A Comparative Scanning Electron Microscopic Investigation of the Smear Layer after the Use of Sodium Hypochlorite Gel and Solution Forms as Root Canal Irrigants Vahid Zand, DDS, MSc,* Mehrdad Lotfi, DDS, MSc,* Saeed Rahimi, DDS, MSc,* Hadi Mokhtari, DDS, MSc,* Ali Kazemi, DDS,* and Vahideh Sakhamanesh, DDS † Abstract Introduction: The effect of sodium hypochlorite (NaOCl) gel along with EDTA on the removal of the smear layer has not been studied; therefore, the aim of the present study was to compare the efficacy of gel and solution forms of NaOCl in removal of the smear layer from root canal walls. Methods: A total of 40 single- rooted teeth with minimum curvature (<5  ) were selected and divided into two experimental groups, each containing 15 teeth and one positive control group containing 10 teeth. The canals of all the teeth were prepared with rotary RaCe instruments up to #35. In the NaOCl solution group, the root canals were flushed with 2.5% NaOCl solution during instrumentation and in NaOCl gel group, the instruments were coated with gel form of NaOCl and used inside the root canals; then saline was used for root canal irrigation. Finally, 1 mL of 17% EDTA was used to rinse inside the root canals and remained in the root canals for 2 minutes in the both experimental groups; in the saline group, only saline was used for irrigation. The amount of the smear layer was quantified according to the Torabinejad method using a scanning electron microscope. Data were analyzed by the Kruskal-Wallis and Mann- Whitney tests. All the statistical analyses were set with a significance level of a = 0.05. Results: There were no significant differences between NaOCl gel and solu- tion forms in the coronal, middle, and apical thirds of root canals. There were significant differences between NaOCl solution and saline groups in the three parts of root canal walls and between NaOCl gel and saline in the coronal, middle, and apical thirds. Conclusion: The use of NaOCl gel can be as effective as NaOCl solu- tion along with EDTA in smear layer removal in the three parts of root canal walls. (J Endod 2010;36:1234–1237) Key Words EDTA, NaOCl, scanning electron microscope, smear layer S uccessful endodontic therapy requires shaping and cleaning of root canal systems (1). During canal preparation of infected teeth, special attention must be given to the elimination of bacteria and their toxins from the root canal system (2). The smear layer is produced during root canal preparation by the manipulation of the surface elements of the dentinal walls of canals. It is believed, from a biologic point of view, that the presence of a smear layer contributes to leakage, and it is a source of nutrients for microorganisms (3). One of the most popular root canal irrigants is sodium hypochlorite (NaOCl) because of its physicochemical and antibacterial properties (2). NaOCl has the unique capacity to dissolve necrotic tissue and the organic components of the smear layer (4). However, there are some disadvantages for NaOCl. It is toxic to tissues, causing hemo- lysis, ulceration, and inhibition of neutrophil migration; it results in damage to endo- thelial and fibroblast cells, facial nerve weakness, allergic reaction, and necrosis after extrusion during root canal treatment (5). NaOCl is an alkaline solution with a pH of approximately 11 to 12. It causes injury primarily by oxidation of proteins. Heggers et al studied the toxic effects of NaOCl and found that at a concentration of 0.025%, the solution was both bactericidal and nontoxic. However, at a concentration of 0.25%, the solution became significantly toxic to tissues. They found severe inflamma- tion and cellular destruction in all tissues except for heavily keratinized epithelium (6). Inadvertent injection of sodium hypochlorite beyond the apical foramen may occur in teeth with wide apical foramina or when the apical construction has been de- stroyed during root canal preparation or by resorption. In addition, extreme pressure during irrigation or binding of the irrigation needle tip in the root canal may result in contact of large volumes of the irrigant with the apical tissues (7). If this occurs, excel- lent tissue-dissolving capacity of NaOCl will lead to tissue necrosis (7). The use of the gel form of NaOCl can reduce the risk of extrusion of NaOCl into periapical tissues. The effect of NaOCl gel along with EDTA on the removal of the smear layer has not been studied; therefore, the present study tested the hypothesis that a gel formulation of 2.5% NaOCl as a lubricant accompanied with saline is as effective as 2.5% NaOCl solu- tion in removing the smear layer when 17% EDTA is used as a final rinse. Materials and Methods A total of 40 extracted human anterior single-rooted teeth with mature apices and minimum curvature (<5  ) were selected for the purpose of this study. The degree of canal curvature was determined using the Schneider method (8). After access cavity preparation, the pulp tissues were removed with a barbed broach (Dentsply, Maillefer, Switzerland), and the size of the apical foramen was gauged with a #15 K-file (Dentsply, From the *The Department of Endodontics,Tabriz Dental School,Tabriz University of Medical Sciences, Tabriz, Iran; and † Private Practice, Tabriz, Iran. Address requests for reprints to Dr Mehrdad Lotfi, Department of Endodontics, Tabriz Dental School, Tabriz University of Medical Sciences, Tabriz, Iran. E-mail address: mehrlotfi@yahoo.com. 0099-2399/$0 - see front matter Copyright ª 2010 American Association of Endodontists. doi:10.1016/j.joen.2010.02.033 Basic Research—Technology 1234 Zand et al. JOE — Volume 36, Number 7, July 2010