Hydroxyapatite in oral care products: in vitro effects on erosion/abrasion and analysis of formulation components.

Introduction: Hydroxyapatite (HAP) is promoted as biomimetic material in dentistry. The aim of the study was to investigate whether HAP-containing formulations can reduce erosive/abrasive tissue loss and to analyse components in these formulations.

Methods: Two HAP toothpastes with and two without fluoride and a HAP mouthrinse were investigated, controls were active-agent-free toothpaste, SnF2 toothpaste and F/Sn mouthrinse. For 10 d, human enamel samples were eroded for 2 min, 6x/d in 0.5% citric acid and immersed for 2 min, 2x/d in toothpaste slurries or mouthrinse. Half were additionally brushed for 15s, 2x/d. The particulate fraction was extracted and examined morphologically and with element analyses. Other parameters were REA, RDA, fluoride and calcium content.

Results: The F/Sn mouthrinse almost completely prevented tissue loss; none of the HAP formulations reduced tissue loss compared to the negative control, two increased it instead. Brushing increased tissue loss in all groups except the F/Sn mouthrinse. All toothpastes contained amorphous particles of different sizes. Elemental analysis identified Si and O, additionally Ca and P were present in small amounts on the particles of the HAP toothpastes and one HAP+F toothpaste. In the liquid phase, elevated calcium levels were found in one HAP toothpaste and in both HAP+F toothpastes; in the formulation with the highest value, the fluoride concentration was low. REA and RDA values were not associated with tissue loss.

Conclusions: Whether alone or in combination with fluoride, HAP formulations had either no or a detrimental effect on erosive tissue loss and could not reduce abrasion. In the context of erosive tooth wear, HAP seems to be neither an alternative to fluoride nor a suitable supplement to it.