Measuring Parameters for Working Length Determination with the Impedance Method

Objectives: To determine the best range of measurement frequencies and optimal impedance ratio value for working length determination with the impedance ratio method, and to evaluate its accuracy. Methods: Electrical impedance was measured in vitro on 14 extracted human teeth with single canals using a QuadTech 1920 precision impedance analyzer. A file electrode was inserted into the root canal; the second electrode was placed in the saline solution surrounding the tooth. Measurements were performed in a frequency range from 20 Hz up to 1 MHz, and were repeated with different distances of the file tip from the major apical foramen (from 8 mm inside the canal to 2 mm beyond the foramen). The measured impedances were analyzed as a function of distance of the file tip to the major apical foramen. Parameters evaluated were measurement frequencies as well as impedance ratio values that would result in optimal determination of the root canal length according to the two criteria: to result in minimal average distance with the lowest standard deviation of the file tip to the major apical foramen, and in maximal number of measurements within ±0.5 mm distance. Results: The optimal determination of the major apical foramen position was obtained at frequencies of 5 kHz and 1 kHz for the impedance ratio between 0.75 and 0.79 (95% confidence interval: -0.63–0.49). The strict clinically acceptable limit of ±0.5 mm was attained in 86% of all measurements. Conclusion: With the obtained optimal measuring parameters, the impedance ratio method determined the working length with acceptable accuracy and consistency.

Supported by the Slovenian Ministry of Science, Education and Sport (No. J3-8713-0381-99).