A new structural model for enamel

Despite over half a century of research, there remains controversy over the exact role of fluoride in the prevention of tooth decay. It has been known for a long time that fluoride helps to harden the enamel that protects teeth from attack from acid produced by decay-causing bacteria.

More recent studies suggest that fluoride doesn’t penetrate the enamel as deeply as was previously believed, but hardens a much thinner layer of the teeth. This suggests that, in addition to hardening the enamel, fluoride has other protective effects. Research using artificial teeth, in the form of hydroxyapatite pellets, revealed that fluoride also affects the adhesion force of bacteria, reducing their ability to stick to the surface of the tooth. This facilitates removal of the bacteria by the action of saliva, brushing, or other activity. The saliva washes away carious substances such as sugars, as well as neutralising the acids which cause decay.

Many drugs can also promote tooth decay, usually as a result of reducing saliva production — for example, through anti-cholinergic side effects. Research showed that rats receiving clonidine developed 84% more cavities on the smooth surfaces of their teeth than those not receiving the drug.

Researchers at the Northwestern University in Illinois, studying the teeth of various species of rodents, discovered a new structural model for enamel. Rodent enamel is similar in structure to human enamel. It was previously known that enamel was made up of a core of hydroxyapatite “nanowires”, but the team discovered it was the material surrounding the nanowires that determines the enamel’s acid resistance and mechanical properties.

It was the first time that the composition of this unstructured, amorphous phase had been studied. This material makes up only a small percentage of enamel and it was found to contain minerals rich in iron and magnesium. It was shown that it was these metal ions that afford protection from decay. Using a technique called atom-probe tomography, the researchers were able to map the structure of the enamel atom by atom, and it was found that the pigmented enamel of beaver teeth is both harder and more acid-resistant than human enamel.

When the teeth were subjected to acid exposure, it was discovered that it was the amorphous biominerals in the structure which dissolved, rather than the nanowires themselves. In beavers it is iron (which gives the enamel a red-brown pigmentation) that is the predominant mineral; in humans it is magnesium. It was found that the pigmented beaver incisors were more resistant to acid even than fluoride-treated human enamel, and it is the presence of iron that improves the acid-resistant qualities.

It was stressed the enamel in the teeth of a beaver is chemically but not structurally different from that of humans. It is hoped that focusing on the amorphous material surrounding the nanowires could lead to improvements in current fluoride treatment and dental health.