It was first described in 1975 and research has been performed since then to evaluate its importance in bacteria penetration into the dentinal tubules and its effects on endodontic treatment.
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.
They observed an amorphous layer of debris, with an irregular and granular surface, on instrumented dentinal walls using a scanning electron microscope (SEM).
[1][2] The authors stated that "most standard instrumentation techniques produced a canal wall that was smeared and packed with debris.
"[3] 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.
[4] The smear layer is a physical barrier that decreases the penetration of disinfecting agents into dentinal tubules, and consequently, their efficacy.
[6] The most important cause of endodontic failure is the residual microorganisms that are harboured within the root canal system and hard-to-reach areas.
In vivo, intra pair comparisons of bacterial invasion into dentinal tubules beneath ground, fractured and acid treated surfaces were made.
Olgart came to a conclusion that acid produced by microorganisms may dissolve the smear layer allowing bacteria to pass into dentinal tubules.
This set-up was then placed into a mounting jig which was designed for the Instron Universal Testing Machine so that only a tensile load was applied without shearing.
Because the smear layer produced during endodontic instrumentation contains both inorganic and organic material, it cannot be removed by any of the presently available root canal irrigants alone.
[21] Clark-Holke et al. (2003) focused on determining the effect of the smear layer on the magnitude of bacterial penetration through the apical foramen around obturating materials.
Standardized bacterial suspensions containing Fusobacterium nucleatum, Campylobacter rectus and Peptostreptococcus micros were inoculated into the upper chambers.
[22] Kokkas et al. (2004) examined the effect of the smear layer on the penetration depth of three different sealers (AH Plus, Apexit, and a Grossman type-Roth 811) into the dentinal tubules.
After complete setting, the maximum penetration depth of the sealers into the dentinal tubules was examined in upper, middle, and lower levels.
Çobankara et al. (2004) determined the effect of the smear layer on apical and coronal leakage in root canals obturated with AH26 or RoekoSeal sealers.
It was determined that that removal of the smear layer has a positive effect in reducing apical and coronal leakage for both AH26 and RoekoSeal root canal sealers.
[24] However Bertacci et al. (2007) evaluated the ability of a warm gutta-percha obturation system Thermafil to fill lateral channels in the presence or absence of the smear layer.
Specimens were cleared in methyl salicylate and analyzed under a stereomicroscope to evaluate the number, length, and diameter of lateral channels.
[26] Saleh et al. (2008) studied the effect of the smear layer on the penetration of bacteria along different root canal filling materials.
Survival analyses were performed to calculate the median time of leakage and log-rank test was used for pairwise comparisons of groups.
The root canals were obturated with lateral condensation of gutta-percha and the specimens were cleared, allowing for observation under the microscope.