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”.
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.
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.