It appears that the mesiolingual canal is missing for RCT (Fig.2 (coronal section at the mesial root)).  Bony defect is buccodistal; try to place an implant as mesiolingual as possible (Fig.3 (axial section)).

4 mm Native Bone

Xin Wei, DDS, PhD, MS 1st edition 10/05/2017, last revision 10/07/2017