Use of Apex Locators

One of the innovations in the root canal treatment has been the development and production of electronic devices for detecting the canal terminus. The preoperative radiograph is essential in endodontics to determine the anatomy of the root canal system, the number and curvature of the roots, the presence or absence of disease, and to act an initial guide for working length. The electronic apex locator is an instrument, which when used with appropriate radiographs, allows for much greater accuracy of working length control.

History of Electronic Apex Locators

An electronic method for root length determination was first investigated by Custer in 1918. The electrical conductivity of the tissues surrounding the apex of the root is greater than the conductivity inside the root canal system provided the canal is either dry or filled with a non-conductive fluid. It has been found that the electrical resistance between a root canal instrument inserted into a canal and an electrode applied to the oral mucous membrane registered consistent values and speculated that this would measure the canal length. There are a number of apex locaters. There can be first generation, second generation, third generation and fourth generation.

First Generation

Second Generation

Third Generation

Fourth Generation

Other Uses of Apex Locators

All modern apex locators are able to detect root perforations to clinically acceptable limits and are equally able to distinguish both large and small perforations. This aids in decision making and consideration of treatment options. Suspected periodontal and pulpal perforation during the pinhole preparation can be confirmed by all apex locators, as a patent perforation will cause the instrument to complete a circuit and indicate the instrument is beyond the ‘apex’. Any connection between the root canal and the periodontal membrane such as root fracture, cracks and internal or external resorption will be recognised by the apex locator which serves as an excellent diagnostic tool in these circumstances. Multiple function apex locators are becoming more common and have several vitality testing functions as well.

Problems Associated with the Use of Apex Locators

The biological phenomena such as inflammation can have an effect on the accuracy of the apex locators. Intact vital tissue, inflammatory exudates and blood can conduct electric current and cause inaccurate readings so their presence should be minimised before accepting apex readings. Other conductors that can cause short-circuiting are metallic restorations, caries, saliva and instruments in a second canal. Care must be taken if any of these variables exist.

Lack of patency, the accumulation of dentine debris and calcifications can affect accurate working length determination with electronic apex locators. It has been suggested that pre-flaring of root canals as used in modern crown-down preparation techniques would increase the accuracy of readings. Canal patency appears to be more important, as dentine debris may disrupt the electrical resistance between the inside of the canal and the periodontal ligament. Constant recapitulation and irrigation ensures accurate electronic length readings during instrumentation. The size of the apical foramen also has an influence on electronic length determination. Immature or ‘blunderbuss’ apices tend to give short measurements electronically due to the instruments not touching the apical dentine walls. Other methods for length determination such as the use of paper points are deemed to be more effective in these cases.

Electromagnetic interference from dental equipment including electronic apex locators has the potential to interfere with cardiac pacemakers. The manufacturers of electronic apex locators specifically warn against their use with patients with cardiac pacemakers, as there are many therapeutic uses and types of pacemakers some may not be influenced by apex locator use. To use apex locator safely in patients with pacemakers it might be prudent to confer with the patients’ cardiologist prior to treatment.