structure and components of the smear layer/ rotary endodontic courses by indian dental academy

Structure & Components
STRUCTURE:
When the root canals are instrumented during endodontic therapy, a smear
layer composed of dentine, remnants of pulp tissue and odontoblastic processes,
necrotic materials, and sometimes bacteria, is always formed on the canal walls.
Under low magnification, it has an amorphous, irregular and granular appearance
under the scanning electron microscope (Branstrom et al,1980; Yamada et
al,1983; Pashley et al, 1988) and the dentinal tubules are obscured. This
appearance may be formed by translocating and burnishing the superficial
components of the dentin walls during endodontic instrumentation (Baumgartner
&Mader, 1987). At higher magnification, the smear layer exhibits a granular
substructure composed of particles with an approximate diameter of 0.05-0.1 µm
(Pashley et al, 1988; Love, 1994). These particles represent an enormous surface
area-to-mass ratio, rendering the smear layer prone to dissolution by acids and
chelating agents.
McComb and Smith (1975) suggested that the smear layer associated with
root canal treatment consisted not only of dentin as in coronal smear layer, but
also remnants of odontoblastic processes, pulp tissue and bacteria. Hence, it may
contain both organic and inorganic material. They also observed that
instrumentation with K-reamers, K-files and Giromatic files created similar
surfaces, as observed under the scanning electron microscope.
According to Cameron (1982) ,the smear layer on the wall 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 the root canal. In addition, its
composition is influenced by the area in dentin in which it is generated, because
of differing organic to inorganic component ratios (Causton, 1984) and
percentage area of dentinal tubuli (Suzuki & Finger, 1988).
Clinically produced smear layers have an average depth of from 1 to 5 µm.
The depth entering the tubules may vary from a few µm upto 40 µm (Pashley,
1984). The smear layer consists of two separate layers. According to Mader et al
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(1984) and Cameron (1983), the smear layer consists of a superficial layer on
the surface of the canal wall approximately 1 to 2 µm in thickness (which is thin,
loosely adherent and easy to remove) and a deeper layer which is the
intradentinal layer and is packed into the dentinal tubules to a depth of upto 40
µm. Dentin debris enters the orifices of the dentinal tubules and acts as plugs to
occlude the ends of the tubules (Cameron, 1987) and strongly adheres to the
canal walls. The components of the smear layer can be forced into the dentinal
tubules to varying distances. This can occur as a result of the linear movement and
rotation of instruments and because of capillary action generated between the
dentinal tubules and the smear material (Cengiz, 1990; Brannstrom and
Johnson, 1974; Mader et al, 1984). In case of burs, their rotation causes
centrifugal scattering of the smear material, which would penetrate the dentinal
tubules if they were appropriately oriented. Whereas, endodontics instruments,
which makes linear up and down motion perpendicular to the dentinal tubules
during filing, contact the smear layer material which forces it into the tubules.
Fig. 11
Smear layer (SL) in cross section. Smear plugs (SP) are formed from cutting debris forced into
the tubules. The smear layer and plugs greatly reduce the permeability of cut dentin surface.
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One can conclude that the smear layer is present on all restoratively or
endodontically prepared teeth unless the dentin surface was treated with an acid or
a chelating agent. And initially, it is far more tenacious than one would expect.
Several factors may cause the depth of the smear layer to vary from tooth to tooth:
1. Dry or wet cutting of the dentin (Pashley, 1984).
2. The size and shape of the cavity or root canal.
3. The type of instrument used.
4. The amount and chemical makeup of the irrigating solution.
5. 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.
6. Use of coarse diamond burs produce a thicker smear layer than the use of
carbide burs.
Perhaps the thickest smear layers that have been produced (∼10-15 µm
thick) were produced in vitro with a coarse diamond blade mounted on a
metallurgical saw. This device tends to pack and burnish the debris into a
smooth, highly glossy finish (Pashley, Michelich & Kehl, 1981)
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.
Dentin is also composed of two different layers. Superficial dentin is dentin
near the enamel. Deep dentin is near the pulp. Smear layers found on deep dentin
contain more organic material than those found on superficial dentin. This may be
explained by the greater number of proteoglycans lining the tubules or by 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.
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Eick et al (1970) showed that the smear layer was made of tooth particles
ranging from less than 0.5 µm to 15 µm.
Pashley et al (1988) found that these particles were also composed of
globular subunits, approximately 0.05-0.1 µm in diameter which originated from
mineralized fibers.
Goldman et al (1981) and Mader et al (1984) reported the thickness of the
smear layer to be l-5 µm. This thickness depended on the type and sharpness of
the cutting instruments and whether the dentin was cut dry or wet (Barnes, 1974;
Gilboe et al, 1980).
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
(Jodaikin and Austin, 1981) and thus the amount produced during automatic
preparation, as with Gates Glidden post drills will be greater in volume than that
produced by hand-filing (Czontskowsky, 1990).
Cengiz et al (1990) proposed that the penetration of the smear layer into the
dentinal tubules could be caused by capillary action as a result of adhesive forces
between the dentinal tubules and the smear material. This hypothesis of capillary
action may explain the packing phenomenon observed by Aktener et al (1989)
who showed that this penetration was increased upto 110 µm by the use of
surface-active reagents as a working solution during endodontic instrumentation.
Varvara (1997) analysed the relationship between the use of rotating
instruments, the production of a smear layer and the presence of alterations to
enamel microstructures. The rotating instruments used were carbide (8-12 blade)
and diamond tipped (30-15 m) cutters. The results obtained showed on the one
hand that carbide cutters leave a smoother surfacer than diamond tipped cutters,
and on the other that the smear layer is eliminated better by carbide cutters
compared to diamond tipped cutters.
20

The combined study by SEM and TEM suggests that smeared dentin, when
cut and abraded has an altered structure ranging from 0-3 µm in depth. This depth
is not uniform. Greater depth of structural alteration might be expected, depending
on the way the surface is cut or abraded. Intertubular collagen appears to be
denatured in this region of altered surface to a depth upto 1 µm. There was some
evidence of micro cracking of surface material upto 2 or 3 µm below the
outermost disturbed surface. There may have been some loosening or lifting of
surface material, which would correspond to the edge of the flakes.
COMPONENTS:
The exact proportionate composition of the endodontic smear layer has not
been determined, but scanning electron microscopic examinations have disclosed
that its composition was both organic and inorganic although Goldman and
colleagues (1981) and Lester & Boyde (1977) have demonstrated that the smear
layer, created by endodontic instrumentation, is primarily calcific (inorganic) in
nature. The inorganic material was made up of tooth structure and some non-
specific inorganic contaminants. Cutting instrument particles, however, were not
found. The organic components may consist of heated coagulated proteins
(gelatin formed by the deterioration of collagen heated by cutting temperatures),
glycosaminoglycans from the organic extracellular matrix, necrotic or viable pulp
tissue and odontoblastic processes plus saliva, blood cells and micro
organisms.This serves as a matrix for the seemingly predominant inorganic phase.
Once a root canal has been instrumented, the high magnification of the
scanning 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. Superficial
debris and cracks may be present in the dentin. Occasionally, the cracks can be
attributed to the process of preparing the specimens for scanning electron
microscopic examination.
21

A profile view of the specimen may show inconsistencies, disclosing fine
particulate material, densely or loosely packed to various depths into the dentinal
tubules. In all specimens, however, the packing material will show a segmented
appearance as if it had been packed in increments.
Tubule packing was seen most often where less than half the circumference
of the tubule has been fractured away. Specimens showing a fracture of more than
half the circumference rarely will display this packing phenomenon.
Instrumentation by other means than finger files and irrigating solutions may
produce a packing phenomenon with a different appearance (Mader et al, 1984).
The older methods of root canal preparation, especially chelation and
irrigation, did not produce a clean surface for permanent sealing but with the
newer chelating agents (EGTA - Calt & Serper, 2000, Viswanath et al, 2003)
the canal surface was clean which aids in the sealing. It has a superior action even
in the very important apical third.
The flushing action of the irrigating solution was imperative for the debris
removal, but scanning electron microscopic studies after instrumentation still
showed the presence of a smear layer. Some impaling solutions will compound
this disadvantage by causing a dense, amorphous precipitate to form on the smear
layer.
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structure and components of the smear layer/ rotary endodontic courses by indian dental academy

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