5.8. Corrosion of NiTi

5.8.1. Corrosion in vitro

In the fibroblast cell culture media, the amount of nickel in the NiTi group was 129 µg/l on the second day (first analysis), 23 µg/l on the fourth day (second analysis), 9 µg/l on the sixth day (third analysis) and 8 µg/l on the eighth day (fourth analysis). The titanium concentration was below 20 µg/l in all cases, and there were no measurable concentration variations.

Nickel release from stainless steel was 7 µg/l on the second day, 3 µg/l on the fourth day, 2 µg/l on the sixth day and 2 µg/l on the eighth day. The titanium concentrations assessed from the titanium cell culture media were below 20 µg/l in all samples and showed no variations.

Metal dissolution measured from osteoblast media correlated quite well with the results of fibroblast media. Again, the most pronounced increase in nickel ion concentration in the media of the osteoblast cultures was observed in the first NiTi medium analysis, 87 µg/l. It then decreased rapidly: 14 µg/l (second sample), 5 µg/l (third sample), 5 µg/l (fourth sample). The titanium concentrations were all below 20 µg/l in both the NiTi and the titanium medium samples. The nickel concentrations in stainless steel exposed culture media were 7 µg/l (first sample), 1 µg/l (second sample), 11 µg/l (third sample) and 1 µg/l (fourth sample) (Figure 5-19).

Although the amount of nickel in both fibroblast and osteoblast cell culture media was found to be higher in the NiTi than in the stainless steel cell culture media at the baseline, it had no effect on or correlation with cell proliferation or cell growth near the implant surface at the concentration levels measured in this study.

5.8.2. Trace ions in various organs

In study IV the amounts of nickel measured in various organs of NiTi rats did not stand out in the stainless steel rats. The overall levels were very low (< 2 µg/g/dry weight) and near the detection limit of the GFAAS-based sensitive method. In the NiTi group at 26 weeks, the highest Ni levels were observed in the kidney (1.4 ± 1.0 g/g). In the StSt group at 26 weeks, the highest Ni concentration was observed in the brain tissue (2.0 ± 0.1 g/g). The values decreased by 60 weeks. In the spleen, the Ni ion concentration slightly increased from 26 (NiTi 0.17 ± 0.06 g/g, StSt 0.12 ± 0.07 g/g) to 60 (NiTi 1.4 ± 1.1 g/g, StSt 0.7 ± 1.1 g/g) weeks in both groups (Figure 5-20A and B). There were no statistically significant differences in the Ni concentration between the NiTi and StSt groups or between the 26- and 60-week time points in any of the organs. The spleen Fe concentrations at 26 weeks (6010 ± 1950 g/g) vs. 60 weeks (14200 ± 4370 g/g) were statistically significantly (p < 0.05) different in the StSt group, but not in the NiTi group. Comparing 26 to 60 weeks, an increased Fe content in the kidneys of both groups (NiTi 502 ± 95 vs. 915 ± 71 g/g, p < 0.005) (StSt 572 ± 83 vs. 886 ± 153 g/g, p < 0.05) was noted. The concentrations of Cr in organs were similar in both groups (< 0.4 g/g).

5.8.3. Corrosion analysis of retrieval implants

In study II only one stainless steel implant had rust streaks and some minor pitting corrosion visible to the bare eye. There was no evidence of corrosion on visual inspection in any of the NiTi, Ti-6Al-4V or stainless steel implants the studies III or IV. Overall corrosive changes, as shown by FESEM analysis in study IV, were more evident in StSt implants. Some corrosion pits were observed in the StSt surface after 60 weeks of implantation at magnifications of 200x or higher. The pits were uniformly distributed over the implant. No such pits were seen in the NiTi implants, which, in turn, contained small, longitudinal, irregular enlarged microstructures. Some surface contaminants were present in both materials, especially at 60 weeks (Figure 5-21A, B, C and D).