Electronic Apex Locators (EAL) were introduced to the dental profession in the early ’60s by Sunada in 1962 based on the work of Suzuki first described in 1942, where he found that the electrical resistance between a file inserted into a root canal and an electrode attached to the oral mucosa was relatively constant. The first apex locators operated on a dc current and the accuracy was dependant on a relatively dry canal without electrolytes, blood, or inflammatory exudates. In the intervening 40 years new generations of EAL have been developed which use multiple alternating currents and frequencies to measure root canal length. These 4th generation units have proven to be very accurate, varying by 0.5- 1.0mm. from the apical constriction in In Vivo tests (Moshonov et al (2004), Pagavino (1998), Dunlap (1998), Shabahang (1996). New miniature EAL recently introduced have proved to be as accurate as the larger models after 2005. These EAL are able to operate in the presence of intact or necrotic tissue, inflammatory exudates, and irrigant solutions such as sodium hypochlorite and EDTA. However since these devices are not infallible, and often may give misleading readings, this article will discus some of the common problems encountered during routine endodontic treatment.
Like most electronic devices, the EAL are powered by an electrical source which are usually replaceable or rechargeable batteries. Some of the larger units use a small transformer connected to a standard electrical circuit. If your EAL is not functioning normally check the batteries and or power source, as some units will not operate effectively when the charge falls below 50%. All units rely on electric wires to conduct the current and these can be checked very quickly using an external test model NRG Test tooth (Fig. 1) (Clinical Research Dental) or the “Endo-Q training device” (Acadental). Often the EAL will not register and these devices can be used to check both the integrity of the wiring and the state of the battery. In the model (Fig. 1) the lip electrode is attached to the metal bar and the file clip attaches to the k-file. As the file is advanced toward the apex of the tooth the apex locator will indicate its position. This testing device can be used with any apex locator as the “lip bar” is a standard diameter.
MEASURING LENGTH OF THE CANAL
Measurement of the length of the canal should only be done once the coronal 2/3rds of the canal has been shaped and cleaned. This will eliminate dentinal shelves in molars as well as, eliminating considerable debris and bacteria from the canal system. Often in roots with severe curvatures, the length is shortened slightly during routine shaping, thus confirming the length at this stage will enhance the accuracy of the readings. Like any electronic device short circuits will render the unit inoperative. The current generation of EAL demand that the measuring electrode (usually a k-file) does not touch any metallic restorations, or be in contact with electrolyte solutions in the coronal pulp chamber. If the unit registers on insertion of the file into the access opening, check that the file is not contacting any metallic restorations, and that the coronal pulp chamber is dry. Often solutions in the pulp chamber can cause a short circuit. This is especially important in multi rooted teeth, as the canals must be isolated from one another. If the unit registers as the file is placed in the canal even after drying, some ethanol can be placed in the coronal pulp chamber and gently blown dry. This will dehydrate the coronal pulp chamber and the coronal few mm of root canal allowing you to get a more accurate reading. If isolation from a metallic restoration is a problem you can paint the upper part of the k-file with nail varnish thus providing an insulating barrier. Another alternative is to fill the access cavity with a non conducting gel (iso-gel: Acadental).
In cases where the access is very difficult due to coronal calcifications you can check for possible perforations during this difficult procedure by using your apex locator to determine if you have made a small perforation. If you discover a small catch, which you are not sure is the canal or a perforation into the furcation or periodontal ligament space, use of the EAL can confirm this prior to creating an insolvable problem. Hook up your apex locator and place the tip of the k-file into the catch. If you get a reading that the apex has been reached, you might be dealing with a perforation. This can be confirmed with radiographs. (Ensure the file is not touching a metallic restoration as this will give the same result.) At the mid root level a similar reading might be indicative of a horizontal root fracture, or even a large lateral canal.
The authors use a 31mm. k-file with calibration rings (Fig. 2). This allows better vision, and an accurate length can be measured immediately against a reproducible reference point (cavo- surface or cusp tip) with out using rubber stops which are prone to move.
DIFFICULTIES NEAR THE APEX
How do we know that the apical constriction has indeed been located? The authors rely on the audible signals generated by the EAL, as it is often difficult to ensure that the file is not touching metallic restorations and at the same time observe the graphical display on the unit. Some miniature units NRG (Clinical Research) or Sybron mini (Sybron endo) can be positioned just below the chin and can be observed while manipulating the file. The file is slowly advanced until the audible signal changes to a higher frequency, which then changes to a continuous signal on advancing the file further. At this point, we know that we are beyond the apex. The file is slowly withdrawn until the sound returns to a slow pitch. It is again inserted into the canal while watching the length using the calibration rings as a reference. If the frequency changes at the same length as before, this is taken as the WL (working length). NOTE. You have to calibrate your EAL. For the first ten cases always take a confirmation radiograph when you believe the apical constriction has been reached. This will give you a very good idea of your EAL idiosyncrasies. Even EAL from the same manufacturer may give slightly different readings. Each unit is relatively unique. Once you have calibrated the unit, all future readings will be similar. Often you may not get a reading. Try using a larger file as most manufacturers advise the largest file that will fit the apical constriction should be used. Often this will generally not be larger than a size 20 k-file and is often smaller. When working on an upper tooth whose roots are very close to the maxillary sinus you need to exercise caution as the file may penetrate into the sinus and you will get an inaccurate reading. Advance the file very slowly until the unit registers the apex has been reached. Immediately pull the file back and repeat the procedure, if the same length is registered take this as the reading. If you repeatedly get a variable length take a radiograph.
TEETH WITH OPEN APICES
In very young teeth the apices are often wide open. EAL are not very accurate in these situations and radiographs should be used for length confirmation.
In summary, keep the access cavity dry. When possible use a “crown down” shaping technique and use the largest file that will reach the apical constriction. Make sure the batteries are charged and ensure metallic restorations are not contacted during length determination. Note this can also occur as a result of conductive fluids in the pulp chamber. Always recheck your length once the canal has been shaped as curvatures may have been straightened resulting in a slightly shorter length. a difference of 0.5mm. can result in non healing)
Manfred Friedman B.D. S., BCh.D hons. graduated from the University of The Witwatersrand in 1971, and comp[leted his Hons., in 1981. He e
migrated to Canada in 1987 taking a full time position as an assistant professor at the University of Western Ontario. He is a member of the Canadain Academy of endodontics, and The american Association of endodontists. He is a Fellow of the Pierre Fuchard Academy. Dr. Friedman is currently in Full time private practice limited to endodontics, and is an adjunct clinical Professor in the division of operative dentistry at the Schulich School of medicine and dentistry. He can be reached at firstname.lastname@example.org.
Dr. Len Boksman earned his DDS degree in 1972 from the University of Western Ontario. He is a fellow of the Academy Dentistry International and the International College of Dentists. He has received the ODA Award of Merit and was recognized by his alma mater with the award of Alumni of Distinction . He has memberships in the Canadian Academies of Esthetic and Cosmetic Dentistry and the American Academy of Cosmetic Dentistry. Dr. Boksman has published over 90 refereed articles, chapters in dental texts and is an international lecturer on Restorative Dentistry. He currently holds a part time consulting position as Director of Clinical Affairs for Clinical Research Dental and is an adjunct clinical professor in the Division of Operative Dentistry at the Schulich School of Medicine and Dentistry, London, Ontario. He can be reached at email@example.com.
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