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Teeth enamel is permeable, you can see this by people who smoke having yellowed teeth, people who drink sugary drinks having yellowed teeth etc

Visually it's quite amazing how rough the enamel is upon magnification!

Fluoride, being a mineral suspended in water or, in this case, toothpaste, can go into the small gaps and divets in the enamel. It bonds into the little divets and acts like a shield against bacteria and other gunk.

When the pH on the tooth surface becomes acidic, the enamel starts to demineralize so your tooth is more susceptible to bacteria. Fluoride in the mouth enhances remineralization of the enamel using the calcium and phosphate ions in the saliva. There's different sources of oral fluoride. Stannous fluoride in certain toothpaste gels sticks to the enamel surface. It's supposed to be stop bacterial proliferation > biofilm > plaque and kill bacteria. Sodium fluoride works better in a toothpaste because it is more abrasive.

In Europe there's biomimetic hydroxyapatite to create a new layer around teeth to harden the existing enamel so fluoride or lack thereof isn't as big a deal.

'stronger' = more resistant to acid attack.

Teeth are made of a form of hydroxyapatite, mostly composed of calcium and phosphate (with some other traces of magnesium, carbonate etc). Enamel is 99% mineral, with a tiny bit of water and proteins. Acids from bacterial fermentation of sugars are what dissolves tooth enamel and causes tooth decay. The addition of fluoride turns the mineral into fluorapatite, which is much less soluble by acids. It doesn't make the teeth stronger in the physical sense like being able to withstand stronger chewing.

The fluorapatite enamel can still dissolve if there's constant acid attack, just less readily than unfluoridated tooth apatite. It's not invincible

While I'm at it, might as well go deeper into the science and mechanism of how fluoride works in other ways.

When enamel dissolves, the calcium and phosphate are released into the solution around it. In a highly acidic environment, the Ca and PO4 do not readily form a solid structure (i.e. crystals). With fluoride, the formation of crystals happens more readily and the required pH to inhibit crystallization is much lower. Hence, fluoride helps encourage remineralisation of teeth after an acid attack.

Calcium and phosphate also exists in other forms such as mono-, di-, tri- and octocalcium phosphates. These are low-energy crystalline forms, less ordered and more prone to dissolution than hydroxyapatite. Calcium and phosphate tend to organise themselves into these low-energy forms, e.g. tartar on teeth. With fluoride, the ions actually tend to want to form fluorapatite instead of the weaker, low-energy forms

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Featherstone, J. D. B. (2008). "Dental caries: A dynamic disease process". Australian Dental Journal. 53 (3): 286–291. doi:10.1111/j.1834-7819.2008.00064.x

https://onlinelibrary.wiley.com/doi/10.1111/j.1834-7819.2008.00064.x

I'm also a dentist, and did my training at a dental school where the research behind remineralisation and the dental caries process was heavily promoted and taught

That's a great review with clear photomicrographs and schematics. Enjoyed reading it. There are updated reviews available via Pubmed, too, for those that want the newest in-depth information--just search "dental caries pathophysiology" and screen for free full text.

Some examples are: https://pubmed.ncbi.nlm.nih.gov/35129809/ and https://pubmed.ncbi.nlm.nih.gov/35129809/

It is interesting that osteoporosis and dental caries have overlapping mechanisms, and together represent "the most prevalent chronic disease in the world" (per the first paper cited above). Osteoporosis affects approx 10% of the global populace and dental caries affect 100%...

Underneath the enamel surface the dentin layer contains an organized network of tubules that can be clearly seen on micrographs.

Here's a very cool article on the difference between narwhal tusks and human teeth :=} Towards the end it has a figure/photo of the dental tubules. https://biologicalexceptions.blogspot.com/2015/03/the-search-for-unicorn-slightly-off.html

It's a bit of a stretch for this article to say dental caries has a prevalence of 100% of people. (Somewhat technically correct, but really stretching it and trying to make a dramatic effect)

For it to be called dental caries, there has to be a bacteriological element to it. Meaning the demineralisation is due to acid-producing plaque bacteria.

Yes, if you were to hypothetically extract ALL the teeth of a random sample of people and examine every surface of those teeth, you'll find that everyone will likely have a spot on a tooth somewhere that has lost some mineral, even if it's a tiny, minuscule area. So what?

It's certainly near 100%, but I'd disagree with the idea that absolutely everyone has active decay in their mouth and is suffering the effects of it. As long as you're alive and eating food, there will be *some* demineralisation of teeth, but in most cases the minerals reconstitute back into the tooth shortly after. There's always a constant cycle of demineralisation and remineralisation every day, and it's not always due to bacteria. A lot of fruits have a pH below the critical pH for demineralisation.

Sure, at some point in anyone's life, the demineralisation can outstrip remineralisation and there is a net loss of mineral from a certain tooth surface. For the demineralisation to be clinically significant (i.e. detectable by visual or radiographic means), it takes months or years to develop. And if it's an early stage, shallow lesion, there's still a good chance for it to remineralise completely if the causative factors are addressed.

Bones are also in a constant state of resorption and deposition. When someone's bone density goes below a certain threshold (T score -2.5), suffered a pathological fracture, or has a combination of factors that puts them at high risk of pathological fracture, only then can it be called osteoporosis.

Whereas someone can technically be diagnosed with dental caries the moment a small spot of demineralisation is detected, regardless of where it is in the continuum of the demineralisation-remineralisation dynamic.

If the diagnostic criteria for osteoporosis were changed to "detection of any resorptive loss of bone mineral in any bone in the body", you could then technically say that 100% of people have osteoporosis too.