Enamel structure
Enamel development is conventionally described in five histologically recognizable stages: secretion (matrix deposition and transition), cell organization, preabsorption, early maturation, and late maturation. In later work, only four stages are described, based on chemical composition. Developmental disturbances may occur at any stage.
There is, however, no clear clinical evidence that aberrations in enamel structure affect resistance to dental caries, unless the defects are major and result in rough surfaces that enhance plaque retention. Trace elements may be essential for the development of normal enamel structure. For example, selenium is essential for proper matrix formation, but an excess may adversely affect calcification. Thus, either an excess or a deficiency of selenium, or other trace elements, may affect tooth resistance. This will be considered later, in more detail.
Opacities are isolated disturbances of the internal structure of enamel, appearing clinically as white patches, with or without an associated defect in surface contour.
Opacities are common on the facial surfaces of permanent maxillary incisors and may be sequelae to trauma to the primary incisors. Some have demonstrated that opacities may be associated with water composition. Because opacities affect only small areas of a tooth, they will not influence caries resistance (Weatherell et al, 1977). On the contrary, whereas there is limited posteruptive uptake of fluoride in sound enamel, fluoride is found in relatively high concentrations in opacities, probably because of the greater porosity.
So-called Turner’s teeth, malformed premolars resulting from infection in the preceding primary molar, are caries susceptible. The condition is, however, too uncommon to be used in identifying susceptible individuals.
So-called mottled enamel involves most teeth in the mouth of an affected individual.
It arises from a disturbance of enamel formation at one or several of the developmental stages and appears to be systemic rather than local in origin. The four stages of enamel development based on chemical composition are (1) the secretion of a partially mineralized matrix; (2) selective withdrawal of amelogenin components; (3) massive selective loss of amelogenins with maturation and selective mineralization; and (4) the production of hard, mature enamel. Mottling of enamel that is caused by high levels of ingested fluoride is related to the presence of less wellmineralized enamel on a per volume basis, resulting from a disturbance of stage 3. Incorporation of fluoride into the enamel may produce hypomineralized subsurface layers. Mottled enamel as a result of ingestion of relatively high levels of fluoride has been investigated intensively since Dean and Elvove (1935) showed that it was related to the concomitant presence of low dental caries experience.
Studies show that caries prevalence is lower in groups of individuals with fluorosis
than it is in nonfluorotic groups (Axelsson and El Tabakk, 2000c), not because
fluorosis prevents caries, but because individuals with fluorosis will continue to
benefit from the posteruptive effect of fluoride as long as they continue to live in an
area with high water fluoride concentration. Today it is well known that the cariespreventive
effect of fluoride is almost 100% posteruptive. However, in communities
in the United States with up to four times the optimal level of fluoride in the drinking
water, caries protection from the fluoride is compromised by severe fluorosis. If the
fluorosis becomes so severe as to cause substantial hypoplasia, possibly aggravated by
posteruptive loss of enamel, then the prevalence of dental caries starts to increase
again because of exposed dentin and increased plaque retention.
Although dental researchers have shown that mottling could be ascribed to the
presence of dietary fluoride, other trace elements might also be responsible, and by
extrapolation there might also be an association between other elements and caries
susceptibility. Data on the trace element composition of various US water supplies,
when matched with those for dental caries prevalence and enamel mottling in the
areas supplied, show almost as significant a relationship between dental caries and the
concentration of strontium as that between caries and fluoride. Other trace elements
may also give rise to tooth resistance and enamel mottling. Elements such as
strontium could give rise to enamel mottling identical to that produced by fluoride.
Defects of enamel have been shown to occur with high levels of yttrium and lithium.
In humans, strontium is the only trace element other than fluoride shown to be
associated with increased enamel mottling (Curzon and Spector, 1977). Children
consuming high levels of strontium at 33 ppm (mg/L) had a significantly higher
prevalence of enamel mottling than did those using drinking water with lower
strontium concentrations. The level of mottling was greater than that expected from
the concentration of fluoride in the drinking water (1.0 to 1.2 ppm). Dental caries was
found to be exceptionally low in these communities (Curzon et al, 1978), and it was
suggested that this was the result of the combination of fluoride and strontium in the
drinking water. The combination of the two trace elements could be said to be a
marker of considerable tooth resistance to dental caries as well as associated with
increased enamel mottling.
Although mild-to-moderate fluoride mottling of the enamel is associated with
resistance to caries on a population basis, at the individual level it is at best a rough
indicator of resistance. It cannot be overemphasized, however, that the overwhelming
caries-preventive effect of systemic administration of fluoride from drinking water is
posteruptive and that dental caries is a multifactorial disease. In most developing
countries with natural fluoride and other trace elements in the drinking water, the
climate is tropical or subtropical, and the frequency of water intake (topical effect)
and the daily volume of drinking water consumed (daily systemic dose) are high
compared to those in regions with temperate climates. In most developing countries,
particularly Africa and Asia, the normal diet (vegetables, rice, and fish) has low
cariogenicity. It is of interest to note that most studies on the relationship between
mottled enamel and caries prevalence have been carried out in such regions.
