What are Apex locators their Classification, Uses and types

Apex Locators are the instruments used in Endodontics to determine the Working length in Root Canal Treatment as an adjunct to Radiography. They are used to locate the Apical foramen by determining the Apical constriction or Cementodentinal junction. These are divided into 4 types based on the way the apex is located

Cemento dentinal junction location for Apex locators

Classification of Apex locators based on Generations:

1st generation Apex locators:

These are also known as Resistance Apex locators. This measures the opposition to flow of direct current i.e resistance. It is based on the principle that resistance offered by periodontal ligament and oral mucous membrane is the same i.e. 6.5 K ohms. It is called as Sonoexplorer. These apex locators give false readings in the presence of Blood, pus, chelating agents, irrigants in the root canal.

Resistance based 1st generation apex locator

Advantages of 1st generation Apex locators:

  • Easily operated
  • Digital read out
  • Audible indication
  • Dtects perforation
  • Can be used with K-file
  • May incorporate pulp tester
Disadvantages of 1st generation Apex locators:
  • Requires a dry field
  • Patient sensitivity
  • Requires calibration
  • Requires good contact with lip clip
  • Cannot estimate beyond 2mm


  • Neosono Apex locator

2nd generation apex locators:

It works on the principles of Impedance, hence they are also called as impedance apex locators. They work by measuring opposition to flow of alternating current or impedance. A major disadvantage of second generation apex locator is that root canal has to be free of electro-conductive materials to obtain accurate readings. Presence of electro-conductive materials to obtain accurate readings. Presence of electro-conductive irrigants changes the electric characteristics and lead to inaccurate readings. Examples of Second generation apex locators.


  • Endocater
  • Sono explorer
  • Apex finder endoanalyzer it is a combination of Apex locator and pulp tester
  • Digipex
  • Extract a pex
  • Foramatron IV
  • Pio apex locator
Advantages of 2nd generation Apex locators:
  • Does not require lip clip
  • No patient sensitivity
  • Analog method
Disadvantages of 2nd generation Apex locators:
  • No digital read out
  • Difficult to operate
  • Canal should be free of elecro-conductive irrigants and tissue fluids

3rd generation apex locator:

They are also called as Frequency dependent apex locator. Works on the principle of frequency or Comparitive impedence. They are based on the fact that different sites in canal give difference in impedance between high (8 KHz) and low (400 Hz) frequencies.The difference in impedance is least in the coronal part of canal. As the probe goes deeper into canal, difference increases. It is the greatest at cemento dentinal junction.

Advantages of 3rd generation apex locator:

  • Easy to operate
  • Uses K-type file
  • Audible indication
  • Can operate in presence of fluids
  • Analog read out
Disadvantages of 3rd generation apex locator:
  •  Requires lip clip
  • Chances of short circuit


  • Endex
  • Root ZX
  • Neosono Ultima
  • Mark V plus
  • Justwo or Justy II
  • Apex finder A.F.A

4th Generation Apex Locators: 

These measure the resistance and capacitance separately rather than the resultant impedance value. There can be difference in the combination of values of capacitance and resistance that provides the same impedance, thus the same foraminal reading. But by using 4th generation apex locators this can be broken down into primary components and measures saperately for better accuracy and thus less chances of occurance of errors.

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