The danger zone is an area of the root canal where the primary thickness of dentin before preparation is under 1 mm. In general, the danger zone is located 4 to 6 mm below the canal chamber orifice. A danger zone can appear in the buccal root of maxillary premolars and molars and in the distal area of the mesial root and mesial region of the distal root of the mandibular molars. The cause of forming C-shaped canals and the prevalence of the danger zone in C-shaped roots is a failure of Hertwig’s epithelial root sheath to fuse on the lingual or buccal root surface. If Hertwig’s epithelial root sheath fails to fuse on the lingual surface, then the groove and danger zone occur on the lingual side of the root. If the root sheath fails to unite on the buccal surface-danger zone appears on the buccal side. 

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