Oral Health Group

ENDODONTICS: The Use of Apex Locators in the Diagnosis of Perforations

December 1, 2000
by Yosi Nahmias, DDS, MSc.

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’s2 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).

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

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.


A 53-year-old patient was undergoing conventional root canal therapy for tooth #4.5, which was suspected to have sustained an iatrogenic perforation. (Fig. 3).

There was no clear-cut evidence to substantiate this clinical impression. A #10 file connected to an apex locator was inserted into the buccal canal. A gradual increase in the electrical resistance was observed, and when the apical foramen was reached the apex locator gave us a reading indicating it.

The canal length was determined to be 15mm. When the file was inserted in the “lingual canal,” the apex locator indicated that the foramen had been reached as soon as the file was inserted into the canal opening. This confirmed the clinical impression that a perforation was present in the root canal system.

The character and location of the defect precluded a surgical correction. The tooth was extracted, and the perforation was visually verified. I have to add that this case was treated previous to the use of MTA. Nowadays this perforation could have been repaired by using this material!


A 35-year-old patient presented with a recently completed root canal treatment and a post cemented in the distal root. It was suspected that the post had perforated the middle area of the root. Radiographs failed to provide a definite answer (Fig. 4).

No other clinical signs or symptoms were present. It was decided to connect one of the electrodes of the apex locator to the post in order to demonstrate a communication with the periodontal membrane. The apex locator registered a typical “in canal” reading. This and evidence of total healing after one year confirmed the accuracy of the reading with the apex locator (Fig. 5).


A 75-year-old patient required endodontic treatment on an upper bicuspid that had been recently crowned. The radiograph revealed that the canals appeared to be calcified (Fig. 6).

Endodontic therapy was started using the surgical microscope. The access opening was made through the crown and the tooth was transiluminated in order to find the orifices into the canals. A white dot was seen at high magnification. A #8 file was introduced into what appeared to be a canal and it was connected to one of the electrodes of an apex locator.

The device registered an “in canal” reading. A second canal was found using the same technique. A radiograph was taken to confirm the working length and the location of the canals (Fig. 7).

The only way I know to prevent a perforation 100% is by not doing any endodontics. All of us who like doing endodontics take this risk on a daily basis. Careful examination and thorough knowledge of the internal and external anatomy of each tooth can prevent disasters from happening.

Techniques that utilize high magnification, transilumination, dyes and ultrasonics can be extremely useful. However, sometimes we can encounter situations where all the anatomical landmarks are gone and the only thing that we can see is a small “red” or “white” dot. The question is, how can we tell, if these dots are canals or perforations!

If we are able to determine that a perforation has occurred we can proceed to repairing it immediately and thus increase the chances of repair dramatically. Materials such as MTA can be used quite predictably in situations like this one. However, the success of a perforation repair depends on the size of the defect and how quickly the defect is sealed. Prompt diagnosis is then paramount. The use of an apex locator in this scenario can be extremely advantageous.

Dr. Nahmias currently maintains a private practice specializing in endodontics in Oakville, ON, Canada. He is also the creator of Endoweb (www.endoweb.com) an endodontic electronic magazine. He can be reached at nahmias@endoweb.com

Oral Health welcomes this original article.


1.Nahmias Y, Aurelio A and Gerstein H. Expanded Use of the Electronic Canal Length Measuring Devices; J Endo 1983, 9:347-349.

2.Sunada I. New Method for Measuring the Length of the Root Canal. J Dent Res. 1962, 41:375-387.

3.Kobayashi C. The Evolution of Apex-Locating Devices. Alpha Omegan. 1997, 90 #4:21-27.