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Enamel Acid Etching:
A Review
Abstract
Guilherme Carpena Lopes, DDS,
MS, PhD
Assistant Professor
Department of Operative Dentistry
Universidade Federal de Santa Catarina
School of Dentistry
Florianópolis, Brazil
Daniela Greenhalgh Thys, DDS, MS
Bonding to enamel has over 50 years of history. Efforts have been made to develop or
introduce a simplified alternative, but enamel acid etching remains the most effective
procedure for stable enamel bonding. Although acid etching is considered the most
popular procedure in dentistry, there are characteristics that deserve special attention
because of how crucial they can be in many clinical situations. This article reviews
some of these aspects of enamel bonding using the acid-etching technique.
Research Assistant
Department of Orthodontics
Universidade Federal de Santa Catarina
School of Dentistry
Florianópolis, Brazil
Learning Objectives
Pricila Klauss, MS
• explain the characteristics of hard tissue.
Research Assistant
Materials Engineer
Universidade Federal de Santa Catarina
Florianópolis, Brazil
Gustavo Mussi, DDS
Clinical Instructor
Department of Operative Dentistry
Universidade Federal de Santa Catarina
School of Dentistry
Florianópolis, Brazil
Nicolas Widmer, DDS
Clinical Instructor
Department of Operative Dentistry
Universidade Federal de Santa Catarina
School of Dentistry
Florianópolis, Brazil
After reading this article, the reader should be able to:
• discuss aspects of enamel bonding
with the acid-etching technique.
A
clinician’s ability to bond a restoration to enamel has influenced changes in prosthetic
and cavitary preparations, restorative
approaches for esthetic corrections,
bonding techniques for orthodontic
devices, and the treatment of caries.1-3
Bonding to dentin has been the
objective of several studies over the last
2 decades.1-3 In the beginning, the research was to find an adhesive system
that could interact with efficacy in
dentin, and more recently, the research
has focused on simplifying the systems.1-3 Because a major part of bonding procedures requires simultaneously
treating enamel and dentin, it is important to understand that the foundation
of enamel bonding strongly influenced
the development of dental adhesive
systems. This article will review some
relevant aspects of bonding to enamel
using the total-etch technique.
Tooth Enamel
Enamel is the hardest tissue in the
human body. Its mineral portion is
662
Compendium • January 2007;28(1):662-669
• describe the current knowledge to
overcome saliva contamination during bonding to enamel.
approximately 96% of its weight,4 the
rest is organic components and water.
The mineral elements include hydroxyapatite crystals, approximately 0.03
µm to 0.2 µm, surrounded by a thin
film of firmly bound water.4 In prismatic enamel, which constitutes the
main fraction, the crystals are densely
grouped and arranged in 3 directions.
With this arrangement, lengthy prisms
(a diameter of about 5 µm) from the
dentoenamel junction to near the outermost surface of enamel are formed
(Figure 1). The prisms maintain their
integrity and support because of their
transverse arrangement, irregular morphology, and overlapping patterns. At
the moment of enamel instrumentation, the prisms are exposed in several
planes according to their direction
(Figure 2).
In intact enamel, water comprises
up to 4% of the weight (11% in volume),4 either in the hydration layer of
enamel around each crystal of apatite
or in the pores between prisms.4 After
tooth eruption, there is an enamel
Figure 1—Scanning electron microscope (SEM) micrograph showing a composite-enamel interface. Note the orientation of the enamel prisms. Final magnification 150x.
Figure 2—SEM micrograph showing fractured enamel. Final magnification
2000x.
Figure 3—SEM micrograph showing the smooth enamel surface. Final magnification 1000x.
Figure 4—SEM micrograph showing the enamel etching pattern type I after a
32% phosphoric acidc etch applied for 15 seconds. Final magnification 4000x.
carried out to verify whether enamel changes over time
might have clinical relevance when acid etched.
Acid-Etching Technique
Figure 5—SEM micrograph showing the enamel etching pattern type II after a
32% phosphoric acidc etch applied for 15 seconds. Final magnification 4000x.
maturation process that makes it more resistant to demineralization. This maturation consists of mineral deposition from oral fluids in interprism spaces that were previously filled with water.5 Because all hard tissue is in
continuous ionic change with the environment, it could
be expected that human enamel will respond to acid conditioning differently, depending on age and factors related mainly to saliva and diet. Further research should be
During the 32nd Annual Meeting of the International Association for Dental Research in 1954,6
Buonocore suggested that using 85% phosphoric acid
solution resulted in an adhesion of acrylic resin to enamel that lasted 1070 hours to debond when stored in
water.7 Similar to other conceptual and technologic innovations, this procedure was introduced in dentistry ahead
of its time and only 10 years later the bonding mechanism was described,8 Bis-GMA based adhesive systems
and com-posite resins were developed,9 and the first clinical application, as a pit-and-fissure sealant, was reported
in the literature.10
Chemical treatment by acid etching enhances the
topography of enamel, changing it from a low-reactive
surface (Figure 3) to a surface that is more susceptible to
adhesion (Figure 4 and 5). The demineralization is selective because of the morphological disposition of the
prisms. The difference of angulation of the prism crystals
causes the acid to have higher demineralization potential
at certain microregions. After cavitary instrumentation,
Compendium • January 2007;28(1):662-669
663
Figure 6—SEM micrograph showing the resin-enamel interface after a laboratorial demineralization in hydrochloride 6N for 30 seconds. The low-viscosity fluid resin wets this high-energy surface by capillary attraction into the
microporosities created by the etching. After polymerization, the tags formed
by this extension of resin into the microporosities form a strong micromechanical interlocking with the enamel. Final magnification 1500x.
depending on the angulation of the prisms, demineralization can be greater at the prism head (Figure 4) or at the
periphery (Figure 5). These features are respectively
known as type I and type II acid-etching patterns. This
feature is important in understanding the fundamentals
of adhesion though it is not clinically relevant.
Acid etching removes appoximately 10 µm of enamel
surface and creates a morphologically porous layer (5 µm
to 50 µm deep).11 The surface free energy is doubled,12 and
as a result, the low-viscosity fluid resin contacts the surface
and is attracted to the interior of these microporosities created by conditioning through capillarity (capillary attraction).13 Therefore, resin tags are formed into microporosities of conditioned enamel (Figure 6) that after adequate
polymerization, provide a resistant, long-lasting bond by
micromechanical interlocking with this tissue.8,14,15
Bonding to Enamel with Hydrophilic Primers
In most clinical situations, the restoration is bonded to
enamel and dentin. To interact with this intrinsically moist
tissue, an adhesive system was needed that could diffuse
under this condition. The introduction of hydrophilic
primers has allowed for adequate bonding to dentin.16 The
modern adhesive systems that use the total-etch technique
combine the functions of primer and adhesive in 1 bottle,
intended for application on moist dentin surfaces.17
For this reason, most adhesive systems that use the
total-etch technique have in their formulation low-viscosity hydrophilic monomers diluted in organic solvents with
a high potential of volatilization (acetone, water, or a mixture of both)18,19 to displace the moisture of conditioned
dentin. The primer components (eg, hydroxyethyl methacrylate [HEMA], bis-phenyl dimethacrylate [BPDM], 4methacryloxyethyl trimellitate anhydride [4-META]) are
called bipolar because they have 2 functional groups
(hydrophilic and hydrophobic). The hydrophilic end has
the ability to interact even under moist conditions, and the
hydrophobic end has chemical affinity with the methacry-
664
late group of the bonding resin matrix (also called hydrophobic adhesive) or with the composite resin.20
Regarding bonding to enamel, the rapid volatilization of the solvent allows for the complete interdiffusion
of the adhesive system through the extension of the conditioning,21 with a more intimate contact of the composite resin to enamel, which results in high bond strengths
to this tissue18 and adequate marginal sealing,22 even in
moist conditions.19,22
It is not an easy procedure to keep dentin moist and
dry only the enamel. It is possible that acetone-and
ethanol-based solvents, included in mostly 1-bottle adhesives, remove residual moisture and promote a superior
flow of the restorative material to acid-etched enamel.19,21,23
Hydrophilic primers become essential when enamel is
moist and work very well when enamel is dry.18,24 However,
because the presence of residual moisture and organic solvents interferes negatively in the complete polymerization
of monomers, it is important after the application to properly dry with air spray of a triple syringe. Also, it has been
theorized that the stability of bonding to enamel is also
compromised with time because of the formation of adhesive layers similar to semipermeable membranes.25
Isolating the operatory field is important to achieve
a contaminant-free working area. There is a lack of clinical evidence that says that bonding procedures with the
use of rubber dam isolation results in better clinical performance than relative isolation, but it has been the
authors’ experience that rubber dam isolation should be
preferred to relative isolation because of the interference
of patients’ breath moisture. A recent study simulating
the oral environment compared the bond strength of 2
bonding agents (hydrophilic and hydrophobic) to enamel under temperature and air humidity conditions of the
oral cavity (35˚C, 90% to 95% of relative humidity).26 The
simulated contamination with high levels of air moisture
did not compromise the bonding to enamel, whether it
was accomplished with hydrophilic primer or a
hydrophobic adhesive.26
Viscosity of the Adhesive System
Some adhesives have inorganic filler particles to
increase their film thickness and cohesive strength (filler
loading ranges between 8.5% and 25% weight, depending
on the brand). It seems that an intermediary flexible zone
allows for a better transmission of the interfacial stresses
induced by masticatory forces, differences in modulus of
elasticity, water sorption presented by composite resins,
and polymerization shrinkage.27 Filled sealants and flowable composite resins can bond to conditioned enamel
without an intermediary adhesive agent.28 However, it has
been reported that highly viscous sealants have greater difDentsply, York, Pa; www.dentsply.com
ESPE, St. Paul, Minn; www.3m.com/espe
c Bisco Dental Products, Schaumburg, Ill; www.bisco.com
d Coltène/Whaledent Inc., Cuyahoga Falls, Ohio;www.coltenewhaledent.com
a
b 3M
Compendium • January 2007;28(1):662-669
Figure 7—SEM micrograph showing the enamel etching pattern after a 15%
phosphoric acidd etch applied for 15 seconds. Final magnification 3000x.
ficulty penetrating enamel as much as the conditioning
depth.29 Studies have demonstrated that filled adhesive systems present lower bond strengths to enamel.18,19,30 This
seems to be related to the high viscosity of these adhesives,18,30 which makes it difficult to penetrate in interprismatic areas as deeply as unfilled adhesives.31 Adhesive systems containing nanofillers have been developed (Prime &
Bond NTa, Adper Single Bond 2b). The nanofillers are
designed to reinforce resin tags and the adhesive layer,32 and
it seems that the low molecular weight does not compromise the interdiffusion in conditioned enamel.31
Concentration of the Conditioner
When the acid-etching technique was extended to
dentin (total-etch technique), the manufacturers included
low concentration acid conditioners in adhesive systems
possibly because of lack of information, knowledge, or
marketing strategy, which supposedly would cause less
aggression to pulpal tissue. In the early 1990s, it was common to find etchants such as 10% phosphoric acid, 10%
maleic acid, 10% citric acid, 2.5% oxalic acid, and 2.5%
nitric acid. Some of these acids do not result in a dull,
frosty-white appearance typical of conditioned enamel,33
but a few studies show that this doesn’t negatively affect
immediate adhesive bonding to instrumented enamel.33,34
However, other research shows a significant decrease of the
bond strengths.35 Because of the lack of long-term clinical
evidence on the durability of bonding to enamel using
these less aggressive conditioners, the previous use of 32%
to 40% phosphoric acid conditioner is still the best option
to achieve predictable bonding to enamel.36 Figures 4 and
7 compare the 32% phosphoric acidc etch pattern (Figure
4) with the 15% phosphoric acidd etch pattern (Figure 7)
applied with the same 15-second etching time.
Enamel Acid-Etching Time
When enamel etching was introduced in 1955, the
recommended time was 30 seconds for 85% phosphoric
acid.7 Then, at the time of its first clinical use (the 1960s),
it was extended to 60 seconds.8,10 In the 1980s, it was
reduced to a 30-second application and has remained
today.37-39 Some authors recommend reducing the etching
time to 15 seconds when a 32% to 40% phosphoric acid is
used.40 Most of the manufacturers of adhesive systems
have recommended 15 seconds because it is saves time
without compromising the adhesive performance.
Reducing times has been suggested because it presents 3 advantages. First, because acid conditioning causes superficial tissue loss, it is desirable that minimal tooth
structure be dissolved; therefore, minimal acid-application time should be used.41 The difference between 15-second and 30-second application time of phosphoric acid on
enamel dissolution is very small. The chemical reaction of
the conditioning occurs quickly and, as mineral components are lost, the acid potential decreases by buffering.
Second, when one is dealing with cavities involving enamel and dentin, the expansion of the etching technique to
dentin (also called total-etch technique) is controversial
because the time should not be longer than 15 seconds in
dentin, but this is the minimum time required to achieve
a suitable bonding to enamel.3 As a result, it has been suggested that the conditioning time be reduced to 15 seconds, which is considered adequate for creating a retentive enamel surface with no difference in the enamel etching pattern42 or decrease of the bond strengths to instrumented enamel.39-41 In vitro studies have demonstrated
that a 15-second conditioning time is also adequate for
orthodontic adhesive procedures.43,44 The third advantage
is saving chair time.
A 15-second etching time is sufficient when using
32% to 40% phosphoric acid to achieve a proper bond
strength when it is applied to instrumented enamel surfaces or when a cavitary preparation has been accomplished. However, in the presence of intact enamel, a different approach may be preferred, which will be discussed later in the article.
Type of Enamel
An important clinical factor in bonding to enamel is
the tissue to be bonded. The surface instrumentation, the
patient’s age, and environmental factors can lead to subtle
differences in enamel characteristics and influence the
ability of an acid conditioner to properly demineralize.
Several materials have been analyzed in studies on instrumented enamel surfaces.45 However, it should be taken into
account that restorations are commonly extended beyond
the margins of the cavity preparation. Also, a number of
conservative restorative treatments (eg, diasthema closure,
tooth recontouring, restoration of fractured teeth, pit-andfissure sealing, and bonding of orthodontic devices) are all
performed without tissue instrumentation. Because of
higher inorganic content, the intact enamel surface presents some unique features. First, young patients’ teeth have
an aprismatic layer of approximately 30 µm that covers the
entire crown.4 This layer is lost with time; however, the
hard tissue of the teeth becomes more mineralized when
exposed to the oral environment in patients with equilibri-
Compendium • January 2007;28(1):662-669
665
Figure 8—SEM micrograph showing the cervical enamel before (A) and after (B) etched with 35% phosphoric acidb for 15 seconds. Note the presence of
hypermineralized enamel not removed. (A) Final magnification 2000x. (B) Final magnification 3000x.
um in the demineralization process. This causes the surface layer of enamel to present hypermineralization features when compared with the innermost enamel.46 These
2 differences can influence the feature of the etching pattern and result in less homogeneous etching patterns,40
compromising the quality of bonding. Figure 8 shows cervical enamel before and after etching with 35% phosphoric acidb for 15 seconds.
The instrumentation of the tissue by a cavity preparation, microetching, or placement of a bevel can change
the response of the tissue to the acid etching. Several
studies report that the removal of the surface layer of
enamel enhances the etching result, and consequently,
the bond strength.47,48 A recent study showed that optimal bond strength to aprismatic enamel is achieved by
increasing the time of acid etching suggested by the manufacturer from 15 seconds to 30 seconds (with 35%
phosphoric acidb).49 It is the authors’ belief that the
chemical and morphological characteristics of intact
enamel would influence this difference. Further studies
must be performed to identify the response of enamel
with different mineral features when acid etched.
Cleaning Enamel
A relevant factor in bonding to enamel is the cleaning of the substrate to be bonded. Some advantages clinicians attribute to acid etching are bactericidal action and
cleansing potential. Phosphoric acid has an antibacterial
effect.50 To potentiate this effect, some acid conditioners
with antibacterial agents (eg, 3% cetypyridinium chloridec) have been made available. Without questioning
the confidence of this fact, this is not the objective of the
acid etching when taking into consideration exclusively
bonding to enamel. For a proper etching, the surface of
enamel must be clean. This cleaning must be accomplished before etching using cotton pellets soaked in
agents such as chlorhexidine gluconate and benzalkonium chloride. Alternatively, the air/water spray of a triple
syringe is an option in easy access locations. When the
e Ultradent
f
Products Inc, South Jordan, Utah; www.ultradent.com
Global Dental Products, North Bellmore, NY; www.gdpdental.com
666
clinician prefers to perform the cleaning after the acid
etching, chlorhexidine gluconate (Consepsis Scrube) and
benzalkonium chloride (Tubulicidf) solutions should be
preferred to disinfection pastes, because they do not
influence the marginal seal.51 Floss and gauze are recommended for proximal surfaces. Prophylactic pastes and
Robinson brushes are useful on noninstrumented enamel surfaces. A suitable alternative is a bicarbonate jet.
Bonding to Contaminated Enamel
It has been established that a main reason for debonding is the contamination of the conditioned enamel surface
before the application of the adhesive.37 Thus, it is important to control the oral fluids throughout the adhesive procedures to avoid possible contamination of the operatory
field. Moisture or contamination with oral fluids impairs
bonding of resins to enamel52-62 because the etched enamel
surface is chemically and physically modified.37,56,63
Because rubber dam isolation is impratical in some
instances, there is a risk of surface contamination from
blood or saliva and difficulty keeping the surfaces completely dry. This difficulty can be observed in the surgical
bonding of traction devices to unerupted teeth,64 or sealing of occlusal surfaces in erupting teeth. These situations are critical because bonding failure requires a new
surgical intervention or allows for early marginal
microleakage, respectively.
To achieve suitable clinical results, most of the adhesive
systems require a contamination-free surface.65-67 There are
reports that the bond strength is reduced to 50% when the
adhesive is applied directly to contaminated enamel compared with uncontaminated surfaces.60,61,68 To overcome contamination, some manufacturers have intoduced hydrophilic adhesives, suggesting their potential to successfully
bond to a contaminated enamel surface. The difficulty of
bonding to contaminated enamel can be minimized if the
adhesive is hydrophilic and has the potential to polymerize
completely, even in the presence of moisture.18,19,23,69,70 A few
studies have shown that adhesive agents containing
hydrophilic monomers can overcome the effect of contaminants on bond strength to contaminated enamel.68,71,72
Compendium • January 2007;28(1):662-669
3.
Durability of Bonding to Enamel
Bonding to enamel is considered a long-lasting procedure.3,16 When the durability of the adhesion is simulated in the laboratory under extensive thermocycling
(30,000 cycles), the acid-etching technique in enamel
allows for the maintenance of a stable bond strength.
Theoretically, an extensive thermocycling (30,000
cycles) is comparable (with limitations) to 3 years of
clinical function.73 However, this stability depends on the
adhesive system used.74 Some systems could be less stable because of excessive concentration of solvents, and
the presence of hydrophilic monomers that present high
wettability potential. Although these monomers allow
for immediate high bond strengths, they are more susceptible to hydrolytic degradation with time when compared with hydrophobic monomers. Another significant
variable is the mode of application. The active application (eg, scrubbing the primer on conditioned enamel
with a disposable applicator) has a negative effect on the
bond strength, mainly regarding durability.75
Conclusion
Bonding to enamel is a simple procedure in the dental
practice; however, some details can influence its durability.
High enamel bond strengths are achieved with previous
acid etching. This bonding is sufficiently high to compensate for the polymerization shrinkage of composite resins
and an effective marginal seal. This is paramount to obtain
a suitable clinical performance of composite resin restorations, preventing marginal leakage, and allowing for adequate retention. Knowing the factors that can influence
bonding to enamel is essential when selecting the most
appropriate materials and techniques for each situation.
For exampe, it was recently observed that when enamel
prisms are exposed perpendicularly, the bond strength is
reduced to 50% compared with the parallel exposure of the
prisms.76 Therefore, extreme caution is recommended in
the gingival margins of Class II restorations, with the
smoothing of enamel with manual instruments, rubber
dam isolation, application of etchant beyond all margins,77
and insertion and polymerization of composite resin in
small increments.78 Taking these details into consideration,
more predictable restorations will be obtained.
Acknowledgment
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
The first author would like to thank Dr Jorge
Perdigão and Ms Manuela Lopes for the training on scanning electron microscopy at the University of North
Carolina at Chapel Hill from August to September, 1998,
and for helping every year to train the Master of Science
students at Universidade Federal de Santa Catarina.
24.
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Compendium • January 2007;28(1):662-669
Quiz1
1. Enamel’s mineral portion is approximately:
a. 96% of its weight.
b. 16% of its weight.
c. 4% of its weight.
d. 75% of its weight.
2. Chemical treatment by acid etching does which
of the following, changing it from a low-reactive surface into a surface that is more susceptible to adhesion?
a. enhances the topography
b. lowers the free energy
c. minimizes the micromechanical interlocking
with the enamel
d. increases its free energy
3. Acid etching removes approximately 10 µm of
enamel surface and creates a morphologically
porous layer with how much depth?
a. 0.5 µm to 5 µm
b. 0.05 µm to 0.5 µm
c. 5 µm to 50 µm
d. less than 0.5 µm
4. Most adhesive systems that use the total-etch
technique have in their formulation:
a. water.
b. acetone.
c. low-viscosity hydrophilic monomers.
d. fluoride.
5. In the early 1990s, it was common to find
etchants such as:
a. 10% maleic acid.
b. 10% citric acid.
c. 2.5% nitric acid.
d. all of the above
6. The use of which concentration of phosphoric
acid conditioner is still the best option to
achieve predictable bonding to enamel?
a. 32% to 40%
b. 2.5% to 10%
c. 85% to 100%
d. 10% to 15%
7. Cleaning can be accomplished with:
a. air/water spray.
b. chlorhexidine gluconate.
c. benzalkonium chloride.
d. all of the above
8. There are reports that the bond strength is
reduced to _____ when the adhesive is applied
directly to contaminated enamel compared
with uncontaminated surfaces.
a. 5%
b. 90%
c. 50%
d. 75%
9. The durability of bonding to enamel depends on:
a. the adhesive system used.
b. excessive concentration of solvents.
c. the mode of application.
d. all of the above
10. It was recently observed that when enamel
prisms are exposed perpendicularly, the bond
strength is reduced to:
a. 50%.
b. 10%.
c. 80%.
d. 75%.
Please see tester form on page 680.
This article provides 1 hour of CE credit from Ascend Dental Media, in association with the University of Southern
California School of Dentistry and the University of Pennsylvania School of Dental Medicine, representatives of which have
reviewed the articles in this issue for acceptance. Record your answers on the enclosed answer sheet or submit them on a
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