The effectiveness of ultrasonic instruments has been noted with the increased use of microscopes in the field of endodontics. The optimal EDTA concentration for removing the smear layer (SL) and smear plugs (SP) caused by ultrasonic instruments under various conditions, however, is still unclear. The aim of the present study is to determine the optimal EDTA concentration for removing those SL and SP. Fifty sectioned single-rooted specimens without SL and SP were used. In examination 1, the SL and SP were formed using a diamond tip (DT) with water at power level 3. Specimens with SL and SP were immersed for 2 min in 2 ml solutions of EDTA at different concentrations (1%, 5%, 10%, and 15%; pH = 7.4). The specimens were observed by scanning electron microscopy (SEM) and scored. In examination 2, specimens were shaped by DT with water at power level 3 for making SL and SP. The specimens were then prepared using a stainless steel tip with water for reducing SL. After preparation, specimens were immersed for 2 min in 2 ml solutions of EDTA at different concentrations (1%, 5%, 10%, and 15%; pH = 7.4). The specimens were observed by SEM and scored. Specimens in examination 1 retained SL and SP even when using 15% EDTA solution. SL was not observed when using 10% and 15% EDTA solutions in examination 2, but SP was observed when 1% and 5% EDTA solutions were used. The optimal EDTA concentration differed according to the conditions used to form the SL and SP. In conclusion, 10% EDTA for 2 min can remove the smear layer and smear plugs when the root canal is shaped using a smooth-surface ultrasonic tip, if water is applied and the tip slightly (5–10 gf) contacts the root canal wall.
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