Impedance between the electrode inserted in a root canal of a human tooth and the outer electrode placed on the oral mucosa serves as a measure of the root canal length, a vital parameter necessary for efficient endodontic procedure in dentistry. For better understanding of current conduction through the tooth, the impedance has been measured on extracted teeth (in vitro) and further used to develop corresponding electrical lumped element models. For modeling the metal/solution interface and complex structure of the tooth, Fricke's constant phase elements are employed. More detailed insight into current conduction is given by numerical simulation. Numerical simulation demonstrates the influence on the impedance of several important parameters, such as dentin conductance, canal preparation, and solution conductance.
The performance of a monolithic instrumentation amplifier used as an interface for a four-electrode bioimpedance measurement is examined with a commercially available impedance meter based on an auto-balancing bridge. The errors due to particularities in the input stage of the impedance meter, when used without a front-end, were several orders of magnitude higher than the measured quantity. The analysis was performed on an electrical circuit model of the skin and electrodes over a frequency range of 20 Hz to 1 MHz. The achieved accuracy with balanced electrode impedances for the frequencies up to 100 kHz can be below 0.2% for impedance magnitude and 0.1 degrees for impedance phase, which is within the specified basic accuracy range of the LCR-meter used for the measurements. At frequencies above 100 kHz the errors are increasing and are higher than the LCR-meter's basic accuracy. This study indicates that use of an instrumentation amplifier as a front-end with the particular LCR-meter can significantly improve the measurement accuracy of the four-electrode bioimpedance measurement at low frequencies.
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