The mandibular incisors are four in number. The mandibular central incisors are centered in the mandible, one on either side of the median line, with the mesial surface of each one in contact with the mesial surface of the other. The right and left mandibular lateral or second incisors are distal to the central incisors. They are in contact with the central incisors mesially and with the canines distally.
The mandibular incisors have smaller mesiodistal dimensions than any of the other teeth. The central incisor is somewhat smaller than the lateral incisor, which is the reverse of the situation in the maxilla.
These teeth are similar in form and have smooth crown surfaces that show few traces of developmental lines. Mamelons on the incisal ridges are worn off soon after eruption, if the occlusion is normal, which leaves the incisal ridges smooth and straight (compare Figure 7-9, 7 and 8). The contact areas are near the incisal ridges mesially and distally, and lines drawn through the contact areas are near the same level on both central and lateral incisors; here also the situation is unlike that of the maxillary incisors. The mandibular incisors show uniform development, with few instances of malformations or anomalies (see Figure 7-12).1,2
The anatomical form of these teeth differs entirely from that of the maxillary incisors. The inclination of the crowns differs from the mesial and distal aspects; the labial faces are inclined lingually so that the incisal ridges are lingual to a line bisecting the root. After normal wear has taken place, obliterating the mamelons, the incisal surfaces thus created show a labial inclination when the occlusion has been normal. Note that the incisal surfaces of maxillary incisors have a lingual inclination. With this arrangement, the incisal planes of the mandibular and maxillary incisors are parallel with each other, fitting together during incising action.
Mandibular Central Incisor
Figures 7-1 through 7-12 illustrate the mandibular central incisor in various aspects. Generally, the mandibular central incisor is the smallest tooth in the dental arches (Table 7-1). The crown has little more than half the mesiodistal diameter of the maxillary central incisor; however, the labiolingual diameter is only about 1 mm less. The lines of greatest masticatory stress are brought to bear on the mandibular incisors in a labiolingual direction, which makes this reinforcement necessary.
The single root is very narrow mesiodistally and corresponds to the narrowness of the crown, although the root and crown are wide labiolingually. The length of the root is as great as, if not greater than, that of the maxillary central incisor.
Detailed Description of the Mandibular Central Incisor From All Aspects
Labial Aspect
The labial aspect of the mandibular central incisor is regular, tapering evenly from the relatively sharp mesial and distal incisal angles to the apical portion of the root (see Figures 7-7 through 7-9). The incisal ridge of the crown is straight and is at approximately a right angle to the long axis of the tooth.
FIGURE 7-1 Mandibular right central incisor, lingual and incisal aspects. IR, Incisal ridge; DMR, distal marginal ridge; LF, lingual fossa; CL, cervical line; C, cingulum; MMR, mesial marginal ridge.
FIGURE 7-2 Mandibular right central incisor, labial aspect. (Grid = 1 sq mm.)
FIGURE 7-3 Mandibular right central incisor, lingual aspect. (Grid = 1 sq mm.)
FIGURE 7-4 Mandibular right central incisor, mesial aspect. (Grid = 1 sq mm.)
FIGURE 7-5 Mandibular right central incisor, distal aspect. (Grid = 1 sq mm.)
FIGURE 7-6 Mandibular right central incisor, incisal aspect. (Grid = 1 sq mm.)
Figure 7-7 Mandibular right central incisor. Graph outlines of five aspects are shown. (Grid = 1 sq mm.)
Usually, the mesial and distal outlines of the crown make a straight drop downward from the incisal angles to the contact areas, which are incisal to the junction of incisal and middle thirds of the crown. The mesial and distal sides of the crown taper evenly from the contact areas to the narrow cervix.
The mesial and distal root outlines are straight with the mesial and distal outlines of the crown down to the apical portion. The apical third of the root terminates in a small, pointed taper, in most cases curving distally. Sometimes the roots are straight (see Figure 7-9, 2 and 10).
The labial face of the mandibular central incisor crown is ordinarily smooth, with a flattened surface at the incisal third; the middle third is more convex, narrowing down to the convexity of the root at the cervical portion.
Except in newly erupted teeth, central incisors show few traces of developmental lines. The labial surface of the root of the mandibular central incisor is regular and convex.
Lingual Aspect
The lingual surface of the crown is smooth, with very slight concavity at the incisal third between the inconspicuous marginal ridges (see Figures 7-1, 7-3, 7-7, and 7-8). In some instances, the marginal ridges are more prominent near the incisal edges (see Figure 7-11, 2 and 8). In these cases, the concavity between the marginal ridges is more distinct.
The lingual surface becomes flat and then convex as progression is made from the incisal third to the cervical third.
FIGURE 7-8 Mandibular right central incisor.
No developmental lines mark the cingulum development on this tooth at the cervical third. No other tooth in the mouth, except the mandibular lateral incisor, shows so few developmental lines and grooves. The outlines and surfaces of the mandibular incisors are regular and symmetrical.
