Caries Management - Science and Clinical Practice. Группа авторов

pH, plaque fluid is actually supersaturated. This means that the pH must fall to a certain value before the plaque fluid becomes just saturated with respect to the dental mineral. This pH is called the critical pH.²⁴ For enamel mineral, this is calculated from analyses of plaque fluid to be 5.2–5.5. Dentin mineral is known to be more soluble than enamel mineral,²⁷ so the critical pH must be higher than for enamel, but as the solubility of dentin is not accurately known a good estimate has yet to be made. Once the pH of plaque fluid falls below the critical pH, demineralization can occur. Conversely, once the pH of plaque fluid rises above the critical pH, dissolution will cease. Therefore, only the fraction of the Stephan curve between the critical pH and the minimum pH represents a cariogenic challenge (see Figs. 2.2 and 2.13).

      During the second phase of the Stephan curve, pH returns to the resting value, but this typically takes longer than the pH fall in the first phase of the curve. During this phase, H⁺ ions are released from the immobile buffers associated with the bacterial cell walls and diffuse slowly outward into the saliva. H⁺ ions are also removed from plaque by mobile buffers—small, diffusible ions such as phosphate and bicarbonate—which act as H⁺ “carriers” between the plaque fluid and the saliva³⁸ (Fig. 2.14). During the second phase of the Stephan curve, the pH will rise above the critical pH and precipitation of mineral will become possible. This can occur either as crystal growth within the caries lesion—where it will help to replace lost mineral (remineralization)—or as precipitation within the plaque (calculus formation).

      Acids are not the only products of sugar utilization by plaque bacteria. In particular, many plaque bacteria synthesize polysaccharides from dietary sugar (see Chapter 11). These include extracellular polymers of glucose (glucans) or fructose (fructans) and intracellular glycogen-like storage polysaccharides. All are relevant to the caries process. Intracellular polysaccharides (IPS) can be utilized for energy between intakes of sugar, as can soluble extracellular fructans, and the resulting acid production may extend the recovery phase of the Stephan curve. IPS-producing bacteria are more abundant in the plaque of caries-active individuals, owing to the selection pressure of frequent sugar ingestion.¹⁹ Extracellular glucans tend to be insoluble and are not utilized for energy. The insoluble, sticky glucans synthesized by S. mutans are important in the adhesion of these bacteria to tooth surfaces and in their retention in plaque. They may also increase the cariogenicity of plaque in another way. In plaque with abundant extracellular glucans, the number of bacteria per unit volume is reduced. Consequently, sugar diffusing into plaque at the start of a cariogenic challenge is utilized less rapidly and so can diffuse further into the plaque. This brings acid production closer to the tooth surface, thereby creating a more cariogenic environment³⁹ (see Fig. 2.12). Sucrose might induce other alterations in plaque matrix composition which lead to depletion of mineral ions.³⁹

      Fig. 2.13 Stephan curve. The dashed horizontal lines indicate the range for the critical pH. The shading indicates the period of the cariogenic challenge, when the plaque fluid is undersaturated with respect to enamel mineral. The denser shading below the lower estimate of critical pH indicates the increasing severity of the challenge with falling pH (see Fig. 2.7).

      Fig. 2.14 Clearance of H⁺ ions from plaque during the second phase of the Stephan curve. H⁺ ions released from fixed (nondiffusible) buffers associated with bacterial cell walls (left) diffuse out slowly (dashed arrow, bottom). Clearance is accelerated by diffusible buffers from saliva—bicarbonate (top) and phosphate (middle)—which act as H⁺-transporters.

      Fig. 2.15 Release of calcium ions bound to bacteria during a cariogenic challenge. At neutral pH, Ca²⁺ ions are bound to carboxyl groups (top left) and phosphate groups (bottom left), associated with bacterial cell walls. When plaque pH falls, Ca²⁺ ions are displaced by H⁺ ions (top right, bottom right). The resulting fall in H⁺ ion concentration and the rise in Ca²⁺ concentration raise the degree of saturation of the plaque fluid and hence ameliorate the cariogenic challenge. Some Ca²⁺ ions act as bridges between neighboring plaque bacteria (middle) and increase plaque cohesion; these bridging Ca²⁺ ions are also released under acidic conditions.

      The overall severity of a cariogenic challenge is influenced by several factors. It is mainly the overall concentration of sugar in a food item that determines how far the pH will fall in a single exposure, so high-sugar foods pose a greater challenge (see Fig. 2.2). Similarly, the more frequently sugar is consumed, the longer the plaque pH can remain below neutrality, so that the potential for remineralization is reduced (see Fig. 2.2). As predicted by the ecological plaque hypothesis, frequent exposure to sugar causes selection for acidogenic, aciduric bacteria and for bacteria producing intracellular polysaccharides, which will intensify the cariogenic challenge (greater, more prolonged pH fall). If the main sugar to which the plaque is exposed is sucrose (the most abundant sugar in sweetened foodstuffs), the plaque cariogenicity will be increased as explained above. Although complex dietary carbohydrates, such as starch, are thought to be relatively noncariogenic, starch probably increases the stickiness of sugar-rich foods, thereby retaining them close to the tooth surface and prolonging the cariogenic challenge.⁴⁰

      Some members of the plaque flora mitigate the effects of lactic acid production. Veillonella, which is consistently found in plaque, derives energy from metabolizing lactic acid to acetic and propionic acids. As these are weaker acids than lactic acid, they will remove H⁺ from the plaque fluid and thus tend to raise pH.³ Other bacteria can produce ammonia from nitrogen-containing substances—amino acids, for example—and this will tend to raise the pH. Plaque bacteria bind appreciable quantities of calcium⁴¹ and when the pH falls, Ca²⁺ ions are released (Fig. 2.15; Table 2.2) in exchange for H⁺ ions. This will increase the degree of saturation.

      The main host factors influencing the cariogenic challenge are the flow rate and buffering capacity of saliva.^1,38 The pH rise in the second phase of the Stephan curve is facilitated by good salivary flow, as this helps to clear H⁺ ions away from the plaque. A mitigating factor in the caries process is that consumption of foods stimulates salivary flow and this is accompanied by increased salivary bicarbonate concentration: both enhance H⁺ ion removal from the plaque.

       NOTE

      The presence of dental plaque, made up of densely-packed bacteria, on sheltered regions of the tooth prevents flow of oral fluids over the tooth surface, so exchange of substances between the tooth surface or plaque and the oral fluids occurs by the slow process of diffusion. Consequently, metabolism