In vivo assessment of the impedance ratio method used in electronic foramen locators

Abstract

Background: The results of an in vivo study on the "ratio method" used in electronic foramen locators (EFL) are presented. EFLs are becoming widely used in the determination of the working length (WL) during the root canal treatment. The WL is the distance from a coronal reference point to the point at which canal preparation and filling should terminate. The "ratio method" was assessed by many clinicians with the aim of determining its ability to locate the apical foramen (AF). Nevertheless, in vivo studies to assess the method itself and to explain why the "ratio method" is able to locate the apical foramen and is unable to determine intermediate distances were not published so far.

Methods: A developed apparatus applies an electrical current signal with constant amplitude of 10 μARMS through the endodontic file within the root canal. The applied current signal is composed by summing six sine waves, from 250 Hz to 8 kHz. Data were acquired with the endodontic file tip at 7 different positions within root canals. In the frequency domain the quotients between the amplitude of a reference frequency and the amplitudes of the other frequencies components were calculated. Twenty one root canals were analyzed in vivo, during the endodontic treatment of twelve teeth of different patients, with age between 20 to 55 years.

Results: For the range of frequencies used in the commercial EFLs and for distances ranging from -3 mm to -1 mm of the AF, the impedance of the root canal is mainly resistive. However, when the file tip gets closer to AF, the root canal electrical impedance starts to change from a mainly resistive to a complex impedance. This change in the measured root canal impedance starts when the file tip is near -1.0 mm from the AF, getting stronger as the file tip gets closer to the AF. This change in the impedance behavior affects the ratio (quotient) of the impedance measured at different frequencies. Through graphic analysis it is demonstrated why EFLs based on the ratio method are unable to accurately measure any distances between - 3.0 and -0.5 mm from the apical foramen. The only reliable measurement is the 0 mm distance, which is when the file tip is at the AF.

Conclusions: The electrical impedance values of 21 root canals were in vivo studied. The results confirm the ability of EFLs that are based on the ratio method to accurately locate the AF position and explain why they are unable to determine the file tip position along the root canal.

Figure 1

Sketch of the tooth root-terminal structure, illustrating the root apex, apical foramen and the apical constriction (AC), also known as minor foramen.

Figure 2

Block diagram of the Impedance Ratio Modulus Analyzer (IRMA) built for in vivo studies of the root canal bio-impedance.

Figure 3

Result of one of the four experiments to validate IRMA. The root canal impedance was simulated with a 5 kΩ resistor in series with a 27 nF capacitor. The graphic shows the simulated and measured Fourier spectrum. To make the comparison, both Fourier spectra were normalized by the 250 Hz frequency component amplitude.

Figure 4

Results of the in vivo experiments performed to study of the ratio method used in EFLs. A constant electrical current amplitude of 10 μA_RMS was used in the experiments. Vertical axis: Ratio between the impedances or components amplitudes. Horizontal axis: distances of -3.0, -2.5,-2.0, -1.5, -1.0, -0.5 and zero millimeters from the AF. (a) Ratio between the impedances at 8 kHz (Z1) and those at: 4 (Z2), 2 (Z3), 1 (Z4), 0.5 (Z5) and 0.25 kHz (Z6). (b) To make possible a comparison between the ratios, all curves were divided by their maximum values in graphic (a) that occur at 250 Hz. (c) to (g) are the same curves shown in (b) but with the bar standard deviation and maximum and minimum values included (I). The positive value distance corresponds to the one reading taken with the file tip positioned further the AF.