RESUMO
The purpose of this study was to compare the accuracy of Root ZX II to locate the apical constriction with the display meter set at "0.5" and "1'"reading. Seventy single-rooted teeth were soaked in an alginate model and randomly distributed in 2 groups (n = 35). Measurements were taken following canal irrigation with 1% NaOCl. The length was established using a #20 K-file attached to the holder when the display indicator reached the marks "0.5" (group I) or "1" (group II), after the meter read "Apex." Then, the file was fixed in position and the teeth removed from the alginate. The apical portion of the root was shaved until the tip of the file could be seen, the distance to the apical constriction verified by means of a stereomicroscope and the measurements compared. Statistical analysis was performed by using Student t test with the null hypothesis set as 5%. The mean positions of the file tip relative to the apical constriction were -0.23 +/- 0.39 mm and -0.42 +/- 0.45 for groups I and II, respectively, with no statistical difference (P > .05). The accuracy was 90.5% and 83.78% for the Root ZX II "0.5" and "1" readings, respectively. It was concluded that the meter reading "1" of Root ZX II reduced the risk of working length overestimation.