Hydroxyapatite weakens acids and is a calcium-source for the teeth during acid attacks.

Acids can erode our teeth leading to tooth wear. The stronger an acid, the lower its pH. In a healthy person, the oral cavity has a pH in the neutral range, i.e. a pH ≈ 7. Our teeth can be damaged at pH < 5.5. However, the pH level is not the only important factor for acidic tooth wear. The presence of other ions or components in the oral cavity is also key. As our enamel is comprised of around 97% hydroxyapatite, a calcium phosphate compound, acids can be weakened by enriching the environment with calcium.


Can hydroxyapatite enrich the environment in the case of contact with bacteria and trigger a protective action, preventing acid formation and as a result, dental cavities?

Materials and methods

This in-vitro study used the bacterium Streptococcus mutans, which is present in the oral cavity and in dental cavities. The influence of hydroxyapatite on the pH and calcium levels was investigated in fluid cultures and in S. mutans biofilms. Fluid specimens were cultured for 48 hours with the addition of either 5% hydroxyapatite or 5% silica, with silica used as the control substance in order to exclude the influence of the substances on bacterial growth. The pH and the amount of calcium ions in the culture were measured at the start, after 24 hours and after 48 hours. After 24 hours, the cavities biofilms (acidic pH) were adjusted using 0.5% hydroxyapatite or 0.5% silica (control).  After 72 hours in total, the pH and the calcium content were determined in both the culture supernatant and the biofilm.


In the measurements of the fluid cultures, the biofilm supernatant and the biofilm, the calcium level is greatly increased when hydroxyapatite is added versus silica or the negative control. After 24 hours and 48 hours, the pH of the fluid culture shows no significant differences between hydroxyapatite and silica, with a trend towards a slightly elevated pH when hydroxyapatite is added. After 72 hours, the pH of the biofilm is significantly higher for biofilms in which hydroxyapatite is present versus silica and the control. This shows that the buffering action of hydroxyapatite is efficient especially in acidic conditions, as is the case with cariogenic oral biofilms.

Figure 1: Hydroxyapatite dissolves in acidic (cariogenic) biofilms and weakens the acids. The pH value is higher in the hydroxyapatite group, indicating that the biofilm is less acidic, i.e. weaker, than in the control groups.
Figure 2: Hydroxyapatite dissolves in acidic (cariogenic) biofilms. This releases calcium, which can protect and repair the tooth.


Hydroxyapatite is a source of calcium and protects our teeth from acids.

The published study can be read here.

Source:  Cieplik F. et al. Ca2+ release and buffering effects of synthetic hydroxyapatite following bacterial acid challenge. BMC Oral Health, 1-8 (2020).