The Permanent Mandibular Molars (Dental Anatomy, Physiology and Occlusion) Part 1

The mandibular molars are larger than any other mandibular teeth. They are three in number on each side of the mandible: the first, second, and third mandibular molars. They resemble each other in functional form, although comparison of one with another shows variations in the number of cusps and some variation in size, occlusal design, and the relative lengths and positions of the roots.

The crown outlines exhibit similarities of outline from all aspects, and each mandibular molar has two roots, one mesial and one distal. Third molars and some second molars may show a fusion of these roots. All mandibular molars have crowns that are roughly quadrilateral, being somewhat longer mesiodistally than buccolingually. Maxillary molar crowns have their widest measurement buccolingually.

The mandibular molars perform the major portion of the work of the lower jaw in mastication and in the comminution of food. They are the largest and strongest mandibular teeth, both because of their bulk and because of their anchorage.

The crowns of the molars are shorter cervico-occlusally than those of the teeth anterior to them, but their dimensions are greater in every other respect. The root portions are not as long as those of some of the other mandibular teeth, but the combined measurements of the multiple roots, with their broad bifurcated root trunks, result in superior anchorage and greater efficiency.

Usually the sum of the mesiodistal measurements of mandibular molars is equal to or greater than the combined mesiodistal measurements of all the teeth anterior to the first molar and up to the median line.


The crowns of these molars are wider mesiodistally than buccolingually. The opposite is true of maxillary molars.

Mandibular First Molar

Figures 12-1 through 12-17 illustrate the mandibular first molar from all aspects. Normally, the mandibular first molar is the largest tooth in the mandibular arch. It has five well-developed cusps: two buccal, two lingual, and one distal (see Figure 12-1). It has two well-developed roots, one mesial and one distal, which are very broad buccolingually. These roots are widely separated at the apices.

The dimension of the crown mesiodistally is greater by about 1 mm than the dimension buccolingually (Table 12-1). Although the crown is relatively short cervico-occlusally, it has mesiodistal and buccolingual measurements that provide a broad occlusal form.

The mesial root is broad and curved distally, with mesial and distal fluting that provides the anchorage of two roots (see Figure 13-22). The distal root is rounder, broad at the cervical portion, and pointed in a distal direction. The formation of these roots and their positions in the mandible serve to brace the crown of the tooth efficiently against the lines of force that might be brought to bear against it.

Detailed Description of the Mandibular First Molar From All Aspects

Buccal Aspect

From the buccal aspect, the crown of the mandibular first molar is roughly trapezoidal, with cervical and occlusal outlines representing the uneven sides of the trapezoid. The occlusal side is the longer (see Figures 12-3, 12-4, 12-12, 12-13, and 12-14).

If this tooth is posed vertically, all five of its cusps are in view. The two buccal cusps and the buccal portion of the distal cusp are in the foreground, with the tips of the lingual cusps in the background. The lingual cusps may be seen because they are higher than the others.

Two developmental grooves appear on the crown portion. These grooves are called the mesiobuccal developmental groove and the distobuccal developmental groove. The first-named groove acts as a line of demarcation between the mesiobuc-cal lobe and the distobuccal lobe. The latter groove separates the distobuccal lobe from the distal lobe (see Figures 12-2 and 12-3).

The mesiobuccal, distobuccal, and distal cusps are relatively flat. These cusp ridges show less curvature than those of any of the teeth described so far. The distal cusp, which is small, is more pointed than either of the buccal cusps. Flattened buccal cusps are typical of all mandibular molars. In most first molar specimens the buccal cusps are worn considerably, with the buccal cusp ridges almost at the same level. Before they are worn, the buccal cusps and the distal cusp have curvatures that are characteristic of each one (see Figures 12-4 and 12-14, 4).

 Mandibular right first molar, occlusal aspect. DBCR, Distobuccal cusp ridge; DBDG, distobuccal developmental groove; DCR, distal cusp ridge; DTF, distal triangular fossa (shaded area); DLCR, distolingual cusp ridge; LDG, lingual developmental groove; MLCR, mesiolingual cusp ridge; MTF, mesial triangular fossa (shaded area); SG, a supplemental groove; MBCR, mesiobuccal cusp ridge; MBDG, mesiobuccal developmental groove.

Figure 12-1 Mandibular right first molar, occlusal aspect. DBCR, Distobuccal cusp ridge; DBDG, distobuccal developmental groove; DCR, distal cusp ridge; DTF, distal triangular fossa (shaded area); DLCR, distolingual cusp ridge; LDG, lingual developmental groove; MLCR, mesiolingual cusp ridge; MTF, mesial triangular fossa (shaded area); SG, a supplemental groove; MBCR, mesiobuccal cusp ridge; MBDG, mesiobuccal developmental groove. 

The mesiobuccal cusp is usually the widest mesiodistally of the three cusps. This cusp has some curvature but is relatively flat. The distobuccal cusp is almost as wide, with a cusp ridge of somewhat greater curvature. The two buccal cusps make up the major portion of the buccal surface of the crown. The distal cusp provides a very small part of the buccal surface, because the major portion of the cusp makes up the distal portion of the crown, providing the distal contact area on the center of the distal surface of the distal cusp. The distal cusp ridge is very round occlusally and is sharper than either of the two buccal cusps.

Mandibular right first molar, occlusal aspect. Shaded area is the central fossa. CP, Central pit; DMR, distal marginal ridge; DP, distal pit; CDG, central developmental groove; MP, mesial pit; MMR, mesial marginal ridge.

Figure 12-2 Mandibular right first molar, occlusal aspect. Shaded area is the central fossa. CP, Central pit; DMR, distal marginal ridge; DP, distal pit; CDG, central developmental groove; MP, mesial pit; MMR, mesial marginal ridge.

Mandibular right first molar, buccal aspect. MBDG, Mesiobuccal developmental groove; CL, cervical line; MR, mesial root; DR, distal root; DBDG, distobuccal developmental groove; DC, distal cusp; DLC, distolingual cusp; DBC, distobuccal cusp; MLC, mesiolingual cusp; MBC, mesiobuccal cusp.

Figure 12-3 Mandibular right first molar, buccal aspect. MBDG, Mesiobuccal developmental groove; CL, cervical line; MR, mesial root; DR, distal root; DBDG, distobuccal developmental groove; DC, distal cusp; DLC, distolingual cusp; DBC, distobuccal cusp; MLC, mesiolingual cusp; MBC, mesiobuccal cusp.

Table 12-1 Mandibular First Molar

First evidence of calcification

At birth

Enamel completed

tmp57-14

Eruption

6-7 yr

Root completed

9-10 yr

Ieasurement Table

Cervico-

OCCLUSAL

Length of Crown

Length of Root

Mesiodistal Diameter of Crown

Mesiodistal Diameter of Crown at Cervix

Labio- or Buccolingual Diameter of Crown

Labio- or Buccolingual Diameter of Crown at Cervix

Curvature of Cervical Line—Mesial

Curvature of Cervical Line—Distal

Dimensions* suggested for carving technique

7.5

14.0

11.0

9.0

10.5

9.0

1.0

0.0

 

Mandibular right first molar, buccal aspect. (Grid = 1 sq mm.)

Figure 12-4 Mandibular right first molar, buccal aspect. (Grid = 1 sq mm.)

Mandibular right first molar, lingual aspect. DLC, Distolingual cusp; LDG, lingual developmental groove; CL, cervical line; MLC, mesiolingual cusp.

Figure 12-5 Mandibular right first molar, lingual aspect. DLC, Distolingual cusp; LDG, lingual developmental groove; CL, cervical line; MLC, mesiolingual cusp.

Mandibular right first molar, lingual aspect. (Grid = 1 sq mm.)

Figure 12-6 Mandibular right first molar, lingual aspect. (Grid = 1 sq mm.)

Mandibular right first molar, mesial aspect. (Grid = 1 sq mm.)

Figure 12-7 Mandibular right first molar, mesial aspect. (Grid = 1 sq mm.)

Mandibular right first molar, mesial aspect. MMR, Mesial marginal ridge; MLC, mesiolingual cusp; MCA, mesial contact area; CL, cervical line; DD, developmental depression; CR, cervical ridge; MBC, mesiobuccal cusp.

Figure 12-8 Mandibular right first molar, mesial aspect. MMR, Mesial marginal ridge; MLC, mesiolingual cusp; MCA, mesial contact area; CL, cervical line; DD, developmental depression; CR, cervical ridge; MBC, mesiobuccal cusp.

Mandibular right first molar, distal aspect. MMR, Mesial marginal ridge; MBC, mesiobuccal cusp; DBC, distobuccal cusp; DC, distal cusp; DBDG, distobuccal developmental groove; DCA, distal contact area; CL, cervical line; DMR, distal marginal ridge; DLC, distolingual cusp; MLC, mesiolingual cusp.

Figure 12-9 Mandibular right first molar, distal aspect. MMR, Mesial marginal ridge; MBC, mesiobuccal cusp; DBC, distobuccal cusp; DC, distal cusp; DBDG, distobuccal developmental groove; DCA, distal contact area; CL, cervical line; DMR, distal marginal ridge; DLC, distolingual cusp; MLC, mesiolingual cusp.

Mandibular right first molar, distal aspect. (Grid = 1 sq mm.)

Figure 12-10 Mandibular right first molar, distal aspect. (Grid = 1 sq mm.)

Mandibular right first molar, occlusal aspect. (Grid = 1 sq mm.)

Figure 12-11 Mandibular right first molar, occlusal aspect. (Grid = 1 sq mm.)

Mandibular right first molar. Graph outlines of five aspects are shown. (Grid = 1 sq mm.)

Figure 12-12 Mandibular right first molar. Graph outlines of five aspects are shown. (Grid = 1 sq mm.)

Mandibular right first molar.

Figure 12-13 Mandibular right first molar.

Mandibular first molar, occlusal aspect. Ten typical specimens are shown.

Figure 12-14 Mandibular first molar, occlusal aspect. Ten typical specimens are shown.

These three cusps have the mesiobuccal and distobuccal grooves as lines of demarcation. The mesiobuccal groove is the shorter of the two, having its terminus centrally located cervico-occlusally. This groove is situated a little mesial to the root bifurcation buccally. The distobuccal groove has its terminus near the distobuccal line angle at the cervical third of the crown. It travels occlusally and somewhat mesially, parallel with the axis of the distal root.

The cervical line of the mandibular first molar is commonly regular in outline, dipping apically toward the root bifurcation.

The mesial outline of the crown is somewhat concave at the cervical third up to its junction with the convex outline of the broad contact area. The distal outline of the crown is straight above the cervical line to its junction with the convex outline of the distal contact area, which is also the outline of the distal portion of the distal cusp.

The calibration of this tooth at the cervical line is 1.5 to 2 mm less mesiodistally than the mesiodistal measurement at the contact areas, which of course represents the greatest mesiodistal measurement of the crown.

The surface of the buccal portion of the crown is smoothly convex at the cusp portions with developmental grooves between the cusps. Approximately at the level of the ends of the developmental grooves, in the middle third, a developmental depression is noticeable. It runs in a mesiodistal direction just above the cervical ridge of the buccal surface (see Figure 12-14, 6 and 8). This cervical ridge may show a smooth depression in it that progresses cervically, joining with the developmental concavity just below the cervical line, which is congruent with the root bifurcation buccally.

The roots of this tooth are, in most instances, well formed and constant in development.

When the tooth is posed so that the mesiobuccal groove is directly in the line of vision, part of the distal surface of the root trunk may be seen, and in addition, part of the distal area of the mesial root is visible because the lingual portion of the root is turned distally. These areas may be seen in addition to the buccal areas of the roots and root trunk.

The mesial root is curved mesially from a point shortly below the cervical line to the middle third portion. From this point, it curves distally to the tapered apex, which is located directly below the mesiobuccal cusp. The crest of curvature of the root mesially is mesial to the crown cervix. The distal outline of the mesial root is concave from the bifurcation of the root trunk to the apex.

The distal root is less curved than the mesial root, and its axis is in a distal direction from cervix to apex. The root may show some curvature at its apical third in either a mesial or a distal direction (see Figure 12-14, 1 and 8). The apex is usually more pointed than that of the mesial root and is located below or distal to the distal contact area of the crown. Considerable variation is evident in the comparative lengths of mesial and distal roots (see Figure 12-14).

Both roots are wider mesiodistally at the buccal areas than they are lingually. Developmental depressions are present on the mesial and distal sides of both roots, which lessens the mesiodistal measurement at those points. They are somewhat thicker at the lingual borders. This arrangement provides a secure anchorage for the mandibular first molar, preventing rotation. This I-beam principle increases the anchorage of each root (see Figure 13-22).

The point of bifurcation of the two roots is located approximately 3 mm below the cervical line. A deep developmental depression is evident buccally on the root trunk, which starts at the bifurcation and progresses cervically, becoming shallower until it terminates at or immediately above the cervical line. This depression is smooth with no developmental groove or fold.

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