Incisors: Structure and function
The incisors are chisel shaped teeth located anteriorly within the oral cavity. Their name originates from the Latin word ‘incidere’, which means ‘to cut’. Hence, their main function is to cut food during mastication.
There are four corresponding maxillary and four mandibular incisors, yielding a total of eight. They are numbered 7 through 10 and 23 through 26 respectively, according to the universal dental numbering system. The incisors on each dental arcade are further divided into central and lateral ones, depending on their location relative to the midline. The deciduous incisors are amongst the first teeth to erupt in an infant, appearing anywhere between 6 and 16 months of age.
|Types||Maxillary central and lateral
Mandibular central and lateral
|Numbers||8 in total - 4 maxillary and 4 mandibular|
|Surfaces||Labial, lingual, mesial, distal, incisal|
|Functions||Cutting food, talking, supporting lips|
While the eight incisors share a common, general structure, various nuances differentiate them. This depends if they are central or lateral and mandibular or maxillary. This article will examine these specific variations, including the detailed structure of the incisor teeth.
All eight incisors share a general tooth anatomy, consisting of a crown that is continued by the root. The crown is covered by enamel, while the root is covered by cementum. The body of the tooth consists almost entirely of dentine which surrounds a central pulp cavity. This cavity expands into a pulp chamber, which narrows at the end into the root canal.
A healthy, adult oral cavity contains thirty-two teeth in total, sixteen on each dental arcade. There are eight incisors in total located on either side of the midline. Half of them are located on the maxilla (maxillary incisors), while the remaining four are situated on the mandible (mandibular incisors). According to the universal dental numbering system, they are numbered 7 through 10 and 23 through 26, respectively. The incisors on each dental arch can be further classified according to their relationship to the midline. The ones closest to it are called central incisors, while the ones located more laterally are the lateral incisors.
Each incisor has five surfaces, each one named according to the anatomical structure that it faces:
- Labial surface - faces the lips.
- Lingual surface - faces of the mandibular incisors face the tongue. The corresponding maxillary incisors have a palatal surface instead of a lingual one that faces the hard palate of the oral cavity.
- Mesial surface - medial surfaces of the incisors. This surface is located closer to the front of the mouth and midline.
- Distal surface - the surface located lateral or distal to the midline and closer to the back of the mouth. The mesial and distal surfaces of two adjacent incisors create a contact zone. The exceptions are the central incisors, where the contact zone is formed by their two mesial surfaces.
- Biting or incisal surface - the portion of the crown furthest away from the apex. This portion of incisors is sharp and straight to facilitate cutting food. The junction of the incisal and labial surfaces is called the incisal edge.
If you want to master the general anatomy and types of teeth before diving into further details, take a look below.
Maxillary central incisor
After seeing the general characteristics of incisors, let’s learn about the subtle differences between the sub-groups. As you know, the two maxillary central incisors are part of the maxillary dental arcade, either side of the midline.
They are the most prominent teeth, having a rectangular or square shape. Each maxillary central incisors measures approximately 22.5 mm in length, half of which (10-11 mm) represents the crown.
Maxillary central incisors have the largest mesiodistal distance out of all members of this group, but the least convex labial surface. However, the crown surface of the labial face is smooth. The mesial and distal crests of curvature on this face provide the contact points between neighboring incisors. The crown portion located mesially on the labial face has a small degree of convexity. In turn, the portion located distally is a lot more convex than the mesial part. The mesial and distal portions of the crown contribute to the formation of the mesioincisal and distoincisal angles, respectively.
The incisal portion of the crown may have protuberances (mamelons), but they are usually obliterated in adults. Therefore, this portion is usually straight and regular. The cervical portion of the crown is semicircular, following the curvature of the root. The root appears conical when viewed from the labial surface, with a blunt apex. The mesial and distal outlines of the root are regular.
In contrast to the smooth labial surface, the lingual surface is full of convexities and concavities. In addition, a convexity (cingulum) is located below the cervical portion of the crown. Marginal ridges extend mesially and distally away from the cingulum. The shallow lingual fossa is located between the marginal ridges and below the cingulum. Apart from that, the remaining crown lines are identical to the ones on the labial surface, discussed previously.
The mesial aspect of the maxillary central incisors is triangular. The base is located at the cervix and the apex at the incisal ridge. A characteristic feature of these specific incisors is that the incisal ridge of the crown and the center of the tooth are perfectly aligned.
The crests of curvature of the mesial surface are coronal to the cervical line of the crown. They are located on the labial and lingual portions of the mesial aspect. After curving for 0.5 mm, the crests of curvature continue as the labial and lingual outlines. The former is slightly convex. However, the lingual outline is convex above the intersection point with the cingulum, then becomes concave, and ultimately finishes convex again close to the incisal edge. The cervical line of the mesial surface has the greatest curvature out of all surfaces and teeth in the oral cavity. The line points incisally. From the perspective of the mesial surface, the root appears cone shaped with a bluntly rounded apex.
The crown outlines present on the distal surface are almost identical to the ones on the mesial surface. The main difference is the curvature of the cervical line, which is less on the distal surface. In addition, the maxillary central incisors appear bigger when viewed from this perspective because a greater portion of the labial surface is visible from this angle.
The crown of the maxillary central incisors appear bulkier when viewed from the incisal surface. The mesial and distal contact areas are marked by broad surfaces. The triangular outline of the incisal surface is quite uniform, except at the lingual portion which exhibits some irregularities.
The labial portion of the crown on the incisal surface is flat and broad in comparison to the lingual portion. The cervical portion is broad and convex. The incisal edge and the remaining portion of the incisal ridge appear clearly differentiated and distinguishable. The lingual portion of the crown tapers off in a lingual direction towards the cingulum. The latter represents the cervical portion of the lingual surface. The mesiolabial and distolabial angles are markedly visible from the incisal surface.
The pulp chamber of the maxillary central incisors is located equidistant from the walls, within the centre of the crown. The chamber is ovoid mesiodistally and narrow buccopalatally, forming the root canal.
These incisor types have only one root and one straight, root canal. The root canal is wide buccopalatally and round apically. It has a constriction apical to the cervix.
Maxillary lateral incisor
Now that we’ve finished analyzing the maxillary central incisors, let’s take one step laterally on the same maxillary dental arch and learn about the maxillary lateral incisors.
They have a very similar structure to their group neighbors, but are smaller. To be exact, the maxillary lateral incisors measure 21 mm in length. In addition, the proportion of root to crown lengths is greater in the maxillary lateral incisors. They have the second most varied structure in the entire oral cavity, after the third molar.
The labial surface is more curved compared to the maxillary central incisors. However, they have similar proportions. The incisal ridge and angles are rounded on the mesial and distal portions of the labial surface of the crown. The mesial outline has a rounded mesioincisial angle and a crest. The distal outline is round, having a cervical crest of contour in the center of the middle third. The labial portion of the crown has a greater convexity compared to the maxillary central incisors.
The root of these teeth tapers off apically from the cervical line. Two thirds down its length, the root curves distally, becoming pointed at its apex.
The lingual aspect of the crown contains prominent distal and mesial marginal ridges. In addition, the cingulum is also markedly developed and joins the lingual fossa. The latter exhibits several deep developmental grooves and it is more concave and circumscribed than its counterpart on the maxillary central incisors. The cingulum can also contain developmental grooves, especially on its distal side. They can extend up to the entire root.
The mesial surfaces of the maxillary lateral incisors share similar features to their central neighbors, with some exceptions. They have slightly shorter crowns and labiolingual distances, but longer roots. The cervical line of the mesial surface curves towards the incisal ridge, but to a lesser extent than the one on the maxillary central incisors. The incisal ridge is also thicker. If you take a look at the root from the mesial surface, it resembles a cone with a rounded apex. Its labial outline also appears straight.
From the distal surface, the crown appears wide compared to the other surfaces. It also contains a developmental groove distally, which projects onto the root.
The incisal surface of the maxillary lateral incisors is almost identical to the corresponding surface on the maxillary central incisors. However, their labial and lingual convexities are more pronounced, hence it is not as straight or uniform. In addition, the incisal surface of these teeth can resemble small canines.
The pulp chamber of the maxillary lateral incisors is rounder than that of the maxillary lateral ones. In addition, they also have two pulp horns.
These incisors also have only one root canal. Its shape is also straight, but has a smaller diameter. It starts wider in the labiopalatal region and constricts apically to the cervix. The apical region has a palatally oriented curvature.
Mandibular central incisor
As you are now an expert in maxillary incisors, the mandibular ones should be a piece of cake. While they are far from being identical, they are pretty similar in terms of structure.
Mandibular incisors are part of the mandibular dental arch and correspond to the maxillary ones. Upon normal occlusion of the mouth, the incisal surfaces of the four pairs of incisors end up parallel to each other. The mandibular central incisors are located on either side of the midline of the mandible. They are the smallest teeth out of all of them, averaging approximately 21 mm in length.
The labial surface is regularly shaped and smooth. As it travels towards the root, it flattens and then becomes convex. The labial surface contains sharp distal and mesial incisal angles which taper off into the apical part of the root.
The incisal ridge travels straight and perpendicular to the long axis of the tooth. The distal and mesial portions of the crown connect the incisal angles to the contact areas of the teeth. Then, they taper off below the contact areas until the cervix. They continue straight until the apical portion of the root, where they usually curve distally. The labial surface of the root is convex and regular.
The smooth lingual surface of the crown features a concavity at the incisal third between the marginal ridges. As the surface progresses towards the cervical third, it first becomes flat, then convex. The maxillary central incisors is almost devoid of developmental lines and grooves.
The mesial surface of the crown is quite varied, ranging from convex and smooth to broad and flat cervical to the contact area. It becomes concave up until a point above the cervical line. When looking at the mesial surface, the outline of the labial face starts off straight, before sloping between the crest of the curvature and incisal ridge. The lingual outline is partially inclined labially above the cingulum, continuing straight further down the crown. The shape of the incisal ridge is round and its center is located lingually with regards to the centre of the tooth. The cervical line shows a distinguishable curvature.
From the perspective of the mesial surface, the root appears straight, flat and with a uniform diameter. However, the root tapers off close to the apical third, becoming round or pointed at the end. The mesial surface of the root has developmental depressions along its length
The distal surface has a cervical line that curves in an incisal direction. On the root, there is a developmental depression with a deeply marked central groove. Apart from that, the distal surface is very similar to the incisal surface described below.
The two mesial halves of the crown are identical, meaning that this entire surface of the mandibular central incisor is symmetrical. The incisal edge runs perpendicular to the labiolingual axis. The latter has greater dimensions compared to the mesiodistal axis. In the cervical third of the crown, the labial portion is wide. The lingual area of the crown contains the cingulum. In contrast, the labial surface of the incisal third is convex, while the lingual surface of the same region is concave.
The pulp chamber of the mandibular lateral incisor points incisally, has a wide labiolingual axis and three pulp horns. The pulp chamber appears ovoid in cross section.
These teeth can have either one or two root canals. In cross section, they appear ovoid labiolingually and round in the apical third. In addition, the canal narrows as one moves buccopalatally.
Mandibular lateral incisor
Last but not least, let’s take a look at the mandibular lateral incisors. They are located on the same mandibular dental arch but more laterally. They closely resemble the mandibular central incisors and perform their function as a team, hence only the differences will be pointed out. The mandibular lateral incisors measure approximately 21 mm in length.
Labial and lingual surfaces
These two crown surfaces have a greater mesiodistal diameter in the distal half, by about 1 mm, compared to the mandibular central incisors.
Incisal surface and internal structure
The mandibular lateral incisors contain an identifiable feature on their incisal surfaces. In contrast to their central counterparts, the incisal edge of these incisors is straight, following the trajectory of the mandibular dental arch. Apart from these, their incisal surfaces are almost identical.
The pulp chambers of the mandibular lateral incisors are identical to the mandibular central ones. The only exception is that they are larger. These incisors also have either one or two roots that can curve either labially or distally.
As form follows function, the anatomy that you’ve learned above can easily explain the functions of this set of teeth. Since they are the most anterior teeth in the mouth, the functions of the incisors involves cutting or shearing food. During occlusion of the mouth, the incisors will close and insert their sharp incisal edges into the food. They do not feature cusps like the teeth located at the back of the mouth, which are involved in grinding or crushing food. Instead, incisors prepare the food for grinding by breaking it down into smaller, manageable pieces that can be chewed during the process of mastication.
However, the incisors are involved in two other, less obvious functions. Thanks to them, you can pronounce letters, words and sounds such as ‘th’ in words like ‘the’, ‘thunderstorm’ and so on. The incisors also help support the lips, maintaining them in their correct position.