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 environment makes hydroxyapatite less soluble.1,11-13 In a healthy oral environment that is not undergoing an acid challenge due to dietary-, gastric-, or medicinal-related acids, plaque fluid and saliva are supersaturated with calcium, phosphate, and hydroxyl ions, preventing dissolution of tooth enamel. (Despite this supersaturation, however, hydroxyapatite crystals do not continuously grow on the tooth surface. This is because saliva contains protein inhibitors of hydroxyapatite crystal growth, such as statherin, as well as proline-rich proteins that coat the enamel surface and prevent seeding by exposed crystals.1)

If an acid challenge causes plaque fluid and saliva to become increasingly more acidic, calcium, phosphate, and hydroxyl ions combine with hydrogen, removing these ions from the solution. The solution, therefore, becomes undersaturated with respect to hydroxyapatite, and tooth hard structure dissolves. The more undersaturated plaque fluid and saliva is, the greater the amount of dissolution.1,13 In general, the solubility of dental hard tissues increases by a factor of 10 with a drop of each single pH unit. Dissolution continues until saturation is established once again.

Because dental plaque is in close proximity to the tooth, and generally prevents access of saliva to enamel, more attention is paid to the level of supersaturation in the interbacterial fluid in dental plaque. This fluid loses its supersaturation very quickly in response to an exposure to sucrose, and becomes more unsaturated as the concentration of sucrose increases. Frequent sucrose exposures that cause the pH of dental plaque fluid to cycle up and down repeatedly and quickly have been found to deplete calcium and phosphate reservoirs in plaque. This promotes pH-induced undersaturation, which increases the cariogenic potential of plaque fluid. This is why the frequency of sugar intake is considered more harmful than total sugar intake when it comes to caries.8,14