Table 1 The exclusion and inclusion criteria of specimen

a. Fully formed apex without fracture.

a. Long oval-shaped canals ( the ratio of the long to short canal diameter was > 2).

b. One independent severely curved root canal with a curvature of 25° to 50° according to the Schneider method [17].

b. Root resorption (internal, external, or apical) or calcifications.

c. Previous endodontic treatment.