the dental tissues with an acidic solution is a standard clinical procedure
that results in the demineralization of the superficial layer of enamel and
dentin. Different acids have been proposed over the years: phosphoric acid
(from 10% to 50%), fluoridated phosphoric acid, pyruvic acid, citric acid, maleic
acid, oxalic acid, tannic acid,
EDTA, trichloracetic acid, and polyacrylic acid, among others.1-6
Recent studies have shown that the pH
and the pKa of the acid solution are important parameters that influence the
aggressiveness of the acids and their ability to demineralize the surface.7,8
The physical status of the solution (either
gel or liquid) is an important parameter. The gel type etching agent is easier
to apply on the enamel surface than the liquid form, and it is reported to
produce a wider and deeper enamel penetration.8 As to methods of
application, a continuous brushing technique may be useful when applying an
acid-etching gel to obtain a more defined etched pattern for an improved
marginal adaptation of the composite resin restoration.9
Etching of the enamel results in 3
types of patterns.10 A preferential removal of prism core material,
leaving the periphery intact, constitutes the Type I etching pattern (Figure 1B). Type II is defined as a preferential removal of periphery core
material, leaving the prism core relatively unaffected (Figure 1A). The
Type III pattern represents a more random etching morphology in which adjacent
areas of the tooth surface correspond to Types I and II, mixed with regions in
which the pattern could not be related to prism morphology (Figure 2).
The consequences of etching dentin
are different from etching enamel. Acids remove the superficial mineral dentin
to expose the organic matrix, in particular, the collagen dentin network. The
characteristic collagen banding is formed by a longitudinal overlapping of the
molecules that involve approximately one quarter of the length of the fibril
monomer, leaving a space between the extremity of one collagen triple helix and
the beginning of the next.11 This “hole” has been considered a site
for accumulation of hydroxyapatite crystals inside the collagen molecule.33
Collagen banding has been also reported in the peritubular region.12
The lack of collagen banding in some areas of etched dentin may be caused by
the aggressiveness of the etchant, since dentin collagen, after
demineralization, is susceptible to proteolytic degradation.13
In spite of assumptions that etching
dentin results in collagen denaturation and precludes the establishment of
durable bonding, it has never been validated, nor has it been proven that
changes in collagen ultrastructure are detrimental for the performance of dentin
adhesives. The results from a recent immunocytochemical study provided
additional evidence that application of phosphoric acid for 15 seconds results
in a mineral dissolution of the crystals, enveloping the superficial collagen
fibers without damaging the collagen ultrastructure.14
molecules of the dentin matrix that might have an important role in adhesion
are the proteoglycans (PG). The complex molecular conformation of the collagen
fibrils network that is exposed by the etching agent involves the presence of
lateral chains of proteoglycans that have a fundamental role in stabilizing the
fibrillar arrangement.15 Proteoglycans are important components of
the extracellular matrix of all human connective tissues; keratan sulfate (KS)
and chondroitin sulfate (C4-S and C6-S) are present in predentin, dentin, and
cement.15,16 Due to the ability of PG to bind water molecules,17
PG regulate the biophysical properties of the collagen fibrils and are able to
imbibe a great amount of water. This peculiarity is transferred to the
nonmineralized connective tissues as a high resistance to a load force.18 On
the other hand, the presence of the collagen fiber in the connective dentin
matrix gives the typical high resistance to traction forces. Collagen fibers
and PG interact closely, creating a complex network of fibrillar structures
that act as the scaffold of the dentin matrix and react with the adhesive agent
during infiltration.19 The role of proteoglycans is crucial, as
water is fundamental in preventing collagen collapse after etching.
(Continued from page 1 )
A group of simplified dentin adhesive
systems has been recently developed and received considerable attention from
clinicians, due to a simplified clinical application. In these one-bottle
dental adhesive systems, enamel and dentin are etched with phosphoric acid
simultaneously, followed by the application of a single solution that combines
primer and adhesive resin from the early generation. These simplified systems
contain hydrophilic (ie, HEMA, PENTA, BPDM, or PMDM) and hydrophobic resins (ie,
BisGMA, UDMA), dissolved in high vapor pressure solvents, such as acetone and
ethanol. The solvents may be able to displace water from the dentin surface and
from the moist collagen network and allow the monomers to intermingle with the
filigree of collagen fibers to form a “hybrid layer” (Figure 3).20,21
However, the idea that organic solvents lead the monomers into direct contact
with the moist collagen fibers and prevent their collapse has never been
substantiated. In fact, some organic solvents may actually cause shrinkage of
moist demineralized dentin and interfere with the penetration of resin monomers
The demineralized dentin matrix
collapses when dentin is air dried following etching.23 When the
“wet-bonding” technique24 was first considered as a standard
procedure associated with total-etch, the moist dentin surface was deemed as
the ideal bonding substrate, and most manufacturers recommend it.25
The concept behind it is to obtain high bond strengths in vitro, supporting the idea that moist dentin provides a porous
collagen network that allows infiltration of adhesives into the nanospaces
between adjoining collagen fibers(Figures 4 and 5).26-28
The immediate high-bond strengths obtained, decrease when the specimens
are stored in aqueous environments.29,30
The demineralized dentin matrix
collapses easily in vitro when it is
air-dried after being rinsed with water.23,31-33 After dissolution
of the hydroxyapatite crystals by the acid, it is essential to keep dentin
moist, so that air-drying and consequent collapse of the fibrillar structure of
the collagen scaffold is prevented, in
Air-drying of etched preparations was
taught in dental schools as a method to check for etched aspect of enamel. Some
clinicians still dry the preparation after rinsing the etching gel. As a result
of air-drying enamel, dentin is also dried, which causes dentin collagen to
collapse resulting in the closing of the pores in intertubular collagen in vitro. Re-wetting dried dentin with
water raises the collapsed collagen to a level compared to a “wet bonding”
technique and restores the bond strengths when dentin is re-wet for twice as
long as the time spent with drying.27,34,35
Data obtained from in vitro
studies focused on different degrees of moisture have not been corroborated by
clinical findings. As opposed to laboratory conditions, dentin is an inherently
hydrated tissue in vivo, penetrated
by a system of 1.0 to 2.5 mm diameter fluid-filled dentin
tubules. Flow of fluid from the pulp to the dentin-enamel junction is a result
of a slight, but constant, pulpal pressure,36 which has a magnitude
of 25 mm to 30 mm Hg or 34 mm to 40 cm H2O.37,38
Six-month results of a clinical trial of two dentin adhesives, applied on moist
versus dry dentin, have shown that
the moisture level of the substrate may not be as important clinically as it is
under laboratory conditions.39 In this study, an ethanol- and
water-based adhesive and an acetone-based adhesive, were applied in
non-carious, non-beveled, Class V lesions, either after removing the excess of
water with a damp cotton pellet, or after drying enamel and dentin with air for
2 to 3 seconds. Retention rates for both adhesives applied on dried dentin were
100% at 6 months, which challenges the clinical relevance of the in vitro findings and the experimental
setup of current laboratory experiments.
- Garcia-Godoy F, Dodge WW,
Donohue M, O’Quinn JA. Effect of a fluoridated etchant on the shear bond
strength of a composite resin to enamel. Int J Paediatr Dent 1992;2(1):25-30.
- Sharma A, Chandra S, Jaiswal JN, Bajpai
of conditioning the enamel surface of primary teeth with citric acid: A SEM
Clin Pediatr Dent 1992;16(3):207-212.
- Breschi L, Gobbi P, Mazzotti G, et al. High resolution SEM evaluation of dentin
etched with maleic and citric acid. Dent Mater 2002;18(1):26-35.
- Prati C, Pashley DH, Chersoni S, Mongiorgi
R. Marginal hybrid layer in Class V restorations. Oper Dent 2000;25(3):228-233.
- Bitter NC. The effect of
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- Prati C, Montanari G, Biagini
G, et al. Effects of dentin surface
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- Misra DN. Interaction of citric acid with
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- Baharav H, Cardash HS, Pilo R, Helft M. The
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- Ben-Amar A, Baharav H, Liberman R,
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- Silverstone LM, Saxton CA, Dogon IL,
Fejerskov O. Variation in the pattern of acid etching of human dental enamel examined
by scanning electron microscopy. Caries Res 1975;9(5):373-387.
- Prockop DJ, Kivirikko KI,
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- Perdigão J, Lambrechts P, van
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- Okamoto Y, Heeley JD, Dogon IL, Shintani H. Effects
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- Breschi L, Perdigão J, Gobbi P,
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- Goldberg M, Takagi M. Dentine proteoglycans:
Composition, ultrastructure and functions. Histochem J 1993;25(11):781-806.
- Cheng H, Caterson B, Yamauchi M.
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tooth cementum. Connect Tissue Res 1999;40(1):37-47.
- Scott JE. Proteoglycan: Collagen
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the development and ageing of connective tissues. J Anat 1990;169:23-35.
- Ruoslahti E. Structure and biology of
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- Breschi L, Gobbi P, Lopes M, et
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- Eick JD, Gwinnett AJ, Pashley DH, Robinson
SJ. Current concepts on adhesion to dentin. Crit Rev Oral Biol Med 1997;8(3):306-335.
- Jacobsen T, Söderhold KJ. Some effects of
water on dentin bonding. Dent Mater
- Nakajima M, Okuda M, Pereira PN, et al. Dimensional
changes and ultimate tensile strengths of wet decalcified dentin applied with
one-bottle adhesives. Dent Mater 2002;18(8):603-608.
- Pashley DH, Horner JA, Brewer PD.
Interactions of conditioners on the dentin surface. Oper Dent 1992;Suppl 5:137-150.
- Kanca J. Resin bonding to wet substrate. 1.
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- Kanca J. Effect of resin primer solvents and
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- Tam LE, Pilliar RM. Fracture surface
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- Gwinnett AJ. Dentin bond strength after air
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- Hashimoto M, Ohno H, Sano H,
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- Tay FR, Gwinnett JA, Wei SH.
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- Perdigão J, Frankenberger R. Effect of
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- Tay FR, Gwinnett AJ, Wei SH. Ultrastructure
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