Detailed Description of the Maxillary Central Incisor From All Aspects
Labial Aspect
The crown of the average central incisor will be 10 to 11 mm long from the highest point on the cervical line to the lowest point on the incisal edge (see Figures 6-2 and 6-9). The mesiodistal measurement will be 8 to 9 mm wide at the contact areas. The mesiodistal measurement, where the root joins the crown, will be 1.5 to 2 mm less. The crests of curvature mesially and distally on the crown represent the areas at which the central incisor contacts its neighbors. Any change in the position of this crest of contour affects the level of the contact area (see Figure 5-15, A).
The mesial outline of the crown is only slightly convex, with the crest of curvature (representing the contact area) approaching the mesioincisal angle.
The distal outline of the crown is more convex than the mesial outline, with the crest of curvature higher toward the cervical line. The distoincisal angle is not as sharp as the mesioincisal angle, the extent of curvature depending on the typal form of the tooth.
The incisal outline is usually regular and straight in a mesiodistal direction after the tooth has been in function long enough to obliterate the mamelons. The incisal outline tends to curve downward toward the center of the crown outline, so that the crown length is greater at the center than at the two mesial angles.
The cervical outline of the crown follows a semicircular direction with the curvature rootwise, from the point at which the root outline joins the crown mesially to the point at which the root outline joins the crown distally.
The root of the central incisor from the labial aspect is cone-shaped, in most instances with a relatively blunt apex,and the outline mesially and distally is regular. The root is usually 2 or 3 mm longer than the crown, although it varies considerably. (See illustrations of typical central incisors and those of variations from the labial aspects in Figures 6-9 and 6-12.)
Figure 6-9 Maxillary central incisor, labial aspect. Ten typical specimens are shown.
Figure 6-10 Maxillary central incisor, mesial aspect. Ten typical specimens are shown.
FIGURE 6-11 Maxillary central incisor, incisal aspect. Ten typical specimens are shown.
FIGURE 6-12 Maxillary central incisor. Ten specimens with uncommon variations are shown. 1, Extralingual inclination of incisal portion of crown. Note developmental (palatoradicular) groove traversing root and part of crown. 2, Root extremely long. 3, Specimen small in all dimensions. 4, Crown extremely long, root very short. 5, Specimen malformed; crown unusually long; cervix very wide. 6, Root short and tapering. 7, Same as specimen 6. 8, Crown nearly as wide at the cervix as at contact areas, crown long, root short. 9, Root with unusual curvature. 10, Crown and root narrow labiolingually; root comparable with that of specimen 2.
Table 6-1 Maxillary Central Incisor
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First evidence of calcification |
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3-4 mo |
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Enamel completed |
4-5 yr |
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Eruption |
7-8 yr |
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Root completed |
10 yr |
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Measurement Table |
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Cervicoincisal Length of Crown |
Length of Root |
Mesiodistal Mesiodistal Diameter of Diameter Crown at of Crown Cervix |
Labio- or Buccolingual Diameter of Crown |
Labio- or Buccolingual Diameter of Crown at Cervix |
Curvature of Cervical Line—Mesial |
Curvature of Cervical Line—Distal |
|
|
Dimensions* suggested for carving technique |
10.5 |
13.0 |
8.5 7.0 |
7.0 |
6.0 |
3.5 |
2.5 |
A line drawn through the center of the root and crown of the maxillary central incisor tends to parallel the mesial outline of the crown and root.
Lingual Aspect
The lingual outline of the maxillary central incisor is the reverse of that found on the labial aspect (see Figure 6-3). The lingual aspect of the crown is different, however, when we compare the surface of the lingual aspect with that of the labial aspect. From the labial aspect, the surface of the crown is smooth generally. The lingual aspect has convexities and a concavity. The outline of the cervical line is similar, but immediately below the cervical line a smooth convexity is to be found; this is called the cingulum (see Figure 6-1).
Mesially and distally confluent with the cingulum are the marginal ridges. Between the marginal ridges, below the cingulum, a shallow concavity is present called the lingual fossa. Outlining the lingual fossa, the linguoincisal edge is raised somewhat, being on a level with the marginal ridges mesially and distally, completing the lingual portion of the incisal ridge of the central incisor.
From the foregoing description, we note that the lingual fossa is bordered mesially by the mesial marginal ridge, incisally by the lingual portion of the incisal ridge, distally by the distal marginal ridge, and cervically by the cingulum. Usually there are developmental grooves extending from the cingulum into the lingual fossa.
The crown and root taper lingually, so that the crown calibration at the two labial line angles is greater than the calibration at the two lingual line angles, and the lingual portion of the root is narrower than the labial portion. A cross section of the root at the cervix shows the root to be generally triangular with rounded angles. One side of the triangle is labial, with the mesial and distal sides pointing lingually. The mesial side of this triangle is slightly longer than the distal side (see Figure 13-8, C, 3, 4, 5, and 6).
Mesial Aspect
The mesial aspect of this tooth has the fundamental form of an incisor. The crown is wedge-shaped, or triangular, with the base of the triangle at the cervix and the apex at the incisal ridge (see Figure 4-16, A and Figures 6-4 and 6-10).
Usually a line drawn through the crown and the root from the mesial aspect through the center of the tooth will bisect the apex of the root and also the incisal ridge of the crown. The incisal ridge of the crown is therefore on a line with the center of the root. This alignment is characteristic of maxillary central and lateral incisors. A straight line drawn through the center of the crown and root from the mesial or distal aspects will rarely if ever pass lingual to the incisal edge. Maxillary incisors are occasionally seen with the incisal ridges lingual to the bisecting line (see Figure 6-12, 1).
Labially and lingually, immediately coronal to the cervical line are the crests of curvature of these surfaces. These crests of contour give the crown its greatest labiolingual measurement.
Normally, the curvature labially and lingually is approximately 0.5 mm in extent (see Figure 6-4) before continuing the outlines to the incisal ridge.
The labial outline of the crown from the crest of curvature to the incisal ridge is very slightly convex. The lingual outline is convex at the point where it joins the crest of curvature at the cingulum; it then becomes concave at the mesial marginal ridge and slightly convex again at the lin-guoincisal ridge and the incisal edge.
The cervical line outlining the cementoenamel junction (CEJ) mesially on the maxillary central incisor curves incisally to a noticeable degree. This cervical curvature is greater on the mesial surface of this tooth than on any surface of any other tooth in the mouth. The curvature varies in extent, depending on the length of the crown and the measurement of the crown labiolingually. On an average central incisor of 10.5 to 11 mm in crown length, the curvature is 3 to 4 mm (see Figure 5-26).
The root of this tooth from the mesial aspect is cone-shaped, and the apex of the root is usually bluntly rounded.
Distal Aspect
Little difference is evident between the distal and mesial outlines of this tooth (see Figure 6-5). When looking at the central incisor from the distal aspect, it may be noted that the crown gives the impression of being somewhat thicker toward the incisal third. Because of the slope of the labial surface distolingually, more of that surface is seen from the distal aspect; this creates the illusion of greater thickness. Actually, most teeth are turned a little on their root bases to adapt to the dental arch curvature. The maxillary central incisor is no exception.
The curvature of the cervical line outlining the CEJ is less in extent on the distal than on the mesial surfaces. Most teeth show this characteristic.
Incisal Aspect
The specimen of this tooth is posed in the illustrations so that the incisal edge is centered over the root (see Figures 6-6 and 6-11). A view of the crown from this aspect superimposes it over the root entirely so that the latter is not visible.
From this aspect, the labial face of the crown is relatively broad and flat in comparison with the lingual surface, especially toward the incisal third. Nevertheless, the cervical portion of the crown labially is convex, although the arc described is broad.
The incisal ridge may be seen clearly, and a differentiation between the incisal edge and the remainder of the incisal ridge, with its slope toward the lingual, is easily distinguished.
The outline of the lingual portion tapers lingually toward the cingulum. The cingulum of the crown makes up the cervical portion of the lingual surface.
The mesiolabial and distolabial line angles are prominent from the incisal aspect. The relative positions of these line angles should be compared with the mesiolingual and dis-tolingual line angles, which are represented by the borders of the mesial and distal marginal ridges. The mesiodistal calibration of the crown at the labial line angles is greater than the same calibration at the lingual line angles.
The crown of this tooth shows more bulk from the incisal aspect than one would expect from viewing it from the mesial or distal aspect. Relatively broad surfaces are at the site of contact areas mesially and distally. Comparison should also be made between the dimensions of the crown labiolingually and mesiodistally. The labiolingual calibration of the crown is more than two thirds as great as the mesiodistal calibration. A cursory examination would not reveal this detail.
Bilaterally, the outline of the incisal aspect is rather uniform. The lingual portion shows some variation, however, in that a line drawn from the mesioincisal angle to the center of the cingulum lingually will be longer than one drawn from the same point on the cingulum to the distoincisal angle. The crown conforms to a triangular outline reflected by the outline of the root cross section at the cervix mentioned earlier.
Maxillary Lateral Incisor
Figures 6-13 through 6-21 illustrate the maxillary lateral incisor in various aspects. Because the maxillary lateral incisor supplements the central incisor in function, the crowns bear a close resemblance. The lateral incisor is smaller in all dimensions except root length (Table 6-2). Because it resembles the maxillary central incisor in form, direct comparisons are made with the central incisor in its description.
This tooth differs from the central incisor in its development, which may vary considerably. Maxillary lateral incisors vary in form more than any other tooth in the mouth except the third molar. If the variation is too great, it is considered a developmental anomaly. A common situation is to find maxillary lateral incisors with a nondescript, pointed form; such teeth are called peg-shaped laterals (see Figure 6-21, 7 and 8). In some individuals, the lateral incisors are missing entirely7; in these cases, the maxillary central incisor may be in contact distally with the canine. The presence of a palatogingival groove in maxillary incisors may be a predisposing factor in localized periodontal disease8 (see Figure 6-21, 3). This groove is also referred to as the pala-toradicular groove.9
FIGURE 6-13 Maxillary right lateral incisor, labial aspect. (Grid = 1 sq mm.)
FIGURE 6-14 Maxillary right lateral incisor, lingual aspect. (Grid = 1 sq mm.)
One type of malformed maxillary lateral incisor has a large, pointed tubercle as part of the cingulum; some have deep developmental grooves that extend down on the root lingually with a deep fold in the cingulum; and some show twisted roots, distorted crowns, and so on (see Figure 6-21).
