Dentin and Enamel--Chemistry and Histology
Jorge Perdiago, DMD, MS, PhD
The recent advances in adhesion
technology have substantially reduced the need for extensive tooth
preparations.1 The acid-etch technique has changed in predictability
from its beginning.2 The ability of clinicians to bond resin
materials to enamel and dentin has changed the concepts of cavity preparation,
orthodontic treatment, caries prevention, and cementation of fixed prostheses,
including tooth-colored carbon-fiber posts. Procedures used for aesthetic enhancement,
such as porcelain laminates, (Figures 1 and 2) rely on new adhesive
materials and techniques. Improved marginal sealing around bonded restorations
has reduced the frequency of unfavorable postoperative responses. Improvements
in adhesive materials have expanded the indications for tooth-colored
restorations from anterior to posterior teeth (Figures 3 and 4).
The word “adhesion” is derived from
the latin term adhaerere (to stick
to). Adhaerere itself is composed of ad
(to) and haerere (to stick).3
Adhesion is defined by specification D907 of the American Society for Testing
and Materials as “the state in which two surfaces are held together by
interfacial forces, which may consist of valence forces or interlocking forces
The adhesive, frequently a fluid,
joins two substrates and solidifies. The adherend is the material or initial
substrate to which the adhesive is applied. Adhesion or bonding is the process
of forming an adhesive joint,3 consisting of two substrates joined
together. Most adhesive joints involve only two interfaces; a bonded composite
restoration is an example of a more complex adhesive joint
Contemporary adhesive techniques
allow dentists to restrict operative procedures to removal of diseased tooth
tissue, thereby preserving sound tissue. The short lifetime of restorations,
frequently assessed by methods not based on clinical evidence,4,5
requires clinicians to replace restorations recurrently.6-8 Each
time the restoration is replaced, part of the remaining sound tooth structure
is unavoidably removed, and a more complex restoration is required.9
Increasing the lifetime of a restoration is one of the primary goals of current
DENTIN AND ENAMEL--CHEMISTRY AND HISTOLOGY
Human enamel is composed by weight of
96% hydroxyapatite crystals (Figures 5-6-7-8), being the remainder of
the tissue formed by an organic matrix consisting of proteins (ie,
amelogenins and enamelins) and trace amounts of water.10 The
uniqueness of dentin as a histologic entity makes this hard tissue a challenge
when trying to adhere restorative materials. Dentin is formed by the
interaction between the ectodermal and ectomesenchymal components that induce
odontoblast differentiation and dentinogenesis.11 As odontoblasts
fabricate dentin, they leave a track throughout the dentin forming the dentinal
tubules (Figures 9 and 10).11
The dentin tubules have different density and orientation in distinct locations
of the tooth.12
The tubular structure of
dentin is responsible for its intrinsic hydration, due to the communication
with the pulp tissue, which is under vascular pressure.13,14 The
intertubular dentin is made of a complex lattice of collagen fibers with
hydroxyapatite crystals filling up the spaces between fibers and surrounding
the tubular or peritubular dentin (Figures 11 and 12).14
The scaffold of all the
tubules and intertubular dentin is represented by the collagen fibrils produced
by the odontoblasts, while the hydroxyapatite precipitates in-between the
fibers and inside the nanospaces of each fiber during dentinogenesis. Mineral,
in the form of carbonate-rich apatite, constitutes approximately 50% of the
dentin volume.13 The precipitation of mineral substance on the
collagen fibrils during dentinogenesis results in the final mineralized
Bonding to dentin has been one of the
most challenging issues since the introduction of the acid-etch technique nearly
50 years ago.2 Acid etching transforms the smooth enamel into an
irregular surface (Figures 10-11-12-13A-13B). A mixture of resin monomers or adhesives is
applied on the enamel surface (Figure 14). The resin is drawn into the
microporosities of the enamel surface by capillary action. The monomers in the
fluid resin polymerize and become mechanically interlocked with the enamel
structure. The formation of resin microtags (Figure 14) within the enamel
structure has been considered the essential mechanism of adhesion of resin to
When tooth structure is
prepared with a bur or other instrument, residual organic and inorganic
components form a “smear layer” of debris on the surface.17 The
smear layer obstructs the entrance of dentin tubules (Figure 15), decreasing
dentin permeability by up to 86%.18 Submicron porosity of the smear
layer still allows flow of dentinal fluid.19 Although the smear layer
acts as a physical obstacle that decreases the permeability of dentin, it can also
be considered a barrier that must be removed or made permeable, so that
monomers can contact and interact with the dentin surface. Manufacturers of
dentin/enamel adhesives use one of two major strategies for overcoming the
barrier (Figure 16):
Adhesives that treat the dentin and enamel surface with a non-rinsing solution
of acidic monomers in water. These bonding systems do not remove the smear
layer (Figures 17 and 18), but make it permeable to the monomers subsequently
Adhesives that include an acid gel to treat dentin and enamel for 15 to 30
seconds. The acid dissolves the smear layer and the top 1 µm to 6 µm of
hydroxyapatite (Figure 19). Total-etch has led to significant improvements
in the in vitro bond strengths of
resins to dental substrates.20,21
It is clear that dentin and enamel etching is moving forward before our eyes. By studying the methods used in the past, dentists will be better suited to deal with the methods that will ultimately be used in the future. The ability of dentists to bond material to both enamel and dentin has reshaped the field of dental adhesion and the potential of restorative care.
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