Home Dentistry Articles Tooth enamel
Tooth enamel is the most
highly mineralized and hardest substance of the body.
Enamel is one of the three major parts of the human tooth,
the others being dentin and cementum. It 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 water and organic material composing
the rest . 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, enamel is thickest at the cusp, up to 2.5
mm, and the thickness tapers down to a miniscule amount
at its border, which is clinically seen as the cementoenamel
Enamel's primary mineral component is
hydroxyapatite, which is a crystalline calcium phosphate
. The very large amount of minerals in enamel accounts
for its strength, 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 among other functions .
The basic unit of enamel
is called an enamel rod . Measuring 4 Ám wide to 8 Ám
high, 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 and is not fully explained
here. Both the cells which initiate 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 .
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 under the microscope in the crown stage.
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 Ám 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 contribute
to what is known as the enamel matrix, which is almost immediately
mineralized 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 in 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
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, 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 color gradually darkens with age. Additionally, enamel becomes
less permeable to fluids, less soluble to acid, and contains less
Progress of Enamel Formation for Primary Teeth
||Amount of Enamel Formed at
||Enamel Mineralization Completed
||1.5 months after birth
||2.5 months after birth
||9 months after birth
||Cusps united; occlusal completely calcified
and 1/2 to 3/4 crown height
|6 months after birth
||Cusps united; occlusal incompletely calcified;
calcified tissue covers 1/5 to 1/4 crown height
|11 months after birth
||2.5 months after birth
||3 months after birth
||9 months after birth
||cusps united; occlusal
|5.5 months after birth
||cusps united; occlusal
|10 months after birth
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.
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, 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 bacteria involved
with tooth decay is Streptococcus mutans, but the number
and type of bacteria varies with the progress of tooth
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.
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 extent to which tooth decay is
likely, known as cariogenicity, depends on factors such
as how retentive the sugar is to the teeth. Contrary to
common belief, not the amount of sugar ingested but the
frequency of sugar ingestion 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,
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 dinnertime than to snack on a single, small
bag of candy throughout the entire workday.
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
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, whcih 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 demineralize
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.
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
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 . 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, one of the greatest successes in dental health
care has been the inclusion of fluoride in public water to decrease
Effects of Dental Procedures on Enamel
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
Nonetheless, enamel can sometimes be removed
before there is any decay present. The process of placing dental
sealants involves removing healthy enamel in the deep fissures
and grooves of a tooth and replacing it with a restorative material
. 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 .
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, 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 . 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
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.
Other anatomical features of enamel
The rod sheath is the border where the crystals
of enamel rods and the crystals of interrod enamel meet.
Formed from changes in diameter of Tomes' processes,
striae of Retzius are stripes that appear on enamel when viewed
microscopically in cross-section . These stripes demonstrate the
growth of enamel similar to the annual rings on a tree. Darker
than the other stripes, 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 where the striae
of Retzius end .
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 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. As a result, tetracycline is contraindicated
in pregnant women.
Enamel in non-human Animals
For the most part, research has shown that enamel
does not vary consistently between humans and non-humans. Enamel
formation in animals is described almost identically to formation
in humans. The enamel organ, including the dental papilla, and
ameloblasts function similiarly . The variations of enamel that
are present are infrequent but sometimes important. Differences
exist, however, in the morphology, number, and types of teeth
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 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 than 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.