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CONTENTS
INTRODUCTION
HISTORY
DEFINITIONS OF SMEAR LAYER
SMEAR LAYER IN CONSERVATIVE DENTISTRY AND ITS
IMPLICATIONS
PHYSIOLOGICAL CONSIDERATIONS
PATHOLOGICAL AND TREATMENT
CONSIDERATIONS
FUNCTIONAL IMPLICATIONS
SMEAR LAYER IN ENDODONTICS AND ITS
IMPLICATIONS
REMOVAL OF SMEAR LAYER
BONDING AND THE SMEAR LAYER
CONCLUSION
REFERENCES
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MORPHOLOGICAL CONSIDERATIONS
Literature Review: The earliest studies on the effects of various instruments on dental tissues were
those reported by Lammie and Draycott (1952) and Street (1953). After the use of
different burs and abrasive stones, these authors, using powdered graphite, disclosed
ridges and troughs on the cut surfaces. Viewed with a light microscope, the pattern
and magnitude of the grooves varied, with diamond abrasives producing the most
striking anomalies. This technique for disclosing the topographical dental has a
significant limitation, namely, that the powdered graphite weight tends to obscure
more surface detail than it would highlight.
Peyton and Mortell (1956) understood this problem and substituted a thin
metal coating for the graphite. Employing a technique of metal vaporization described
by Scott and Wyckoff (1946), they deposited copper on cut surfaces of teeth and
examined them with reflected light microscopy. Significant differences were noted
between burs and stones, through different speeds, with and without coolant,
produced no notable differences.
Diamonds produced relatively deep and uniform grooves where as burs
showed less evidence of grooves and tendency toward nonuniform, uneven cutting.
Scott and O’ Neil (1961) reported a transmission electron microscope study
that a major advance was made in the description of the morphological detail of cut
surfaces of teeth. They observed the microscopic anomalies lest from the action of the
tool and found no marked differences in surface texture with different instruments.
Repeated replication of the surfaces with collodial continued to extract cutting
debris, identified as apatite by electron diffraction. While the prismatic structure of
enamel was recorded in replicas, cut surfaces of dentin were usually irregular and
without any evidence for the tubular nature of this tissue. This study was conducted
during the advent of research into adhesive restorative materials. In this context, and
importantly, Master (1961) and Skinner (1961) emphasized Scott and O’ Neil’s
conclusion cut surfaces of teeth is a key to formulating adhesive restorative systems.
Boyde, Switsur and Stewart (1963) appear to have been among the first to
describe in greater detail, using SEM, the nature of the surface deposits insitu, which
Scott and O’ Neil (1961) removed with their replication procedures. Boyde and his
coworkers also appear to have presence of what they called a “swear layer” on
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surfaces of cut enamel. Such a layer was readily removed with sodium hypochlorite,
leading then to conclude that an organic layer containing apatite particles was
deposited or smeared on the enamel through frictional heat generated during cutting.
They believed that heterogenous nature of enamel was the source of the smeared
components.
Boyde and his Coworkers (1963) attributed smearing of enamel to melting of
the tissue by frictional heat. Indeed, studies have shown that temperature will rise up
to 6000 C in dentin when it is cut without a coolant. This value is significantly lower
than the melting point of apatite (1500 – 18000 C) and has led most to conclude that
smearing is a physicochemical phenomenon rather than a thermal transformation of
apatite involving mechanical shearing and thermal degradation of the protein.
Smear layer (Definitions):
1) Smear Layer :
The smear layer is a layer of debris, comprising both organic and inorganic
components, found on canal walls after endodontic instrumentation.
(Endodontics in clinical practice 5th edition. HARTY’S).
2) Smear Layer:
When the tooth surface is altered by rotary and manual instrumentation during
cavity preparation, cutting debris, forming what is termed the smear layer.
Or
The smear layer has been defined as “any debris, calcific in nature, produced
by reduction or instrumentation of dentin, enamel or cementum”, or as a
“contaminant” that precludes interaction with the underlying pure tooth tissue.
(Fundamentals of OD) James B. Summitt.
3) Smear Layer:
When cutting or abrading procedures are applied to a dentin surface, an
amorphous layer of organic film and debris is created that has been termed the smear
layer.
(Principles and practice of operative dentistry (Charbeneau) 3rd edition.)
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4) Smear Layer :
The instrumentation involved in cutting a cavity preparation produces a
tenacious layer of debris, particularly on the dentin. This thin layer,
approximately 5 to 10 m, is referred to as the smear layer.
(Text book of operative Dentistry) 3rd edition. (Baum, Phillips, Lund).
5) Smear layer:
Cut dentinal surfaces are covered with a thin, deranged layer called the smear
layer.
(Minimally invasive Restorations with bounding) Michel Degrange.
6) Smear layer:
The cutting of both enamel and material, rich in calcium that is smeared over
the surface of the enamel and dentin is known as smear layer.
(Restorative dentistry an integrated approach) Peter Jacobsen.
7) Smear Layer:
A layer consists primarily of tooth debris, other contaminants such as plaque,
pellicle, saliva and possibly blood, which partially occludes the tubule orifices, when
dentin is cut or polished during dental treatment is known as smear layer.
(Preservation and Restoration of tooth structure) Graham J. Mount.
8) Smear layer:
A layer after scaling, abrasion, attrition, caries, and cavity preparation leaves
microcrystalline debris that extends slightly into the dentinal tubules, covers the
dentinal surface, and is usually several microns in thickness called as smear layer.
(Endodontic therapy 4th edition) Franklin S. Wine.
9) Smear layer:
The cutting of dentin during cavity preparation produces microcrystalline
grinding debris that coats the dentin and clogs the orifices of the dentinal tubules. This
layer of debris is termed the smear layer.
(Pathways of the pulp 8th edition) Cohen.
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SMEAR LAYER: PHYSIOLOGICAL CONSIDERATIONS DESCRIPTION
AND PRODUCTION:
Whenever dentin is cut with either a hand instrument or a rotary instrument, the
mineralized matrix shatters rather than being uniformly sheared or cleaved, producing
considerable quantities of cutting debris. Much of debris, made up of very small
particles of mineralized collagen matrix, is spread over the surface of the dentin to
form what has been called a ‘smear layer’ (Elick and Others, 1970). It is analogous to
wood being covered by wet sawdust. Although a similar phenomenon occurs in
enamel.
- The smear layer is absent from specimens of demineralized teeth examined by
light microscopy because the smear layer is dissolved during demineralization.
- When examined in undemineralized specimens by SEM the smear layer looks
like an amorphous, relatively smooth, featureless surface. Its constituents are
below the resolution of the SEM.
- TEM may provide important new information about the size of due particles
constituting the smear layer as well as their packing density and the
dimensions of the diffusion channels between the particles.
- The depth of smear layer varies widely depending upon whether the dentin is
cut dry or wet, the amount and composition of the irrigating solution used, the
size and shape of the cavity (or root canal), and the type of instrument
employed (Gilboe and others, 1980).
- Generally speaking, cutting without water spray generates a thicker layer of
debris (smear layer) than cutting with a copious spray of air and water.
- Further, coarse diamond burs tend to produce thicker smear layers than
carbide fissure burs (Brannstron, Hantz and Nordenvall, 1979, Shortall, 1981).
- Smear layer increases the resistance to movement of fluid across dentin discs,
both in vivo and invitro.
- The ease with which fluid could flow through etched dentin (dentin free of
smear layer) termed ‘hydraulic conductance’’.
- Brushing etched dentin with an orangewood stick decreased hydraulic
conductance 66 %. The use of a rotary rubber cup containing prophylaxsis
paste was even more effective at reducing hydraulic conductance. These
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pastes are much more abrasive than dentifrices and hence and far more
effective at creating a smear layer.
STRUCTURE:
The smear layer has anamorphous, irregular and granular appearance when –
viewed under SEM. McComb and Smith (1975) suggested that smear layer associates
with root canal treatment consisted of not only dentin as in coronal smear layer but
also remnants of odontoblastic processes, pulp tissue and bacteria. Hence it may
contain organic or inorganic material.
According to Cameron (1988) the smear layer on the walls of the root canal
could have a relatively high organic content in the early stages of instrumentation
because of necrotic and /or viable pulp tissue in root canals.
Eick et al (1970) showed that smear layer was made of tooth particles ranging
from less than 0.5 m to 15 m. Pashley et al (1988) found out that there particle
were also composed of globular subunits, approximately 0.05 to 0.01 m in diameter
which originated from mineralized fibers. The reported thickness of this layer is 1.5
m (Goldman et al 1981 ; Mader et al 1984). This thickness may depend on the type
of sharpness of cutting instrument and whether the dentin is cut dry or wet.
Cameron (1983) and Manderal et al (1984) described the smear material in two
parts. First, superficial smear layer and second, the smear material packed in to the
dentinal tubules. The extension of thus packed material into dentinal tubules was
calculated as extending up to 40 m. it was also calculated that this tubular packing
phenomenon of this meal layer was due to action of burs and endodontic instruments.
However penetration of smear material into dentinal tubular could be capillary
action as a result of adhesive forces between the dentinal tubular and smear material.
This hypothesis of capillary action may explain the packing phenomenon
observed by AK tense et al (1989) that showed that this penetration was increased
upto 110 m by use of surface reagents as working solution during endodontic
instrumentation.
FORMATION:
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The exact mechanism of the formation of the smear layer is incompletely
understood.
Boyde et al observed the smear layer using SEM and considered that frictional
heat during cavity preparation was the most important.
Various studies showed that temperature up to 6000 were obtained when
cutting was carried out uncooled. Apatite melts at 1500 – 18000. It would appear
therefore that smear formation is a physicochemical rather than a simple thermal
phenomenon. Plastic flow of hydroxy apatite may occur below its melting point.
Elrich and Koblitz et al considered that smear layers were formed by brittle
and ductile transition and alternating rupture and transfer of apatite and collagen
matrix on to the surface. Dentin (35 % collagen matrix) was a much richer source of
protein than enamel, dentin matrix may contribute to smear layer formed on enamel.
Gwinnett considered that smear layer was formed on enamel and dentin
surface by cutting when energy was expended. Frictional heat, plastic and elastic
deformation resulted in the alteration and deterioration of dentin. Contaminants
lowered the surface energy and hence reactivity. Frictional heat produced
temperatures well below the melting point of appetite, therefore swearing was
probably a physiochemical phenomenon, involving mechanical shearing and thermal
degradation of protein.
The exact composition and mechanisms of formation of smear layer,
particularly over dentin are therefore unknown. It would appear likely that
composition and attachment mechanism of the smear may vary with in one wall of
any cavity. There will be differences between the smear layer created in sound tooth,
preparation and that created during the instrumentation of carious dentin. Carious
dentin differs in size of its tubules and degree of mineralization from sound dentin.
These changes depend on the host response and the rate of progress of carious lesion
thro’ the dentin.
SIZE / THICKNESS:
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In one of the earliest studies –Eick et al described the smear layer as an
organic film less than 0.5 – 15 microns in size. It was present on all cut surfaces but
not necessarily in continuous layers.
In the current literature a thickness of 1 – 5 microns is commonly noted.
Pashly et al have observed rather thicker smear layers in their studies and give
a value of 10 – 15 microns.
ATTACHMENT TO DENTIN:
The exact mechanism of the attachment of the smear layer to the dentin
surface is poorly understood.
The degree of attachment of the smear layer to the underlying dentin is
variable. The smear layer may change the morphological and physicochemical
properties of the dentin surface and hence influence retention of restorative materials.
In some area it is firmly attached while in others it may lift free, according to
Gwinnett.
Several authors have attached adhesive materials to dentin covered with smear
layer and then in the course of measuring the bond strengths achieved observed that
mode of failure of the bond.
EFFECTS ON MOVEMENT OF FLUID IN AND OUT OF DENTINAL
TUBULES:
The smear layer will reduce dentin permeability and provide resistance to fluid
movement in dentin according to Pashely et al, the reduction of dentin.
Permeability may be an important factor of the pulp to a given restoration. The
movement of fluid in the dentin tubules is considered to be an important phenomenon
related to dentin sensitivity.
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Pashley distinguished between fluid movement inwards from the dentin
surface and outwards from the dentin tubules. He defined diffusion as the movement
of fluid from a high to a low concentration and gave the examples of microbial toxins
on the cavity floor moving inwards towards the pulp. The rate of such movement
varies with the square of radius. The defined pressure gradient in the tubules which
results in a tendency for fluid outflow from the tubule ends. This varies width 4 th
power of radius an hence is much more sensitive to reduction of diameter of tubules
as a result of smear form.
The smear layer given 86 % of the total resistance to fluid movement to
tubules towards the pulp. Pashley calculated that the diffusion surface is 1 % of the
dentin surface area at the DEJ and 22 %of dentin surface area near the pulp when the
dentin is etched.
Pashley postulated that if the smear layer is removed, diffusion is increased by
5 – 6 times. With the smear layer present the diffusion area is 1.7 % in tubules 1 mm
from pulp. Etching increase this to 7.9 %. The smear in this case had occluded 78.5 %
of the tubule orifices with debris.
POTENTIAL ADVANTAGES / DISADVANTAGES OF THE SMEAR LAYER:
The main advantages of the presence of a smear layer on dentin are :
1) Reduction of dentin permeability to toxins and oral fluids.
2) Reduction of diffusion of fluids prevents wetness of cut dentin surfaces
according to Brannstorm et al and Johnson et al.
3) Bacterial penetration of dentinal tubules is prevented.
The main disadvantages are:
1) It may harbour bacteria, either from original carious lesion or saliva which
may multiply taking nourishment from smear layer or dentinal fluid.
2) Smear layer is permeable to bacterial toxins.
3) The smear may prevent the adhesion of composite resin system, bonding
agent, GIC and poly carboxylate cements.
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SMEAR LAYER: PATHOLOGICAL AND TREATMENT
CONSIDERATIONS:
1) BACTERIA IN THE SMEAR LAYER UNDER RESTORATIONS:
The pathological consequences of the smear layer and whether it should be
present or absent under restorations are rather complicated questions. To a great
extent they seem to be related to the presence of bacteria under the restoration.
One question was: Is it possible that bacteria entrapped in the smear layer
survive and multiply under these restorations?
Brannstrom and Nyborg, 1973 done a study, those facial cavities were
prepared in 20 contralateral pairs of human premolars. One cavity, randomly selected
after preparation, was cleaned with water spray, while the other was cleaned with an
antiseptic detergent. Both cavities were then filled with composite and allowed to set.
In both teeth, the outer part of the filling was removed and replaced with zinc oxide
and eugenol or cavity cement. In this way we prevented the growth of bacteria into
the contraction gap between the resin and the cavity walls.
The teeth were extracted after three to six weeks. They were coded and
histologic evaluation was made by two observers.
The histologic evaluation revealed that in 17 of the water – cleaned cavities,
with the smear layer remaining, numerous bacteria were present; in the antiseptically
cleaned cavities, bacteria were absent. These results were highly significant and
showed that a few bacteria entrapped in the smear layer may survive and multiply.
There was also pulpal inflammation under these cavities.
The fact that bacteria may multiply on cavity walls even if there is no
appreciable communication to the oral cavity seems to indicate that certain
microorganisms get sufficient nourishment from the smear layer and dentinal fluid.
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The presence of a smear layer may also affect the retention of a lining and of
luting cements. Their retention is obtained mainly through mechanical interlocking
into microundercuts in the dentin. It is possible that the presence of a superficial
smear layer will weaken mechanical retention between the lining and the surface of
the cut dentin.
Major 1974 suggested that bacteria are not present in freshly prepare smear
layers, in vitro study.
On the other hand, in normal clinical procedures, especially when operating on
carious teeth, usually with low – speed or hand instruments in the final preparation,
we must consider the great risk of bacteria surviving in the smear layer. Bacteria may
even be left in the narrow gap between the enamel and dentin at the lateral walls, as
well as in single tubules in mineralized dentin underneath.
Bases of ZOE and eugenol and Ca (OH)2 may have good antiseptic effects but,
unfortunately, under permanent restorations these bases of Ca(OH)2, such as Dycal,
may disappear when leakage occurs, leaving a fluid space for bacteria to enter.
SMEAR LAYER ON DENTIN EXPOSED TO THE ORAL CAVITY :
Another question concerns what may happen to the smear layer on surfaces
exposed to the oral cavity and left unrestored e.g., in root planning, after superficial
grinding, or under poorly fitting temporary crowns.
When a smear layer is produced experimentally on human dentin, and
left exposed, it disappears after a couple of days and is replaced by bacteria, and
after a week almost all tubules are opened and some even widened.
There may be 10,000 – 20,000 tubules per square millimeter exposed
on a superficial, hypersensitive exposure.
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The consequence is the invasion of bacteria. Bacteria may plug the
apertures of the tubules. After two weeks, however, we have occasionally seen a
mineralized pellicle blocking the apertures of the tubules. (Brannstron, 1982).
2) REMOVAL OF THE SMEAR LAYER UNDER RESTORATIONS:
We cannot expect a mineralized pellicle to develop under a restoration where
saliva does not circulate. However outward flow of fluid in dentinal tubules and
around fillings may be reduced with time. The pulpal ends of the tubules may be
partly blocked by irregular dentin.
As reported by Pashley (1984), accumulation of solids in tubules and at their
outer apertures may contribute to a reduced flow of fluid. Under favourable
conditions a mineralized pellicle may develop at the outer aperture of the contraction
gap. The same has been observed in the apertures of tubules of cut dentin left
unprotected.
The smear layer may be detached and follow the outward flow of fluid in the
contraction gap. In a vital tooth this flow is directed outward due to the
pressure gradient – a higher pressure of fluid in the pulp. The size of the gap
around the restoration may vary from 5 to 20 m.
Certain bacteria may directly dissolve enamel and the highly mineralized
peritubular dentin and may remove at least parts of the smear layer.
Histologic sections sometimes reveal that the bacterial layer is closely oriented
to the surface of the cut dentin; the bacteria have, in other words, occupied due
smear layer.
Sometimes the whole bacterial layer is detached from the cavity and no
bacteria are seen in the dental tubules because of the presence of smear plugs
in the tubule apertures. Thus is one reason why we may not always find a
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correlation between pulpal inflammation and the presence of bacteria on
cavity walls.
3) THE PROTECTIVE EFFECT OF SMEAR PLUGS IN TUBULE
APERTURES AND THE CONSEQUENCE OF REMOVING THE PLUGS :
The degree of inflammation in the pulp seems to depend on the amount and
type of toxin, from both live and dead bacteria, reaching the pulp, rather than the
presence of bacteria with in the tubules. From opened tubules, bacteria many easily
reach the pulp and multiply. Therefore, removal of smear plugs should be avoided.
Pashley (1984) has also demonstrated that smear plugs reduce permeability of dentin.
Another important consequence of etching and the removal of smear plugs and
peritubular dentin at the surface is that the area of wet tubules may increase from
about 10 to 25 % of the total. Subsequently it is difficult to get the dentin dry because
fluid, continues to be supplied from below thro’ the tubules.
In sensitive dentin, the tubules are open all the way. It is better to keep them
occluded with disinfected smear and with peritubular dentin preserve at the surface.
The permeability is reduced and the cut dentin can be more easily desiccated with a
blast of air.
4) PULPAL IRRITATION DUE TO REMOVAL OF THE SMEAR LAYER
:
Application of 50 % citric acid or 37% phosphoric acid for even five seconds
is sufficient to remove smear plugs and peritubular dentin at the surface.
Acid etchants, detergents, a thin mix of phosphate cement, silicate, GIC, and
resins donot produce any appreciable damage and inflammation to the pulp, not even
when applied to exposed pulps (Brannstron, 1982, 1984).
But various acids and EDTA are capable of removing the smear layer but,
unfortunately, they also removed the smear plugs and peritubular dentin. Several
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investigations were performed to find a suitable cleanser that would retain the smear
plugs and remove only the superficial smear layer.
A detergent should remove the superficial smear layer, so that an antiseptic
component in the cleanser can reach and kill any bacteria present in smear plugs.
One acceptable solution contain a surfactant combined with 0.2 % EDTA and
benzyglkonium chloride to which 1 % sodium fluoride was added. Fluoride in this
concentration is antibacterial and gets a fluoride impregnation of cavity walls and
remaining smear plugs.
5) SMEAR LAYER IN ROOT CANALS AFTER REANSING :
The morphology of the canal wall is interesting. In adult teeth, the wall may be
partly covered with a tubular, irregular dentin and thus the tubules are blocked in the
same way as under erosion and abrasion. Infection may not be seen in the tubules in
such an area.
In young teeth, have large areas with primary dentin facing the root canal.
From a necrotic and infected canal, bacteria enter the dentin and can be found rather
deep in the tubules. Infected tubules with fluid communication to the exterior may
cause pathological complications such as external resorption of roots and periapical
pathosis.
In the treatment of infected roots there is a good reason to remove smear plugs
from the apertures of the tubules by using, for instance, EDTA. In this way the
bacteria with in the tubules at some distance can be more easily destroyed by an
intracanal dressing. On the other hand, if the asepsis or the sealing is poor, we may
run risk of reinfecting dentinal tubules opened and widened by treatment with EDTA.
The situation is similar to that for cavities.
The absence of superficial smear may facilitate good contact between the
sealing material and the wall of cut dentin.
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Tight seal is must to prevent the contamination of root canal from oral cavity.
FUNCTIONAL IMPLICATIONS
DENTAL MATERIALS:
Dental materials scientists have been concerned about the smear layer insofar
as it masks the underlying dentin matrix and may interfere with the bonding of
adhesive dental cements such as the polycarboxylates and glass ionomers being
developed, which may react chemically with the dentin matrix.
Dahl (1978) demonstrated that simply pumicing the dentin surface produced a
threefold increase in tensile strength of bond between dentin and polycarboxylate
cement, which relies strictly upon mechanical roughness for retention. Presumably
allowing cement to react chemically with the smear layer, rather than with the matrix
of sound intertubular dentin, produces a weaker bond due to the fact that the smear
layer can be torn away form the underlying matrix.
When the cements are applied to dentin covered with a smear layer and then
tested for tensile strength, the failure can be either adhesive (between cement and
smear layer) or cohesive (between constituents of the smear layer).
If one wants to increase the tensile strength of a cement – dentin interface
there are several approaches to due problem.
1) Remove the smear layer by etching with acid. This seemingly extreme
procedure does not injure the pulp, especially if dilute acids are used for short
periods of time. Etching dentin with 6 % citric acid for 60 seconds removes all of
the smear layer (and smear plugs) as does 15 seconds of etching with 37 %
phosphoric acid. The advantages are that smear layer is entirely removed, the
tubules are open and available for increased retention, and the surface collagen is
exposed for possible covalent linkages with new experimental primers for
cavities.
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Further, with the smear layer gone, one doesn’t have to worry about it
slowly dissolving under a leaking restoration or being removed by acid produced
by bacteria, leaving avoid between the cavity wall and the restoration, which
might permit bacterial colonization.
The disadvantage of removing the smear layer is that, in its absence, there
is no physical barrier to bacterial penetration of dentinal tubules. Further, with
nothing occluding the orifices of the tubules, the permeability of dentin increases
four to nine fold depending upon the size of the molecule.
2) Another entirely different approach would be to use resin that would infiltrate
through the entire thickness of the smear layer and either bond to the underlying
matrix or penetrate into tubules.
Smear layers on deep dentin may have more organic material in them than
those on superficial dentin. This may be due to the greater number of
odontoblastic processes or to the greater amount of proteoglycans lining the
tubules.
3) Another approach is to try to fix the smear layer with glutaraldehyde or
tanning agents such as tannic acid or ferric chloride.
The idea is to increase the cross linking of exposed collagen fibers with in
the smear layer and between it and the matrix of the underlying dentin to improve
its cohesion.
4) A fourth and most convenient approach to the problem is to remove the smear
layer by etching with acid and replace it with an artificial smear layer composed
of a crystalline precipitate (Causton and Johnson, 1982).
Bowen has used this approach by treating dentin with 5 % ferric oxalate,
which replaces the original smear layer with a new complex permitting extremely
high bond strengths to be produced between resin and dentin.
ENDODONTICS:
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The presence or absence of the smear layer is of interest not only to restorative
dentists; but to endodontists as well whenever dentin is filed, a smear is produced on
its surface (fig 3, pg –17). If a smear layer containing bacteria or bacterial products
were allowed to remain with in the pulp chamber or root canals, it might provide a
reservoir of potential irritants. The removal of smear layer from the dentin lining the
pulp chamber and root canals has been subject of numerous investigations.
Goldman and others (1982) recommend alternate use of NaOCl and EDTA to
remove smeared dentin. The sodium hypochlorite removes organic material, including
the collagenous matrix of dentin and EDTA removes the mineralized dentin, thereby
exposing more collagen. Such preparative treatment of root canals presumably
permits a better adaptation of obturating materials and sealers to the dentin.
Goldman’s group has recently demonstrated that removing the smear layer
from the root canal permits increased tensile strength of plastic posts. The increased
retention was associated with penetration of the resin into the open dentinal tubules.
(fig. 5).
PERIODONTICS:
Periodontics produce a smear layer on root dentin during deep scaling
or root planning. (Register (1973) found, empirically, that etching radicular
dentin with saturated citric acid facilitated reattachment following periodontal
flap surgery.
Register (1973), Register and Burdick (1975, 1976), Ririe, Crigger and
Selvig (1980), and Nalbandian and Cote (1982) have shown that this procedure
(etching with citric acid) stimulates cementogenesis and the subsequent
intertwining of collagenous fibers of the periodontal ligament with fibers of the
matrix of dentin or cementum. They also demonstrated that cementum did not
form as readily on dentin covered with a smear layer.
The those cases where repair did take place in the presence of a smear
layer, the cementum or periodontal fibers, or both, pulled away from the
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underlying dentin during histologic processing, indicating a very weak bond or
attachment.
Careful examination of published transmission electron micrographs
taken of mineralized sections of roots that were planed but not etched with acid
reveals the presence of a finely granular organic layer interposed between root
dentin and developing cementum. Authors have called it zone 3’ or granular
junctional cementum. It probably represents simply a fine, this, smear layer
created on the surface of radicular dentin during root planning. Its presence
clearly modifies local reaction of tissue in that it apparently inhibits attachment
of firm new connective tissue white permitting migration of the epithelium over
its surface.
Etching effectively removes the smear layer in addition to exposing
collagen fibers in the matrix of radicular dentin. Even after removal of the
mineral phase of the smear layer by saturated citric acid, there still remains an
organic smear layer, which may interfere with subsequent interdigitation of
collagen fibers of periodontal ligament and dentin matrix.
The organic smear layer is easily rubbed off with a cotton pellet and
this indicates how important it may be to standardize techniques of etching,
namely, specifying concentration of acid, time of exposure, time of rinsing,
dabbing, or rubbing, and so forth.
RESTORATIVE DENTISTRY:
Whenever castings are cemented into place, patients are asked to bite down on
a cotton roll or seating aid that places all of the masticatory force on that one tooth.
The maximum biting force that is comfortable for a patient is about 9-
12 kg in the incisor region and 200 kg in due molar region.
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If for the sake of simplicity, we assume that only 10 % of that
maximum force is concentrated on 1 cm2 of a molar crown, then the force per
unit area, that is, pressure, generated on and inside the casing would be 20 kg /
cm2. Since the cement is an incompressible liquid, it will transfer this pressure to
fluid on and in dentin. There is even danger that the cement may enter the
dentinal tubules before it sets, displacing an equal volume of dentinal fluid into
the pulp. This may be responsible for the pain that some unanesthetized patients
feel during cementation of crowns and can be explained by hydrodynamic
theory of dentin sensitivity. Thus, it may be movement of fluid per se, rather
than the acidity of the cement, that produces pain and pulpal irritation.
The pressure generated during the seating of castings can be even higher if
the surface area of cavity is smaller.
The case with which fluid can forced across dentin is formalized by a tern
called the hydraulic conductance (Lb). This term describes the volume of fluid
transported across a known area of surface per unit time under an gradient of
unit pressure.
The question of microleakage restorative material is beyond the scope.
It is worth mentioning however that there are atleast 2 or 3 routes by which
substances can leak into the pulp.
First, even if there were no gap between dentin and a restorative
material, bacterial products could theoretically diffuse around the material via
small channels and interstices with in the smear layer. Unfortunately, one cannot
perfectly adapt amalgam or any other restorative material to the walls of a
prepared cavity. Thus, there avoids and spaces between amalgam and dentin that
allow considerable microleakage.
Most clinicians use a cavity varnish or liner to seal dentin. These
organic films are placed on moist dentin, which, microscopically, has pools of
liquid on it, which produce an uneven layer of film of variable thickness and
permeability. Each layer provides potential routes for micro leakage.
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If one could produce a truly adhesive felling material that had no
shrinkage upon polymerization and a coefficient of thermal expansion close to
that of tooth structure, then one would want to remove the smear layer and omit
the use of any cavity liner or varnish that did not react chemically with both the
dentin and the resin.
INFLUENCE ON SENSITIVITY OF DENTIN:
Etching the dentin of roots, whether done therapeutically or by the
action of microorganisms of plaque, can rewove the thin lager of covering
cementum or smear layer, or both, there by exposing patent dentinal tubules to
the oral cavity. This can lead to sensitivity of dentin to the point where it
interferes with the patient’s oral hygiene. As movement fluid is central to the
hypothesis, several careful studies have been made of the most important
variables influencing movement of fluid through dentin. Theses studies indicate
that most of the resistance to the glove of fluid across deities is due to the
presence of the smear layer.
Etching dentin greatly increases the ease with which fluid can move
across dentin. This is accompanied clinically by increased sensitivity of dentin
to osmotic, thermal and tactile stimuli.
If dentin is sensitive, then according to the hydrodynamic theory of
dentin sensitivity; the dentinal tubules must be patent and must allow movement
of fluid across dentin. If fluid can move, it seems reasonable to assume that
bacterial products from plaque covering those surfaces of sensitive dentin may
also permeate dentin into the pulp. The presence of a smear layer will prevent
bacterial penetration of the tubules but will permit bacterial produce a mild, low
– grade inflammatory response that lowers the pain threshold in the affected
teeth, making them more sensitive than they would be in the absence of plaque.
INFLUENCE ON PERMEABILITY OF DENTIN
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The presence of a swear layer has a large influence on permeability of dentin.
Substances diffuse across dentin at a rate that is proportional to their
concentration gradient and the surface area available for diffusion.
The area available for diffusion in dentin is determined by the density of
dentinal tubules, that is, the number of tubules per square millimeter, and by
due diameter of these tubules. Both of these values vary as a function of
distance from the pulp chamber.
The actual area of diffusional surface is the product of tubule density
and the area of each tubule.
If one looks at the surface of a smear layer in SEM, one would predict
that it Wight be impermeable. However, experiments both in vitro and in vivo
have demonstrated that is topically labeled solutes of various molecular sizes
easily penetrate the smear layer
Removal of the smear layer by etching with acid increased the area of
diffusional surface of the tubules to 7.9%.
The Smear Layer in Endodontics
Researchers became aware of the endodontic smear layer about 1975.
Tidmarsh, in 1978, treated instrumented teeth with the use of 50% citric acid
and found the dentin to be generally clean of the smear layer and the
dentinal tubules wide open.
In endodontics, the smear layer results directly from the instrumentation
used to prepare the canal wall and are found only where the walls are
prepared and not in uninstrumented areas. The amount of smear layer
produced by automatic preparation will be greater in volume than that
produced by finger filing.
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Filing a canal without irrigation will produce a thicker layer of dentin debris
than similar situations in which a copious spray or constant canal irrigation
is used.
The presence or absence of the smear layer in endodontics is just as
important. When a canal is instrumented, the swear layer produced will
remain with in the canal and pulp chamber. The bacteria and bacterial
products found in the smear layer can provide a reservoir of potential
irritants.
Components of the Smear Layer
The exact proportionate composition of the endodontic smear layer has not been
determined, but SEM examinations have disclosed that its composition is both organic
and inorganic
The inorganic material in the smear layer is made up of troth structure and
some non specific inorganic contaminants.
The organic components may consist of heated coagulated proteins, necrotic
or viable pulp tissue and odontoblastic processes plus saliva, blood cells and
micro – organisms.
Once a root canal has been instrumented, the high magnification of the
electron microscope will disclose that the normal canal anatomy has been lost
by the instrumentation and that a thick smear layer has been found. The dentin
surface of the canal appears granular amorphous and irregular.
A profile view of the specimen may show in consistency , disclosing fine
particulate material, densely or loosely packed to various depths in to the
dentinal tubules.
Tubule packing is seen most often where less than half the ciramference of the
tubule has been fractured away.
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Instrumentation by other means than finger filings and irrigating solutions may
produce a packing phenomenon with a different appearance.
PHYSICAL BARRIER FOR BACTERIA AND DISINFECTANT
The advantage and disadvantages of smear layer and whether it should be
removed or not from the instrumented root canals is still controversial. The role of
smear layer acting as a physical barrier to bacteria and bacterial by products has been
supported by many researchers.
Vojinovic et al (1973) showed that dentinal plugs stopped bacterial invasion
into dentinal tubules.
Michelich et al (1980) and diamond and Carrell (1984) also stated that bacteria
could not penetrate into dentin in the presence of smear layer.
Conversely Baker et al (1975) and Yamada et al (1983) observed that bacteria
could remaining the smear layer and dentinal tubules despite instrumentation
of the root canal and thus they may survive and multiply and can grow into
dentinal tubules.
It has also been shown that removal of smear layer facilitated passive
penetration of bacteria. The extent of this bacterial invasion is dependent on
the type of bacterial species and on time.
William and gold man (1985) showed that swear layer delayed the penetration
of proteus vulgaris, but was not a complete barrier to this bacteria.
Smear layer itself is permeable even to large molecules such as albumin.
After degradation of the smear layer by proteolytic enzymes released by
certain bacteria a gap will develop between the filling material and the canal
Page 24
wall, permitting the leakage of other bacterial species and their by products
along the canal walls into dentinal tubules and the periradicular tissues.
When the root canal becomes heavily infected, bacteria may be found deep in
the dentinal tubules given after chemo mechanical instrumentation of the root
canal, some bacteria still remain in the canal and dentinal tubules. For this
reason, chemomechanical cleansing is often supported by the use of
disinfectant.
According to some authors (Goldbery and Abramovich 1977, Wayman et al
1979, Yamada et al 1983, and Madev 1987), The presence of the smear layer
may block the antimicrobial effects of inter canal disinfectants in to the
tubules.
In 1980, Happa concluded that the smear did delay, but not abolish the action
of the disinfectants. However, following the removal of smear layer, bacteria
in dentinal tubules can be easily destroyed and in this way, it may be
beneficial to use lower concentration and /or amounts of anti bacterial agent
since all of these agents show some degree of toxicity to viable host cell.
Apical Leakage
Plasticized GP can enter the dental tubules when the smear layer is absent.
This can establish a mechanical lock between the gutta perch and the canal
wall. Coupled with the increased surface area at the interface between filling
and canal wall, this lock should create an impermeable seal.
Thus, the use of injected thermoplastic zed GP should be accompanied by the
use of a sealer regardless of whether or not the smear layer has been removed.
On the other hand, Kennedy directly contraindicated most of Evan’s
conclusions. Kennedy stated that an absence of the smear layer causes less
apical leakage than GP –filled canals with the smear layer intact.
Page 25
He also stated that the use of a chelating agent on the smear layer would increase
apical leakage.
Furthermore, he stated that 7- day duration between instrumentation and
obturation allows for an increased amount of apical leakage. He concluded that
removal of smear layer would improve GP seals if the master cones are softened
with chloroform and used with a sealer and lateral condensation.
The greater the degree of canal preparation, the smatter the amount of
apical leakage.
With situations in which apical leakage existed in the presence of dentin plugs,
it must be concluded that the plugs were permeable. Their porosity allowed
them to fall short of the goal of creating a hermetic apical seal.
In addition to being porous, dentin plugs allowing leakage exhibited large
amounts of shrinkage. Scanning electron microscopic examination of
unsatisfactory apical plugs always showed marginal and structural defects.
Further considerations for advocating smear layer removal in endodontics
are the importance of creating a good apical plug and the effects the two
main types of sealers have on the canal walls.
SEALERS
Because of the bacterial content of the swear layer any apical extrusion of the
smear layer during instrumentation or obturation can defeat one of the goals of
endodontic therapy:
To be considered an ideal sealer, a material should not of itself cause or
further irritation in this tissue. Some root canal filling materials, especially N2 paste
and silver points, are not biocompatible.
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Endodontic sealers act as a glue to ensure a good adaptation of gutta percha to
the canal walls. If the smear layer is not removed, the GP may
occasionally be glued to the dentin in the smear layer as well as to exposed
parts of due canal wall. Not being firmly attached to the dentin, the smear
layer may laminate off the canal wall and create a false seal, voids in the fill,
and an expelled environment for microleakage.
Smear layer induced inflammation of the periapical area can be caused by over
instrumentation or by the careless measurement and fitting of a master cone.
The type of sealer used has different implications once the smear layer is
removed. A powder-liquid combination, the most common of which is
Grossmen’s sealer, contains small particles in the powder that could enter the
orifices of due dentinal tubules and help create a secure interface between
sealer and canal wall.
Ca(OH)2 based sealers have the advantage of promoting the apposition of
cementum at the canal apex and sealing it off against micro leakage.
Although Ca(OH)2 has dentin- regenerating properties, the formation of secondary
dentin along the canal wall is prevented by the absence of vital pulp tissue.
Dentin filling occur during instrumentation, but the formation of an apical
plug from them is often an inadvertent or accidental occurrence.
The use of some dentin- bonding agents to harden the smear layer to the canal
wall and to harden the apical plug is a subject for research. It is doubtful that
the bonding agent would be ant microbial to the bacteria in the smear layer.
If the smear layer is removed, the use of a Ca(OH)2 sealer will not promote
enough effect on bone to seal lateral canals. The calcium ion is used in the
formation of asteroid or dentoid type material. Circulation of blood (Which is
Page 27
absent in filled canals) is needed for the calcium ion to promote new tissue;
thus the Ca(OH)2 sealers are effective for sealing only at the root – apex)
POST CEMENTATION
Recent research has embraced the modalities of composite cements, GIC, and
dentin bonding agents, trying to sort these out, hoping to find a technique to
improve the tensile strenp the of cemented posts.
Removal of smear layer increase the cementation on bond and the
tensile strength of the cementing medium. GIC are effective in post
cementation after smear layer removal because the glass ionomer has a better
union with tooth structure.
These has been no significant difference between cements when the final
canal rinse was 2CC of 5.25% sod hypochlorite.
When an unfilled Bis-GMA resin was used after sod tryochlorite rinse, the
strength of the resin bond was better than that of poly carboxyl ate cement.
When the swear layer was removed by blushing with EDTA and Sod.
Hypochlorite rinse, the Bis-GMA resin flowed into the exposed dentinal
tubules and into the serrations on the post, vastly improving retention.
The use of a dentin bonding agent prior to cementing a post with a
composite cement or a GIC may or may not dictate removal of the smear
layer, depending upon which bonding agent is used or whether a GI is
used.
REMOVAL OF SMEAR LAYER
Smear layer removal is a controversy that fluctuates with the various modalities of
restorative dentistry. In operative dentistry, it may depend on the type of dentin
adhesive used or on the use of glass inomers.
Page 28
But, in endodontics, its removal is becoming unequivocal. In operative
techniques; the concept of removing most of the smear layer over the tubules is an
ideal that is difficult to achieve clinically because of the complex geometry of many
cavities and the difficulty of obtaining adequate success.
The most recent thinking veers toward retaining the smear layer even if it
limits the strength of dentin bonding agents because it is a natural cavity liner that
reduces the permeability of the dentin for more than any cavity varnish.
Bonding, or obturating to the smear layer must be considered a weak union
because the smear layer can be torn away from the underlying matrix. In endodontics,
once the layer is removed, a better adaptation of the obturating materials and sealers
becomes possible. Dentin permeation by diffusion is increased five to six times and
by convection 25 to 36 times. This attribute allows an improved penetration of
disinfecting agents, medicaments and obturating materials.
The smear layer’s presence plays a significant part in an increase or decrease
in apical leakage. Its absence makes adaptation of the Gp to the canal will be
significantly increased. Without the smear layer, the leakage will still occur but at a
decreased rate.
Some products, used singly or in combination, will remove it.
The quality of the smear layer removal will vary with the type of solvent
used.
The solvents-organic or inorganic-may or may not be effective when
used by themselves but their action may be enhanced when acting in
combination with another irrigant.
Neither hand nor automated instrumentation will provide a clean canal.
Instead of eliminating one. The character of the debris formed by hand
filing is granular in contrast with the automated formed debris that
appears finer and caked.
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Irrigating solutions have been used during and after instrumentation to
increase cutting efficacy of root canal instruments and to flush away
debris. The efficacy of the irrigating solution is dependent not only on
the chemical nature of the solution but also on the quantity and temp, the
contact time, the depth of penetration of irrigation needle, the type and
the guage of needle , the surface tension of irrigating solution and the age
of the solution (ingle 1985)
SODIUM HYPOCHLORITE
The organic tissue dissolving activity of NaOCl is well known and increases
with rising temp. However the capacity to remove smear layer from the instrumented
root canal walls has been found to be insufficient.
Many authors have concluded that use of NaOCl during or after
instrumentation produces superficial clean canal walls with smear layer present.
The alternate use of H202 and NaOCl solutions was often advocated in the
past.
MC Combe and Smith (1975) and Better (1989) showed that this combination
was not more effective in removing smear layer.
Adding surface active reagents to NaOCl to increase its action proved also
not to be effective (Camerson 1986)
CHELATING AGAENTS
The most common chelating solutions are based on Ethylene – Diamine tetra
acetic acid (EDDTA) which reacts with calcium ions in dentin and forms soluble
calcium chelates (Grossman et al 1988).
Nygaurd – Ostby (1963) found that EDTA decalcified dentin to a depth of
20-30 mm in 5min
Fraser (1974) stated that the chelating effect was almost negligible in the
apical third of the root canals.
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Different preparation of EDTA have been used as a root canal irrigant. In a
combination, urea peroxide was added to float the dentinal debris from the
root canal. However it appeared that despite further instrumentation and
irrigation, and irrigation, a residue of this mixture (RC-Prep) was left on the
canal walls.
A quaternary ammonium bromide (cetrimide) has been added to EDTA
solution to reduce surface tension and increase penetrability of the solutions.
MC-Combe and Smith (1975) reported that when this combination (REDTA)
was used during instrumentation, there was no smear layer except in the
apical part of the canal. After in –VIVO use of REDTA it was shown that
root canal surfaces were uniformly occupied by patent dentinal tubules with
very little superficial debris. When used during and after instrumentation,
remnants of odontoblastic processes could still be seen with in the tubules
even though there was no smear layer present. (Goldman et al 1981)
It was indicated that optimal working time of EDTA into root canal
was 15min and no more chelating action could be expected after this period.
Another root canal chelating agent is salvisol which is based on properties
similar to materials of the quarternary ammonium group and possess the
combined action of chelation and organic debridement.
One study smear layer removal by EGTA ethylene glycol –N,N,N’,N’ – tetra
acetic acid was done in 2000 by Semra Calt and Ahmet Serper.
They evaluated the effects of EGTA on removal of the smear layer on the
canal wall as an alternative to EDTA by using SEM. Smear layer was
removed from the instrumented root canals by irrigation with 17% EGTA or
17% EDTA, followed by 5% NaOCl. The effects were compared.
Root canals which were irrigated with EDTA followed by NaOCl, it
was observed that the smear layer was completely removed from the
instrumented root surfaces obtained from the middle and the apical third. Inter
Page 31
tubular and peritubular dentinal erosion was observed in the middle third of
the root canals. In some areas, this excessive erosion lead to conjugation of
two or more tubules that indicated the destructive effect of EDTA .
The combination of EGTA and NaOCl irrigation was effective in
removing the Smear layer from the dentin walls. In these specimens, dentinal
tubules seemed to be completely open to the canal surface, and they; were not
obscured by the smear layer in the middle third, this combination didn’t cause
erosion of the intertubular and peritubular dentin. But didn’t completely
remove the superficial smear layer in the apical third, and some of the dentinal
tubular orifices were clogged.
A chelator reacts with calcium ions in the hydroxyapatite crystals to
produce a metallic chelate. Removal of calcium ions from the dentin softens
the dentinal tissue, especially the hydroxyapatite – rich peritubular dentin and
increases the diameter of exposed dentinal tubules
The erosion of the exposed globular surface of the calcospherites and the
enlargement of orifices of the dentinal tubules probably resulted from the
alternating action of NaOCL, which dissolved the organic component of
the dentin, and EDTA, that demineralized the inorganic component. And
in some areas dentinal tubules were conjugated in some areas. However,
this effect was not observed during EGTA administration.
The main advantage of EGTA over EDTA is that is somewhat effective
in removing the smear layer without inducing erosive action.
EGTA was not as effective as EDTA in the important apical third. Further it is
not clear that the erosion and joining of orifices from EDTA action is deleterious.
These results seem to indicate that EDTA action is simply stronger than that of
EGTA.
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ORGANIC ACIDS
Citric acid appeared to be an effective root canal irrigant (Coel 1975) and
even more effective than NaOCl alone in removing the smear layers (Baumegartner et
al 1984). This acid removed smear layer better than many acids such as polyacrylic
acid, lactic acid and phosphoric acid except EDTA.
Wayman et al (1979) showed that canal walls treated with 10%,25% and
50% citric acid solutions were generally free of smeared appearance, but they had
the best results in removing smear layer with sequential use of 10% citric acid
solution and 2.5%. NaOCl solution, then again followed by 10% solutions of citric
acid.
However it was also observed that 25%. Citric acid, NaOCl group was not
as effective as 17% EDTA – NaOCl combination.
Besides Citric Acid left precipitated crystals in the root canal which might be
disadvantageous in root canal obturation. With 50% lactic acid, the canal walls were
generally clean, but the opening of dentinal tubules didn’t appear to be completely
patient.
Bitter (1989) introduced the use of 25%. Tannic acid solution as root canal
irrigant cleanser. It was demonstrated that the canal walls irrigated with this solution
appeared significantly cleaner and smoother than the walls treated with a combination
of H202 and NaOCl and that smear layer was removed.
REMOVAL OF SMEAR LAYER IN THE ROOT CANAL USING
OXIDATIVE POTENTIAL WATER
It is used as an irrigant for the efficacy of removing the smear layer.
Bactericidal and demineralizing effects have recently been noted to occur in
the tooth structure when OPW is used during dental treatment. Inoue et al investigated
the ability of OPW to etch the ground tooth surface for composite bonding. The
showed it could condition both enamel and dentin for bonding with composite resin.
Page 33
OPW has been developed in Japan and is defined as an electrolytically
obtained highly acid water having accumulated in the anode-containing compartment
after sodium chloride-added H20 has consumed OH-ions.
It constitutes the counterpart of alkaline water forming in the cathode-
containing compartment after the water therein has consumed H+ ions.
It kills Viruses as well as bacteria pH is 2.7 or less, and oxidation-reduction
potential as high as 1050MV or more in contrast to that of tap water.
It also has several activated oxygen-containing antimicrobial constituents,
such as HOCl and O3. OPW is safe enough for patients to hold in the oral
cavity.
ULTRASONICS
After the introduction of ultrasonic devices, the use of ultrasound was
investigated is endodontic. A continues flow of sodium hyprochlorite solution
activated by and ultra sound delivery system was used for the preparation and
irrigation of the root canal. It was observed that this method produced smear free root
canal surfaces.
Camerson (1988) showed that while conc. Of 2% to 4% NaOCl in
combination with ultrasonic energy, were able to remove smear layer, lower
concentrations of solution were unsatisfactory. However Ahmad et al (1987)
classified that their technique of modified ultra sonic instrumentation using 1%
NaOCl removed the debris and smear layer more effectively than the technique
recommended by Martin and Cunninghav (1983) .
Camerson (1983) also compared the effect of different ultrasonic irrigation
periods on removing smear layer and found that a 3 and 5 win irrigation produced
swear free canal walls while a l min irrigation was ineffective.
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OTHER AGENTS
Two commercial formulas from Sweden, Tubuliced Blue Label and Tubulicid
Red Label will remove most of the smear layer without affecting the smear plugs in
the dentinal tubules.
LASERS:
Lasers have been tried on tooth structure for several years.
The effects of lasers exposure on dentin and its potential application in
endodontics have been explored by a number of investigators.
Nd: YAG laser to irradiate root canal walls and showed melted, recrystallized,
and glazed surfaces.
Moshonov et al demonstrated that organ Laser irradiation of the root canal
system was efficient in removing intracanal debris.
Koba reported a histopathological and clinical examination of pulsed Nd:
YAG laser application to one-visit treatment of infected root canals.
Study done by Tomomi Harashima et al in 1998 checked the efficacy G Er:
YAG laser irradiation in removing debris and smear layer on root canal walls.
Er: YAG laser irradiation produced melted and sealed tubules, accompanied
by evaporation of the organic matrix, and could result in the reduction of fluid
permeability, sterilization of the contaminated root apex and a increased resistance to
root resorption.
Wigdor et al compared the thermal increase in teeth caused by exposure to
CO2, Nd:YAG and Er: YAG laser caused less thermal damage than either the
Nd:YAG or Co2 laser.
Morita and Koba reported that pulsed Nd: YAG laser had the capability of
evaporating debris and remnant pulp tissue pain wouldn’t occur. For this
purpose, the laser tip has to be improved.
JOE Vol 24; No8, Aug 1998
Page 35
Giromatic cleaning
Giromatic handpiece produces oscillation of root canal instruments through a
900 are. It has been reported to be an effective method for cleansing root canals
(Fromme and Gelttfit, 1972). However, numerous other structure shave indicated that
hand instrumentation was superior to the giromatic tech. Further more, hand
instrumentation caused lesser extrusion of debris through the apical foramen, a
possible factor in endo “flare ups” than the giromatic tech. In the final analysis,
neither hand nor giromatic instrumentation is capable of removing tissue in
irregularities, resorption lacunal and lateral canals.
BONDING AND THE SMEAR LAYER
In general, diamonds, through the introduction of grooved anomalies, produce
a greater surface area than buss. This has implications in bonding where differences in
the bond strength of resin attached to enamel have already been reported to be higher
for diamonds compared to burs.
The increased surface area probably offered a larger number of reaction or
retentive sites. These sites in enamel are primarily micromechanical and the retention
mechanism for this tissue lies in the multitude of superficial micropores enhanced
following acid conditioning of the tissue. Acids are among several agents that can
remove the smear layer. Ex; phosphoric acid in gel or solution in a concentration
ranging from 30 to 65% is the most popular agent.
The application of this agent to dentin removes the smear layer and by
dissolution of the peritubular dentin, the luman of the dentinal tubule is significantly
enlarged. When phosphoric acid removes the smear layer and enlarges the dentinal
tubules, it also appears to degrade the collagen matrix.
Some of the degradation products may be removed with water but the surface
of the acid-conditioned dentin appears relatively smooth with a gelatinous quality
even after a thorough lavage.
Page 36
Treatment with sodium hypochlorite brings about a significant morphological
change. It dissolves the organic material to produce a rougher texture to the surface,
which is dependent upon the time of application of this agent. When tubules are
exposed in longitudinal section, lateral of sodium hypochlorite.
In addition the composition of dentin and its surface following instrumentation
also dictates choice of treatment. We are presently pursuing different chemical
treatment.
The observation that smear layers could occlude the tubular structure of dentin
and bone was first made by Van Heuwenhock in 1677(O’Sulliuan and Flannelly
1990), although he did not call them smear layers. More recently dentinal smear layer
was described by Boyde et al.
The composition of smear layer was demonstrated by Eick et al (1970) to
consist of calcium and phosphate plug, organic material (containing sulfur, nitrogen
and carbon).
The composition of the smear layer reflects the composition of dentin from
which it is formed. Thus the smear layer is superficial normal dentin may have a
composition close to that of intertubular dentin, where as the composition of the
smear layer in deep dentin would reflect its lesser degree of mineralization. Similarly
smear layers created on caries affected tissue may contain collagen that has been
denatured by the action of proteo-lytic enzymes from cariogenic bacteria. Caries
affected dentin has also been found to contain whitelockite than normal dentin as has
sclerotic cervical dentin. Thus smear layer created on caries affected dentin and
sclerotic tissue may contain intratubular whitlockite.
It self – etching primers are used without the sensing step it is possible that the
demineralized collagen from the smear layer will remain on the demineralized
surface where it may become incorporated into the hybrid layer.
Dentin adhesives such as scotch bond dual cure (3m dental product), Dentin
Adhesiv (vivadent), Bondlite (kerr), Prisma Universal bond (Dentsply), didn’t
treat the smear layer with an acid to remove it prior to resin application.
Page 37
These adhesives were thus applied directly to the smear layer. They were not
very successful and bond strengths of 3-7 Mpa were commonly reported.
The interaction of bonding agents with smear layers thus deserves continued
consideration. Little is known regarding any correlation between the depth of resin
penetration into the smear layer and the resulting bond strength.
When manufacturers began adding HEMA to their bonding agents, shear bond
strengths increased from about 5Mpa to 10-12 Mpa. One presumption was that
the filled channels around the particles of grinding debris that make up the smear
layer.
The presence of smear plug into the dentinal tubule may provide anchorage of
smear layer to the underlying dentin matrix in a manner analogous to that of
epithelial projections in to connective tissue strengthening the dermal epidermal
interface.
If the smear layer is thin enough i.e., 1MM, or the ability of the bonding agent to
penetrate it slightly. This may lead to large bond strength from 10-12 Mpa to 20-
24 Mpa.
Gwinnett (1994 measured the bond strength of the all bond 2, Optibond (kerr)
and Scotch bond multipurpose dentin boning system to acid etched dentin before
and after treatment with 5% NaOCl to remove the exposed collagen fibrils.
Although the thickness of hybrid layer varied depending upon the bonding
system, no variance in bond strength was recorded.
Some investigators have advocated using an abrasive system to remove
the loose smear layer (Gwinnett 1994) with the hope that such treatment might
improve bond strengths.
Nikaido et al (1995) reported no change in resin enamel bond strength
following the abrasion of either substrate with sodium bicarbonate powder.
Page 38
Nakabayashi et al (1995) suggested using a polishing paste containing
hydroxyapatite particles so that the smear layer might be removed without
demineralizing the substrate. Both air abrasive system and such polishing
techniques would not been practical under many clinical conditions.
Thus appropriately treated smear layers and acid condition dentin surface will
likely remain the most clinically relevant surfaces for bonding.
The smear layer occupies a strategic position in restorative dentistry. It exists
at the interface of most restorative materials and the dentin matrix.
Our knowledge of the smear layer, its structure and function, is rapidly
growing and will influence all areas of clinical dentistry in the wear future. Much
work need to be done, but promise of greater understanding of the smear layer should
provide increased benefits thro’ improved dental therapy.
Smear Layer – Modifying Adhesives
Dentin adhesives that modify the smear layer are based on the concept that the
smear layer provides a natural barrier to the pulp, protecting it against bacterial
invasion and limiting the outflow of pulpal fluid that might impair bonding efficiency.
Efficient wetting and in situ polymerization of monomers infiltrated into the smear
layer are expected to reinforce the bonding of the smear layer to the underlying
dentinal surface, forming a micromechanical and perhaps chemical bond to the
underlying dentin. Most typical in this group are the primers that are applied before
the application of polyacid-modified resin composites, or compomers.
The interaction of these adhesives with dentin is very superficial, with only a
limited penetration of resin into the dentinal surface. This shallow interaction of the
adhesive system with dentin, without any collagen fibril exposure, confirms the weak
acidity of these smear layer- modifying primers. The dentinal tubules commonly
remain plugged by smear debris.
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Smear Layer – Modifying Adhesives
Most of today’s adhesive systems operated for a complete removal of the
smear layer, using a total – etch concept.
Their mechanism is principally based on the combined effect of hybridization and
formation of resin tags.
These systems are in their original configuration, applied in three consecutive
steps and subsequently categorized as three – step smear
Layer- removing systems has been reduce to two steps by combining the
primer with the adhesive resin in one solution.
Three – step adhesives
Advantages
i) Separate application of conditioner, primer and adhesive resin.
ii) Low technique sensitive.
iii) Proven effectiveness of adhesion to enamel and dentin in vitro and
in vivo.
iv) Most effective and consistent results.
v) Possibility for particle – filled adhesive (“shock absorber”)
Disadvantages
1) Risk of overetching dentin (highly concentrated phosphoric acid etchants).
2) Time – consuming three – step application procedure.
3) Post conditioning rinsing required.
4) Sensitive to over wet or over dry dentin surface conditions.
Two – step (“one – bottle”) adhesive
Advantages
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i) Basic features of three – step systems.
ii) Application procedure simple with one less step.
iii) Possibility for single – dose packaging.
iv) Consistent and stable composition.
v) Hygienic application (unidose, to prevent cross contamination)
vi) Possibility for particle- filled adhesive (“shock absorber”)
Disadvantages
i) Application not substantially faster (multiple layers)
ii) More technique sensitive (multiple layers)
iii) Risk of a too – thin bonding layer (no glossy film, no stress – relieving
“shock absorber”)
iv) Effects of total etch technique.
Risk of overetching dentin
Post conditioning sensing required.
Dentin – wetness sensitive.
v) Insufficient long – term clinical results.
SMEAR LAYER – DISSOLVING ADHESIVES
A simplified application procedure is also a feature of the smear layer – dissolving
adhesives or “self-etching primers.
These primers partially demineralize the smear layer and the underlying dentin
surface without removing the dissolved smear layer remnants or unplugging the
tubule orifices.
Concept of self – etching primers has already been introduced with an earlier
generation of scotch bond 2 – like systems, such as ART Bond, ecusit Primer –
Mono (DMG)and syntac. However, these systems are advocated for dentin
bonding only and, therefore, require selective enamel etching in a separate step.
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The current two – step smear layer – dissolving adhesives provide self –
etching primers for simultaneous conditioning and priming of both enamel and dentin.
The actual rational behind these systems are to superficially demineralize dentin
and to simultaneously penetrate it to the depth of demineralization with monomers
that can be polymerized in situ.
REFERENCES
1) Fundamentals of operative dentistry – a contemporary approach, JAMES B SUMMITT, 2nd
edition.
2) Seltzer and Bender’s Dental Pulp, KENNETH M HARGREAVES
3) Pulp dentin biology in restorative dentistry, IVAR A MJOR
4) Enododontics in clinical practice, HARTY’S , 5th edition.
5) Minimall invasive restorations with bonding, MICHEL DEGRANG
6) Introduction, Oper Dent, Suppl. 3, 1984 y
7) Textbook of Operative Dentistry, LIOYD BAUM, 3rd edition.
8) Pathways of the pulp, 8th edition, STEPHEN COHEN.
9) Endodontic therapy,4th edition, FRANKLIN S.WEINE.
10) Art and science of operative dentistry,4th edition, STURDEVANTS’
11) Modern practice (DCNA) Jan 2004 vol 48 no 1;pg 147
12) Efficacy of Er:YAG laser irradiation in removing debris and smear layer on root
canal walls. JOE, VOL 24 No 8 Aug 1998, 548 – 551.