The electronic determination of working length.

Electronic digital imaging systems provide the clinician with the ability to manipulate, enhance, and store radiographic images for immediate recall. Apex locators are a useful adjunct to endodontic therapy. It is important to understand that they do not replace radiographs completely in treatment. Radiographs still provide the only means by which the clinician can gain an overall visual image of the tooth being treated. It is important to visualize the crown and root anatomy; the relationship of the crown to the root; the size, shape, curvature, and location of the roots; and to estimate the tooth length. These data give the clinician a good guide and provide a baseline from which to use an apex locator. Failure to do this will result in errors of measurement due to a lack of basic treatment information. The current types of apex locators available are accurate for determining working length, providing some basic steps are followed carefully for their use. Current generation units have degrees of accuracy that range from 83.0% to 93.4%. They need to have well-charged batteries and to be calibrated correctly before use. Resistance-type apex locators require a reasonably dry canal--a guideline that must be strictly followed. Files must be matched to fit the canal snugly, contact against metallic restorations must be avoided, and good lip contact for the return electrode must be obtained. The impedance-type units require careful calibration before use, careful matching of the coated probe to the canal diameter, and good patient contact with the hand-held return electrode. Care must be taken to ensure that the insulating layer does not wear off the coated probes during use. The frequency-based units are easy to use. They operate in an electroconductive environment and have the advantage of using conventional K-type files. However, the rechargeable battery must be kept fully charged, and their calibration steps must be followed. Fluid filling the tooth to the level of the pulp chamber will cause the units to become overloaded, and careful matching of the file to the size of the canal is necessary. Strict adherence to these simple set-up and calibration procedures and the use of the apex locator in conjunction with high-quality, standardized diagnostic radiographs provide the clinician with an accurate and useful adjunct to endodontic therapy.