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The Stephan Curve

The impact of saliva in neutralising plaque pH was illustrated as early as 1940 by Dr. Robert Stephan, who measured the changes in plaque acidity following sugar intake. He used indwelling antimony microelectrodes to monitor changes in plaque pH in situ following a sucrose rinse, and plotted the result on a graph, which has become known as the Stephan Curve.

The Stephan Curve, after Dr. Robert Stephan, an officer in the U.S. Public Health Service, in 1940

Characteristically, the Stephan Curve shows a rapid drop in plaque pH, which is followed by a slow rise until resting pH is attained. The critical pH below which demineralisation of enamel generally takes place is 5.5, shown as the shaded part of the curve.

The time taken for these changes to occur varies between individuals and also varies according to the nature of the challenge. The drop in pH usually takes no more than five minutes whereas the recovery for the resting pH can take between fifteen and forty minutes depending on the acid neutralising properties of an individual’s saliva.

The initial sharp drop depends upon the speed with which plaque microbes are able to metabolise sugar. Large molecules, like starch for example, diffuse into the plaque more slowly and take longer to be broken down resulting in a less steep drop in pH.

The rise in pH is dependent upon diffusion of acid by-products out of the plaque and, secondly, salivary bicarbonate diffusing into the plaque buffering the acid by-products. One of the most important factors governing the overall shape of a Stephan Curve, but particularly the pH recovery, is saliva flow rate.

Saliva exerts two effects. First, it dilutes and carries away metabolites diffusing out of the plaque. Second, it supplies bicarbonate ions which diffuse into plaque and neutralise the by-products of fermentation (organic acids) in situ. The bicarbonate-mediated acid neutralisation effect is enhanced by the increase in salivary bicarbonate associated with increased saliva flow, which coincides with eating or chewing e.g. sugarfree chewing gum.

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