A Qualitative In Vitro SEM Study on the Protective Effects of a Self-Antibacterial Nano-Hydroxyapatite Toothpaste Against Acid-Induced Enamel Surface Erosion

This study evaluated the potential protective effect of a synthesized self-antibacterial nano-hydroxyapatite (nano-HA) toothpaste against erosive changes in the enamel surface induced by a cola-based soft drink, based on a qualitative scanning electron microscopy (SEM) study, in comparison with conventional fluoride toothpaste. Thirty extracted human premolars were sectioned to obtain enamel specimens and randomly assigned into a control group and experimental groups in which fluoride or synthesized nano-HA toothpaste was applied either before or after cola exposure (n = 5 per group). Enamel surface morphology was qualitatively assessed using SEM, and surface roughness (Ra) was estimated using a semi-quantitative approach based on SEM image analysis using ImageJ software. Antibacterial activity was evaluated using the agar diffusion method to explore the potential additional benefits of the synthesized self-antibacterial nano-HA formulation. SEM observations showed that, after cola exposure, specimens treated with nano-HA exhibited less surface erosion than fluoride-treated groups. Post-treatment with nano-HA results in a denser and more uniform surface layer with fewer structural defects. Similarly, enamel treated with nano-HA after cola exposure showed a statistically significant reduction in surface roughness compared with the fluoride group (p < 0.05). These findings suggest that nano-HA provides greater protective effects against acid-induced enamel surface erosion. Furthermore, nano-HA indicated potential antibacterial activity against S. aureus and E. coli. Overall, nano-HA toothpaste provided enhanced protection against acid-induced enamel erosion with additional antibacterial effects.