Apex Locators in the diagnosis of perforations

It is Monday morning, you are starting a root canal on an upper first molar. The x ray shows that the canals are calcified. You make your access opening in a careful manner and after half an hour of drilling you see a "red" spot. Your heart rate increases, your blood pressure is up and your palms start to sweat profusely. Is this red dot "good blood" (read "pulpal tissue") or is it " bad blood" (read "perforation"). You try to dry the hemorrhage but you can't. You take a radiograph, you look at it and you tell yourself " it looks like I am in the canal " however, you are not sure!!. You ask yourself "what should I do???". You decide to enlarge the "canal" but the bleeding doesn't stop. You take another x ray, this time it looks like a perforation. Unfortunately this scenario is not uncommon. One of the more perplexing problems in endodontics is the unforeseen perforation of the canal wall or floor of the pulp chamber. These are sometimes difficult to diagnose due to location, film angulation, lack of hemorrhage, and/or subjective symptoms.
About 20 years ago, I described a technique using apex locators in order to determine the existence of a perforation (1). Back then, few dentists were familiar with the use of these devices. Now that technology has taken over our practices, apex locators have become an important part of our armamentarium. That is the reason why I have decided to revisit this subject. This article will describe a technique for the diagnosis of perforations of the root canal wall or pulpal floor with the use of an apex locator.
Apex locators can be used to determine if the perforation communicates with the periodontal membrane, This is based on Sunada's (2) findings that the electrical resistance between the mucous membrane and the periodontium can be considered to have a constant relationship. It can be supposed that the electrical resistance between the oral mucous membrane and the periodontal membrane would register a constant value. Older apex locators worked under this principle (Impedance method)(3). Newer apex locators that work under different principles (gradient method, ratio method etc.) essentially do the same. Once the measuring probe (a file or a reamer) touches the periodontal ligament the apex locator will indicate that the apex has been reached.
When clinical inspection and radiographic evidence are inconclusive in determining whether the root or pulpal floor is perforated, the apex locator should be used in the following manner. A #10 file, connected to the device, is inserted into the suspected perforation. A dramatic increase in the electrical resistance immediately will be noticed if a true perforation is
present (fig#1).


Fig #1 Apex locator's analog readout shows a dramatic increase
in the electrical resistance that is indicative of a perforation.

This is in direct contrast to the gradual increase in the electrical resistance obtained while negotiating an intact root canal system (fig#2).



Fig #2 Typical "in canal" reading. The file is introduced in the canal (A)the analog readout shows little increase of the electrical resistance.
As the file advances
(B) a gradual increase is noted.
The device (C) indicates that the apical foramen has been reached. Experience in the use of the apex locator will allow the clinician to recognize the difference immediately. If in doubt, wash the area thoroughly and dry the site with paper points and repeat the test. A period of familiarization is required for the inexperienced operator to learn the "language" of the machine. All apex locators have equivalent capabilities.