The principle design and development of electronic apex locators date back to research
undertaken by Suzuki in 1942. He found that electrical resistance between the periodontium and
oral mucous membrane in dogs was a constant value.2
Sunada, I. New method for measuring the length of the root canal. J Dent Res 1962; 41:375.
This study carried Suzuki’s findings one step further. Using a simple DC ohmmeter, the electrical
resistance between the periodontium and the oral mucous membrane was measured in 124 teeth.
· Resistance between the periodontium and the oral mucous membrane in humans was constant at
6.5 KW, regardless of the age of the patients or the shape and type of teeth. Contents of the canal
(vital pulp vs. necrotic pulp) also had no effect upon the resistance.
· Bottom Line: It was found that the resistance between the periodontium and the oral
mucous membrane in humans was 6.5 KW
Apex locators are also divided into 1
st
, 2
nd
, 3
rd
, and 4
th
generation.
· First-generation apex location devices, also known as resistance apex locators, measure
opposition to the flow of direct current or resistance. When the tip of the reamer reaches
the apex in the canal, the resistance value is 6.5 kilo-ohms (current 40 mA).
· Second-generation apex locators, also known as impedance apex locators, measure
opposition to the flow of alternating current or impedance.
· The principle on which “third-generation” apex locators are based requires a short
introduction. In biologic settings, the reactive component facilitates the flow of
alternating current, more for higher than for lower frequencies. Thus, a tissue through
which two alternating currents of differing frequencies are flowing will impede the
lower-frequency current more than the higher-frequency current. Since the impedance of
a given circuit may be substantially influenced by the frequency of the current flow, these
devices have been called “frequency dependent ”. Since it is impedance, not frequency
that is measured by these devices, and since the relative magnitudes of the impedances
are converted into “length” information, the term “comparative impedance” may be
more appropriate.
· The proposed “fourth-generation” apex locators are marketed by Sybron Endo and
included the AFA Apex Finder and the Elements Diagnostic Unit. Both are ratio type
Direct Current: Original Ohmmeters used by Suzuki and Sunada
Resistance type:
Root Canal Meter/The Endodontic Meter (Onuki)
Sono Explorer (Salatec)
Neosono-D, MC, and Ultima EZ (Amadent)
Apex Finder (EIE – old version)
Impedance type: Endocator (Hygienic) uses 400 kHz
Subtraction
(difference)
type:
Endex/Apit (Osada) uses 1kHz and
5kHz
Neosono Ultima EZ (Amadent)
2
frequencies
Root ZX (J. Morita) uses 0.4kHz and
8kHz
Alternating
Current:
Frequency type:
Ratio type:
5
frequencies
The AFA Apex Finder (Sybron)
Elements Diagnostic Unit (Sybron)3
apex locators that determine the impedance at 5 frequencies and both have built-in
electronic pulp testers.
.
