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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