Wouldn't it be nice to have some form of GPS to help us determine true working length? Where to terminate root canal fillings reigns as one of the most intense controversies in endodontics. Some advocate filling to an arbitrary point short of the radiographic apex, and others suggest apical patency and filling to the minor constriction of the apical foramen. While this debate will probably continue, several valuable educational points can be drawn from it.
Four methods can be used to determine true working length: 1) tactile sensation, 2) electronic apex location, 3) bleeding-point determination, and 4) radiographs. In my empirical opinion, radiographic length, while time-honored, is the most crude and the least reliable. That said, prior to beginning any case, the clinician should look at the preoperative radiographs and mentally image how long the tooth is - in other words, determine the estimated working length. With experience, it is possible to estimate the true working length from the preoperative radiograph, within 1 mm. Mentally noting how long the tooth is preoperatively can go a long way toward allowing the clinician to later determine if other methods (those listed above) are "in the ballpark."
Tactile sensation is an often overlooked means to help confirm true working length. Said differently, from the tactile sense of the file in the clinician's hands during the first pass of a small K file (6 to10) out the foramen, he or she can feel a tangible "pop" as the instrument exits the minor constriction of the apical foramen. If the K file is properly felt and noted, it can give a very accurate measurement. A mental note should be made at this point of the length at which the pop occurred and the length compared to results obtained from the other methods.
It also is important to determine when the final true working length is determined in the instrumentation sequence. Many endodontists do not determine true working length until just before the final shaping instrument is employed to create the exact "apical capture zone" before obturation.
Apex locators measure the minor constriction of the apical foramen - in other words, where the pulp tissue meets the periodontal ligament. Electronic apex location could fill a small book, but suffice it to say that apex locators generally - and with some exceptions - work best in dry canals (despite manufacturers' claims). They also work best when using the largest K file possible and in situations where the file is not touching metal (crowns or fillings) as it exits the canal. I use the Elements Diagnostic Unit (SybronEndo, Orange, Calif.) for my electronic apex location. The Elements unit uses a different mathematical method for determination of the true working length relative to the other machines on the market. For me, it represents the state of the art. In my hands, the Elements unit provides a stable, reliable, and easily determined length, and one that is not subject to flutter.
In addition, if the operator is employing a philosophy of achieving and maintaining apical patency throughout the process of instrumentation, it should be possible - with the correct size and taper of paper point - to see a small 1/2 mm to 1 mm bleeding or moisture point which is reproducible when the paper point is placed to the true working length. Confirmation of true working length by paper-point determination was popularized by Dr. David Rosenberg of Vero Beach, Fla. Of the four methods to determine true working length, bleeding-point determination is the one I rely on most because, in concert with the Elements unit, it most clearly marks the location of the minor constriction where the canal meets the periodontal ligament.
I stated earlier that radiographs are the least reliable method for determining true working length. This statement does not square with the dental literature. Dental literature places a greater value on confirmation by radiographic methods. Ideally, working length should be determined and confirmed by a combination of the four methods. Radiographic length determination is subject to interpretation by the operator. There also may be inaccuracies inherent in the method (anatomic structures overlapping, angulation difficulties, developing problems, resolution challenges with digital radiography, etc.).
In any event, while we don't have GPS to glide us into home like an airplane, we do have four methods which, if used in harmony, can make the apical level of our endodontic fillings predictable and precise.
Dr. Richard Mounce is in private endodontic practice in Portland, Ore. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material is also available as audio CDs and as a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via email at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.