In dentistry, the smear layer is a layer found on root canal walls after root canal instrumentation. It consists of microcrystalline and organic particle debris. 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. More broadly, it is the organic layer found over all hard tooth surfaces.
Description 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). The thin, granular microcrystalline layer of debris was 2–5 micrometres thick and was found packed onto the canal wall. The authors stated that "most standard instrumentation techniques produced a canal wall that was smeared and packed with debris."
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 aspect looked "down onto" the smear layer and the second from the side. Photomicrographs obtained by SEM showed that the smear layer consists of two confluent components. These were described as a thin superficial layer 1–2 micrometres thick overlying a densely packed layer and a second 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.
Contents
Composition 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 organo-mineral content is composed of two distinct superimposed 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.
Contents of the smear layer
Dentine particles Residual vital pulp tissue Residual necrotic pulp tissue Erythrocytes Remnant of odontoblast process Saliva Bacterial components
Thickness of the smear layer The smear layer is a physical barrier that decreases the penetration of disinfecting agents into dentinal tubules, and consequently, their efficacy. The most important cause of endodontic failure is the residual microorganisms that are harboured within the root canal system and hard-to-reach areas. Studies were conducted into the thickness of smear layer created by different instruments, to enhance the understanding and aid the removal of the smear layer, and therefore aid the removal of any bacteria that may otherwise have been entombed by the smear layer. Results of the study showed that the Protaper series of rotary instruments caused the maximum amount of smear layer, followed by the Profile series of rotary instruments. The hand instruments caused the least amount of smear layer. Increasing the roughness of instruments has been found to increase the thickness of the smear layer as well.
Bacterial Penetration Olgart et al. (1974) examined the penetration of bacteria into dentinal tubules of ground, fractured and acid treated dentin surfaces. In vitro the penetration of bacteria into tubules of intact dentin exposed by fracture was compared in pairs of teeth, one of which in each pair was mounted with intrapulpal hydrostatic pressure (30 mmHg). In vivo, intra pair comparisons of bacterial invasion into dentinal tubules beneath ground, fractured and acid treated surfaces were made. They observed that an outward flow of fluids into the tubules due to intrapulpal pressure mechanically hindered bacterial growth and that the debris and smear layer produced from grinding obstructed the bacterial invasion into tubules. However this barrier seemed to be removed after a few days which allowed bacterial growth into intact dentin. Olgart came to a conclusion that acid produced by microorganisms may dissolve the smear layer allowing bacteria to pass into dentinal tubules.Olgart L, Brännström M, Johnson G. Invasion of bacteria into dentinal tubules: Experiments in vivo and Invitro. Acta Odontologica Scandinavica 1974; 32:61-70.
However, when Pashley et al. (1981) studied the scanning electron microscope (SEM) appearance of dentin before and after removing successive layers of the smear layer they came to a different conclusion. Twenty dentin disks were cut from human extracted third molars. The dentin surface of the disks was etched with 6% citric acid for 5, 15, 30, 45 and 60 seconds. SEM examination showed that citric acid was able to remove smear layer in successive layers according to etching time finally exposing the dentinal tubules. Pashley concluded that the maintenance of the smear layer established a protective diffusion barrier.Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981; 46:531-7.
Gettleman et al. (1991) assessed the influence of a smear layer on the adhesion of sealer cements to dentin. A total of 120 teeth was tested, 40 per sealer namely AH26, Sultan, and Sealapex; 20 each with and without the smear layer. The teeth were split longitudinally, and the internal surfaces were ground flat. In the smear layer-free specimens the smear layer was removed by washing for 3 minutes with 17% EDTA followed by 5.25% NaOCl. Using a specially designed jig, the sealer was placed into a 4-mm wide × 4-mm deep well which was then set onto the tooth at a 90-degree angle and allowed to set for 7 days. 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. The set-up was subjected to a tensile load at a crosshead speed of 1 mm per min. The only significant difference with regard to the presence or absence of the smear layer was found with AH26, which had a stronger bond when the smear layer was removed.Gettleman BH, Messer HH, ElDeeb ME. Adhesion of sealer cements to dentin with and without the smear layer. J Endod 1991; 17:15-20.
Removal of the smear layer
Reasons for removal The smear layer can affect bonding, disinfection as well as obturation hence why it is considered important to remove. As discussed earlier this is a result of the fact that bacteria can be left entombed within the smear layer, if not removed.
Impaired bonding Removing the smear layer exposes the underlying bulk and the orifices of the dentine tubules which are occluded following mechanical tooth preparation. This allows interaction between the bulk dentine and restorative resin ensuring an effective bond and seal. Failure to remove the smear layer may compromise the bond strength and sealing ability as it may not be strongly bound to the underlying dentine. Impaired disinfection If the smear layer is not removed during dental procedures, the disinfection process will be compromised as the disinfectant is unable to penetrate the infected dentinal tubules. The presence of bacteria in dentinal tubules can reduce the outward flow of dentinal fluid, promoting disease and an increased diffusion rate of substrates into tubules. Disease promotion can lead to inflammatory changes in the pulpo-dentinal complex, leading to pulpitis, pulpal necrosis, infection of the root canal system and periapical disease. This can cause pain, and discomfort and further complications if left unchecked. Impaired obturation Following mechanical tooth preparation during endodontic procedures the smear layer can act as a barrier between filling materials and the root canal wall. This compromises the formation of a satisfactory seal, leading to possible coronal leakage. Bacteria left underneath a material can populate and allow the invasion of more bacteria into dentinal tubules infection and failure of treatment. Teeth prepared using GI sealant and lateral condensation of GP showed higher rates of leakage when smear layer was not removed.
Methods for removal
Endodontic irrigation 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. Therefore, the recommended protocol for smear layer removal is NaOCl followed by EDTA (ethylenediaminetetraacetic acid) or citric acid. Water, saline, chlorhexidine (CHX), or iodine compounds have no dissolving effect on the smear layer.
Sodium hypochlorite (NaOCl) The most widely used root canal irrigation solution for several decades because it is inexpensive, can dissolve infected necrotic tissue and is bactericidal. The antimicrobial effectiveness is due to its high pH. This interferes with the cytoplasmic membrane integrity with irreversible enzymatic inhibition, biosynthetic alterations in cellular metabolism and phospholipid degradation. When hypochlorite contacts proteins it causes amino acid degradation and hydrolysis through the action of chloramine molecules. Thus necrotic tissue and pus are dissolved. Both citric acid and EDTA immediately reduce the available chlorine in solution, rendering the sodium hypochlorite irrigant ineffective on bacteria and necrotic tissue. Hence, citric acid or EDTA should never be mixed with sodium hypochlorite. Ethylenediaminetetraacetic acid (EDTA) This is the most widely used chelating agent. Its prominence as a chelating agent arises from its ability to sequester di and triactionic metal ions such as Ca2+ and Fe3+. With direct exposure for extended time EDTA extracts bacterial surface proteins by combining with metal ions from the cell envelope, which can eventually lead to bacterial death. Effect of phosphoric acid etching and self-etching primer application methods on dentinal shear bond strength. Testing and clinical evidence has shown that 17% EDTA needs to be placed inside the root canal for one minute to effectively dissolve organic components and smear layer. If the EDTA is placed within the root canal for less than one minute, the smear layer will not optimally be removed. The recommended time for smear layer removal is two minutes. EDTA alone cannot remove the smear layer completely. The inorganic portion is removed but the organic matter is still left partially blocking the dentin canal openings. EDTA effectively abolishes the tissue‐dissolving effect of NaOCl and should therefore not be used until at the end of the treatment as the final rinse. Citric acid 10% citric acid can be used as an alternative to EDTA as the final rinse to remove the smear layer after use of NaOCL. Citric acid is more potent than EDTA. Citric acid is used as a component in MTAD and Tetraclean, the combination products for smear layer removal. In the MTAD preparation citric acid helps remove the smear layer by allowing the doxycycline to enter the dentinal tubules and exert an antibacterial effect.
Paste lubricants
Gel based lubricants can be placed on the instrument before insertion into the root canal to reduce friction. Examples include "Glyde" and "Fileze" which both contain the chelating agent EDTA which can help enlarge narrow root canals by softening the canal walls.
Dentine conditioners
These are generally acidic solutions which dissolve or at least solubilize the smear layer in attempt to expose the underlying dentine to the bonding agent. Examples include: phosphoric acid, nitric acid, maleic acid, citric acid, EDTA. Most manufacturers now supply a single agent to simultaneously etch enamel and condition the dentine.