Corrosion and biocompatibility testing of palladium alloy castings.

OBJECTIVE: The biocompatibility of palladium-copper alloys has been questioned in the literature. The intention of the present work was to study: (a) the release of ions in an immersion test from a copper-containing alloy, Option((R)) (79% Pd, 10% Cu, 9% Ga, 2% Au), compared with an alloy without Cu, IS85 (82% Pd, 6% Ga, 3.5% Sn, 3.5% In, 2.5% Ag, 2.5% Au); (b) the effect of oxide films produced by preoxidation on corrosion resistance; and (c) the possibility of biologically synergetic effects of ions released.

METHODS: Specimens of both alloys were cast, rubbed and ultrasonically cleaned. Metallographic specimens were prepared after (a) casting and (b) preoxidizing treatment at approximately 1000 degrees C and studied by SEM and EDS. Immersion tests were carried out in a solution of 0.1mol/l of NaCl and 0.1mol/l of lactic acid at 37 degrees C for 7 days. The alloy specimens were tested in the following three steps: (1) as preoxidized; (2) after subsequent removal of a 0.1mm thick layer by grinding; and (3) after an additional removal of approximately 0.1mm by grinding. The test solutions were analyzed by means of ICP to record the amounts of ions that had leached out from the alloy specimens. The biocompatibility was studied by cell culture tests and the HET-CAM method. Test solutions were prepared by dissolving PdCl(2) and CuCl(2) to appropriate concentrations.

RESULTS: The metallographic investigations revealed moderate segregations in interdendritic regions and grain boundaries. After preoxidation in air a zone of oxidation from 25 up to 200 microm thickness for Option and from 5 to 10 microm for IS85 was found. Oxidation was observed along a rim for both alloys and for Option also along interdendritic positions. The oxides were seen as small, dark spots <1 microm in a metallic matrix. These results indicate that: (1) the oxidation of the copper-containing palladium alloy is far more severe than that of the alloy with no copper; and (2) the elemental release from these oxides is substantially larger than that from the corrosion of the metallic structure. The results of the cell culture testing showed that Cu was most toxic, followed by Cu(2+)+Pd(2+) (1:2), based on the determination of the concentration that caused 50% cytotoxicity. The HET-CAM testing showed Cu(2+)+Pd(2+) (1:2) to have the highest irritation score.

SIGNIFICANCE: The copper-containing Pd alloy developed a 0.1mm thick rim with small oxide particles in a metallic matrix during preoxidation. If this rim is not completely removed, significantly more Cu, Ga and Pd ions have been shown to leach into the test solution than from the alloy structure. No synergetic effect of Cu and Pd ions was observed in cultured cells, while the mixture Pd(2+)+Cu(2+) (1:2) was most irritating to mucous membrane as evaluated by the HET-CAM method.