1st GENERATION APEX LOCATORS

These apex locators use the resistance method for determining the WL.. Basically these instrumentsmeasured the opposition to the flow of direct current (resistance) and hence the name Resistance based apexlocators, Initially an alternating current of 150 Hz Sine wave was used (Root canal meter, 1969) but the painwas felt by the patient due to high currents, Therefore modifcations were made and new machines whichused current less than 5 micro amperes were introduced (Endodontic Meter S ll, Kobayashi, 1995). Sincethese machines were not found to be accurate, the research work continued to develop in this field.

2nd GENERATION APEX LOCATORS

These apex locators use the Impedance method for determining the Wl. Basically these instrumentsmeasure the opposition to the flow of alternating current (impedance) and hence the name Impedance basedapex locators. 'These units utilize the current of a single frequency to perform the task. Formatron Ivzl, .Son0Explorer'! and Endocater are a few examples of this generation, almost all having the similar drawback ofinaccurate readings especially in the presence of irritants in the canal

3rd GENERATION APEX LOCATORS

These apex locators use two frequencies insteadof a single one to measure the impedance in order tose should be called “comparative impedance” type apexdetermine the WL. With this scientifc rationale thlocators. However, the impedance of any given circuit is influenced by the frequency of the current flow,e credit of being the first apex locator in this categoryhence the name frequency based apex locators. Thgoes to Endex'". However it had the drawback of rejuiring calibration for each canal before use. Later cameRoot ZX, which did not require any calibration!]It uses two different frequencies of 400 Hz and 8 kHzsimultaneously to measure the impedance in thecanal. Then it determines a quotient value by dividing8 kHz impedance value by 400 Hz impedance vale. The reading of minor diameter is revealed when thequotient value is 0.67s!. These apex locators had theupper hand over their predecessors in terms of accuracyand reliability, Other units falling into this categoryare AFA, Neosono Ultima EZ, Justy Il, etc.

4th GENERATION APEX LOCATORS

These apex locators use multiple frequencies (2-5 frequencies) to measure the impedance in order todetermine the W,’!. Multi-frequency measurement system is used to calculate the distance from the tipof the file to the foramen by measuring changes in impedance between two electrodes. Unlike the thirdgeneration, these ones do not use the impedance value as a mathematical algorithm only to assess theWl, but instead utilize the resistance and capacitance measurements and thereafter compare them with adatabase to measure the distance of the fle to the apex of the canal. This technology presumably leads to lesssampling error and more consistent readings. Canal pro apex locator (Coltene) belongs to this category. Themeasurements in Canal Pro apex locator are performed using AC signals at two frequencies. The frequenciesare alternated rather than mixed, as it is done in other apex locators, thus canceling the need for signal flteringand eliminating the noise caused by non-ideal flters. The RMS (Root Mean Square) level of the signal ismeasured, rather than its amplitude or phase, The RMS value is much more immune to various kinds ofnoises than other parameters of the measured signal. The two-feld display with file tracking over the wholecanal length and enlarged apical Zoom makes this apex locator uniquely different from the existing thirdgeneration ones. 'The apex locators of this generation, so far, are the best in their category owing to their highaccuracy and reliability, For a clinician, looking for high accuracy and reliability in their Wl determinationthe fourth generation apex locators would be the most ideal, for they can be trusted upon the most.

Fifth Generation:

Fifth-generation EALs brought about advantages such as safety, reliability, clinician andof working length, especially in the presence ofpatient friendliness, and accurate detectioleven if 5th generation performs very well in theexudates or weeping in the canal. Howevepresence of blood and exudate they experence considerable difficulties while operating indry canals

Sixth Generation Apex Locators:

The sixth generation, also termed adaptive apex locators, represents a modification of thefifth generation and exhibits the highest consistency in measurements, particularly in casesof root perforation or apical root resorption, Measurement with the adaptive apex locatorhas made it possible to eliminate the necessity of drying or moistening the canal. Theprecision of readings in this generation is not affected by the presence of blood, sodiumhypochlorite, or while manipulating dry canals. One of the specific characteristics of thisgeneration is that those units can continuously adapt to the level of humidity in the rootcanal.

Standard： IEC60601-1-2020

section 6.2 Protection against electric shock