The apical third exhibited higher amounts of residual CH than the coronal and middle thirds in all experimental groups, except for LAI. This finding is in line with the results of previous studies12,21. This observation may be related to accumulation and transfer of residual CH to the apical region, which has smaller canal area and smaller volume of irrigation solution, as well as the anatomic complexity of apical third9,12. The action and circulation of irrigants may therefore be hindered.