The in situ potential of synthetic nano-hydroxyapatite for tooth enamel repair

This study was designed to evaluate whether nano-hydroxyapatite toothpastes with or without fluoride would be more advantageous than a fluoride toothpaste in the repair of eroded enamel in situ. Twenty-one subjects participated in this single-blind, randomized, cross-over design study with three 7-day treatment phases. In each phase, the subjects wearing a palatal appliance containing five sterilized enamel specimens used either one of the two test regimens (1% nano-hydroxyapatite toothpaste and 2.25% nano-hydroxyapatite/1450 parts per million (ppm) fluoride toothpaste) or one control (1400 ppm fluoride toothpaste). Enamel specimens were extraorally demineralized (4 × 5 min/day) and were intraorally treated with the toothpastes (2 × 2 min/day). The nano-hydroxyapatite toothpaste groups exhibited significantly higher surface microhardness than did the standard fluoride toothpaste group (p < 0.05). Enamel surface hardness was increased only by nano-hydroxyapatite toothpastes after in situ treatment compared with the baseline (p < 0.05). Morphological analysis demonstrated an apatite-type crystal deposition on the eroded enamel surface produced by nano-hydroxyapatite toothpastes, while fluoride toothpaste failed to show any significant surface deposition. Chemical analysis showed a higher content of calcium and phosphorus in the enamel surface treated with nano-hydroxyapatite toothpastes compared with that in the control one (p < 0.05). It is concluded that home use of nano-hydroxyapatite containing toothpastes may have a protective effect against erosion at the enamel surface.