Vertical root fractures in endodontically treated teeth most commonly occur along the mesiodistal root surface of mandibular molars because this area is narrower and subjected to higher occlusal forces, especially during mastication. The root anatomy, combined with extensive dentin removal during canal preparation, predisposes these roots to fracture vertically in the mesiodistal plane. Additionally, the presence of multiple canals and thinner dentinal walls in mandibular molars increase susceptibility. Recognizing this common fracture location is crucial for accurate diagnosis and management of vertical root fractures.

Reference: Pathways of the Pulp, 12th Edition.