SMEAR LAYER :
Introduction :
Unknown and unrecognized for years, the smear layer has become a
force to be reckoned with during the last decade. Most dentists know now it
exists but are often puzzled as to whether or not they should cope with it.
Since the smear layer has been recognized, dentist have come to realize that
they must renew their acquaintance with the science of dental materials so
they can understand the relationships of the products they work with to the
smear layer.
History :
Boyde et al (1963) were the first to describe and demonstrate the
presence of a “smear layer” on surfaces of cut enamel such a layer was
readily removed with sodium hypochlorite, leading them to conclude that
an organic layer containing apatite particles was deposited or smeared on
the enamel surface, through functional heat generated during cutting.
They believed that the heterogenous nature of enamel was the source of
the smeared components.
Provenza and Sardana (1996) evaluated means of removing debris
from enamel and dentin after the use of steel burs, diamond stones and
hand instruments. They reported variations in the degree to which debris
was removed. Detergents were relatively ineffective, EDTA left behind a
film, 0.1N hydrochloric acid was considered too destructive in its action,
hydrogen peroxide appeared to be most effective.
Nelsen and Zisman (1966) described the dynamics of cutting dental
tissues and appeared to imply the existence of an altered surface layer
due to elastic and plastic deformation of the tissue.
Eick et al (1970) found that surfaces cut dry are rougher and more
smeared than those in which water is used as a coolant. In the absence of
coolant, smeared debris does not form a continuous layer but exists
rather as localized islands with discontinuities exposing the underlying
dentin. If the diamond is allowed to clog with cutting debris, the smear
layer appears to cover a wider area. Water coolant does not prevent
smearing but significantly reduces the amount and distribution of it.
Boyde et al (1963) attributed smearing of enamel to melting of the
tissue by functional heat. Studies have shown that temperatures will rise
upto 6000C in dentin when it is cut without a coolant. This value is
significantly lower than the melting point of apatite (i.e. 1500 – 18000C)
and has led to the conclusion that smearing is a physiochemical
phenomenon rather than a thermal transformation of apatite involving
mechanical shearing and thermal degradation of the protein. Plastic flow
of hydroxyapatite is believed to occur at lower temperatures than its
melting point and may also be a contributing factor to smearing.
Eick et al (1970) found the smear layer to be composed of an organic
film less than 0.5 m thick, included within it were particles of apatite
ranging from 0.5 – 15 m. They also found that coarse diamond burs
produced more smearing than tungsten carbide bur.
Eirich and Koblitz (1976) accounted for the formation of smear
layers especially in dentin by a brittle and ductile transition and
alternating rupture and transfer of apatite and collagen matrix onto the
surface. Dentin, comprising approximately 35% collagen matrix and
water, is a more abundant source of protein than enamel, which contains
approximately 2% protein matrix and water.
Researchers become aware of the endodontic smear layer by about
1975. It was first reported by Baker.
Tidmarsh in 1978, treated instrumented teeth with 50% citric acid
and found the dentin clear of smear layer and the dentinal tubules wide
open.
Goldman in 1979, demonstrated that the smear layer was tenacious
regardless of flushing with both a conventional and a perforated needle.
Two years, later, he tested various solutions individually and in
combination and concluded that chelating agent EDTA and sodium
hypochlorite was the best to remove the debris when used as a final
flush.
Baumgartner showed similar results with citric acid and sodium
hypochlorite.
Kennedy used warmed solution of sodium hypochlorite.
Cameron, used ultrasonics to produce a smear layer, found that the
layer was composed of two separate layers, each having a different effect
on the tubules and dependent on the time the ultrasonic was used.
White et al found that plastic filling material could penetrate dentinal
tubule when smear layer was removed.
Goldman discovered that smear layer removal improved the tensile
strength of post retention when the posts were cemented with a BIS –
GMA resin.
Mader, by SEM investigation measured thickness of the smear layer
and the depth of its penetration into the dentinal tubules.
Evans, injected thermoplasticized gutta-percha into canals after smear
layer removal and concluded that the presence or absence of the smear
layer had no significant effect on the apical seal.
Definition of smear layer :
According to Shwartz – “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”.
What is smear layer ?
When tooth structure is cut, instead of being uniformly sheared, the
mineralized matrix shatters and considerable quantities of cutting debris
made of small particles of mineralized collagen matrix are produced that is
scattered over the enamel and dentin surfaces at the interface of restorative
materials and dentin matrix and is known as the “Smear layer”.
In endodontics, the smear layer results directly from the
instrumentation used to prepare the canal wall and is found only where the
wall is instrumented and not in uninstrumented areas.
Because it is a very thin layer and is soluble in acid it is not very
apparent. It cannot be seen in demineralized teeth as it dissolves in the
process of demineralization. It is only visible under SEM or TEM.
The smear layer has an amorphous, irregular and granular appearance
when viewed under the scanning electron microscope. This appearance may
be formed by translocating and burnishing the superficial components of the
dentin walls during endodontic instrumentation.
The smear layer consists of two separate layers:
1) A superficial layer.
2) Loosely attached layer to dentin.
Dentin debris enters the orifices of the dentinal tubules and acts as a
plug (smear plug) to occlude the ends of the tubules.
The smear layer is made up of tooth particles ranging from less than
0.5 m to 15 m. The particles are composed of globular subunits
approximately 0.05 – 0.1 m in diameter which originated from mineralized
fibers. The thickness of this layer is 1-5 m. The depth entering the tubules
may be from a few m upto 40m. This tubular packing phenomenon is due
to action of burs and endodontic instruments. However Cengiz et al
proposed that the penetration of smear material into dentinal tubules could
be caused by capillary action as a result of adhesive forces between the
dentinal tubules and the smear layer. This hypothesis of capillary action
may explain the packing phenomenon observed by Aktener et al who
showed that this penetration was increased upto 110m.by the use of
surface achieve reagents as a working solution during endodontic
instrumentation.
One can conclude that a smear layer is present on all restoratively or
endodontically prepared teeth unless the dentin surface was treated with an
acid or a chelating agent.
Several factors may cause the depth of the smear layer to vary from
tooth to tooth – i.e. :
1) Dry or wet cutting of the dentin.
2) The size is shape of the cavity or root canal.
3) The type and sharpness of instrument used.
4) The amount and chemical make up of the irrigating solution.
5) Increased centrifugal forces resulting from the movement and the
proximity of the instrument to the dentin wall form a thicker and
more resistant smear layer.
6) The amount produced during automatic preparation as with gates –
glidden or post drills will be greater in volume than that produced by
hand filing. Instrumentation with K files and reamers and giromatic
files created similar surfaces.
If there is a difference in the rate of flow of fluid across dentin before
and after removal of the smear layer, the magnitude of rate change is an
indication of the thickness or density of the smear layer.
Filing a canal without irrigation or cutting without water spray will
produce thicker debris than otherwise similarly coarse diamond burs
produce thicker debris than carbide burs.
Dentin is composed of 2 different layers:
1) Superficial dentin (near the enamel).
2) Deep dentin (near pulp).
Smear layer on deep dentin contains more organic material than
superficial dentin. This is because of greater number of proteoglycans lining
the tubules or the greater number of odontoblastic processes near the pulp.
The adhesive strength of all cements is always 50% greater in
superficial dentin. This may indicate that the quality or quantity of the smear
layer found on superficial dentin may be greater than that produced in deep
dentin.
The movement of fluid across dentin meets a resistance directly
proportional to the quantity and quality of smear layer present. In vital teeth
the smear layer restricts the dentinal fluid from flushing the dentin surface.
It also hinders the chemical process that produces marginal seal. The
presence of smear layer, however, does not appear to restrict the adaptation
of freshly condensed amalgam to cavity surface.
In non-vital teeth, marginal seals are improved because of the lack of
moisture within the dentinal tubules. When the acid etch technique is used
the retention of the smear layer is not an important factor in the
development of a marginal seal around composite resin restorations. The
initial sealing process occurring under amalgam restorations may be
compromised because of the instability of the smear layer and its penchant
for leaching under the amalgam. This leaching process will produce a
widening of the amalgam – tooth micro crevice and ultimately weaken the
sealing mechanism.
Jodaikin proposed a conflicting theory about the smear layer in the
sealing mechanism of a restoration. He believed that a chemical effect was
in force that provided a substrate that interacted with the restoration
substrates or other substances that might find their may into the micro
crevices at the restoration – tooth interface. He theorized that the smear
layers presence provided an environment that was conducive to the
initiation and progression of the sealing mechanism. By restricting the
dentinal fluid from flushing the molecules that affected the seal from the
restoration – tooth interface, the smear layer may also play a physical as
well as chemical role in margin sealing.
According to some investigators, after a canal is instrumented the
smear layer produced can harbour bacteria and bacterial products that can be
a reservoir of potential irritants. The smear layer is a separate structure from
the underlying dentin and may crack open and pull away from the
underlying dentinal tubules. A situation like this could be harmful to the
foundation of gutta-percha obturated over the smear layer. Hence they
thought it best to remove the smear layer, though controversy still remains.
COMPONENTS OF THE SMEAR LAYER :
Though the exact proportion of the composition is not certain.
Is composed of
i) Organic component.
ii) Inorganic component.
Inorganic component is made up of tooth structure and some non-
specific inorganic components.
Organic component consists of heated coagulated proteins (gelatin
formed by the deterioration of collagen heated by cutting temperatures),
necrotic or viable pulp tissue, odontoblastic processes, Saliva, blood cells
and micro-organisms.
Advantages of smear layer :
1) Reduction of dentin permeability to toxins and oral fluids.
2) Reduction of diffusion (usually inwards by convection and outwards
by hydrostatic pressure) of fluids and prevents wetness of cut dentin
surface.
3) Bacterial penetration of dentinal tubules is prevented.
Disadvantages of smear layer :
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 layer may prevent the adhesion of composite resin system,
bonding agents, GIC and polycarboxylate cements.
PHYSICAL BARRIER FOR BACTERIA AND DISINFECTANTS :
When pathologic changes occur in the dental pulp, the root canal
system can harbour several species of bacteria, their toxins and by products.
These bacteria are predominantly gram-negative anaerobes. The
morphology of the root canals is very complex therefore the mechanically
prepared canals contain areas not accessible by endodontic instruments and
bacteria will be found more in number in these areas.
Available evidence shows that bacteria and its by products present in
infected root canals may invade the dentinal tubules. Investigators have
reported the presence of bacteria in the dentinal tubules of infected teeth at
approximately half the distance between the root canal walls and the
cemento-dentinal junction. Bacterial penetration into the dentinal tubules is
seen upto 150 m. in the apical 2/3rd of the root. Thus even after
chemomechanical 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 disinfectants.
Drake et al showed that removal of the smear layer opened the
tubules, allowing bacteria to colonize in the tubules to a much higher degree
(10 fold) compared with roots with an intact smear layer, removal of smear
layer facilitates passive penetration of bacteria. It was shown that smear
layer delayed the penetration of proteus vulgaris but it was also found that
pseudomonas aeruginosa penetrated even thicker dentin slices by removing
the smear layer it self and by opening the orifices of dentinal tubules after
possible collagenase production. A. Viscosus, corynebacterium spp. and S.
sanguis also digested the smear layer and facilitated their penetration. Smear
layer is permeable even to large molecules such as albumin. Therefore this
layer is not a strict barrier to bacteria.
According to some authors the presence of smear layer may block the
antimicrobial effects of intracanal disinfectants into the tubules, various
medicaments have been proposed for disinfection of root canals, they are:
1) Traditional phenolic or fixative agents like camphorated mono
chlorophenol (CMCP), formacresol and cresatin.
2) Non – phenolic compounds like iodine potassium iodide & calcium
hydroxide.
Researchers found that in absence of smear layer, liquid camphorated
monochlorophenol disinfected the dentinal tubules rapidly and completely
but calcium hydroxide failed to eliminate enterococcus faccalis even after 7
days of incubation and hence concluded that smear layer did delay but not
abolish the action of the disinfectants. However following removal of smear
layer, bacteria in dentinal tubules can be easily destroyed and in this way, it
may be beneficial to use lower concentrations of antibacterial agents since
all these agents show some degree of toxicity to viable host cells.
Smear layer and microleakage :
An important consideration in endodontics is the ultimate seal of root
canals in order to prevent possible microleakage which may be the cause of
future failure of the root filling. Prepared dentin surfaces should be very
clean to increase the sealing efficiency of obturation. Smear layer on root
canal walls acts as an intermediate physical barrier and may interfere with
adhesion and penetration of sealers into dentinal tubules.
Investigators observed that plastic filling materials and sealers
penetrated into the dentinal tubules after removal of smear layer, and its
presence obstructed their penetration. The penetration in smear free groups
ranged from 40-60m.They concluded that tubular penetration may increase
the interface between the filling and the dentinal structures and thus may
prevent leakage. However there is no strong evidence to this statement.
Pashley et al observed extensive reticular network of micro-channels
with thickness of 1-20m around restorations that had been placed in
cavities with intact smear layer and this provided a passage for
microleakage to occur. They concluded that removal of smear layer
decreased microleakage but increased dentin permeability.
In a recent study it was definitely shown that coronal leakage of root
canal filling was less in smear free groups than those with smear layer.
Microleakage in root canal is very complicated and many variables
may contribute to it like anatomy and instrumented size of the root canal,
irrigating solutions, root filling techniques, physical and chemical properties
of the sealers and the infectious state of the canal etc.
Apical leakage :
According to Evan et al, the use of injected thermoplasticized gutta-
percha should be accompanied by the use of sealer regardless of whether or
not the smear layer has been removed. But Kennedy stated that an absence
of the smear layer causes less apical leakage than gutta-percha filled canal
with the smear layer intact. He also stated that the use of chelating agents on
the smear layer would increase apical leakage. He concluded that removal
of smear layer would improve gutta-percha seals if the master cones are
softened with chloroform and used with a sealer and lateral condensation,
technique.
The greater the degree of canal preparation, the smaller the amount of
apical leakage.
It is still inconclusive whether the presence of dentinal fillings or
plugs will enhance the seal of root canal filling as the dentinal plugs were
porous and permeable and apical leakage existed in some situations.
Sealers :
Endodontic sealers act as a glue to ensure good adaptation of gutta-
percha to the canal walls. If the smear layer is not removed then the gutta-
percha is not firmly attached to the dentin and the smear layer may laminate
off the canal wall and create a false seal, voids in the fill and an
environment for microleakage.
The type of sealer used has different implications once the smear
layer has been removed. For example Grossman sealer which is a powder
liquid combination, contains small particles in the powder that enter the
dentinal tubule orifices and create a secure interface between sealer and
canal wall, after the removal of smear layer, calcium hydroxide based
sealers promote the apposition of the cementum at the canal apex and seal it
off against microleakage by the formation of osteoid or dentoid type
material. Circulation of blood is needed for the calcium ion to promote new
tissue thus the calcium hydroxide sealers are effective for sealing only at the
root apex. If more cementum is going to form to create a better apical seal,
dentin chips at the apex of a root canal acts as a nidus for formation of hard
tissue. Bacterial contamination by the presence of a smear layer can prevent
this.
There is no practical advantage to the use of auto cure unfilled resin
as a seal over the tubules before gutta-percha oburation, as the resin would
be susceptible to moisture though lateral canals and the apex and upon
polymerization the resin would shrink creating a gap between the fill and
the canal wall.
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 of research and is
doubtful that the bonding agent would be antimicrobial to the bacteria in the
smear layer.
Post cementation :
Removal of smear layer increases the cementation bond and the
tensile strength of the cementing medium for post cementation.
Glass ionomer cements are effective in post cementation after smear
layer removal because the glass ionomer has better union with tooth
structure.
When the smear layer was removed by flushing with EDTA and
sodium hypochlorite rinse, the unfilled BIS. GMA resin (cementing media)
flowed into the exposed dentinal tubules and into serrations on the post,
improving retention vastly, and with the removal of smear layer and an
unfilled resin bonding agent, shorter posts can be used.
Functional implications :
1) Dental materials :
The presence of smear layer masks the underlying dentin matrix and
may interfere with the bonding of adhesive dental cements such as
polycarboxylates and glass ionomer that reacts chemically with the dentin
matrix. Zinc phosphate require dentin matrix for mechanical roughness to
aid in retention.
The cements that react chemically to smear layer rather than the
matrix of sound intertubular dentin produce a weaker bond as the smear
layer can be torn away from the underlying matrix, and when these cements
are tested for tensile strength, the failure can be either adhesive (between
cement and smear layer) or cohesive (between constituents of smear layer).
To increase the tensile strength of a cement dentin interface there are several
approaches:
1) Remove the smear layer by etching with acids. This procedure does
not injure the pulp if dilute acids are used for shorter periods of time ex:
etching dentin with 6% citric acid for 60secs removes all the smear layer
as does 15secs of etching with 37% phosphoric acid. The advantages are
that the smear layer is entirely removed, the tubules are open and
available for increased retention and the surface collagen is exposed for
covalent linkage with new experimental primers for cavities.
The disadvantage is that there is a physical barrier for bacterial
penetration and the permeability of dentin increases.
2) Another approach would be to use a resin that would infiltrate
through the entire thickness of the smear layer and either bond to the
underlying matrix or penetrate into the tubules. The impressive tensile
strength of Scotch bond is due to this process. The bond is stronger
between resin and pumiced dentin that between resin and etched dentin.
Removing smear layer with acid etching exposes surface collagen and
removes peritubular dentin from the top 5-10 m. of the tubules, yielding
a tubule with a funnel shaped orifice. If the resin penetrates only into the
funneled portion of tubule rather than where the tubules are normal and
of uniform diameter then retention would be less due to diverging tubule
walls rather than normal parallel walls of unetched tubules. Additionally,
acid etching demineralizes the surface which lowers the adhesive bond
between cement and minieralized dentin.
The adhesive strength of clearfil resin to dentin with a smear layer
present was as high as polycarboxylate and glass – ionomer cement. Etching
dentin doubled the adhesion of clearfil resin to etched superficial dentin.
Acid etching and removal of smear layer increases the adhesive
strength of composite resin to superficial dentin by 800 – 1000% over that
of deep dentin even though far more tubules are available for penetration of
resin. This indicates that composite resins probably do not derive their
adhesiveness from penetration of resin into the tubules but rather by
interacting with mineralized intertubular dentin.
3) Another approach is to fix the smear layer with gluteraldehyde or
tanning agents such as tannic acid or ferric chloride. This increases the
cross linking of exposed collagen fibers within the smear layer and
between it and the matrix of the underlying dentin to improve its
cohesion.
4) A fourth and most convenient approach is to remove the smear layer
by etching with acid and replace it with an artificial smear layer
composed of a crystalline precipitate. Bowen used this approach by
treating dentin with 5% ferric oxalate which replaces the original smear
layer with a new complex permitting extremely high bond strength to be
produced between resin and dentin.
ENDODONTICS :
Smear layer might provide a reservoir of potential irritants. Alternate
use of sodium hypochlorite and EDTA is employed to remove smear layer.
The sodium hypochlorite removes organic material and collagenous matrix
of dentin and EDTA removes mineralized dentin thereby exposing more
collagen.
Removal of smear layer results in better adaptation of obturating
materials and sealers to dentin. Goldman also demonstrated increased
tensile strength of plastic posts after smear layer removal as these is more
penetration of resin into the open dentinal tubules.
Restorative dentistry :
When cementing a casting or condensing amalgam and during normal
mastication there is considerable force or pressure applied to the tooth. This
pressure is transferred to dentin which causes movement of dentinal fluid
and displacement of fluid into the pulp that might cause pain. The presence
of smear layer can prevent this phenomenon to a certain extent.
Influence on sensitivity and permeability of dentin :
Sensitivity of dentin is due to movement of fluid (hydrodynamic
theory of pain) most of the resistance to the flow of fluid across dentin (86%
of total resistance) is due to the presence of smear layer. Etching dentin to
remove the smear layer greatly increases the ease with which fluid can
move across dentin. This is accompanied by increased sensitivity of dentin
to osmotic, thermal and tactile stimuli.
Though the smear layer is permeable to bacterial toxins and its by
products to a certain extent but it still reduces the permeability of bacteria as
compared to smear layer free area. Transport of material across dentin can
be by 2 ways :
1) Diffusion.
2) Convection.
In diffusion, there is movement of substance from higher to lower
concentration and the concentration of the substance is dissipated over a
distance.
In convection, movement of substance is due to a pressure gradient
but no dissipation of concentration occurs.
It is shown that removal of smear layer increases dentin permeability
by diffusion by about 5-6 times and convection by 25-36 times.
Different treatment modalities on smear layer :
1) No treatment of the smear layer at all eg- PRISMA.
2) No removal of smear layer, just partial demineralization.
3) Complete removal of smear layer.
4) Modification of smear layer by keeping smear plugs intact.
5) Removal of natural smear layer and replacement by artificial smear
layer. eg – ferric oxalate.
Removal of smear layer :
Irrigating solutions have been used during and after instrumentation
to increase cutting efficiency and flush away debris. The efficacy of the
irrigating solution is not only dependent on the chemical nature of the
solution but also on the quantity and temperature, the contact time, depth of
penetration of the irrigating needle, type and gauge of the needle, surface
tension of the irrigating solution and age of the solution.
Sodium hypochlorite :
The organic tissue dissolving activity of Naocl is well known and its
increases with rising temperatures. However, the capacity to remove smear
layer from instrumented canal has been found to be insufficient. It produces
a superficially Clean canal wall with smear layer present.
Alternating use of hydrogen peroxide and Naocl solutions that was
advocated in the past was no more effective than Naocl used alone. It was
seen that the effect produced by Naocl was similar to that produced by
water. Adding surface active reagents also did not improve the situation.
Chelating agents :
The most common chelating solutions are based on ethylene diamine
tetra acetic acid (EDTA) which reacts with calcium ions in dentin and forms
soluble calcium chelates.
It was shown that EDTA decalcified dentin to a depth of 20 – 30 m.
in 5 min, but its chelating effect was almost negligible in the apical third of
root canals.
Another preparation i.e. EDTA in combination with urea peroxide
(RC-prep) was used to float the dentinal debris from the root canal but
despite further instrumentation and irrigation a residue of this mixture was
left on the canal walls which was a disadvantage in hermetic sealing of the
root canal.
A quarternary ammonium bromide (utrimide) has been added to
EDTA (REDTA) solutions to reduce surface tension and increase
penetrability of the solution. When this combination was used during
instrumentation, there was no smear layer left except in the apical third of
the canal.
Another combination used was EDTAC i.e. EDTA with cetavlon. It
was seen that optimal working time of EDTAC is 15mins after which no
more chelating action takes place.
Another root canal chelating agent is Salvizol – based on amino
quinaldinum diacetate. It has surface acting properties similar to materials of
the quaternary ammonium group and possess the combined action of
chelation and organic debridement.
It was shown that REDTA was the most efficient irrigating solution
in removing smear layer.
Organic acids :
Citric acid appeared to be an effective root canal irrigant and was
more effective than Naocl alone in removing the smear layer, it was also
better than polyacrylic acid, lactic acid and phosphoric acid but not EDTA.
It was shown that canal walls treated with 10%, 25% and 50% citric
acid solutions were free of smear layer, but the best results were with
sequential use of 10% citric acid and 2.5% Naocl solution then again
followed by 10% citric acid. It was however 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 canals which was a
hinderence during root canal obturation, with 50% lactic acid, the canal
walls were generally clean but the dentinal tubules openings were not
completely patent.
Another root canal irrigant cleanser used was 25% tannic acid. It was
demonstrated that the canal walls irrigated with this solution appeared
significantly cleaner and smoother than the wall treated with a combination
of Naocl and hydrogen peroxide and that the smear layer was removed.
The use of 20% polyacrylic acid was found as better than REDTA.
5% and 10% polyacrylic acids also removed smear layer but only in
accessible regions.
Sodium hypochlorite and EDTA : (method of choice)
The purpose of irrigation is two folds:
a) To remove gross debris originating from pulp tissue and
bacteria -organic component.
b) To remove smear layer – inorganic component.
Because there is no single solution which has the ability to dissolve
organic tissues and to demineralize the smear layer, a sequential use of
organic and inorganic solvents have been recommended. For example the
alternate use of EDTA and Naocl for removal of smear layer and soft tissue
debris.
According to a study it was found that the most effective working
solution was 5.25% Naocl and the most effective final flush was 10 ml of
17% EDTA followed by 10 ml of 5.25% Naocl.
Ultrasonics :
According to Matrin and Cunningham, a continuous flow of sodium
hypochlorite solution (2.4%) activated by ultrasound delivery system that
was used for preparation and irrigation of the root canal also produced
smear free root canal surfaces.
Ahmad et al showed that with modified ultrasonic instrumentation
and 1% Naocl, smear layer could be removed.
It was observed that the apical region of the canals showed less debris
and smear layer than the coronal aspects, depending on the acoustic
streaming, which was more intense in magnitude and velocity at the apical
region of the file. It was seen that a 3-5min. Irrigation produced smear free
canal walls.
In contrast to these results. It has been found by other investigators,
that ultrasonic preparation was not able to remove the smear layer. This
contradiction may be due to physical contact of the file with the canal wall
that reduced the acoustic streaming.
Lasers :
Takeda et al found that lasers can be used to vapourize tissues in the
main canal, remove the smear layer and eliminate residual tissue in the
apical portion of the root canals. Effectiveness of lasers depends on many
factors including the power level, the duration of exposure, the absorption of
light in the tissues, the geometry of the root canal and the tip target distance.
Takeda et al using the erbium – yttrium – aluminium – garnet (Er:
YAG) laser demonstrated optimal removal of smear layer without the
melting, charring and recrystallization associated with other laser types like
neodymium – yttrium – aluminium garnet (Nd:YAG) laser, carbon dioxide
laser, organ fluoride excimer laser and organ laser. Although there was
removal of smear layer it showed destruction of peritubular dentin. The
main difficulties with lasers is the access to small canals as only large
probes are available for delivery of laser beam.
CONCLUSION :
The problem of smear layer is yet a controversy. To keep it or
remove it is still a problem , the solution of which still eludes us. It is upto
the dentists judgement , knowledge and understanding to treat the smear
layer or not.
Introduction :
History Definition of smear layer :
What is smear layer ?COMPONENTS OF THE SMEAR LAYER :Advantages of smear layer :
Disadvantages of smear layer : PHYSICAL BARRIER FOR BACTERIA AND DISINFECTANTS Smear layer and microleakage Apical leakage Sealers :Post cementation Functional implications :
1) Dental materials
ENDODONTICS :
Restorative dentistry :Influence on sensitivity and permeability of dentin :Different treatment modalities on smear layer Removal of smear layer
CONCLUSION :