In reviewing the literature, I found an article that has important clinical relevance with regard to the determination of TWL and that is worthy of wider discussion (“Frequency of Over instrumentation with an Acceptable Radiographic Working Length” by El Ayouti, et al, JOE, Vol. 27, No.1, January 2001). The purpose of this study was to determine how frequently a seemingly accurate working length (0-2mm from the radiographic apex) resulted in instrumentation beyond the apical foramen. The TWLs of 169 canals (from the coronal reference to apical foramen observed visually) from extracted teeth were compared to a clinically acceptable (0-2mm short of the radiographic apex) standardized (radiographically determined) working length. It was found that the file tip using a clinically acceptable (radiographically determined) working length was beyond the apical foramen in 51% and 22% of the premolar teeth and molar teeth respectively. The authors concluded that their data “… indicate that seemingly accurate radiographic working lengths… more often than expected, lead to unintentional over instrumentation in premolars and molars.” In other words “… the results of this study suggest that a working length ending radiographically 0-2mm short of the radiographic apex does not guarantee that instrumentation beyond the apical foramen will be avoided in premolars and molars.”
It is critical to determine TWL from multiple sources (radiographic verification and apex locators primarily, tactile sensation and hemorrhage on a paper point secondarily) instead of relying on just one method. The various methods should confirm one another. It is well established in the endodontic literature that the distance from the anatomical root apex (radiographic apex) to the apical foramen can range anywhere from coincident to 4.0mm up the root end. The clinical significance is subtle but significant. Apparently short filled roots may in fact be filled close or to the apical foramen and roots filled to the radiographic terminus as advocated by some may in fact be filled significantly beyond the apex regardless of radiographic appearances.
It has value before beginning treatment to determine an estimated working length (EWL). In the context of protecting the minor constriction (MC) of the apical foramen from over instrumentation, the clinician should make a mental note of where they are as they advance down the canal with hand files and RNT files. There will be greater tactile resistance of the files as the clinician approaches the MC.
Accurate determination of TWL is critical as the MC is the point above which all instrumentation, irrigation and obturation should be kept, to the greatest extent possible. In addition, keeping the MC at its original position and size is a key value in creating excellent endodontic results, along with achieving and maintaining apical patency. Advancing a RNT too quickly (and inadvertently placing the file out of the apical foramen) can easily transport the MC amongst other iatrogenic events.
Once the first file, RNT or hand K file reaches the EWL, an electronic apex locator should be used to make the first determination of TWL. I use the Elements Diagnostic Unit (SybronEndo, Orange, CA, USA) for its accuracy relative to the alternatives amongst other attributes. This first TWL reading should be verified repeatedly throughout the process until instrumentation is completed. A final confirmation of TWL might ideally take place with a small paper point (placed to the TWL) that shows a small spot of moisture (blood or clear tissue fluid) and provides a very accurate determination of the MC and the natural termination point for both instrumentation and obturation.
In summary, multiple methods of TWL verification are needed to assure the clinician that they have in fact found the correct position of the MC of the apical foramen. Relying solely strictly upon radiographic means is fraught with potential inaccuracies.
I welcome your questions and feedback.
Dr. Mounce lectures globally and is widely published. He is in private practice in endodontics in Vancouver, WA, USA. Amongst other appointments, he is the endodontic consultant for the Belau National Hospital Dental Clinic in the Republic of Palau. Korror, Palau (Micronesia). He can be reached at RichardMounce@MounceEndo.com.
