Physical model for plaque action in the tooth-plaque-saliva system

Plaque fluid and diffusion: study of the cariogenic challenge by computer modeling

Every cariogenic challenge involves a mixture of convective transport, diffusive transport, and biochemical reactions, plus physico-chemical reactions (including charge-coupling of diffusion), all of which together require numerical methods for their analysis. This presentation describes a one-dimensional finite-difference computer model of the cariogenic process, and some conclusions obtained from it. Sugar clearance from the mouth, together with site-dependent exchange between the bulk saliva and plaque surface via a salivary film, is combined with a finite-difference model of events occurring within the dental plaque. The latter includes: sugar diffusion and pH-dependent acid production; diffusion and dissociation equilibria for two acid end-products of fermentation and their anions (acetate and lactate); diffusion and dissociation equilibria of phosphate buffer; diffusion of potassium and chloride; diffusion of protons and simultaneous equilibration with fixed and mobile buffers. So that proper concentration distributions consistent with local charge neutrality can be ensured, an algorithm called Q-couple is used to impose charge-coupling between the fluxes of different ions including fixed charges. Mineral dissolution and precipitation are modeled as part of the same equilibrium calculations. The predictions of the model are compared with those of an earlier, much simpler one, in which fixed buffers were not included. It is shown that the known concentration of fixed buffer greatly extends the low pH of a Stephan curve. The isoelectric point of the plaque bacteria also appears to be of importance. The effects of various concentrations of mobile buffers, including acetate, are investigated. It is also shown that varying plaque/saliva contact over the known range derived from published studies has a profound effect on the modeled cariogenic challenge.

The effects of experimental variations in dietary sugar intake and oral hygiene on the biochemical composition and pH of free smooth-surface and approximal plaque

The biochemical compositions of free smooth-surface and approximal plaque were compared after 48 hours in the presence and absence of twice-daily toothbrushing and during ingestion of either high- or low-sugar diets in ten young adults. In addition, the effect of a single sugar intake on the pH of both types of plaque was investigated. The results confirmed previously reported differences in biochemical composition between free smooth-surface and approximal plaque, the concentrations of inorganic ions and carbohydrate fractions being generally lower in the approximal sample after all experimental regimens. The concentrations of calcium and phosphorus were doubled on the low-sugar diet compared with the high-sugar diet in both types of plaque. A significant decrease in carbohydrate concentrations was observed in free smooth-surface plaque in subjects on the low-sugar diet, but not in approximal plaque. Little variation was observed in the biochemical composition which could be attributed to the presence or absence of toothbrushing. The pH in free smooth-surface plaque was lower than that in approximal plaque five minutes after a sugar intake, but the situation was reversed two hours later. The results indicate that differences in composition and pH between free smooth-surface and approximal plaque are associated with reduced access to salivary and dietary influences due to diffusion limitation in the deeper layers of plaque.

Collection and biochemical analysis of human dental plaque from the approximal tooth surfaces and comparison with plaque from free smooth surfaces

The composition of approximal plaque was compared with that normally collected from the free smooth surfaces in investigation of 45 males aged 11-14 years and 75 females aged 13 years. Statistically significant differences in calcium, inorganic and organic phosphorus, magnesium, fluoride and carbohydrate were observed between free smooth-surface and approximal plaque. The concentrations were always lower in the approximal sample, ranging from 55 to 70 per cent of those in free smooth-surface plaque. A subsequent comparison in 12 students indicated that the concentration of total lipids in approximal plaque was about twice that of free smooth-surface plaque, a difference which could explain the quantitative disparity in total composition between the two types of plaque in the studies with children. The differences are consistent with approximal plaque having reduced access to a range of environmental factors due to diffusion limitation in its deeper layers.

The influence of salivary flow rate on diffusion of potassium chloride from artificial plaque at different sites in the mouth

The rate at which substances diffuse from dental plaque influences the rate of clearance of acid and bacterial toxins from plaque into saliva. The aim of this study was to compare the rates of clearance of potassium chloride, as a model substance, from artificial plaque of 3-, 4-, and 6-mm-diameter, positioned bilaterally at different locations in the mouth. The diffusant was KCl (1 mol/L) in a 1.0% agarose matrix, placed in wells 1.5 mm deep, in small acrylic devices 3 mm thick, which could be fastened to the teeth with dental floss and removed after different time periods. The half-time for clearance was determined from the best-fitting least-squares line of the potassium concentration remaining in the gel plotted against the square root of time. For 14 subjects, half-times for the lower anterior lingual and upper posterior lingual regions averaged about 2.5 times greater than those for clearance into a large, stirred volume in vitro, whereas those for the upper and lower anterior buccal regions averaged about 12.8 times greater. This difference may be due to the fact that anterior buccal sites are exposed only to minor rather than to major salivary gland secretions. When salivary flow was stimulated by the sucking of sour lemon drops, all in vivo half-times were reduced by about one-half. The half-times were also directly related to the surface areas of the chambers, which implies that rates of diffusion from plaque of substances such as acid or bacterial toxins are inversely related to the surface area of the plaque at a particular site.

A theoretical analysis of the effects of plaque thickness and initial salivary sucrose concentration on diffusion of sucrose into dental plaque and its conversion to acid during salivary clearance

A mathematical model, written in FORTRAN, has been developed to simulate the interrelated processes of salivary sucrose clearance from the mouth, diffusion of sucrose into dental plaque, and conversion of sucrose to acid and glucan. Reaction of acid with enamel is not included in the model. A total of 28 parameters can be varied by the user, and the relative importance of the different factors affecting acid formation can be assessed. The output of the program gives sucrose and acid concentrations and pH at different depths within the plaque. The initial variables studied were plaque thickness, the salivary sucrose concentration, and the duration of exposure of the plaque to sucrose. Stephan curves typical of those recorded in vivo were generated by the model. With any particular salivary sucrose concentration, there was an optimum plaque thickness at which a minimum pH was achieved at the enamel surface, with very thin or thick plaque samples producing a smaller pH fall. With thick plaque, the minimum pH was often not achieved at the inner surface but at some intermediate depth, which may explain the location of early caries lesions in fissures. The extent of the pH fall at the inner surface and the duration of the pH-minimum region of the Stephan curve were directly related to the initial salivary sucrose concentration and to the duration of exposure to sucrose prior to normal salivary clearance. Simulation of a water rinse at as short a time as two min after the beginning of normal salivary sugar clearance showed that this procedure had only a very small effect on the shape of the Stephan curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Dental caries: prospects for prevention

Combined utilization of measures now available or imminent could reduce caries of the crowns of the teeth to the point of negligibility as a public health problem, if public desire were great enough to motivate changes in some of our habits (64). Universal optimum application of fluoride and substitution of starchy foods for sugary ones (or even simply judicious consumption of sugar) would alone do most of the job. Sealing of susceptible occlusal areas with adhesive polymers promises to protect the sites where fluoride evidently cannot be maximally effective. It seems unlikely that any single measure will be found sufficient to control this multifactorial disease. Consequently, we must continue the search for new means to increase the caries resistance of teeth, to reduce the cariogenicity of foodstuffs, and to check the deleterious activities of cariogenic bacteria. Anticaries food additives and antibacterial agents for intraoral use seem to be approaching practicability. Past performance warrants expectation that ongoing fundamental investigations will produce leads for future development and application.