10 The Permanent Mandibular Premolars
The mandibular premolars number four: two are situated in the right side of the mandible and two in the left side. They are immediately posterior to the mandibular canines and anterior to the molars.
The mandibular first premolars are developed from four lobes, as were the maxillary premolars. The mandibular second premolars are, in most instances, developed from five lobes, three buccal and two lingual lobes.
The first premolar has a large buccal cusp, which is long and well formed, with a small, nonfunctioning lingual cusp that in some specimens is no longer than the cingulum found on some maxillary canines (see Figure 10-10, 3 and 8; and Figure 10-12, 4 and 7). The second premolar has three well-formed cusps in most cases, one large buccal cusp and two smaller lingual cusps. The form of both mandibular premolars fails to conform to the implications of the term bicuspid, a term that implies two functioning cusps.
The mandibular first premolar has many of the characteristics of a small canine, because its sharp buccal cusp is the only part of it occluding with maxillary teeth. It functions along with the mandibular canine. The mandibular second premolar has more of the characteristics of a small molar, because its lingual cusps are well developed, a fact that places both marginal ridges high and produces a more efficient occlusion with antagonists in the opposite jaw. The mandibular second molar functions by being supplementary to the mandibular first molar.
The first premolar is always the smaller of the two mandibular premolars, whereas the opposite is true, in many cases, of the maxillary premolars.
Mandibular First Premolar
Figures 10-1 through 10-12 illustrate the mandibular first premolar from all aspects. The mandibular first premolar is the fourth tooth from the median line and the first posterior tooth in the mandible. This tooth is situated between the canine and second premolar and has some characteristics common to each of them.
Figure 10-1 Mandibular right first premolar, mesial and occlusal aspects. BC, Buccal cusp; BTR, buccal triangular ridge; LC, lingual cusp; MLDG, mesiolingual developmental groove; CL, cervical line; BCR, buccal cervical ridge; MCA, mesial contact area; MMR, mesial marginal ridge; DBCR, distobuccal cusp ridge; DMR, distal marginal ridge; CDG, central developmental groove; MBCR, mesiobuccal cusp ridge.
Figure 10-2 Mandibular right first premolar, buccal aspect. The specimen in this photograph shows a mesial inclination of the root. Mandibular premolars and canines have this tendency, although most of the roots of these teeth will curve, if at all, in a distal direction. (Grid = 1 sq mm.)
Figure 10-7 Mandibular right first premolar. Graph outlines of five aspects are shown. (Grid = 1 sq mm.)
Figure 10-12 Mandibular first premolar. Ten specimens with uncommon variations are shown. 1, Crown oversized. 2, Crown and root diminutive. 3, Mesial and distal sides of crown straight; cervix wide mesiodistally; root extra long. 4, Unusual formation of lingual portion of crown; root with deep developmental groove mesially. 5, Bifurcated root. 6, Lingual cusp long; little lingual curvature; root of extra length. 7, No lingual cusp; root bifurcated. 8, Dwarfed root. 9, Crown poorly formed; root unusually long. 10, Very long curved root for crown so small.
The characteristics that resemble those of the mandibular canine are as follows:
The characteristics that resemble those of the second mandibular premolar are as follows:
Although the root of the mandibular first premolar is shorter generally than that of the mandibular second premolar, it is closer to the length of the second premolar root than it is to that of the mandibular canine (Table 10-1).
DETAILED DESCRIPTION OF THE MANDIBULAR FIRST PREMOLAR FROM ALL ASPECTS
From the buccal aspect, the form of the mandibular first premolar crown is nearly symmetrical bilaterally (see Figures 10-2, 10-7, 10-8, and 10-9). The middle buccal lobe is well developed, which results in a large, pointed buccal cusp. The mesial cusp ridge is shorter than the distal cusp ridge.
The contact areas are broad from this aspect; they are almost at the same level mesially and distally, this level being a little more than half the distance from cervical line to cusp tip. The measurement mesiodistally at the cervical line is small compared with the measurement at the contact areas.
From the buccal aspect, the crown is roughly trapezoidal (see Figure 4-16, C). The cervical margin is represented by the shortest of the uneven sides.
The crown exhibits little curvature at the cervical line buccally, because of the slight curvature of the cervical line on the mesial and distal surfaces of the tooth. The crest of curvature of the cervical line buccally approaches the center of the root buccally.
The mesial outline of the crown is straight or slightly concave above the cervical line to a point where it joins the curvature of the mesial contact area. The center of the contact area mesially is occlusal to the cervical line, a distance equal to a little more than half the crown length. The outline of the mesial slope of the buccal cusp usually shows some concavity unless wear has obliterated the original form.
The tip of the buccal cusp is pointed and, in most cases, is located a little mesial to the center of the crown buccally (see Figure 10-9, 3, 7, 8, and 9). The mandibular canine has the same characteristic to a greater degree.
The distal outline of the crown is slightly concave above the cervical line to a point where it is confluent with the curvature describing the distal contact area. This curvature is broader than that describing the curvature of the mesial contact area. The distal slope of the buccal cusp usually exhibits some concavity.
The cervix of the mandibular first premolar crown is narrow mesiodistally when compared with the crown width at the contact areas.
The root of this tooth is 3 or 4 mm shorter than that of the mandibular canine, although the outline of the buccal portion of the root bears a close resemblance to that of the canine.
The buccal surface of the crown is more convex than in the maxillary premolars, especially at the cervical and middle thirds.
The development of the middle buccal lobe is outstanding, ending in a pointed buccal cusp. Developmental depressions are often seen between the three lobes (see Figure 10-9, 2, 3, 8, and 10).
The continuous ridge from the cervical margin to the cusp tip is called the buccal ridge.
In general, the enamel of the buccal surface of the crown is smooth and shows no developmental grooves and few developmental lines. If the latter are present, they are seen as very fine horizontal cross lines at the cervical portion.
The crown of the mandibular first premolar tapers toward the lingual, since the lingual measurement mesiodistally is less than that buccally. The lingual cusp is always small (see Figures 10-3, 10-7, and 10-8). The major portion of the crown is made up of the middle buccal lobe (see Figure 10-11). This makes it resemble the canine.
The crown and the root taper markedly toward the lingual so that most of the mesial and distal surfaces of both may be seen from the lingual aspect.
The occlusal surface slopes greatly toward the lingual in a cervical direction down to the short lingual cusp. Most of the occlusal surface of this tooth can therefore be seen from this aspect.
The cervical portion of the crown lingually is narrow and convex, with concavities in evidence between the cervical line and the contact areas on the lingual portion of mesial and distal surfaces. The contact areas and marginal ridges are pronounced and extend out above the narrow cervical portion of the crown.
Although the lingual cusp is short and less developed (resembling a strongly developed cingulum at times), it usually shows a pointed tip. This cusp tip is in alignment with the buccal triangular ridge of t/>