Introduction - Materials and methods.
M. Dartar Oztan, A. A. Akman, L. Zaimoglu & S. BilgicDepartment of Endodontics, Faculty of Dentistry, AnkaraUniversity.
Department of Physical Chemistry, Faculty of Science, Ankara University, Ankara,Turkey.Introduction.
Chemomechanical canal preparation during the rootcanal treatment involves cleaning and shaping procedures with endodontic instruments and irrigating solutions. The aim of the root-canal instrumentation is to obtain a continuous tapering funnel shape, flowing with the original canal from the coronal access to the apex (Walcott & Himel 1997). The functions of irrigants are to act as lubricants during the mechanical debridement of pulpal and dentinal tissues, a medium to remove debris, a solvent to dissolve tissue, an agent to promote root-canal sterility and patent dentinal tubules on the root-canal walls (Mueller1983).
Many solutions, such as sodium hypochlorite, hydrogen peroxide, citric acid, ethylene diamine tetraacetic acid and physiological saline, have been used for irrigating root canals. In recent years, chlorhexidine gluconate, because of its antimicrobial effects, was also advocated as an irrigating solution (Delany et al. 1982, Jeansonne & White 1994). Even though the benefits of irrigating solutions are essential for chemomechanical preparation, the chemical and electrochemical aggressiveness of these solutions on the instruments must also be taken into account to prevent the decreased service performance of the instruments with usage. The cutting ability of endodontic instruments is affected by several factors, being a complex interaction of different parameters, such as material, metallurgical properties, cross-sectional design, number of flutes, chip-removal capability and helical angle (Scha' ffer 1999). The chemical effects of the irrigating solutions on endodontic files may also hinder their performance (Mueller 1983). Corrosion adversely affects the metallic surfaces by causing pitting and porosity, and decreases the cutting efficiency of endodontic files (Stokes et al. 1999). Neal et al. (1983) evaluated the effect of sterilization and irrigants on the cutting ability of stainless-steel files and concluded that sodium hypochlorite, hydrogen peroxide, and EDTA-urea peroxide irrigants caused a decrease in the cutting ability of the files.
Stokes et al. (1999) evaluated the corrosive effect of 5.25% NaOCl on stainless-steel and Ni-Ti files using five commercial brands. They reported that no relationship existed between the metal alloy and corrosion, as both the corroding- and non-corroding files were present in the same package. They also concluded that the difference could possibly be due to the variations in the manufacturing process and quality control.
The purpose of the present study was to evaluate and compare the electrochemical corrosion rate of stainless- steel endodontic files when immersed in different irrigating solutions. Materials and methods.
Twenty five mm ISO size 25 stainless-steel K-files (Mani Inc.,Tochigi-Ken, Japan) were used for the present study. The irrigating solutions studied were:
- 0.2% Chlorhexidine gluconate (pH 5.72) (prepared in the laboratory);
- 5.25% Sodium hypochlorite solution (NaOCl, pH 12.10) (Sultan Chemists, Inc., Englewood, NJ, USA);
- Chlorinated soda with KOH (5.25% NaOCl ÑKOH, pH 12.09) (Sultan Chemists Inc., Englewood, NJ, USA); and
- 17% EDTA, pH 5.25 (prepared in the laboratory).
The corrosion rates of the stainless-steel K-files in the irrigating solutions were determined electrochemically by the Tafel extrapolation method. Electrochemical experiments were carried out in a Pyrex cell with three compartments. The cell was water-jacketed, and connected to a constant temperature circulator. The experiments were carried out at a stable 37 8C. Stainless-steel files were used as experiment electrodes. The cutting flutes of the files were immersed in the irrigating solutions, and kept for 20 min in the cell prior to each experiment, so that the rest potential of the electrode could be attained. A saturated calomel electrode (SCE) was used as a reference and a platinum plate as counter electrode. All potentials were referred to SCE. During each experiment, the solutions were mixed with a magnetic stirrer to ensure the contact of solutions with the whole file surface.
Data was obtained using a combined system containing a potentiostat (Wenking LB 75 L, Gottingen, Germany), a voltage scan generator (Wenking VSG 72) and a recorder (Yokogawa 3077, Tokyo, Japan). The potential scan rate was chosen as 2.5 mV.
In order to determine corrosion rates, the linear part of anodic currents, obtained from electrochemical current potential curves (Elog) was extrapolated to corrosion potentials. Seven experiments were done for each solution, and statistical analysis of the data was performed using the Kruskal-Wallis one-way anova. The files from the each group, and also one untreated control file (Fig.1) were photographed under the scanning electron microscope (SEM).
Figure 1. SEM of stainless-steel control file revealed no evidence of corrosion (1000x).