Topographic

Topographic anatomy of deciduous teeth at various stages of development. Topographic anatomy of permanent teeth in different stages of development. Modeling teeth with plastic and hard materials. Physiology and pathology of teething. Terms of formation andresorption of roots temporary and permanent teeth. 

Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into themouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth weeks, and permanent teeth begin to form in the twentieth week. If teeth do not start to develop at or near these times, they will not develop at all.

For several decades, the developing tooth organ has served as a valuable paradigm for studying the fundamental processes involved in organogenesis. These processes are the determination of position, when the precise site of tooth initiation is established, the determination of form, or morphogenesis, when the size and shape of the tooth organ is set, and cell differentiation, when organ-specific tissues are formed by defined cell populations, each with unique properties. The dental literature is enriched with excellent reviews on tooth development, and the reader is encouraged to study the topic in further detail.

Teeth develop in distinct stages that are easily recognizable at the microscopic level. Hence, stages of odontogenesis are described by the histologic appearance of the tooth organ. From early to late, these stages are described as the lamina, bud, cap, early bell, and late bell stages of tooth development.” Although the following description will use these common terms, modern literature uses functional terminology to describe odontogenesis as occurring in four phases: initiation, morphogenesis, cell differentiation (or cytodifferentiation), and matrix apposition.

Lamina stage

The dental lamina is the first morphologic sign of tooth development and is visible at approximately 5 weeks of human development and at embryonic day 11 in mouse gestation. This thickening of the oral epithelium lining the frontonasal, maxillary, and mandibular arches occurs only at sites where tooth organs will develop.At the lamina

stage, cells in the dental epithelium and underlying ectomesenchyme are dividing at different rates, the latter more rapidly. As will be explained later, the dental lamina has the full potential to induce tooth formation by dictating the fate of the underlying ectomesenchyme”.

Bud stage

As the dental lamina continues to grow and thicken to form a bud, cells of the ectomesenchyme proliferate and condense to form the dental papilla. At this stage, the inductive or tooth-forming potential is transferred from the dental epithelium to the dental papilla.

Cap stage

At this stage, the tooth bud assumes the shape of a cap that is surrounded by the dental papilla.The ectodermal compartment of the tooth organ is referred to as the dental or enamel organ. The enamel organ and dental papilla become encapsulated

by another layer of mesenchymal cells, called the dental follicle, that separates the tooth organ papilla from the other connective tissues of the jaws.

The transition from the bud stage to the cap stage is an important step in tooth development, because it marks the onset of crown formation. Recent studies have pointed to the role of the enamel knot as an important organizing center that initiates cuspal patterning. Formally described as a transient structure with no ascribed functions, the enamel knot is formed by the only cells within the central region of the dental organ that fail to grow. As will be described later, the enamel knot expresses a unique set of signaling molecules that influence both the shape of the crown and the development of the dental papilla. In incisors, the enamel knot initiates the first folding of dental epithelium. Secondary enamel knots determine the site of new cusps in molars. Similar to signaling centers in other organizing tissues, such as the developing limb bud, the enamel knot undergoes programmed cellular death, or apoptosis, after cuspal patterning is completed at the onset of the early bell stage.

Early bell stage

The dental organ assumes the shape of a bell as cells continue to divide but at differential rates. A single layer of cuboidal cells, called the external or outer dental epithelium, lines the periphery of the dental organ; cells that border the dental papilla and are columnar in appearance form the internal or inner dental epithelium. The inner epithelium gives rise to the ameloblasts, cells responsible for enamel formation. Cells located in the center of the dental organ produce high levels of glycosaminoglycans that are able to sequester fluids as well as growth factors that lead to its expansion. This network of star-shaped cells is named the stellate reticulum. Interposed between the stellate reticulum and the internal dental epithelium is a narrow layer of flattened cells, termed the stratum intermedium. These

cells express high levels of alkaline phosphatase. The stratum intermedium is believed to influence the biomineralization of enamel. In the region of the apical end of the tooth organ, the internal and external dental epithelial layers meet at a junction

called the cervical loop. At the early bell stage, each layer of the dental organ has assumed special functions. The reciprocal exchange of molecular information between the dental organ and dental papilla influences the important events that lead to cell differentiation at the late bell stage.

Late bell stage

The dental lamina that connects the tooth organ to the oral epithelium gradually disintegrates at the late bell stage. The cells of the internal dental epithelium continue to divide at different rates to determine the precise shape of the crown. Shortly after, cells of the internal dental epithelium at the sites of the future cuspal tips stop dividing and assume a columnar shape. The most peripheral cells of the dental papilla enlarge and become organized along the basement membrane at the tooth’s epithelial-mesenchymal interface. These newly differentiated cells are called odontoblasts, cells that are responsible for the synthesis and secretion of dentin matrix. At this time, the dental papilla is termed the dental pulp. After odontoblasts deposit the first layer of

predentin matrix, cells of the internal dental epithelium receive their signal to differentiate further into ameloblasts, or enamel-producing cells. As enamel is deposited over dentin matrix, ameloblasts retreat to the external surface of the crown and are believed to undergo programmed cell death. In contrast, odontoblasts line the inner surface of dentin and remain metabolically active throughout the life of a tooth.

In summary, development of the tooth rudiment from the lamina to the late bell stages culminates in the formation of the tooth crown. As root formation proceeds, epithelial cells from the cervical loop proliferate apically and influence the differentiation of odontoblasts from the dental papilla as well as cementoblasts from follicle mesenchyme.This leads to the deposition of root dentin and cementum, respectively. The dental follicle that gives rise to components of the periodontium, namely the periodontal ligament fibroblasts, the alveolar bone of the tooth socket,

and the cementum, also plays a role during tooth eruption, which marks the end phase of odontogenesis.

 

A significant amount of research has focused on determining the processes that initiate tooth development. It is widely accepted that there is a factor within the tissues of the first branchial arch that is necessary for the development of teeth.

In vertebrates, several specializations of epithelial tissue (‘phanères’) generate after thickening specific structures: keratinized structure (hair, nails) or exoskeletons structure (scales, teeth). Placoids scales and teeth of sharks are considered homologous organs.

The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth.These cells are derived from the ectoderm of thefirst branchial arch and the ectomesenchyme of the neural crest. The tooth bud is organized into three parts: the enamel organ, the dental papilla and the dental follicle.

The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The location where the outer enamel epithelium and inner enamel epithelium join is called the cervical loop. The growth of cervical loop cells into the deeper tissues forms Hertwig’s Epithelial Root Sheath, which determines the root shape of the tooth.

The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.

The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum

Human tooth development timeline

The following tables present the development timeline of human teeth. Times for the initial calcification of primary teeth are for weeks in utero. Abbreviations: wk = weeks; mo = months; yr = years.

 

 

 

The developing tooth bud                          

One of the earliest steps in the formation of a tooth that can be seen microscopically is the distinction between the vestibular lamina and the dental lamina. The dental lamina connects the developing tooth bud to the epithelial layer of the mouth for a significant time.

Tooth development is commonly divided into the following stages: the bud stage, the cap stage, the bell stage, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histologic sections of the same developing tooth, which can appear to be different stages.

Bud stage

The bud stage is characterized by the appearance of a tooth bud without a clear arrangement of cells. The stage technically begins once epithelial cells proliferate into the ectomesenchyme of the jaw. Typically, this occurs when the fetus is around 6 weeks old. The tooth bud itself is the group of cells at the end of the dental lamina.

Along with the formation of the dental lamina, 10 round epithelial structures, each referred to as a bud, develop at the distal aspect of the dental lamina of each arch. These correspond to the 10 deciduous teeth of each dental arch, and they signify the bud stage of tooth development. Each bud is separated from the ectomesenchyme by a basement membrane. Ectomesenchymal cells congregate deep to the bud, forming a cluster of cells, which is the initiation of the condensation of the ectomesenchyme. The remaining ectomesenchymal cells are arranged in a more or less haphazardly uniform fashion.

Cap stage

Histologic slide of tooth in cap stage.

The first signs of an arrangement of cells in the tooth bud occur in the cap stage. A small group of ectomesenchymal cells stops producing extracellularsubstances, which results in an aggregation of these cells called the dental papilla. At this point, the tooth bud grows around the ectomesenchymal aggregation, taking on the appearance of a cap, and becomes the enamel (or dental) organ. A condensation of ectomesenchymal cells called the dental follicle surrounds the enamel organ and limits the dental papilla. Eventually, the enamel organ will produce enamel, the dental papilla will produce dentin and pulp, and the dental follicle will produce all the supporting structures of a tooth.

Histologic slide of tooth in early bell stage. Note cell organization.

Bell stage

The bell stage is known for the histodifferentiation and morphodifferentiation that takes place. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of their star-shaped appearance. The bell stage is divided into the early bell stage and the late bell stage. Cells on the periphery of the enamel organ separate into three important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium. The columnar cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop. In summary, the layers in order of innermost to outermost consist of dentine, enamel (formed by inner enamel epithelium, or ‘ameloblasts’, as they move outwards/upwards), inner enamel epithelium and stratum intermedium (specialised stratified cells that support the synthetic activity of the inner enamel epithelium) What follows is part of the initial ‘enamel organ’, the middle of which is made up of stellate reticulum cells. This is all encased by the outer enamel epithelium layer.

Other events occur during the bell stage. The dental lamina disintegrates, leaving the developing teeth completely separated from the epithelium of the oral cavity; the two will not join again until the final eruption of the tooth into the mouth.

Histologic slide of tooth in late bell stage. Note disintegration of dental lamina at top.

The crown of the tooth, which is influenced by the shape of the internal enamel epithelium, also takes shape during this stage. Throughout the mouth, all teeth undergo this same process; it is still uncertain why teeth form various crown shapes—for instance, incisors versus canines. There are two dominant hypotheses. The “field model” proposes there are components for each type of tooth shape found in the ectomesenchyme during tooth development. The components for particular types of teeth, such as incisors, are localized in one area and dissipate rapidly in different parts of the mouth. Thus, for example, the “incisor field” has factors that develop teeth into incisor shape, and this field is concentrated in the central incisor area, but decreases rapidly in the canine area. The other dominant hypothesis, the “clone model”, proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the “progress zone”. Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it is postulated that both models influence tooth development at different times.

Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.

Histologic slide of developing hard tissues. Ameloblasts are forming enamel, while odontoblasts are forming dentin.

Crown stage

Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.

The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.

After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth’s enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.

Enamel

Enamel formation is called amelogenesis and occurs in the crown stage of tooth development. “Reciprocal induction” governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation. Generally, enamel formation occurs in two stages: the secretory and maturation stages. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization.

In the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. The appearance of this mineralized tissue, which occurs usually around the third or fourth month of pregnancy, marks the first appearance of enamel in the body. Ameloblasts deposit enamel at the location of what become cusps of teeth alongside dentin. Enamel formation then continues outward, away from the center of the tooth.

In the maturation stage, the ameloblasts transport some of the substances used in enamel formation out of the enamel. Thus, the function of ameloblasts changes from enamel production, as occurs in the secretory stage, to transportation of substances. Most of the materials transported by ameloblasts in this stage are proteins used to complete mineralization. The important proteins involved are amelogenins,ameloblastins, enamelins, and tuftelins. By the end of this stage, the enamel has completed its mineralization.

Dentin

Dentin formation, known as dentinogenesis, is the first identifiable feature in the crown stage of tooth development. The formation of dentin must always occur before the formation of enamel. The different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin.

Odontoblasts, the dentin-forming cells, differentiate from cells of the dental papilla. They begin secreting an organic matrix around the area directly adjacent to the inner enamel epithelium, closest to the area of the future cusp of a tooth. The organic matrix contains collagen fibers with large diameters (0.1–0.2 μm in diameter).^[15] The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process. Thus, dentin formation proceeds toward the inside of the tooth. The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.

Whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process. Odontoblasts increase in size, eliminating the availability of any extracellular resources to contribute to an organic matrix for mineralization. Additionally, the larger odontoblasts cause collagen to be secreted in smaller amounts, which results in more tightly arranged, heterogeneous nucleation that is used for mineralization. Other materials (such as lipids, phosphoproteins, and phospholipids) are also secreted.

Secondary dentin is formed after root formation is finished and occurs at a much slower rate. It is not formed at a uniform rate along the tooth, but instead forms faster along sections closer to the crown of a tooth. This development continues throughout life and accounts for the smaller areas of pulp found in older individuals. Tertiary dentin, also known as reparative dentin, forms in reaction to stimuli, such as attrition or dental caries.

Cementum

Cementum formation is called cementogenesis and occurs late in the development of teeth. Cementoblasts are the cells responsible for cementogenesis. Two types of cementum form: cellular and acellular.

Acellular cementum forms first. The cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth’s root once Hertwig’s Epithelial Root Sheath (HERS) has begun to deteriorate. The cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth. As the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers. Noncollagenous proteins, such as bone sialoprotein and osteocalcin, are also secreted. Acellular cementum contains a secreted matrix of proteins and fibers. As mineralization takes place, the cementoblasts move away from the cementum, and the fibers left along the surface eventually join the forming periodontal ligaments.

Cellular cementum develops after most of the tooth formation is complete and after the tooth occludes (in contact) with a tooth in the opposite arch. This type of cementum forms around the fiber bundles of the periodontal ligaments. The cementoblasts forming cellular cementum become trapped in the cementum they produce.

The origin of the formative cementoblasts is believed to be different for cellular cementum and acellular cementum. One of the major current hypotheses is that cells producing cellular cementum migrate from the adjacent area of bone, while cells producing acellular cementum arise from the dental follicle.  Nonetheless, it is known that cellular cementum is usually not found in teeth with one root. In premolars and molars, cellular cementum is found only in the part of the root closest to the apex and in interradicular areas between multiple roots.

Formation of the periodontium

The periodontium, which is the supporting structure of a tooth, consists of the cementum, periodontal ligaments, gingiva, and alveolar bone. Cementum is the only one of these that is a part of a tooth. Alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called a “socket“. Periodontal ligaments connect the alveolar bone to the cementum, and the gingiva is the surrounding tissue visible in the mouth.

Periodontal ligament

Cells from the dental follicle give rise to the periodontal ligament (PDL). Specific events leading to the formation of the periodontal ligament vary between deciduous (baby) and permanent teeth and among various species of animals. Nonetheless, formation of the periodontal ligament begins with ligament fibroblasts from the dental follicle. These fibroblasts secrete collagen, which interacts with fibers on the surfaces of adjacent bone and cementum. This interaction leads to an attachment that develops as the tooth erupts into the mouth. The occlusion, which is the arrangement of teeth and how teeth in opposite arches come in contact with one another, continually affects the formation of periodontal ligament. This perpetual creation of periodontal ligament leads to the formation of groups of fibers in different orientations, such as horizontal and oblique fibers.

Alveolar bone

As root and cementum formation begin, bone is created in the adjacent area. Throughout the body, cells that form bone are called osteoblasts. In the case of alveolar bone, these osteoblast cells form from the dental follicle.^[19] Similar to the formation of primary cementum, collagen fibers are created on the surface nearest the tooth, and they remain there until attaching to periodontal ligaments.

Like any other bone in the human body, alveolar bone is modified throughout life. Osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. As is the case when movement of teeth is attempted through orthodontics, an area of bone under compressive forcefrom a tooth moving toward it has a high osteoclast level, resulting in bone resorption. An area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation.

Gingiva

The connection between the gingiva and the tooth is called the dentogingival junction. This junction has three epithelial types: gingival, sulcular, and junctional epithelium. These three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth.

Much about gingival formation is not fully understood, but it is known that hemidesmosomes form between the gingival epithelium and the tooth and are responsible for the primary epithelial attachment. Hemidesmosomes provide anchorage between cells through small filament-like structures provided by the remnants of ameloblasts. Once this occurs, junctional epithelium forms from reduced enamel epithelium, one of the products of the enamel organ, and divides rapidly. This results in the perpetually increasing size of the junctional epithelial layer and the isolation of the remnants of ameloblasts from any source of nutrition. As the ameloblasts degenerate, a gingival sulcus is created.

Nerve and vascular formation

Frequently, nerves and blood vessels run parallel to each other in the body, and the formation of both usually takes place simultaneously and in a similar fashion. However, this is not the case for nerves and blood vessels around the tooth, because of different rates of development.

Nerve formation

Nerve fibers start to near the tooth during the cap stage of tooth development and grow toward the dental follicle. Once there, the nerves develop around the tooth bud and enter the dental papilla when dentin formation has begun. Nerves never proliferate into the enamel organ.

Vascular formation

Blood vessels grow in the dental follicle and enter the dental papilla in the cap stage. Groups of blood vessels form at the entrance of the dental papilla. The number of blood vessels reaches a maximum at the beginning of the crown stage, and the dental papilla eventually forms in the pulp of a tooth. Throughout life, the amount of pulpal tissue in a tooth decreases, which means that the blood supply to the tooth decreases with age. The enamel organ is devoid of blood vessels because of its epithelial origin, and the mineralized tissues of enamel and dentin do not need nutrients from the blood.

Tooth eruption

Main article: Tooth eruption

Tooth eruption occurs when the teeth enter the mouth and become visible. Although researchers agree that tooth eruption is a complex process, there is little agreement on the identity of the mechanism that controls eruption. Some commonly held theories that have been disproven over time include: (1) the tooth is pushed upward into the mouth by the growth of the tooth’s root, (2) the tooth is pushed upward by the growth of the bone around the tooth, (3) the tooth is pushed upward by vascular pressure, and (4) the tooth is pushed upward by the cushioned hammock. The cushioned hammock theory, first proposed by Harry Sicher, was taught widely from the 1930s to the 1950s. This theory postulated that a ligament below a tooth, which Sicher observed under a microscope on a histologic slide, was responsible for eruption. Later, the “ligament” Sicher observed was determined to be merely an artifact created in the process of preparing the slide.

The most widely held current theory is that while several forces might be involved in eruption, the periodontal ligaments provide the main impetus for the process. Theorists hypothesize that the periodontal ligaments promote eruption through the shrinking and cross-linking of their collagen fibers and the contraction of their fibroblasts.

Although tooth eruption occurs at different times for different people, a general eruption timeline exists. Typically, humans have 20 primary (baby) teeth and 32 permanent teeth. Tooth eruption has three stages. The first, known as deciduous dentition stage, occurs when only primary teeth are visible. Once the first permanent tooth erupts into the mouth, the teeth are in the mixed (or transitional) dentition. After the last primary tooth falls out of the mouth—a process known as exfoliation—the teeth are in the permanent dentition.

Primary dentition starts on the arrival of the mandibular central incisors, usually at eight months, and lasts until the first permanent molars appear in the mouth, usually at six years. The primary teeth typically erupt in the following order: (1) central incisor, (2) lateral incisor, (3) first molar, (4) canine, and (5) second molar. As a general rule, four teeth erupt for every six months of life, mandibular teeth erupt before maxillary teeth, and teeth erupt sooner in females than males. During primary dentition, the tooth buds of permanent teeth develop below the primary teeth, close to the palate or tongue.

Mixed dentition starts when the first permanent molar appears in the mouth, usually at six years, and lasts until the last primary tooth is lost, usually at eleven or twelve years. Permanent teeth in the maxilla erupt in a different order from permanent teeth on the mandible. Maxillary teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) first premolar, (5) second premolar, (6) canine, (7) second molar, and (8) third molar. Mandibular teeth erupt in the following order: (1) first molar (2) central incisor, (3) lateral incisor, (4) canine, (5) first premolar, (6) second premolar, (7) second molar, and (8) third molar. Since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. If any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost in the mouth. This may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as malocclusion. Orthodontics may be required in such circumstances for an individual to achieve a straight set of teeth.

The permanent dentition begins when the last primary tooth is lost, usually at 11 to 12 years, and lasts for the rest of a person’s life or until all of the teeth are lost (edentulism). During this stage, third molars (also called “wisdom teeth“) are frequently extracted because of decay, pain or impactions. The main reasons for tooth loss are decay and periodontal disease.^[34]

Immediately after the eruption enamel is covered by a specific film: Nasmyth’s membrane or ‘enamel cuticle’, structure of embryological origin is composed of keratin which gives rise to theenamel organ.

Nutrition and tooth development

As in other aspects of human growth and development, nutrition has an effect on the developing tooth. Essential nutrients for a healthy tooth include calcium, phosphorus, and vitamins A, C, andD. Calcium and phosphorus are needed to properly form the hydroxyapatite crystals, and their levels in the blood are maintained by Vitamin D. Vitamin A is necessary for the formation ofkeratin, as Vitamin C is for collagen. Fluoride is incorporated into the hydroxyapatite crystal of a developing tooth and makes it more resistant to demineralization and subsequent decay.

Deficiencies of these nutrients can have a wide range of effects on tooth development. In situations where calcium, phosphorus, and vitamin D are deficient, the hard structures of a tooth may be less mineralized. A lack of vitamin A can cause a reduction in the amount of enamel formation. Fluoride deficiency causes increased demineralization when the tooth is exposed to an acidic environment, and also delays remineralization. Furthermore, an excess of fluoride while a tooth is in development can lead to a condition known as fluorosis.

Celiac Disease can cause dental enamel defects in children. 

Abnormalities

There are a number of tooth abnormalities relating to development.

Anodontia is a complete lack of tooth development, and hypodontia is a lack of some tooth development. Anodontia is rare, most often occurring in a condition called Hypohidrotic ectodermal dysplasia, while hypodontia is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population (not including third molars). The absence of third molars is very common, occurring in 20–23% of the population, followed in prevalence by the second premolar and lateral incisor. Hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces, such as infection and chemotherapy medications, and is also associated with many syndromes, such as Down syndrome and Crouzon syndrome.

Hyperdontia is the development of extraneous teeth. It occurs in 1–3% of Caucasians and is more frequent in Asians. About 86% of these cases involve a single extra tooth in the mouth, most commonly found in the maxilla, where the incisors are located. Hyperdontia is believed to be associated with an excess of dental lamina.

Dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. As a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. Cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed upward by trauma into the gingiva where it moves the tooth bud of the permanent tooth.

Regional odontodysplasia is rare, but is most likely to occur in the maxilla and anterior teeth. The cause is unknown; a number of causes have been postulated, including a disturbance in the neural crest cells, infection, radiation therapy, and a decrease in vascular supply (the most widely held hypothesis). Teeth affected by regional odontodysplasia never erupt into the mouth, have small crowns, are yellow-brown, and have irregular shapes. The appearance of these teeth in radiographs is translucent and “wispy,” resulting in the nickname “ghost teeth”.

Molecular biology

In fish hox gene expression regulate mechanisms for teeth initiation.

In mouse WNT signals are required for the initiation of teeth development.

NGF-R was present in the condensing ecto-mesenchymal cells of the dental papilla in the early cap stage tooth germ and play multiple roles during morphogenetic and cytodifferentiation events in the tooth. There is a relationship between tooth agenesis and absence of the peripheral trigeminal nerve (see Hypodontia).

All stages (bud, cap, bell, crown), growth and morphogenesis of the teeth are regulated by a protein: sonic hedgehog.

During tooth development there are strong similarities between keratinization and amelogenesis. Keratin is also present in epithelial cells of tooth germ  and a thin film of keratin is present on the tooth erupted recently (Nasmyth’s membrane or enamel cuticle).

Enamel knots as a signaling center in the tooth morphogenesis and odontoblast differentiation.

Various phenotypic inputs modulate the size of the teeth.

The shape of the teeth in prehistoric man was different from that of modern man.

In some dermoid teratomas (particularly ovarian, lung, pancreas, testes) develop complete teeth.

For the tooth eruption is necessary parathyroid hormone.

Tooth development in animals

Invertebrate “teeth”

True teeth are unique to vertebrates, although some invertebrates have structures have analogous structures sometimes called “teeth” – the organism with the simplest genome bearing such “teeth” is probably the worm genus Ancylostoma (Ancylostoma duodenale, Necator americanus). Molluscs have a structure called a radula which bears a ribbon of chitinous “teeth”. However, these are histologically and developmentally different from vertebrate teeth, and are unlikely to be homologous. For example, vertebrate teeth develop from a neural crest mesenchyme-deriveddental papilla, and the neural crest is specific to vertebrates, as are tissues such as enamel.

Tooth development in vertebrates

Teeth is atavic structure and their development is similar in many vertebrates.

Fish have many specialized bony structures, it exist with (Archosargus probatocephalus order Perciformes, family Sparidae) and without teeth (Caristiidae order Perciformes, family Caristiidae, teeth in traces present in juveniles).

Unlike most animals, sharks continuously produce new teeth throughout life via a drastically different mechanism. Because shark teeth have no roots, sharks easily lose teeth when they feed (zoologists estimate that a single shark can lose up to 2,400 teeth in one year)—they must therefore be continually replaced. Shark teeth form from modified scales near the tongue and move outward on the jaw in rows until they fully develop, are used, and are eventually dislodged.

Snakes generally have teeth, with some exception (African Egg-eating Snake).

Today, birds do not have teeth, though it is speculated that prehistoric birds, such as archaeopteryx, did.

In order Tubulidentata (Class Mammalia) teeth are without enamel, they lack incisors and canines and the molars molars are growing continuously from the root.

Generally, tooth development ion-human mammals is similar to human tooth development. The variations lie in the morphology, number, development timeline, and types of teeth, not usually in the actual development of the teeth.

Enamel formation ion-human mammals is almost identical to that in humans. The ameloblasts and enamel organ, including the dental papilla, function similarly. Nonetheless, while ameloblasts die in humans and most other animals—making further enamel formation impossible—rodents continually produce enamel, forcing them to wear down their teeth by gnawing on various materials. If rodents are prevented from gnawing, their teeth eventually puncture the roofs of their mouths. In addition, rodent incisors consist of two halves, known as the crown and root analogues. The labial half is covered with enamel and resembles a crown, while the lingual half is covered with dentin and resembles a root. Both root and crown develop simultaneously in the rodent incisor and continue to grow for the life of the rodent.

The mineral distribution in rodent enamel is different from that of monkeys, dogs, pigs, and humans.  In horse teeth, the enamel and dentin layers are intertwined, which increases the strength and decreases the wear rate of the teeth.

Supporting structures that create a “socket” are found exclusively in Mammalia and Crocodylia. In manatees, mandibular molars develop separately from the jaw, and are encased in a bony shell separated by soft tissue. This also occurs in elephants‘ successional teeth, which erupt to replace lost teeth.

Dental anatomy is a field of anatomy dedicated to the study of human tooth structures. The development, appearance, and classification of teeth fall within its purview. (The function of teeth as they contact one another falls elsewhere, under dental occlusion.) Tooth formation begins before birth, and teeth’s eventual morphology is dictated during this time. Dental anatomy is also a taxonomical science: it is concerned with the naming of teeth and the structures of which they are made, this information serving a practical purpose in dental treatment.

Usually, there are 20 primary (“baby”) teeth and 28 to 32 permanent teeth, the last four being third molars or “wisdom teeth“, each of which may or may not grow in. Among primary teeth, 10 usually are found in the maxilla (upper jaw) and the other 10 inthe mandible(lower jaw). Among permanent teeth, 16 are found in the maxilla and the other 16 in the mandible. Most of the teeth have distinguishing features.

Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans. For human teeth to have a healthy oralenvironment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero. If teeth do not start to develop at or near these times, they will not develop at all.

A significant amount of research has focused on determining the processes that initiate tooth development. It is widely accepted that there is a factor within the tissues of the first branchial arch that is necessary for the development of teeth. The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth and is organized into three parts: the enamel organ, thedental papilla and the dental follicle.

The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium. These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The growth of cervical loop cells into the deeper tissues forms Hertwig’s Epithelial Root Sheath, which determines the root shape of the tooth. The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. The dental follicle gives rise to three important entities:cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.

Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histologic sections of the same developing tooth, which can appear to be different stages.

Numbering systems

Main article: Dental notation

There are several different dental notation systems for associating information to a specific tooth. The three most commons systems are the FDI World Dental Federatiootation, Universal numbering system (dental), and Palmer notation method. The FDI system is used worldwide, and the universal is used widely in the USA.

Although the Palmer notation was supposedly superseded by the FDI World Dental Federatiootation, it overwhelmingly continues to be the preferred method used by dental students and practitioners in the United Kingdom. It was originally termed the “Zsigmondy system” after the Austrian dentist Adolf Zsigmondy who developed the idea in 1861, using a Zsigmondy cross to record quadrants of tooth positions. The Palmer notation consists of a symbol (┘└ ┐┌) designating in which quadrant the tooth is found and a number indicating the position from the midline. Permanent teeth are numbered 1 to 8, and primary teeth are indicated by a letter A to E. The universal numbering system uses a unique letter or number for each tooth. The uppercase letters A through T are used for primary teeth and the numbers 1 – 32 are used for permanent teeth. The tooth designated “1” is the right maxillary third molar and the count continues along the upper teeth to the left side. Then the count begins at the left mandibular third molar, designated number 17, and continues along the bottom teeth to the right side. The FDI system uses a two-digit numbering system in which the first number represents a tooth’s quadrant and the second number represents the number of the tooth from the midline of the face. For permanent teeth, the upper right teeth begin with the number, “1”. The upper left teeth begin with the number, “2”. The lower left teeth begin with the number, “3”. The lower right teeth begin with the number, “4”. For primary teeth, the sequence of numbers goes 5, 6, 7, and 8 for the teeth in the upper right, upper left, lower left, and lower right respectively.

As a result, any given tooth has three different ways to identify it, depending on which notation system is used. The permanent right maxillary central incisor is identified by the number “8” in the universal system. In the FDI system, the same tooth is identified by the number “11”. The palmer system uses the number and symbol, ¹┘, to identify the tooth. Further confusion may result if a number is given on a tooth without assuming (or specifying) a commootation method. Since the number, “12”, may signify the permanent left maxillary first premolar in the universal system or the permanent right maxillary lateral incisor in the FDI system, the notation being used must be clear to prevent confusion.

Victor Haderup of Denmark in 1891 devised a variant of eight tooth quadrant system in which plus(+) and minus(-) were used to differentiate between upper and lower quadrants, and between right and left quadrants (e.g., +1=upper right central incisor; 1-=lower left central incisor). Primary teeth were numbered as upper right (05+ to 01+), lower left (-01 to -05). This system is still taught in Denmark.

Anatomic landmarks

Crown and root

The tooth is attached to the surroundinggingival tissue and alveolar bone (C) by fibrous attachments. The gingival fibers (H)run from the cementum (B) into the gingiva immediately apical to the junctional epithelialattachment and the periodontal ligament fibers (I), (J) and (K) run from the cementum into the adjacent cortex of the alveolar bone.

The term “crown” of a tooth can be used in two ways. The term “anatomic crown” of a tooth refers to the area above the cementoenamel junction (CEJ) or “neck” of the tooth. It is completely covered in enamel. The term “clinical crown” often is convenient in referring to any part of the tooth visible in the mouth, but as a rule the unqualified term “crown” refers to the anatomic crown. The bulk of the crown is composed of dentin, with the pulp chamber within. The crown is enclosed within bone before the tooth erupts, but after eruption the crown is almost always visible in an anatomically normal and clinically healthy mouth.

The anatomic root is found below the cementoenamel junction and is covered with cementum, whereas the clinical root is any part of a tooth not visible in the mouth. Similarly, the anatomic root is assumed in most circumstances. Dentin composes most of the root, which normally has pulp canals. The roots of teeth may be single iumber (single-rooted teeth) or multiple. Canines and most premolars, except for maxillary first premolars, usually have one root. Maxillary first premolars and mandibular molars usually have two roots. Maxillary molars usually have three roots. The tooth is supported in bone by an attachment apparatus, known as the periodontium, which interacts with the root.

Surfaces

Surfaces that are nearest the cheeks or lips are referred to as facial, and those nearest the tongue are known as lingual. Facial surfaces can be subdivided into buccal (when found on posterior teeth nearest the cheeks) and labial (when found on anterior teeth nearest the lips). Lingual surfaces can also be described as palatal when found on maxillary teeth beside the hard palate.

Surfaces that aid in chewing are known as occlusal on posterior teeth and incisal on anterior teeth. Surfaces nearest the junction of the crown and root are referred to as cervical, and those closest to the apex of the root are referred to as apical. The words mesial and distal are also used as descriptions. “Mesial” signifies a surface closer to the median line of the face, which is located on a vertical axis between the eyes, down the nose, and between the contact of the central incisors. Surfaces further away from the median line are described as distal.

Cusp

A cusp is an elevation on an occlusal surface of posterior teeth and canines. It contributes to a significant portion of the tooth’s surface. Canines have one cusp. Maxillary premolars and the mandibular first premolars usually have two cusps. Mandibular second premolars frequently have three cusps— one buccal and two lingual. Maxillary molars have two buccal cusps and two lingual cusps. A fifth cusp that may form on the maxillary first molar is known as the cusp of Carabelli. Mandibular molars may have five or four cusps.

Cingulum

A cingulum is a convexity mesiodistally resembling a girdle,encircling the lingual surface at the cervical third, found on the lingual surface of anterior teeth. It is frequently identifiable as an inverted V-shaped ridge, and its appearance is comparable to a girdle. All anterior teeth are formed from four centers of development, referred to as lobes. Three are located on the facial side of the tooth, and one on the lingual side. The cingulum forms from this lingual lobe of development. The majority of a lingual surface’s cervical third is made up of the cingulum.  On lower incisors, a cingulum usually is poorly developed or absent. Maxillary canines have a large, well-developed cingulum, whereas the cingulum of mandibular canines is smoother and rounded.

Ridges

Ridges are any linear, flat elevations on teeth, and they are named according to their location. The buccal ridge runs cervico-occlusally in approximately the center of the buccal surface of premolars. The labial ridge is one that runs cervico-incisally in approximately the center of the labial surface of canines. The lingual ridge extends from the cingulum to the cusp tip on the lingual surface of most canines. The cervical ridge runs mesiodistally on the cervical third of the buccal surface of the crown. These are found on all primary teeth but only on the permanent molars.

Cusp ridges are ridges that radiate from cusp tips. There are two marginal ridges, mesial and distal, present on all teeth. On anterior teeth, they are located on the mesial and distal borders of the lingual surface; on posterior teeth, they are located on the mesial and distal borders of the occlusal surface. Triangular ridges are those that project from the cusp tips of premolar and molars to the central groove. Transverse ridges are formed by the union of two triangular ridges on posterior teeth. The joining of buccal and lingual triangular ridges is usually named as an example. The oblique ridge is found on the occlusal surfaces 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. The oblique ridges usually forms the distal boundary of the central fossa.

Developmental groove

The teeth demonstrating the least number of developmental grooves are the mandibular central and lateral incisors. However, the canines show the most prominent developmental grooves, because they have strong anchorage to the bone.

Embrasures

Embrasures are triangularly shaped spaces located between the proximal surfaces of adjacent teeth. The borders of embrasures are formed by the interdental papilla of the gingiva, the adjacent teeth, and the contact point where the two teeth meet. There are four embrasures for every contact area: facial (also called labial or buccal), lingual (or palatal), occlusal or incisal, and cervical or interproximal space. The cervical embrasure usually is filled by the interdental papilla from the gingiva; in the absence of adequate gingival tissue a black angle, or Angularis Nigra is visible.

Embrasures have three functions. They form spillways between teeth to direct food away from the gingiva. Also, they provide a mechanism for teeth to be more self cleansing. Lastly, they protect the gingiva from undue frictional trauma but also providing the proper degree of stimulation to the tissues.

Mammelons

Mammelons are usually found as three small bumps on the incisal edges of anterior teeth. They are the remnants of three lobes of formation of these teeth, the fourth lobe represented by the cingulum. Since this surface of the tooth is the first to wear away from attrition, mammelons may not be visible on teeth of older people. Instead, the best chance to see this characteristic is soon after eruption of the tooth into the mouth. Note, the presence of mammelons in adults is an indication of malocclusion.

Distinguishing characteristics of teeth

Incisor

8 incisors are anterior teeth, 4 in the upper arch and 4 in the lower. Their function is for shearing or cutting food during chewing. There are no cusps on the teeth. Instead, the surface area of the tooth used in eating is called the incisal ridge or incisal edge. Though similar, there are some minor differences between the primary and permanent incisors.

Maxillary central incisor

A permanent maxillary central incisor

The maxillary central incisors are usually the most visible teeth, since they are the top center two teeth in the front of a mouth, and they are located mesial to the maxillary lateral incisor.The overall length of the deciduous maxillary central incisor is 16 mm on average, with the crown being 6 mm and the root being 10 mm. In comparison to the permanent maxillary central incisor, the ratio of the root length to the crown length is greater in the deciduous tooth. The diameter of the crown mesiodistally is greater than the length cervicoincisally, which makes the tooth appear wider rather than taller from a labial viewpoint.

The permanent maxillary central incisor is the widest tooth mesiodistally in comparison to any other anterior tooth. It is larger than the neighboring lateral incisor and is usually not as convex on its labial surface. As a result, the central incisor appears to be more rectangular or square in shape. The mesial incisal angle is sharper than the distal incisal angle. When this tooth is newly erupted into the mouth, the incisal edges have three rounded features called mammelons. Mammelons disappear with time as the enamel wears away by friction.

Maxillary lateral incisor

The maxillary lateral incisor is the tooth located distally from both maxillary central incisors of the mouth and mesially from both maxillary canines.

Mandibular central incisor

The mandibular central incisor is the tooth located on the jaw, adjacent to the midline of the face. It is mesial from both mandibular lateral incisors.

Mandibular lateral incisor

The mandibular lateral incisor is the tooth located distally from both mandibular central incisors of the mouth and mesially from both manibular canines.

Canine

Both the maxillary and mandibular canines are called the “cornerstone” of the mouth because they are all located three teeth away from the midline, and separate the premolars from the incisors. The location of the canines reflect their dual function as they complement both the premolars and incisors during chewing. Nonetheless, the most common action of the canines is tearing of food. There is a single cusp on canines, and they resemble the prehensile teeth found in carnivorous animals. Though similar, there are some minor differences between the deciduous and permanent canines.

Maxillary canine

The maxillary canine is the tooth located laterally from both maxillary lateral incisors of the mouth but mesially from both maxillary first premolars. It is the longest tooth in total length, from root to the incisal edge, in the mouth.

Mandibular canine

The mandibular canine is the tooth located distally from both mandibular lateral incisors of the mouth but mesially from both mandibular first premolars.

Premolar

Premolars are found distal to canines and mesial to molars. They are divided into first and second premolars. The functions of premolars vary. There are no deciduous premolars. Instead, the teeth that precede the permanent premolars are the deciduous molars.

Maxillary first premolar

The maxillary first premolar is the tooth located laterally from both the maxillary canines of the mouth but mesially from both maxillary second premolars. The function of this premolar is similar to that of canines in regard to tearing being the principal action during chewing. There are two cusps on maxillary first premolars, and the buccal cusp is sharp enough to resemble the prehensile teeth found in carnivorous animals. There is a distinctive concavity on the cervical third of the crown extending onto the root.

Maxillary second premolar

The maxillary second premolar is the tooth located laterally from both the maxillary first premolars of the mouth but mesially from both maxillary first molars. The function of this premolar is similar to that of first molars in regard to grinding being the principal action during chewing. There are two cusps on maxillary second premolars, but both of them are less sharp than those of the maxillary first premolars.

Mandibular first premolar

The mandibular first premolar is the tooth located laterally from both the mandibular canines of the mouth but mesially from both mandibular second premolars. The function of this premolar is similar to that of canines in regard to tearing being the principal action during mastication. Mandibular first premolars have two cusps. The one large and sharp is located on the buccal side of the tooth. Since the lingual cusp is small and nonfunctional, which means it is not active in chewing, the mandibular first premolar resembles a small canine.

Mandibular second premolar

The mandibular second premolar is the tooth located distally from both the mandibular first premolars of the mouth but mesially from both mandibular first molars. The function of this premolar is to assist the mandibular first molar during mastication. Mandibular second premolars have three cusps. There is one large cusp on the buccal side of the tooth. The lingual cusps are well developed and functional, which means the cusps assist during chewing. Therefore, whereas the mandibular first premolar resembles a small canine, the mandibular second premolar is more like the first molar.

Molar

Molars are the most posterior teeth in the mouth. Their function is to grind food during chewing. The number of cusps, and thus the overall appearance, vary among the different molars and between people. There are great differences between the deciduous molars and those of the permanent molars, even though their functions are similar. Permanent maxillary molars are not considered to have any teeth that precede them. Despite being named “molars”, the deciduous molars are followed by permanent premolars. The third molars are commonly called “wisdom teeth.”

Maxillary first molar

The maxillary first molar is the tooth located laterally from both the maxillary second premolars of the mouth but mesially from both maxillary second molars. There are usually four cusps on maxillary molars, two on the buccal and two palatal.

Maxillary second molar

The maxillary second molar is the tooth located laterally from both the maxillary first molars of the mouth but mesially from both maxillary third molars. This is true only in permanent teeth. In deciduous teeth, the maxillary second molar is the last tooth in the mouth and does not have a third molar behind it. The deciduous maxillary second molar is also the most likely of the deciduous teeth to have an oblique ridge. There are usually four cusps on maxillary molars, two buccal and two palatal.

Maxillary third molar

The maxillary third molar is the tooth located laterally from both the maxillary second molars of the mouth with no tooth posterior to it in permanent teeth. In deciduous teeth, there is no maxillary third molar. There are usually four cusps on maxillary molars, two buccal and two palatal. Nonetheless, for this tooth, there are great variances among third molars, and a specific description of a third molar will not hold true in all cases.

Mandibular first molar

The mandibular first molar is the tooth located distally from both the mandibular second premolars of the mouth but mesially from both mandibular second molars. It is located on the mandibular arch of the mouth, and generally opposes the maxillary first molars and the maxillary 2nd premolar. This arrangement is known as Class I occlusion. There are usually five well-developed cusps on mandibular first molars: two on the buccal, two palatal, and one distal.

Mandibular second molar

The mandibular second molar is the tooth located distally from both the mandibular first molars of the mouth but mesially from both mandibular third molars. This is true only in permanent teeth. In deciduous teeth, the mandibular second molar is the last tooth in the mouth and does not have a third molar behind it. Though there is more variation between individuals to that of the first mandibular molar, there are usually four cusps on mandibular second molars: two buccal and two palatal.

Mandibular third molar

The mandibular third molar is the tooth located distally from both the mandibular second molars of the mouth with no tooth posterior to it in permanent teeth. In deciduous teeth, there is no mandibular third molar. For this tooth, there are great variances among third molars, and a specific description of a third molar will not hold true in all cases.

The following chart shows when your child’s primary teeth (also called baby teeth or deciduous teeth) should erupt and shed. Eruption times vary from child to child.

As seen from the chart, the first teeth begin to break through the gums at about 6 months of age. Usually, the first two teeth to erupt are the two bottom central incisors (the two bottom front teeth). Next, the top four front teeth emerge. After that, other teeth slowly begin to fill in, usually in pairs — one each side of the upper or lower jaw — until all 20 teeth (10 in the upper jaw and 10 in the lower jaw) have come in by the time the child is 2 ½ to 3 years old. The complete set of primary teeth is in the mouth from the age of 2 ½ to 3 years of age to 6 to 7 years of age.

Primary Teeth Development Chart
Upper Teeth	When tooth emerges	When tooth falls out
Central incisor	8 to 12 months	6 to 7 years
Lateral incisor	9 to 13 months	7 to 8 years
Canine (cuspid)	16 to 22 months	10 to 12 years
First molar	13 to 19 months	9 to 11 years
Second molar	25 to 33 months	10 to 12 years
Lower Teeth
Second molar	23 to 31 months	10 to 12 years
First molar	14 to 18 months	9 to 11 years
Canine (cuspid)	17 to 23 months	9 to 12 years
Lateral incisor	10 to 16 months	7 to 8 years
Central incisor	6 to 10 months	6 to 7 years

                                                                                                                          

Other primary tooth eruption facts:

·                     A general rule of thumb is that for every 6 months of life, approximately 4 teeth will erupt.

·                     Girls generally precede boys in tooth eruption.

·                     Lower teeth usually erupt before upper teeth.

·                     Teeth in both jaws usually erupt in pairs — one on the right and one on the left.

·                     Primary teeth are smaller in size and whiter in color than the permanent teeth that will follow.

·                     By the time a child is 2 to 3 years of age, all primary teeth should have erupted.

Shortly after age 4, the jaw and facial bones of the child begin to grow, creating spaces between the primary teeth. This is a perfectly natural growth process that provides the necessary space for the larger permanent teeth to emerge. Between the ages of 6 and 12, amixture of both primary teeth and permanent teeth reside in the mouth.

Anatomy and development of the mouth and teeth:

Children’s teeth begin developing in the fetus. Good nutrition from the mother during pregnancy is important in the development of the teeth. The mother’s diet should have adequate amounts of calcium, phosphorus, vitamin C, and vitamin D. Certain medications, such as tetracycline, should not be taken by the mother while she is pregnant as this can cause harm to the developing teeth of the embryo. There are four main stages of development of the tooth:
The first stage begins in the fetus at about 6 weeks of age. This is when the basic substance of the tooth forms.

• Next, the hard tissue that surrounds the teeth is formed, around 3 to 4 months of gestation.

• After the child is born, the next stage occurs when the tooth actually protrudes through the gum.

• Finally, there is the loss of the primary “baby” teeth.

Parts of the tooth:

Each tooth has four main parts, including the following:
enamel – the outer layer of the tooth.

dentin – the inner layer and the main part of the tooth.

pulp – part of the inside of the tooth that contains the nerve.

root – the part of the tooth that secures it into the jaw.

The teeth on the upper jaw usually erupt one to two months after the same tooth on the lower jaw. There are a total of 20 primary teeth. Usually, about one tooth erupts per month once the teeth have started coming in. There is normally a space between all the baby teeth. This leaves room for the larger permanent teeth to erupt.
 Eruption of teeth happens at different times for each child.

Teeth of humans are small, calcified, hard, whitish structures found in the mouth. They function in mechanically breaking down items of food by cutting and crushing them in preparation for swallowing and digestion. The roots of teeth are embedded in the maxilla (upper jaw) or the mandible(lower jaw) and are covered by gums. Teeth are made of multiple tissues of varying density and hardness.

Teeth are among the most distinctive (and long-lasting) features of mammal species. Humans, like other mammals, are diphyodont, meaning that they develop two sets of teeth. The first set (also called the “baby”, “milk”, “primary”, and “deciduous” set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months is known as teething and can be painful.

Dental trauma refers to trauma to the face, mouth, and especially the teeth, lips and periodontium. The study of dental trauma is called dental traumatology.

Permanent teeth of right half of lowerdental arch, seen from above.

Anatomy

A third molar.

Main article: Dental anatomy

Dental anatomy is a field of anatomy dedicated to the study of tooth structure. The development, appearance, and classification of teeth fall within its field of study, though dental occlusion, or contact among teeth, does not. Dental anatomy is also a taxonomic science as it is concerned with the naming of teeth and their structures. This information serves a practical purpose for dentists, enabling them to easily identify teeth and structures during treatment.

The anatomic crown of a tooth is the area covered in enamel above the cementoenamel junction (CEJ) or “neck” of the tooth. Most of the crown is composed of dentin (dentine in British English) with the pulp chamber inside. The crown is within bone before eruption. After eruption, it is almost always visible. The anatomic root is found below the CEJ and is covered with cementum. As with the crown, dentin composes most of the root, which normally have pulp canals. A tooth may have multiple roots or just one root (single-rooted teeth). Canines and most premolars, except for maxillary first premolars, usually have one root. Maxillary first premolars and mandibular molars usually have two roots. Maxillary molars usually have three roots. Additional roots are referred to as supernumerary roots.

Models of human teeth as they exist within the alveolar bone.

Humans usually have 20 primary (deciduous, “baby” or “milk”) teeth and 32 permanent (adult) teeth. Teeth are classified as incisors, canines,premolars (also called bicuspids), and molars. Incisors are primarily used for biting pieces from foods such as raw carrots or apples and peeled but uncut bananas, while molars are used primarily for grinding foods after they are already in bite size pieces inside the mouth.

Most teeth have identifiable features that distinguish them from others. There are several different notation systems to refer to a specific tooth. The three most common systems are the FDI World Dental Federatiootation, the universal numbering system, and Palmer notation method. The FDI system is used worldwide, and the universal is used widely in the United States.

Anatomy

A third molar.

Main article: Dental anatomy

Dental anatomy is a field of anatomy dedicated to the study of tooth structure. The development, appearance, and classification of teeth fall within its field of study, though dental occlusion, or contact among teeth, does not. Dental anatomy is also a taxonomic science as it is concerned with the naming of teeth and their structures. This information serves a practical purpose for dentists, enabling them to easily identify teeth and structures during treatment.

The anatomic crown of a tooth is the area covered in enamel above the cementoenamel junction (CEJ) or “neck” of the tooth. Most of the crown is composed of dentin (dentine in British English) with the pulp chamber inside. The crown is within bone before eruption. After eruption, it is almost always visible. The anatomic root is found below the CEJ and is covered with cementum. As with the crown, dentin composes most of the root, which normally have pulp canals. A tooth may have multiple roots or just one root (single-rooted teeth). Canines and most premolars, except for maxillary first premolars, usually have one root. Maxillary first premolars and mandibular molars usually have two roots. Maxillary molars usually have three roots. Additional roots are referred to as supernumerary roots.

Models of human teeth as they exist within the alveolar bone.

Humans usually have 20 primary (deciduous, “baby” or “milk”) teeth and 32 permanent (adult) teeth. Teeth are classified as incisors, canines,premolars (also called bicuspids),^] and molars. Incisors are primarily used for biting pieces from foods such as raw carrots or apples and peeled but uncut bananas, while molars are used primarily for grinding foods after they are already in bite size pieces inside the mouth.

Most teeth have identifiable features that distinguish them from others. There are several different notation systems to refer to a specific tooth. The three most common systems are the FDI World Dental Federatiootation, the universal numbering system, and Palmer notation method. The FDI system is used worldwide, and the universal is used widely in the United States.

Primary teeth

Among deciduous (primary) teeth, ten are found in the maxilla (upper jaw) and ten in the mandible (lower jaw), for a total of 20. The dental formula for primary teeth is .

In the primary set of teeth, there are two types of incisors – centrals and laterals, and two types of molars – first and second. All primary teeth are normally later replaced with their permanent counterparts.

Permanent teeth

Among permanent teeth, 16 are found in the maxilla and 16 in the mandible, for a total of 32. The dental formula is .

The maxillary teeth are the maxillary central incisor, maxillary lateral incisor, maxillary canine, maxillary first premolar, maxillary second premolar, maxillary first molar, maxillary second molar, and maxillary third molar. The mandibular teeth are the mandibular central incisor, mandibular lateral incisor, mandibular canine, mandibular first premolar, mandibular second premolar, mandibular first molar, mandibular second molar, and mandibular third molar. Third molars are commonly called “wisdom teeth” and may never erupt into the mouth or form at all. If any additional teeth form, for example, fourth and fifth molars, which are rare, they are referred to as supernumerary teeth (hyperdontia).^] Development of fewer than the usual number of teeth is called hypodontia.

Parts

Section of a human molar

Enamel

Main article: Tooth enamel

Enamel is the hardest and most highly mineralized substance of the body. It is one of the four major tissues which make up the tooth, along withdentin, cementum, and dental pulp. It is normally visible and must be supported by underlying dentin. 96% of enamel consists of mineral, with water and organic material comprising the rest. The normal color of enamel varies from light yellow to grayish white. At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue tone. Since enamel is semitranslucent, the color of dentin and any restorative dental material underneath the enamel strongly affects the appearance of a tooth. Enamel varies in thickness over the surface of the tooth and is often thickest at the cusp, up to 2.5mm, and thinnest at its border, which is seen clinically as the CEJ.

Enamel’s primary mineral is hydroxylapatite, which is a crystalline calcium phosphate. The large amount of minerals in enamel accounts not only for its strength but also for its brittleness. Dentin, which is less mineralized and less brittle, compensates for enamel and is necessary as a support.Unlike dentin and bone, enamel does not contain collagen. Instead, it has two unique classes of proteins called amelogenins and enamelins. While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as framework support among other functions.^[11]

Dentin

Main article: Dentin

Dentin is the substance between enamel or cementum and the pulp chamber. It is secreted by the odontoblasts of the dental pulp. The formation of dentin is known as dentinogenesis. The porous, yellow-hued material is made up of 70% inorganic materials, 20% organic materials, and 10% water by weight. Because it is softer than enamel, it decays more rapidly and is subject to severe cavities if not properly treated, but dentin still acts as a protective layer and supports the crown of the tooth.

Dentin is a mineralized connective tissue with an organic matrix of collagenous proteins. Dentin has microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp cavity to the exterior cementum or enamel border.^[14] The diameter of these tubules range from 2.5 μm near the pulp, to 1.2 μm in the midportion, and 900 nm near the dentino-enamel junction. Although they may have tiny side-branches, the tubules do not intersect with each other. Their length is dictated by the radius of the tooth. The three dimensional configuration of the dentinal tubules is genetically determined.

Cementum

Main article: Cementum

Cementum is a specialized bone like substance covering the root of a tooth. It is approximately 45% inorganic material (mainly hydroxyapatite), 33% organic material (mainly collagen) and 22% water. Cementum is excreted by cementoblasts within the root of the tooth and is thickest at the root apex. Its coloration is yellowish and it is softer than either dentin or enamel. The principal role of cementum is to serve as a medium by which the periodontal ligaments can attach to the tooth for stability. At the cementoenamel junction, the cementum is acellular due to its lack of cellular components, and this acellular type covers at least ⅔ of the root. The more permeable form of cementum, cellular cementum, covers about ⅓ of the root apex

Pulp

Main article: Pulp (tooth)

The dental pulp is the central part of the tooth filled with soft connective tissue. This tissue contains blood vessels and nerves that enter the tooth from a hole at the apex of the root. Along the border between the dentin and the pulp are odontoblasts, which initiate the formation of dentin. Other cells in the pulp include fibroblasts, preodontoblasts, macrophages and T lymphocytes. The pulp is commonly called “the nerve” of the tooth.

Development

Main article: Tooth development

Radiograph of lower right third, second, and first molars in different stages of development.

Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse specieshave teeth, non-human tooth development is largely the same as in humans. For human teeth to have a healthy oral environment, enamel, dentin,cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero. If teeth do not start to develop at or near these times, they will not develop at all.

A significant amount of research has focused on determining the processes that initiate tooth development. It is widely accepted that there is a factor within the tissues of the first branchial arch that is necessary for the development of teeth.

Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histologic sections of the same developing tooth, which can appear to be different stages.

The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth. It is organized into three parts: the enamel organ, the dental papilla and the dental follicle. The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.^[22] These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The growth of cervical loop cells into the deeper tissues forms Hertwig’s Epithelial Root Sheath, which determines a tooth’s root shape. The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.

Eruption

Bottom teeth of a seven-year old, showing primary teeth (left), a lost primary tooth (middle), and a permanent tooth (right)

Tooth eruption in humans is a process in tooth development in which the teeth enter the mouth and become visible. Current research indicates that the periodontal ligaments play an important role in tooth eruption. Primary teeth erupt into the mouth from around six months until two years of age. These teeth are the only ones in the mouth until a person is about six years old. At that time, the first permanent tooth erupts. This stage, during which a person has a combination of primary and permanent teeth, is known as the mixed stage. The mixed stage lasts until the last primary tooth is lost and the remaining permanent teeth erupt into the mouth.

There have been many theories about the cause of tooth eruption. One theory proposes that the developing root of a tooth pushes it into the mouth. Another, known as the cushioned hammock theory, resulted from microscopic study of teeth, which was thought to show a ligament around the root. It was later discovered that the “ligament” was merely an artifact created in the process of preparing the slide. Currently, the most widely held belief is that the periodontal ligaments provide the main impetus for the process.

The onset of primary tooth loss has been found to correlate strongly with somatic and psychological criteria of school readiness.^{[28][29][clarification needed]}

Supporting structures

The periodontium is the supporting structure of a tooth, helping to attach the tooth to surrounding tissues and to allow sensations of touch and pressure. It consists of the cementum, periodontal ligaments, alveolar bone, and gingiva. Of these, cementum is the only one that is a part of a tooth. Periodontal ligaments connect the alveolar bone to the cementum. Alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called an alveolus, or “socket”. Lying over the bone is the gingiva or gum, which is readily visible in the mouth.

Periodontal ligaments

The periodontal ligament is a specialized connective tissue that attaches the cementum of a tooth to the alveolar bone. This tissue covers the root of the tooth within the bone. Each ligament has a width of 0.15–0.38mm, but this size decreases over time. The functions of the periodontal ligaments include attachment of the tooth to the bone, support for the tooth, formation and resorption of bone during tooth movement, sensation, and eruption. The cells of the periodontal ligaments include osteoblasts, osteoclasts, fibroblasts, macrophages, cementoblasts, and epithelial cell rests of Malassez. Consisting of mostly Type I and III collagen, the fibers are grouped in bundles and named according to their location. The groups of fibers are named alveolar crest, horizontal, oblique, periapical, and interradicular fibers. The nerve supply generally enters from the bone apical to the tooth and forms a network around the tooth toward the crest of the gingiva. When pressure is exerted on a tooth, such as during chewing or biting, the tooth moves slightly in its socket and puts tension on the periodontal ligaments. The nerve fibers can then send the information to the central nervous system for interpretation.

Alveolar bone

The alveolar bone is the bone of the jaw which forms the alveolus around teeth. Like any other bone in the human body, alveolar bone is modified throughout life. Osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. As is the case when movement of teeth is attempted through orthodontics, an area of bone under compressive force from a tooth moving toward it has a high osteoclast level, resulting in bone resorption. An area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation.

Gingiva

The gingiva (“gums”) is the mucosal tissue that overlays the jaws. There are three different types of epithelium associated with the gingiva: gingival, junctional, and sulcular epithelium. These three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth. The gingival epithelium is not associated directly with tooth attachment and is visible in the mouth. The junctional epithelium, composed of the basal lamina and hemidesmosomes, forms an attachment to the tooth.The sulcular epithelium is nonkeratinized stratified squamous tissue on the gingiva which touches but is not attached to the tooth.

Tooth decay

Plaque

Plaque is a biofilm consisting of large quantities of various bacteria that form on teeth. If not removed regularly, plaque buildup can lead to periodontal problems such as gingivitis. Given time, plaque can mineralize along the gingiva, forming tartar. The microorganisms that form the biofilm are almost entirely bacteria (mainly streptococcus and anaerobes), with the composition varying by location in the mouth.Streptococcus mutans is the most important bacterium associated with dental caries.

Certain bacteria in the mouth live off the remains of foods, especially sugars and starches. In the absence of oxygen they produce lactic acid, which dissolves the calcium and phosphorus in the enamel. This process, known as “demineralisation”, leads to tooth destruction. Saliva gradually neutralises the acids which cause the pH of the tooth surface to rise above the critical pH. This causes ‘remineralisation‘, the return of the dissolved minerals to the enamel. If there is sufficient time between the intake of foods then the impact is limited and the teeth can repair themselves. Saliva is unable to penetrate through plaque, however, to neutralize the acid produced by the bacteria.

Caries (cavities)

Advanced tooth decay on a premolar.

Dental caries (cavities), described as “tooth decay”, is an infectious disease which damages the structures of teeth. The disease can lead to pain, tooth loss, and infection. Dental caries has a long history, with evidence showing the disease was present in the Bronze, Iron, and Middle ages but also prior to the neolithicperiod.The largest increases in the prevalence of caries have been associated with diet changes. Today, caries remains one of the most common diseases throughout the world. In the United States, dental caries is the most common chronic childhood disease, being at least five times more common thanasthma. Countries that have experienced an overall decrease in cases of tooth decay continue to have a disparity in the distribution of the disease. Among children in the United States and Europe, 60–80% of cases of dental caries occur in 20% of the population

Tooth decay is caused by certain types of acid-producing bacteria which cause the most damage in the presence of fermentable carbohydrates such as sucrose,fructose, and glucose. The resulting acidic levels in the mouth affect teeth because a tooth’s special mineral content causes it to be sensitive to low pH. Depending on the extent of tooth destruction, various treatments can be used to restore teeth to proper form, function, and aesthetics, but there is no known method to regenerate large amounts of tooth structure. Instead, dental health organizations advocate preventative and prophylactic measures, such as regular oral hygieneand dietary modifications, to avoid dental caries.

Tooth care

Toothbrushes are commonly used to help clean teeth.

Oral hygiene is the practice of keeping the mouth clean and is a means of preventing dental caries, gingivitis, periodontal disease, bad breath, and other dental disorders. It consists of both professional and personal care. Regular cleanings, usually done by dentists and dental hygienists, remove tartar (mineralized plaque) that may develop even with carefulbrushing and flossing. Professional cleaning includes tooth scaling, using various instruments or devices to loosen and remove deposits from teeth.

The purpose of cleaning teeth is to remove plaque, which consists mostly of bacteria.Healthcare professionals recommend regular brushing twice a day (in the morning and in the evening, or after meals) in order to prevent formation of plaque and tartar. A toothbrush is able to remove most plaque, except in areas between teeth. As a result, flossing is also considered a necessity to maintain oral hygiene. When used correctly, dental floss removes plaque from between teeth and at the gum line, where periodontal disease often begins and could develop caries.

Electric toothbrushes are a popular aid to oral hygiene. A user without disabilities, with proper training in manual brushing, and with good motivation, can achieve standards of oral hygiene at least as satisfactory as the best electric brushes, but untrained users rarely achieve anything of the kind. Not all electric toothbrushes are equally effective and even a good desigeeds to be used properly for best effect, but: “Electric toothbrushes tend to help people who are not as good at cleaning teeth and as a result have had oral hygiene problems.” The most important advantage of electric toothbrushes is their ability to aid people with dexterity difficulties, such as those associated with rheumatoid arthritis.

Fluoride therapy is often recommended to protect against dental caries. Water fluoridation and fluoride supplements decrease the incidence of dental caries. Fluoride helps prevent dental decay by binding to the hydroxyapatite crystals in enamel. The incorporated fluoride makes enamel more resistant to demineralization and thus more resistant to decay. Topical fluoride, such as a fluoride toothpaste or mouthwash, is also recommended to protect teeth surfaces. Many dentists include application of topical fluoride solutions as part of routine cleanings.

Restorations

A restored premolar.

After a tooth has been damaged or destroyed, restoration of the missing structure can be achieved with a variety of treatments. Restorations may be created from a variety of materials, including glass ionomer, amalgam, gold, porcelain, and composite. Small restorations placed inside a tooth are referred to as “intracoronal restorations”. These restorations may be formed directly in the mouth or may be cast using the lost-wax technique, such as for some inlays and onlays. When larger portions of a tooth are lost, an “extracoronal restoration” may be fabricated, such as a crown or a veneer, to restore the involved tooth.

When a tooth is lost, dentures, bridges, or implants may be used as replacements.  Dentures are usually the least costly whereas implants are usually the most expensive. Dentures may replace complete arches of the mouth or only a partial number of teeth. Bridges replace smaller spaces of missing teeth and use adjacent teeth to support the restoration. Dental implants may be used to replace a single tooth or a series of teeth. Though implants are the most expensive treatment option, they are often the most desirable restoration because of their aesthetics and function. To improve the function of dentures, implants may be used as support.

Abnormalities

A broken 1 upper front tooth showing the pink of the pulp.

Tooth abnormalities may be categorized according to whether they have environmental or developmental causes. While environmental abnormalities may appear to have an obvious cause, there may not appear to be any known cause for some developmental abnormalities. Environmental forces may affect teeth during development, destroy tooth structure after development, discolor teeth at any stage of development, or alter the course of tooth eruption. Developmental abnormalities most commonly affect the number, size, shape, and structure of teeth.

Environmental

Alteration during tooth development

Tooth abnormalities caused by environmental factors during tooth development have long-lasting effects. Enamel and dentin do not regenerate after they mineralize initially. Enamel hypoplasia is a condition in which the amount of enamel formed is inadequate. This results either in pits and grooves in areas of the tooth or in widespread absence of enamel. Diffuse opacities of enamel does not affect the amount of enamel but changes its appearance. Affected enamel has a different translucency than the rest of the tooth. Demarcated opacities of enamel have sharp boundaries where the translucency decreases and manifest a white, cream, yellow, or brown color. All these may be caused by a systemic event, such as an exanthematous fever.Turner’s hypoplasia is a portion of missing or diminished enamel on a permanent tooth usually from a prior infection of a nearby primary tooth. Hypoplasia may also result from antineoplastic therapy. Dental fluorosis is condition which results from ingesting excessive amounts of fluoride and leads to teeth which are spotted, yellow, brown, black or sometimes pitted. Enamel hypoplasia resulting from syphilis is frequently referred to as Hutchinson’s teeth, which is considered one part of Hutchinson’s triad

Destruction after development

Tooth destruction from processes other than dental caries is considered a normal physiologic process but may become severe enough to become a pathologic condition. Attrition is the loss of tooth structure by mechanical forces from opposing teeth. Attrition initially affects the enamel and, if unchecked, may proceed to the underlying dentin. Abrasion is the loss of tooth structure by mechanical forces from a foreign element. If this force begins at the cementoenamel junction, then progression of tooth loss can be rapid since enamel is very thin in this region of the tooth. A common source of this type of tooth wear is excessive force when using a toothbrush. Erosion is the loss of tooth structure due to chemical dissolution by acids not of bacterial origin. Signs of tooth destruction from erosion is a common characteristic in the mouths of people with bulimia since vomiting results in exposure of the teeth to gastric acids. Another important source of erosive acids are from frequent sucking of lemon juice. Abfraction is the loss of tooth structure from flexural forces. As teeth flex under pressure, the arrangement of teeth touching each other, known as occlusion, causes tension on one side of the tooth and compression on the other side of the tooth. This is believed to cause V-shaped depressions on the side under tension and C-shaped depressions on the side under compression. When tooth destruction occurs at the roots of teeth, the process is referred to as internal resorption, when caused by cells within the pulp, orexternal resorption, when caused by cells in the periodontal ligament.

Discoloration

Discoloration of teeth may result from bacteria stains, tobacco, tea, coffee, foods with an abundance of chlorophyll, restorative materials, and medications. Stains from bacteria may cause colors varying from green to black to orange. Green stains also result from foods with chlorophyll or excessive exposure to copper or nickel. Amalgam, a common dental restorative material, may turn adjacent areas of teeth black or gray. Long term use of chlorhexidine, a mouthwash may encourage extrinsic stain formatioear the gingiva on teeth. This is usually easy for a hygienist to remove. Systemic disorders also can cause tooth discoloration. Congenital erythropoietic porphyria causes porphyrins to be deposited in teeth, causing a red-brown coloration. Blue discoloration may occur with alkaptonuria and rarely with Parkinson’s disease. Erythroblastosis fetalis and biliary atresia are diseases which may cause teeth to appear green from the deposition of biliverdin. Also, trauma may change a tooth to a pink, yellow, or dark gray color. Pink and red discolorations are also associated in patients with lepromatous leprosy. Some medications, such astetracycline antibiotics, may become incorporated into the structure of a tooth, causing intrinsic staining of the teeth.

Alteration of eruption

Tooth eruption may be altered by some environmental factors. When eruption is prematurely stopped, the tooth is said to be impacted. The most common cause of tooth impaction is lack of space in the mouth for the tooth. Other causes may be tumors, cysts, trauma, and thickened bone or soft tissue. Ankylosis of a tooth occurs when the tooth has already erupted into the mouth but the cementum or dentin has fused with the alveolar bone. This may cause a person to retain their primary tooth instead of having it replaced by a permanent one.

A technique for altering the natural progression of eruption is employed by orthodontists who wish to delay or speed up the eruption of certain teeth for reasons of space maintenance or otherwise preventing crowding and/or spacing. If a primary tooth is extracted before its succeeding permanent tooth’s root reaches ⅓ of its total growth, the eruption of the permanent tooth will be delayed. Conversely, if the roots of the permanent tooth are more than ⅔ complete, the eruption of the permanent tooth will be accelerated. Between ⅓ and ⅔, it is unknown exactly what will occur to the speed of eruption.

Developmental

Abnormality iumber

Some systemic disorders which may result in hyperdontia include Apert syndrome, Cleidocranial dysostosis, Crouzon syndrome, Ehlers–Danlos syndrome, Gardner syndrome, and Sturge–Weber syndrome. Some systemic disorders which may result in hypodontia include Crouzon syndrome, Ectodermal dysplasia, Ehlers–Danlos syndrome, and Gorlin syndrome.

Abnormality in size

Microdontia of a single tooth is more likely to occur in a maxillary lateral incisor. The second most likely tooth to have microdontia are third molars. Macrodontia of all the teeth is known to occur in pituitary gigantism and pineal hyperplasia. It may also occur on one side of the face in cases of hemifacial hyperplasia.