Tooth enamel is the most highly mineralized and hardest substance of the body. Among enamel, dentin, and cementum, enamel is the dental tissue of a tooth which usually is visible in the mouth and must be supported by underlying dentin. Minerals compose 96% of enamel, with the rest being water and organic material. Since enamel is semi-translucent, the color of dentin and any restorative dental material underneath the enamel highly affects the outer appearance of the tooth. The color of enamel is a light yellow to grayish white. It varies in thickness over the surface of the tooth. Often, the cusp is the location where the enamel is thickest, up to 2.5 mm, and the thickness tapers down to a miniscule amount at its border, which is clinically seen as the cementoenamel junction (CEJ).
The primary mineral component is hydroxyapatite , which is a crystalline calcium phosphate. This very large amount of minerals accounts for the strength of enamel, but also for its brittleness. Thus, dentin, which is less mineralized and less brittle, compensates for enamel and is necessary as a support.
The organic portion of enamel does not contain collagen, as dentin and bone does. Instead, it has two unique classes of proteins called amelogenins and enamelins . The role of these proteins is not understood fully at this time, but it is believed that these proteins aid in the development of enamel as a framework support and other mechanisms.
Structure
The basic unit of enamel is called an enamel rod . Measuring 4 μm wide to 8 μm high, an enamel rod (the antiquated term being 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.
It should be noted that the arrangement of the crystals within each enamel rod is highly complex and is not fully explained here. Both the cells which initiated enamel formation, known as ameloblasts, and Tomes’ processes affect the crystals’ pattern. Basically, the enamel crystals are oriented parallel to the long axis of the rod when the crystal is in the head of the enamel rod. When found in the tail of the enamel rod, the crystal’s orientation diverges slightly from long axis.
The arrangement of enamel rods is understood more clearly. Enamel rods are found in rows along the tooth. Within each row, the long axis of the enamel rod generally is perpendicular to the underlying dentin. In permanent teeth, the enamel rods near the cementoenamel junction (CEJ) tilt slightly more toward the root of the tooth than would be expected. Knowing the orientation of enamel is very important in restorative dentistry because enamel unsupported by underlying dentin is prone to fracture and usually is avoided.
The area around the enamel rod is known as interrod enamel . Interrod enamel has the same composition as the enamel rods. Nonetheless, a histologic distinction is made between the two because crystal orientation is different in each.
Development
Amelogenesis , or enamel formation, occurs after the first establishment of dentin and via cells known as ameloblasts. Human enamel forms at a rate of ~4 μm per day and begins at the future location of cusps around 3-4 months in utero. As in all human processes, the creation of enamel is complex, but it would be fitting to view the process in two stages. The first stage, called the secretory stage, involves proteins and the organic matrix and results in a partially mineralized enamel. The second stage, called the maturation stage, completes enamel mineralization .
In the secretory stage, the ameloblasts are polarized columnar cells . In the rough endoplasmic reticulum of these cells, enamel proteins are released into the surrounding area and contributes to what is known as the enamel matrix , which almost immediately mineralizes partially 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 resembling a pit into which the Tomes’ process lies, and around the end of the Tomes’ process, resulting in deposition of matrix inside the pits. The matrix within the pits will eventually become enamel rods, and the walls will eventually become interrod enamel. The only distinguishing factor between the two is the orientation of the crystals.
In the maturation stage, the ameloblasts act as cells transporting substances for 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 compose most of the transported material. These are used for the final mineralization process. 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.
Sometime before the tooth erupts into the mouth, but after the maturation stage, the ameloblasts are broken down. Consequently, enamel has no way of regenerating itself as many other tissues of the body. After destruction of enamel from decay or injury, neither the body nor a dentist can restore the enamel tissue.
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 moths 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 ¼ 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
The high mineral content of enamel, which makes this tissue the hardest in the human body, also makes it susceptible to the demineralization which often occurs as dental caries, otherwise known as cavities. There are many reasons for demineralization to occur. The most important cause of tooth decay is the ingestion of sugars.
Sugars from candies, soft drinks, and even fruit juices play a significant role in tooth decay, and consequently in enamel destruction. There is already a great number and variety of bacteria residing in the mouth. When sucrose, the most common of sugars, coats the surface of the mouth, some intraoral bacteria interact with it and form lactic acid. Acid decreases the pH in the mouth and the hydroxyapatite crystals of enamel demineralize, allowing for greater bacterial invasion further into the tooth. The most important bacteria 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 the most likely locations for decay to occur. The most common site for the initiation of dental caries is in the deep grooves and pits of enamel. This is expected because these locations are impossible to reach with a toothbrush and allow for bacteria to reside. When demineralization of enamel occurs, a dentist can use a sharp instrument, such as an explorer (dental), 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 extent to which tooth decay is likely, known as cariogenicity , depends upon factors such as how retentive sugar is to the teeth. Further, there is a false belief among the general public that the amount of sugar ingested is the deciding cause of tooth decay. In truth, it is not the amount but the frequency of sugar ingestion that is the most important factor. A great quantity of sugar at one time in the day will be less detrimental than a very small quantity ingested in many intervals throughout the day. For example, in terms of oral health, it is better to eat a very large dessert at dinner time than to snack on a single, small bag of candy throughout the entire workday.
In addition to bacterial invasion, enamel is susceptible to other means of destruction as well. Bruxism, also known as clenching of or grinding on teeth, destroys enamel very quickly. The wear rate of enamel, called attrition, is 8 micrometres a year from normal factors. A common misperception 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, and can cause irreversible damage to the enamel.
Other non-bacterial processes of enamel destruction include abrasion (involving foreign elements, such as a 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, whose purpose is to reduce the number of bacteria and food particles particularly on enamel. Some isolated societies in the world 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 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, general oral health can usually prevent enough bacterial growth to keep tooth decay from starting.
These methods of oral hygiene have been helped greatly with the use of fluoride. Fluoride can be found in many locations naturally. It can be found in a variety water sources, the ocean being a notable example. Consequently, many seafood dishes contain fluoride as well. 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.
Many groups of people have spoken out against fluorinated 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. In spite of this, most substances, even helpful ones, taken in extreme are detrimental. 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. The acute toxic dose of fluoride is ~5 mg/kg of body weight. Furthermore, whereas topical fluoride, such as found in toothpaste, does not cause fluorosis, it is also not as successful in preventing tooth decay as systemic fluoride, such as drinking fluorinated water. Thus, one of the greatest successes in dental health care has been the inclusion of fluoride in public water to decrease tooth decay.
Other features of enamel
The rod sheath is the border where the crystals of enamel rods and the crystals of interrod enamel meets.
Formed from changes in diameter of Tomes’ processes, striae of Retzius are stripes that appear on enamel when views microscopically in cross-section. These stripes demonstrate the growth of enamel similar to the annual rings on a tree. Darker than all the rest, the neonatal line is the stripe that separates the enamel formed before and after birth.
Gnarled enamel is found over the cusps of teeth. The twisted appearance results from the orientation of enamel rods and the rows in which they lie.
Perikymata are shallow furrows into which the striae of Retzius end.
Important enamel disorders
Amelogenesis imperfecta has many different types. The hypocalcification type, which is the most common, is an autosomal dominant condition resulting 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.
Fluorosis leads to mottled enamel and occurs from overexposure to fluoride as explained earlier.
Tetracycline staining leads to brown bands on the areas of developing enamel. As a result, tetracycline is contraindicated in pregnant women.
See also
References
- Ash, Major M. and Stanley J. Nelson. Wheeler’s Dental Anatomy, Physiology, and Occlusion. 8th edition. 2003.
- Biology of the Human Dentition
- Cate, A.R. Ten. Oral Histology: development, structure, and function. 5th ed. 1998.
- Diagnosis and Managment of Dental Erosion
- Harris, Edward F. Craniofacial Growth and Development. 2002.
- Ross, Michael H., Gordon I. Kaye, and Wojciech Pawlina. Histology: a text and atlas. 4th ed. 2003.
- Tooth: Structure of a Normal Tooth
- Why Teeth Fossilize Better Than Bone
