Tooth enamel

Enamel is the hardest substance in the human body and contains the highest percentage of minerals (at 96%), with water and organic material composing the rest. The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. 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. Since enamel is semitranslucent, the color of dentin and any material underneath the enamel strongly affects the appearance 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. Tooth enamel ranks 5 on Mohs hardness scale (between steel and titanium) and has a Young's modulus of 83 GPa. Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support. 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. Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins: 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 for minerals to form on, among other functions. == Structure ==

The basic unit of enamel is called an enamel rod. 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 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 ==

Enamel formation is part of the overall process of tooth development. Under a microscope, different cellular aggregations are identifiable within the tissues of a developing tooth, 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. 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. 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 which gives rise to the enamel organ. :* 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, otherwise known as cavities. Enamel is also lost through tooth wear and enamel fractures. ::Ca10(PO4)6(OH)2(s) + 8H+(aq) → 10Ca2+(aq) + 6HPO42−(aq) + 2H2O(l) Sugars and acids from candies, soft drinks, and 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. 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. 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 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 dissolving by soft drinks or lemon and other juices), and possibly abfraction (involving compressive and tensile forces). Though enamel is described as tough, it has a similar brittleness to glass, making it, unlike other natural crack-resistant laminate structures such as shell and nacre, vulnerable to fracture. 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 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. Fluoride therapy is used to help prevent dental decay. Fluoride ions, as an antimicrobial, may activate bacterial genes associated with fluoride riboswitches. 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. 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 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. These codes are supported by the American Dental Association and the American Academy of Pediatric Dentistry. Furthermore, whereas topical fluoride, found in toothpaste and mouthwashes, 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 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 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. 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 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. 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. 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 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 in the enamel and dentin. The agents most commonly used to intrinsically change the color of teeth are hydrogen peroxide and carbamide peroxide. 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. 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 ==

discoloration, or other causes. The National Institutes of Health include a dental exam in the diagnostic protocol of celiac disease. 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. 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 color 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 tetracycline. As a result, tetracycline is contraindicated in pregnant women. Celiac disease, a disorder characterized by an auto-immune response to gluten, 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. 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. 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 staining. Consequently, this risk must be accounted for when tetracycline antibiotic therapy is administered to young 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 wear resistance of those teeth. == Other organisms ==

Enamel or enameloid is found in the dermal denticles of sharks and many early vertebrates, and it appeared there before gnathostome teeth evolved. The ganoin that covers the scales of many actinopterygians is probably derived from enamel. Enamel-like substances also coat the jaws of some crustacea, but this is not homologous with vertebrate enamel. Enameloid 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 three times greater than that of geological hydroxyapatite, 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 in enamel as both components have different mechanical properties. The interrod enamel has around 53% and 74% decreased hardness and elastic modulus 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 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 ==