Biocompatibility of nickel-titanium shape memory metal and its corrosion behavior in human cell cultures

Nickel-titanium alloy (Nitinol) is a metallic biomaterial that has a unique thermal shape memory, superelasticity, and high damping properties. Nitinol is potentially very useful in orthopedic surgery, for example. At present, there are not enough confirmative biocompatibility data available on Nitinol. The aim of our study was to clarify the primary cytotoxicity and corrosion rate of Nitinol in human cell cultures. Comparisons were made with stainless steel (Stst), titanium (Ti), composite material (C), and control cultures with no test discs. Human osteoblasts (OB) and fibroblasts (FB) were incubated for 10 days with test discs of equal size, 6 x 7 mm. The cultures were photographed and the cells counted. Samples from culture media were collected on days 2, 4, 6, and 8, and the analysis of metals in the media was done using flameless atomic absorption spectrophotometry. The proliferation of FB was 108% (Nitinol), 134% (Ti) (p < 0.02), 107% (Stst), and 48% (C)(p < 0.0001) compared to the control cultures. The proliferation of OB was 101% (Nitinol), 100% (Ti), 105% (Stst), and 54% (C) (p < 0.025) compared to the controls. Initially, Nitinol released more nickel (129-87 micrograms/L) into the cell culture media than Stst (7 micrograms/L), but after 2 days the concentrations were about equal (23-5 micrograms/L versus 11-1 micrograms/L). The titanium concentrations from both Nitinol and Ti samples were all < 20 micrograms/L. We conclude that Nitinol has good in vitro biocompatibility with human osteoblasts and fibroblasts. Despite the higher initial nickel dissolution, Nitinol induced no toxic effects, decrease in cell proliferation, or inhibition on the growth of cells in contact with the metal surface.