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Tooth enamel

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

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   Tooth enamel is the hardest and most highly mineralized substance of
   the body ^, and with dentin, cementum, and dental pulp is one of the
   four major tissues which make up the tooth. It is the normally visible
   dental tissue of a tooth and must be supported by underlying dentin.
   Ninety-six per cent of enamel consists of mineral, with water and
   organic material composing 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 colour 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
   2.5 mm, and thinnest at its border, which is seen clinically as the
   cementoenamel junction (CEJ)^ .

   Enamel's primary mineral is hydroxyapatite, 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 a
   framework support, among other functions ^.

Structure

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   The basic unit of enamel is called an enamel rod ^. Measuring 4 μm - 8
   μm in diameter an enamel rod, formerly called an enamel prism, is a
   tightly packed mass of hydroxyapatite crystals 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 crystals within each enamel rod is highly
   complex. Both ameloblasts (the cells which initiate enamel formation)
   and Tomes' processes affect the crystals' pattern. Enamel crystals 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 crystals'
   orientation diverges slightly 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 ^. 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 ^.

   The area around the enamel rod is known as interrod enamel. Interrod
   enamel has the same composition as enamel rod, however a histologic
   distinction is made between the two because crystal orientation is
   different in each ^. The border where the crystals of enamel rods and
   crystals of interrod enamel meet is called the rod sheath ^.

   Striae of Retzius are stripes that appear on enamel when viewed
   microscopically in cross section ^. Formed from changes in diameter of
   Tomes’ processes, these stripes demonstrate the growth of enamel,
   similar to the annual rings on a tree. Perikymata are shallow furrows
   where the striae of Retzius end ^. Darker than the other stripes, the
   neonatal line is a stripe that separates enamel formed before and after
   birth ^.

   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

   Histologic slide showing a developing tooth. The mouth would be in the
   area of space at the top of the picture.
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   Histologic slide showing a developing tooth. The mouth would be in the
   area of space at the top of the picture.

   Enamel formation is part of the overall process of tooth development.
   When the tissues of the developing tooth are seen under a microscope,
   different cellular aggregations can be identified, including structures
   known as the enamel organ, dental lamina, and dental papilla ^. 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, 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.
   Histologic slide showing enamel formation.
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   Histologic slide showing enamel formation.

   In the secretory stage, ameloblasts are polarized columnar cells. In
   the 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 ^. 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 crystals.

   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 amelogenins, ameloblastins, enamelins, and tuftelins ^.
   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. The discoloration of
   teeth over time can result from exposure to substances such as tobacco,
   coffee, and tea ^. This is partly due to material building up in the
   enamel, but is also an effect of the underlying dentin becoming
   sclerotic ^. As a result, tooth colour gradually darkens with age.
   Additionally, enamel becomes less permeable to fluids, less soluble to
   acid, and contains less water ^.

   CAPTION: Progress of Enamel Formation for Primary Teeth ^

     Amount of Enamel Formed at Birth  Enamel Mineralization Completed
   Primary
   Maxillary
   Tooth Central Incisor 5/6 1.5 months after birth
   Lateral Incisor 2/3 2.5 months after birth
   Canine 1/3 9 months after birth
   1st Molar Cusps united; occlusal completely calcified
   and 1/2 to 3/4 crown height 6 months after birth
   2nd Molar Cusps united; occlusal incompletely calcified;
   calcified tissue covers 1/5 to 1⁄4 crown height 11 months after birth
   Primary
   Mandibular
   Tooth Central Incisor 3/5 2.5 months after birth
   Lateral Incisor 3/5 3 months after birth
   Canine 1/3 9 months after birth
   1st Molar Cusps united; occlusal
   completely calcified 5.5 months after birth
   2nd Molar Cusps united; occlusal
   incompletely calcified 10 months after birth

Destruction

   Destruction of enamel by cervical decay from dental caries.
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   Destruction of enamel by cervical decay from dental caries.

   The high mineral content of enamel, which makes this tissue the hardest
   in the human body, also makes it susceptible to a demineralization
   process which often occurs as dental caries, otherwise known as
   cavities ^. Demineralization occurs for several reasons, but the most
   important cause of tooth decay is the ingestion of sugars. Tooth
   cavities are caused when acids dissolve tooth enamel:^

   Ca[10](PO[4])[6](OH)[2](s) + 8H^+(aq) → 10Ca^2+(aq) + 6HPO[4]^2-(aq) +
   2H[2]O(l)

   Sugars from candies, soft drinks, and even fruit juices play a
   significant role in tooth decay, and consequently in enamel
   destruction. The mouth contains a great number and variety of bacteria,
   and when sucrose, the most common of sugars, coats the surface of the
   mouth, some intraoral bacteria interact with it and form lactic acid,
   which decreases the pH in the mouth. ^. Then, the hydroxyapatite
   crystals 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.
   The effects of bruxism on an anterior tooth, revealing the dentin and
   pulp which are normally hidden by enamel.
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   The effects of bruxism on an anterior tooth, revealing the dentin and
   pulp which are normally hidden by enamel.

   The extent to which tooth decay is likely, known as cariogenicity,
   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 at dinner
   time than to snack on a bag of candy 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, 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 ligaments
   (pdl) and the arrangement of dental occlusion. 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 (involving
   chemical processes, such as lemon juice), and possibly abfraction
   (involving compressive and tensile forces) ^.

Oral hygiene and fluoride

   Considering the vulnerability of enamel to demineralization and the
   daily menace of sugar ingestion, prevention of tooth decay is the best
   way to maintain the health of teeth. Most countries have wide use of
   toothbrushes, which can reduce the number of bacteria and food
   particles on enamel. Some isolated societies do not have access to
   toothbrushes, but it is common for those people to use other objects,
   such as sticks, to clean their teeth. In between two adjacent teeth,
   floss is used to wipe the enamel surfaces free of plaque 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.
   Common dentistry trays filled with fluoride.
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   Common dentistry trays filled with fluoride.

   These methods of oral hygiene have been helped greatly by the use of
   fluoride. Fluoride can be found in many locations naturally, such as
   the ocean and other water sources. Consequently, many seafood dishes
   contain fluoride. The recommended dosage of fluoride in drinking water
   is 1 part per million ( ppm) ^. Fluoride helps prevent dental decay by
   binding to the hydroxyapatite crystals in enamel ^. The incorporated
   fluoride makes enamel more resistant to demineralization and, thus,
   resistant to decay ^. Fluoride therapy is used to help teeth prevent
   dental decay.

   Many groups of people have spoken out against fluoridated drinking
   water. One example used by these advocates is the damage fluoride can
   do as fluorosis. Fluorosis is a condition resulting from the
   overexposure to fluoride, especially between the ages of 6 months to 5
   years, and appears as mottled enamel ^. Consequently, the teeth look
   unsightly and, indeed, the incidence of dental decay in those teeth is
   very small. However, it is important to note that most substances, even
   beneficial ones, are detrimental when taken in extreme doses. 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. The acute toxic dose of fluoride is ~5 mg/kg of body weight.
   Furthermore, whereas topical fluoride, found in toothpaste and
   mouthwashes, does not cause fluorosis, its effects are also less
   pervasive and not as long-lasting as those of systemic fluoride, such
   as when drinking fluorinated water ^. For instance, all of a tooth's
   enamel gains the benefits of fluoride when it is ingested systemically,
   through fluorinated water or salt fluoridation (a common alternative in
   Europe). Only some of the outer surfaces of enamel can be reached by
   topical fluoride. Thus, despite fluoridation's detractors, most dental
   health care 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.

Effects of dental procedures

   An X-ray showing enamel and dentin replaced by an amalgam restoration.
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   An X-ray showing enamel and dentin replaced by an amalgam restoration.

Dental restorations

   Most dental restorations involve the removal of enamel. Frequently, the
   purpose of removal is to gain access to the underlying decay in the
   dentin or inflammation in the pulp. This is typically the case in
   amalgam restorations and endodontic treatment.

   Nonetheless, enamel can sometimes be removed before there is any decay
   present. The most popular example is the dental sealant. The process of
   placing dental sealants in the past involved removing enamel in the
   deep fissures and grooves of a tooth and 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 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 and veneers to enhance the appearance
   of teeth. In both of these instances, it is important to keep in mind
   the orientation of enamel rods because it is possible to leave enamel
   unsupported by underlying dentin, leaving that portion of the prepared
   teeth 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 ^. By dissolving minerals in enamel, etchants remove the outer
   10 micrometers on the enamel surface and makes a porous layer 5–50
   micrometers deep ^. 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 crystal orientation in the enamel ^.

Tooth whitening

   Tooth whitening or tooth bleaching are procedures that attempt to
   lighten a tooth's colour in either of two ways: by chemical or
   mechanical action ^.

   Working chemically, a bleaching agent is used to carry out an oxidation
   reaction in the enamel and dentin ^. The agents most commonly used to
   intrinsically change the colour of teeth are hydrogen peroxide and
   carbamide peroxide ^. 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 colour 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.

Systemic conditions affecting enamel

   There are many different types of Amelogenesis imperfecta. The
   hypocalcification type, which is the most common, is an autosomal
   dominant condition that results in enamel that is not completely
   mineralized ^. Consequently, enamel easily flakes off the teeth, which
   appear yellow because of the revealed dentin. The hypoplastic type is
   X-linked and results in normal enamel that appears in too little
   quantity, having the same effect as the most common type ^.

   Chronic bilirubin encephalopathy, which can result from
   erythroblastosis fetalis, is a disease which has numerous effects on an
   infant, but it can also cause enamel hypoplasia and green staining of
   enamel ^.

   Enamel hypoplasia 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 is a genetic disease resulting in the
   deposition of porphyrins throughout the body. These deposits also occur
   in enamel and leave an appearance described as red in colour and
   fluorescent ^.

   Fluorosis leads to mottled enamel and occurs from overexposure to
   fluoride ^.

   Tetracycline staining leads to brown bands on the areas of developing
   enamel. Children up to age 8 can develop mottled enamel from taking
   tetracylicne. As a result, tetracycline is contraindicated in pregnant
   women.

   Celiac disease, an auto-immune disorder triggered by gluten allergies,
   also commonly results in demineralization of the enamel.

Enamel in animals

   For the most part, research has shown that enamel does not vary
   consistently between humans and nonhumans. Enamel formation 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.
   Teeth of a rottweiler
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   Teeth of a rottweiler

   Dogs are less likely than humans to have tooth decay due to the very
   high pH of dog saliva, which prevents an acidic environment from
   forming and the subsequent demineralization of enamel which would occur
   ^. In the event that 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 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 enamel is different from that of
   monkeys, dogs, pigs, and humans ^. In horse teeth, the enamel and
   dentin layers are intertwined with each other, which increases the
   strength and decreases the wear rate of those teeth ^.

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