Microleakage is a major cause of restorative failure and occurs due to microscopic gaps between filling materials and tooth structure. The smear layer, a byproduct of cavity preparation, may provide a pathway for microleakage along its granular constituents or through microchannels in its structure. While some studies found removal of the smear layer decreased microleakage, others found it increased permeability or had no effect. In endodontics, the smear layer's presence impacts the seal of root canal fillings and ability of sealers to adhere, but results are conflicting on whether its removal improves or worsens the seal. Overall, the smear layer's role in microleakage is complex and depends on the materials and techniques

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Microleakage in Restorative Dentistry:
Microleakage is a major cause of restorative failure. There are atleast two or
three routes by which substances can leak into the pulp. It can occur because there
are microscopic gaps at the interface of the filling material and the tooth. 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
within the smear layer.
Pashley et al (1989) observed an extensive reticular network of
microchannels around restorations that had been placed in cavities with smear
layer. The thickness of these channels was 1-10 m. Smear layer may thus present
a passage for substances to leak around or through its particles at the interface
between the filling material and the tooth structure. Pashley & Depew (1986)
found that microleakage decreased after the removal of the smear layer, but dentin
permeability increased.
Fig 40
Schematic representation of the interface of dentin and restorative material in a typical cavity.
The granular constituents of the smear layer have been exaggerated out of their normal
proportion for emphasis. Three theoretical routes for microleakage are indicated by arrows.

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Unfortunately, one cannot perfectly adapt amalgam or any other restorative
material to the walls of a prepared cavity. Thus, there are voids and spaces
between amalgam and dentin that allow considerable microleakage (Going,
1972). 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.
One wonders how well these films adapt to dentin and how well the restorative
material adapts to them. Each layer provides potential route for microleakage.
Jodaikin & Austin (1981) tested whether the smear layer restricts the
adaptation of freshly condensed dental amalgams to unvarnished tooth cavity
surfaces and whether the smear layer plays an essential role in the sealing
mechanism which develops around aged dental amalgam restorations. It was
thought that the smear layer may play a chemical role in the sealing mechanism
by providing a substrate (which interacts with the amalgam substrates or other
substances which migrate into the microcrevices at the amalgam-tooth interface)
or by providing an environment which is conducive to the initiation and
progression of the sealing mechanism. The smear layer may also play a physical
role by restricting the dentinal fluid from flushing the molecules which effect the
seal from the amalgam-tooth interface. It was found that all the short-term
restorations leaked and the long-term restorations demonstrated some sealing.
Thus the smear layer does not appear to restrict the adaptation of freshly
condensed amalgams to tooth cavity surfaces. The improved marginal seal,
which was obtained around the aged restoration margins of the etched cavities,
suggests that the smear layer may in fact, hinder the initial sealing process. The
reason for this maybe that the smear layer is unstable and leaches out and this
leaching process may tend to widen the amalgam-tooth microcrevice and
interfere with the sealing mechanism.
Srisawasdi et al (1988) evaluated the effect of removal of smear layer on
microleakage of Class V restorations using 3 restorative techniques: a composite
resin with its dentin bonding agent, a composite based with glass-ionomer lining

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cement, and a glass-ionomer restorative cement. 10% polyacrylic acid was used
for the removal of the smear layer. The results of the study favour the removal of
the smear layer only when a glass-ionomer liner is used under the composite
restoration. The microleakage of the glass-ionomer restorations was greater than
either the composite or composite based with glass-ionomer liner.
In a study done by Yu et al (1992) on a composite restorative material that
utilizes a smear layer-mediated dentinal bonding agent, it was found that
microleakage occurred at the smear-layer dentin interface and progressed into
both the smear layer and dentinal tubules, suggesting that the smear layer acts as
a pathway for microleakage.
Viewed in this theoretical, perspective, if one could produce a truly adhesive
filling 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.
Microleakage in Endodontics:
Another important consideration in endodontics is the ultimate seal of root
canals in order to prevent possible microleakage which may be the cause of the
future failure of the root fillings. Microleakage is defined as the passage of
bacteria, fluids and chemical substances between the root structure and fillings of
any type. It occurs because there are microscopic gaps at the interface of the
filling material and the tooth. Microleakage in root canals is a more complicated
subject as many variables may contribute such as anatomy and instrument size of
the root canal, irrigating solutions, root-filling techniques, physical and chemical
properties of the sealers, and the infectious state of the canal. Allen (1964), Ingle
(1976) and Strindberg (1956) have shown inadequate obturation of the root canal
system to be one of the major causes of endodontic failure. Ingle (1976)
determined that 63% of failures resulted form inadequate obturation. Cohen &

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Burns (1987) said that an inadequate seal at the apex accounts for 60% of failures
of root canal therapy.
However, coronal leakage is now considered to be a more important reason
for failure (Madison et al, 1987; Madison & Wilcox, 1988; Saunders &
Saunders, 1990). It has been shown that the quality of the permanent restoration
plays a significant role in the success of endodontically treated teeth, probably
more so than the quality of root canal filling. When the coronal portion of the root
canal system is exposed to oral flora, it may allow ingress of bacteria to the
periapical tissues. Coronal leakage provides a constant source of microorganisms
and nutrients that initiate and maintain periradicular inflammation and may very
well be the largest cause of failure of non-surgical endodontic therapy (Saunders
& Saunders, 1994). Since the path of leakage may be affected by the presence of
smear layer, it is seen that the smear layer could influence coronal leakage.
Prepared dentin surfaces should be very clean to increase sealing efficiency
of obturation (McComb & Smith, 1975; Combe, 1986). Smear layer on root
canal walls acts as an intermediate physical barrier and may interfere with
adhesion and penetration of sealers into dentinal tubules. Its absence makes the
dentin more conducive to a better and closer adaptation of the gutta percha to the
canal wall. Lester & Boyde (1977) found that ZOE-based root canal sealer failed
to enter into dentinal tubules in the presence of smear layer. In 2 consecutive
studies, White et al (1984, 1987) observed that plastic filling materials and
sealers penetrated into the dentinal tubules after removal of the smear layer.
Oksan et al (1993) also found that smear layer obstructed the penetration of
filling materials, and that penetration in smear-free groups ranged from 40-60 m.
It may be concluded that such tubular penetration may increase the interface
between the filling and the dentinal structures, and this process may improve the
ability of a filling material to prevent leakage (White et al, 1984). This
mechanical lock between the gutta percha and the canal wall, coupled with the
increased surface area at the interface between filling and canal wall, should
create an impermeable seal. Dye tests, however failed to substantiate this thesis

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when tested after the removal of the smear layer. The dye penetrated accessory
canals and spread laterally along the filling canal interface. Thus the injection of
thermoplasticized gutta percha should be accompanied by the use of a sealer
regardless of whether or not the smear layer has been removed. Follow-up dye
tests, with the smear layer intact and the use of sealer and lateral condensation,
showed no dye penetration.
If the aim is maximum penetration into the dentinal tubules to avoid
microleakage, root canal filling materials should be applied at a surface free of
smear layer and they should have a low surface activity (Aktener et al, 1989).
However, there has been no direct correlation between microleakage and
penetration of filling materials into dentinal tubules.
The presence or absence of a smear layer may play an important role in the
adhesiveness of some sealers to the root canal walls. Studies have shown a
significant increase in adhesive strength and resistance to microleakage of AH26
sealer when the smear layer was removed (Gettleman et al, 1991; Economides
et al, 1999). Gettleman et al (1991) did not find any change in adhesive strengths
when Sultan And Sealapex sealers were evaluated with or without the smear layer
intact. Several investigators have shown less dye leakage after removal of the
smear layer with various obturation techniques and root canal sealers. It was
found that 80% of obturated teeth will leak after 96 hours regardless of the
presence or removal of the smear layer (Goldman et al, 1986). With the smear
layer intact, apical leakage will be significantly increased. Without the smear
layer, leakage will still occur but at a decreased rate (Kennedy et al, 1986).
Kennedy et al (1986) also stated that the use of a chelating agent on the smear
layer would increase apical microleakage. Furthermore, he stated that 7-day
duration between instrumentation and obturation allows for an increased amount
of apical leakage. He concluded that removal of the smear layer would improve
gutta percha seals if the master cones were softened with chloroform and used
with a sealer and lateral condensation. Cergneux et al (1987) demonstrated that
when the smear layer was not eliminated there was a tendency for greater

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infiltration of dye. The complete elimination of the smear layer improved the seal
of the root canal obturation. Karagoz-Kucukay (1994) also showed by means of
an electrochemical technique, that the incidence of apical leakage reduced
significantly in the absence of the smear layer. Goya et al (2000) evaluated the
removal of smear layer at the apical stop by pulsed Nd:YAG laser irradiation with
or without black ink, and the degree of apical leakage after obturation in vitro.
Irradiation vaporized the debris and tissue remnant from root canal surface and
smear layer was evaporated, melted, fused and recrystallized. Root canal walls
were left clean. The results of this study suggest that pulsed Nd:YAG laser
irradiation with black ink increased the removal of the smear layer compared with
that without black ink, and reduces apical leakage after obturation significantly.
Several investigations done regarding coronal leakage also showed that
smear layer removal is beneficial and that it resulted in less leakage than those in
which smear layer was left intact (Saunders & Saunders, 1992 & 1994;
Vassiliadis et al, 1996; Taylor et al, 1997- as demonstrated by dye leakage
models). Leonard, Gutmann &Guo (1996) found that there was a significantly
better seal in both the apical and coronal directions when using a dentin bonding
agent and resin obturation material (C & B Metabond) following smear layer
removal and dentin conditioning with 10:3 citric acid-ferric chloride solution, as
compared to obturation using a glass-ionomer sealer. The C & B Metabond
interface revealed the presence of the characteristic hybrid layer along with
microtags of resin penetrating deep into the dentinal tubules. Behrend et al
(1996) and Clark-Holke et al (2003) showed that removal of smear layer reduced
the leakage of bacteria through the root canal system. De Souza et al (2005)
found that the use of procedures to remove the smear layer (17% EDTA or
Er:YAG laser) led to less microleakage because this permits greater contact of the
sealer with the dentine wall. They also suggested that use of liquid adhesives
reduced coronal microleakage significantly.

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Fig 41 (Leonard et al, 1996)
Cross-sectional SEM view of the dentine adhesive interface hybrid layer (H), resin filling
material (R), and demineralized dentin are visible with resin tags extending deep into the dentinal
tubules. x 940.
Other investigators have reported that the removal of the smear layer did not
have any significant effect on the microleakage of the root canals when various
sealers and obturation techniques were used. Evans & Simons (1986) evaluated
the apical seal produced by injected thermoplasticized gutta percha in the absence
of smear layer, and found that smear layer had no significant effect on apical seal
whereas the sealer was necessary to prevent apical leakage. Madison & Krell
(1984) also found no difference in apical leakage after use of chelating agent
irrigation. Timpawat et al (1998) also found that there was no difference in the
apical leakage in canals obturated with Thermafil with different sealers, with or
without the smear layer. Froes et al (2000) found that found that there was no
significant difference in the degree of apical leakage with and without the smear
layer when 4 different obturation techniques were compared. Cook et al (1976)
and Biesterfield & Taintor (1980) examined apical leakage of canals after
potentially affecting the smear layer with a chelating agent. Cooke et al (1976)
found that apical leakage increased with chelating agent use. Biesterfield &
Taintor (1980) found that apical leakage increased in specimens obturated 1
week after instrumentation. Neither study documents smear removal, apical
patency prior to obturation, or effective leakage evaluation techniques.
Chailertvanitkul et al (1996) found no significant difference in microbial coronal
leakage of obturated root canals when the smear layer was removed or intact.

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In contrast to these findings, Timpawat et al (2001) have reported that
removal of the smear layer has adverse effects on the apical microleakage of filled
root canals and in fact caused more microleakage than when the smear layer was
left intact.
These conflicting results might be attributable to differences in the types of sealer
and obturation techniques, the means of producing a smear layer, different forms of
chelating agents to remove the smear layer, and the diversity of bacteria used under
various laboratory conditions.
When the smear layer is not removed, the durability of the apical seal should
be evaluated over a long period. Since this layer is a nonhomogenous and weakly
adherent structure (Mader et al 1984), it may slowly disintegrate, dissolving
around a leaking filling material, thus creating a void between the root canal wall
and the sealer. If the endodontist does not wish to worry about these possible
disadvantages, the smear layer can be removed in ways that will be discussed
later. However, it should also be borne in mind that there is a risk of reinfection of
dentinal tubules by microleakage if the seal should fail after removal of the smear
layer (Brannstrom, 1984).
Another important factor is that, the greater the degree of canal preparation,
the smaller the amount of apical leakage (Yee et al, 1984). It is still inconclusive
whether or not the presence of dentinal filings will enhance the seal of a root canal
filling. When it was noted that stoppage of leakage occurred, it was related to
smaller file sizes. 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 (Jacobson
et al, 1985). In addition to being porous, dentin plugs allowing microleakage
exhibited large amounts of shrinkage. Scanning electron microscopic examination
of unsatisfactory apical plugs always showed marginal and structural defects (Yee
et al, 1984). 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.