根尖孔徑對電子式根管長度測量之影響

Abstract

工作長度之測量為根管治療之重要步驟，臨床上最常用方式為電子式根尖定位器與放射線攝影之配合使用。現行電子式根尖定位器之運作，多以複數頻率交流電流求取根管內阻抗值，經運算後判定根管銼是否抵達根尖孔位置。根尖孔型態對電子式根尖定位器準確度之影響，在以往研究中顯示出兩極看法，然而此類研究均缺乏標準化實驗模型，亦未針對根管系統電學結構進行分析，因此難以釐清根尖孔徑此參數所造成之影響。 本研究建立之自然牙實驗模組，具備根尖孔徑、相對根尖孔位置與測量用交流頻率三項參數，可對根管系統電學結構、阻抗特性以及市售電子式根尖定位器之準確度進行分析。研究結果顯示，市售電子式根尖徑定位器之準確度因根尖孔徑增加而降低，當根尖孔徑大於0.6 mm時，其測量誤差將大於根管治療臨床操作可接受之範圍。此外，因20 kHz以上之交流頻率將引起測量電極 (根管銼) 與中性電極 (唇鉤) 之電感效應，而不建議使用。根管系統阻抗特性之研究結果則顯示，以低頻交流電流進行阻抗值量測，其測量值與根尖孔徑呈現低相關性，且對根尖孔之存在不敏感；以高頻交流電流進行量測，於根尖孔徑小於0.6 mm者，可見阻抗測量值於根尖周圍產生明顯變化，此變化量隨根尖孔徑增加而減少，至0.8 mm與1.0 mm則不顯著。近一步針對阻抗值變化趨勢進行分析，可知根尖孔徑影響阻抗值及電子式根尖定位器之讀值，為根管系統之電容效應所致。 本研究證實新提出之根管系統等效電路模型可用於描述自然牙實驗模組之複雜電學結構，並首次以電化學阻抗頻譜分析呈現根尖孔徑與不同頻率阻抗值量測之相互關係。未來展望則在於以此理論方法消弭根尖孔徑之干擾，研發更為精確之電子式根尖定位器。

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.