| Summary: | 碩士 === 國立臺灣大學 === 臨床牙醫學研究所 === 96 === The determination of working length plays an important role during endodontic treatment and the usual method combines application of electronic apex locator (EAL) and radiographs. The operating principle of modern EAL is calculating the impedance magnitude in the root canal which is measured by multiple frequencies of alternating current (AC) to determine the position of apical foramen. According to the researches, the effect from the morphology of apical foramen to the accuracy of electronic apex locator is still under debate. It is attributed to that the studies are lack of standardized experimental model and do not analyze the electrical structure of root canal system.
To investigate the accuracy of two commercialized EALs and the impedance characteristic of root canal system, we establish the nature teeth experimental model with three parameters as the size of apical foramen, the relative distance to apical foramen, and the testing frequency of AC. The results show that the accuracy of commercialized EALs decreased with larger sizes of apical foramen. The erroneous reading from the apical diameter more than 0.6mm makes the EALs unreliable in clinical use. In addition, the testing frequency more than 20kHz is not suitable for measurement because of the inductive effect from measuring electrode (file) and neutral electrode (lip clip). By analyzing the impedance characteristic, it demonstrates that the low-frequency AC is not sensitive to the existence of apical foramen and the correlation between the impedance magnitude and the size of apical foramen is low. When it is performed by high-frequency AC, the impedance change around apical foramen becomes significant only if the apical diameter is smaller than 0.6mm. In conclusion, the diameter of apical foramen affected the impedance magnitude and EAL reading because of the capacitive effect of root canal system is altered.
This study proved that the innovating equivalent-circuit model could describe the complicated electrical feature of root canal system, and it is the first time to present the relationship between the diameter of apical foramen and the testing frequency of impedance measurement by means of electrochemical impedance spectroscopy. This theoretical method would be applied to the invention of the new EAL with better accuracy.
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