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To compare the effectiveness of different irrigation procedures on the removal of the smear layer, 40 curved canals were manually prepared in vitro under copious irrigation with 3 per cent NaOCl. Ten canals served as controls. The remaining 30 were equally distributed in three experimental groups and subsequently irrigated with the following: ultrasound with NaOCl; EDTA; ultrasound with EDTA. In the scanning electron microscope, the appearance of the smear layer was rated over the distribution of dentinal tubule openings for the coronal, middle and apical segments of the canals. Irrigation with NaOCl produced consistently smeared surfaces. Ultrasonic stirring of NaOCl removed the smear layer moderately, while EDTA produced almost smear-free surfaces. Ultrasound in association with EDTA did not enhance the dissolving capability of this chelating agent. A definite decline in the efficiency of the irrigation procedures was also observed along the apical part of the canals.

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Summary. The sealing of obturated root canals which had previously been cleaned chemically by EDTA or mechanically by ultrasound was compared using 60 single‐rooted teeth in an in vitro study. At first, the canals were manually prepared under copious irrigation with NaOCl. Then the specimens were divided into three experimental groups: in group I (control group) the canals were immediately obturated without further preparation; in group II (ultrasound group) the canals were subjected to ultrasound before being obturated, and in group III (EDTA group) the canals were irrigated with EDTA before being obturated. All speciments were then subjected to dye infiltration before being transversely sectioned at various levels from the apex. The amount of leakage was scored on an arbitrary four‐point scale. The results showed some differences in leakage between the three groups at levels close to the apex: EDTA‐treated canals showed the least infiltration, while those treated with ultrasound showed significantly less compared with the control group. The role of the smear layer and its removal is discussed in the light of these results.