THE INFLUENCE OF EXCITATION CURRENT ON THE ROOT CANAL
LENGTH MEASUREMENT
T. Protulipac, Z. Stare
Faculty of Electrical Engineering and Computing, University of Zagreb, Croatia
Abstract: Root canal length measurement was simulated by placing the tip of the active electrode (Kerr
file or Miller-Donalds needle) on the gingiva and
measuring the impedance between the active electrode and large area electrode fixed on the lip. The
impedance was measured for the series of voltage
excitation amplitudes at the frequencies in range
20 Hz – 1 MHz. It has been shown that measured
values of impedance tend to decline when current
density higher than critical one is applied. This
matches with the same measurements performed on
the electrode/electrolyte interface, but critical values
of current density are about 10 times lower for
measurement in electrolyte. In the construction of
electronic apex locator it is important to ensure that
current density is lower than critical one for every
needle used.
Keywords: apex location, current density,
endodontics, bioimpedance, electrodes
Introduction
The root canal length measurement is important procedure in endodontic treatment. Accuracy of the measurement directly affects the success of the entire treatment
Nowadays, there is a tendency to use electronic devices to perform such measurements, because their use
minimizes the need for x-ray examination. These electronic devices (often called apex locators) use the measurement of electric impedance between the file introduced into the endodontically prepared root canal and
the large area electrode that is fixed on the lip. The file
is moved in the canal until the instrument indicates that
the tip of the file is positioned at the apical foramen.
When constructing electronic apex locator there is a
question how large excitation (voltage or current) could
be used without significant influence on the measured
value of impedance.
This study intends to determine the influence of the
excitation level on the measured value of impedance.
area numbers 0,096 mm2. Because the Kerr file has the
drill shape, the area of the tip was approximated by
comparing the results of measurement with results obtained with the M-D needle.
Large area passive electrode is made of stainless
steal. The impedance of this electrode is considered
negligible.
Electrodes were cleaned in alcohol and active (test
electrodes) were insulated so that only the tips were able
to constitute the interface with tissue. The length of
uninsulated segment was 0,05 mm for M-D needle and
0,2 mm for the Kerr file.
Passive electrode was fixed on the lip in the corner
of the mouth in order to make contact with mucosa.
Active electrode was placed on the gingiva in the way
that the full area of the tip was in contact with tissue.
Hewlet-Packard HP4284A LCR meter was used to
measure impedance. Measurement and data storage
were computer controlled via HPIB (IEEE-488) bus.
Software to control measurement was built in HP VEE
development environment.
Rp and Cp parameters of impedance were measured
for both M-D needle and Kerr file at 6 logarithmically
distributed frequencies in the range from 20 Hz to
1 MHz. At every frequency impedance parameters were
obtained for excitation voltages 0.5, 20, 50, 100, 200,
500 and 1000 mVrms. Before the change of frequency a
5 s pause was taken. Voltage excitation was used because the instrument used is not capable of supplying
sinusoidal current of amplitude small enough.
Results
Parallel equivalent resistance Rp and capacitance Cp
were measured as described on ten young adult persons.
Materials and Methods
When measurement with apex locator is conducted
during endodontic treatment of the root canal, files and
needles used for the treatment are commonly used as
active electrodes too. In this experiment Miller-Donalds
(M-D) needle and Kerr file nr. 35 were used as active
electrodes. The diameter of the tip of both electrodes is
0,35 mm, the Miller-Donalds needle tip is flat, and it’s
Fig. 1: Rp results for one person.
Fig. 1 shows measurement results for the Rp component of impedance. One can see that measured value
of Rp depends on the excitation frequency and amplitude. [1]
Due to the usage of voltage excitation measurement
results were obtained at diverse current densities. To
analyse results measured on all subjects it was necessary
to interpolate the Rp-J and Cp-J curves for every person.
For this purpose Matlab environment and spline interpolation were used. Values of Rp and Cp were calculated for every person for the current densities in the range
0.2  200 mA/cm2.
After interpolation average values and standard deviations were calculated for Rp and Cp.
Fig. 2 and 3 show average values of Rp and Cp vs.
current density J for M-D needle.
ducted in the electrolyte [2], also the critical values of
current density increase with the excitation frequency.
As presented in Fig. 3 the relative change of Cp is
smaller than for Rp, which means that the phase angle is
changing significantly with the current density.
Fig. 4 shows that measurements with the Kerr file
have the same character of Rp component as measurements with M-D needle.
Fig. 4: Comparison of average values of Rp for M-D
needle and Kerr file.
Fig. 2: Average values of Rp and 2 standard deviations ranges for 20 Hz, 100 Hz and 1 kHz.
The difference between the average Rp values for
the Kerr file and M-D needle is in the range of 10% of
the Rp value for the M-D needle. The Differences for
the Cp component are higher (40%). From the similarity of the Rp and Cp for Kerr file and M-D needle
comes that the approximation of the Kerr file tip area
with the area of the flat tip is acceptable.
Conclusions
The measurement shows that the values of Rp and
Cp depend significantly on the excitation current density below 1 kHz. For the root canal length measurement
it is important that for every electrode used (d= 0,1 
0,85 mm typically) the current density is lower than
critical one. For example, if the Kerr file nr. 10 is used
at 1 kHz, then the excitation voltage must be below
100 mVrms.
Fig. 3: Average values of Cp and and 2 standard deviations ranges for 20 Hz, 100 Hz and 1 kHz.
Discussion
From Fig. 2 and 3 one can see that for the M-D needle the value of element Rp decreases and value of Cp
increases as the current density increases. Therefore, the
value of impedance declines with the increase of current
density. The value of impedance also declines with the
increase of frequency. The relative decrease of Rp is the
highest at 20 Hz, and becomes smaller as the frequency
increases. At the frequency of 10 kHz it becomes negligible. If we define the critical value of current density as
the one at which the value of Rp decreased and Cp increased for 10% of 20Hz value, then these values are
about 10 times higher than for the measurements con-
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