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Images were obtained from intact and acid etched enamel surfaces (both before and after growth of surface apatite) with electron diffraction, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Labial surfaces (intact) of bovine incisor teeth were slightly abraded and pumiced. They were then etched by a 14-hour immersion in 2 mM sodium acetate-acetic acid buffer at pH 5.0; an avenge of 31 μMol/cm2 calcium and 24 μMol/cm2 phosphate was dissolved from the surfaces. Thu SEM and TEM showed loss of minerals to a depth of 10 μm. Dissolution was largely in central areas of enamel prisms, and prismatic structure in the etched surfaces was retained. Electron diffraction, SEM, and TEM showed the growth of crystalline apatites at the etched surfaces after 24 to 96 hours immersion in neutral pH solution (metastable) which contained 1.5 mM Ca+2, 0.9 mM H2PO4/HPO-24, 0.05 mM F-, and 150 mM NaCl. Indirect analysis, at tho loss of calcium and phosphate concentrations in the metastable solution after immersion periods, indicated deposition of approximately 24 μMol/cm2 calcium and 14 μMol/cm2 phosphate on the etched surfaces. Results show induction or apatite crystal growth and recrystallization at enamel surfaces in otherwise stable solutions containing 0.05 mM F-. [ABSTRACT FROM AUTHOR]

The presence of trimetaphosphate ions (TMP) in calcium phosphate solutions inhibited the rehardening of softened enamel surfaces. TMP decreased, whereas fluoride increased, the rates of calcium, phosphate, and fluoride reactions with etched enamel surfaces. The mechanisms of action of TMP and fluoride were independent of each other and possibly synergistic. [ABSTRACT FROM AUTHOR]