![Blausen 0863 ToothAnatomy 02](https://upload.wikimedia.org/wikipedia/commons/9/99/Blausen_0863_ToothAnatomy_02.png)
Tooth enamel is one of the four major
tissues that make up the
tooth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the
crown
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
. The other major tissues are
dentin
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...
,
cementum
Cementum is a specialized calcified substance covering the root of a tooth. The cementum is the part of the periodontium that attaches the teeth to the alveolar bone by anchoring the periodontal ligament.Illustrated Dental Embryology, Histology, a ...
, and
dental pulp
The pulp is the connective tissue, nerves, blood vessels, and odontoblasts that comprise the innermost layer of a tooth. The pulp's activity and signalling processes regulate its behaviour.
Anatomy
The pulp is the neurovascular bundle centr ...
. It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink. Calcium hardens the tooth enamel. In rare circumstances enamel fails to form, leaving the underlying dentin exposed on the surface.
Features
Enamel is the hardest substance in the human body and contains the highest percentage of minerals (at 96%),
[Ross ''et al.'', p. 485] with water and organic material composing the rest.
[Ten Cate's Oral Histology, Nancy, Elsevier, pp. 70–94] The primary mineral is
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
, which is a
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line
calcium phosphate
The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are whi ...
.
Enamel is formed on the tooth while the tooth develops within the jaw bone before it
erupts into the mouth. Once fully formed, enamel does not contain blood vessels or nerves, and is not made of cells.
Remineralisation of teeth
Tooth remineralization is the natural repair process for non-cavitated tooth lesions, in which calcium, phosphate and sometimes fluoride ions are deposited into crystal voids in demineralised enamel. Remineralization can contribute towards res ...
can repair damage to the tooth to a certain degree but damage beyond that cannot be repaired by the body. The maintenance and repair of human tooth enamel is one of the primary concerns of
dentistry
Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions o ...
.
In humans, enamel varies in thickness over the surface of the tooth, often thickest at the
cusp
A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth.
Cusp or CUSP may also refer to:
Mathematics
* Cusp (singularity), a singular point of a curve
* Cusp catastrophe, a branch of bifurc ...
, up to 2.5 mm, and thinnest at its border with the
cementum
Cementum is a specialized calcified substance covering the root of a tooth. The cementum is the part of the periodontium that attaches the teeth to the alveolar bone by anchoring the periodontal ligament.Illustrated Dental Embryology, Histology, a ...
at the
cementoenamel junction
The cementoenamel junction, frequently abbreviated as the CEJ, is a slightly visible anatomical border identified on a tooth. It is the location where the enamel, which covers the anatomical crown of a tooth, and the cementum, which covers the ...
(CEJ).
[Ten Cate's Oral Histology, Nanci, Elsevier, 2013, p. 122]
The normal color of enamel varies from light yellow to grayish (bluish) white .It has been suggested that the color is determined by differences in the translucency of enamel, yellowish teeth having a thin, translucent enamel through which the yellow color of the dentin is visible and grayish teeth having a more opaque enamel. The translucency may be attributable to variations in the degree of calcification and homogeneity of the enamel. . At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue or translucent off-white tone, easily observable on the
upper incisors
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wh ...
. Since enamel is
semitranslucent, the color of dentin and any material underneath the enamel strongly affects the
appearance
Appearance may refer to:
* Visual appearance, the way in which objects reflect and transmit light
* Human physical appearance, what someone looks like
* ''Appearances'' (film), a 1921 film directed by Donald Crisp
* Appearance (philosophy), or p ...
of a tooth. The enamel on primary teeth has a more opaque crystalline form and thus appears whiter than on permanent teeth.
The large amount of mineral in enamel accounts not only for its strength but also for its brittleness.
[Ten Cate's Oral Histology, Nanci, Elsevier, pp. 70–94] Tooth enamel ranks 5 on
Mohs hardness scale
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by th ...
(between steel and titanium) and has a
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
of 83 GPa.
Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support.
[Johnson] On radiographs, the differences in the mineralization of different portions of the tooth and surrounding periodontium can be noted; enamel appears lighter than dentin or pulp since it is denser than both and more
radiopaque
Radiodensity (or radiopacity) is opacity to the radio wave and X-ray portion of the electromagnetic spectrum: that is, the relative inability of those kinds of electromagnetic radiation to pass through a particular material. Radiolucency or hypod ...
.
[Illustrated Dental Embryology, Histology, and Anatomy, Bath-BaloghFehrenbach, Elsevier, 2011, p. 180]
Enamel does not contain
collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
, as found in other hard tissues such as dentin and
bone
A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
, but it does contain two unique classes of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s:
amelogenin
Amelogenins are a group of protein isoforms produced by alternative splicing or proteolysis from the ''AMELX'' gene, on the X chromosome, and also the ''AMELY'' gene in males, on the Y chromosome. They are involved in amelogenesis, the developme ...
s and
enamelin
Enamelin is an enamel matrix protein (EMPs), that in humans is encoded by the ''ENAM'' gene. It is part of the non- amelogenins, which comprise 10% of the total enamel matrix proteins. It is one of the key proteins thought to be involved in ame ...
s. While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework for minerals to form on, among other functions.
Once it is mature, enamel is almost totally without the softer organic matter. Enamel is avascular and has no nerve supply within it and is not renewed, however, it is not a static tissue as it can undergo mineralization changes.
Structure
![Labeledandfulltooth](https://upload.wikimedia.org/wikipedia/commons/a/a1/Labeledandfulltooth.jpg)
The basic unit of enamel is called an
enamel rod
An enamel prism, or enamel rod, is the basic unit of tooth enamel. Measuring 3-6 μm in diameter, enamel prism are tightly packed hydroxyapatite crystals structures. The hydroxyapatite crystals are hexagonal in shape, providing rigidity to the p ...
.
Measuring 4–8
μm in diameter, an enamel rod, formally called an enamel prism, is a tightly packed mass of
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
crystallites in an organized pattern.
In cross section, it is best compared to a keyhole, with the top, or head, oriented toward the crown of the tooth, and the bottom, or tail, oriented toward the root of the tooth.
The arrangement of the crystallites within each enamel rod is highly complex. Both
ameloblast
Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown.
Structure
Each ameloblast is a columnar cell approximately 4 micrometers in ...
s (the cells which initiate enamel formation) and
Tomes' process Tomes's processes (also called Tomes processes) are a histologic landmark identified on an ameloblast, cells involved in the production of tooth enamel. During the synthesis of enamel, the ameloblast moves away from the enamel, forming a projectio ...
es affect the crystallites' pattern. Enamel crystallites in the head of the enamel rod are oriented parallel to the long axis of the rod.
When found in the tail of the enamel rod, the crystallites' orientation diverges slightly (65 degrees) from the long axis.
The arrangement of enamel rods is understood more clearly than their internal structure. Enamel rods are found in rows along the tooth, and within each row, the long axis of the enamel rod is generally perpendicular to the underlying dentin.
[Ten Cate's Oral Histology, Nanci, Elsevier, 2013, pp. 122–128] In permanent teeth, the enamel rods near the cementoenamel junction (CEJ) tilt slightly toward the root of the tooth. Understanding enamel orientation is very important in restorative dentistry, because enamel unsupported by underlying dentin is prone to fracture.
![Enamel and dentine - ground section](https://upload.wikimedia.org/wikipedia/commons/5/56/Enamel_and_dentine_-_ground_section.jpg)
The area around the enamel rod is known as
interrod enamel Interrod enamel is histologically identified on microscopic views of tooth enamel. Because interrod enamel is located around enamel rods, the areas of interrod enamel enhances the "keyhole" appearance of enamel rods by acting as its border. The lo ...
. Interrod enamel has the same composition as enamel rod, however a
histologic
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
distinction is made between the two because crystallite orientation is different in each.
The border where the crystallites of enamel rods and crystallites of interrod enamel meet is called the
rod sheath Rod sheath is an area identified in histologic sections of a tooth. It is found where enamel rods, the functional unit of enamel, meet interrod enamel. The crystals of both types of enamel meet at sharp angles and form the appearance of a space ...
.
Striae of Retzius
The striae of Retzius are incremental growth lines or bands seen in tooth enamel. They represent the incremental pattern of enamel, the successive apposition of different layers of enamel during crown formation.
There are 3 types of incremental ...
are incremental lines that appear brown in a stained section of mature enamel. These lines are composed of bands or cross striations on the enamel rods that, when combined in longitudinal sections, seem to traverse the enamel rods.
Formed from changes in diameter of Tomes' processes, these incremental lines demonstrate the growth of enamel, similar to the annual rings on a tree on transverse sections of enamel. The exact mechanism that produces these lines is still being debated. Some researchers hypothesize that the lines are a result of the diurnal (circadian), or 24-hour, metabolic rhythm of the ameloblasts producing the enamel matrix, which consists of an active secretory work period followed by an inactive rest period during tooth development. Thus, each band on the enamel rod demonstrates the work/rest pattern of the ameloblasts that generally occurs over a span of a week.
[Bath-Balogh, Fehrenbach, p. 186]
Perikymata
Perikymata (Greek language, Greek plural of περικύμα, perikyma) are incremental growth lines that appear on the surface of tooth enamel as a series of linear grooves. In anatomically modern humans, each perikyma takes approximately 6–12 ...
which are associated with the Striae are shallow grooves noted clinically on the nonmasticatory surfaces of some teeth in the oral cavity.
Perikymata are usually lost through tooth wear, except on the protected cervical regions of some teeth, especially the permanent maxillary central incisors, canines, and first premolars, and may be confused as dental calculus.
Darker than the other incremental lines, the
neonatal line The neonatal line is a particular band of incremental growth lines seen in histologic sections of both enamel and dentin of primary teeth. It belongs to a series of a growth lines in tooth enamel known as the Striae of Retzius denoting the prolonged ...
is an incremental line that separates enamel formed before and after birth. The neonatal line marks the stress or trauma experienced by the ameloblasts during birth, again illustrating the sensitivity of the ameloblasts as they form enamel matrix. As one would expect, the neonatal line is found in all primary teeth and in the larger cusps of the permanent first molars. They contain irregular structures of enamel prisms with disordered crystallite arrangements basically formed by the abrupt bending of the prisms towards the root; usually, the prisms gradually bent back again to regain their previous orientation.
Gnarled enamel is found at the cusps of teeth.
Its twisted appearance results from the orientation of enamel rods and the rows in which they lie.
Development
![Toothhistology11-17-05](https://upload.wikimedia.org/wikipedia/commons/6/66/Toothhistology11-17-05.jpg)
Enamel formation is part of the overall process of
tooth development
Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate stag ...
. Under a microscope, different cellular aggregations are identifiable within the tissues of a developing tooth, including structures known as the
enamel organ
The enamel organ, also known as the dental organ, is a cellular aggregation seen in a developing tooth and it lies above the dental papilla. The enamel organ which is differentiated from the primitive oral epithelium lining the stomodeum.The enam ...
,
dental lamina
The dental lamina is a band of epithelial tissue seen in histologic sections of a developing tooth. The dental lamina is first evidence of tooth development and begins (in humans) at the sixth week in utero or three weeks after the rupture of th ...
, and
dental papilla
In embryology and prenatal development, the dental papilla is a condensation of ectomesenchymal cells called odontoblasts, seen in histologic sections of a developing tooth. It lies below a cellular aggregation known as the enamel organ. The ...
.
[Ross ''et al.'', p. 443] The generally recognized stages of tooth development are the bud stage, cap stage, bell stage, and crown, or calcification, stage. Enamel formation is first seen in the crown stage.
Amelogenesis
Amelogenesis is the formation of enamel on teeth and begins when the crown is forming during the advanced bell stage of tooth development after dentinogenesis forms a first layer of dentin. Dentin must be present for enamel to be formed. Amelobla ...
, or enamel formation, occurs after the first establishment of dentin, via cells known as ameloblasts. Human enamel forms at a rate of around 4 μm per day, beginning at the future location of cusps, around the third or fourth month of pregnancy.
As in all human processes, the creation of enamel is complex, but can generally be divided into two stages.
The first stage, called the secretory stage, involves proteins and an organic matrix forming a partially mineralized enamel. The second stage, called the maturation stage, completes enamel mineralization.
![Enamelmineralization11-17-05](https://upload.wikimedia.org/wikipedia/commons/1/1c/Enamelmineralization11-17-05.jpg)
In the secretory stage, ameloblasts are polarized columnar
cells. In the
rough endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
of these cells, enamel proteins are released into the surrounding area and contribute to what is known as the enamel matrix, which is then partially mineralized by the enzyme
alkaline phosphatase
The enzyme alkaline phosphatase (EC 3.1.3.1, alkaline phosphomonoesterase; phosphomonoesterase; glycerophosphatase; alkaline phosphohydrolase; alkaline phenyl phosphatase; orthophosphoric-monoester phosphohydrolase (alkaline optimum), systematic ...
.
[Ross ''et al.'', p. 445] When this first layer is formed, the ameloblasts move away from the dentin, allowing for the development of Tomes' processes at the apical pole of the cell. Enamel formation continues around the adjoining ameloblasts, resulting in a walled area, or pit, that houses a Tomes' process, and also around the end of each Tomes' process, resulting in a deposition of enamel matrix inside of each pit.
The matrix within the pit will eventually become an enamel rod, and the walls will eventually become interrod enamel. The only distinguishing factor between the two is the orientation of the calcium phosphate crystallites.
In the maturation stage, the ameloblasts transport substances used in the formation of enamel. Histologically, the most notable aspect of this phase is that these cells become striated, or have a ruffled border.
These signs demonstrate that the ameloblasts have changed their function from production, as in the secretory stage, to transportation. Proteins used for the final mineralization process compose most of the transported material. The noteworthy proteins involved are
amelogenin
Amelogenins are a group of protein isoforms produced by alternative splicing or proteolysis from the ''AMELX'' gene, on the X chromosome, and also the ''AMELY'' gene in males, on the Y chromosome. They are involved in amelogenesis, the developme ...
s,
ameloblastin
Ameloblastin (abbreviated AMBN and also known as Sheathlin or Amelin) is an enamel matrix protein that in humans is encoded by the AMBN gene.
Function
Ameloblastin is a specific protein found in tooth enamel. Although less than 5% of enamel con ...
s,
enamelin
Enamelin is an enamel matrix protein (EMPs), that in humans is encoded by the ''ENAM'' gene. It is part of the non- amelogenins, which comprise 10% of the total enamel matrix proteins. It is one of the key proteins thought to be involved in ame ...
s, and
tuftelin Tuftelin is an acidic phosphorylated glycoprotein found in tooth enamel. In humans, the Tuftelin protein is encoded by the ''TUFT1'' gene. It is an acidic protein that is thought to play a role in dental enamel mineralization and is implicated in ...
s. How these proteins are secreted into the enamel structure is still unknown; other proteins, such as the
Wnt signaling
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling ...
components
BCL9
B-cell CLL/lymphoma 9 protein is a protein that in humans is encoded by the ''BCL9'' gene.
Function
BCL9, together with its paralogue gene BCL9L (BCL9 like or BCL9.2), have been extensively studied for their role as transcriptional beta-cateni ...
and
Pygopus
''Pygopus'' is a genus belonging to the family of Australian legless lizards (Pygopodidae). Members of this genus are also commonly called scaly-foot.
Species
Within the genus ''Pygopus'' the following five species are recognized as being valid ...
, have been implicated in this process.
During this process, amelogenins and ameloblastins are removed after use, leaving enamelins and tuftelin in the enamel. By the end of this stage, the enamel has completed its mineralization.
At some point before the tooth erupts into the mouth, but after the maturation stage, the ameloblasts are broken down. Consequently, enamel, unlike many other tissues of the body, has no way to regenerate itself. After destruction of enamel from decay or injury, neither the body nor a dentist can restore the enamel tissue. Enamel can be affected further by non-pathologic processes.
Enamel is covered by various structures in relation to the development of tooth:
:*
Nasmyth membrane or enamel cuticle, structure of embryological origin is composed of
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
which gives rise to the
enamel organ
The enamel organ, also known as the dental organ, is a cellular aggregation seen in a developing tooth and it lies above the dental papilla. The enamel organ which is differentiated from the primitive oral epithelium lining the stomodeum.The enam ...
.
:* Acquired pellicle, structure acquired after tooth eruption is composed of food debris, calculus, dental plaque (organic film).
Enamel loss
The high mineral content of enamel, which makes this tissue the hardest in the human body, also makes it
demineralize in a process that often occurs as
dental caries
Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
, otherwise known as cavities.
Demineralization occurs for several reasons, but the most important cause of tooth decay is the ingestion of
fermentable carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s.
Tooth cavities are caused when acids dissolve tooth enamel: Enamel is also lost through
tooth wear
Tooth wear refers to loss of tooth substance by means other than dental caries. Tooth wear is a very common condition that occurs in approximately 97% of the population. This is a normal physiological process occurring throughout life; but with i ...
and
enamel fracture An enamel fracture, or chip, is a complete fracture of the tooth enamel without the involvement of the dentine and pulp. A fracture occurs when a tooth contacts a hard object with enough force to break a section of enamel. Chips form with minimal pl ...
s.
::Ca
10(PO
4)
6(OH)
2(''s'') + 8H
+(''aq'') → 10Ca
2+(''aq'') + 6HPO
42−(''aq'') + 2H
2O(''l'')
Sugars and acids from
candies,
soft drink
A soft drink (see § Terminology for other names) is a drink that usually contains water (often carbonated), a sweetener, and a natural and/or artificial flavoring. The sweetener may be a sugar, high-fructose corn syrup, fruit juice, a su ...
s, and
fruit juices
Juice is a drink made from the extraction or pressing of the natural liquid contained in fruit and vegetables. It can also refer to liquids that are flavored with concentrate or other biological food sources, such as meat or seafood, such as ...
play a significant role in tooth decay, and consequently in enamel destruction. The mouth contains a great number and variety of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, and when
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
, the most common of sugars, coats the surface of the mouth, some intraoral bacteria interact with it and form
lactic acid
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natu ...
, which decreases the pH in the mouth.
[Ross ''et al.'', p. 453] The critical pH for tooth enamel is generally accepted to be pH 5.5. When acids are present and the critical pH is reached, the hydroxyapatite crystallites of enamel demineralize, allowing for greater bacterial invasion deeper into the tooth. The most important bacterium involved with tooth decay is ''
Streptococcus mutans'', but the number and type of bacteria varies with the progress of tooth destruction.
Furthermore, tooth morphology dictates that the most common site for the initiation of dental caries is in the deep grooves, pits, and fissures of enamel. This is expected because these locations are impossible to reach with a toothbrush and allow for bacteria to reside there. When demineralization of enamel occurs, a dentist can use a sharp instrument, such as a
dental explorer, and "feel a stick" at the location of the decay. As enamel continues to become less mineralized and is unable to prevent the encroachment of bacteria, the underlying dentin becomes affected as well. When dentin, which normally supports enamel, is destroyed by a physiologic condition or by decay, enamel is unable to compensate for its brittleness and breaks away from the tooth easily.
![Topviewtooth](https://upload.wikimedia.org/wikipedia/commons/3/3f/Topviewtooth.jpg)
The extent to which tooth decay is likely, known as
cariogenicity
Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
, depends on factors such as how long the sugar remains in the mouth. Contrary to common belief, it is not the amount of sugar ingested but the frequency of sugar ingestion that is the most important factor in the causation of tooth decay. When the pH in the mouth initially decreases from the ingestion of sugars, the enamel is demineralized and left vulnerable for about 30 minutes. Eating a greater quantity of sugar in one sitting does not increase the time of demineralization. Similarly, eating a lesser quantity of sugar in one sitting does not decrease the time of demineralization. Thus, eating a great quantity of sugar at one time in the day is less detrimental than is a very small quantity ingested in many intervals throughout the day. For example, in terms of oral health, it is better to eat a single
dessert
Dessert is a course (food), course that concludes a meal. The course consists of sweet foods, such as confections, and possibly a beverage such as dessert wine and liqueur. In some parts of the world, such as much of Greece and West Africa, and ...
at dinner time than to snack on a bag of
candy
Candy, also called sweets (British English) or lollies (Australian English
Australian English (AusE, AusEng, AuE, AuEng, en-AU) is the set of varieties of the English language native to Australia. It is the country's common language an ...
throughout the day.
In addition to bacterial invasion, enamel is also susceptible to other destructive forces.
Bruxism, also known as clenching of or grinding on teeth, destroys enamel very quickly. The wear rate of enamel, called
attrition
Attrition may refer to
*Attrition warfare, the military strategy of wearing down the enemy by continual losses in personnel and material
**War of Attrition, fought between Egypt and Israel from 1968 to 1970
**War of attrition (game), a model of agg ...
, is 8 micrometers a year from normal factors. A common misconception is that enamel wears away mostly from chewing, but actually teeth rarely touch during chewing. Furthermore, normal tooth contact is compensated physiologically by the
periodontal ligament
The periodontal ligament, commonly abbreviated as the PDL, is a group of specialized connective tissue fibers that essentially attach a tooth to the alveolar bone within which it sits. It inserts into root cementum one side and onto alveolar b ...
s (pdl) and the arrangement of dental
occlusion
Occlusion may refer to:
Health and fitness
* Occlusion (dentistry), the manner in which the upper and lower teeth come together when the mouth is closed
* Occlusion miliaria, a skin condition
* Occlusive dressing, an air- and water-tight trauma ...
. The truly destructive forces are the
parafunctional movements, as found in bruxism, which can cause irreversible damage to the enamel.
Other nonbacterial processes of enamel destruction include
abrasion (involving foreign elements, such as toothbrushes),
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
(involving chemical processes, such as dissolving by soft drinks or lemon and other juices), and possibly
abfraction
Abfraction is a theoretical concept explaining a loss of tooth structure not caused by tooth decay ( non-carious cervical lesions). It is suggested that these lesions are caused by forces placed on the teeth during biting, eating, chewing and gri ...
(involving compressive and tensile forces).
Though enamel is described as tough, it has a similar
brittleness
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...
to
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
, making it, unlike other natural crack-resistant
laminate structures such as
shell
Shell may refer to:
Architecture and design
* Shell (structure), a thin structure
** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses
** Thin-shell structure
Science Biology
* Seashell, a hard o ...
and
nacre, vulnerable to
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
. In spite of this it can withstand bite forces as high as 1,000
N many times a day during chewing. This resistance is due in part to the microstructure of enamel which contains
enamel tufts
Enamel tufts are hypomineralized ribbon-like structures that run longitudinally to the tooth axis and extend from the dentinoenamel junction (DEJ) one fifth to a third into the enamel. They are called ‘‘tufts’’ due to their wavy look wit ...
that stabilize such fractures at the dentinoenamel junction.
The configuration of the tooth also acts to reduce the
tensile stresses that cause fractures during biting.
Gastroesophageal reflux disease
Gastroesophageal reflux disease (GERD) or gastro-oesophageal reflux disease (GORD) is one of the upper gastrointestinal chronic diseases where stomach content persistently and regularly flows up into the esophagus, resulting in symptoms and/ ...
can also lead to enamel loss, as acid refluxes up the esophagus and into the mouth, occurring most during overnight sleep.
Oral hygiene
Because enamel is vulnerable to demineralization, prevention of tooth decay is the best way to maintain the health of teeth. Most countries have wide use of
toothbrush
A toothbrush is an oral hygiene tool used to clean the teeth, gums, and tongue. It consists of a head of tightly clustered bristles, atop of which toothpaste can be applied, mounted on a handle which facilitates the cleaning of hard-to-reach ar ...
es, which can reduce the number of dental biofilm and food particles on enamel. In isolated societies that do not have access to toothbrushes, it is common for those people to use other objects, such as sticks, to clean their teeth. In between two adjacent teeth,
floss
Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...
is used to wipe the enamel surfaces free of
plaque
Plaque may refer to:
Commemorations or awards
* Commemorative plaque, a plate or tablet fixed to a wall to mark an event, person, etc.
* Memorial Plaque (medallion), issued to next-of-kin of dead British military personnel after World War I
* Pla ...
and food particles to discourage bacterial growth. Although neither floss nor toothbrushes can penetrate the deep grooves and pits of enamel, good general oral-health habits can usually prevent enough bacterial growth to keep tooth decay from starting. Structural integrity of the enamel is genetic, and so is its predisposition to demineralization or attack from bacteria.
Fluoride remineralization
Fluoride catalyzes the diffusion of calcium and phosphate into the tooth surface, which in turn
remineralizes the crystalline structures in a dental cavity. The remineralized tooth surfaces contain fluoridated
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
and
fluorapatite
Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, bl ...
, which resist acid attack much better than the original tooth did.
Fluoride therapy
Fluoride therapy is the use of fluoride for medical purposes. Fluoride supplements are recommended to prevent tooth decay in children older than six months in areas where the drinking water is low in fluoride. It is typically used as a liquid, ...
is used to help prevent dental decay.
![FluorideTrays07-05-05](https://upload.wikimedia.org/wikipedia/commons/f/fb/FluorideTrays07-05-05.jpg)
Fluoride ions, as an antimicrobial, may activate bacterial genes associated with fluoride
riboswitch
In molecular biology, a riboswitch is a regulatory segment of a messenger RNA molecule that binds a small molecule, resulting in a change in production of the proteins encoded by the mRNA. Thus, an mRNA that contains a riboswitch is directly invo ...
es. The combination of fluoride ions and QAS (quaternary ammonium salts) was found to have a stronger antimicrobial effect on many oral bacteria associated with dental decay, including ''
S. mutans
''Streptococcus mutans'' is a facultatively anaerobic, gram-positive coccus (round bacterium) commonly found in the human oral cavity and is a significant contributor to tooth decay.
It is part of the " streptococci" (plural, non-italic lowercase ...
''.
Fluoride in drinking water
Most dental professionals and organizations agree that the inclusion of fluoride in public water has been one of the most effective methods of decreasing the prevalence of tooth decay. Fluoride can be found in many locations naturally, such as the ocean and other water sources. The recommended dosage of fluoride in
drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
does not depend on air temperature.
Some groups have spoken out against
fluoridated drinking water, for reasons such as the neurotoxicity of fluoride or the damage fluoride can do as
fluorosis. Fluorosis is a condition resulting from the overexposure to fluoride, especially between the ages of 6 months and 5 years, and appears as mottled enamel.
Consequently, the teeth look unsightly, although the incidence of dental decay in those teeth is very small. Where fluoride is found naturally in high concentrations, filters are often used to decrease the amount of fluoride in water. For this reason, codes have been developed by dental professionals to limit the amount of fluoride a person should take. These codes are supported by the American Dental Association and the American Academy of Pediatric Dentistry;
Furthermore, whereas topical fluoride, found in
toothpaste
Toothpaste is a paste or gel dentifrice used with a toothbrush to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from the teeth, ...
and
mouthwash
Mouthwash, mouth rinse, oral rinse, or mouth bath is a liquid which is held in the mouth passively or swilled around the mouth by contraction of the perioral muscles and/or movement of the head, and may be gargling, gargled, where the head is ti ...
es, does not cause fluorosis, its effects are now considered more important than those of systemic fluoride, such as when drinking fluorinated water. However, systemic fluoride works topically as well with fluoride levels in saliva increase also when drinking fluoridated water. Lately, dental professionals are looking for other ways to present fluoride (such as in varnish) or other mineralizing products such as
Amorphous calcium phosphate
Amorphous calcium phosphate (ACP) is a glassy solid that is formed from the chemical decomposition of a mixture of dissolved phosphate and calcium salts (e.g. (NH4)2HPO4 + Ca(NO3)2). The resulting amorphous mixture consists mostly of calcium and ph ...
to the community in the form of topical procedures, either done by professionals or self-administered. Mineralization of the incipient lesion instead of restoration later is a prime goal of most dental professionals.
Dental procedures
Dental restorations
Most
dental restoration
Dental restoration, dental fillings, or simply fillings are treatments used to restore the function, integrity, and morphology of missing tooth structure resulting from caries or external trauma as well as to the replacement of such structure sup ...
s involve the removal of enamel. Frequently, the purpose of removal is to gain access to the underlying decay in the
dentin
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...
or inflammation in the
pulp
Pulp may refer to:
* Pulp (fruit), the inner flesh of fruit
Engineering
* Dissolving pulp, highly purified cellulose used in fibre and film manufacture
* Pulp (paper), the fibrous material used to make paper
* Molded pulp, a packaging material
...
. This is typically the case in
amalgam
Amalgam most commonly refers to:
* Amalgam (chemistry), mercury alloy
* Amalgam (dentistry), material of silver tooth fillings
** Bonded amalgam, used in dentistry
Amalgam may also refer to:
* Amalgam Comics, a publisher
* Amalgam Digital
...
restorations and
endodontic treatment
Root canal treatment (also known as endodontic therapy, endodontic treatment, or root canal therapy) is a treatment sequence for the infected pulp of a tooth which is intended to result in the elimination of infection and the protection o ...
.
Nonetheless, enamel can sometimes be removed before there is any decay present. The most popular example is the
dental sealant
Dental sealants (also termed pit and fissure sealants, or simply fissure sealants) are a dental treatment intended to prevent tooth decay. Teeth have recesses on their biting surfaces; the back teeth have fissures (grooves) and some front teeth h ...
. In the past, the process of placing dental sealants involved removing enamel in the deep fissures and grooves of a tooth, followed by replacing it with a restorative material. Presently, it is more common to only remove decayed enamel if present. In spite of this, there are still cases where deep fissures and grooves in enamel are removed in order to prevent decay, and a sealant may or may not be placed depending on the situation. Sealants are unique in that they are preventative restorations for protection from future decay, and have been shown to reduce the risk of decay by 55% over 7 years.
Aesthetics is another reason for the removal of enamel. Removing enamel is necessary when placing
crowns
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
and
veneers to enhance the appearance of teeth. In both of these instances, when unsupported by underlying dentin, that portion of the enamel is more vulnerable to fracture.
Acid-etching techniques
Invented in 1955, acid-etching employs dental etchants and is used frequently when bonding dental restoration to teeth. This is important for long-term use of some materials, such as
composites and
sealants
Sealant is a substance used to block the passage of fluids through openings in materials, a type of mechanical seal. In building construction ''sealant'' is sometimes synonymous with ''caulking'' and also serve the purposes of blocking dust, sou ...
.
By dissolving minerals in enamel, etchants remove the outer 10 micrometers on the enamel surface and make a porous layer 5–50 micrometers deep.
[Summitt ''et al., p. 193] This roughens the enamel microscopically and results in a greater surface area on which to bond.
The effects of acid-etching on enamel can vary. Important variables are the amount of time the etchant is applied, the type of etchant used, and the current condition of the enamel.
There are three types of patterns formed by acid-etching.
Type 1 is a pattern where predominantly the enamel rods are dissolved; type 2 is a pattern where predominantly the area around the enamel rods are dissolved; and type 3 is a pattern where there is no evidence left of any enamel rods. Besides concluding that type 1 is the most favorable pattern and type 3 the least, the explanation for these different patterns is not known for certain but is most commonly attributed to different crystallite orientation in the enamel.
Tooth whitening
The discoloration of teeth over time can result from exposure to substances such as
tobacco
Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
,
coffee
Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulant, stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world.
S ...
, and
tea
Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of ''Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of southwestern China and north ...
. The staining occurs in the interprismatic region internally on the enamel, which causes the tooth to appear darker or more yellow overall. In a perfect state, enamel is colorless, but it does reflect underlying tooth structure with its stains since light reflection properties of the tooth are low.
Tooth whitening or
tooth bleaching
Tooth whitening or tooth bleaching is the process of lightening the color of human teeth. Whitening is often desirable when teeth become yellowed over time for a number of reasons, and can be achieved by changing the intrinsic or extrinsic color o ...
procedures attempt to lighten a tooth's color in either of two ways: by chemical or mechanical action. Working chemically, a bleaching agent is used to carry out an
oxidation reaction
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
in the enamel and dentin. The agents most commonly used to intrinsically change the color of teeth are
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
and
carbamide peroxide
Hydrogen peroxide - urea (also called Hyperol, artizone, urea hydrogen peroxide, and UHP) is a solid composed of equal amounts of hydrogen peroxide and urea. This compound is a white crystalline solid which dissolves in water to give free hydroge ...
. Oxygen radicals from the peroxide in the whitening agents contact the stains in the interprismatic spaces within the enamel layer. When this occurs, stains will be bleached and the teeth now appear lighter in color. Teeth not only appear whiter but also reflect light in increased amounts, which makes the teeth appear brighter as well. Studies show that whitening does not produce any ultrastructural or microhardness changes in the dental tissues.
Studies show that patients who have whitened their teeth take better care of them. However, a tooth whitening product with an overall low pH can put enamel at risk for decay or destruction by demineralization. Consequently, care should be taken and risk evaluated when choosing a product which is very acidic. Tooth whiteners in toothpastes work through a mechanical action. They have mild abrasives which aid in the removal of stains on enamel. Although this can be an effective method, it does not alter the intrinsic color of teeth. Microabrasion techniques employ both methods. An acid is used first to weaken the outer 22–27 micrometers of enamel in order to weaken it enough for the subsequent abrasive force. This allows for removal of superficial stains in the enamel. If the discoloration is deeper or in the dentin, this method of tooth whitening will not be successful.
Related pathology
![Enamel celiac](https://upload.wikimedia.org/wikipedia/commons/6/68/Enamel_celiac.jpg)
There are 14 different types of
amelogenesis imperfecta
Amelogenesis imperfecta (AI) is a congenital disorder which presents with a rare abnormal formation of the enamel or external layer of the crown of teeth, unrelated to any systemic or generalized conditions. Enamel is composed mostly of mineral, ...
.
The
hypocalcification type, which is the most common, is an
autosomal dominant
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
condition that results in enamel that is not completely mineralized.
[Harris, p. 7: see section titled "X-Linked Inheritance"] Consequently, enamel easily flakes off the teeth, which appear yellow because of the revealed dentin. The hypoplastic type is
X-linked
Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, ...
and results in normal enamel that appears in too little quantity, having the same effect as the most common type.
Chronic bilirubin
encephalopathy
Encephalopathy (; from grc, ἐνκέφαλος "brain" + πάθος "suffering") means any disorder or disease of the brain, especially chronic degenerative conditions. In modern usage, encephalopathy does not refer to a single disease, but r ...
, which can result from
erythroblastosis fetalis
Hemolytic disease of the newborn, also known as hemolytic disease of the fetus and newborn, HDN, HDFN, or erythroblastosis foetalis, is an alloimmune condition that develops in a fetus at or around birth, when the IgG molecules (one of the five ...
, is a disease which has numerous effects on an
infant
An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
, but it can also cause enamel hypoplasia and green staining of enamel.
Enamel hypoplasia
Enamel hypoplasia is a defect of the teeth in which the enamel is deficient in quantity, caused by defective enamel matrix formation during enamel development, as a result of inherited and acquired systemic condition(s). It can be identified as m ...
is broadly defined to encompass all deviations from normal enamel in its various degrees of absence. The missing enamel could be localized, forming a small pit, or it could be completely absent.
Erythropoietic
porphyria
Porphyria is a group of liver disorders in which substances called porphyrins build up in the body, negatively affecting the skin or nervous system. The types that affect the nervous system are also known as acute porphyria, as symptoms are ra ...
is a genetic disease resulting in the deposition of
porphyrin
Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical com ...
s throughout the body. These deposits also occur in enamel and leave an appearance described as red in color and fluorescent.
Fluorosis leads to mottled enamel and occurs from overexposure to fluoride.
Tetracycline
Tetracycline, sold under various brand names, is an oral antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis.
Common side effects in ...
staining leads to brown bands on the areas of developing enamel. Children up to age 8 can develop mottled enamel from taking tetracycline. As a result, tetracycline is contraindicated in
pregnant
Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occur ...
women.
Celiac disease
Coeliac disease (British English) or celiac disease (American English) is a long-term autoimmune disorder, primarily affecting the small intestine, where individuals develop intolerance to gluten, present in foods such as wheat, rye and barle ...
, a disorder characterized by an auto-immune response to
gluten
Gluten is a structural protein naturally found in certain cereal grains. Although "gluten" often only refers to wheat proteins, in medical literature it refers to the combination of prolamin and glutelin proteins naturally occurring in all grain ...
, also commonly results in demineralization of the enamel.
Other mammals
For the most part, research has shown that the formation of tooth enamel in animals is almost identical to formation in humans. The enamel organ, including the dental papilla, and ameloblasts function similarly. The variations of enamel that are present are infrequent but sometimes important. Differences exist, certainly, in the morphology, number, and types of teeth among animals.
![RottweilerTeeth01](https://upload.wikimedia.org/wikipedia/commons/0/00/RottweilerTeeth01.jpeg)
Dogs are less likely than humans to have tooth decay due to the high
pH of dog saliva, which prevents an acidic environment from forming and the subsequent demineralization of enamel which would occur.
[Pinney, p. 187] If tooth decay does occur (usually from trauma), dogs can receive dental fillings just as humans do. Similar to human teeth, the enamel of dogs is vulnerable to
tetracycline
Tetracycline, sold under various brand names, is an oral antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis.
Common side effects in ...
staining. Consequently, this risk must be accounted for when tetracycline antibiotic therapy is administered to young dogs.
Enamel hypoplasia may also occur in dogs.
The mineral distribution in
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
enamel is different from that of monkeys, dogs, pigs, and humans. In
horse teeth
Horse teeth refers to the dentition of equine species, including horses and donkeys. Equines are both heterodontous and diphyodontous, which means that they have teeth in more than one shape (there are up to five shapes of tooth in a horse's ...
, the enamel and dentin layers are intertwined with each other, which increases the strength and wear resistance of those teeth.
Other organisms
Enamel or
enameloid
Enameloid, also known as durodentine or vitrodentine, is an Tooth enamel, enamel-like Tissue (biology), tissue found in many fish. It is the primary outer component of shark Odontode, odontodes (teeth and dermal denticles). Although the origin of e ...
is found in the
dermal denticles of
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
s and many early vertebrates, and it appeared there before
gnathostome
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
teeth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
evolved. The
ganoin
Ganoine or ''ganoin'' is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some non-teleost ray-finned fishes, such as gars, bichirs, and coelacanths. It is composed of rod-like, pseudoprismat ...
that covers the scales of many
actinopterygians
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or hor ...
is probably derived from enamel. Enamel-like substances also coat the jaws of some crustacea, but this is not homologous with vertebrate enamel.
Enameloid
Enameloid, also known as durodentine or vitrodentine, is an Tooth enamel, enamel-like Tissue (biology), tissue found in many fish. It is the primary outer component of shark Odontode, odontodes (teeth and dermal denticles). Although the origin of e ...
covers some fish scales.
Mechanical properties
Due to the unique structure of enamel, the mechanical properties are very interesting. Enamel is the hardest material in the body and is one of the most durable load-bearing biological materials. It has been shown to have a
fracture toughness
In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a c ...
three times greater than that of geological
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
, the main mineral in enamel.
Consisting of rod and interrod regions within the microstructure of enamel, the mechanical properties of enamel vary depending on the location within the microstructure.
The rod and interrod structure causes
anisotropy
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
in enamel as both components have different mechanical properties. The interrod enamel has around 53% and 74% decreased
hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
and
elastic modulus
An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...
when compared to the rod structures. This leads to a composite like hierarchical structure of enamel. The hardness and stiffness parallel to the rod axis results in high hardness and modulus, shown to have modulus values of 85-90 GPa and hardness values of 3.4-3.9 GPa.
The hardness and modulus in the direction perpendicular to the rod directions are lower values, shown to have an elastic modulus between 70 and 77 GPa and a hardness value of 3.0-3.5 GPa.
The anisotropy between the two directions can be as high as 30%.
This is due partially to the structure of the material and the directionality of the rods in the c-direction.
The structure of enamel is also
composite
Composite or compositing may refer to:
Materials
* Composite material, a material that is made from several different substances
** Metal matrix composite, composed of metal and other parts
** Cermet, a composite of ceramic and metallic materials
...
in nature between the interrod and rods which also leads to anisotropy. Single crystallite hydroxyapatite, the mineral enamel is based on, is also anisotropic. Single crystallite hydroxyapatite has a higher hardness and young's modulus, which may be due to the defects present in enamel, such as substitutional ions as well as the presence of organic materials.
The mechanical properties of enamel not only are anisotropic due to the structure of the rods and interrods. They are also varying across the length of enamel from the enamel at the surface of the tooth, the outer enamel, to the junction between the dentin and enamel, DEJ. The elastic modulus increases as the distance between the dentin-enamel junction (DEJ) increases within enamel. The fracture toughness is also anisotropic. The fracture toughness can vary by up to a factor of three due to the orientation of the rods. Also in enamel, Cracks do not easily penetrate the dentin, which may lead to the higher fracture toughness.
Overall, enamel is a highly anisotropic material due to its microstructure, which leads to properties needed for the effective use of our teeth.
See also
*
Ameloblast
Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown.
Structure
Each ameloblast is a columnar cell approximately 4 micrometers in ...
*
Odontoblast
In vertebrates, an odontoblast is a cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is dentinogenesis, which is the formation of dentin, the substance beneath the tooth enamel on t ...
*
Amorphous calcium phosphate
Amorphous calcium phosphate (ACP) is a glassy solid that is formed from the chemical decomposition of a mixture of dissolved phosphate and calcium salts (e.g. (NH4)2HPO4 + Ca(NO3)2). The resulting amorphous mixture consists mostly of calcium and ph ...
*
Dental caries
Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
*
Ivory
Ivory is a hard, white material from the tusks (traditionally from elephants) and teeth of animals, that consists mainly of dentine, one of the physical structures of teeth and tusks. The chemical structure of the teeth and tusks of mammals is ...
*
Tooth development
Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate stag ...
References
Bibliography
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External links
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{{Authority control
enamel
Tissues (biology)