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Authors: Dr. Annil Dhingra

Cleaning and Shaping of the root canal system are among the most important phases of endodontic treatment because they ease pain and eliminate debris and bacterial pathogens. The most challenging step in cleaning and shaping is determining working length, which is defined as the distance from a coronal reference point to the point at which canal preparation and obturation should terminate." After establishing the working length, the clinician can eliminate many etiologies of endodontic pathosis and prepare the root canal system for obturation. If a proper working length cannot be determined,the canal cannot be cleaned properly, shaped, or obturated. If instruments and obturating materials are not kept within the canal space, unnecessary inflammation can occur. More importantly, when endodontic filling is either too long or too short, the prognosis is decreased.“

V-CORONAL REFERENCE POINT Before calculating working length, the clinician should remove all caries, unsupported cusps, and restorations from t e tooth being treated. lf occlusal reduction is required for patient comfort or to prevent continued propagation of cracks, it should be completed before the working length is determined. This ensures the reference point to be used will remain unchanged. The most common method of marking the instruments used for cleaning and shaping is with manufactured silicone “stops.” Several devices have been developed to assist in dispensing these stops and Setting them on individual files at predetermined lengths. The stop should be placed perpendicular to the file to ensure an exact measurement.“ Alternatively,the clinician can make rubber stops from rubber bands, using a rubber dam punch set at the largest setting. One benefit of this method is that it produces smaller, more rigid stops, which improve visualization when several instruments are in the canals simultaneously during radiographic working length determination. As an alternative to rubber stops, some files are manufactured with hash marks etched at various levels on the file shank. Endometric probes are etched at millimeter increments and can be identified on radiographs.

The reference point can either be the cusp tip of the canal being measured or the same cusp tip for all canal. If the file is deflected away from a particular cusp tip, the cavosurface margin of that particular cusp may also be used. If no particular definable point can be located, a ledge can be made in the tooth structure that can act as a reference point. When treating a tooth that is longer than the longest set of files available, the clinician may need to find the reference in this fashion. Alternatively, if a crown restoration is planned, the cusp tip can be reduced before length calculation.


Radiographs are often misinterpreted because of the difficulty of discerning radicular anatomy and pathosis from normal structures. Electronic apex locators are used for working length determination as an adjunct to radiography. They should be used when the apical portion of the canal system is obstructed by impacted teeth, tori, the malar process, the zygomatic arch, excessive bone density, overlapping roots, shallow palatal vaults, or even normal medullary and cortical bone patterns. In these cases they can provide information that radiography cannot.They may also be used in the treatment of pregnant patients to reduce radiation exposure, in children who may not tolerate taking radiographs, and in disabled or heavily sedated patients. If a patient does not tolerate radiograph placement because of the gag reflex, electronic apex locators can be a valuable tool. Sewerin evaluated full mouth radiographic series taken on 478 patients and found that 13% exhibited a significant gag reflex and 1.3% were unable to tolerate completion of the radiographic examination. Patients with disabilities or debilitating disorders such as Parkinson’s disease may not be able to hold the film in place. Children also may have difficulty with this task. Because as many as 40% Of deviations of canal foramen from the apical center are in the buccal or lingual plane and apical canal curvature is in the buccal or lingual plane, electronic apex locators can provide good information Where unknown curves may otherwise go undetected with radiographic film.

Electronic devices such as electronic apex locators should not be used on patients who have cardiac pacemakers. The “demand” type pacemakers (the most commonly used today) that stimulate the heart only when necessary are most affected by electronic equipment.

Because of the problems caused by the interaction of electronic apex locators with moisture and endodontic irrigants in the canals, devices were developed to operate in moist conditions. This insulated the measuring device from canal irrigants and tissue, but the instruments were not always fine enough to pass through the apical constriction. Also, the Teflon coating tended to break away from the probes with time, altering the accuracy of the device. Most recently,fifth -generation electronic apex locators have entered the market. They can operate in a wet environment, even one containing sodium hypochlorite,and appear to be much more accurate compared with previous devices. They use two different frequencies and average the change as the apex is reached to provide a much smoother reading in different conditions. This apex locator must be used in a moist environment and is calibrated by being reset with the file a few millimeters into the canal before a measurement is taken of each canal. The apical terminus is reported with a constant tone as well as a meter that reflects the position in the canal. It uses a tone as well as a digital readout. This Apex Locator has been demonstrated in the literature to be the most accurate apex locator on the market.

The working length measured to clean, shape and obturate a canal space may not always be measured to apical constriction.In case of root perforation,the point of exit of the canal to the periodontal ligament space is a crucial measurement. If the perforation occurs on the buccal or lingual or furcal surface of the tooth, it may be difficult to detect. Apex locators can be reliable instruments to detect perforations and the length to the area Where the perforation exits the tooth structure. An electronic apex locator can also be placed on a post to confirm that the post is perforated or on a pin to detect Whether it has been placed into the pulp space. When electronic apex locators are compared for accuracy in the literature, scientific methods used include simulated acrylic canals, animal, cadaver, and human studies. A defined acceptable range of accuracy among Working length determinations is plus or minus 0.5 mm from the apical constriction. Researchers averaged the accuracy of 12 studies assessing the SonoExplorer, a second-generation apex locator, and found that 76% of the measurements fall within this range.

The most clinically relevant data can be gathered from blinded studies where working length is determined with any test method, cementation of the file in position, extraction of the tooth, and direct measurement of the file tip from the apical constriction. When using electronic apex locators to determine working length, the clinician should keep in mind that metallic restorations may interfere with the reading of the device if the working length file comes into Contact with the restoration or fluid that is in contact with the restoration. This problem can be avoided by initially drying the coronal canal space and chamber before measurement or by using a silicone-containing material within the canal spaces. The most important thing to do when determining working length is to use as many of the techniques as possible during the course of treatment. First the clinician should secure a stable coronal reference point(s).The next step is to estimate working length from the historical average lengths and the preoperative radiograph and keep this estimate in mind throughout treatment. Finally the clinician should use rational thought in combining tactile sense, radiography, and electronic devices to arrive at the desired apical terminus of the endodontic Preparation.

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