Human incisors have thin, blade-like crowns which are adapted for the cutting and shearing of food . There are two incisors per quadrant, four per arch. The first incisor, the central incisor, is next to the midline. The second incisor, the lateral incisor, is distal to it.
Maxillary incisors by definition arise in the premaxilla (which is merged into the maxilla in humans); mandibular incisors are the teeth that articulate with them.
Maxillary Central Incisor
Facial: It is the most prominent tooth in the mouth. It has a nearly straight incisal edge and a gracefully curved cervical line. The mesial presents a straight outline; the distal aspect is more rounded. Mamelons are present on freshly erupted, unworn central incisors.
Lingual: The lingual aspect presents a distinctive lingual fossa that is bordered by mesial and distal marginal ridges, the incisal edge, and the prominent cingulum at the gingival.
Proximal: Mesial and distal aspects present a distinctive triangular outline. This is true for all of the incisors. The incisal ridge of the crown is aligned on the long axis of the tooth along with the apex of the tooth.
Incisal: The crown is roughly triangular in outline; the incisal edge is nearly a straight line, though slightly crescent shaped.
Contact Points: The mesial contact point is just about at the incisal, owing to the very sharp mesial incisal angle. The distal contact point is located at the junction of the incisal third and the middle third.
Right and Left: Viewed from the labial, the distal incisal angle is more rounded that the mesial. In many specimens, a cross-section mid-root reveals a right triangle outline. The hypotenuse is toward the mesial.
Variation: The maxillary central incisor usually develops normally. Variations include a short crown or, on occasion, and unusually long crown. This tooth is rarely absent. The Hutchinson incisor is a malformation due to congenital syphilis in utero.
An important non-metric variation of the upper incisors is the shovel shaped incisor trait. It presents with large, robust marginal ridges and a deep lingual fossa. This feature is significant in Chinese, Eskimo-Aleuts, and North American Indians. It is an important clue to population movements, especially those peoples who moved into the Americas from Siberia since the end of the Ice Age.
Maxillary Lateral Incisor
Facial: The maxillary lateral incisor resembles the central incisor, but is narrower mesio-distally. The mesial outline resembles the adjacent central incisor; the distal outline--and particularly the distal incisal angle is more rounded than the mesial incisal angle (which resembles that of the adjacent central incisor. The distal incisal angle resembling the mesial of the adjacent canine.
Lingual: On the lingual surface, the marginal ridges are usually prominent and terminate into a prominent cingulum. There is often a deep pit where the marginal ridges converge gingivally. A developmental groove often extends across the distal of the cingulum onto the root continuing for part or all of its length.
Proximal: In proximal view, the maxillary lateral incisor resembles the central except that the root appears longer--about 1 1/2 times longer than the crown. A line through the long axis of the tooth bisects the crown.
Incisal: In incisal view, this tooth can resemble either the central or the canine to varying degrees. The tooth is narrower mesiodistally than the upper central incisor; however, it is nearly as thick labiolingually.
Contact Points: The mesial contact is at the junction of the incisal third and the middle third. The distal contact is is located at the center of the middle third of the distal surface.
Right and left: The distoincisal angle is more rounded than the mesial incisal angle. The tip of the root may incline distally, but this is not a consistent finding.
Variation: This tooth is quite variable. Often the tooth is narrow, conical, and peg-shaped. It is absent either singly or bilaterally in 1-2% of individuals. Only the lower second premolar is more frequently missing.
The lingual pit when present can be very deep and is prone to early caries in many individuals.
Mandibular Central Incisor
Facial: The mandibular central incisor is the smallest tooth in the dental arch. It is a long, narrow, symmetrical tooth. The incisal edge is straight. Mesial and distal outlines descend apically from the sharp mesial and distal incisal angles.
Lingual: The lingual surface has no definate marginal ridges. The surface is concave and the cingulum is minimal in size.
Proximal: Both mesial land distal surfaces present a triangular outline.
Incisal: The incisal edge is at right angles to a line passing labiolingually through the tooth reflecting its bilateral symmetry.
Right and Left: The symmetry of this tooth makes a judgement on right and left unreliable.
Variation: This tooth is consistent in development and is is rarely absent. The upper incisor region is a common site for supernumerary teeth which may occasionally occur in the midline; such a variant is called a mesodens.
Mandibular Lateral Incisor
Facial: This tooth resembles the central incisor, but is somewhat larger in most proportions. It is a more rounded tooth; this is especially evident in the distal incisal angle in unworn speciments. There is a lack of the bilateral symmetry seen in the central.
Lingual: Except for the lack of symmetry, this tooth resemble the central.
Proximal: Like the central, the crown presents a triangular outline. When viewed critically, the rotation of the incisal edge can be seen.
Incisal: The incisal edge 'twisted' from the 90 degree angle with a line passing labiolingually through the tooth.
Right and Left: Two significant features assist in identification, even in a worn tooth. The incisal edge is 'twisted' relative to a line passing from the labial to the lingual anticipating the curvature of the dental arch. Also, the cingulum will be shifted toward the side from whence the tooth has come.
Variation: This tooth is stable, but variations in root length and direction are occasionally seen.
Comparative Anatomy: Incisors of Interest in other Species
In the animal kingdom, incisors have a variety of functions, such as fur combing in lemurs, tree chopping in certain rodents, and meat stripping in the carnivores.
1. Rodent incisors are deeply implanted in the jaws, are curved, and continue to grow throughout life. The enamel is limited to the labial surface. In life, the diastem between the incisors and molars is filled by the cheeks and tong, effectively creating separate incising and grinding chambers. There is a sphincter muscle to facilitate this action.
The lagomorphs, the rabbits and hares, have a very small second incisor immediately behind the main one on each side.
2. Lemurs have comb-like lower incisors adapted to grooming--a paramasticatory adaptation.
3. The Aye Aye, unlike humans and the majority of living primates have two upper and two lower incisors in each quadrant of the jaw. A curious adaptation in the Aye Aye of Madagascar is the acquisition of continuously growing rodent-like incisors.
4. Sheep have no upper anterior teeth at all. There is instead a horny pad which meets the six lower incisors (and two canines) to 'crop' the grass on which the animal feeds.
The ruminants, such as cows, crop and swallow. Later they regurgitate the contents of the caecum to chew their cud. A large volume of saliva buffers and protects the teeth.
5. The most extreme adaptation is the Monodon, the narwhale. It is a rare example of a unilateral tooth. In the adult male, there is a large left incisor which reaches a length of nine to twelve feet and is grooved in a left handed spiral. The one to the right is vestigial. Rare males have two tusks; the incisors tusks of females calcify, but do not erupt and thus remain edentulous. It has no other teeth besides the tusks. The narwhale has another distinction: it is the most extreme example of dental sexual dimorphism in mammals.
6. The largest incisors are the tusks of elephants and their extinct relatives, the mammoth and mastodon which died out at the end of the last Ice Age. The tusk incisors grow from persistent pulps and continue to grow throughout life. They have an enamel tip upon eruption; however, cement covers the remainder of the crown part of the tusk and extends onto the root. The adult tusks have deciduous predecessors.
7. Horses have three incisors per quadrant. Their unusual design and wear throught life permits their use in judging the age of an animal. When the teeth first erupt they have a central pit. With wear the enamel becomes worn off the area around the pit so that the enamel bordering the central (the mark) becomes isolated from the enamel of the crown surface by exposed dentin. With further wear the bottom of the pit (the mark) is reached and passed, so that in old animals the central part of the occlusal surface is made up of secondary dentin filling in the roof of the pulp cavity.