How does fluorine actually help prevent tooth decay?

[TheExibo]

Fluorine ions are negatively charged, and help prevent tooth decay by forming fluoraptite when substituting the lost hydroxide ions in the tooth enamel. Since they are both negative, it is successfully substituted. However, it was told that acid on the tooth causes that decay. My question is, how do the fluroine ions not react with the acid on the tooth, or is that what they also do?
Thanks!

 

[Greg Bernhardt]

My question is, how do the fluroine ions not react with the acid on the tooth, or is that what they also do?

This article (dated 1999) apparently suggests that they do
http://www.scientificamerican.com/article/how-does-the-fluoride-in/