Figure 1 Schematic diagrams showing Vertucci’s classification of root canal morphology and canal configuration of permanent second molars of the upper jaw with fused roots. (A) Classification of the anatomic morphology of canals. (B) Teeth with one (B1), two (B2), and three (B3) roots. (C) Fused root with one root canal (C1, coronal 1/3; C2, middle 1/3; and C3, apical 1/3). (D) Fused root with two root canals (D1, coronal 1/3; D2, middle 1/3; D3, apical 1/3). (E) Fused root with three root canals (E1, coronal 1/3; E2, middle 1/3; E3, apical 1/3). (F) Fused root with four root canals (F1, coronal 1/3; F2, middle 1/3; F3, apical 1/3).