下颌第一磨牙根管形态：Mandibular First Molar

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 preparations. It will then be necessary to remove cuspal hard tissue or restoration to locate the orifice. As part of the access preparation, the unsupported cusps of posterior teeth must be reduced. Remember, this tooth, like all posterior teeth, should always receive full occlusal coverage after endodontic therapy. Therefore a wider access cavity to locate landmarks and orifices is better than ignoring one or more canals for the sake of a "conservative" preparation, which may lead to failure. Skidmore and Bjørndal stated that approximately one third of the mandibular first molars studied had four root canals. When a tooth contained two canals, "they cither remained two distinct canals with separate apical foramina, united and formed a common apical foramen, or communicated with each other partially or completely by transverse anastomoses. If the traditional triangular outline were changed to a more rectangular one, it would permit better visualization and exploration of a possible fourth canal in the distal root." Multiple accessory foramina are located in the furcation areas of mandibular molars.  They are usually impossible to clean and shape directly and are rarely seen, except occasionally on postoperative radiographs if they have been filled with root canal sealer or warmed gutta-percha. It would be proper to assume that if irrigating solutions have the property of "seeking out" and disposing of protein degeneration products, then the furcation area of the pulp chamber should be thoroughly exposed (calcific adhesions removed, etc.) to allow the solutions to reach the tiny openings. All caries, leaking restorations, and pulpal calcifications must be removed prior to endodontic treatment, and full cuspal protection and internal reinforcement are recommended (Burns RC, Buchanan LS. Tooth Morphology and Access Openings. Part One: The Art of Endodontics in Pathway of Pulp, 6th Ed. p. 160).