Tooth decay begins when the outer surface of the tooth is attacked by acid. The acid is produced by bacteria which live on the surfaces of the teeth as a layer called plaque. When foods or drinks containing sugars enter the mouth, the bacteria within the plaque rapidly convert the sugars into acid. The plaque can hold the acid in contact with the tooth surface for up to 2 hours before it is neutralised by saliva.
During the time that the plaque is acidic, some of the calcium and phosphate minerals, of which enamel is largely composed, are dissolved out of the enamel into the plaque. This process is called demineralisation. However, once the plaque acid has been neutralised the minerals can return to the enamel surface – a process called remineralisation. This whole process is often described as an “ionic see-saw” in which mineral ions constantly move back and forth between the tooth surface and the plaque.
The capacity for remineralisation is, however, limited, and if sugars enter the mouth too frequently a net loss of mineral from the enamel surface results in a cavity through which bacteria can penetrate and infect the inner structure of the tooth. This is tooth decay and, if left untreated, will gradually destroy the tooth causing pain and often the formation of an abscess.
How does fluoride protect our teeth?
The relationship between fluoride and tooth decay is complex and probably not yet fully understood. However, it is known that fluoride interferes with the process of tooth decay in at least four ways:
If children ingest sufficient fluoride during the period of enamel development (up to 7 years of age) the fluoride alters the structure of the developing enamel making it more resistant to acid attack. This was originally thought to be the most important mechanism of fluoride; however, with advances in knowledge this is now understood to be the least important mechanism.
When teeth are subjected to alternating demineralisation and remineralisation as described above, the presence of low levels of fluoride in the plaque and saliva both encourages remineralisation and ensures that the enamel crystals that are laid down are of improved quality. In other words, low levels of fluoride in the mouth gradually improve the strength of the tooth enamel and its ability to resist acid attack. This important mechanism was first described in 1966 and means that early patches of decay can be arrested and damaged enamel will ‘heal’. This explains the dramatic improvement in dental health since the introduction of fluoride into toothpaste formulations in the mid-1970s.
The third way in which fluoride works is by reducing the ability of the plaque bacteria to produce acid. This is a major factor in the prevention of tooth decay. It results from the ability of the plaque bacteria to concentrate the low levels of fluoride at the tooth surface up to a level which inhibits the function of some enzymes which are essential to the bacteria’s ability to produce acid.
A fourth, and probably minor effect of fluoride is that, if sufficient fluoride is ingested during childhood when the teeth are developing, it affects the depth of the fissures (grooves) on the biting surfaces of the teeth. In children who grow up in areas where the drinking water is fluoridated these grooves in the teeth tend to be shallower, thus reducing the ability of plaque to remain undisturbed.
Work to further develop our knowledge of the mechanism of fluoride continues. However, our knowledge is sufficiently well developed to be able to say that of the four mechanisms the second – the remineralisation effect – is the most important. The emphasis in health promotion terms is now focussed on the goal of maintaining low levels of fluoride in everybody’s mouths for as long as possible – particularly during waking hours when foods and drinks containing sugars are likely to be ingested.
For this reason control of sugars in the diet, water fluoridation, and the regular use of fluoride toothpaste remain the cornerstones of tooth decay prevention.