Mandibular First Molar

The mandibular first molar generally has two separate and distinct roots, one mesial and the other distal. The mesial root always has two distinct canals leaving the floor of the pulp chamber, which exit as separate apical foramina in approximately 85 per cent of the cases (Type III canal configuration) but merge to form one apical foramen (Type II) in the remaining cases. A mesio-distal

section through the tooth reveals that the orifices of both the mesial and the distal canals lie in the mesial two-thirds of the crown and the canals are well centered in their roots. A bucco-lingual section shows that the pulp chamber is in the centre of the crown and that the distal canal is wide and ribbon shaped, whereas the mesial canals are thin. The cervical cross section shows that the chamber floor is trapezoidal. This means that the access cavity for this tooth is also trapezoidal with rounded corners. The shortest side is to the distal aspect, and the mesial side is slightly longer. The buccal and lingual sides are of approximately the same length and taper towards each other distally. The mesiolingual canal lies beneath the mesio-lingual cusp. The mesio-buccal canal is the most difficult to locate, but it is usually found on a straight line to the buccal from the mesio-lingual orifice and is deep beneath the mesio-buccal cusp.

A tapered fissure bur is used in the central pit and the preparation is increased in depth by moving the bur mesially and distally from the central pit to the mesial pit. Once the roof of the chamber is penetrated, the safe-tipped bur is used to remove the remaining overhanging tooth structure. The distal canal is the largest and easiest to find. Therefore, the orifice of the distal canal is located first with the endodontic explorer. The safe-tipped bur is placed in this area and moved mesially and slightly to the lingual side to uncover the mesio-lingual canal. After the latter is located, the bur is moved buccally to expose the mesio-buccal orifice. The entire preparation is confined to the mesial two-thirds of the crown.