Smear layer

The smear layer is a layer of microcrystalline and organic particle debris that is found spread on root canal walls after root canal instrumentation. It was first described in 1975 and much research has been performed since then to evaluate its importance in assisting or preventing the penetration of bacteria into the dentinal tubules. More broadly, it is the organic layer found over all hard tooth surfaces.

Early studies of dentinal walls after cavity preparation performed by Brännström and Johnson (1974) showed the presence of a thin layer of debris that was 2 to 5 micrometres thick.

In 1975 McComb and Smith first described the smear layer. They observed an amorphous layer of debris, with an irregular and granular surface, on instrumented dentinal walls using a scanning electron microscope (SEM). This smear layer was composed of dentin, pulp and bacterial remnants. The authors stated that “most standard instrumentation techniques produced a canal wall that was smeared and packed with debris.”

In 1984 Pashely described the smear layer as being composed of two phases; an organic phase, composed of collagen residues and glycosaminoglycans from extracellular matrix of pulp cells, which acts as a matrix for an inorganic phase. This organomineral content is composed of two distinct superposed layers. The first layer covers the canal wall and is loosely adherent and easy to remove. The second layer however occludes the dentinal tubules and strongly adheres to the canal walls.

In the same year Mader et al. studied the morphological characteristics of the smear layer in teeth that were endodontically instrumented with k type files and irrigated with 5.25% NaOCl. The smear layer was examined from two aspects; the first “downonto” the smear layer and the second from the side or profile view. Photomicrographs obtained by SEM showed that the smear layer consists of two confluent components. A thin superficial layer 1-2 micrometres thick overlying a densely packed layer that penetrated into the dentinal tubules for distances of up to 40 micrometres. The packed material showed finger like structures projecting into the tubules from the canal wall. There is still controversy over whether the smear layer should be removed and whether the disadvantages of leaving it in place overcome its benefits.

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