Monofluorophosphate Interaction with Hydroxyapatite and Intact Enamel

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Abstract
Hydroxyapatite was exposed to solutions of monofluorophosphate (MFP) at pH 5 and pH 7, for periods up to 48 h, and fluoride (F––) and F as MFP were measured in solution and solid. The exposed solid was dissolved in 0.5 m HC104 at 2 °C and F–– measured immediately, and after hydrolysis had been completed 24 h later, employing a fluoride electrode. Solutions were analyzed in a similar manner. Solution F–– (3% of total F) decreased rapidly and then gradually increased to concentrations above the original level. Solution MFP decreased continuously, the rate decreasing with time. The corresponding deposition of F on the solid was in the forms of both F–– and MFP. The fraction of deposited F–– increased with increased time of exposure while the fraction of F as MFP decreased. MFP was removed more readily by washing than F––. MFP in MFP exposed apatite decreased while forming F–– under humid conditions, but remained constant under dry conditions. Only small amounts of F were deposited from MFP in intact enamel. MFP retained in the mouth after a MFP rinse was subjected to rapid hydrolysis. The findings showed that the uptake of F from MFP was greater at pH 5 than at pH 7 and that MFP is hydrolyzed by two mechanisms, one operating at the solid solution interface of the apatite and the other in saliva and/or plaque. It is concluded that the caries-reducing effect of MFP is due to F–– released by hydrolysis.

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