Effects of indium and iron ions on in vitro calcium phosphate precipitation and crystallinity

The elution characteristics of indium and iron ions from six kinds of dental casting alloys were studied in vitro. A certain amount of indium (0.01-1.70 mg/cm2) was solubilized from the surface of only one prosthetic semiprecious brand (Miro Bright) in either 1% lactic acid or 0.05% hydrochloric acid solutions. The elution of iron was below the detection limit or < 0.01 mg/cm2. We also studied the stimulatory effects of indium and ferric ions on the calcium phosphate precipitation in the absence and presence of an inhibitor (phosvitin, a phosphoprotein purified from egg yolks). Indium and ferric ions promoted the reaction, and their stimulatory effects were stronger than the inhibitory effects of phosvitin (250 micrograms/ml). These facts suggest that indium, but not iron, could be eluted into the mouth, and that the eluted indium may stimulate the formation of oral calcium phosphate precipitates, counteracting the inhibition by saliva-derived phosphoproteins. The effects of these metal ions on calcium phosphate precipitates were also studied using both X-ray diffractometry and infrared spectrophotometry. It was shown that both indium and ferric ions decreased the grade of crystallinity of the calcium phosphate precipitates (hydroxyapatite).