12 The Permanent Mandibular Molars
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.
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.
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 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.
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 mesiobuccal 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).
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.
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.
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.
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.
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.
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 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.
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.
From the lingual aspect, three cusps may be seen: two lingual cusps and the lingual portion of the distal cusp (see Figures 12-5, 12-6, 12-12, and 12-13). The two lingual cusps are pointed, and the cusp ridges are high enough to hide the two buccal cusps from view. The mesiolingual cusp is the widest mesiodistally, with its cusp tip somewhat higher than the distolingual cusp. The distolingual cusp is almost as wide mesiodistally as the mesiolingual cusp. The mesiolingual and distolingual cusp ridges are inclined at angles that are similar on both lingual cusps. These cusp ridges form obtuse angles at the cusp tips of approximately 100 degrees.
The lingual developmental groove serves as a line of demarcation between the lingual cusps, extending downward on the lingual surface of the crown for a short distance only. Some mandibular first molars show no groove on the lingual surface but show a depression lingual to the cusp ridges. The angle formed by the distolingual cusp ridge of the mesiolingual cusp and the mesiolingual cusp ridge of the distolingual cusp is more obtuse than the angulation of the cusp ridges at the tips of the lingual cusps.
The mesial outline of the crown from this aspect is convex from the cervical line to the marginal ridge. The crest of contour mesially, which represents the contact area, is somewhat higher than the crest of contour distally.
The distal outline of the crown is straight immediately above the cervical line to a point immediately below the distal contact area; this area is represented by a convex curvature that also outlines the distal surface of the distal cusp. The junction of the distolingual cusp ridge of the distolingual cusp with the distal marginal ridge is abrupt; it gives the impression of a groove at this site from the lingual aspect. Sometimes, a shallow developmental groove occurs at this point (see Figure 12-10). Part of the mesial and distal surfaces of the crown and root trunk may be seen from this aspect because the mesial and distal sides converge lingually.
The surface of the crown lingually is smooth and spheroidal on each of the lingual lobes. The surface is concave at the side of the lingual groove above the center of the crown lingually. Below this point, the surface of the crown becomes almost flat as it approaches the cervical line.
The roots of the mandibular first molar appear somewhat different from the lingual aspect. They measure about 1 mm longer lingually than buccally, but the length seems more extreme (see Figures 12-6 and 12-7). This impression is derived from the fact that the cusp ridges and cervical line are at a higher level (about 1 mm). This arrangement adds a millimeter to the distance from root bifurcation to cervical line. In addition, the mesiodistal measurement of the root trunk is less toward the lingual surface than toward the buccal surface. Consequently, this slenderness lingually, in addition to the added length, makes the roots appear longer than they are from the lingual aspect (see Figure 12-9).
As was mentioned, the root bifurcation lingually starts at a point approximately 4 mm below the cervical line. This developmental depression is quite deep at this point, although it is smooth throughout and progresses cervically, becoming more shallow until it fades out entirely immediately below the cervical line. The depression is rarely reflected in the cervical line or the enamel of the lingual surface of the crown, as is found in many cases on the buccal surface of this tooth.
When the mandibular first molar is viewed from the mesial aspect, with the specimen held with its mesial surface at right angles to the line of vision, two cusps and one root only are to be seen: the mesiobuccal and mesiolingual cusps and the mesial root (see Figures 12-7, 12-8, 12-12, 12-13, and 12–15).
The buccolingual measurement of the crown is greater at the mesial portion than it is at the distal portion. The buccolingual measurement of the mesial root is also greater than the same measurement of the distal root. Therefore, because the mesial portions of the tooth are broader and the mesial cusps are higher, the distal portions of the tooth cannot be seen from this angle.