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

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

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.)

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.

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

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:

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:

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.

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.

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.

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.

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

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

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.

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.

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:

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

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.

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.

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

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.

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.

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

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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

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.

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.