The eight permanent incisors are four maxillary (upper) and four mandibular (lower). The maxillary incisors consist of two central and two lateral incisors, as do the four mandibular incisors (Fig. 12-1).
The most prominent teeth in the mouth are the maxillary incisors. The maxillary central incisors are larger than the lateral incisors, but these teeth complement each other in form and function. The central incisors erupt in the seventh or eighth year, and the lateral incisors 1 year or more later.
A maxillary central incisor (Fig. 12-2) is the widest mesiodistally of any of the anterior teeth. Its labial appearance is less rounded than that of a maxillary lateral incisor or canine. The crown usually looks symmetrical and normally formed, having a nearly straight incisal edge, a mesial side with a straight outline, and the distal side that is more curved. The mesioincisal angle is relatively sharp, and the distoincisal angle is much more rounded.
Maxillary central incisors usually develop normally. Two anomalies that sometimes occur are a short root or an unusually long crown. A third is gemination, where the tooth shows evidence of attempting to divide itself.
Labial aspect (Fig. 12-3; see also Fig. 12-8, A)
The labial surface of the crown of the maxillary central incisor is slightly convex, bulging out from the cervical portion of the crown. The enamel surface is very smooth. When the tooth first erupts, mamelons can be seen on the incisal ridge. These mamelons are rounded portions of the incisal ridge of newly erupted teeth. Each mamelon forms the incisal ridge portion of one of the labial primary lobes. Developmental lines on the labial face divide the surface into three parts, each developmental line separating a primary lobe.
From this facial view, the distal outline of the crown is more rounded or convex than the mesial outline, the height of curvature being higher toward the cervical line. The cervical line crests slightly distal to the center of the tooth.
The incisal outline is usually regular and straight across the incisal ridge after the tooth has been in function long enough to wear down the mamelons. When an incisor first erupts, the incisal portion of the crown is rounded and the mamelons are quite distinct. This ridge portion is then called the incisal ridge. However, normal use eventually wears down the rounded ridge into a flat edge, and therefore the term incisal edge is more appropriate than ridge.
The root of a central incisor from the labial aspect is cone shaped and has a blunt apex in most instances. The root is usually 2 to 3 mm longer than the crown, although the root-to-crown ratio varies considerably.
Lingual aspect (Fig. 12-4; see also Fig. 12-8, B)
The lingual outline of a maxillary central incisor is the reverse of that found on the labial aspect. The labial surface of the crown is smooth, whereas the lingual surface is bordered by rounded convexities and a concavity. The outline of the cervical line is similar, but immediately below the cervical line is a smooth convexity called the cingulum.
Mesially and distally confluent with the cingulum is a shallow concavity called the lingual fossa. The marginal and incisal ridges, which are rounded convexities, border the lingual fossa. Usually there are developmental grooves extending from the cingulum into the lingual fossa.
Mesial aspect (Fig. 12-5; see also Fig. 12-8, D)
The crown of a maxillary central incisor is triangular, with the base of the triangle at the cervix and the apex at the incisal ridge. The incisal ridge of the crown is centered over the middle of the root. This alignment is characteristic of maxillary central and lateral incisors.
The labial outline of the crown from the crest of curvature to the incisal edge is slightly convex, with the height of curvature about one third of the way down from the cervical line. The cervical curvature is greater on the mesial surface of these teeth than on any surface of any other teeth in the mouth.
Distal aspect (Fig. 12-6; see also Fig. 12-8, E)
Little difference is evident between the distal and mesial outlines of maxillary central incisors. The cervical line indicating the cementoenamel junction (CEJ) is less curved on the distal surface than on the mesial. It is generally true that if a difference exists in the curvatures of the mesial and distal cervical lines of the same tooth, the mesial curvature is greater. For example, if mesial curvature is 2.5 mm, then the distal might be 1.5 mm.
Incisal aspect (Fig. 12-7; see also Fig. 12-8, C)
The incisal ridge tends to slope lingually as a result of the lower incisors coming more frequently into contact with the lingual edge than with the facial edge of the maxillary incisor. From an incisal view, the crown shows a triangular shape, with its apex on the lingual surface. (See Fig. 12-8 for all of the views of a maxillary right central incisor.)
Maxillary lateral incisors complement the central incisors in function and resemble each other in form. Lateral incisors are small in all dimensions except root length. The features—curvatures, concavities, and convexities—of the lateral incisors are more prominent and show more distinction and contrast than those of the central incisors. These teeth differ from the central incisors in that their individual development varies considerably. Maxillary lateral incisors vary in form more than any other teeth in the mouth except the third molars. If the variation is too great, it is considered a developmental anomaly. A common situation is to find maxillary lateral incisors that have a nondescript, pointed form; such teeth are called peg-shaped lateral incisors. For some, the lateral incisors are missing entirely. Maxillary lateral incisors are more likely to be congenitally missing than any other teeth except the third molars. One type of malformed maxillary lateral incisor displays a large, pointed tubercle as part of the cingulum; some have deep developmental grooves that extend down the root lingually with a deep fold in the cingulum. Other maxillary lateral incisors show twisted roots or distorted crowns.
Although the labial aspect (Fig. 12-9, A) of a maxillary lateral incisor may appear to resemble that of a central incisor, it usually has more curvature, with a rounded incisal ridge and rounded angles mesially and distally. The distal outline is always more rounded, and the height of contour is more cervical than the mesial outline.
The labial surface of the crown is more convex than that of a central incisor, and as a rule the root length is greater in proportion to the crown length than that of a central incisor. The root is often about 1.5 times the length of the crown.
The lingual view (Fig. 12-9, B) of a lateral incisor shows more contrast than the same view of a central incisor. Mesial and distal marginal ridges are pronounced, and the cingulum is usually prominent, with a tendency toward deep developmental grooves within the lingual fossa where it joins the cingulum. The linguoincisal ridge is better developed, and the lingual fossa is more concave and circumscribed than that of the central incisor. A lingual pit is frequently present.
The incisal aspect (Fig. 12-9, C) of these teeth sometimes resembles that of the central incisors or a small canine. The cingulum and the incisal ridge, however, may be large; the labiolingual dimension may be greater than usual in comparison with the mesiodistal dimension. If these variations are present, the teeth show a strong resemblance to the canines.
The maxillary central incisor usually has a straight, thick, cylindrically shaped root. The maxillary lateral incisor has a more narrow root mesiodistally; the root is as long as that of the central incisor but appears thinner. The apical portion of the lateral incisor’s root often curves distally and ends in a sharp apex rather than in a blunt, straight apex as in the central incisor.
The pulp cavity varies in size with the age of the tooth. When the tooth first erupts, the pulp cavity is very large, and the root is incompletely formed, so the canal becomes funnel shaped in the region of the apical foramen. As the tooth develops completely, the entire pulp cavity to the apex of the root becomes smaller, and the dentin becomes thicker in both the crown and the root. The apical foramen is then very small. This process continues throughout the life of the tooth. In very old people, it is not unusual to find that the entire pulp cavity has become calcified and solid dentin is filling the entire root canal. This variation in the size of the pulp cavity with aging is common to all of the permanent teeth.
The pulp cavity of the maxillary central incisor (Fig. 12-10) mirrors the configuration of the tooth. Only one root canal is evident, which is rather large. The pulp chamber lies in the coronal portion of the tooth and presents three sharp elongations: mesial, distal, and central pulp horns. The central pulp horn is usually shorter and more rounded than the other two.
The pulp cavity of the maxillary lateral incisor is quite simple, comprising only a pulp chamber and a single pulp canal. The chamber is similar to that of the maxillary central incisor but usually does not have three sharp pulp horns. More often, the pulp chamber ends incisally as one rounded form or two less sharp pulp horns, a mesial and distal (Fig. 12-11).