Introduction: Radiography is the most common method for measurement of working length, though it is not the ideal method because of technical problems, radiation hazard, superimposition of permanent teeth bud on primary teeth, and so on. Electronic Apex Locator (EAL) is a device for determination of working length. Therefore, this study aimed to compare the accuracy of an electronic apex locator and radiography for determining working length of primary necrotic molars. Methods: sixty canals of twenty necrotic mandibular primary molars were used in children with 5-7 years old. There were no calcified canal, previous root canal therapy and perforation of pulp chamber floor. Access cavity was prepared and working length was determined by means of NSK EAL and conventional radiography and then was compared. Moreover, Pulpectomy was completed. Data were analyzed by Paired Samples T test. Results: In 56(93.3%) of the canals, the length measured by NSK EAL was determined to be equal to the length measured by radiography. In 3 canals(5%), NSK measurements were found to be less than radiography measurements for 1mm and in 1 canal (1.7%), it was greater than radiography measurements for 1mm. T test analysis indicated no significant difference between the two measurement methods (p=0.85). Conclusions: Electronic apex locator can accurately assess the length of canals in primary molars.

Keywords: Electronic Apex Locator; Primary Molar Tooth; Pulpectomy; Radiography; Working Length

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