acid etching/ rotary endodontic courses by indian dental academy

ACID ETCHING
CONTENTS
Page no
INTRODUCTION 1
HISTORY 4
ENAMEL 10
Formation of enamel 10
Composition of enamel 11
Structure of enamel 12
DENTIN 23
Physical properties 24
Composition of dentin 24
Structure of dentin 24
ACID ETCHING OF ENAMEL 41
Steps in acid etching technique 41
Effects of etching on enamel 45
Role of fluorides in etching of enamel 47
Factors affecting etching of enamel 50
Variation in acid etching methodologies 52
ACID CONDITIONING Of DENTIN 55
Goals of acid conditioning of dentin 56
Effects of acid conditioning of dentin 58
Factors affecting dentin conditioning 68
Conditioners on dentin surface 77
BIOCOMPATIBILITY 79
VARIOUS ACID CONDITIONERS 86
SELF ETCHING PRIMERS 95
EFFECT OF ACID ETCHING ON PRIMARY TEETH 98
APPLICATIONS OF THE ACID CONDITIONERS 100
FACTORS TAKEN INTO CONSIDERATION 102
ACID CONDITIONERS FOR GI CEMENTS 103
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BIBLIOGRAPHY 105
INTRODUCTION
The possibility of bonding restorative materials to the hard
dental tissues intrigued the dental professionals for many years. The
development and regular use of adhesive materials has begun to
revolutionize many aspects of restorative and preventive dentistry.
Attitudes towards cavity preparation are altering since, with
adhesive materials, it is no longer necessary to produce large
undercuts in order to retain the filling. These techniques are
therefore responsible for the conservation of large quantities of sound
tooth structure, which would otherwise be victim of the dental bur.
Buonocore (1955) was the first to report the positive effects of
application of 85% phosphoric acid to enamel for the retention of
acrylic resin restorations. Gwinnett, Matsui and Buonocore (1969),
further explored the effect of acid solutions and this came to be
accepted as an integral part of any direct tooth coloured restorative
technique.
Bonding of restorative materials to hard dental tissues would be
impossible without the use of acid solutions. It is the effect of these
various acid solutions and pretreatments that results in the hard
dental tissues being characterized by numerous microscopic
porosities, which allows the resin to readily wet the surface and
penetrate into these micro porosities. Once the resin penetrates into
these micro porosities it can be polymerized to a form a mechanical
bond to the hard dental tissues.
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The success of acid etching of enamel led Buonocore et al (1956)
to try to acid etch dentin using 7% HCL for one minute. Unlike
enamel when dentin is etched, surface becomes mineral poor, protein
rich, and it tends to become wetter (Brannstrom and Norden Vall
1977). Unfortunately, success with dentin was never realized,
because the relatively cured resin materials that were available at that
time would not wet dentin very well. Buonocore, however, was very
much aware of the requirements for good bonding.
The term conditioner or etchant is used to describe agents that
are washed off the dentin. The word “etchant” has, until recently,
been taboo in western dentistry, for describing the action of various
acidic materials on dentin. Etching of the dentin can be defined as
“any alteration to the dentin done after the creation of dentin cutting
debris, termed the smear layer” (Eick et al 1970). One of the objectives
of dentin etching is to create a surface capable of micro-mechanical
bonding to a dentin-bonding agent. Several acids have been
researched as dentin etchants. These include hydrochloric acid,
oxalic and pyruvic acid in addition to the better known acid such as
phosphoric, maleic, citric and nitric acid. Fusayama et al (1979) were
the first to report the successful use of Phosphoric acid to remove the
smear layer, etch the dentin and restore with adhesive composite
resin.
Acid etching of dentin is used by many bonding systems to
remove the smear layer and permit bonding directly to the dentin
matrix. Although early animal studies indicated that acid etching
caused moderate to severe pulpal reactions, there is a high probability
that the pulpal irritation may have been due to micro leakage of
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bacteria and their products. As these reactions are not seen following
acid etching of dentin bonding systems.
It is clear that one can acid etch dentin if, and only if, one can
seal the dentin with subsequently placed bonding systems. Because
acid etching increases dentin permeability and dentin wetness,
successful bonding of adhesive resins to acid etched dentin requires
the use of hydrophilic resins that bond equally well to both peri
tubular and inter tubular dentin. The trend seems to be toward
lowering both the concentration of acids and the time of etching of
dentin. While all bonding systems should be carefully scrutinized
prior to marketing, the future looks very promising for the use of
adhesive resins on both enamel and dentin through the effective use
of acid-etch technique.
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HISTORY
There are names in the lexicon of dental adhesion that those of
us in the field should always remember to acknowledge because it was
on their shoulders that we stood as we grappled with our own
research problems.
Dr. Michael Buonocore was certainly one of such best-known
pioneers in adhesive bonding of resins to teeth. He found that lightly
etching enamel created a micro porous surface into which direct filling
liquid resins could flow polymerize and make a micro mechanical
attachment.
He thereby achieved his primary objective of bonding, a
conservative means of sealing developmental pits and fissures. One of
Dr. Buonocore’s contemporary Dr. George Newman developed similar
methods to bond orthodontic brackets directly to the enamel of teeth.
Another distinct advantage of effective acid etching technique
and its resultant adhesive bonding to dentin is the prevention of
removal of healthy dentin for mechanical retention of composite
restorations, a process that is painful without an anesthetics.
The mechanism of enamel bonding is well understood and
involves a micro-mechanical union between enamel and the resin,
which occupies tissue microspores enlarged by the action of an acidic
conditioning agent. However, acid etching of the pulp dentin complex
and its resultant bonding has undergone a number of changes over
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the last thirty years and both the bond strength values and
biocompatibility to pulp dentin organ have tremendously improved.
Brannstrom et al (1984) suggested that on a number of
occasions, they inadvertently acid etched teeth with small pulp
exposures at the base of deep cavities. These were not often
discovered until subsequent histo-pathological examination of the
extracted teeth. Unless there was concomitant infection there was no
particular damage or inflammation to the pulp. However, they noticed
that when restorations leaked, and bacteria colonized the cavity
surface, the teeth that had been etched exhibited more severe pulpal
responses, than those that were not etched.
Many attempts have been made to synthesize different coupling
agents for tooth surfaces. One of the earliest successful compounds
tested was NPG-GMA, the reaction product of N-phenyl-glycine and
glycidyl methacrylate (Bowen 1965). The use of this surface-active co-
monomer alone improved the water resistant bonding between resins
and enamel and dentin to a degree that was statistically but not
clinically significant (Bowen 1965).
Removal of the structurally weak smeared layer, pellicle, or
other superficial layers of the tooth surface by use of acidic (Fusayama
& others, 1979, Fusayama 1980) or chelating agents might reduce the
availability of calcium ions on dentin surfaces for interaction with a
chelating surface active co-monomer like NPG-GMA or other coupling
agent with, preferably, multiple-bonding ligand groups. To
supplement calcium ion sites for improved bonding, certain
appropriate metal cations were evaluated for use on tooth surfaces.
Experiments indicated that the most effective agent might be ferric
oxalate, primarily because of the iron ion’s high tendency to be bound
strongly by denting and enamel and its high chelate stability
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constants with molecules that have linked groups similar to those of
NPG-GMA (Bowen 1978).
Furthermore, the oxalate would form an insoluble precipitate
with calcium ions, which, together with insoluble ferric phosphate,
would seal the dental tubules to provide pulp protection and
desensitization.
Nakabayashi (1982) introduced the concept of hybridization.
The technique consists of applying an acid, ranging in concentration
from 10% to 30% to the surface of dentin. Within 15 minutes the acid
selectively dissolves away the inorganic component of the dentin to a
depth of 5 to 10 microns. It then flows in to the dentinal tubule for
upto 100 microns at which point it diffuses laterally into the
peritubular dentin for up to 10 microns. As in the previous case the
calcium component is selectively eliminated. Then these spaces are
replaced by an insoluble resin component that completely
encapsulates all exposed collagenous fiber.
It was then discovered that the additional use of a relatively
hydrophilic monomer containing two free carboxyl groups in addition
to two polymerizable groups on each molecule dramatically improved
bond strengths to levels of clinical significance (Bowen and others,
1982). This monomer was called “PMDM” (the reaction products of
pyromellitic dianhydride and hydroxyethyl-methacrylate). There was a
synergistic interaction between the NPG-GMA and the PMDM (Bowen
and others, 1984).
The original adhesive system developed was a sequential
application of aqueous acidic ferric oxalate, followed by an acetone
solution of NPG-GMA or NTG-GMA (the reaction product of N-
ptolyglycine and glycidyl methacrylate), and then an acetone solution
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of PMDM. This system was effective only if placed in the described
sequential order utilizing all three components. The acidic ferric
oxalate solution was removing the original smear layer, the disturbed
surface layer caused by mechanical abrasion in preparing a
restoration site (Bowen and others, 1984), and laying down a layer of
precipitation product that was plugging up the lumina of dentinal
tubules. The latter function significantly reduced tooth sensitivity to
the subsequent procedure. The NTG-GMA was necessary to induce
polymerization of the PMM, but the exact mechanism of this free
radial initiation is still not clear.
During subsequent experimentation, it was discovered that the
smear- removing capabilities of ferric oxalate were due primarily to the
presence of small amounts of nitric acid left over from the synthesis of
the oxalate (Cobb and others, 1989). Controlled additions of nitric
acid to the aqueous oxalate solution were made to determine the
optimum acid concentration for this solution (Blosser and Bowen,
1988). A small increase in the concentration of nitric acid to about
2.5% HNO3 by weight also improved the simultaneous etching of
instrumented enamel.
However, an adverse side effect of the application of the ferric
oxalate solution was discovered; the occasional appearance of black
staining at the adhesive interface in early animal trails (Stanley,
Bowen and Cobb, 1988). This could be reproduced in the laboratory
by applying a sodium sulfide solution to ferric oxalate treated dentin.
The cause of this staining in vivo is probably (although not proven to
be) the reduction of ferric to ferrous ions by sulfide forming anaerobic
microorganisms resulting in the formation of black ferrous sulfide
pigments.
To eliminate this, acidic aluminum oxalate was substituted, and
it produced no staining on dentin. Aqueous solution of aluminum
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oxalate and nitric acid were then applied, and no staining problems
occurred in animal trails (Blosser and others 1989).
There was some evidence in vitro that aluminum oxalate did not
produce as much of the reaction products plugging the dentin
tubules, as has ferric oxalate. Eventually, the first successful transfer
of the adhesion technology developed by scientists at the ADA Health
Foundation’s Paffenbarger Research Center involved the development
of a product that incorporated aluminum oxalate in a conditioning
solution.
In the continuing research, it was found that the aluminum
oxalate could be eliminated entirely from the experimental system
without loss in adhesion, if the dilute nitric acid solution was
retained. None of the other acids evaluated, in a wide range of
concentrations, were as good or better than the dilute nitric acid
(which should be distinguished from concentrated nitric acid, a strong
oxidizing agent). It was then surprisingly discovered that NPG (N-
phenyl-glycine) could be substituted for NPG-GMA or NTG-GMA. The
experimental system was then reduced to the three components of
♦ Dilute nitric acid
♦ NPG acetone solution
♦ PMDM acetone solution
The three components still had to be applied individually in
sequence to adhesive adhesion, and efforts were concentrated on ways
to simplify application. It was then suspected and verified that NPG
would be soluble in the dilute aqueous nitric acid solution. This
permitted a simplification of the procedure to the application of two
solutions.
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♦ An acidic NPG solution
♦ PMDM acetone solution
However, preparation and storage of the first solution was difficult
because of the reactivity of the NPG molecule to atmospheric oxygen.
Storage times were very short if the acid NPG solution was exposed to
air. If the solution was used shortly after mixing, adhesion was
effective. Methods were then developed for preparing the solution
under an inert atmosphere and protecting it from subsequent oxygen
exposure. These protected solutions were solutions were stable under
normal storage conditions. Some commercial products are currently
based on this two-solution system. Current experimentation with the
system is focusing on optimizing the individual components. Nitric
acid concentrations will continue to be refined to yield optimal
treatment of both dentin and enamel.
Different analogues of the NPG molecules are being synthesized
toward improving effectiveness, storage, stability and ease of synthesis
(Johnson, Asmussen and Bowen 1989). PMDM is being investigated
to isolate more effective linking agents between tooth surfaces and the
overlying restorative resins. Many years of experience in etching
enamel with phosphoric acid have shown bonding by this method to
be most reliable clinically. However, it is noteworthy that the use of
the chemically functional and more hydrophilic dentin bonding
agents, significantly increases bond strengths to acid etched enamel
at least in laboratory tests.
A number of bonding systems already available to practioners
are beneficial for increased versatility toward improving the
performance of restorative materials. And, given the high tensile
strength of dentin (Bowen and Rodriguez 1962), the progress made in
the last decade, and the currently recognized need for dentin as well
as enamel bonding, it is reasonable to expect that before the end of
this decade the intensive and extensive research efforts will succeed in
providing clinicians with completely satisfactory materials and
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methods for preventive and restorative dentistry by way of adhesive
bonding to both dentin and enamel through the reliable use of acid
treatments on both enamel and dentin.
ENAMEL
Enamel is the most highly calcified and hardest tissue of the
body. Enamel contains 96% inorganic portion and 4% organic
portion. Unlike dentin, cementum and bone, cells of ectodermal origin.
In the human tooth, the enamel normally forms a covering layer for
the whole of the crown, but varies considerably in thickness in
different parts of the crown (FIG .1).
Enamel is a composite material consisting of two phases:
1: Mineral.
2: Organic.
The mineral phase, an apatite calcium phosphate, is the major
component and accounts for the hardness of the tissue. The
properties of the mineral phase are modulated dramatically because it
is divided into microscopic whiskers or fibers known as crystals. The
crystals are cemented together by the organic phase, which is a matrix
of protein polymer. The composite resists brittle fracture far better
than does crystalline apatite alone.
Formation of Enamel
The long, thin, lathe like crystals that compose enamel are
oriented roughly perpendicular to its surface. These crystals grow in a
gel of protein matrix, which disappears to a large extent as the
crystals grow within it. Eventually, the protein matrix takes the form
of extremely thin layers, which both glue and separate the enamel
crystals. The basic orientation of the enamel crystals is perpendicular
to the tooth surface.
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This orientation results from their tendency to grow
perpendicularly to the surface on which they develop. The developing
surface is not simply flat, but is pitted by the secretory poles of the
ameloblasts. A good three-dimensional picture of the sub-microscopic
structure of enamel can be obtained by visualizing crystals
perpendicular to this peculiarly shaped, pitted surface.
However, it is probably of more significance and greater interest
to understand the discontinuities in the enamel structure, which
develop at the sharp concavities of the boundaries, or floors and walls,
of these pits. It is the arrangement of the crystals at the developing
surface that causes the discontinuities in crystal orientation, which
we know as the prism boundaries or junctions.
These locations acquire a more concentrated organic matrix
during maturation and in the adult tissue are distinguished by the
name “prism sheaths”.
Composition of Enamel
The enamel consists mainly inorganic material (96%) and only a
small amount of organic substance and water (4%). The inorganic
material is apatite. The nature of the organic constituents of enamel is
incompletely understood. In development and histological staining
reactions the enamel matrix resembles keratinizing epidermis. More
specific methods have revealed sulfydryl groups and other reactions
suggestive of keratin. (FIG. 2)
However chemical analysis of the matrix of mature enamel
indicate that the amino acid composition is not closely related to the
keratin and is distinctly different from collagen. Proteins can be
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isolated in several different fractions; they generally contain high
percentages of serine, glutamic acid and glycine.
Roentgen-ray diffraction studies reveal that the molecular
structure is typical of the group of proteins called cross-beta proteins.
In addition histochemical reactions have suggested that the enamel-
forming cells of developing teeth also contains a polysaccharide-
protein complex and that an acid mucopolysaccharide enters the
enamel itself at the time when calcification becomes a prominent
feature.
Tracer studies have indicated that the enamel of erupted teeth
of rhesus monkeys can transmit and exchange radioactive isotopes
originating from the saliva and the pulp. Considerable investigation is
still required to determine the normal physiologic characteristics and
the age changes that occur in the enamel.
Brudevold et al (1960) reported the inorganic components
inorganic components of enamel are principally apatite in its hydroxy,
fluoride or carbonate ions.
Minor variations occur in composition in which aluminium,
barium, magnesium, strontium, radium and vanadium among others
can be found in the lattice.
Structures of Enamel
Enamel Prism or Rod
The prism or rod is the fundamental structural unit of enamel,
each prism extends from its site of origin at the DEJ to the outer
enamel surface crystals of hydroxyapatite (FIG. 3). All enamel with few
exceptions (eg: very thin enamel) is made up of super assemblies of
these structures, combined with varying amounts of interprismatic
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material. Changes in the orientation of the crystals, relative to each
other, mark the boundaries of the prisms. In the human enamel, the
boundary of the prism body is incomplete cervically. Here the prism is
continuous with a wedge-shaped ‘tail’, which comparative studies
(BOYDE 1965) show to be interprismatic enamel. The combined shape
of the prism body and the tail is that of a keyhole (FIG. 4 and 5).
The body of the prism is approximately 5micron meter wide and
the prism plus tail keyhole is approximately 9micron meter long (FIG.
6). The apatite crystals are most closely packed in the prism bodies,
which occupy 60-65% v/v of enamel (Shellis 1984) when considered
(FIG. 7).
The configuration of enamel crystals is related to the
organization of the ameloblast and it’s tome’s processes. The forming
surface of enamel consists of pits, each defined by a wall made up of
newly formed interprismatic enamel. During active secretion, each of
these walled pits is occupied is occupied by a tomes processes. The
inter prismatic walls are formed slightly earlier than the prism
enamel, which constitutes the floors of the pits and are formed by
secretion sites at the ameloblasts peripheries.
The presumptive prism boundary is defined by the position of
the junction between the pit wall and the floor. In human enamel the
pit is at it’s deepest occlusally, and rises to become confluent with the
wall cervically, thus eliminating the boundary in this region. Each
wall (inter prismatic region) is formed as a cooperative effort by
adjacent secretory ameloblasts. Based on current knowledge of
enamel formation, it is clear that each ameloblast is responsible for
the formation of one prism at its central secretary site and a portion of
the surrounding inter prismatic region at its cooperative peripheral
sites. Inter prismatic enamel contains more enamel protein than the
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prism bodies, because the crystals meet at different angles and thus
cannot be packed as tightly together.
15
Fig 2. Composition of enamel
by volume percentage
Fig 1. Distribution of enamel (A. Dental
enamel covering anatomical crown, B.
Dentinoenamel junction, C. Cemento
enamel junction)
Fig 3. Individual enamel
rods inter digitizing with
neighboring rods
Fig 4. Orientation of
crystals in forming
rod head & tail
Fig 5. Orientation of enamel rods

The consistent arrangement of the inter prismatic enamel, with
its greater protein content, accounts for the fish scale appearance
observed in ground sections.
Due to its ultra-structural organization, enamel despite it’s
hardness and density- has appreciable porosity. The pore affects the
mechanical and optical properties of enamel; the formation of carious
lesions is strongly influenced by the pathways for diffusion and by
electro chemical effects arising from the charge on the pore wall.
The prism junctions or boundaries, which are the sites where
crystals of the tail region of one prism meet with those in the body of
another, are sites where there is an abrupt change in the crystal
orientation. Consequently, prism junctions have enlarged pores, filled
with matrix and hence increased porosity. (Hamilton et al 1973).
In human enamel the incomplete prism junctions form laminar
pores with curved cross-section running from the dentinoenamel
junction to the outer surface. In outer enamel the prism junctions
tend to separate, and thus exist as independent channels, whilst
those in inner enamel (especially in molars) interconnect to form a
three-dimensional network of laminar spores (Boyde 1989, Shellis
1996).
Enamel mineral is composed of relatively small crystals, the
arrangement of which results in internal pores that are small and
variable in form, orientation and distribution. Chromium soleplate
demineralization has been used to provide ultra structural
information on the distribution of matrix (Sundstrom and Zelander
1968) used this technique, and reported individual crystals with a
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coating of matrix. Matrix is more apparent in the tail region than in
the body region.
The material at prism junctions has a raised solubility (Shellis
1996), which may be due to the deposition of the mineral with
increased magnesium and carbonate content during amelogenesis,
leading to the formation of sites with defective, more soluble apatite
(Shellis 1996). The increased solubility at the prism junctions,
combined with faster diffusion in this region, accounts for the
demineralization pattern observed in advancing carious lesions.
At such lesions sites demineralization occurs preferentially via
these prism junctions and then spreads laterally into the inter
prismatic regions. While the largest pores in enamel are associated
with the prism junctions, they only contribute in a small way to the
total porosity, most of which is associated with prism bodies and tails.
Here, the pores exist as very narrow gaps between closely packed
crystals but some, while small, are elongated and tubule like and may
communicate with the prism junction pores only through narrow inter
crystalline pores.
Enamel rods follow a wavy, spiraling course, producing an
alternating arrangement for each group or layer of rods as they
change direction in progressing from the dentin toward the enamel
surface where they end a few micrometers short of tooth surface (FIG.
8) Enamel rods rarely run a straight radial course because it appears
there is an alternating clockwise and counterclockwise deviation of the
rods from the radial course at all levels of the crown. They initially
follow a curving path through one third of the enamel next to the
dentino-enamel junction. After that, the rods usually follow a more
direct path through the remaining two thirds of the enamel to the
enamel surface.
Boyde (1976) stated that the keyholes shape of the prisms in
cross section tends to prevent slip across prism boundaries under
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lateral shear. The keyhole configuration results from the unique
shape of the typical pit produced on the development surface by
ameloblast.
Gnarled Enamel
There are groups of enamel rods that may entwine with adjacent
groups of rods, and they follow a curving irregular path towards the
tooth surface. These comprise gnarled enamel, which occurs near the
cervical regions and the incisal and occlusal areas (FIG. 5) Gnarled
enamel is not subject to cleavage as is regular enamel. This type of
enamel formation does not yield readily to pressure of bladed, hand
cutting instruments in tooth preparation (FIG. 9)
Hunter Schreger Bands
The changes in the direction of the enamel prisms that
minimize cleavage in the axial direction produce an optical
appearance called Hunter Schreger bands (FIG.10 and 11). These
bands appear to be composed of alternate light and dark zones of
varying widths that are slightly different permeability and organic
content. These bands are found in different areas of each class of
teeth. Since the enamel rod orientation varies in each tooth, Hunter –
Schreger bands also have a variation in the number present in each
tooth. In the anterior they are located near the incisal surface. They
increase in the number and areas of the teeth from, the canines to the
premolars in the molars the bands occur from near the cervical region
to the cusp tips. The orientation of the enamel rod heads and tails and
gnarling of the enamel rods provide strength by resisting, distributing,
and dissipating impact forces. In the inner one-half to two thirds of
the enamel, curvature of the prisms is responsible for the formation of
HUNTER-SCHREGER BANDS. Each band consists of 10-13 prisms,
which in alternate bands are sectioned approximately longitudinally or
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approximately transversely. However the transition between alternate
bands is gradual.
19
Fig 6. Key hole shaped enamel
rods
Fig 7. Enamel rods in cross
section
Fig 8. Enamel rods appear
wavy in section of enamel
Fig 9. Gnarled
enamel
Fig 10. Photomicrograph of
enamel illustrating
phenomenon of light & dark
bands (Hunter Shregar Bands)
Fig 11. Hunter – Shregar Bands
when enamel is viewed under
polarized light

Enamel Tufts
Enamel tufts are hypo-mineralized structures of enamel rods
and inter-rod substance that project between adjacent groups of
enamel rods from the dentino-enamel junction (FIG. 12 and 13) these
projections arise in the dentin, extend into the enamel in the direction
of the long axis of the crown, and may play a role in the spread of
dental caries. These regions are of high porosity, as they cut across
the prism structure, in which crystals are small and dispersed and
protein abundant (Orams et al 1976).
Enamel Lamellae
They are thin leaf faults between enamel rod groups that extend
from the enamel surface towards the dentino-enamel junction,
sometimes extending into the dentin (FIG. 12). They contain mostly
organic material, which is a weak area predisposing a tooth to the
entry of bacteria and dental caries (FIG. 14).
Enamel Spindles
Odontoblastic processes sometimes cross the dentino-enamel
junction into the enamel; these are termed enamel spindles when
their ends are thickened (FIG. 12). They may serve as pain receptors,
there by explaining the enamel sensitivity experienced by some
patients during tooth preparation (FIG. 15).
Incremental Lines of Enamel - Striae of Retzius
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Enamel rods are formed linearly by successive opposition of
enamel in discrete increments (FIG. 16). The resulting variations in
structure and mineralization are called the Incremental Striae of
retzius and can be considered growth rings (FIG. 12).
21
Fig 12. Photomicrograph exhibiting enamel tuft, enamel
lamellae, enamel spindle, striae of retzius, Dentino enamel
junction
Fig 13. Transmitted light micrograph of DE junction
showing enamel tufts
Fig 14. Enamel lamellae
Fig 15. Enamel spindles

In horizontal sections if the tooth, the Striae of Retzius appear
as concentric circles. In vertical sections, the lines transverse the
cuspal and incisal areas in a symmetric arc pattern descending
obliquely to the cervical region and terminating at the dentino-enamel
junction. When these circles are incomplete at the enamel surface, a
series of alternating grooves, called the imbrication lines of Pickerill,
are formed. The elevations between the groves are called Perikymata;
these are continuous around the tooth and usually lie parallel to the
cemento-enamel junction and each other.
The enamel of deciduous teeth develops partly before and partly
after birth. The boundary between the two portions of enamel in the
deciduous tooth is marked by an accentuated incremental line of
retzius, the neonatal line or neonatal ring (FIG. 17). It appears to be
the result of abrupt change in the environment and nutrition of the
newborn infant. The prenatal line is usually well developed than the
postnatal enamel. This is explained by the fact that the foetus
develops in a well-protected environment with an adequate supply of
all the essential materials, even at the expense of the mother.
In addition, it has been reported that there is locally increased
porosity at the incremental growth lines (Newman and Poole 1974).
As a result, enamel structure is altered along these lines and electron
microscopy has reveled a possible decrease in the number of crystals
in the striae. There is also increased porosity on the cross striations
(Boyde 1989), which are a pattern of periodic banding noted at 2-6
micron meter intervals along the length of the prisms, and which
represent the circadian variation in secretory activity of the
ameloblast. Shellis (1996) produced methacrylate replicas of some
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cross striations in inner enamel, but was unable to do so in outer
enamel, suggesting that the pores at most striations are very small or
inaccessible.
In cuspal enamel the prism curvature gives rise to a related but
often apparently more complicated appearance of gnarled enamel.
Bands in which the prisms run parallel with the section plane reflect
the light to a different degree compared with those in which the
prisms are perpendicular to the section plane (Silverstone 1982).
Because of the deviations in prism orientation, inner enamel is
relatively porous. It is thought that the relatively complicated prism
arrangement within the Hunter-Schreger bands to reduce the
propagation of fractures (Osborn 1968, Boyde 1989).
In the outer enamel, the prisms are straight and parallel in the
cuspal and lateral regions; so do not show Hunter-Shreger banding.
The angle at which prisms reach the surface varies with the
anatomical location on the tooth. At the cervical margin, the prisms
follow an undulating course and approach the surface at very variable
times acute angles (Boyde 1989). Occlusally different orientation is
noted, with prisms on the lateral surface of the crown being angled at
approximately 70°, whilst on the cuspal surface the angle returns to
approximately 90°.
Prism Shape and Crystal Orientation
The cross sectional appearance of prisms is by the inter-
relationship of prismatic and inters prismatic enamel (FIG. 18). Three
classical prism patterns have been defined, termed (1-3) (Boyde 1989).
Pattern 1
Is characterized by prisms with complete boundaries, separated
by well defined inter prismatic regions.
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Pattern 2
The prisms have incomplete outlines and are arranged in rows.
Within each rows narrow bridges of inter prismatic enamel separate
the rows.
Pattern 3
Is the structure observed in human enamel, containing
alternating prisms with horseshoe shaped boundaries.
Although pattern 3 is predominant in human enamel (Boyde
1989), the other patterns can be found in restricted areas. In
particular, pattern 1 enamel, occurs close to the dentinoenamel
junction and also near the outer surface i.e., in the enamel formed at
the beginning and end of the ameloblast life cycle.
Comparative studies show that there is no correlation between
prism pattern and incremental rate. In all the three patterns, crystals
in the inter prismatic regions are oriented approximately
perpendicular to the general forming surface (i.e., perpendicular to the
plane of the retzius lines), while the crystals within the prisms form
perpendicular to the floor of the Tome’s process pit.
In human enamel, this results in a gradual divergence of the
crystals in the tail region from the parallel intra prismatic
arrangement by angles of about 15°-45° in the cervical direction (Poole
and Brookes 1961). In pattern 2 enamel it results in a large angle
between the interprismatic crystals and those in the prism sheets.
This distinction between pattern 2 and pattern 3 is important because
of the widespread use of rodent and bovine enamel (pattern 2) in
dental research.
24

Crystal Size and Morphology
The crystals of mature enamel appear to grow and fill the bulk
of the space available within the prism. The apatite crystals
characteristically exhibit considerable irregularity of outline, but are
roughly hexagonal in cross-section, with a mean width of 68.3 nm and
mean thickness of 26.3nm. Many of the crystals in mature enamel
show evidence of crystallographic defects (Ichijo et al. 1993).
Aprismatic Enamel
Aprismatic enamel, up to 100-micron meter thick, has been
reported to be present at the surface of both permanent and
deciduous human enamel (Boyde 1989, Kodaka et al. 1989) (FIG. 19).
The thickness of aprismatic enamel varies both within and between
tooth types. Within aprismatic surface enamel, the crystals are
arranged parallel to each other and perpendicular to the surface,
although some deviation in crystal orientation, due to the presence of
remnants of prism boundaries, may be detectable in some areas
(Kodaka et al. 1989).
Because of the parallel alignment of crystals and the absence of
prism boundaries, the surface layer is generally more highly
mineralized than the subsurface enamel (Robinson et al 1971). This
relatively featureless layer is thought to be result from the loss of the
tomes ‘process by the ameloblast; thus the structural feature which
directs the deposition of crystal into prisms and interprismatic
material is lost, altering enamel structure as a consequence.
25

Dentino Enamel Junction
The interface of the enamel and dentin is called the
dentinoenamel junction (FIG 12). It is scalloped or wavy in outline,
with the crest of waves penetrating toward the enamel. The rounded
projections of the enamel fit into the shallow depressions of the
dentin. This inter digitations seems to contribute to affirm attachment
between dentin and enamel. The dentino-enamel junction is also a
hypo-mineralized zone about 30 micrometer thick (FIG. 20).
Enamel is incapable of repairing itself once destroyed because
the ameloblast cell degenerates following formation of the enamel rod.
The final act of the ameloblast cell is secretion of a membrane
covering the end of the enamel rod. This layer is referred to as
Nasmyth membrane, or the primary enamel cuticle. This membrane
covers the newly erupted tooth and is worn away by mastication and
cleaning. The membrane is replaced by an organic deposit called a
pellicle, which is a precipitate of salivary proteins. Microorganisms
may invade the pellicle to form bacterial plaque, a potential precursor
to dental disease.
26

DENTIN
28
Fig 16. Ground section of
enamel viewed under
transmitted light showing striae
of retzius
Fig 17. Photomicrograph showing
prenatal and post natal enamel in primary
teeth
Fig 18. Different prism patterns in
transverse section
Fig 19. Aprismatic enamel
Fig 20. The Scalloped appearance of dentino enamel
junction

Dentin provides the bulk and general form of the tooth and is
characterized as a hard tissue with tubules throughout the thickness.
It forms slightly before the enamel; it determines the shape of the
crown, including the cusps and ridges and the number and size of the
roots (FIG. 21). Along the crown, the dentin is covered by enamel,
along the root by cementum. It encloses the dental pulp, with which it
shares a common origin from the dental papilla. The dentin and pulp
can be considered as a single development and functional unit, often
described as pulpodentinal complex.
Dentin can be defined as porous biological composite composed
of apatite crystal filler particles in a collagen matrix (Pashley 1996).
The apatite crystallites are thought to provide strength, where as the
collagen matrix provides toughness.
Dentin contains dentinal tubules surrounded by highly
mineralized (95% volume mineral phase) intratubular dentin
embedded within a partially mineralized (30% volume mineral phase)
collagen matrix (inter tubular dentin) (Marshall et al. 1997).
The majority of tooth structure is composed of dentin, which is
the vital component of the tooth. When compared with the enamel
(Knoop hardness number KHN 343), dentin is much softer (KHN 68)
(Craig 1993), a characteristic explains why dentin exhibits much
faster wear. In addition the modulus of elasticity of enamel is
approximately 84 Gpa (Craig 1993) compared with a value of 13-17
Gpa reported for dentin.
Physical Properties
29

It is light yellow in colour and becomes darker with age and less
translucent. It is harder than bone and cementum but softer and less
brittle than enamel. Dentin has greater compressive strength and
tensile strength than enamel because it is traversed by tubules. The
dentin is readily permeable. Specific gravity – 2 .1g/ml. Dentin is
elastic and subject to slight deformation and acts as a shock absorber
to overlying enamel. The lower mineral salt content in dentin renders
it more radiolucent than enamel. Compressive strength of dentin - 40
– 50,000 PSI. Modules of resilience vital dentin – 100-140 LBS/Inch.
Modules of vital dentin – 1,90,000 psi.
Composition of Dentin
70% - In organic material
20% - Organic Materials
10% - Water
The inorganic substance consists of hydroxyapatite crystals and
small amount of phosphate, carbonates and sulfates (FIG. 23). The
organic substance consists of type-1, collagen containing 20% of
matrix with proteoglycans between the fibres.
Structure of Dentin
Dentinal Tubules
The dentinal matrix contains tubules, each or which ranges
from about 1 to 2micro meter in diameter at its outer end and 3 to
4micrometer at is pulpal side. The number of tubules are about
15,000 /mm2
near the dentinoenamel junction and it is 65,000mm2
near the pulpal surface.
The dentinal tubules are fine canals that extend across entire
width of the dentin. They contain odontoblastic process. The course
of the dentinal tubules follows a gentle curve, which is “S” Shaped.
30

They show two curvatures - primary curvature and secondary
curvatures (FIG. 22).
Primary curvature start at right angle from the pulpal surface,
the convexity of this curved course is directed towards the apex of the
root and the curvature in the outer half is directed towards the
occlusal or incisal surface. These tubules end perpendicular to the
dentino-enamel junction and cemento-dentinal junction. It is almost
straight at the root apex, incisal edges and cusps. Over their entire,
length, the tubules exhibit minute relatively regular secondary
curvatures (FIG. 24).
The fore most morphological characteristic of dentin is it’s
tubular branched structure the pulp to the dentino-enamel junction.
Under normal conditions the tubules are filled with fluid, may be
important in hydraulically transferring and relieving stresses imparted
to dentin through the supporting structures of the periodontium and
the enamel. Indeed this may explain why endodontically treated teeth
are more brittle than vital teeth. When isolated from the dentin, each
individual dentinal tubule would have the appearance of an inverted
cone; with the smallest dimension being recorded at the dentino-
enamel junction end the largest dimension adjacent to the cell body in
the pulp.
Canaliculi or Microtubules
The dentinal tubules have lateral branches throughout the
dentin termed as canaliculi. These canaliculi are1micrometer or less
in diameter and originate more or less at right angle to the main
tubule.
31
Fig 21. Structures seen in dentin Fig 22. S-shaped
dentinal tubules
Fig 23. Composition of dentin by volume
percentage
Fig 24. Dentinal tubules seen in longitudinal ground section showing
primary and secondary curvatures

Near the dentino-enamel junction, the dentinal tubules divide
into several terminal and form an inter communicating and
anastomosing network. Some dentinal tubules extend into the enamel
for several millimeters. These are formed as enamel spindles (FIG. 15).
Peritubular Dentin
The dentin that immediately surrounds the dentinal tubules is
called peritubular dentin. This dentin forms the walls of the tubules.
It is more highly mineralized about 9% than the intertubular dentin. It
is completely broken down and disappears on being subjected to
routine decalcification methods.
Intertubular dentin
The main body of the dentin is composed of intertubular dentin.
It is located between the dentinal tubules or between the zones of
peritubular dentin. Although it is highly mineralized this matrix, like
bone and cementum is retained after decalcification. About one half of
its volume is organic matrix, specially collagen fibres which are
randomly oriented around the dentinal tubules. The fibres have a
lattice like arrangement coursing in gentle curves between the tubules
and their peri-tubular zones. The fibres also exhibit cross-bonding.
Hydroxyapatite crystals are formed along the fibres.
Within each tubule is a collagen-deficient, hyper mineralized
layer of dentin, which has been termed as peritubular dentin, and
which may be more accurately termed periluminal (Pashley 1996) or
intratubular dentin, which is calcium deficient carbonate rich
hydroxyapatite.
The small crystals present have a higher crystallinity and are
five times harder than the intertubular dentin, with KHN of 250
33

compared with a KHN of 52 for intertubular dentin. The presence of
this intertubular dentin narrows the lumen of the tubule from its
original 3-µm to as little as 0.6- 0.8 µm in superficial dentin near the
dentino-enamel junction. The width of intratubular dentin decreases
in a pulpward direction, where there is a zone in which there is no
intratubular dentin present and the tubule (luminal) diameter is
approximately 3µm (Garberoglio and Brannstrom 1976).
There is little published information on the biological control of
intra tubular apposition, but it is known to be a slow process, slower
than the incremental formation of secondary dentin in the pulp
chamber.
Pre dentin
Predentin is located adjacent to the pulpal tissue and is 2µm to
6µm wide. It is the first formed dentin and is not mineralized. As the
collagen fibres undergo mineralization at the pre-dentin front, the
predentin then becomes dentin and a new layer of predentin forms
circumpulpally (FIG. 25).
Odontoblasts
The cells, which are related to the deposition of dentin, are the
odontoblasts. The odontoblasts are a layer of specialized cells, which
lie on the surface of the pulp against the internal surface of the
dentin. In a fully formed tooth, the odontoblasts are arranged at a
single layer of closely packed cells, which are pyriform, in shape. As
the cells are the cells are at different levels in the layer, on erroneous
impression of stratification results.
Each odontoblast possesses a long process (Tome’s Fibres),
which passes from the distal end of the cell into the substance of the
34

dentin where it is housed in a fine canal, the dentinal tubules. The
odontoblastic processes are largest in diameter near the pulp (3 to
4µm) and taper upto 1mm further into dentin (FIG. 26, 27 and 28).
Primary Dentin
The dentin that forms the initial shape of the tooth is called
primary dentin. It is usually completed three years after tooth
eruption. It consists of mantle and circumpulpal dentin (FIG. 29).
Mantle Dentin
Mantle dentin is the name of the first formed dentin in the
crown underlying the dentino-enamel junction. It is thus the outer (or)
most peripheral part of the primary dentin and it is about 20µm thick.
The fibrils formed in this zone are perpendicular to the dentino-
enamel junction and the organic matrix is composed of the collagen
fibrils (FIG. 30).
Circumpulpal Dentin
Circumpulpal dentin forms the remaining primary dentin or
bulk of the tooth. It is circumpulpal dentin that represents all of the
dentin formed prior to root completion. The fibrils in circumpulpal
dentin are much smaller in diameter and are more closely packed
together. The circumpulpal dentin may contain slightly more mineral
than mantle dentin.
35
Fig 25. Predentine
Fig 28. Extension of odontoblast
process in dentinal tubule
Fig 26. Odontogenic zone comprising
odontoblasts, cell rich zone, cell free
zone
Fig 27. SEM of deep dentin
showing odontoblastic process
Fig 29. Primary dentin and secondary
dentin
Fig 30. Histology of mantle

Secondary dentin is a continuation of primary dentin that forms
at a slower rate as the tooth ages physiologically. It is a narrow band
of dentin bordering the pulp and represents the dentin formed after
the root completion. Secondary dentin formation takes place without
any external stimuli. In secondary dentin, the tubules take a different
directional pattern in contrast to primary dentin (FIG. 29).
Incremental Lines
The incremental lines von ebner or imbrication lines appear as
fine lines (or) striations in dentin (FIG. 31). They run at right angles to
the dentinal tubules and correspond to the incremental lines in
enamel (or) bone. These lines reflect the daily rhythmic, recurrent
deposition of dentin matrix as well as hesitation in the daily formative
process. The distance between lines varies form 4 to 8µm. In the
crown to much less in the root. The course of the lines indicates the
growth pattern of the dentin.
Contour lines of Owen
Occasionally some of the incremental lines are accentuated
because of the disturbances in the matrix and mineralization process.
Such lines are readily demonstrated in ground sections and are
known as contour lines. The most consistently seen contour lines is at
the junction of the primary and secondary dentin (FIG. 32).
Neonatal Lines
In the deciduous teeth and in the first permanent molars, where
dentin is formed partly before and partly after birth, the prenatal and
postnatal are separated by an accentuated contour line. This is
termed as neonatal line and is seen in enamel and as well as dentin.
37

This line reflects the abrupt change in the environment that
occurs at birth. The dentin matrix formed prior to birth is usually of
better quality than that formed after birth and neonatal line may be a
zone of hypo-calcification (FIG. 33).
Inter Globular Dentin
Some times mineralization of dentin begins in small globular
areas that fail to fuse into a homogenous mass. This results in zone of
hypo-mineralization between the globules. These zones are known as
interglobular dentin. Inter globular dentin forms in the crown of teeth
in the circumpulpal dentin just below the mantle dentin, and it
follows the incremental pattern (FIG. 34).
The dentinal tubules pass un-interruptedly through
interglobular dentin, thus demonstrating defects of mineralization and
not of matrix formation. In dry ground sections some of the
interglobular dentin black in transmitted light. However, spaces in
interglobular dentin are not believed to occur naturally.
Granular Layer
When dry ground section of the root dentin is visualized in
transmitted light, there is a zone adjacent to the cementum that
appears granular. This is known as tomes (or) granular layer (FIG.
35). This zone increases slightly in amount from the Cementoenamel
junction to the root apex and is believed to be caused by a coalescing
and looping of the terminal portions of the dentinal tubules. The
cause of development of this zone is probably similar to the branching
and beveling of the tubules at the dentinoenamel junctions.
38

39
Fig 32. Contour lines of
Owen
Fig 31. Von Ebner’s lines
Fig 33. Neonatal line in
dentin
Fig 34. Ground section of dentin
viewed under transmitted light
showing interglobular dentin
Fig 35. Ground section of dentin,
viewed under polarized light showing
granular layer

AGE AND FUNCTIONAL CHANGES
Reparative Dentin
Reparative dentin is formed by the replacement (or) secondary
odontoblast in response to irritation caused by attrition, abrasion,
erosion, trauma, dental caries, some operative procedures and other
irritants (FIG. 36 and 37). Reparative dentin is formed when Tomes
Process are cut within 1.5 mm from the pulp. The cut fibres die along
with the corresponding odontoblasts leaving dead tracts. New
odontoblasts are differentiated from mesenchymal cells of the pulp in
about 15 days and these replacement odontoblasts lay down the
reparative dentin.
Dead Tracts
This is a type of reaction dentin, which appears to result from
irritation of greater severity. The odontoblast process in the whole
length of the injured tubule degenerates and at the same time is
sealed off at the pulpal end by a deposit of reactionary dentin (FIG.
38).
In dried ground section of normal dentin the odontoblast
processes disintegrate and the empty tubules are filled with air. They
appear black in transmitted and white in reflected light. Loss of
odontoblast process may also occur in teeth containing vital pulp as a
result of caries, attrition, abrasion, cavity preparation (or) erosion,
(When the tomes process are cut more than 1.5mm). These areas
demonstrate decreased sensitivity and appear to a greater extent in
older teeth.
40

Sclerotic Dentin
Sclerotic dentin results from aging or mild irritation (such as
slowly advancing caries) and causes a change in the composition of
the primary dentin. The peritubular dentin becomes wider, gradually
filling the tubules with calcified material, progressing from the D.E.
Junction pulpally. These areas are harder, denser, less sensitive, and
more protective of the pulp against subsequent irritations (FIG. 39).
The deposition of intratubular dentin, as a result of ageing or in
response to attrition, results in a progressive reduction in the tubule
lumen, and if continued, obliterates the tubule. If this occurs in
several tubules in adjacent areas, the dentin assumes a glassy
appearance. The term used to describe this progressive deposition and
obliteration of the tubule is SCLEROSIS, resulting in sclerotic dentin.
This process begins in root dentin of 18 – year old premolars
without any external influence. It can therefore be assumed that this
is a physiological response and the occlusion of the tubules is
achieved by continued intratubular deposition. The mechanism by
which intratubular dentin is formed are poorly understood and three
possible mechanisms have been suggested (Torneck 1994).
Firstly, it has been suggested that there may be a passive
redistribution of mineral from inter tubular dentin into the tubules
around the pre-existing components of the tubule. Secondly there
may be an active response on the part of the odontoblast process,
resulting in an organic matrix that is actively mineralized as a result
of odontoblast activity. Finally, it has been suggested that the
odontoblast may produce an organic matrix that becomes mineralized
by redistribution of mineral from intertubular dentin, as in the first
case. In which ever way it is formed, the net result is that intratubular
dentin is deposited at the expense of the odontoblast process, which is
either retracted or shortened by the loss of it’s distal extremity.
41

42
Fig 37. Types of reparative dentin
Fig 36. Reparative dentin
Fig 38. Dead tracts - ground section of
dentin viewed under transmitted light Fig 39. Sclerotic dentin

The amount of sclerosed dentin increases with age and is most
frequently encountered in the apical third of the root. Sclerosis
reduces the permeability of dentin and thus may help prolong pulp
vitality.
Processes, which contribute to sclerotic dentin in the crown in
response to attrition and caries, may differ from the physiological
deposition of sclerotic (translucent) dentin in the root, which is age-
dependent and whose rate of deposition is not altered by attrition.
Although there is a little evidence in the literature, it is thought
sclerosis resulting from aging is physiological dentin sclerosis and
that resulting from mild irritation is reactive dentin sclerosis.
Eburnating dentin is a term referring to the outward portion of
reactive sclerotic dentin where slow caries has destroyed formerly
overlying tooth structure, leaving, a hard, darkened, cleanable
surface.
The refractive indices of dentin in which the tubules are
occluded are equalized and such areas become transparent.
Transparent (or) sclerotic dentin can be observed in the teeth of the
elderly people, especially in the roots.
Sclerotic dentin may also be found under slowly progressing
caries. Mineral density is greater in this area of dentin as shown both
by radiography and permeability studies. It appears transparent or
light in transmitted light and dark in reflected light.
43

INNERVATIONS OF DENTIN
Intertubular nerves
Dentinal tubules contain numerous nerve endings in the
predentin and inner dentin no further than 100 to 150µm from the
pulp. Most of these small vesiculates endings are located in the
tubules in the coronal zone, specifically in the pulp horns. The nerves
and their terminals are found in close association with the
odontoblast process within the tubule (FIG. 28).
Nerve grows into the papilla in the bell stage of tooth
development (Byers 1980) both afferent neurons and efferent
automatic nerves that innervate pulpal blood vessels are present. The
number of myelinated axons in permanent teeth increases with age
and /or tooth development, reaching a plateau value of about 500
myelinated axons per human premolar at age 15, which remains
constant upto 60 years.
There may be single terminals or several dilated and constricted
portion. In either case, the nerve endings are packed with small
vesicles, either electron dense or lucent, which probably depends on
whether there as been discharge of their neuro transmitter substance.
In any case, they interdigitate with the odontoblast process, indicating
an intimate relationship to this cell. It is believed that most of these
are terminal processes of the myelinated nerve fibres of the dental
pulp. The primary afferent somato sensory nerves of the dentin and
pulp project to the main sensory nucleolus of the midbrain.
Extent of Odontoblastic Process
During tooth development, at the bell stage, odontoblast
processes extend from the odontoblast cell body through predentin to
the dentino-enamel junction. As the thickness of dentin increases,
the cellular processes must elongate.
44

However, the true length of the processes in mature dentin, in
the absence of blood vessels or supporting cells, is an issue that is
open to debate (FIG. 28)
In human teeth, the thickness of dentin is about 3-3.5mm Such
that if an odontoblastic process were to pass the entire distance from
the pulpal border to the DEJ, then the volume of the cellular process
would be four fold larger than that of the cell body (Pashley 1996).
This difference in volume between the cell body and the process is
even greater if the situation with cuboidal or flattened odontoblasts is
considered, as seen in the root towards the apex. It is generally agreed
that the process of most odontoblasts is between 0.1 and 1.0mm
(Byers 1996).
The question of how far the odontoblast process penetrates
dentin is of vital importance when considering dentin sensitivity. If
odontoblasts were to participate directly in the sensitivity of dentin to
surface stimuli, then the stimuli must interact directly with the
process, which is unlikely to be the case.
Normally dentin is covered coronally with and on the root
surface by cementum. When these surface coverings are lost, dentin is
subjected to a variety of stimuli, including mechanical, chemical,
thermal and smaller mechanical stimuli to which intact teeth are
responsive. When exposed, it is proposed that the fluid filled tubules
allow minute fluid shifts across the dentin when exposed to thermal,
tactile, evaporative or osmotic stimuli. The effect of this is that
mechanoreceptors in the pulp are stimulated (Pashley 1996).
These fluid shifts can directly irritate odontoblasts, pulpal
nerves and sub odontoblastic blood vessels by applying large sheer
forces on their surface as the fluid streams through narrow spaces.
The effect of fluid shift on the release of neuro peptides has been
45

assessed (Kimberly and Byers 1988, Byers et al 1990, Byers 1996),
and results in the release of calcitonin gene related peptide (CGRP) or
substance p (SP) from the pulpal nerves to generate a local neurogenic
inflammatory condition.
Dentin Characteristics Change With Depth
Both primary and secondary dentin contains tubules. The
circumference of the dentin at the most peripheral part of the crown
or root is much greater than that of the final circumference of the pulp
chamber or root canal space this results in the odontoblasts being
much more crowded as they approach their final position, thus
leading to the appearance of a columnar layer of odontoblasts,
especially over the pulp horns. The convergence of odontoblasts
towards the pulp creates a unique structural organization, with
functional consequences. The convergence has been estimated to be
4:1.
The number of tubules per unit area and the radius of the
tubules increases in the direction from the dentino-enamel junction to
the pulp, thus the area occupied by tubule lumina also increases.
Pashley (1984) calculated the area occupied by tubule lumina at
the dentino-enamel junction to be approximately 1% of the total
surface area of the dentino-enamel junction and 22% of the pulp. As
this area is occupied by dentinal fluid, which is 95% water. (Pashley
1996), the surface area figures are also approximately equal to the
tubule water content of these regions.
Therefore, the water content or wetness of dentin increases 20
fold from superficial to deep dentin. This factor has clinical
implications; in terms of dentin bonding of restorative materials to
deep dentin the water competes with resin monomers for surface
collagen fibrils (Pashley and Carvalho 1997).
46

Fluid Flow
In clinical conditions there is an outward fluid flow across
exposed dentin in response to the low but positive pulpal tissue
pressure. The composition of this fluid is uncertain, but must have an
ion product of calcium and phosphate, which is above or near the
solubility product constants for a number of forms of calcium
phosphate (Pashley 1996).
This would in turn lead to the formation of mineral deposits in
dentinal tubules which have many forms (Mjor 1985), as the dentinal
fluid moves outwards, larger amounts of mineral ions are presented to
the walls of tubules than would occur in sealed tubules. Indeed,
Shellis (1994) used this principle to reduce the depth of
demineralization in vitro under stimulated caries forming conditions,
by using a supersaturated surrogate dentinal fluid, which was
perfused through the pulp chamber. When examined microscopically,
translucent bands resembling sclerotic dentin were sometimes
observed.
Clinically, patients who complain of dentin sensitivity report
that a cold stimulus elicits a greater response than evaporative, tactile
or osmotic stimulation (Orchardson and Collins 1987). Outward direct
fluid movement (in response to cold) is far more effective at activation
pulpal mechanoreceptors than is the inward movement of fluid (seen
following a hot stimulus).
Dentin Permeability
The structure of dentin is tubular, as previously stated, and it is
this characteristic that provides the channels for the permeation of
solutes and channels for the permeation of solutes and solvents
across dentin.
47

The density of tubules per mm square varies from 15,000 at the
dentino-enamel junction to 65,000 at the pulp boundary be predicted
from tubule density and diameters, due to the presence of intra
tubular material such as collagen fibrils and mineralized constrictions
of the tubules (Pashley 1996).
Dentin permeability can be subdivided into two broad categories
(Pashley 1996):
Transdentinal movements of substances through the entire
thickness of dentin via dentinal tubules (such as fluid shifts in
response to hydro dynamic stimuli).
Intradentinal movement of exogenous substances into the
infiltration of hydrophilic adhesive resins into demineralized dentin
surfaces during resin bonding or demineralization of inter tubular
dentin by bacterially derived acids (Kinney et al 1995), where the
material enters the tubules but does not travel across the tubules.
The presence of the smear plugs and / or intra tubular deposits
(i.e. sclerotic dentin) is thought to lower intratubular permeability to
minimal values (Pashley et al 1991).
Dentin permeability (Transdentinal or intratubular) is not
uniform across the tooth. Coronal dentin permeability is much higher
than that of the root.
This can be attributed to the convergence of tubules towards the
pulp chamber, the tubule density increases about four fold in coronal
dentin, but only two fold in root dentin.
48

Thus, within any location on the tooth peripheral dentin has a
lower permeability than deeper dentin. The permeability of
intertubular dentin has never been quantified, but it must be very low
and limited to patent lateral canals that branch off from tubules
(Chappell et al 1994, Mjor and Nordahl 1996).
Numerous methods have been used to assess dentin
permeability (Pashley 1990). The easiest method of measuring trans
dentinal permeability is to quantify its hydraulic conductance. This
measures the ease with which fluid can filter across a unit surface
area of dentin in a unit time under a unit pressure gradient (Pashley
1990). It has been reported, in unobstructed dentin, that the
hydraulic conductance increases as dentin thickness decreases.
However, the presence of intratubular dentin and hence lowers its
permeability (Pashley 1996).
The structure of dentin makes it act both as a barrier and a
permeable structure, depending on its thickness, age and other
variables (Pashley and Pashley 1991). Dentin is very porous because
of its tubular structure and the minimum porosity of normal
peripheral coronal dentin is about 15000 tubules per square. If the
dentin is uncovered, then the tubules provide a diffusion channel from
the surface to the pulp.
The rate at which diffusional flux of exogenous material crosses
dentin to the pulp is highly dependent on dentin thickness and upon
the hydraulic conductance of dentin (Pashley 1985, 1990).
The Pulpo-Dentinal Complex
Dentin and pulp are embroyologically, histologically and
functionally united and there is much evidence to support the concept
of viewing the dentin and pulp as a functionally coupled unit, which
49

act as an integrated system. As soon as the tissues, which normally
cover dentin, are lost, then normal compartmentalization between the
tissues is lost (Pashley 1996) and they become functionally
continuous. The pulp responds to the stimuli generated by the loss of
dentinal covering, in the short term, by mounting an outward
movement of fluid (Vongsavan 1994, Mathews 1996) and
macromolecules (Byers 1996). The long-term response to the stimulus
is the production of tertiary dentin, which is a biological response to
reduce the permeability of the dentin of the dentin –pulp complex.
50

ACID ETCHING ON ENAMEL
The developed materials that adhere or bond to tooth structure
would minimize removal of healthy tissue, thus allowing a more
conservative preparation and providing for an impenetrable seal at the
margin between the tissue and restoration.
Criteria for Bonding
Three basic criteria necessary for bonding. The surface with
which the bonding is to occur should be:
1: Similar to the surface.
2: Free of contamination.
3: Smooth and uniform.
Steps In Acid Etch Technique
Enamel Prophylaxis
The mechanical cleaning of the enamel is an important first step
clinically in the bonding procedure. Maximum bond strength was
developed only when an oral prophylaxis was done before etching. An
examination of etched enamel surfaces not receiving an oral
prophylaxis shows pellicular remnants and microorganisms
contaminating the enamel. Clearly, acid alone cannot remove all
contaminants. This especially true of calculus and a careful inspection
should be made for the presence of this accretion which should be
removed by scaling.
51

Because there is concern for interference of flavoring oils,
glycerin and fluorides with the etching process, the use of watery
slurry of flow pumice has been recommended.
There is no significant difference in the retention rate of
sealants with or with out pre-etch pumice prophylaxis (Donnan and
Ball 1988). However, no clinical or laboratory evidence has been
presented to preclude the use of commercial pastes, even those
containing fluoride. Studies in 1980’s have showed no difference in
the clinical performance of a sealant whether fluoridated or non-
fluoridated toothpaste was used for the prophylaxis. Further research
is indicated.
Pellicle Removal
An oral prophylaxis should remove all gross deposits and
accretions from enamel, but it may not remove all integuments such
as subsurface pellicle. In addition, some protein may become smeared
over the surface during the prophylaxis.
Some of this pertinacious constituent may go into the solution
in the acid while the remainder may be floated away mechanically as
the phase of enamel is solubilized.
Application of Etchant (FIG. 40)
In the next step, with the teeth dried and properly isolated from
saliva, the acid is applied by one of several means including a cotton
pellet, brush or minisponge. The object is to gently agitate the acid for
a minute for maximal effect. This can be achieved using a gentle
swabbing motion. Clinical reports have suggested extending the
etching time upto 2 minutes in relatively high fluoride areas and
highly calcified mature enamel as for an adult.
52

53
Fig 40
Pre operative - silver amalgam
restoration
After cavity preparation
Acid Etching Rinsing with water
Blot excess water using mini
sponge or cotton
Application of bonding agent
Placement of composite restoration Finished composite restoration

It is important not to rub the enamel during acid application,
since burnishing the friable rods and their crystallites will reduce the
surface area available for bonding. This has been shown to reduce
bond strength. Scrubbing or rubbing may push the decalcified
material back into the pores that are being formed.
No apparent difference exists in the degree of irregularity after
etching acid solution compared with an acid gel. Gels provide better
control for restricting the etch area but may require more through
rinsing afterward. The most popular enamel / dentin etchant in
general dentistry is phosphoric acid blue gel. This gel is syringe
dispensed, as adequate colour contrast, smooth consistency and
almost ideal viscosity for application and rinsing off cleanly, and
provides and even, nicely demarcated white frosted appearance. This
etchant is recommended whenever extra good etching of enamel is
desired, such as deciduous teeth.
Studies and clinical experience indicate the 15 seconds is
probably adequate for etching most young permanent teeth. However,
individual variation exists in enamel solubility between patients,
between teeth, and with in the same tooth, and 30 to 60 seconds may
recommended for molars and adult teeth. Longer periods provide no
more, but actually less, retention because of loss of surface structure.
Caution should be exercised when etching over acquired and
developmental demineralization. It is best to avoid it. If this is
impossible a short etching time the applicant of the sealant, and the
use of direct bonding with extra attention to not having areas of
adhesive deficiency are important. The presence of avoids, together
the poor oral hygiene, can lead to indelible staining of underlying
developmental white spots.
54

Washing
There is a significant increase in bond strength values when
enamel is washed for 60 secs compared to 15 secs. These observations
were made using phosphoric acid in concentration of 30% and lower.
The chemical composition of the rinsing solution did not affect the
bond strength. 1% potassium Chloride solution was found to improve
bond strength. The presence of contaminants in the post etch rinsing
solutions could adversely affect the composite bond strength. Given
the size of dentinal tubules any contaminant that is small enough to
penetrate or obstruct the flow of monomers into the dentinal tubules
may influence the process of polymerization and ultimately affect the
development of hybrid layer and potential bond strength. Significant
reduction in the bond strength was demonstrated when saline was
used as rinsing solution, due to the presence of ions, which interfered
with the formation of hybrid layer (Eric C. Sung et al 2002).
In clinical procedure involving the etching of dentin with
phosphoric acid, it requires complete removal of etchant and reaction
products that are formed on the etched dentin surface, as in complete
removal of reaction products will intervene with bond strength (Bates
et al 1982). At the end of etching period the etchant is rinsed off the
teeth with abatement water spray. A high-speed evacuator is strongly
recommended for increased efficiency in collecting the etchant - water
rinse and to reduce moisture contamination on teeth and Dri-Angles.
Salivary contamination of the etch must not be allowed (If it occur,
rinse with the water spray or re-etch for few seconds; the patient must
not rinse).
55

Drying
Next, the teeth are thoroughly dried with a moisture-and-oil-free
air source to obtain the well-known dull, frosty appearance. Teeth
that do not appear dull and frosty white should be re-etched. Cervical
enamel, because of its different morphology, usually looks somewhat
different from the centre and incisal portions of a sufficiently etched
tooth. It should not be re-etched in attempts to produce a uniform
appearance over the entire enamel surface.
Effects Of Etching On Enamel
A routine etching removes from 3 to 10µm of surface enamel.
Another 25µm reveals subtle histologic alterations, creating the
necessary mechanical interlocks. Deeper localized dissolution will
generally cause penetration to a depth of 100µm or more. Although
laboratory studies indicate that enamel alterations are largely (though
not completely) reversible, it can be stated that the overall effect of
applying etchant to healthy enamel is not detrimental. This is
augmented by the fact that normally enamel is from 1000 to 2000µm
thick. (except as it tapers toward the cervical margin), abrasive wear
of facial enamel is normal and proceeds at the rate of upto 2µm per
year, and facial surfaces are self-cleaning and not prone to caries. On
the other hand, caution should be exercised when etching damaged
teeth with exposed dentin, deep enamel cracks or external or internal
demineralization.
Pattern of Etching
Silverstone et al (1975) studied the morphological changes
produced on the acid etched enamel surface scanning electron
microscope. Exposure of human enamel to conditioning solutions
produces three basic etching patterns (FIG. 42a).
56

57
Fig 41. Etching pattern of enamel after acid etching
Fig 42a. Different types of etching pattern
Fig 42b. Acid etched enamel rod core
dissolved to greater extent than rod sheath
Type I Type II Type III

Type 1
Prism core material is preferentially removed, leaving the prism
peripheries relatively intact, resulting in a honeycomb appearance
(FIG. 42b). The average diameter of the hollowed prism cores
measures about 3µm. This pattern is most common of the three types
observed.
Type 2
The peripheral regions of the prism are dissolved preferentially,
leaving the prism cores relatively intact, resulting in a cobblestone
appearance.
Type 3
Etching pattern contains areas, which resembles both type 1
and type 2 along with some distinct areas where the pattern of etching
appears to be unrelated to the enamel prism morphology.
Studies with polarized light microscope showed that sound
enamel etched with phosphoric acid to be affected at 3 distinct levels
and may be described in terms of three specific zones (Silverstone
1974). A superficial etched zone, which is a narrow zone of enamel of
about 10 microns in depth that is removed by etching. A Qualitative
porous zone of about 20 microns in depth. It is rendered porous by the
acid attack and may be identified qualitatively using polarized light. A
Quantitative porous zone of about 20 microns depth that qualitatively
indistinguishable form adjacent enamel.
Enhancement of Enamel Porosity
Enamel is a porous tissue that contains approximately 0.1% to
0.2% by volume of space. Many of the pores communicate to allow for
transport of tissue fluid and ions in solution. Poole and his coworkers
(1961) showed that enamel behave like a molecular sieve, allowing
58

passage of only the smallest molecules comparable in size to that of
water. Acid etching enhances not only the size of the pores to permit
access of relatively large resin molecules, but does so far distances
approximately 20 to 30 micrometer in from the tissue surface.
Decreased concentration of phosphoric acid enhances porosity to
greater depths in the enamel. This observation holds significance for
the depth to which resin may penetrate into the tissue.
Antimicrobial Property of Etchants
Lsettembrine et al (1997) at the university college of dentistry
New York concluded that all phosphoric acid etchant materials tested
demonstrated antimicrobial activity against several bacteria commonly
found in the oral cavity. They also reiterated that addition of
antimicrobial agents to etchant or cavity preparation may not be
necessary given the antimicrobial activity of the etchant, if the current
bonding systems can provide and sustain sealed tooth restorative
interface.
Role Of Flourides In Etching Of Enamel
Enamel is soluble when exposed to an acid medium, but the
dissolution is not uniform. Solubility of enamel increases from, the
enamel surface to the dentino-enamel junction. When fluorides are
present during enamel formation or are topically applied to the enamel
surface, the solubility of the surface enamel is decreased. Flouride
concentration decrease towards the dentino-enamel junction.
Flouride additions can affect the chemical and physical properties of
the apatite mineral and influence the hardness, chemical reactivity,
and stability of enamel while preserving the apatite structures. Trace
amount of fluorides stabilize enamel by lowering acid solubility,
decreasing the rate of demineralization and enhancing the rate of
59

remineralization. Evidence also shows that topical fluorides alter the
oral bacterial flora, there by increasing resistance to dental caries.
It has been accepted to etch apparently normal enamel for 15
secs and enamel that shows signs of fluoridation for double that time
or more.
The use of prophylaxis pastes containing fluorides and topical
fluoride treatments prior to etching is slowly diminishing. There is
virtually no evidence that the fluoride incorporated in enamel prior to
etching will significantly interfere with etching or will significantly
affect bond strengths.
It is well known that even the fluoride-acquired from acidulated
topical fluoride solutions is poorly retained and easily removed under
oral conditions in a short time. There is no contraindication to the
use prior to etching because the fluoride from these agents including
acidulated or nonacidulated sodium fluoride and stannous fluoride,
will most likely find its way into the deep recesses of the pits and
fissures and benefit in sealing them.
This solid fluoride may conceivably be retained in the pits and
fissures even after etching. Once sealed in the fissure by a sealant,
the fluoride may gradually react with enamel (and perhaps with
dentin, which in accessible at the base of some fissures) to produce a
resistant tooth structure that can afford protection against caries even
when sealant application is no longer provided.
Fluorides should be avoided as part of the etching solution or
immediately prior to regular bonding. Studies have shown that
fluorides react with etched surface to produce reaction products that
may interfere with bonding.
These reaction products appear to interfere with optimal
adhesive penetration resulting in weaker bond and/or bonds that will
60

not survive as long under conditions of oral moisture. It should be
noted that washing away the acid conditioning solution with water
containing 1or 2 parts per million of fluoride is not expected to
interface with achieving high bonding strengths.
Uptake of Fluorides in Etched Enamel
Most important use of fluoride is after bonding procedures of all
types. During etching more enamel surface usually is intentionally or
unintentionally etched than is subsequently covered by adhesive,
such as inter proximal areas etched by acid spillover. Etched enamel
is highly reactive and readily combines with and better retains many
times more fluoride than a natural unetched enamel surface. The
large amount of fluoride thus acquired by etched but uncovered
enamel from a topical fluoride application, may confer on the etched
enamel surface a greater resistance to cavities, normally have this
capacity. Infact, it has been suggested that a mild acid etch
(independent of bonding procedures) be employed prior to application
of acidulated sodium fluoride in order to enhance the acquisitions and
retention of fluoride from this source.
Retief et al (1986) has stated that fluoride is not evenly
distributed through out enamel, the fluoride concentration follows a
negative exponential distribution being highest in the surface enamel.
The loss of fluoride rich surface enamel during the etching procedure
may make the enamel more caries susceptible in the oral
environment.
Factors Affecting Etching On Enamel
61

TIME
Increased Time Application
High fluoride content and primary teeth require longer etching
time. The increased etching time is needed to enhance the etching
pattern on enamel that is more a prismatic than that of permanent
enamel. Currently 15 sec, a sufficient time to produce a bond
equivalent to that produce by a 60 sec etching time is used routinely.
Shorter Etching Time
C.J. Guba et al (1994) highlighted that etching times and
etchant consistency were not critical to enamel bond strengths. It
yields acceptable bond strength. It conserves enamel and saves time.
They also found that on microscopic examination of a 10 secs
etch versus a 60 sec etch showed that etching the enamel for 10 sec
produced a very superficial etch compared to a very deep etch with a
60 sec etch. However this did not have significant impact on the
tensile strength. Though some researchers suggested that the etching
effect is reduced when the etching viscosity of the acid is high, this
study showed no significant difference as related to their viscosities.
ACID CONCENTRATION
An interesting and important phenomenon is the existence of an
inverse relationship between the etching effect of phosphoric acid and
it’s concentration. The phenomenon was first observed and reported in
1965 and subsequently confirmed by others. The same etch time
lower concentrations of acid tend to be more destructive of the enamel
than higher concentrations (FIG 43a to f).
62

Concentrations of phosphoric acid over 65% tend to show
minimal changes. The concentrations of acid, producing consistent,
63
Fig 43a. 10% H3PO4
Fig 43e. 36% H3PO4 for 30 secs
Fig 43f. 37% H3PO4
Fig 43b. 32% H3PO4
Fig 43c. 35% H3PO4
Fig 43d. 35% H3PO4 for 2 minutes
Etching pattern with various concentration of phosphoric acid (SEM)

more or less evenly distributed relatively deep etch pattern, appear to
be in the range of 30 to 50%. Bond strengths are greater with 30 to
50% acid concentration, the difference between their values and those
obtained on surfaces etched with 10 to 70% acid were not as great.
The higher concentration of acid may not produce a sufficient in depth
etch to provide adequate resin penetration (tag formation) and / or
sufficient bonding area to resist repeated long-term masticatory and
other dislodging stresses encountered in the oral environment.
In an in vitro study carried out on bovine substrate by
M.J.Shingi et al (2000) it was concluded that milder concentrations of
phosphoric acid or less aggressive acids could be used to pretreated
enamel for orthodontics adhesive systems and sealants if the diffusion
potential of applied monomers is high enough.
According to Unos (1996) depths of demineralization increased
by both acid concentration and conditioning times following a
logarithmic relationship.
Chow and Brown (1973) demonstrated that the application of
phosphoric acid solutions greater than 27% Phosphoric acid resulted
in the formation of monocalcium phosphate monohydrate while
dicalcium phosphate dehydrate was formed with phosphoric acid
concentrations less than 27%. The former product is readily soluble
and would be completely washed away in the clinical situations.
VARIATIONS IN ACID ETCHING METHODOLOGIES
64

Currently phosphoric acid is the acid of choice, but it is possible
that other acidic etching agents such as pyruvic acid may be used in
the future. A controversial issue, however, is the optimal
concentration of phosphoric acid. The most widely used
concentrations of phosphoric acid used in clinical practice exceed 30%
phosphoric acid. This is partly based on the findings the phase
diagram of the phosphoric acid ± calcium hydroxide ± water ternary
system. They demonstrated that the application of phosphoric acid
solutions greater than 27% phosphoric acid resulted in the formation
of monocalcium phosphate monohydrate. While dicalcium phosphate
dihydrate was formed with phosphoric acid concentrations less than
27% phosphoric acid. The former product is readily soluble and
would be completely washed away in the clinical situation, while the
latter product is less soluble. The reaction products, if not completely
removed after the etching procedure, may interfere with the bonding
of composite resins to etched enamel surfaces.
The effect of phosphoric acid concentration on the tensile bond
strength of a conventional composite resin to enamel surfaces etched
with 10, 20, 30, 40, 50, 60, and 70% phosphoric acid was determined.
The tensile bond strength to enamel surfaces etched with 70%
phosphoric acid was significantly lower than the bond strengths
recorded to enamel surfaces etched with other phosphoric acid
concentrations.
The application of a phosphoric acid etching solution to freshly
cut dentin may elicit a pulpal response. To prevent the flow of
phosphoric acid applied to the enamel walls of preparations to the
freshly exposed dentin at the floors of the preparations phosphoric
acid gels were recently introduced. The objective was to confine the
acid-etching agent to the intended site of application. It is
recommended that the etching agent should be applied to the enamel
surface using a dabbing action as opposed to rubbing. Another issue
65

that has not been resolved is the optimal duration of etching with
phosphoric acid.
It is surprising that some authors recommend that the etchant
should remain on the tooth surface for at least 60seconds to develop
an appropriate etched pattern. The etch duration is of particular
importance in acid etching enamel prior to the direct bonding of
orthodontic attachments, as it is practically impossible to confine the
bonding site. Fluoride is not evenly distributed in enamel but allows a
negative exponential distribution with fluoride concentration being in
the surface enamel.
The loss of fluoride rich enamel surface during prolonged
etching may make the adjacent enamel more susceptible to enamel
decalcification during orthodontic treatment. The reaction products
that are formed on the enamel surface after phosphoric acid etching
should be removed completely, as incomplete removal may interfere
with bond strength. Etched surface should be washed for at least 15
secs to remove the reaction products.
The tooth to be restored should be isolated with a rubber dam to
prevent saliva contamination prior to acid etching and the placement
of composite resin. It is generally recommended that saliva
contaminated etched enamel should be washed and retched. O’Brien
and others showed, however that it was not necessary to re-etch an
enamel surface contaminated briefly with saliva, as a thorough
washing of such a surface did not have a detrimental effect on bond
strength.
Buonocore M (1955) introduced a simple conservative technique
for bonding restorative resins to enamel. He placed a drop of self-
curing acrylic resin on the labial enamel surface of upper central
incisor of ten subjects. One surface was treated prior to resin
66

placement with 85% phosphoric acid for 30 seconds. He noted that
the acid conditioning of the enamel resulted in on uncontioned control
surfaces lasted less than 12 hours. After three decades of laboratory
and clinical research, Buonocore’s method is widely adopted and has
added a new and exciting technical dimension to the practice of
dentistry.
Gwinnett, Matsui and Buonocore (1969) suggested that
formation of resin tags was the primary attachment mechanism of
resin to phosphoric acid. Acid etching removes about 10 microns of
the enamel surface and creates a porous layer ranging from 5-50
microns deep.
When a low viscosity resin is applied, it flows into the micro
porosities and channels to this layer and polymerizes to form a micro
mechanical bond with the enamel.
Fusayama et al (1979) introduced an etching technique for both
the enamel and the dentin cavity wall using 37% phosphoric acid
followed by a dentin-bonding agent containing methacryloxyethyl
hydrogen phenyl phosphate (phenyl-P). This improved bond strength
greater extent and dentinal etching has become fairly common
practice in Japan. However, the concept of total etching only recently
has gained acceptance in the United States.
ACID CONDITIONING OF DENTIN
Any discussion of the effects of acid conditioning of dentin must
begin with the acid etching of enamel. This was first proposed by
67

Buonocore (1955) as an attempt to clean enamel, increase the
microscopic surface area for bonding, and infiltrate unfilled resins into
enamel porosities. Many investigators were alarmed at what was then
regarded as an unconventional and even reckless approach to the
problem.
Buonocore, Wilernan and Brudevold (1956) not only introduced
the acid etching of enamel to dentistry, they also were among the first
to attempt to bond resins to acid-etched (7% HCL, one minute) dentin.
Their success with acid etching of enamel led them to try to acid
etch dentin. Unlike enamel, when dentin is etched, its surface
becomes mineral -poor protein rich; and it tends to become wetter
(Brannstrom and Nordenvall 1977). Unfortunately, Buonocore and his
colleagues' success with dentin was never realized, because the
relatively crude resin materials that were available at that time would
not wet dentin very well. Buonocore, however, was very much aware
or the requirements for good bonding.
For clinical success, the conditioned dentin must be sealed to
prevent sensitivity and to prevent the pathology (Brannstrom, 1981)
associated with the increased permeability of the dentinal tubules.
Conditioning of dentin will be defined as any alteration of dentin done
after the creation of dentin cutting debris, termed the smear layer.
The objective of dentin conditioning is to create a surface
capable of micro-mechanical and possible chemical bonding to a
dentin-bonding agent.
Goals of acid conditioning of dentin
♦ Remove the intrinsic weakness of the smear layer to permit
bonding to underlying dentin.
68

♦ Demineralize the superficial dentin matrix to permit resin
infiltration into surface.
♦ Uncover both intertubular and peritubular dentin.
♦ Clean the dentin surface free of any biofilms.
It is important to define the purpose of the acid etching of
dentin so that once identified, these goals can be tested in a
systematic scientific manner.
As the smear layer is intrinsically weak, the first goal is to
loosen it or remove it so that subsequently placed adhesive resins can
interact with solid dentin adhesive resins can interact with solid
dentin matrix. Most smear layers are 1-2 µm thick; they are composed
of the cutting debris of the materialized tissue on which they lie. (Ruse
and Smith 1991)
The reason for acid etching is to demineralize the solid dentin
matrix (both intertubular and peritubular dentin) to increase the
porosity of the dentin. While this is analogous to why enamel is
etched, the porosities that are produced are of the order of 0.05 – 1-3
µm in peritubular dentin rather than the 5-7 µm diameter of enamel
prisms. Further, acid-etched enamel can be thoroughly dried, while
that foal is much more difficult in vital, normal dentin. Enamel
contains little protein that is at risk of being denatured by acid
treatment. Dissolving away hydroxyapatite mineral crystallites from
the collagen component of dentin matrix creates dentin porosities. The
crystals tend to stabilize collagen and prevent its denaturation.
There is a risk that the acid used to demineralize the dentin
may denature or weaken the collagen. As denatured proteins generally
change their dimensions, the pores may become smaller if the
collagen is denatured. This may interfere with subsequent resin
infiltration and prevent the formation of a hybrid layer (Nakabayashi,
69

Nakamura and Yasuda 1991). Another danger in the etching step is
that the demineralized zone may extend, for instance, 5 µm into the
dentin, while the resin infiltration may only extend 4 µm, leaving a 1
µm demineralized zone at the base of the hybrid layer that is unpro-
tected by mineral or resin and that may be structurally weak. If the
pulpodentin complex can re-mineralize this unprotected basal 1 µm of
demineralized dentin (Tatsumi, 1989; Tatsumi and others 1992), then
the layer may become as strong as normal dentin, rather than be a
zone of debonding that has been seen in vitro (Nakabayashi and
others 1991).
Another purpose of acid etching dentin is to clean the dentin
surface. Often dentin is inadvertently contaminated with blood during
the cavity preparation. Acid etchant, by dissolving most of the smear
layer, tend to float these biofilms on the dentin when it is rinsed. The
low pH of the etchant may also denature the plasma proteins and
hemoglobin. The purpose of acid etching may vary depending upon
the material. If the intention is simply to remove the smear layer but
leave the smear plugs in place, as when one uses glass-ionomer
cements, then short etching times with dilute acids would seem to be
indicated (Bowen 1978; Pashley and Others 1981; Hamlin and Others
1990a).
However, if one wants to create a resin hybrid layer
(Nakabayashi and Others 1991) in the dentin rather than on the
dentin, then one must demineralized more deeply and in the process,
removes smear plugs. This can still be accomplished using dilute
acids, but the etching time may have to be extended.
Effects Of Conditioning Of Dentin
The principal effects of conditioning of dentin may be classified as
a) Physical changes
b) Chemical changes
70

Physical changes
Increases or decreases in the thickness and morphology of
smear layer changes in the shape of dentinal tubules.
Chemical changes
a) Modification of the fraction of organic matter
b) Decalcification of the inorganic portion
Conditioning of dentin may be done by several means
1) Chemical
a) Acids
b) Calcium chelators
2) Thermal
a) Lasers
3) Mechanical
a) Abrasion
When dentin is cut for cavity preparation, the wrenched cutting
debris of the dentin forms a thin smear layer on the surface. It is also
driven into the dentinal tubule apertures displacing the odontoblast
process and forming a smear plug at a depth of less than 10-micron
meter. Etching dissolves the smear layer and part of the peritubular
dentin, leaving tapered cylindrical holes of that depth.
In an experiment on monkey, dentin wall demineralized with a
phosphoric acid jelly etchant for 60 sec was completely re-mineralized
after 4 months. This results indicates that etching did not result in
deleterious effect upon either the collagen fibers or the odontoblast
processes, because the presence of collagen fibers maintaining their
proper cross bonded structure as a base for apatite crystals to attach
71

to and of the vital odontoblast processes to supply the calcium
phosphate from the pulp is essential for remineralization of dentin.
A.J.Gwinnett and M.D.Jendresen (1978) have concluded from
their experiments and observations that the surface of acid
conditioned eroded dentin is significantly different from that of acid
conditioned normal dentin. They further observed the depth of
penetration of resin is also less in acid treated eroded dentin where
many tubules remain partially occluded by intratubular insoluble
deposits.
Ruse and Smith (1991) found when common conditioning
agents were used, it has been found by X-ray photo electron
microscopy that the outermost surface contains only 10% or less of
the calcium and phosphorus initially present. They concluded that the
treatment of dentin with acidic conditioners leaves the surface so
depleted of calcium and enriched by organic residues that
subsequently placed bonding systems should be based upon agents
able to interact with organic components of dentin. Bonding agents
that rely on chelation to calcium are unlikely to be successful when
applied to acid etched dentin unless they penetrate into the
demineralized matrix to reach normal, mineralized dentin.
Acid etching of dentin is not harmless but represents one more
source of acute irritation to the pulpodentin complex in addition to the
vibratory, thermal, mechanical and evaporative stimuli that
accompany cavity a preparation. However, it is not as irritating as has
been previously thought.
Nakabayashi (1982) introduced the concept of hybridization.
The technique consists of applying an acid, ranging in concentration
from 10% to 30% to the surface of dentin. Within 15 minutes the acid
selectively dissolves away the inorganic component of the dentin to a
depth of 5 to 10 microns. It then flows in the dentinal tubule for up to
72

100 microns at which point it diffuses laterally into the peri-tubular
dentin for up to 10 microns.
As in the previous case the calcium component is selectively
eliminated. Then these spaces are replaced by an insoluble resin
component that completely encapsulates all exposed collagenous
fiber.
He also reported that dentin conditioning by citric acid
containing ferric chloride followed by a dentin bonding agent
containing 4 META (methacryloxyethyl trimellitate anhydride) was
effective method of dentinal bonding.
Concerning the bonding mechanism, he proposed that diffusion
and impregnation of monomers into the subsurface of pretreated
dentinal substrate and their polymerization, creating a hybrid layer of
resin reinforced dentin. This newly formed hybrid layer may be
thought of as an admixture of polymer and dentinal components,
creating a resin dentin composite. This technique not only enhances
the shear bond strength of the resin to the dentin but also increase
the potential against micro leakage and postoperative sensitivity.
Nakabayashi (1985) suggested that the acidic treatment
partially demineralized a zone of the dentin near the surface,
facilitating an infiltration process of compatible monomers. The
polymerized resin forms a reinforced zone of dentin on which a resin
based restorative material can be bonded. The bond strength is not
dependent upon interlocking at the dentinal tubules.
Kurosaki et al (1987) found that etching of dentin of the clinical
cavity floor allows the chemically adhesive composite resin to produce
resin tags of tapered, cylindrical or tubular form as well as
73

impregnated dentinal layers. These changes will considerably improve
the bond strength as well as the tubule aperture seal.
Surface Interactions Of Dentin Conditioners
Smear layer removal
One purpose of a dentin conditioner is removal of the smear
layer to provide a surface that is more suitable for adhesion; however
this does not necessarily apply to all systems. For example, the All-
Bond system can be used with the SA-HEMA conditioner, which is
weakly acidic and probably modifies the smear layer without removing
it, except where the smear layer is quite thin.
Dentin permeability changes
Removal of the smear plugs results in increased permeability of
the dentin, and the rate of removal by conditioner can be examined by
measurements of permeability increase for different application times.
This is controlled by the strength of the acid, it’s concentration, and
whether there are modifying components in the conditioning solution
(FIG. 44).
Because acid etching increases dentin permeability and dentin
wetness, successful bonding at adhesive resins to acid etched dentin
requires the use of hydrophilic resins that bond equally well to both
peritubular and intratubular dentin. Future trends seem to be toward
lowering both the concentration of acids and the time of etching
dentin.
Conditioner Application Time To Achieve Maximum Permeability
CONDITIONER Time (sec)
74

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acid etching/ rotary endodontic courses by indian dental academy