Removing bacteria and pulp tissue from the root canal system is necessary to achieve endodontic success. (1,2) The only way this can be accomplished is with an accurate determination of the working length. Historically, this has been done through the use of radiographs. However, the actual canal terminus (minor constriction) can vary from the radiographic apex by as much as three millimeters, and the apical foramen coincides with the radiographic apex in only 17% to 46% of cases. (3,4,5,6) When using radiographs to determine working length, an arbitrary measurement within the canal is chosen as the desired end point. For example, most clinicians choose to fill the root canal between 0.5 mm and 1.0 mm from the radiographic apex. The clinical significance of this arbitrary choice can have dire effects on clinical success. If the canal terminates more than 1.0 mm from the apex, an overfill will result. This may or may not be an issue depending on the cleanliness that was achieved during the instrumentation and irrigation phases of treatment. On the other hand, if the canal exits at the radiographic apex and the instrumentation and obturation are terminated 0.5 mm short of the canal terminus, bacteria will persist in the root canal system and may result in post-treatment disease.
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According to the endodontic literature, using an electronic apex locator (EAL) is more accurate than radiographs in determining the location of the apical constriction. (7,8) An in vivo study reported an EAL could measure the foramen to within +/- 0.5 mm in 96% of teeth. (9) Although it was a problem with first-generation apex locators that measured length via impedance or resistance, the presence of fluids or canal lubricant in the canal will not lead to inaccurate readings with modern apex locators. (10) Additionally, the accuracy of an EAL is not dependent on pulpal status, as there is no difference in the accuracy of an EAL if the pulp is vital or necrotic. (11,12) Many practitioners incorrectly consider file size to be an important determinant of accuracy for an EAL, and there was a historical, but unfounded concern about the safety of the EAL in patients with cardiac pacemakers. (13,14,15)
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There is a learning curve involved in the use of an EAL, and the use of a radiograph to confirm the accuracy of the EAL reading would be recommended until the clinician has complete confidence in this technique. The best time to initially determine working length with an EAL is following straight-line access, opening of the orifice, and establishment of an unobstructed glide path. This will provide the most consistently accurate readings, because the orifice will be centered over the canal. (16,17) Taking this initial reading following access, prior to final instrumentation of the apical portion of the root canal, and immediately in advance of obturation will provide the most accurate results. Taking several readings will take time, especially in multirooted teeth. However, increased accuracy will make the time well spent, and multiple EAL measurements can be taken in the time required to obtain one well angled and clinically useful working length radiograph.Clinicians rely on a variety of methods to determine working length including, but not limited to, the tactile sensation of feeling the file “pop” through the apex, bleeding on paper points, and radiographs. However, proper use of the EAL, in conjunction with a well-exposed and properly angled preoperative radiograph, will result in the most accurate and reproducible means of determining the true working length. Ideally, this will lead to more predictable and efficient treatment outcomes to maximize both patient comfort and root canal success.
On routine six-month recall, the patient was asymptomatic and the access opening was restored. The apical and crestal bone was unremarkable. The follow-up radiograph was taken from different horizontal and vertical angles. Now the fill-in palatal root looks nearly flush with the radiographic apex. A case like this clearly illustrates the limitations of two-dimensional radiographs and the benefits of an EAL for the most accurate working length determination.
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