Effect of varying pH on Nickel release from dental alloys

Objectives: Nickel (Ni) based dental alloys are widely used in dentistry. They are a cheap alternative to the more expensive precious or semi precious alloys. Although some of its properties are superior to the precious or semi precious alloys, its biocompatibility has always been of concern as Ni is the most common allergen. Methods: One Ni-Manganese-Chromium (NiMnCr) alloy (VeraSoft), Four NiCr-Molybdenum (Mo) alloys (Remanium, VeraBond, Wiron 99 and Heranium NT) and one NiCr-Iron (Fe) alloy (Wirolloy) were placed into Phosphate Buffered Saline (PBS) solution. Each alloy was placed into PBS with a varying pH from 7 down to 2 at intervals of 0.5. After placing test pieces into solution they were agitated and incubated at 37°C for 5 days. The test pieces were removed and the amount of Ni released into the solution was detected using graphite furnace atomic absorption spectrophotometry. Each test piece was re-polished and cleaned before being placed back into fresh PBS at a different pH. Results: The NiMnCr alloy showed a rise in the amount of Ni released as the pH of the PBS became more acidic compared to the other 5 alloys. The other 5 alloys released similar amounts of Ni with a slight increase in Ni release as the pH of the PBS became more acidic. One of the NiCrMo alloys contained less Cr than the NiMnCr alloy but did not show the same increase in Ni release as the pH became more acidic. Conclusion: The composition of the alloy as well as the pH of the solution determined the amount of Ni release. The amount of both Cr and Mo is important to passivate the surface to prevent Ni release as the NiMnCr released high amounts of Ni while a NiCrMo with lower Cr did not release such a high amount of Ni.