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Summary. Curved root canals from extracted human teeth were prepared using K‐type files. The resultant shapes were compared both quantitatively and qualitatively with canals that were formed in polyester clear casting resin and similarly prepared. Moderately curved canals generally demonstrated a funnel‐shaped canal preparation. Severely curved canals showed a high incidence of hour‐glass shaped preparations. When the proportions of the hour‐glass shaped canals from extracted teeth and simulated root canals were compared, there were no significant differences.

Summary. This study reports the effect of the step‐bad enlargement technique of root canal preparation on the shape of the curved root canal of extracted human teeth. This was achieved by superimposing tracings made from photographic prints obtained by radiographs of teeth before canal preparation, after apical preparation and after coronal flaring. After apical preparation, the root canal sometimes demonstrated an hour‐glass shape. Flaring the coronal portion of the canal was often successful in opening out the narrowest portion, the elbow, of this hour‐glass shape.