Several studies have been conducted to determine a
constant electrical resistance or impedance in order to
diagnose the position of the canal terminus, e.g.
Sunada (1962) who determined R ¼ 6.5 kX. However,
Meredith & Gulabivala (1997) reported there is neither
constant reference impedance nor constant resistance
for all root canals. The reason that, for example,
resistance-based devices work in a reasonable number
of cases is that there is a substantial difference between
the resistance (or the impedance) value at the pulp and
periodontal junction compared with intracanal positions.
Indeed, this is the property Custer (1918)
described many years ago.
Based on this fact, Ushiyama (1983) proposed a
method to measure the variation in impedance when a
file was inserted into the root canal. Using bipolar
electrodes and applying a 400-Hz alternating current,
this device monitored variations in the impedance
value. Ushiyama (1983) concluded that a sharp
variation in the value, determined the position of the
file at the apical constriction, the narrowest portion of
the root canal system. Ushiyama (1983) also reported
that in the presence of strong electrolytes the ‘voltage
gradient method’ could accurately detect the apical
constriction. However, use of a special bipolar electrode
is one of the main disadvantages of this device, as the
electrode will not fit into narrow canals. No commercial
product of the experimental device developed by
Ushiyama (1983) appears to be available.