Mandibular Molars Root Anatomy and Access Cavities Dr. Mohammad Hammad

Lower first Molar Internal anatomy The earliest permanent tooth to erupt. Usually has 2 roots and 3 or 4 canals. MB and ML in the mesial root and D (DB and DL) in the distal root. Percentage of a MM canal ranges from 1% to 15%. 4 pulp horns; mesiobuccal, mesiolingual, distobuccal and distolingual. The pulp chamber cross-section is rhomboid or trapezoid in shape.

Mesial root is wider than the distal root. Roots curves mesially from cervical line to middle third then angles distally to the apex. Thin concave distal wall of the mesial root. MB canal usually more curved than ML canal.

Mesial curve Distal curve Thin concave wall

Number of canals in the Mesial root: 1.One canal 40 (28)%, 43 (29)%. 2.Two canals 59 (8)%, 50 (6)%. 3.Three canals 1%, 7%. (Vertucci et al., 1984; Gulabivala et al., 2001)

Orifices of all canals lie in the mesial 2/3 of the crown. MB orifice is usually underneath the MB cusp. ML orifice is usually lingual to the central grove. MM might be found in the groove between the ML and MB.

Mesial Distal Lingual Buccal

If a single distal orifice, usually oval buccolingually and located distal to the buccal groove. If two distal canals (DL and DB), they are round from orifice to apex.

Number of canals in the distal root: 1.One canal 85 (15)%, 83.3 (4)%. 2.Two canals 15 (8)%, 16 (3)%. 3.Three canals 0%, 0.7%. (Vertucci et al., 1984; Gulabivala et al., 2001)

Access cavity is usually rhomboid or trapezoid. Mesially the access cavity should not invade the mesial marginal ridge. Distal extension should allow straight- line access to the distal canal(s).

Access cavity of lower first molar Removal of caries and restorations. Access cavity has mesial and distal boundary limitations. Mesial boundary is a line connecting the mesial cusp tips. Distal boundary is a line connecting the buccal and lingual grooves. Starting location is on the central groove halfway between the mesial and distal boundaries.

Round or fissure bur placed perpendicular to occlusal table. Penetration should be directed towards the largest canal, distal canal until drop-in is felt. Unroofing is carried out. Number and location of orifices determine shape and location of access cavity. Removal of cervical dentine bulge and finishing.

Lower second molar Internal anatomy Smaller than the lower first molar and more symmetrical. Pulp chamber and canal orifices are smaller than that of the first molar. The tooth might have one, two, three or four canals. The two mesial canals are closer than that of the lower first molar.

Number of canals in the mesial root: 1.One canal 65 (38)%, 56 (52)%. 2.Two canals 35 (9)%, 40%. 3.Three canals 0%, 0%. (Vertucci et al., 1984; Weine et al., 1988)

Number of canals in the distal root: 1.One canal 95 (3)%, 94.6 (9)%. 2.Two canals 4 (1)%, 1.3%. 3.Three canals 0%, 1.3%. (Vertucci et al., 1984; Weine et al., 1988)

Access cavity is similar to that of the first lower molar when three orifices are located. When two orifices are located, the orifices would be equal in size and line up in the buccolingual centre of the tooth. Access cavity would be rectangular in shape. Access cavity is oval in the presence of only one orifice, and usually lined up in the centre of the occlusal surface.

Lower third molar Internal anatomy It has unpredictable anatomy. Careful examination of root morphology before treatment is essential. It might have one to four roots and one to six canals. Access cavity might be oval, triangle, rhomboid,…etc. Depends on number and location of orifices.

Teeth with C-shaped root canal system First reported in Usually found in lower second molars but can be found in any molar tooth. The pulp chamber is a single ribbon-shaped orifice with an arc of 180 degrees or more. Starts usually at the mesiolingual line angle and sweeps towards the distal aspect of the pulp chamber.

Below level of orifice either continue as arc shape until apex or divide into canals. Incidence of c-shaped molars have great ethnical variation. About 32% in Chinese and Japanese, 19% in Lebanese, 33% in Koreans, 10% Jordanians, 2% Sri Lankan. Access cavity shape depends on location and size and shape of pulp chamber. Very difficult to treat such teeth.