Introduction: The exact measurement of the root canal length is of great importance in root canal therapy. Although, determination of the root canal length through radiography is the most common method, recently electronic apex locators are being used to determine working length and decrease the number of radiographs. The purpose of this study was to compare the accuracy of an electronic apex locator (Root ZX) with conventional radiography to determine the working length of curved mandibular molars.

Materials and methods: 35 intact mandibular first molars with rote curve above 10° were selected. The access cavity was prepared and the root canal length was measured using apex locator (Root ZX) and conventional radiography. Data was analyzed by using the paired-sample t-test.(P<0.05)

Results: The working lengths measured by apex locater were equal to the actual working lengths in 42.86%, 0.5-1 mm shorter in 31.43%, and 0.5 mm longer in 25.71%. The working lengths measured by radiography were equal to the actual working length in 51.14%, 0.5-1mm shorter in 39.99% and 0.5mm longer in 2.86%. There was no significant difference between these two methods in the measurement of the working length (p=0.951).

Conclusion:  The apex Locator and conventional radiography were equally effective in determining of the working length of the curved canals.