Impact of sodium hexametaphosphate in fluoride toothpaste on initial enamel erosion

This in vitro study evaluated the effect of fluoride (F) toothpaste, containing or not sodium hexametaphosphate (HMP), on inhibiting or rehardening initial enamel erosion. Sound (n = 48) and softened (n = 48) bovine enamel blocks were selected by surface hardness (SH) and treated with experimental toothpaste (12 blocks/group): Placebo, 1,100 ppm F, 1% HMP, and 1,100 ppm F + 1% HMP. Enamel blocks were immersed in toothpaste/human saliva slurry once for 2 minutes followed by 4 erosive challenges in citric acid (pH 3.5, 0.75%, 1 minute each, under stirring). The percentage of surface hardness change (%SH) was calculated immediately after demineralization, after treatment and 1st, 2nd, 3rd, and 4th erosive challenge. To assess each toothpaste's ability to form precipitates on enamel, softened enamel blocks (n = 3/group) were treated with the toothpastes and analyzed by SEM/EDX. Data were submitted to 2-way ANOVA followed by the Student-Newman-Keuls test (p < 0.05). The Placebo group showed the lowest capacity to inhibit enamel erosion and reharden softened enamel (p < 0.001). Hardness loss increased during repeated acid challenges for all groups. Toothpaste containing 1,100 ppm F and 1% HMP maintained lower hardness loss than 1,100 ppm F alone after the fourth challenge in sound (-20.8% vs. -31.0%) and softened enamel (-31.3% vs. -39.6%) (p < 0.001). Toothpastes containing HMP produced a more uniform and thicker precipitate layer. Toothpaste containing 1,100 ppm F and 1% HMP demonstrated greater protection against initial erosion and greater rehardening of softened enamel than 1,100 ppm F.