The earliest permanent posterior tooth to erupt, the mandibular first molar seems to be the most frequently in need of endodontic treatment. It usually has two roots but occasionally three, with two canals in the mesial and one or two canals in the distal root. The distal root is readily accessible to endodontic cavity preparation and mechanical instrumentation, and the clinician can frequently sec directly into the orifice(s). The canals of the distal root are larger than those of the mesial root. Occasionally the orifice is wide from buccal to lingual. This anatomy indicates the possibility of a second canal or a ribbonlike canal with a complex webbing that can complicate cleaning and shaping. The mesial roots are usually curved, with the greatest curvature in the mesiai-buccal canal. The orifices are usually well separated within the main pulp chamber and occur in the buccal and lingual under the cusp tips. This tooth is often extensively restored. It is almost always under heavy occlusal stress; thus the coronal pulp chambers are frequently calcified. The distal canals are easiest to locate; once these locations arc positively identified, the mesial canals will be found in the aforementioned locations in the same horizontal plane. Because the mesial canal openings lie under the mesial cusps, they may be impossible to locate with conventional cavity pr
