The protective role of fluoride in teeth has been confirmed, yet again. A new study nicely demonstrates how incorporation of even a small amount of fluoride into the surface layer of teeth protects them from the acid attack which leads to tooth decay.
Researchers measured the ablation, or loss of surface material from hydroxyapatite before fluoridation and after fluoridation. It showed a clear difference due to inhibition of ablation by fluoride.
The research findings are published in:
Faidt, T., Friedrichs, A., Grandthyll, S., Spengler, C., Jacobs, K., & Müller, F. (2018). Effect of fluoride treatment on the acid-resistance of hydroxyapatite. Langmuir
Samples were etched with a sodium acetate buffer at pH 4.5 which simulated the effect of an acid attack on teeth resulting from the formation of acid when sugars are microbiologically decomposed in the mouth. The degree of ablation was measured using atomic force microscopy (AFM). Part of the sample surface was coated with a gold layer to prevent acid attack and give a reference surface.
Fluoridated surfaces, submerged for five minutes in a sodium acetate buffer at ph 6.0 containing 500 mg/L of sodium fluoride, were compared with unfluoridated surfaces.
Interestingly, the AFM height images showed there were two different areas of the hydroxyapatite surface when it came to ablation – a fast etching area and a slow etching area. The authors attributed this to the different orientations of crystallites in the hydroxyapatite sample. The image below is for an unfluoridated sample
Ablation of fluoridated samples was quite different – no ablation occurred until after 330 seconds – the image below is for a fluoridated sample
The paper summarises the results for the fluoridated and unfluoridated surfaces and the different ablation rates due to crystallite orientation in this figure:
The crystallites that etched slowly (Z2) in the unfluoridated sample did not etch at all in the fluoridated sample. The more rapidly etching crystallites (Z1) did etch in the fluoridated sample but only after a delay.
The authors concluded that some of the fluoride in the surface layer of the fluoridated samples could eventually be removed by soaking in the acid buffer – but only after a delay. This was confirmed by an analysis of the surface concentrations of Ca, P, O and F using X-ray photoelectron spectroscopy (XPS) – see below:
Thickness of the fluoridated surface layer
The authors recall:
So – a very thin layer. One that some anti-fluoridation commenters claimed insufficient to give any protection. As the authors say
” the question arose whether such a thin layer would actually be capable of protecting the surface against acid attacks. “
But, their results definitely show that this thin layer does offer protection. I am sure critics will quickly point to the fact that the experimental study showed the removal of some of the fluoride after about 400 seconds. But this removal should be seen in the light of the dynamic system in the oral cavity where the pH of saliva is changing, dropping due to sugar decomposition and then rising again. The presence of fluoride, together with phosphate and calcium in saliva also leads to repair of areas where acid attack has occurred.
This experimental work confirms the protective role of fluoride in saliva for existing teeth – despite the fact that the fluoridated layer may be extremely thin – of the order of a few nanometers. While some of the fluoride in the surface layers is eventually removed the presence of fluoride in saliva helps replenish these layers and repair areas of acid attack.
The authors conclude their results provide:
“evidence that already thin and low concentrated fluoridated layers have a large effect on the acid resistance of HAp [hydroxyapatite]”
They combine these finding with results from a previous study of theirs showing fluoridation reduced adhesion forces of bacteria on hydroxyapatite (HAp) to finally conclude:
“the caries-preventive effect of fluoride is an interplay of at least two mechanisms: a reduction of the solubility and a reduction of the bacterial adhesion force.”
Hence the figure at the top of this article.
Featured image: Fluoride protects teeth from the attack of acid and microbes. Figure from Faidt et al., (2018)