Toothpaste viscosity decreases rapidly when diluted with saliva during brushing, potentially causing premature washout of high-risk caries areas and reducing the uptake of dental fluoride ions. However, no reports have examined the acid resistance of enamel from the perspective of the toothpaste's physical properties. This study aimed to elucidate the impact of toothpaste dilution on the acid resistance of the enamel, using bovine enamel as the subject. Five diluted toothpaste groups were created: a control group without toothpaste, and 100% (1.00×), 67% (1.50×), 50% (2.00×), and 25% (4.00×) dilution groups. Acid resistance was evaluated through pH cycling after toothpaste application. The results revealed a significant increase in substantial defects, compared to 67% (1.50×) at dilutions of 50% (2.00×) or higher, accompanied by a decrease in Vickers hardness. Moreover, the mineral loss increased with dilution, and a significant difference was observed between 67% (1.50×) and 50% (2.00×) (p < 0.01). This study revealed that the acid resistance of the enamel decreased when the dilution of toothpaste during brushing exceeded 67% (1.5×). Therefore, delivering toothpaste with a lower dilution to high-risk caries areas, including interproximal spaces and adjacent surfaces, could maintain a higher concentration of active ingredients in the toothpaste, thereby enhancing its medical effects.
Keywords: enamel; pH-cycling; preventive dentistry; toothpaste; toothpaste technique.