Figure 1 CBCT image of second mandibular premolar with joined roots at coronal third (b) then separated at the apical third (c and d) (yellow arrow). Canal configuration is 2-3-2 (a, b, c and d). The roots of the first premolar are fused with type II canal configuration

Figure 2 CBCT image of second mandibular premolar with C-shaped root canal configuration at different levels (a, b, c and d)

Figure 3 Mesiodistal CBCT images of mandibular right first premolar. The image of the canal disappeared in the middle-third (a). Cross-section CBCT image showing two separated roots at the middle-third (b-f). Axial CBCT image at the cervical third showing 2 canals (g), more towered the middle showing 2 canals (h), close to the apical third showing 3 canals with fused roots (i) and 3 canals at the apex (j). Notice the lingual root fused to the distobuccal root. (Arrows points to the canals). Canal configuration is 2-3

Figure 4 CBCT image of radicular groove located in the mesial aspect of the first mandibular premolar root surfaces at different levels (a, b, c and d)

Figure 5 Cross-section CBCT images of mandibular left first premolar showing one root with two canals (a, b). Axial CBCT image at the cervical third showing 2 canals (c), the middle third shows 3 canals (d), close to the apical third showing 4 canals (e) and 2 canals at the apex (f). (Arrows points to the canals). Canal configuration is 2-3-4-2

Figure 6 Cross-section and axial CBCT images of mandibular right first premolar showing one root with two separated canals at the middle-third (a-d). Axial CBCT image at the middle third level (d) showing 5 canals (e) that was traced with black dots (f) and shown with virtual reconstruction image (g). More towered the apical third 4 canals can be seen (h) and 2 canals at the apex (i). See the radicular groove located at the mesio-lingual area (f). (Arrows points to the canals). Canal configuration is 2-5-4-2