Calcium cyclohexaphosphate as a novel bioactive condensed phosphate: synthesis, physicochemical characterization, and protective effects on sound and softened enamel under erosive conditions

Calcium-modified cyclophosphates may enhance protection against enamel demineralization. This study synthesized and characterized calcium cyclohexaphosphate (CaCHP) and evaluated its concentration-dependent protective effect against initial enamel demineralization under erosive conditions in vitro. CaCHP was obtained from sodium cyclohexaphosphate (NaCHP) by ion-exchange resin and characterized by SEM, EDS, XRD, and FTIR. Sound and softened bovine enamel blocks (n = 12/group) were treated with toothpaste slurries prepared in human saliva containing 0.25%, 0.5%, or 1% NaCHP; 0.25%, 0.5%, or 1% CaCHP; 0.24% NaF; or placebo. Blocks underwent four consecutive citric acid challenges (pH 3.5, 1 min), interspersed with surface hardness measurements. Data were analyzed by three-way ANOVA and Tukey tests ( p < 0.05). Enamel surfaces were also examined by SEM/EDS. Characterization confirmed Na replacement by Ca within a cyclic structure and greater crystallinity for CaCHP than NaCHP. All groups showed decreased hardness during acid challenges. However, softened enamel treated with NaF or cyclophosphates showed a smaller decrease in surface hardness than sound enamel ( p < 0.05). Placebo showed the greatest decrease in hardness ( p < 0.05), regardless of initial enamel condition. In sound enamel, 1% CaCHP and 0.24% NaF showed comparable protection through the third acid challenge, but neither differed from placebo after the fourth challenge. In softened enamel, 1% CaCHP remained comparable to 0.24% NaF and significantly more protective than placebo and NaCHP after the fourth challenge ( p < 0.001). In conclusion, CaCHP was synthesized from NaCHP, and 1% CaCHP provided protection comparable to 0.24% NaF, with persistence depending on enamel condition.