Tooth

Teeth (singular, tooth) are structures found in the jaws of many vertebrates. The primary function of teeth is to tear and chew food, and in some animals, particularly carnivores, for fighting. The roots of the teeth are covered by gums. The protective enamel covers the teeth and helps prevent cavities. Adult teeth naturally darken with age as the pulp within the tooth shrinks and dentin is deposited in its place.

Teeth are among the most distinctive features of mammal species and fossilize well. Paleontologists use them to identify fossil species and their relationships. The shape of the teeth is related to the animal's diet and evolutionary descent. For example, plant matter is hard to digest, so herbivores have many molars for chewing. Carnivores need canines to kill and tear and they can swallow without molars to chew since meat is easy to digest.

While humans only have two sets of teeth, some animals have many more. Sharks grow a new set of teeth every two weeks, and other animals grow just one set. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining constant length.

Human teeth

3 infant (deciduous) teeth of an 8 year old. Note the absence of a tooth root, which degenerates before the deciduous tooth falls out

Humans grow two sets of teeth. The first set (the "baby," "milk," "primary" or "deciduous" set) appears at about six months of age, although some babies are born with one or more teeth already visible. This stage is known as teething and can be quite painful for an infant. Human children have 20 milk teeth evenly distributed across the mouth's quadrants. Each quadrant of five teeth has a:

• central incisor
• lateral incisor
• cuspid (canine)
• first molar
• second molar

Section of a human molar

Permanent teeth are evenly distributed across the mouth's quadrants. Each quadrant of eight teeth has a:

• central incisor
• lateral incisor
• cuspid (canine)
• first premolar
• second premolar
• first molar
• second molar
• third molar (wisdom teeth)

Teeth are attached to the underlying bone of the jaw via the periodontal ligament, though the teeth themselves are not made of bone. The white part of the tooth, which can be seen in the mouth, is the enamel. Immediately one to three mm below the enamel is a slightly softer, yellow tissue called dentin. Dentin is supported by the pulp (commonly called 'the nerve', although it contains many other structures which are not nerves), which lies in the center of the tooth. The teeth's composition is specialized to resist the harsh environment of the oral cavity and withstand the large forces imposed upon them by mastication, or chewing.

Dentists use several different notation systems to refer to a specific tooth. The three most commons systems are the Universal numbering system, the Palmer Notation Method, and the two-digit FDI World Dental Federation notation which is widely used internationally [link]. Dentists sometimes refer to the inner surface of teeth as the lingual surface (meaning towards the tongue), and the outer surface as the labial surface (meaning towards the lips) or "buccal" (meaning towards the cheek). Other terms are mesial (toward the midline), distal (away from the midline), occlusal (the top surface), incisal (the cutting surface), "gingival" (toward the gumline), and "pulpal" (toward the center).

Tooth decay

Plaque

Plaque is a soft white layer which forms on teeth, containing large amounts of bacteria of various types, particularly Streptococcus mutans. Left unchecked for a few days plaque will harden, especially near the gums, forming tartar.

Certain bacteria in the mouth live off the remains of foods, especially sugars. In the absence of oxygen they produce lactic acid, which dissolves the calcium and phosphorus in the enamel in a process known as demineralisation. Enamel demineralisation takes place below the critical pH of about 5.5.

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 (two to three hours) and the damage is limited the teeth can repair themselves.

Caries (Cavities)

This section should be [Duplicate articles#How articles should be mergedmerged] into Dental caries.

Radiographs, or X-rays, can reveal if a person has any cavities and some diseases.

In moderation, fluoride is known to protect the teeth against cavities. It toughens the teeth by replacing the hydroxyapatite and carbonated hydroxyapatite minerals of which the enamel is made with fluorapatite, which is harder to dissolve by acid. It also reduces the production of acids by bacteria in the mouth by reducing their ability to metabolize sugars. The addition of fluoride (sodium monofluorophosphate) to toothpaste is now very common, and may explain the decline in dental caries in the Western world in the past 30 years.

Some believe that a diet rich in fluorine salts, particularly in childhood, can lead to a stronger enamel which is less susceptible to decay. Fluoridation of drinking water remains a controversial issue. However, in many parts of the world, the natural water supply may be sufficiently rich in fluorides to supply the needs of children without additional sources being required.

Caries is an infectious disease and is treated only by prevention. Once the decay process begins, caries can be eliminated only through the removal of the bacteria. Fluoride can be used to remineralize decalcified enamel, but new tooth structure cannot be regrown. When a patient gets a filling, the dentist mechanically removes the bacterial flora and dead tooth structure and replaces it with a restorative material (metal or glass). This was, traditionally, achieved using gold or a compound of metals called amalgam. Amalgam fillings have been the cause of some public concern because they contain mercury. Modern tooth-colored composite or ceramic are used in addition to traditional amalgam fillings.

As a last resort, teeth affected by caries may be extracted, preferably under local or general anaesthetic.

Foods

This section should be [Duplicate articles#How articles should be mergedmerged] into oral hygiene.

Better

Worse

Sucrose (table sugar) is most commonly associated with caries, although glucose and maltose seem equally gervic (likely to cause caries). The amount of sugar consumed at any one time is less important than how often food and drinks that contain sugar are consumed. The more frequently sugars are consumed, the greater the time during which the tooth is exposed to low pH levels, at which demineralisation occurs. It is important therefore to try to encourage infrequent consumption of food and drinks containing sugar so that teeth have a chance to repair themselves. Obviously, limiting sugar-containing foods and drinks to meal times is one way to reduce the incidence of caries.

Artificially refined sugar is not the only type that can promote dental caries. There are also sugars found in fresh fruit and fruit juices. These foods (oranges, lemons, limes, apples, etc. ) also contain acids which lower the pH level. On the other hand, carbonic acid found in soda water is very weakly acidic (pH 6.1), and not associated with dental caries (provided the soft drink is sugar free, of course). That said, soft drinks are not as healthy for the teeth as milk, due to their lower pH and lack of calcium. Drinking sugared soft drinks throughout the day raises the risk of dental caries tremendously.

Another factor which affects the risk of developing caries is the stickiness of foods. Some foods or sweets may stick to the teeth and so reduce the pH in the mouth for an extended time, particularly if they are sugary. It is important that teeth be cleaned at least twice a day, preferably with a toothbrush and fluoride toothpaste, to remove any food sticking to the teeth. Regular brushing and the use of dental floss also removes the dental plaque coating the tooth surface.

Tooth care

Notice the temporary crown, and that the wisdom teeth have been removed.

Regular brushing is recommended by healthcare professionals twice a day. [link]. Studies show that the minimum required time interval is once every two days; however, more frequent recommended brushing is to help maintain a healthy habit in the population#redirect [[Template:Fact]]. A mirror may be used to visualize if the plaque is completely removed during brushing. The actual term 'brushing' is rather inaccurate. Though a toothbrush is used, it is not designed to deliver mechanical abrasive action. Rather, the surface of the brush is meant to be held at a forty five degree angle to remove plaque from the periodontal margin (the space in between the gum and tooth). Thus, the term 'cleaning teeth' is far more accurate.

In research, levels of plaque were recorded before and after brushing and found that plaque removal steadily improved as brushing times and pressure were increased. However, their results showed that when people brush for longer than two minutes, at a pressure higher than 8.17 kPa (the equivalent of the pressure produced by the weight of ten U.S. quarters), they are not removing any additional plaque, and may be causing permanent damage to the teeth and gums#redirect [[Template:Fact]].

Brushing teeth immediately after eating acidic foods is not recommended, because acid softens the enamel, which can then be damaged by brushing. It is better to wait at least half an hour after eating acidic foods before brushing. Rinsing the mouth and eating non-acidic foods can also speed up this process.

Electric toothbrushes are no more effective than the manual variety, according to research. However, "rotation-oscillation" electric toothbrushes out-performed manual brushing, removing around 7% more plaque and leading to 17% less gum disease than manual brushes. [link] Any kind of electric toothbrush does tend to help people who are not as good at cleaning their teeth and as a result have had oral hygiene problems.

As noted above, eating certain cheeses such as cheddar soon after eating potentially harmful foods have been noted to be helpful in preventing tooth decay as well.

In the future, tooth decay may be banished by treatment with a genetically modified bacterium, according to research at the University of Florida. [link]

Dentures and \"false\" teeth

Some of the earliest artificial teeth were made by the Etruscans and their use was adopted in Ancient Rome for the wealthy citizens who often dined on food containing damaging sugars.

Abnormalities of the dentition

• Amelogenesis imperfecta - A condition in which the tooth's primary surface, the enamel, does not form properly or at all.
• Dentinogenesis imperfecta - A similar condition to above, but affects the underlying layer of the tooth
• Hypercalcification
• Hypocalcification
• Deossification - Loss of bone tissue
• Dental fluorosis - white spotted, yellow, brown, black and sometimes pitted teeth from over-ingesting fluoride
• Supernumerary roots - More than the normal number of roots for a tooth. Most common in maxillary bicuspids.
• Abnormalities with number of teeth
• * Anodontia (total lack of tooth development)
• * Hyperdontia (More than the average number of teeth)
• * Hypodontia (Missing teeth)
• Abnormalities with size of teeth
• * Microdontia
• * Macrodontia
• Abnormalities with shape of teeth
• * Tooth Gemination
• * Tooth Fusion - the union of two adjacent tooth germs by dentin during formation
• * Concrescence
• * Talon cusp
• * Cusp of Carabelli
• * Dens Envaginus (Cusp-like elevation of enamel)
• * Dens in Dente (also called dens invaginus)
• * Ectopic enamel
• * Taurodontism
• * Hypercementosis
• * Dilaceration - trauma to the tooth during formation causing damage to the root structure
• * Supernumerary Roots (increased number of roots expected on a tooth)

Development of teeth

There are three stages in the embryonic development of teeth, the Bud Stage, the Cap Stage and the Bell Stage. The Bud stage begins at the 7th week of intrauterine life.

Facts about teeth in non-human animals

Section through the ivory tooth of a mammoth

• Rodents' incisors grow continuously throughout their lives.
• Reptiles' and sharks' teeth are replaced constantly, before they wear out. A crocodile replaces its teeth over forty times in a lifetime.
• Elephants' tusks are specialized incisors for digging food up and fighting.
• Turtles and tortoises are toothless
• Narwhals giant unicorn-like tusk is a tooth that contains millions of sensory pathways and may be used for sensing in feeding, navigation and mating. It is the most neurologically complex tooth known.
• Horse teeth can be used to estimate the animal's age, and some horses have a form of premolars called Wolf teeth.

See also

• Fluoride therapy
• Tooth Fairy
• Dental

External links

1. redirect [[Template:Commonscat]]

• [A website about teeth and oral health]
• [An overview of dental anatomy]
• [Tooth eruption chart]
• [Stories from people who have grown a third set of teeth]
• [New Technology to Regrow Teeth] - Azonano.com

[[zh-min-nan:Chhùi-khí]]

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