DESCRIPTION OF PERMANENT TEETH
To describe a tooth it advisable to start with the chronology of the tooth followed by its type and function, number of lobes, relation of the tooth, number of surfaces and roots, geometric outline and anatomy of each surface, root curvatures and anatomy of the pulp cavity. ■
- There are eight permanent incisors, four maxillary (upper) and four mandibular (lower). The maxillary consist of two centrals and two laterals, as do the mandibular.
- When viewed from the labial or the lingual aspect the crown of all incisors is trapezoidal in shape. The longer parallel side of the trapezoid is at the icisal edge and the shorter side is close to the cementoenamel junction. Whrn viewed from the proximal side the crown is triangular in shape with the base represented by the cervical portion.
- The labial and lingual crest of curvatures are at the cervical third of the crown.
- Another common feature of all newly erupted incisors is the presence of rounded portions on the incisal ridge called “Mamelons”. Each mamelon forms the incisal ridge of one of the labial primary lobes (each incisor has four primary lobes, three labial and one lingual). After normal use the mamelons wear down into a flat ridge, therefore the term “Incisal Edge” is more appropriate than ridge.
- The main function of incisors is to incise and cut food material during the process of mastication.
MAXILLARY CENTRAL INCISORS
This tooth has the functions of incising food material as well as esthetic. It has four lobes, three mamelons and a cingulum. The two central incisors make contact mesially with each other and distally with the mesial surface of the lateral incisor. It has four surfaces, labial, palatal, mesial and distal, and incisal aspect. Chronology of the upper central incisor is listed in table I.
The maxillary central incisor is the most widest anterior tooth. The geometric outline of the crown is trapezoid. The mesial outline of the crown is straight or slightly convex with the crest of the curvature at the contact area approaching the mesioincisal angle. The mesioincisal angle is relatively sharp.
The distal outline of the crown is more convex than the mesial outline wit the crest of curvature being higher toward the cervical line as the distal contact area approaching the middle third. The disto-incisal angle is round.
The incisal margin is generally straight and nearly perpendicular to the long axis of the tooth. In newly erupted tooth this margin is characterized by the presence of three mamelons. The cervical line is semicircular with the convexity of the root.
The labial surface is convex in all directions with the highest point of curvature (crest of curvature) located in the cervical third. The convexity tends to decrease to almost a flat surface from the middle one third to the incisal ridge. There are two faint but distinguished shallow grooves extending cervically from the incisal edge and fading out in the middle third. They are termed the mesio-labial and disto-labial developmental grooves and they separate the three mamelons.
The root of the upper central incisor is cone-shaped with blunt apex and regular outline mesially and distally. The root is usually 2-3 mm longer than the crown which is 10-11 mm long.
The crown dimension from this aspect is narrower mesio-distally than from the labial aspect since the mesial and distal surfaces converge toward the lingual surface i.e. lingual convergence.
The lingual outline is the reverse of that of the labial aspect. While the labial surface is smooth the lingual surface is irregular. The cervical line is similar to that on the labial surface. Immediately below it is a smooth large convexity called the “cingulum”, it represents the lingual lobe. The cingulum is located slightly toward the distal surface.
The mesial and distal margins take the form of linear ridges that extend from the incisal line angle to the cingulum. They are refered to as mesial and distal marginal ridges.
The lingual fossa is a shallow smooth concavity below the cingulum that involves the largest part of the middle and incisal part of the lingual surface. It is bordered proximally by the mesial and distal marginal ridges, incisally by the incisal edge and cervically by the cingulum. The lingual fossa may show few irregular lines. In some teeth poorly defined ridges extends from the cingulum to the lingual fossa. The lingual aspect of the root is convex, conical in shape and narrower than the labial aspect.
The mesial surface of the crown is triangular in shape, its base at the cervix and the apex at the incisal ridge. The incisal ridge of the crown is on a line that bisects the center of the root. This alignment is characteristic for maxillary central and lateral incisors.
The labial outline of the crown is convex cervically (the cervical ridge) then it becomes somewhat flattened or slightly curved toward the incisal ridge. The lingual outline, on the other hand, is convex at the cingulum, then becomes concave at the lingual fossa and it become slightly convex again at the linguo-incisal ridge. The mesial surface is convex with the maximum convexity at the junction of the incisal and middle thirds (the contact area). The cervical curvature is greater on the mesial surface than any other tooth in the mouth. For the average crown length the curvature is 3-4 mm.
The root from the mesial aspect is cone-shaped with blunt apex.
There is a little difference between the distal and mesial surfaces. The curvature of the cervical line is less on the distal surface (about 1 mm less). This is a characteristic for most teeth. The distal surface shows maximum convexity located at the center of the middle third (the contact area).
The crown shows a triangular shape with its apex at the lingual surface and the base placed labially. The incisal ridge can be seen clearly slopping lingually. The labial surface of the crown from this aspect is broad and flat in comparison with the lingual surface, especially toward the incisal third. Labially the cervical portion of the crown is convex (the cervical ridge).
The lingual outline tapers lingually toward the cingulum. The cingulum is shifted distally; accordingly a line drawn from the mesio-incisal angle to center of the cingulum is longer than a line drawn from the disto-incisal angle to the center of the cingulum. A view of the crown from this aspect superimposes it over the root entirely, so that the later is not visible.
The Pulp Cavity
The pulp cavity has pulp chamber and one root canal which varies in size with the age of the tooth. When the tooth first erupts, it is very large and the root is incompletely formed, so the canal looks somewhat funnel shaped in the region of the apical foramen. Later as the tooth develops completely, the pulp becomes smaller and the apical foramen is then very small. This process is continuous throughout life.
On a Labio-lingual section the pulp chamber point incisally then follows the increase in the crown dimension cervically. Starting from the cervical level of the crown the root canal tapers gradually as it traverses the root to end in a constriction at the apex which is termed “the apical foramen”
On a mesio-distal section the pulp chamber is wider from this view, confirming the shape of the crown. It tapers from the incisal part, but is further wide at the cervix. It represents three pulp horns corresponding to the three mamelons. The root canal tapers toward the apex. On cervical cross section the pulp chamber is roughly triangular in young teeth and becomes rounded or crescent shaped in old teeth. It is perfectly centered.
MAXILLARY LATERAL INCISORS
The maxillary lateral incisors complement the central incisor in function. It is smaller in all dimensions than the central except for the root length. The crown is about 2 mm narrower mesio-distally and 2 mm shorter and 1 mm less labio-lingually than the central incisor. Chronology of the upper central incisor is listed in table I.
The lateral incisor may show malformation more than any other tooth in the mouth except the third molar. It may show a large pointed tubercle as a part of the cingulum, or deep developmental grooves which extends which extends down with on the root lingually with deep fold in the cingulum. Other laterals may show twisted roots of distorted crowns. Not uncommon situation is to find maxillary lateral incisors that have pointed form. Such teeth are called “peg-shaped” lateral incisor. In some individuals the lateral incisors are missing.
The labial surface of the crown is more convex than that of the central incisor. It shows rounded incisal ridge and rounded incisal angles both mesially and distally. Like the central incisor, labial developmental grooves are present.
The mesial outline of the crown resembles that of the central incisor, but usually shows more curvature with the crest of curvature at the contact areas at the junction of the middle and incisal thirds. The distal outline is more convex and the crest of curvature is more cervically, usually at the center of the middle third. The disto-incisal angle is more rounded than the mesio-incisal angle. The incisal outline is more curved than that of the central because of the more rounded incisal angles. The cervical line curves apically with about the same depth of curvature as the case with the central incisor.
The root is about one and half the length of the crown. It tapers evenly from the cervical line to a point approximately two thirds of its length then it usually curves sharply in a distal direction and end in a pointed apex.
The outline is the reverse of the labial aspect. Mesial and distal marginal ridges and cingulum is usually prominent, with tendency toward deep developmental grooves within the lingual fossa, where as it joins the cingulum. It is common to find a deep developmental groove at the distal side of the cingulum, which extends up to the root for part or all of its length. The lingual fossa is more concave than that of the central.
The crown outline is triangular. It is similar to, but smaller than, that of the central incisor. The root appears longer and the labio-lingual measurement of the crown and the root is about one millimeter less than that of the central. The contact area is located near the junction of the incisal and middle thirds. It is slightly wider, larger and closer to the incisal edge than the distal contact area. The cervical line is less curved than that of the central incisor.
A line drawn through the central of the root tends to bisect the incisal ridge of the crown. The root is conical in shape and shows shallow depression.
The distal surface is smaller, but more convex in all dimensions than the mesial surface. The distal marginal ridge is irregular. The contact area is narrower and more cervically placed than the mesial one. The cervical line shows less curvature than the mesial surface.
It usually resembles that of the central incisor, but rarely it may resemble that of a small canine. The cingulum and incisal edge or ridge may be large. The labio-lingual dimension may be greater than usual in comparison with that of the central incisor, resembling that of a canine. All maxillary lateral incisors show more convexity labially and lingually from the incisal aspect than the maxillary central incisors and the cingulum is centered.
It consists of pulp chamber and a root canal. The chamber is quite similar to that of the central incisor, but without the three sharp pulp horns. More often the pulp chamber ends incisally as one round horn or two less sharp plup.
MANDIBULAR CENTRAL INCISORS
The mandibular incisors are four in number and have smaller mesio-distal dimensions than any other tooth. The contact areas are near the incisal edge both distally and mesially. The labial surfaces are inclined lingually so that the incisal ridges are lingual to a line bisecting the root.
The mandibular central incisor is the smallest in the dental arch. The crown has little more than half of the mesio-distal dimension of the maxillary central incisor; however, the labio-lingual diameter is only about one millimeter less than that of the maxillary central incisor. The crown is shorter than that of the maxillary central by about 1.5 mm.
It is trapezoidal in outline with smallest side at the cervix. The mesial and distal outline of the crown make a straight drop downward from the incisal angles to the contact areas which are close to the incisal edge. The mesial and distal sides then taper evenly from the contact area to the narrow cervix. The mesio-incisal and disto-incisal angles are sharp. The incisal margin is straight and at right angle to the long axis of the tooth.
The labial surface is convex both mesio-distally and inciso-cervically with definite convexity in the cervical one third where he height of contour is located (cervical ridge) and a flattened surface at the incisal third. Mesio-labially and disto-labially developmental grooves are very faint if present. The cervical line is symmetrically curved towardthe root with distal diviation.
The mesial and distal root outlines are straight and are continuous with the mesial and distal outlines of the crown. They slop down to the apical portion and terminate in small pointed taper in most cases curving distally.
The outline of the crown is the reverse of the labial surface, and is narrower. It presents a cingulum much smaller than that of the maxillary anteriors. The lingual fossa is shallow and the mesial and distal marginal ridges are less prominent. The cingulum is placed more cervically and is centered. It is smooth withno accessory ridges, grooves or pits. The root is slightly narrower than labially.
Labial outline of the crown is straight above the cervical curvature, sloping rapidly from the crest of curvature to the incisal edge. The lingual outline shows smooth convexity at the cingulum then it becomes straight line inclined labially for a short distance to join a concave line at the middle third of the crown. This extends upward to join the rounded outline of a narrow incisal edge. The curvatures above the cervical line labially and lingually are less than the maxillary incisor.
The incisal margin is straight of slightly rounded, and its center is located just lingual to the center of the root. The contact area is very close to the incisal edge. The cervical line shows a marked curvature incisally about one third the length of the crown.
The root outlines labially and lingually are straight with the crown outline from the cervical line. The root start to taper in the middle third to either a bluntly rounded or pointed root end. The mesial surface of the root is flat just below the cervical line. Most of these roots have a broad developmental depression for the most of the root length which is deeper at the junction of the middle and apical thirds. In rare cases the root apex is bifid.
It is the reverse of the mesial aspect. The cervical line curves incisally about 1 mm less than on the mesial. The developmental depression is more marked with well defined developmental groove at its center.
Form this view, the tooth is four sided or diamond shaped. The incisal edge is straight and the mesial and distal halves are identical. The cingulum is slightly shifted towards the distal portion The crown appeared centered over the root. The incisal ridge is perpendicular to a line bisecting the crown labiolingually.
In labio-lingual section the outline of the pulp cavity conform to the crown and root outline. The mesiodistal section is narrow and has two pulp horns directed to the mesial and distal angles of the incisal edge. Crow section of the root at the cervical line shows an oval canal usually constricted nesiodistally and wide labiolingually.
MANDIBULAR LATERAL INCISORS
The mandibular lateral incisor has almost the same form as the mandibular central incisor, however, some variations exist. Table IV list the chronological data of mandibular lateral incisor.
The tooth resembles the central incisor except that it is slightly larger by o.5 mm in all directions and is fan shaped. The mesial side is often longer than the distal side, causing distal sloping of the incisal edge. The distoincisal angle is more rounded than its counterpart in the mandibular central.
The distal contact area is more toward the cervical line than the mesial contact area to contact properly with the canine. The crown is larger than that of the central and the root is longer by about 1.5 mm.
Similar to that of the central incisor but the mesial outline and the mesial marginal ridge are longer than the distal.
The cingulum is deviated distal to the center of the lingual surface.
Proximal Aspects (Mesial and Distal)
Differ from the center counterpart in the following:
- The distal surface of the lateral incisor is shorter than the mesial surface.
- Both cervical line curvatures are slightly less than that of the central incisor.
- The distal contact area is more cervically located than the mesial one.
- Root depressions are seen on both the mesial and distal surfaces.
The mandibular lateral incisor appears to be rotated over their root axes because the distal developmental lobe is larger and more mesially located than the distal lobe. This is because the tooth has to curve distally to fit into mandibular arch because it has to fit inside the maxillary arch. The incisal edge follows the curvature of the mandibular dental arch. The cingulum is shifted distally and the incisal ridge follows the curvature of the dental arch.■