As far as I understand, fluoride helps in hardening the enamel of our teeth by replacing the calcium element in hydroxyapatite to convert it in the stronger fluorapatite, like explained here. Wether that's worth other potential risks of using fluor is debatable, but let's keep that aside. I'm curious if it is still beneficial to keep using fluorised toothpaste if you already did so for like 10 years. I suppose all hydroxyapatite is long converted into fluorapatite already after so much brushing. Why would it still be recommendable to keep using it?
Fluoride can be absorbed into the teeth and form fluoroapatite only in children up to 6-8 years of age (WebMD).
Later, fluoride from toothpaste may still be helpful, because it stimulates the incorporation of calcium and phosphorus into the enamel that has been demineralized (PubMed Central, 2006). So, fluoride stimulates remineralization and thus slows down the development of caries; it doesn't mean that it cures caries.
A summary of RCTs on fluoride concentration in toothpastes showed a positive dose response: Pastes with 1000–1500 ppm F showed 23% caries reduction compared to fluoride-free placebo; this value increased to 36% for pastes with around 2500 ppm F. For pastes having below 1000 ppm F, no significant difference was found with placebo, probably due to the small number of studies.
^^ The above means, there was less caries after fluoridated paste use, and not that the established caries was cured.
The main protective effect of fluoride is outside the tooth, not inside.
Small amounts of fluoride in solution around the tooth inhibit demineralization more effectively than incorporated fluoride and have a much greater caries-protective potential than a large proportion of fluorapatite in enamel mineral. Schweiz Monatsschr Zahnmed 122: 1030–1036 (2012)
For example, even an incredible amount of fluoride has limited protective effect. In a classic study, Ogaard compared the resistance of fluoroapatite (shark enamel) and hydroxyapatite (human enamel) against a high caries challenge in a human in vivo model. Two samples of shark enamel and human enamel were each placed in removable appliances in six children and carried for 1 month and a plaque retentive device was placed over each enamel sample. The results showed that the mean total mineral loss (delta Z) was 1680 vol% micron in human enamel and 965 vol% micron in shark enamel. The corresponding mean values for lesion depth were 90 micron and 36 micron, respectively. It was concluded that even shark enamel containing 30,000 ppm F has a limited resistance against caries attacks.
In a later review, the same author concludes that
The fluoride concentration in the apatitic structure of enamel does not have as significant an effect on reducing caries as a continuous presence of fluoride in the plaque liquid.
Hence, to receive the protective effect of the fluoride, we require to keep it near the tooth surface all time.