A grown tooth has a fixed structure. See picture. On the outside you find a white hard glossy surface layer. This is the enamel. It is mineralised for 96%. Underneath this enamel you find the tooth bone or Dentin. This is mineralize for 70%. And further inside you find a space containing blood vessels and nerves forming the so called pulp. Take note that the blood vessels and nerves do not reach into the enamel. So, any ingested fluoride cannot get via the blood stream to the enamel. Therefore ingesting fluoride to protect your teeth from decay is useless.
Graphic from: http://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/Tooth_Section.svg/496px-
Tubules
The dentin contains thousands of tiny canals or tubules, which allow fluid and nutrients from the pulp to reach the dentin throughout. These tubules are running from the pulp with a wavy S-form to the dentin/enamel boundary. Through this tubular structure filled with fluid the dentin gets some flexibility and resilience. The tubules are wide near the pulp and taper to very narrow towards the enamel and end there as dead end streets near the boundary line dentin/enamel. They have also numerous little side branches and some of them interconnect the tubules. This arrangement forms a gigantic mineralised inner surface paved with calcium phosphate in the hydroxy-apatite form. Any fluoride getting into these tubules would immediately react with the calcium and change hydroxy-apatite into fluoro-apatite, which is much more brittle.
In the 1950s and 1960s research was done to see whether fluoride intake would have an effect on the inside of teeth. It turned out that fluoride was indeed deposited in the dentin. The highest fluoride concentration was found in the wider section of the tubules near the pulp. This indicated that the fluoride came from the pulp blood supply. It was also found that when the tooth ages, the tubules were filled in with a hydroxy-apatite material and became calcified. The pre
