Impedance Characteristics of Mimic Human Tooth Root Canal and Its Equivalent Circuit Model Jui-Hsiung Huang, a Shi-Chern Yen, a,z and Chun-Pin Lin b,z a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b School of Dentistry and Graduate Institute of Clinical Dentistry, National Taiwan University and National Taiwan University Hospital, Taipei 10051, Taiwan This investigation examined the complicated impedance characteristics of the human tooth root canal using a mimic system that comprised a cylindrical tubule and 5% agar-gel. The in vitro impedance measurements of root canals were made using a frequency response analyzer and a tubule filled with distilled water or normal saline. An equivalent circuit model of this bioelectric system was proposed; it closely corresponds to the experimental results. On the basis of the proposed equivalent circuit model, the experimental results can be explained reasonably and the effect of the filling of the canal with the electrolytes can be clearly understood. The modified impedance quotient  Q and the variation ratio of the impedance quotient  S Q  are taken as the useful indices of the apex. Different impedance behaviors have been identified from Q values in under-apex and over-apex regions, respectively. The S Q was 0.15 at the apex location as well as in the overapex region for all electrolytes and could be used to eliminate the effect of the electrolytic conductivity. This study elucidates the complicated electric mechanisms of electrolytic systems and improves clinical applications in the endodontic treatment. © 2008 The Electrochemical Society. DOI: 10.1149/1.2844440 All rights reserved. Manuscript submitted April 12, 2007; revised manuscript received November 26, 2007. Available electronically March 11, 2008. The success of endodontic treatment depends on the accurate measurement of root canal length; 1-3 various approaches, such as tactile sense and radiographic and electronic techniques, have been employed to locate the canal apex of a diseased tooth. The tactile sense is empirical and relatively unreliable. The radiographic tech- nique is also applied in endodontic treatment, but opens to observer interpretation that may result in under-measurement or over- measurement of canal apex position. 4,5 Accordingly, the accuracy of these two methods is limited and depends on the experience of the dentist. The electronic technique for assessing root canal length has at- tracted considerable attention over the last few decades. Instead of the visual inspection as used in the radiographic technique, it utilizes measurements of the electrical resistance or impedance between the measuring electrode file, which is inserted into the diseased root canal, and the other large-surface neutral electrode lip clip, which is placed on the buccal mucosa or held in the patient’s hand. Suzuki 6 measured the electrical resistance between the periodontal ligament and oral mucosa and registered a consistent value of 6.5 k, in- dependently of the age of the patient or the shape and type of tooth. Sunada 7 employed this principle and designed a clinical instrument for measuring root canal length. Subsequently, several instruments called electronic apex locators based on alternating current with a single frequency or multifrequencies have been developed. 8-10 These frequency-dependent instruments are based on the electrical prin- ciple that the impedance changes substantially when the file ap- proaches the apex of the tooth, in which the root canal is at its narrowest. Although the electronic apex locators can reduce the treatment time and the radiation dose, two main factors, the presence of electrolyte in the root canals and the apical size of the root canal, hamper the precision of the electronic apex locator. 11,12 The compli- cated electrical mechanisms associated with the electrical compo- nents must be better understood, based on an electrochemical view of the nonfaradaic impedance system, to improve the accuracy of electronic apex locator. However, most dentistry-related studies have evaluated the accu- racy of commercially available instruments in vivo or in vitro. 13-17 Surprisingly, little fundamental work has been performed on related electrical characteristics and mechanisms. In this study, the com- pleted equivalent circuit model for the complicated electrical mecha- nism of the endodontic treatment is developed and used to simulate the experimental results. On the basis of the conditions of the elec- trolyte in the endodontic treatment, the impedance characteristics of a mimic root canal are examined from the perspective of the elec- trochemical system. Furthermore, new indices numbers Q and S Q  are defined to support more precise location of the apex. This inves- tigation will help us to elucidate the complicated mechanisms of electronic endodontic treatment and improve clinical dental applica- tions. Experimental An acrylic test cell with an area of 6.0  6.0 mm and length of 15.0 mm shown in Fig. 1a, was used to mimic a human tooth. A cylindrical hollow tubule to mimic the root canal with a 0.2 mm bottom apex diameter and a 1.4 mm top diameter was prepared us- ing an acrylic cell under a stereoscopic microscope. Although the electrical characteristics of the acrylic test cell with the cylindrical tubule differ from those of a human tooth, the use of an acrylic tubule to mimic root canals may help eliminate bias in the imped- ance measurement that results from canal curvature, canal taper, or the latent lateral canals. In addition to the mimic cell, an extracted human tooth with a straight single canal was also prepared by re- moving crowns to the cementum-enamel junction level and enlarg- ing the corona to a top diameter of 1.0 mm with an orifice opener. As presented in Fig. 1b, the length and apex diameter of the ex- tracted human tooth were 11 and 0.2 mm, respectively. Following preparation, the extracted human tooth was stored in normal saline solution  0.9% NaCl. As shown in Fig. 1c, electrical impedance measurements of the test cell or the extracted human tooth were made. The test cell or extracted human tooth was mounted onto the polypropylene holder  20 mL, and then 5% agar in normal saline solution was gelled within the holder to mimic the periodontal tissue. The bottom sur- face of the test cell just touched the agar gel surface to prevent the agar from moving into the tubules, yielding erroneous measure- ments. Then the electrolyte was injected into the tubules using a syringe until they were completely full. The electrolyte used herein was either distilled water or normal saline. A no. 15 stainless steel file with a length of 18.0 mm 0.02 taper- off, Profile, Densply Maillefer Co. and a curved stainless steel lip chip with a large area, which rested in the agar gel, were used as the measuring electrode and neutral electrode, respectively. The electri- cal impedance between the file and the neutral electrode was mea- sured using an electrochemical impedance measuring system that comprised a potentiostat/galvanostat AutoLab PGSTAT 30, ECO Chemie with a frequency response analyzer FRA module and FRA 4.9 control software. A small output potential signal from the FRA was sinusoidal with an amplitude of 10 mV based on the open- z E-mail: scyen@ntu.edu.tw; pinlin@ntu.edu.tw Journal of The Electrochemical Society, 155 5 P51-P56 2008 0013-4651/2008/1555/P51/6/$23.00 © The Electrochemical Society P51 Downloaded 28 Nov 2008 to 140.112.113.225. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp