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.
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.
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.
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.