Acid and Hydroxyapatite Solubility

The solubility of hydroxyapatite is greatly affected by the pH of oral fluids. In general, a more acidic environment causes hydroxyapatite to become more soluble, while a less acidic (more neutral or basic) environment makes hydroxyapatite less soluble.^14,15-17 In a healthy oral environment not exposed to acid challenges, such as those caused by dietary, gastric, or medicinal acids, both plaque fluid and saliva are typically supersaturated with calcium, phosphate, and hydroxyl ions. This supersaturation helps prevent the dissolution of tooth enamel. However, despite this favorable ionic environment, hydroxyapatite crystals do not continually grow on the enamel surface. This is due to the presence of specific salivary proteins, such as statherin and proline-rich proteins, which inhibit hydroxyapatite crystal growth. These proteins coat the enamel surface and prevent crystal seeding, thus maintaining a balanced mineral state.¹⁴

If an acid challenge causes plaque fluid and saliva to become more acidic, hydrogen ions bind with calcium, phosphate, and hydroxyl ions, effectively removing these ions from the solution. As a result, the solution, becomes undersaturated with respect to hydroxyapatite, leading to the dissolution of dental hard tissues. The greater the degree of undersaturation, the more extensive the demineralization. In general, the solubility of dental hard tissues increases by a factor of 10 with each one-unit drop of pH. Dissolution continues until the solution becomes saturated again.

Because dental plaque is in close proximity to the tooth, and generally prevents access of saliva to enamel, greater attention is given to the level of supersaturation within the interbacterial fluid in dental plaque. This fluid rapidly loses its supersaturation following exposure to sucrose, and becomes increasingly unsaturated as sucrose concentration rises. Frequent sucrose exposures cause repeated and rapid fluctuations in plaque fluid pH, which have been shown to deplete calcium and phosphate reservoirs within the plaque. This depletion promotes pH-induced undersaturation, which increases the cariogenic potential of plaque fluid. Consequently, the frequency of sugar intake is considered more harmful to dental health than the total sugar amount of sugar consumed.^11,18