Teeth, like all mineralised tissue, must be able to withstand chemical as well as physical trauma. The major chemical threat is dissolution. This is countered by using a mineral which is only sparingly soluble and by surrounding it with a solution which is supersaturated with respect to the salts which comprise the mineral. In the case of bone and the roots of teeth, this is interstitial fluid. Those parts of teeth above the gum margin are protected by saliva.

In order to understand how saliva prevents tooth dissolution it is first necessary to understand the concepts of ionic product and solubility product , the meaning of pK in relation to the effect of pH on hydroxyapatite dissolution and the dissociation equilibria of hydroxyapatite which is dealt with below.

There are a number of calcium phosphate salts which differ in the Ca:P ratio and solubility. Some of these are present in tooth mineral to a greater or lesser extent but the most common by far is an impure form of calcium hydroxyapatite commonly called "Biological apatite" or sometimes just "Apatite".

The impurities, which include sodium, magnesium, potassium, lead, strontium, barium and especially carbonate introduce defects into the hydroxyapatite crystal which render it significantly more soluble. More information on biological apatite and solubility is available here. Complicating the issue even further is the fact that the mineral in enamel has been shown to contain 2-3% of its phosphate in the form of mono-hydrogen phosphate which itself is a component of the hydroxyapatite dissociation equilibrium equation.

Biological apatite is not well defined because of the variability in the impurities. For this reason the dissolution of calcium hydroxyapatite will be described.

The dissolution of hydroxyapatite in an aqueous system is governed by the law of mass action. The net loss of calcium phosphate from the solid phase (tooth) is governed by the ionic product of the relevant ions in solution. In the case of hydroxyapatite these ions are calcium, phosphate and hydroxyl. Note that the important phosphate ion in this respect is the unprotonated form.

At equilibrium there is no net loss/gain of ions in solution or in the solid phase and the ionic product is known as the solubility product (Ksp). The Ksp is difficult to measure accurately because of a number of technical problems, however, a commonly used value is 2.34 x 10^-59.

In the mouth teeth are bathed in saliva which is constantly being replenished. The calcium and phosphate ion concentrations in saliva are variable but on average are both about 1.5 mMoles/Litre. Of course, not all the phosphate is in the unprotonated form but this amount can be calculated and at neutral pH is approximately 5 nanomoles/Litre.

At neutral pH the natural level of calcium phosphate in saliva is sufficient to supersaturate it with respect to hydroxyapatite.

Teeth, however, are made from biological apatite which is more soluble than hydroxyaptite. When powdered human enamel is allowed to equilibrate in aqueous solution the ionic product is significantly greater than the Ksp of hydroxyaptite and significantly less than the the ionic product of saliva.

This means that at neutral or near neutral pH saliva is supersaturated with respect to both hydroxyapatite and biological apatite.

The dissociation equilibrium of hydroxyapatite is very sensitive to the pH of the surrounding medium and exerts its effect by altering the concentration of both unprotonated phosphate ions and hydroxyl ions.

At neutral pH (7.0) saliva is supersaturated with calcium phosphate with most of the phosphate present in either the mono- or di-hydrogen phosphate form. However, as the pH becomes more acid the degree of supersaturation decreases until a point is reached where the saliva ceases to be saturated with respect to the tooth mineral. This is known as the "Critical pH".

Conversely, if the pH becomes more alkaline the degree of saturation with respect to tooth mineral increases and eventually the calcium phosphate in solution becomes unstable and precipitates, not as hydroxyapatite but as the more readily formed mineral, brushite. This precipitation is promoted by nucleating centres within dental plaque and is called calculus.

One of the main functions of saliva is to protect teeth against dissolution via either a cariogenic challenge or dental erosion. This is achieved this by controlling the pH of the oral cavity by means of secreted bicarbonate ions and by maintaining a supersaturated state with respect to the mineral phase of teeth. 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.

Why is mineral not deposited on erupted teeth?
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. Saliva also contains a variety of peptides including the "Proline Rich Peptide" family which serve to stabilise soluble calcium phosphate in vivo.

The degree of calcium phosphate saturation in saliva is so high that the solution borders on instability (technically it is metastable). 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. The calcium phosphate precipitates in these regions, helped by certain plaque bacteria which can act as nucleating centres and further de-stabilising the system.

The profound effect of fluoride on reducing the incidence of caries is well documented. A number of different mechanisms are thought to operate but the main effect is that fluoride promotes the remineralisation of teeth which have been subjected to a cariogenic challenge. These challenges occur at the base of dental plaque adjacent to the tooth surface. The fluid within plaque is high in calcium phosphate and may also contain fluoride ions sourced from the water supply, toothpaste, mouthrinses or the diet.

Calcium fluorapatite is closely related to hydroxyapatite although it is very much more stable with a much lower Ksp. Fluoride can replace hydroxyl ions in hydroxyapatite crystals and such hybrid crystals are sometimes referred to as fluor-hydroxyapatite.

If some fluoride is added to a system in which calcium hydroxyapatite is in equilibrium with the ions in the surrounding aqueous phase the equilibrium will shift quite sharply to favour deposition of mineral (calcium fluorapatite) by acting as a common ion. Only very small concentrations of fluoride are required because at equilibrium the concentration of hydroxyl ions is extremely low.

Even at 1 ppm, the concentration of fluoride used in artificially fluoridated water supplies, fluoride will be many thousands of times more concentrated than hydroxyl ions. This relative difference will increase by a factor of 10 for each drop in pH by 1 pH unit. Furthermore, in dental plaque fluid, the concentration of fluoride can often be orders of magnitudes greater than that found in water especially if it has been recently applied topically in the form of a dentifrice or mouthrinse. Typical dentifrices contain 1500ppm fluoride ion.

Clearly some fluoride will be incorporated at neutral and even alkaline pH but as the pH becomes more acid, the concentration of hydroxyl becomes very much reduced and the effect of fluoride enhanced.

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