Working Length Determination: Knowing Where to Stop

by Richard Mounce, DDS

Electronic length determination… is superior to radiographic localization of the apical constriction” from a recent statement of the German Society for Dental, Oral and Maxillary Medicine.

I agree wholeheartedly with this statement. Of the various methods of determining true working length (TWL), radiographic measurement of the position of the minor constriction (MC) of the apical foramen is, I believe, the least predictable of the various methods. Radiographic means have several shortcomings. First, such images are two-dimensional and never show the true buccal to lingual position of the file in the root unless the tooth is severely rotated. Film angulation has a significant impact on the distance between the file tip and the anatomic apex, i.e. the radiographic terminus (RT) of the root.

The position of the file tip on an x-ray is always somewhat suspect, as the relative position of the file does not tell the clinician where the MC of the apical foramen is in relation, only how far the file tip is from the RT. It must be remembered that the apical foramen can exit the tooth from the anatomic apex and up to 4mm from the RT. This huge variance can lead to a significant deviation in interpretation with regard to the position of the MC. As an aside, interpretation of the position of the file and its significance at the root end is similar in its limitations as looking at a single x-ray of a root canal treated tooth and evaluating whether the apical filling is really placed to the MC. Developmental errors in x-ray processing can also lead to difficulty reading radiographs as can only relying on a limited number of views.

Electronic apex locators (EAL) while highly accurate have some limitations, but if managed, these issues are easily overcome. The measurements obtained with them should always be verified by additional methods. Aside from radiographic means, these methods are: deciding an estimated working length (EWL) before starting, using tactile sense to “feel” for the MC of the apical foramen and determining the bleeding or moisture point at the MC. Relying upon only radiographic means will bring inaccuracy as will relying only on one of the other three methods mentioned above. What is vital is the reliance on multiple methods that confirm one another.

I use the Elements Diagnostic Unit (SybronEndo, Orange, CA), which measures both resistance and capacitance to determine the location of the MC. The Elements unit has compared favorably in the endodontic literature (Plotino, et al., 2006 May; 39(5): 408-14.). Ideally EAL readings should take place in dry canals, be done so with the largest file that will bind at the EWL and a where no part of the file will touch the metal of a crown or filling. The beauty of using an EAL is that at any point in the process of root canal instrumentation, the clinician can know where they are in the shaping of the space, in that they can know (1) if the canal is open, patent and negotiable and (2) almost their exact apical depth of insertion. In the empirical opinion of the author, it is below the standard of care of practice endodontics without such a piece of equipment as the reading is a direct measure of the position of the MC whereas radiographs require an interpretation of the file position to the end of the root. Most savvy clinicians take multiple readings with an EAL at various stages in the process so that at any given time there are no surprises in cone fit or tug back. It is noteworthy that the empirical guess at the start of the treatment of the EWL should be very close to the actual TWL. Such a moisture and bleeding point is obtained when a paper point is entered into a canal which spots at the level of the MC of the apical foramen in a properly prepared canal and the spotting is consistent over several trials. Such spotting should be both reproducible and limited to 1-2mm and should be virtually identical to the measurement given by the EAL.

Many specialists do not take working length pictures with files in them during treatment. For these clinicians, the use of the EAL and bleeding point has replaced such films. A far more accurate and comprehensive picture of canal length is a “cone fit” radiograph that can be taken just before obturation. Digital radiography such as that provided by DEXIS (DEXIS digital radiography, Alpharetta, GA) can make this relatively simple. DEXIS brings many advantages to length determination with its diagnostic software tools, economy, lack of chemicals, time savings, etc. An accurate cone fit picture virtually assures that the tooth will be properly obturated if the clinician has achieved an ideal preparation. Such a tug back film should reveal a cone which is not kinked or deformed in any way, which fills the canal space fully and which has tug back only in the apical 3-4mm of the canal.

In summary, having an EAL and using it correctly can go far toward removing the inaccuracies of relying solely upon a radiograph. Using multiple methods of confirmation to assure an accurate location of the position of the MC is advisable with a combination of tactile sense, bleeding point and electronic apex location being preferable to the use of radiographs. I welcome your questions and feedback.

Dr. Mounce is in private practice in endodontics in Vancouver, Washington, 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 contacted at