The cross section of the root most prone to vertical root fracture is

One of the common anatomic features shared by teeth that typically develop VRFs is an oval cross section of the root, with a buccolingual diameter being larger than the mesiodistal diameter.These teeth include the maxillary and mandibular premolars, the mesial roots of the mandibular molars, and the mandibular incisors. Such anatomy is easily observed in the axial plane of a CBCT scan.The fracture in these teeth typically starts in the buccolingual plane, specifically at the highest convexity of the oval roots. Cohen's Pathways of the pulp ed 11 pg 809