Yamaoka (1984 – quoted in Saito & Yamashita 1990)
developed a measurement device in which two frequencies
were employed in the measurement. This
device measures the impedance value at two different
frequencies (fH and fL) and calculates the difference
between the two values:
Diff ¼ ZðfHÞ ZðfLÞ ð7Þ
In fact, the actual measured signal is the difference
between the voltages in two frequencies that is obviously proportional to the difference in impedance
values. In the coronal portion of the root canal system,
the device must be calibrated to eliminate any effect of
the dielectric material inside the canal.
According to equation 3, the magnitude of the
capacitance of the model is proportional to the distance
between the two nodes shown in Fig. 6. That means,
when the file approaches the canal terminus, the value
of the capacitance sharply increases probably because
of change in the morphology of the apical portion of the
root. On the other hand, the fH frequency used in this
device is five times the fL value. Therefore, according to
equation 4, the change in Z(fL) will be five times larger
than Z(fH), i.e. the difference between two Z(fL) and
Z(fH) impedances rapidly increases at the ‘apical
foramen’. This method has been used in the Apit
device (Osada, Tokyo, Japan).
According to Saito & Yamashita (1990) electrolytes,
such as saline, 5% NaOCl, 14% EDTA and 3% H2O2 did
not interfere with the detection of the apical terminus
regardless of the size of the endodontic file or the size of
‘apical foramen’. Frank & Torabinejad (1993) also
confirmed that the location of the canal terminus could
be detected under moist conditions, but due to the open
electrical circuit the Apit cannot accurately detect the
canal terminus in a dry canal. However, this phenomenon
could be useful for checking the dryness of the
root canal system prior to canal filling (Dahlin 1979).