Bones and teeth are both composed of an inorganic calcium phosphate mineral with a structure similar to hydroxyapatite.
That might not mean a lot if you are not a geologist but hydroxyapatite is a mineral composed of calcium ions, phosphate ions, and a single hydroxide ion. Unfortunately, newspaper printing machines are not very good at subscripts so the formula reads: [Ca]5[PO4]3[OH].
Hydroxyapatite is a crystalline material found in nature with distinctive physical and chemical properties. It is a very hard substance.
In biological systems, the biological apatite or "bioapatite" crystals have a similar chemical composition but differ from the geological samples because of the way that they are formed. The bioapatite found in bones and teeth is highly disordered and non-stoichiometric.
That means it doesn't form the structured crystalline lattice found in hydroxyapatite rocks and the chemical formula has deficiencies or holes in it as carbonates are found substituted for the two of the hydroxide ions. The structure, as a consequence, isn't perfect and this adds strength to the material.
Bones are not made of solid bioapatite. They are composed of a myriad of cells and tissues with distinctive roles and properties. Typically, the mineral content makes up only about 80% of the bone material in an adult human.
Teeth are similar in that they are composed of different components with the interior being living tissues and the outer surface or enamel which is mineralized with little or no protein.
Both bones and teeth act as a ready source of the key regulatory inorganic ions calcium, magnesium, and phosphate. Bones are constantly in a state of flux with osteoblast and osteoclast cells controlling the mineralization and dissolution of the bioapatite.
In teeth, the enamel is predominantly formed in the gum prior to eruption for both baby and adult teeth. Mineralization and demineralization is much more problematic. However, it continues to occur. The material in teeth is continually being replaced.
To understand this we need one of the subjects taught in high school chemistry: solubility. Everything dissolves but everything doesn't dissolve to the same extent.
Consider table salt. It is a simple cation, Na+, combined with a simple anion, Cl-. When put in water, the molecules of water combine to form aquated species. Six water molecules grab each sodium ion while six grab each chloride ion.
But there are only so many water molecules to go around. In a litre of water, this amounts to about 55.5 moles - meaning that you can only dissolve 55.5/12 or 4.5 moles of sodium chloride. It is actually a little bit more than this because not all of the sodium chloride dissociates.
In any case, you reach a solubility limit. Any more sodium chloride added to the mixture doesn't dissolve. It sits on the bottom of the flask.
This is an example of a solubility equilibrium. The relationship between the amount of dissolved sodium chloride and solid sodium chloride can be described mathematically by a solubility product.
One of the harder things to understand about this is that sodium and chloride ions are still dissolving from the solid. However, just as many sodium and chloride ions are precipitating out. Solubility equilibria are dynamic with the constant and continual exchange of mass.
What has this to do with teeth?
Everything. Even my opponent in a recent debate recognizes the value of fluoride in strengthening and protecting teeth. Fluoridation relies on the exchange of the hydroxide and carbonate ions in the tooth enamel for fluoride ions. In a dentist's office, this is accomplished with a variety of fluoridation techniques such as rinses and gels.
Fluoride ions strengthen the bioapatite by remineralizing the material as fluoroapatite. It is both structurally stronger and far more resistant to acid. The result is stronger, healthier teeth that are less susceptible to dental caries.
The problem is fluoroapatite is a mineral made of ions, just like sodium chloride. If exposed to water, it dissolves just like table salt. In the case of fluoroapatite, the solubility product is very, very low so it takes a long time to dissolve. But it will dissolve.
To alter this equation and to help prevent the demineralization of teeth, the simplest and most expedient method is to include fluoride in drinking water. This inhibits the dissolution of the fluoroapatite and, in turn, maintains the strength of the tooth enamel.
This effect is not just topical. It extends to ingested fluoride which is excreted in saliva which provides a further inhibitory effect. It slows down the dissolution of enamel and helps to inhibit bacterial growth in the mouth.
Cavities form when acidic compounds come into contact with teeth because the hydroxide ions in biological apatite are neutralized by the protons in the acid. Fluoridation inhibits this process and teeth last longer.
Fluoride treatments by dentists and fluoride in drinking water are complimentary components of healthy teeth and brilliant smiles.
