Dental anatomy is the branch of anatomy devoted to the structure of human teeth. Its scope includes tooth development, appearance and classification, while the functional relations between contacting teeth belong to dental occlusion. Tooth formation begins before birth, and the final form of a tooth is determined during that period. Dental anatomy is also a classificatory science, because it supplies the names of teeth and of their component structures, and those names have practical use in dental treatment.
Typically there are 20 primary teeth and 32 permanent teeth, the last four being third molars or wisdom teeth, each of which may or may not develop. In the primary dentition, 10 teeth are usually in the maxilla and 10 in the mandible. In the permanent dentition, 16 are in the maxilla and 16 in the mandible. Every tooth has distinguishing features.
Tooth development is the complex process by which teeth form from embryonic cells, grow and erupt into the mouth. Although many species have teeth, non-human tooth development is essentially the same as in humans. For a healthy oral environment, enamel, dentine, cementum and the periodontal tissues must form at the appropriate stages of fetal development. Primary teeth begin to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.[1] If teeth do not begin to develop around those times, they will not develop at all.
Much research has focused on the process that initiates tooth development. It is generally accepted that a factor necessary for tooth development is present in the tissue of the first branchial arch.[2] The tooth bud, sometimes called the tooth germ, is the aggregation of cells that will form a tooth, and it has three parts: the enamel organ, the dental follicle and the dental papilla.[3]
The enamel organ consists of the outer enamel epithelium, the inner enamel epithelium, the stellate reticulum and the stratum intermedium.[3] These cells produce ameloblasts, which in turn produce enamel and the reduced enamel epithelium. Growth of the cervical loop into deeper tissue forms Hertwig's epithelial root sheath, which determines the shape of the root. The dental papilla contains cells that develop into odontoblasts, the cells that form dentine.[3] The junction between the dental papilla and the inner enamel epithelium also determines the shape of the crown.[2] The dental follicle produces three important cell populations: cementoblasts, osteoblasts and fibroblasts. Cementoblasts form the cementum of the tooth, osteoblasts produce the alveolar bone around the root, and fibroblasts develop into the periodontal ligament that attaches the tooth to alveolar bone through cementum.[4]
Tooth development is conventionally divided into the bud, cap, bell and maturation stages. Staging attempts to classify changes along a continuous process, and it is often difficult to assign a particular developing tooth to a stage.[5] Differences in the appearance of sections from the same developing tooth make the assignment even more difficult, because the sections may appear to be at different stages.
Teeth are named by their dentition and arch, class, type and side. A tooth belongs to one of two dentitions, primary or permanent. A succedaneous tooth is a permanent tooth that replaces a primary tooth, namely a permanent incisor, canine or premolar, and succedaneous teeth are usually referred to as a group. The name also depends on the arch, with maxillary used for teeth in the maxilla and mandibular for teeth in the mandible. Teeth fall into four classes: incisors, canines, premolars and molars. Premolars exist only in the permanent dentition, and there are no primary premolars. Within each class, teeth are further distinguished: incisors into central and lateral, and premolars and molars into first, second and third. The side of the mouth may also be included, so that a particular tooth may be called the permanent maxillary left lateral incisor.
Several dental notation systems associate information with a particular tooth. The three most common are the FDI World Dental Federation notation, the Universal Numbering System and the Palmer notation. The FDI system is used worldwide, while the Universal system is widely used in the United States.
Although the Palmer notation is said to have been replaced by FDI notation, it remains the preferred method for dental students and practitioners in the United Kingdom.[6] It was originally called the Zsigmondy system after the Austrian dentist Adolf Zsigmondy, who proposed the idea in 1861 and used the Zsigmondy cross to record dental quadrants.[7] Palmer notation consists of a symbol (┘└ ┐┌) and a number, where the symbol indicates the quadrant as seen by the dentist and the number indicates the position from the midline. Permanent teeth are numbered 1 to 8, and primary teeth are lettered A to E.
The Universal Numbering System gives each tooth a unique letter or number. Upper-case letters A to T are used for primary teeth, and numbers 1 to 32 for permanent teeth. Tooth 1 is the right maxillary third molar, and the count continues along the maxillary teeth to the left. It then begins again at the left mandibular third molar as tooth 17 and continues along the mandibular teeth to the right.
The FDI system uses two digits, the first indicating the quadrant and the second indicating the position from the midline. For permanent teeth, the right maxillary quadrant begins with 1, the left maxillary with 2, the left mandibular with 3 and the right mandibular with 4. For primary teeth the corresponding sequences are 5, 6, 7 and 8 for the right maxillary, left maxillary, left mandibular and right mandibular quadrants.
Any given tooth therefore has three different identifiers depending on the notation used. The permanent right maxillary central incisor is number 8 in the Universal system, number 11 in the FDI system, and the number 1 with the appropriate symbol in Palmer notation. Giving a tooth number without stating the notation can cause confusion, because the number 12 may mean the permanent left maxillary first premolar in the Universal system or the permanent right maxillary lateral incisor in the FDI system.
In 1891 Victor Haderup designed a variant of the quadrant system that used plus (+) and minus (-) signs to distinguish upper from lower and left from right, so that +1 is the right maxillary central incisor and 1- is the left mandibular central incisor. Primary teeth were numbered 05+ to 01+ in the right maxilla and -01 to -05 in the left mandible. The system is still used in Denmark.[8]
The word crown has two uses. The anatomical crown is the region above the cementoenamel junction (CEJ), or neck, of the tooth.[9] It is completely covered by enamel. The clinical crown is conventionally any part of the tooth visible in the mouth, but the unqualified word crown usually means the anatomical crown. The bulk of the crown is dentine surrounding the pulp chamber. Before eruption the crown is enclosed in bone, but after eruption it is almost always visible in an anatomically normal and clinically healthy mouth.
The anatomical root lies below the cementoenamel junction and is covered by cementum, while the clinical root is the part of the tooth not visible in the mouth. Again, the anatomical root is usually assumed. Dentine forms most of the root and usually contains a pulp canal. A root may be single or multiple. Canines and most premolars, except the maxillary first premolar, usually have one root. The maxillary first premolar and mandibular molars usually have two roots, and maxillary molars usually have three. The tooth is supported in bone by an attachment apparatus called the periodontium, which interacts with the root.
The surface near the cheek or lip is the buccal surface on posterior teeth and the labial surface on anterior teeth. The surface near the tongue is the lingual surface, which may also be called the palatal surface on maxillary teeth next to the hard palate.
The surface used in chewing is the occlusal surface on posterior teeth and the incisal edge on anterior teeth. The surface near the junction of crown and root is the cervical surface, and the surface nearest the root tip is the apical surface. The tissue around the root tip is the periapical tissue.
The mesial surface is the surface toward the facial midline, an imaginary vertical line between the eyes that passes down the nose and between the central incisors. A surface away from the midline is the distal surface.
A cusp is a projection on the occlusal surface of posterior teeth and canines, and it forms a large part of the tooth surface. Canines have one cusp. Maxillary premolars and the mandibular first premolar usually have two cusps. The mandibular second premolar usually has three cusps, one buccal and two lingual. Maxillary molars have two buccal and two lingual cusps. A fifth cusp that may form on the maxillary first molar is the cusp of Carabelli. Mandibular molars may have five or four cusps.
The cervical ridge is a band-like elevation running mesiodistally around the cervical third of the lingual surface of anterior teeth. It is usually recognisable as an inverted V-shaped ridge,[10] and it resembles a belt.[11] All anterior teeth develop from four growth centres called lobes, three on the labial side and one on the lingual side. The cervical ridge is formed by the lingual lobe.[12] Most of the cervical third of the lingual surface is made up of the cervical ridge.[13] In mandibular incisors the cervical ridge is usually poorly developed or absent. The maxillary canine has a large, well-developed cervical ridge,[13] while the mandibular canine has a smoother and more rounded ridge.
A ridge is any linear, flat elevation on a tooth,[14] and ridges are named by position. The buccal ridge runs from the cervical region to the occlusal surface in the middle of the buccal surface of a premolar. The labial ridge runs from the cervical region to the incisal edge in the middle of the labial surface of a canine. The lingual ridge runs from the cervical ridge to the cusp on the lingual surface of most canines. The cervical ridge runs mesiodistally in the cervical third of the buccal surface of the crown. These ridges are found on all primary teeth but only on permanent molars.
Cusp ridges radiate from a cusp. Every tooth has two marginal ridges, mesial and distal. On anterior teeth they are on the mesial and distal edges of the lingual surface, and on posterior teeth they are on the mesial and distal edges of the occlusal surface. A triangular ridge runs from a cusp to the central groove of a premolar or molar. A transverse ridge is formed by the union of two triangular ridges on a posterior tooth. The union of a buccal and a lingual triangular ridge is often named. The oblique ridge is found on the occlusal surface of maxillary molars. It is formed by the union of the distal cusp ridge of the mesiolingual cusp and the triangular ridge of the distobuccal cusp, and it usually forms the distal boundary of the central fossa.
The teeth with the fewest developmental grooves are the mandibular central and lateral incisors.[15]
Canines, however, show the most prominent developmental grooves, because they are strongly anchored in bone.
An embrasure is the triangular space between the proximal surfaces of adjacent teeth. Its boundaries are formed by the interdental papilla of the gingiva, the adjacent teeth and the contact point where the two teeth meet. Each contact area has four embrasures: facial, also called labial or buccal, lingual or palatal, occlusal or incisal, and cervical or proximal. The cervical embrasure is usually filled by the interdental papilla, and when there is not enough gingival tissue a dark triangle or Angularis Nigra may be visible.
Embrasures have three functions. They form spillways between teeth that guide food away from the gingiva, they give teeth a more self-cleansing mechanism, and they protect the gingiva from excessive frictional trauma while providing an appropriate degree of stimulation.
Mamelons usually appear as three small projections on the incisal edge of anterior teeth. They are remnants of the three developmental lobes of these teeth, and the fourth lobe forms the cervical ridge. Because this surface is the first to wear away, mamelons may not be visible in older people. The best chance of seeing them is shortly after eruption.
The eight incisors are anterior teeth, four in the maxillary arch and four in the mandibular arch. Their function is to shear or cut food during chewing. They have no cusps. Instead, the surface used in eating is the incisal ridge or incisal edge. Although similar, primary and permanent incisors differ in several subtle ways.
The maxillary central incisor is usually the most prominent tooth, because the two are in the upper centre of the front of the mouth, mesial to the maxillary lateral incisors. The average total length of a primary maxillary central incisor is 16 mm, with a 6 mm crown and a 10 mm root.[16] Compared with the permanent maxillary central incisor, the primary tooth has a larger root-to-crown ratio. Its mesiodistal crown diameter is greater than its cervicoincisal length, so the tooth looks wider than it is tall from the labial aspect.
The permanent maxillary central incisor is the widest anterior tooth mesiodistally. It is larger than the adjacent lateral incisor and is usually less convex on its labial surface, so it looks more rectangular or square. The mesial incisal angle is sharper than the distal angle. When the tooth first erupts, its incisal edge has three rounded features called mamelons.[17] Over time the mamelons disappear through enamel wear.
The maxillary lateral incisor is distal to the maxillary central incisor and mesial to the maxillary canine.
The mandibular central incisor is in the mandible near the facial midline, mesial to the mandibular lateral incisor.
The mandibular lateral incisor is distal to the mandibular central incisor and mesial to the mandibular canine.
The maxillary and mandibular canines are called the cornerstones of the mouth, because both are three teeth from the midline and separate the premolars from the incisors. Their position reflects a dual function, because they assist both premolars and incisors during chewing. Their most common action, however, is tearing food. Canines have one cusp and resemble the grasping teeth of carnivores. Although similar, the maxillary and mandibular canines differ in detail.
The maxillary canine is lateral to the maxillary lateral incisor and mesial to the maxillary first premolar. It is the longest tooth in the mouth from root to incisal edge.
The mandibular canine is distal to the mandibular lateral incisor and mesial to the mandibular first premolar.
Premolars lie distal to the canines and mesial to the molars. They are divided into first and second premolars, and their functions vary. There are no primary premolars. Instead, the primary molars precede the permanent premolars.
The maxillary first premolar is lateral to the maxillary canine and mesial to the maxillary second premolar. Its function resembles that of a canine, and tearing is the main action during chewing. It has two cusps, and the buccal cusp is sharp enough to resemble a carnivore grasping tooth. A distinct depression in the cervical third of the crown extends onto the root.[18] The maxillary first premolar is bifurcated and usually has two roots.
The maxillary second premolar is lateral to the maxillary first premolar and mesial to the maxillary first molar. Its function resembles that of the first molar, and grinding is the main action during chewing. It has two cusps, but neither is as sharp as those of the maxillary first premolar.
The mandibular first premolar is lateral to the mandibular canine and mesial to the mandibular second premolar. Its function resembles that of a canine, and tearing is the main action during chewing. It has two cusps, with a large, sharp buccal cusp. Because the lingual cusp is small and non-functional, the mandibular first premolar resembles a small canine.
The mandibular second premolar is distal to the mandibular first premolar and mesial to the mandibular first molar. Its function is to assist the mandibular first molar during chewing. It has three cusps, with a large buccal cusp and well-developed, functional lingual cusps. Thus the mandibular first premolar resembles a small canine, while the mandibular second premolar resembles a first molar.
Molars are the most posterior teeth in the mouth, and their function is to grind food during chewing. The number of cusps and the overall appearance vary greatly between molars and between individuals. Primary and permanent molars differ substantially, although their function is similar. Permanent maxillary molars are considered to have no predecessors. Despite the name, primary molars are followed by permanent premolars. Third molars are commonly called wisdom teeth.
The maxillary first molar is lateral to the maxillary second premolar and mesial to the maxillary second molar. Maxillary molars usually have four cusps, two buccal and two palatal. Most also have a fifth cusp, the cusp of Carabelli, on the mesiolingual side.
The maxillary second molar is lateral to the maxillary first molar and mesial to the maxillary third molar. This is true only in the permanent dentition. In the primary dentition the maxillary second molar is the last tooth in the mouth and has no third molar behind it. The primary maxillary second molar is also the primary tooth most likely to have an oblique ridge. Maxillary molars usually have four cusps, two buccal and two palatal.
The maxillary third molar is lateral to the maxillary second molar and has no tooth behind it in the permanent dentition. There is no primary maxillary third molar. Maxillary molars usually have four cusps, two buccal and two palatal. Third molars vary so much, however, that a specific description does not hold in all cases.
The mandibular first molar is distal to the mandibular second premolar and mesial to the mandibular second molar. It is on the mandibular arch and usually occludes with the maxillary first molar and maxillary second premolar, an arrangement called Class I occlusion. It usually has five well-developed cusps: two buccal, two lingual and one distal.
The mandibular second molar is distal to the mandibular first molar and mesial to the mandibular third molar. This is true only in the permanent dentition. In the primary dentition the mandibular second molar is the last tooth in the mouth and has no third molar behind it. Although variation is greater than in the mandibular first molar, the mandibular second molar usually has four cusps, two buccal and two lingual.
The mandibular third molar is distal to the mandibular second molar and has no tooth behind it in the permanent dentition. There is no primary mandibular third molar. Third molars vary so much that a specific description does not hold in all cases.
Dental anatomy directly supports caries diagnosis and restoration. Caries favours pits, proximal surfaces and the cervical region, and these sites are closely related to the form of cusps, ridges, developmental grooves and embrasures. During restoration, cusp height, marginal ridges and contact points determine the contour of the filling and the distribution of occlusal load.
Periodontal treatment depends on knowledge of the cementoenamel junction, periodontal ligament and alveolar bone. Pocket depth is related to root form, furcation and cervical anatomy, and the furcation of a multi-rooted tooth is a weak point for attachment loss.
Orthodontic treatment uses dental landmarks to position brackets and plan tooth movement. The incisal edge, cervical ridge and lingual surface of anterior teeth affect aesthetics and function, while the relation between cusps and central fossae determines occlusal contacts in posterior teeth.
Dental anatomy is also the basis of forensic dentistry. Tooth form, restorations, wear and developmental defects can be used for identification, and the eruption times of primary and permanent teeth can estimate a child's age.
In dental anthropology, crown form, cusp number and root number are used to study population relationships and dietary adaptation, and traits such as the cusp of Carabelli differ in frequency between populations.
Individual variation matters clinically. Cusp number, root number, root canal anatomy and enamel thickness vary between people, and extra cusps, fused teeth, dens invaginatus and taurodontism affect endodontic and restorative planning, so preoperative imaging and anatomical knowledge are equally important.
Dental anatomy also informs occlusal reconstruction. A restoration must reproduce or rebuild cusps, marginal ridges, triangular ridges and embrasures to restore chewing efficiency, speech and appearance and to avoid food impaction and gingival trauma.
In dental education, dental anatomy is the common foundation of restorative, endodontic, periodontal and orthodontic courses, and students must build three-dimensional concepts from specimens, models and images.
References