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Smear Layer ManagementEDTA is a decalcifying chelating agent used as a 15% to 17%buffered solution during instrumentation of root canals. Thedecalcifying efficacy of EDTA-containing pastes is variable.160,441EDTA acts as a chelator with calcium ions and removes thedentinal debris produced on the root canal walls during preparation.It thus opens dentinal tubules, promoting better penetrationof disinfectants.154,191,422,485 Whenever the wall of a rootcanal is instrumented, whether by hand or rotating instruments,the parts of a dentin wall touched by an instrument arecovered by a surface layer called the smear layer.261,296 The smear layer, which consists of dentin shavings, cell debris,and pulp remnants,369 can be described as itself having twoseparate layers: a loose, superficial deposit and an attachedstratum that extends into the dentinal tubules, forming occludingplugs.87For some time, clinicians and researchers paid little attentionto the smear layer, partly because it was a thin superficiallayer (1 to 5 m) that might be present or not, depending onthe type of instrument and the sharpness of its cutting blades.369Also, because acids and chelating agents dissolve the smearlayer, it was removed and escaped attention in routinely processedspecimens (Fig. 9-76).109 Smear layers are not seen inunprepared canal areas, which may have calcospherites, buttonlikestructures that are abundant on intracanal surfaces.Some authors have reported that an overlying smear layerdelays but does not eliminate the effect of medicaments.287Others contend that a smear layer may adversely affect disinfectionand may also increase microleakage after canal obturation.369 Although organic substrate in a smear layer may serve

Chelating agents were introduced to endodontic treatmentby Nygaard-Ostby in 1957 for treatment of calcified narrowroot canals.216 EDTA is the chelating solution customarily usedin endodontic treatment. It is available in both liquid and pasteforms with common concentrations between 15% and 17%.143A detergent is frequently added to the liquid to decrease surfacetension, to increase the cleaning ability, and to enhance thebactericidal action of the solution.323 The effectiveness ofEDTA is related to time of application, the pH, and theconcentration.208,Demineralization results in increased dentin permeability119because of the removal of the smear layer and plugs andenlargement of the tubules. It appears that the tubular enlargementis due to selective removal of the peritubular dentin.141The action of chelators and acids appears to be more effectivein the coronal and middle thirds of the root and is reducedapically.143,188 This reduced activity may be a reflection of canalsize.172 This is a clinical concern because of the more irregularstructure of dentin in the apical third. Another investigationdemonstrated marked variations in the apical portion of theroot,204 including accessory root canals, areas of resorption andrepaired resorptions, pulp stones, irregular or absent primarytubules, irregular secondary dentin, and cementum-like tissuelining the apical root canal wall. The variable structure of theapical region of human teeth presents challenges to the use ofendodontic obturation techniques requiring adhesives, becausethis may influence the dentin bonding ability in the apicalregion.204

EDTA appears to be biocompatible when used clinically216;however, irreversible decalcification of periapical bone andneuroimmunologic disturbances have been noted.269 Extrusionof both NaOCl and EDTA in clinical treatment should beavoided.130,228,306The recommended time for removal of the smear layer is 1to 5 minutes.53,143,264 The small particles of the smear layer areprimarily inorganic with a high surface-to-mass ratio that facilitatesremoval by acids and chelators. Investigators have foundthat a 1-minute exposure to 10 ml of EDTA was adequate toremove the smear layer and that a 10-minute exposure causedexcessive removal of both peritubular and intratubular dentin.53The use of EDTA in combination with NaOCl is recommended300,307and may enhance the cleaning188,349 and antimicrobialeffects of these solutions when compared with usingthem alone.49

peritubulardentin is harder than intertubular dentin and therefore is morequickly dissolved in acid than intertubular dentin. By preferentiallyremoving peritubular dentin, acid-etching agents usedduring dental restorative procedures and ethylenediaminetetraaceticacid (EDTA) used in endodontic treatment enlarge theopenings of the dentinal tubules, making the dentin morepermeable.