The Permanent Mandibular Incisors

A. Introduction:

The mandibular incisors are the simplest and least variable teeth in the mouth. They are also the smallest permanent teeth. The central is slightly smaller than the lateral, whereas in the maxillary incisors the central is considerably larger. The mandibular incisors resemble each other to an even greater extent than do the maxillary incisors. Compared to the maxillary incisors, they reveal crowns which are relatively longer incisocervically, and markedly narrower mesiodistally.

B. Permanent Mandibular Central Incisor: 1. General characteristics:

a. Arch position - The mandibular central incisors occupy the position adjacent to the midline in each mandibular quadrant. They share a mesial contact area with each other, while the distal contact is with the permanent lateral incisor.

b. Universal number:

Mandibular right central incisor - #25 Mandibular left central incisor - #24

c. General form and function - The mandibular central incisor normally has the narrowest mesiodistal dimension and the smallest crown size of any permanent tooth. The crown is also quite symmetrical, with mesial and distal halves nearly identical. These teeth function in biting, cutting, incising, and shearing, just as do their maxillary counterparts.

2. Development Table: (Mandibular central incisor)*

Initiation of calcification 3 to 4 months

Completion of enamel 4 to 5 years

Eruption 6 to 7 years

Completion of root 9 years

3. Labial aspect:

a. Mesial outline - The mesial margin normally tapers evenly toward the gingival in a nearly straight line. The mesioincisal angle is quite sharp, normally more so than any of the incisal angles of maxillary incisors. The height of contour is associated with the contact area in the incisal third, very close to the incisal margin.

b. Distal outline - Distally, the outline is-Straight and almost exactly like the mesial outline, with a similarly sharp distoincisal angle. The height of contour is also in the incisal third.

c. Incisal outline - When present, mamelons most always number three. After incisal wear has obliterated the mamelons, the incisal outline is straight, and at right angles to the long axis of the tooth.

d. Cervical outline - The cervical line is symmetrically curved toward the root.

e. Other considerations - The labial surface is generally convex both mesiodistally and incisogingivally, but not to the extent of the maxillary incisors, especially the maxillary lateral. However, like the maxillary incisors, the convexities are much greater in the cervical third. In fact, in some specimens the labial surface may be quite flat incisal to the height of contour. The surface outline is roughly trapezoidal, which in some cases approaches a rectangular shape.

Developmental depressions and imbrication lines are not normally present. Occasionally, there are very faint depressions which only occur near the incisal margin of the labial surface. The height of contour is in the cervical third.

4. Lingual aspect:

a. Mesial, distal and incisal outlines - All three of these margins closely resemble those of the labial aspect.

b. Cervical outline - The CEJ curves evenly toward the root, but is located farther from the incisal edge than it is on the labial surface.

c. Other considerations - The lingual surface is relatively smooth, and its structures are generally less prominent than those of the maxillary incisors. There is usually a slight concavity, or lingual fossa, bordered by indistinct marginal ridges on the mesial and distal. There are normally no grooves, fissures, or pits on the lingual surface.

A cingulum is normally present, although it is not so prominent as in the maxillary incisors.

The height of contour is located in the cervical third of the surface, associated with the greatest convexity of the cingulum.

5. Mesial aspect:

a. Labial outline - The labial margin slopes in a straight to slightly convex line from the incisal ridge to the crest of curvature, and is then convex in the remainder of the gingival third.

b. Lingual outline - The lingual outline is concave in the incisal two-thirds and convex in the cingulum area, or gingival third.

c. Incisal outline - The incisal edge is normally straight, but can be slightly rounded, and is located lingual to the center of the root. The profile of the incisal edge has an inclination toward the labial, which is opposite to the lingual slope of the maxillary incisors. This is due to the wear pattern between the upper and lower incisors.

d. Cervical outline - There is a marked, even curvature incisally of the cervical margin.

e. Other considerations - The mesial surface is roughly triangular, or wedge shaped, like all other anterior teeth. Unlike the maxillary incisors, the crown appears to be slightly offset toward the lingual.

The contact area is located about half way from labial to lingual, and in the incisal third, very close to the incisal edge. It has an ovoid shape, which is long incisogingivally and narrow labiolingually.

The height of contour, at the contact area, is in the incisal third.

6. Distal aspect:

a. The distal surface is similar in all respects to the mesial, except that the cervical margin curves slightly less toward the incisal. Even the contact area has a similar location, a fact which is unique among incisors.

7. Incisal aspect:

a. The most notable features from the incisal aspect are the symmetry of the mesial and distal portions, and the straight incisal edge. Unlike the maxillary central, this tooth is roughly four sided, or diamond-shaped, from this aspect, and the tooth is normally wider labiolingually than mesiodistally.

b. Because the crown is offset toward the lingual, more of the labial surface than the lingual surface is visible from this aspect.

c. Even though the central incisor is described as symmetrical from the incisal aspect, careful scrutiny will reveal that the cingulum is very slightly offset toward the distal, an important feature when attempting to distinguish right from left mandibular central incisors.

a. The root is normally single and straight.

b. From the labial or lingual aspects, the root is generally symmetrical, and tapers gradually to a relatively sharp apex.

c. From the mesial or distal aspects, the root is much wider, and it is slightly convex cervicoapically on both labial and lingual margins. The central portion of the mesial and distal surfaces is usually flattened, or concave. When concave, the surface is said to have a root concavity which is also known as a longitudinal groove. Root concavities are found on the roots of other teeth, and usually extend the majority of the root length, but vary in both length and depth.

d. In cross section at the neck, the outline is roughly a rectangle with rounded corners, but it is slightly wider at the labial than at the lingual. When there are root concavities present, they are reflected as concavities in the mesial and distal outlines. The midroot cross section is similar to the cervical section, only more ovoid.

9. Variations and Anomalies:

a. There is great variability in the lingual inclination of the labial surface of mandibular central incisor specimens.

b. Anomalies are very rare. Occasionally a bifurcated root is found, which means there are two branches, which in mandibular incisors, have labial and lingual locations.

C. Permanent Mandibular Lateral Incisor: 1. General characteristics:

a. Arch position - The mandibular lateral incisor is the second tooth from the midline in each lower quadrant, and it shares a mesial contact area with the central incisor. The distal contact is with the deciduous mandibular canine until that tooth's exfoliation and then contact h shared with the permanent canine.

b. Universal number:

Mandibular right lateral incisor - #26 Mandibular left lateral incisor - #23

c. General form and function - The mandibular lateral incisor is slightly larger in all respects than the mandibular central incisor, but otherwise parallels it very closely in form. It also complements the central in function. Development Table: (Mandibular lateral incisor)*

Initiation of calcification 3 to 4 months

Completion of enamel 4 to 5 years

Eruption 7 to 8 years

Completion of root 10 years

The mandibular lateral incisor so closely resembles the central incisor that a detailed description is unnecessary. Consequently, only the following comparisons need to be made.

a. Labial aspect - The incisal margin may slope slightly gingivally toward the distal, which results in a distoincisal angle that is more rounded than the same angle of the central incisor. This feature creates a slightly shorter distal margin, when compared to the mesial outline. The contact area on the distal is more cervically located than on the mesial, thus creating a more cervically located height of contour on the distal outline. Both heights of contour are still in the incisal third, however.

b. Lingual aspect - The lingual outlines are similar to those of the labial aspect. The structures of the lingual surface are similar to their counter-pans on the central incisor, except the cingulum is more offset to the distal, and as a result, the curvature of the cervical line is also offset distally.

c. Mesial and distal aspects - These two surfaces are similar to their counterparts on the central incisor, with a few minor exceptions. The lateral's distal surface is slightly shorter incisocervically than the mesial surface. Both cervical line curvatures are slightly less than their counterparts in the central, and as would be expected, the mesial cervical line shows greater incisal curvature than does the distal. The distal contact area, and hence the height of contour, is more cervically located than on the mesial. Although still in the incisal third, the distal contact area is very near the junction of the incisal and middle thirds, and is the most cervically located of any mandibular incisor contact.

d. Incisal aspect - From this view, the incisal edge is not straight mesiodist-ally, as it is in the central; rather it curves toward the lingual in its distal portion. Furthermore, the lingual contour (cingulum) appears displaced toward the distal. These factors give the crown the appearance of being slightly twisted on its root. These are the best identifying features, when differentiating this tooth from the central incisor.

e. Root - Root length is normally a little greater than in the central incisor. The root is also slightly thicker and wider. Root concavities may be found on the mesial and distal root surfaces, and if present, the concavity in the distal is usually more pronounced.

Variations and Anomalies:

a. Anomalies are rare, but occasionally a bifurcated root is found.

Maxillary Right Central tncitor
Maxillary Right Lateral Incisor
Maxillary Right Lateral Incisor

Mandibular Right Control Incisor

MESIAL WSTAL

Mandibular Right Lateral Incisor

INCISAL

INCISAL

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