Mandibular Second Premolar
Figures 10-13 through 10-21 illustrate the mandibular second premolar from all aspects. The mandibular second premolar resembles the mandibular first premolar from the buccal aspect only. Although the buccal cusp is not as pronounced, the mesiodistal measurement of the crown and its general outline are similar (Table 10-2). The tooth is larger and has better development in other respects. This tooth assumes two common forms. The first form, which probably occurs most often, is the three-cusp type, which appears more angular from the occlusal aspect (see Figure 10-17). The second form is the two-cusp type, which appears more rounded from the occlusal aspect (see Figure 10-20, 1, 2, 7, and 10).
Figure 10-13 Mandibular left second premolar, buccal aspect. (Grid = 1 sq mm.)
Figure 10-14 Mandibular left second premolar, lingual aspect. (Grid = 1 sq mm.)
Figure 10-15 Mandibular left second premolar, mesial aspect. (Grid = 1 sq mm.)
Figure 10-16 Mandibular left second premolar, distal aspect. (Grid = 1 sq mm.)
The two types differ mainly in the occlusal design. The outlines and general appearance from all other aspects are similar.
The single root of the second premolar is larger and longer than that of the first premolar. The root is seldom, if ever, bifurcated, although some specimens show a deep developmental groove buccally (see Figure 10-18, 3 and 6 ). Often a flattened area appears in this location. Ten specimens with uncommon variations are shown in Figure 10-21.
Figure 10-17 Mandibular left second premolar, occlusal aspect. (Grid = 1 sq mm.)
Detailed Description of the Mandibular Second Premolar From All Aspects
To describe the separate aspects of this tooth, direct comparisons are made with the mandibular first premolar except for the occlusal aspect.
From the buccal aspect, the mandibular second premolar presents a shorter buccal cusp than the first premolar, with mesiobuccal and distobuccal cusp ridges showing angulation of less degree (see Figures 10-13 and 10-18). The contact areas, both mesial and distal, are broad. The contact areas appear to be higher because of the short buccal cusp.
The root is broader mesiodistally than that of the first premolar, the extra breadth appearing for most of its length, and the root ends in an apex that is more blunt. In other respects, the two teeth are quite similar from this aspect.
From the lingual aspect, the second premolar crown shows considerable variation from the crown portion of the first premolar (see Figure 10-14). The variations are as follows:
Figure 10-18 Mandibular second premolar, occlusal aspect. Ten typical specimens are shown.
Figure 10-19 Mandibular second premolar, mesial aspect. Ten typical specimens are shown.
Figure 10-20 Mandibular second premolar, occlusal aspect. Ten typical specimens are shown.
Figure 10-21 Mandibular second premolar. Ten specimens with uncommon variations are shown. 1, Root extremely long. 2, Root dwarfed. 3, Malformed root; developmental groove on buccal surface. 4, Contact areas on crown high and constricted. 5, Crown oversize; developmental groove buccally on root. 6, Root oversize. 7, Root malformed and of extra length. 8, Root very long with blunt apex; extreme curvature at apical third. 9, Crown and root oversized; developmental groove buccally on root. 10, Crown narrow buccolingually; very little curvature buccally and lingually.
Table 10-2 Mandibular Second Premolar
1. The lingual lobes are developed to a greater degree, which makes the cusp or cusps (depending on the type) longer.
2. Less of the occlusal surface may be seen from this aspect. Nevertheless, because the lingual cusps are not as long as the buccal cusp, part of the buccal portion of the occlusal surface may be seen.
3. In the three-cusp type, the lingual development brings about the greatest variation between the two teeth. Mesiolingual and distolingual cusps are present, with the former being the larger and the longer one in most cases. A groove is between them, extending a very short distance on the lingual surface and usually centered over the root (see Figure 10-20, 8).
In the two-cusp type, the single lingual cusp development attains equal height with the three-cusp type. The two-cusp type has no groove, but it shows a developmental depression distolingually where the lingual cusp ridge joins the distal marginal ridge (see Figure 10-20, 2 and 3).
The lingual surface of the crown of all mandibular second premolars is smooth and spheroidal, having a bulbous form above the constricted cervical portion.
The root is wide lingually, although not quite as wide as the buccal portion. Less difference in dimension is evident here than was found on the first premolar, so that much less convergence toward the lingual is seen.
Because in most instances the lingual portion of the crown converges little from the buccal portion, less of the mesial and distal sides of this tooth may be seen from this aspect than are seen from the lingual aspect of the first premolar.
The lingual portion of the root is smoothly convex for most of its length. Considered overall, the second premolar is the larger of the two mandibular premolars.
From the mesial aspect (see Figures 10-15 and 10-19), the second premolar differs from the first premolar as follows:
1. The crown and root are wider buccolingually.
2. The buccal cusp is not as nearly centered over the root trunk, and it is shorter.
3. The lingual lobe development is greater.
4. The marginal ridge is at right angles to the long axis of the tooth.
5. Less of the occlusal surface may be seen.
6. No mesiolingual developmental groove is present on the crown portion.
7. The root is longer and in most cases slightly convex on the mesial surface; however, this convexity is not always present (see Figure 10-19, 6, 7, and 8).
8. The apex of the root is usually more blunt on the second premolar.
The distal aspect of the mandibular second premolar is similar to the mesial aspect, except that more of the occlusal surface may be seen (see Figure 10-16). This is possible because the distal marginal ridge is at a lower level than the mesial marginal ridge when the tooth is posed vertically. The crowns of all posterior teeth are tipped distally to the long axes of the roots, so that when the specimen tooth is held vertically, more of the occlusal surface may be seen from the distal aspect than from the mesial aspect. This is a characteristic possessed by all posterior teeth, mandibular and maxillary. The angulation of occlusal surfaces to long axes of all posterior teeth is an important observation to remember, not only in the study of individual tooth forms but also later, in the study of alignment and occlusion.
As mentioned earlier, two common forms of this tooth are evident. The outline form of each type shows some variation from the occlusal aspect (see Figures 10-17 and 10-20). The two types are similar in that portion that is buccal to the mesiobuccal and distobuccal cusp ridges.
The three-cusp type appears square lingual to the buccal cusp ridges when highly developed (see Figure 10-20, 8). The round, or two-cusp, type appears round lingual to the buccal cusp ridges (see Figure 10-20, 3).
The square type (see Figure 10-20, 8) has three cusps that are distinct; the buccal cusp is the largest, the mesiolingual cusp is next, and the distolingual cusp is the smallest.
Each cusp has well-formed triangular ridges separated by deep developmental grooves. These grooves converge in a central pit and form a Y shape on the occlusal surface. The central pit is located midway between the buccal cusp ridge and the lingual margin of the occlusal surface and slightly distal to the central point between mesial and distal marginal ridges.
Starting at the central pit, the mesial developmental groove travels in a mesiobuccal direction and ends in the mesial triangular fossa just distal to the mesial marginal ridge. The distal developmental groove travels in a distobuccal direction, is somewhat shorter than the mesial groove, and ends in the distal triangular fossa mesial to the distal marginal side. The lingual developmental groove extends lingually between the two lingual cusps and ends on the lingual surface of the crown just below the convergence of the lingual cusp ridges. The mesiolingual cusp is wider mesio-distally than the distolingual cusp. This arrangement places the lingual developmental groove distal to center on the crown.
Supplemental grooves and depressions are often seen, radiating from the developmental grooves. Occasionally, a groove crosses one or both of the marginal ridges. On a tooth of this type, the point angles are distinct. Developmental grooves are often deep.
Figure 10-20, 8 is representative. Variations of this development may be seen in Figure 10-20, 4, 5, 6, and 9.
The round or two-cusp type (see Figure 10-20, 3) differs considerably from the three-cusp type when viewed from the occlusal aspect. It is a true typal form of the two-cusp type. Variations may be seen in Figure 10-20, 1, 2, 7, and 10.
The occlusal characteristics of the two-cusp type are as follows:
1. The outline of the crown is rounded lingual to the buccal cusp ridges.
2. Some lingual convergence of mesial and distal sides occurs, although no more than is found in some variations of the square type.
3. The mesiolingual and distolingual line angles are rounded.
4. One well-developed lingual cusp is directly opposite the buccal cusp in a lingual direction.
A central developmental groove on the occlusal surface travels in a mesiodistal direction. This groove may be straight (see Figure 10-20, 3), but it is most often crescent-shaped (see Figure 10-20, 1, 7, and 10). The central groove has its terminals centered in mesial and distal fossae, which are roughly circular depressions having supplemental grooves and depressions radiating from the central groove and its terminals. The enamel surface inside these fossae and around their peripheries is very irregular, acting as a contrast to the smoothness of cusp ridges, marginal ridges, and the transverse ridge from buccal cusp to lingual cusp.
Some of these teeth show mesial and distal developmental pits centered in the mesial and distal fossae instead of an unbroken central groove (see Figure 10-20, 2).
Although photographs do not demonstrate it very well, most of these two-cusp specimens show a developmental depression crossing the distolingual cusp ridge.