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1. What is Smear Layer?
THE SMEAR PHENOMENON:
Significant amounts of energy are expended at the interface of a substrate
and a tool during cutting and abrading. The generation of frictional heat and
plastic and elastic deformation can all contribute potentially to alteration and
deterioration of the substrate. Grinding debris from the substrate or the tool itself
may be deposited or smeared upon the work surface. Such smeared contaminants
lower the surface energy and therefore have a profound effect upon the reactivity
of the substrate surface.
In a dental context Eirich (1976) stated that smearing occurs when
“hydroxyapatite within (the tissue) is either plucked out or broken, or swept along
and resets in the smeared-out matrix”. Hard dental tissues are heterogeneous,
comprising submicroscopic crystallites of apatite enveloped in an organic matrix.
Significant variations in the proportions of these components exist between
enamel, dentin and cementum, thus contributing to a wide range of topographical
anomalies, which can be related the type of instrumentation and the manner and
conditions under which it is used.
THE SMEAR LAYER:
According to the Operative Dentistry Journal, 1984, the term smear layer
applies to “any debris produced iatrogenically by the cutting, not only of dentin,
but also of enamel, cementum and even the dentin of the root canal”.
It is defined by Anusavice (Philip’s Science of Dental Materials) as:
“Tenacious deposit of microscopic debris that covers enamel and dentin surfaces
that have been prepared for a restoration”
It has been described by Sturdevant as:
“Grinding produces gross mechanical roughness but leaves a smear layer of
hydroxyapatite crystals and denatured collagen that is approximately 1 to 3 µm
thick”.
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2. What is Smear Layer?
According to the American Association of Endodontists’ (1994) glossary
Contemporary Terminology for Endodontics, the smear layer is defined as “a
surface film of debris retained on dentine or other surfaces after instrumentation
with either rotary instruments or endodontic files; consists of dentine particles,
remnants of vital or necrotic pulp tissue, bacterial components and retained
irrigant ”.
It is defined by Cohen as:
“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 as smear layer”
When tooth structures are cut, instead of being uniformly sheared, the
mineralized matrix shatters. Considerable quantities of cutting debris, made up of
very small particles of mineralized collagen matrix, are produced. Existing at the
strategic interface of restorative materials, and the dentin matrix, most of the
debris are scattered over the enamel and dentin surfaces to form what is known as
the smear layer. A much used analogy compares the smear layer to a clump of
wet saw dust on a cut log.
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, as studies have shown that current methods of
cleaning and shaping root canals, may not cleanse the entire root canal system,
especially in irregular and/or curved canals. In addition to superficial debris, it has
been shown, using the scanning electron microscope, that a layer of sludge
material was always formed over the surface of dentinal walls whenever dentin
was cut (Boyde and Knight, 1970; McComb and Smith, 1975; Mader et al,
1984) and this layer was called “smear layer”. The amount of smear layer
produced by automatic preparation will be greater in volume than produced by
finger filing.
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3. What is Smear Layer?
Because it is a very thin layer and is soluble in acid, the smear layer will not
be apparent on routinely processed specimens examined with the light
microscope. this may be the reason why the smear layer received so little attention
by restorative dentists. When examined by the scanning electron microscope, the
smear layer will rarely be discernible on specimens of demineralized teeth
because it will be dissolved during the process of demineralization.
Undemineralized specimens will appear on electron microscopic examination as a
uniform sludge, relatively smooth and featureless.
The presence or absence of the smear layer in Endodontics is important.
When a canal is instrumented, the smear layer produced will remain within the
canal and pulp chamber. The bacteria and bacterial products found in the smear
layer can provide a reservoir of potential irritants. With this thought in mind, the
complete removal of the layer and its corps of inhabitants has been the subject of
numerous investigations.
Another source of indecisiveness about keeping or removing the smear layer
was its long-term stability. The smear layer and its provisional tenacity is a
separate structure from the underlying dentin. It may crack open and pull away
from the underlying dentinal tubules. A situation such as this would be deadly to
the foundation of gutta percha obturated over the smear layer. Thus, it was
axiomatic that removal of the smear layer permits a better adaptation of sealers
and obturating materials in the dentin.
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