AimTo use micro-computed tomography (-CT) to evaluate the amount of calcium hydroxide [Ca(OH)(2)] remaining in the C-shaped root canals of mandibular second molars after attempting to remove it with passive ultrasonic and sonic irrigation. MethodologyThirty mandibular second molars, 15 in C1 and 15 in C2 configurations as first identified by -CT, were divided into three groups (five C1 and five C2 in each group) for the three irrigation methods. All teeth were prepared to ProTaper Universal F2 and filled with Ca(OH)(2) paste. The Ca(OH)(2) was removed with F2 files and irrigation without agitation or with agitation using either EndoActivator or ultrasonics. -CT was used to measure the initial amount of Ca(OH)(2) present. After removal of Ca(OH)(2,) -CT imaging was used to assess the percentage of volume of residual Ca(OH)(2) in the canal. Data were analysed using one-way anova test. ResultsThere was no significant difference in the mean volume of the root canal systems after instrumentation amongst the three groups. The three irrigation techniques left 2-17% of Ca(OH)(2) in the root canals after removal. The mean volume of the remaining Ca(OH)(2) was higher in the group without agitation than in the groups with sonic or ultrasonic agitation (P<0.05). In the apical third, 68% of the canal space remained occupied by Ca(OH)(2) when no agitation was used, whereas 28% and 31% remained filled by Ca(OH)(2) in the EndoActivator and ultrasonic groups, respectively. There was no significant difference in the amount of residual Ca(OH)(2) between the EndoActivator and ultrasonic groups. The proportion of remaining Ca(OH)(2) in the apical canals was higher than in the middle and coronal canals in all groups (P<0.05). ConclusionsA considerable proportion of the apical canal space remained filled with Ca(OH)(2) in the C-shaped root canals after instrumentation and conventional needle irrigation. Although combining rotary instrumentation and irrigation with sonic or ultrasonic agitation reduced the amount of residual Ca(OH)(2) in the C-shaped root canals, the large amount of calcium hydroxide in the critical apical area remains a concern. Alternative strategies should be considered in medication of the apical canal in C-shaped teeth.