Electronic apex locator

The apex of the root has a specific resistance to electrical current, and this is measured using a pair of electrodes typically hooked into the lip and attached to an endodontic file.

Various devices signal this event by a beep, a buzz, a flashing light, digital readouts, or a pointer on a dial.

A problem with these devices was that conductive fluids such as hemorrhage, exudate, or irrigant in the canal would permit current flow and therefore a false reading.

Newer devices are impedance-based, using alternating current of two frequencies;these measure and compare two electrical impedances that change as the file moves apically.

The electrical circuit is completed, when the instrument is introduced into the root canal in an apical direction, and touches the periodontal tissues.