Accuracy of root canal length determination with the impedance ratio method

To provide insight into the principles of operation of electronic apex locators, determine optimal measuring parameters of the impedance ratio method, and to evaluate its accuracy.

Electrical impedance was measured ex vivo on 14 extracted human teeth 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 to 1 MHz, and repeated with different distances of the file tip from the major apical foramen. The measured impedances were analysed as a function of distance of the file tip to the major apical foramen. Parameters (e.g. measurement frequencies, impedance ratio value) that would result in optimal working length determination were evaluated.

The optimal determination of the major apical foramen position was obtained at frequencies of 5 kHz and 0.5 kHz, 10 kHz and 0.5 kHz, and 5 kHz and 1 kHz, for the impedance ratios 0.73 (95% CI: -0.33 to 1.74 mm), 0.66 (95% CI: -0.34 to 1.81 mm) and 0.79 (95% CI: -0.33 to 1.58 mm) respectively. The limit of +/-0.5 mm was attained in 86% of all measurements. Standard deviations decreased as the average measured distance approached and extended beyond the major apical foramen.

With the obtained optimal measuring parameters, the impedance ratio method determined position of the major foramen within +/-0.5 mm. Accuracy varied depending on the set of frequencies used for evaluation as well as on the selected impedance ratio.