicating their higher technique sensitivity despite
them being more user-friendly.33,109,110 Such lower
bonding performance must be ascribed to the many
concerns advanced earlier. Interestingly, Clearfil S3
Bond (Kuraray) and G-Bond (GC) have recently been
shown to clinically perform satisfactorily in short-term
clinical trials.101,111 Retention rates of 98% have been
reported for both G-Bond and Clearfil S3 Bond in a
two-year clinical evaluation.112 From a general point of
view, it seems that the latest generation of one-step selfetch adhesives have performed better in the last years as
they possess superior characteristics when compared to
their earlier versions.101 Long-term clinical evaluation
remains necessary to confirm the good performance of
these adhesives.
CONCLUSIONS
Adhesive dentistry has undergone great progress in the
last decades. Despite the rather disappointing performance of all-in-one self-etch adhesives, conventional
three-step etch-and-rinse adhesives and two-step selfetch adhesives have shown satisfactory results and are
still the benchmark for dental adhesion in routine
clinical practice.
ª 2011 Australian Dental Association
When bonding to enamel, an etch-and-rinse approach is definitely preferred, indicating that simple
micromechanical interaction appears sufficient to
achieve a durable bond to enamel. On the other hand,
the mild self-etch approach seems to provide superior
performance when bonding to dentine, as it additionally chemically interacts with residual hydroxyapatite,
which definitely contributes to bond durability.
Altogether, when bonding to both enamel and
dentine, selective etching of enamel followed by the
application of the self-etch adhesive to both (etched)
enamel and (non-etched) dentine may represent the
best option to effectively and durably bond to the
dental substrate. Moreover, the quality of the substrate
to which the adhesive is applied is of paramount
importance to achieve effective and durable bonding.
In this context, many variables should be taken into
account such as the technique used for cavity preparation and the approach employed during caries
removal. Despite the current trend towards more
conservative intervention, one must be aware that
adhesion to carious tissue reduces the bonding effectiveness and bond durability.
Along with the fast evolution of multi-step adhesives
towards one-step adhesives, luting composites to bond
indirect ceramic restorations underwent a similar
evolution towards simple-to-use and less techniquesensitive one-step luting agents. Hence, these so-called
self-adhesive luting composites do not need any kind of
pre-treatment of the tooth substrate, thereby bringing
the development of self-adhesive composites closer to
reality.
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Address for correspondence:
Dr B Van Meerbeek
Catholic University of Leuven
Department of Conservative Dentistry
Leuven BIOMAT Research Cluster
Kapucijnenvoer 7
3000, Leuven
Belgium
Email: [email protected]
ª 2011 Australian Dental Association