The Stephan Curve describes the change in dental plaque pH in response to a challenge. The type of challenge does not matter but it is usually some element of the diet. On the other hand the challenge could be an inert substance placed in the mouth with the aim of determining its effect on plaque pH. For example, mechanical stimulation of the salivary glands caused by masticating chewing-gum base. This would be of interest in an investigation into the effect of saliva flow on the pH-changes happening in plaque after a challenge by a cariogenic food.

The characteristics of the Stephan Curve are shown in the diagram. In this example, dental plaque was challenged with a fermentable carbohydrate by asking a volunteer to rinse with 10mls of 10% sucrose solution for 10 seconds. Average plaque samples were removed at intervals and the pH recorded.

Characteristically the Stephan Curve reveals a rapid drop in plaque pH, followed by a slower rise until the resting pH is attained. The time course varies between individuals and the nature of the challenge.

The initial drop is usually rapid with the lowest pH being attained within a very few minutes. However, pH recovery can take anything between 15 and 40 minutes depending to a large extent on the acid-neutralising properties of the individual's saliva.

The initial rapid drop in pH is due to the speed with which plaque microbes are able to metabolise sucrose. Larger carbohydrates, such as starch, would diffuse into plaque more slowly and would need to be broken down before assimilation by the microbes. In the case of starch, salivary amylase would produce a mixture of glucose and maltose together with incompletely digested material comprising the branch points of the starch molecule (limit dextrins). The glucose and maltose would then be taken up by plaque bacteria and metabolised. The rate of starch breakdown slows up glycolysis and, therefore, acid production producing a less steep drop in pH.

The lowest pH achieved depends greatly on:

The presence of significant numbers of aciduric, acidogenic bacteria in plaque developing in a sheltered site with a low diffusion rate coupled with a readily fermentable carbohydrate such a sucrose or glucose would produce the lowest pH. Under these conditions a pH in the region of 4.5, or even lower, might be attained.

Conversely, a challenge with a carbohydrate which is metabolised more slowly to a plaque community with fewer aciduric, acidogenic microbes would result in less acid production and a higher terminal pH.

The rate of diffusion of material into and out of plaque is governed by the density of the plaque and access by saliva. Thus less dense plaque fully exposed to saliva flow will more rapidly exchange metabolites with the surroundings. This will enable substrates to diffuse into the plaque rapidly and at the same time allow microbial by-products to diffuse out. The terminal pH following a challenge to the plaque will reflect the relative rates of diffusion of both substrate and metabolites.

The pH starts to rise after a few minutes due to:

In addition, the low pH produced will inhibit microbial metabolism and thereby slow the rate of acid production. This will allow the processes of diffusion and neutralisation to exert a greater effect on plaque pH. Also, as before, plaque which is less dense and fully exposed to saliva flow will show a faster rate of pH recovery.

It normally takes at least 20 minutes for the plaque pH to reach its resting value but it can take considerably longer depending on the factors described above.

In the in vitro method plaque is removed from the mouth at intervals and the pH measured using a micro-pH electrode. Graphs similar to those shown above are obtained by this means. Success with the in vitro method depends to a great extent on the skill of the operator in obtaining 6-7 representative plaque samples and the compliance of the volunteer who has to allow sufficient plaque to accumulate. Obtaining multiple, similar plaque samples requires careful choice of sampling sites and avoiding harvesting too much plaque from any one site. The operator visits about 6 different sites in the mouth harvesting a small amount of plaque from each. The picture shows the amount of plaque required on the end of a microspatula. This plaque is homogenised in a small amount of distilled water and the pH recorded. This is repeated up to 5-6 times for each experiment. Great care must be taken to standardise the time taken to collect plaque and measure the pH to ensure that samples are equivalent.

In the in vivo method plaque pH is measured using electrodes placed within the mouth. The normal method used involves constructing a special dental bridge incorporating the pH electrode. This is placed in the mouth and the volunteer allows plaque to accumulate over the surface of the electrode. The device is connected to recording equipment which can either be carried around with the volunteer or the experiment is carried out within the confines of a laboratory.

The main advantages of the in vivo method are that large amounts of data can be collected in real time for a long period and that the plaque is not disturbed meaning that the response of the same plaque to different challenges can be compared directly.

The main disadvantages are that volunteers are required to have a specific dentition which permits a bridge to be fitted and that the overall expense is high with the result that only a relatively few individuals can be examined.

The main advantage of the in vitro method is that it is cheap and doesn't require much specialised equipment. This means that it can be performed on quite large numbers of volunteers. The main disadvantage is that it requires considerable skill to obtain reliable results.

Read more about Stephan Curves and their Clinical Relevance

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