An Analysis of Factors Influencing Diffusion from Dental Plaque into a Moving Film of Saliva and the Implications for Caries

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Abstract
Recent studies have indicated that saliva in the mouth is present as a film only about 0.1 mm thick, and that this film moves at different rates (about 0.8 to 7.6 mm/min) in different regions of the oral cavity. The clearance rates of KCI, as a model diffusant, from agarose gels at different sites in the mouth have also been found to vary markedly, and it has been proposed that these variations are related to differences in the velocity of the salivary film. A computer model has been developed for prediction of clearance half-times for substances diffusing from plaque of variable dimensions into a film of fluid 0.1 mm thick, moving at different velocities. The results show that over the range of velocities calculated to occur in the mouth, the clearance half-times are directly related to the length of plaque over which the fluid passes, and inversely related to the salivary film velocity. The predictions of the model are in good agreement with experimental results from a physical model. Tests were made of the predicted effect of salivary film velocity on the shape of the pH curve initiated by exposure of plaque to a saturated sucrose solution, followed by normal salivary clearance. With a low salivary film velocity, the fall in pH was greater and more prolonged. These results are relevant to the site-specificity of caries, since they imply that, given two plaque samples of identical area, depth, and microbial composition, the sample located in a region of the mouth where the salivary film velocity is lower will show a more extensive and prolonged pH fall after exposure to fermentable carbohydrate.