Lingual Aspect
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 mesiolin-gual 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 distal cusp is at a lower level than the mesiolingual cusp.
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 distolin-gual 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 cervical line lingually is irregular and tends to point sharply toward the root bifurcation and immediately above it.
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.
This bifurcation groove of the root trunk is located almost in line with the lingual developmental groove of the crown.
Mesial Aspect
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.
As already indicated, all of the posterior mandibular teeth have crown outlines from the mesial aspect that show a characteristic relation between crown and root. The crown from the mesial or distal aspect is roughly rhomboidal, and the entire crown has a lingual tilt in relation to the root axis. It should be remembered that the crowns of maxillary posterior teeth have the center of the occlusal surfaces between the cusps in line with the root axes (see Figure 4-16, E and F).
It is interesting to note the difference between the outline form of the mandibular first molar and the mandibular second premolar from the mesial aspect.The first molar compares as follows:
Figure 12-15 Mandibular first molar, occlusal aspect. Ten typical specimens are shown.
1. The crown is a fraction of a millimeter to 1 millimeter shorter in the first molar.
2. The root is usually that much shorter also.
3. The buccolingual measurement of crown and root of the molar is 2 mm or more greater.
4. The lingual cusp is longer than the buccal cusp. (The opposite is true of the second premolar.)
Regardless of these differences, the two teeth have the same functional form except for the added reinforcement given to the molar lingually. Because of the added root width buccolin-gually, the buccal cusps of the first molar do not approach the center axis of the root as does the second premolar, and the lingual cusp tips are within the lingual outline of the roots instead of being on a line with them.
From the mesial aspect, the buccal outline of the crown of the mandibular first molar is convex immediately above the cervical line. Before occlusal wear has shortened the buccal cusps, this curvature is over the cervical third of the crown buccally, outlining the buccal cervical ridge (see Figure 12-8). This ridge is more prominent on some first molars than on others (see Figure 12-15). Just as on mandibular premolars, this ridge curvature does not exceed similar contours on other teeth as a rule when the mandibular first molar is posed in the position it assumes in the mandibular arch (see Figure 12-7 and Figure 12-15, 1 and 2).
Above the buccal cervical ridge, the outline of the buccal contour may be slightly concave on some specimens (see Figure 12-15, 1 and 2), or the outline may just be less convex or even rather flat as it continues occlusally outlining the contour of the mesiobuccal cusp. The mesiobuccal cusp is located directly above the buccal third of the mesial root.
The lingual outline of the crown is straight in a lingual direction, starting at the cervical line and joining the lingual curvature at the middle third, the lingual curvature being pronounced between this point and the tip of the mesiolingual cusp. The crest of the lingual contour is located at the center of the middle third of the crown. The tip of the mesiolingual cusp is in a position directly above the lingual third of the mesial root.
The mesial marginal ridge is confluent with the mesial ridges of the mesiobuccal and mesiolingual cusps. The marginal ridge is placed about 1 mm below the level of the cusp tips.
The cervical line mesially is rather irregular and tends to curve occlusally about 1 mm toward the center of the mesial surface of the tooth (see Figure 12-15, 1, 4, 9, and 10). The cervical line may assume a relatively straight line buccolin-gually (see Figure 12-15, 3, 6, and 8).
In all instances, the cervical line is at a higher level lingually than buccally, usually about 1 mm higher. The difference in level may be greater. This relation depends on the assumption that the tooth is posed vertically. When the first molar is in its normal position in the lower jaw, leaning to the lingual, the cervical line is nearly level buccolingually.
The surface of the crown is convex and smooth over the mesial contours of the mesiolingual and mesiobuccal lobes.
A flattened or slightly concave area exists at the cervical line immediately above the center of the mesial root. This area is right below the contact area and joins the concavity of the central portion of the root at the cervix. The contact area is almost centered buccolingually in the mesial surface of the crown, and it is placed below the crest of the marginal ridge about one third the distance from marginal ridge to cervical line. (See stained contact area on specimen in Figure 12-7. Before contact wear has occurred, the contact area is not as broad. Refer also to Figure 12-4.)
The buccal outline of the mesial root drops straight down from the cervical line buccally to a point near the junction of cervical and middle thirds of the root. A gentle curve starts lingually from this point to the apex, which is located directly below the mesiobuccal cusp.
The lingual outline of the mesial root is slanted in a buccal direction, although the outline is nearly straight from the cervical line lingually to the point of junction of middle and apical thirds of the root. From this point, the curvature is sharply buccal to the bluntly tapered apex. On those specimens that show a short bifurcation at the mesial root end, the curvature at the apical third lingually is slight (see Figure 12-15, 2 and 10).
The mesial surface of the mesial root is convex at the buccal and lingual borders, with a broad concavity between these convexities the full length of the root from cervical line to apex. If a specimen tooth is held in front of a strong light so that the distal side of the mesial root can be seen from the apical aspect, it is noted that the same contours exist on the root distally as are found mesially, and the root is very thin where the concavities are superimposed. The root form appears to be two narrow roots fused together with thin, hard tissue between.
The mesial surface of the distal root is smooth, with no deep developmental depressions.
Distal Aspect
Because the gross outline of the distal aspect of the crown and root of the mandibular first molar is similar to that from the mesial aspect, the description of outline form will not be repeated. When this aspect is considered from the standpoint of a three-dimensional figure, however, more of the tooth is seen from the distal aspect, because the crown is shorter distally than mesially and the buccal and lingual surfaces of the crown converge distally (see Figures 12-10, 12-12, and 12-13). The buccal surface shows more convergence than the lingual surface. The distal root is more narrow buccolingually than the mesial root.
If a specimen of the first molar is held with the distal surface of the crown at right angles to the line of vision, a great part of the occlusal surface may be seen and some part of each of the five cusps also, which compares favorably with the mandibular second premolar. This is caused in part by the placement of the crowns on the roots with a distal inclination to the long axes. The slight variation in crown length distally does not provide this view of the occlusal surface (see Figures 12-9 and 12-10).
From the distal aspect, the distal cusp is in the foreground on the crown portion. The distal cusp is placed a little buccal to the center buccolingually, with the distal contact area appearing on its distal contour.
The distal contact area is placed just below the distal cusp ridge of the distal cusp and at a slightly higher level above the cervical line than was found mesially compared with the location of the mesial contact area.
The distal marginal ridge is short and is made up of the distal cusp ridge of the distal cusp and the distolingual cusp ridge of the distolingual cusp. These cusp ridges dip sharply in a cervical direction, meeting at an obtuse angle. Often a developmental groove or depression is found crossing the marginal ridge at this point. The point of this angle is above the lingual third of the distal root instead of being centered over the root as is true of the center of the mesial marginal ridge.
The distal contact area is centered over the distal root, which arrangement places it buccal to the center point of the distal marginal ridge.
The surface of the distal portion of the crown is convex on the distal cusp and the distolingual cusp. Contact wear may produce a flattened area at the point of contact on the distal surface of the distal cusp. Just above the cervical line, the enamel surface is flat where it joins the flattened surface of the root trunk distally.
The cervical line distally usually extends straight across buccolingually. It may be irregular, dipping rootwise just below the distal contact area (see Figure 12-10).
The end of the distobuccal developmental groove is located on the distal surface and forms a concavity at the cervical portion of the distobuccal line angle of the crown. The distal portion of the crown extends out over the root trunk distally at quite an angle (see Figure 12-4). The smooth, flat surface below the contact area remains fairly constant to the apical third of the distal root. Sometimes a developmental depression is found here. The apical third portion of the root is more rounded as it tapers to a sharper apex than is found on the mesial root.
The lingual border of the mesial root may be seen from the distal aspect.
Occlusal Aspect
The mandibular first molar is somewhat hexagonal from the occlusal aspect (see Figure 12-2). The crown measurement is 1 mm or more greater mesiodistally than buccolingually. It must be remembered that the opposite arrangement is true of the maxillary first molar.
The buccolingual measurement of the crown is greater on the mesial than on the distal side. Also, the measurement of the crown at the contact areas, which includes the two buccal cusps and the distal cusp, is greater than the mesio-distal measurement of the two lingual cusps. In other words, the crown converges lingually from the contact areas. This convergence varies in individual specimens (see Figure 1216, 1 and 4).
It is interesting to note the degree of development of the individual cusps from the occlusal aspect (see Figures 12-1, 12-2, 12-11, 12-12, 12-13, and 12-16). The mesiobuccal cusp is slightly larger than either of the two lingual cusps, which are almost equal to each other in size; the distobuccal cusp is smaller than any one of the other three mentioned, and the distal cusp is in most cases much the smallest of all.
More variance is evident in the development of the dis-tobuccal and distal lobes than in any of the others (see Figure 12-16, 1, T, and 10).
When the tooth is posed so that the line of vision is parallel with the long axis, a great part of the buccal surface may be seen, whereas only a small portion of the lingual surface may be seen lingual to the lingual cusp ridges. No part of the mesial or distal surfaces is in view below the outline of the mesial and distal marginal ridges. (Compare tooth outlines from the other aspects.)
Figure 12-16 Mandibular first molar, occlusal aspect. Ten typical specimens are shown.
Figure 12-17 Mandibular first molar. Ten specimens with uncommon variations are shown. 1, Root extremely long, crown small. 2, Mesial root longer than average with rounded apex. 3, Crown very wide buccolingually; roots short. 4, Roots short. 5, Crown has no buccal developmental grooves. 6, Crown and roots poorly formed. 7, Roots dwarfed. 8, Roots short; crown wide buccolingually. 9, Crown and root oversize buccolingually. 10, Extra tubercle or cusp attached to mesiolingual lobe.
All mandibular molars, including the first molar, are essentially quadrilateral in form. The mandibular first molar, in most instances, has a functioning distal cusp, although it is small in comparison with the other cusps. Occasionally, four-cusp first molars are found, and more often, one discovers first molars with distobuccal and distal cusps showing fusion with little or no trace of a distobuccal developmental groove between them (see Figure 12-16, 1 and Figure 12-17, 4 and 5). From a developmental viewpoint, all mandibular molars have four major cusps, whereas maxillary molars have only three major cusps (see Figure 11-11).
The occlusal surfaces of the mandibular first molar may be described as follows: there is a major fossa and two minor fossae. The major fossa is the central fossa (see Figure 12-2). It is roughly circular, and it is centrally placed on the occlu-sal surface between buccal and lingual cusp ridges. The two minor fossae are the mesial triangular fossa, immediately distal to the mesial marginal ridge, and the distal triangular fossa, immediately mesial to the distal marginal ridge (see Figure 12-1).
The developmental grooves on the occlusal surface are the central developmental groove, the mesiobuccal developmental groove, the distobuccal developmental groove, and the lingual developmental groove. Supplemental grooves, accidental short grooves, and developmental pits are also found. Most of the supplemental grooves are tributary to the developmental grooves within the bounds of cusp ridges.
The central fossa of the occlusal surface is a concave area bounded by the distal slope of the mesiobuccal cusp, both mesial and distal slopes of the distobuccal cusp, the mesial slope of the distal cusp, the distal slope of the mesiolingual cusp, and the mesial slope of the distolingual cusp (see Figure 12-2).
All of the developmental grooves converge in the center of the central fossa at the central pit.
The mesial triangular fossa of the occlusal surface is a smaller concave area than the central fossa, and the mesial slope of the mesiobuccal cusp, the mesial marginal ridge, and the mesial slope of the mesiolingual cusp bound it. The mesial portion of the central developmental groove terminates in this fossa. Usually a buccal and a lingual supplemental groove join it at a mesial pit within the boundary of the mesial marginal ridge. Sometimes a supplemental groove crosses the mesial marginal ridge lingual to the contact area (see Figure 12-16, 2, 8, 9, and 10).
The distal triangular fossa is in most instances less distinct than the mesial fossa. The distal slope of the distal cusp, the distal marginal ridge, and the distal slope of the distolingual cusp bound it. The central groove has its other terminal in this fossa. Buccal and lingual supplemental grooves are less common here. An extension of the central groove quite often crosses the distal marginal ridge, however, lingual to the distal contact area.
Starting at the central pit in the central fossa, the central developmental groove travels an irregular course mesially, terminating in the mesial triangular fossa. A short distance mesially from the central pit, it joins the mesiobuccal developmental groove. The latter groove courses in a mesiobuc-cal direction at the bottom of a sulcate groove separating the mesiobuccal and distobuccal cusps. At the junction of the cusp ridges of those cusps, the mesiobuccal groove of the occlusal surface is confluent with the mesiobuccal groove of the buccal surface of the crown. The lingual developmental groove of the occlusal surface is an irregular groove coursing in a lingual direction at the bottom of the lingual sulcate groove to the junction of lingual cusp ridges, where it is confluent with the lingual extension of the same groove. Again starting at the central pit, the central groove may be followed in a distobuccal direction to a point where it is joined by the distobuccal developmental groove of the occlusal surface. From this point, the central groove courses in a distolingual direction, terminating in the distal triangular fossa. The distobuccal groove passes from its junction with the central groove in a distobuccal course, joining its buccal extension on the buccal surface of the crown at the junction of the cusp ridges of the distobuccal and distal cusps.
The central developmental groove seems to be centrally located in relation to the buccolingual crown dimension. This arrangement makes the triangular ridges of lingual cusps longer than the triangular ridges of buccal cusps.
Note the relative position and relative size of the distal cusp from the occlusal aspect. The distal portion of it joins the distal contact area of the crown.
