The most critical stage of root canal treatment is precisely determining the millimeter-sized boundary (apical narrowing) where the canal ends and the tooth exits the root canal. In the past, this was only estimated using X-rays. Modern electronic apex locators work by measuring the difference in electrical resistance (impedance) between the tissues of the human body and the hard tissues of the tooth. As the tip of the file advances through the root canal, the device continuously measures; the moment the file tip contacts the living periodontal tissue outside the root, the electrical circuit is completed, and the device audibly and visually indicates the exact “0.0” point (root apex) in millimeters. The reliability rate of modern apex locators is between 95% and 98%. Thanks to these devices, the risk of overflowing the filling or leaving it too short at the root apex has been almost completely eliminated.
