7 The Permanent Mandibular Incisors
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.
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.
Figure 7-12 Mandibular central incisor. Ten specimens with uncommon variations are shown. 1, Crown and root very broad mesiodistally; malformed enamel at incisal third of crown. 2, Crown wide at incisal third, with short crown; root length extreme. 3, Unusual contours at middle third of crown; cervix narrow. 4, Well-formed crown; short root. 5, No curvature labially at cervical third; extreme labial curvature at root end. 6, Specimen well formed but undersized. 7, Contact areas pointed at incisal edge; crown and root very long. 8, Crown long and narrow; root short. 9, Crown measurement at cervical third same as root; crown and root of extreme length. 10, Crown and root very wide labiolingually; greater curvature than average above cervical line at the cervical third of the crown.
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.