Smear Layer & Microleakage

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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Smear Layer & Microleakage

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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Smear Layer & Microleakage

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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Smear Layer & Microleakage

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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Smear Layer & Microleakage

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.

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