A Brief Introduction of the Generation of Dental Apex Locator

The development of the electronic apex locator (EAL) has helped make the assessment of

working length more accurate and predictable, particularly useful when the apical portion of

the canal system is obscured by certain anatomic structures:Impacted teeth, tori ,zygomatic

arch, excessive bone density, overlapping roots and shallow palatal vault.
The objective of working length determination is to establish the length (distance from the

apex) at which canal preparation and subsequent obturation are to be terminated. Methods

for determining working length are radiographs , electronic apex locators, tactile sense,

mathematics method, apical periodontal sensitivity, paper points, microscopic magnification

and average tooth length.


Root canals are surrounded by dentine and cementum that are insulators to electric current.

At the apical foramen there is a small hole in which conductive materials within the canal are

electrically connected to the periodontal ligament that is a conductor of electric current. The

resistive material of the canal (dentine, tissue, fluid) with a particular resistivity forms a

resistor, the value of which depends on the length, cross-sectional area and the resistivity of

the materials .


The first generation: Resistance between the periodontium and the oral mucous membrane in humans was

constant at 6.5 K Ohm, regardless of the age of the patients or the shape and type of teeth.

Contents of the canal (vital pulp tester vs. necrotic pulp) also had no effect upon the resistance.

First-generation apex location devices measure the opposition to the flow of direct current

or resistance. The resistance was measured between the two electrodes to determine

location within a canal. Pain was often felt with this type of apex locator.
Second-generation apex locatorsmeasure the opposition to the flow of alternating current or

impedance.This generation contains 2 types of apex locator: low frequency and high

frequency apex locator. Low frequency AL is based on the assumption that the impedance

between the oral mucous membrane and the depth of the gingival sulcus closely resembles

the impedance between the canal terminus and the oral mucous membrane.


The 3rd generation apex locator has been called “frequency dependent” apex locators. This

type was supplied by 2 frequencies to measure the impedance in the canal. There are 2

types of the 3rd generation ALs: impedance difference type and impedance ratio type.

Impedance difference AL measures the impedance value at two different frequencies and

calculates the difference between the two values (Yamashita, 1990) while impedance ratio

type measured the position of the file from the ratio between these two impedances.


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