Limitations of Dentin and Enamel Adhesion
Jorge Perdiago, DMD, MS, PhD
Numerous studies have suggested that
the resin-dentin interdiffusion zone or hybrid layer is a complex anatomic and
histologic entity. Variation in the extent and reliability of adhesive
infiltration is frequently reported with adhesive systems.1-4 Bond strengths decrease with time, and the
ultrastructure of the resin-dentin interface undergoes changes that may weaken
the bonding.5-8 A reduction in bond strengths up to 48% has been
reported at 1 year with an all-in-one self-etching adhesive.7
Degradation of the bonds may be a result of hydrolysis that occurs either in
the adhesive resin or in the collagen fibers that are not fully enveloped by
the adhesive in the hybrid layer, especially when margins are in dentin.9,6,7
When all margins of the restoration are in enamel, the quality and integrity of
the bonds remain unchanged with time.9
Both total-etch and self-etch adhesive systems may have difficulties to
infiltrate and to embed the collagen fibrils. In total-etch systems the
infiltration of the exposed collagen fibrils by the hydrophilic resin may not
fill all porosities created by etching procedures.1 On the other
hand the self-etch adhesive leaves the solublized mineral phase in the resin as
a suspension (Figures 1 and 2) supporting possible nano-porosities of
the hybrid layer. In all cases the lack of sealing ability of adhesives is
related to the presence of exposed or uninfiltrated collagen fibrils which are
susceptible to proteolytic enzymes and may represent a pathway to secondary
In a recent study the quality of the hybrid layer was evaluated using an
immunocytochemical technique and high resolution SEM.10 This study
evaluated the morphologic labeling pattern of the collagen fibrils within the
hybrid layer using both etching systems. These immunohistochemical data confirm
that in all cases collagen fibrils remain uncovered and not enveloped by resin.
LIMITATIONS OF ENAMEL ADHESION
More recent developments in dental
adhesion include self-etch (SE) adhesive materials;11 they are easy
to apply, and no rinsing is required (Figures 3-4-5-6-7). The same
solution serves as conditioner, primer, and adhesive (Table 11-1). The
rationale behind the action of self-etching agents is the formation of a
continuum between tooth surfaces and adhesive material, which is accomplished
by the simultaneous demineralization and penetration of resin in enamel and
dentin surfaces.11 However, omitting the conventional etching step
with phosphoric acid may result in the absence of the characteristic
demineralization of enamel and dentin (Figure 8).
The enamel etching capability of SE adhesives has been studied profusely.12-21
One of the shortfalls of SE adhesives is that they may not etch enamel to the
same depth achieved with phosphoric acid.18,19 Other
studies, however, demonstrated that enamel bonding with SE adhesives is of the
same magnitude as enamel bonding after phosphoric acid etching.20,21 The low bond strengths of SE
adhesives to intact enamel, which is present in the cervical aspect of some
restorations, raise clinical concerns.22 Optimal bonding to intact
enamel is important to accomplish good clinical performance and to prevent
microleakage.23 Long-term clinical studies with SE adhesives have
demonstrated that these materials do not etch enamel adequately.24,25
The lower adhesion strength of SE adhesives to intact enamel may be a
result of the less pronounced enamel etching pattern obtained with SE adhesives
on intact enamel than the pattern obtained with phosphoric acid.18,20
Intact enamel is less accessible to SE adhesives because it is hypermineralized
and may contain more fluoride than instrumented enamel.14 Changes occur
in the outermost enamel layer after eruption.26 A prismless enamel
layer may be present and prevent further penetration of SE adhesives.27
Longer application times may actually compensate for the higher pH of SE
adhesives compared to that of phosphoric acid etchants.28 In fact,
when a self-etching primer was applied for 60 instead of the recommended 30
seconds, Ferrari et al demonstrated
that an adequate seal is provided in Class V cavities, in vitro and in vivo.13
When the acid-etch technique2 was relatively new, the
formation of tag-like resin extensions into the enamel microporosities was
considered the mechanism of bonding of resin to phosphoric acid-etched enamel,27,31
comparable to that of phosphoric acid etched enamel.32 Other studies
reported that the etching pattern of SE adhesives was not as well defined as
that of total-etch (TE) adhesives on roughened enamel, but resulted in similar
enamel bond strengths.18,20 This similarity in bond strengths for SE
adhesives, in spite of their shallower etching pattern in comparison to TE
adhesives, may seem conflicting. The length of the tags created on phosphoric
acid-etched enamel, however, may contribute little to the bond strength. The
adhesive strength may be a result of the ability of the resin to penetrate
between the enamel crystallites and rods.33 An ideal etch pattern is
not essential to produce a strong bond.34 Nevertheless, etching
enamel with non-rinsing conditioners of pH higher than that of phosphoric acid
remains controversial. The unpredictable behavior of SE adhesives around enamel
margins is the reason why etching with 30% to 40% phosphoric acid is still
recommended clinically.18 Total-etch adhesives bond better to intact
enamel than the corresponding self-etch adhesive from the same manufacturer.
The stability and durability of bonded interfaces formed with
self-etching adhesives has been the subject of some controversy.6-9
The hydrolysis of the hybrid layer formed with self-etch adhesives has now been
established. An NMR study found that the ester portion of the HEMA molecule
undergoes hydrolysis, as a result of the dissociation of the phosphoric acid
component in the MDP molecule.35 Upon hydrolysis, HEMA breaks down
into methacrylic acid and ethylene glycol.
In summary, bonding to dental hard tissues has transformed the conceptualization and practice of operative dentistry. Developments in adhesive materials have not necessarily corresponded to improvements in their clinical success, as manufacturers and clinicians have prioritized the simplification of the clinical technique over the clinical efficacy of the newest materials. Therefore, clinical trials should be mandatory prior to the introduction of new adhesive materials.
- Spencer P, Swafford JR. Unprotected
protein at the dentin-adhesive interface. Quintessence Int 1999;30(7):501-507.
Y, Spencer P. Quantifying adhesive penetration in adhesive/dentin interface
using confocal Raman microspectroscopy. J Biomed Mater Res 2002;59(1):46-55.
Y, Spencer P. Hybridization efficiency of the adhesive dentin interface with
wet bonding. J Dent Res 2003;82(2):141-145.
H, Burrow MF, Tyas MJ. Nanoleakage patterns of four dentin bonding systems.
Dent Mater 2000;16(1):48-56.
- Armstrong SR., Keller JC, Boyer DB. The influence of water
storage and C-factor on the dentin-resin composite microtensile bond strength
and debond pathway utilizing a filled and unfilled adhesive resin. Dent Mater
- Hashimoto M, Ohno H, Kaga M, et al. In vivo degradation of
resin-dentin bonds in humans over 1 to 3 years. J Dent
- Hashimoto M, Ohno H, Sano H, et al. Degradation patterns of
different adhesives and bonding procedures. J Biomed Mater Res B Appl Biomater
- Sano H, Yoshikawa T, Pereira PN, et al. Long-term durability
of dentin bonds made with a self-etching primer, in vivo. J Dent Res
- De Munck J, van Meerbeek B, Yoshida Y,
et al. Four-year water degradation of
total-etch adhesives bonded to dentin. J Dent Res 2003;82(2):136-140.
- Breschi L, Prati C, Gobbi P, et al. Immunohistochemical analysis of the collagen fibrils
within the hybrid layer: A FEISEM study. Oper Dent 2004;29(5):538-546.
- Perdigão J, Frankenberger R, Rosa BT,
Breschi L. New trends in dentin/enamel adhesion. Am J Dent 2000;13(Spec No):25D-30D.
- Perdigão J, Lopes L, Lambrechts P, et al.
Effects of a self-etching primer on
enamel shear bond strengths and SEM morphology. Am J Dent 1997;10(3):141-146.
Erratum in: Am J Dent 1997;10(4):183.
- Ferrari M, Mannocci F, Vichi A, et al. Effect of two etching times on the sealing ability of
Clearfil Liner Bond 2 in Class V restorations. Am J Dent 1997;10(2):66-70.
N, Sano H, Tagami J. Tensile bond strength to and SEM evaluation of ground and
intact enamel surfaces. J Dent 1999;27(7):523-530.
M, Sato M, Onose H. Durability of enamel bond strength of simplified bonding
systems. Oper Dent 2000;25(2):75-80.
M, Hinoura K, Honjo G. Effect of self-etching primer application method on
enamel bond strength. Am J Dent 2002;15(6):412-416.
Y, Tagami J. Effects of regional enamel and prism orientation on resin bonding.
Oper Dent 2003;28(1):20-27.
J, Geraldeli S. Bonding characteristics of self-etching adhesives to intact
versus prepared enamel. J Esthet Restor Dent 2003;15(1):32-42.
PA, Castro PS, Nunes MF, et al. Effect of acid-etching on the enamel bond of
two self-etching systems. J Adhes Dent 2003;5(2):107-112.
- Shimada Y, Senawongse P, Harnirattisai C,
et al. Bond strength of two adhesive
systems to primary and permanent enamel. Oper Dent 2002;27(4):403-409.
M, Reinhardt KJ, Bott B. Self-etching primer vs. phosphoric acid: An
alternative concept for composite-to-enamel bonding. Oper Dent 1999;24(3):172-180.
- Opdam NJ, Roeters FJ, Feilzer AJ, Smale I.
A radiographic and scanning electron
microscopic study of approximal margins of Class II resin composite
restorations placed in vivo. J Dent 1998;26(4):319-327.
EJ, Perdigão J, Heymann HO. Bonding to enamel and dentin: A brief history and
state of the art, 1995. Quintessence Int 1995;26(2):95-110.
Y, Miyatake S, Unemori M, et al. Clinical evaluation of composite resin
restorations with self-etching adhesive systems. J Dent Res 2003;82:908.
N, Takamizu M. Ten-year clinical evaluation of self-etching primer system. J
Dent Res 2004;83:
F. A study of the phosphate solubility of the human enamel surface. J Dent Res 1948;27:320-329.
AJ. The ultrastructure of the “prismless” enamel of permanent human teeth. Arch
Oral Biol 1967;12(3):381-388.
K, Torii Y, Takimura T, et al. Effect of priming time on tensile bond strength
to bovine teeth and morphologic structure of interfaces created by self-etching
primers. Int J Prosthodont 2001;14(3):225-230.
MG. A simple method of increasing the adhesion of acrylic filling materials to
enamel surfaces. J Dent Res 1955;34(6):849-853.
AJ, Matsui A. A study of enamel adhesives. The physical relationship between
enamel and adhesive. Arch Oral Biol 1967;12(12):1615-1620.
CL, Rossi F, Cartz L. Adhesive tag-like extensions into acid-etched tooth
enamel. J Dent Res 1974;53(2):435-441.
- Gordan VV, Vargas MA, Denehy GE. Interfacial ultrastructure of the resin-enamel region
of three adhesive systems. Am J Dent 1998;11(1):13-16.
MJ, Soma K, Nakabayashi N. The effect of phosphoric acid concentration on resin
tag length and bond strength of a photo-cured resin to acid-etched enamel. Dent
RS, McCabe JF. Relationship between enamel etch characteristics and
resin-enamel bond strength. Br Dent J 2002;192(8):463-468.
K, Nishiyama N, Nemoto K. Hydrolysis stability of one-bottle self-etching primer. J Dent Res 2004;83: