Although bicarbonate can exert an independent effect by neutralising acid it is important to remember that it also controls the degree of calcium phosphate saturation of saliva, with respect to tooth mineral, especially since the concentration of these ions does not vary greatly outside of dental plaque. Since saliva is supersaturated with respect to tooth mineral it should follow that the mineral crystals comprising the very surface of teeth, should grow as more calcium phosphate is deposited which is the basis of crystal gardening kits for junior scientists. This does not happen because a solution of calcium phosphate requires a nucleating centre for hydroxyapatite deposition and the very surface of teeth are coated in an aquired pellicle of proteinaceous material derived from saliva which masks the underlying crystals. Dental erosion Caries initially presents as a sub-surface lesion caused by organic acids in the overlying plaque. This lesion may then cavitate and spread to give the characteristic lesion such as the one shown above.
Erosion is a surface event caused by frequent exposure of the tooth to acidic conditions.
Excessive consumption of carbonated drinks can cause erosion as can frequent vomiting. A small rise in pH as a result of, say, increased flow rate when the bicarbonate concentration rises dramatically can be enough to instigate calcium phosphate precipitation. Salivary bicarbonate is in equilibrium with the intra-oral gaseous carbon dioxide. At elevated concentrations it can be rapidly lost with a resulting drop in salivary pH. The highest bicarbonate concentrations are found adjacent to the openings of the salivary ducts where an equilibrium has not had chance to establish. This, therefore, is the region where salivary calcium phosphate is at its most unstable.