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The value of the Stephan Curve is that it provides a means by which the cariogenic challenge to a tooth may be measured. Actually, it only really measures the potential cariogenic challenge because the Critical pH value varies between individulas.

The cariogenic challenge (cariogenicity) is measured as the area delimited by the Critical pH and the Stephan Curve shown in red in the diagram on the right.

Toothfriendly Sweets International, a non-profit association with a membership drawn from academic dentists, dental practioners and nutritionists uses the principle of Stephan Curves to assess the cariogenicity of different foods. It awards the "Toothfriendly Logo" to those it considers not to cause caries or erosion. Care must be taken not to confuse claims accredited by Toothfriendly Sweets International with claims that products are "Tooth Kind" or something similar.

The main use of the in vitro method of measuring cariogenic challenge has been in identifying the risk associated with particular foods or ingredients. The method does not readily lend itself to assessing risk due to an individual's lifestyle. For this the in vivo method which makes use of in-dwelling pH electrodes has proved to be more valuable.

The diagram below shows a typical 24 hour period in which six separate cariogenic challenges can be clearly identified. This could be looked on as the normal "minimum" number. Any additional snacks in the form of confectionary for example would have a significant effect on the overall challenge to the teeth.

The use of in-dwelling electrodes has highlighted the importance of behaviour of the individual. For example, eating small amounts of confectionary frequently, poses a vastly greater cariogenic challenge than consuming the same quantity of fermentable sugar in a single instance.

Compare the behaviour of two mothers. The first insists her child makes a pack of Smarties last a whole day. The second allows her child to eat the whole pack in one 5 minute orgy. Which is better and which would the child prefer?

Although in-dwelling electrodes have been used extensively to follow plaque-pH changes over long periods of time under "physiological" conditions it would be wrong to suggest that this is their only use. They have, for example, been used to identify less cariogenic sweeteners for use in the confectionary industry.

A range of alternatives to sucrose have been discovered. Some are "bulk" sweeteners such as xylitol and sorbose while others are the intense sweeteners such as saccharine.

The Stephan Curve shown below was obtained using an in-dwelling electrode. After each measurement the subject rinsed with a dilute urea solution to help the plaque reach the normal resting pH.

The bulk sweeteners lycasin which is a mixture of dextrins; the sugar alcohols xylitol and sorbitol; and sorbose, a ketose are each compared to sucrose. The subject rinsed with each substance for the same period of time using the same volumes and concentrations.

After each measurement the subject rinsed with a dilute urea solution to help the plaque reach the normal resting pH.

The bulk sweeteners lycasin which is a mixture of dextrins; the sugar alcohols xylitol and sorbitol; and sorbose, a ketose are each compared to sucrose. The subject rinsed with each substance for the same period of time using the same volumes and concentrations.

The Stephan Curve clearly shows that the fall in pH when the subject rinsed with xylitol is very much less than when rinsing with sucrose.

In fact, of all the substance tested, the pH fall was least with xylitol because this is not fermented by plaque microbes whereas the other substances are fermented, albeit at a much reduced level compared with sucrose.

One interesting and remarkable finding was that quite low concentrations of sugar produced a measurable cariogenic challenge. The Stephan Curve below shows the pH changes in plaque when a subject rinsed with different concentrations of sucrose. Between rinsing with the different sugar solutions the subject rinsed with a urea solution to help adjust the pH of the plaque to normal levels.

Although there is a significant drop in pH associated with each of the sucrose concentrations, there is also a clear graded response. The plaque pH fell further with the higher sucrose concentrations. However, this dose-response does not continue indefinitely and concentrations above 10% sucrose do not necessarily continue to cause progressively further drops in pH because the microbes in plaque have a finite fermentation capacity.

At the lower end of the scale, sucrose concentrations below 0.025% would, in this experiment, not be likely to cause the plaque pH to fall below the Critical pH (approx pH 5.5). So, the fall in plaque pH depends to some extent on the amount of sugar available to the plaque microbes and this is not always sufficient to cause the pH to fall below the Critical pH.

It is important to remember that the Stephan Curve reproduced here shows what is happening within the plaque of this individual at this point in time. There is no guarantee that exactly the same results would be obtained if the measurements were repeated using a different subject.

Finally it is worth noting that sucrose concentrations in the region of 1.125% w/v clearly caused a cariogenic challenge yet the individual would have been hard-pressed to identify the solution as sweet.

The use of in-dwelling electrodes has shown that foodstuffs not normally considered as cariogenic can produce significant pH changes in vivo.

In the example shown below the volunteer chewed a small amount of whole wheat bread. The resulting Stephan Curve was similar to that produced by rinsing with a solution of 1% w/v glucose solution.

Presumeably the starch in the bread was broken down by salivary amylase and the resulting dextrins taken up and metabolised by plaque bacteria.

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