Investigation of Nitinol Resistance to Fretting and Crevice Corrosion

Resistance of Nitinol rods (diameter 5.5 mm, 55.8 wt.%Ni, electropolished) locked in commercially available Ti6Al4V pedicle screws of various designs (some of screws were subjected to low voltage microarc oxidation) used for surgical treatment of degenerative spinal diseases to fretting and crevice corrosion was studied. Resistance of Nitinol rods locked in pedicle screws to crevice corrosion was evaluated using ASTM F2129 methodology and long-term (6 months) immersion tests in PBS at 37 degrees Celsius. Fretting corrosion tests were carried out by conducting a series of potentiostatic measurements of peak-to-peak values of fretting corrosion current under bending in a 10% solution of calf serum in PBS. Corrosion patterns on rods after the tests were examined using SEM and EDAX analysis.

The results have shown that Nitinol rods locked in titanium pedicle screws demonstrate inferior resistance to fretting and crevice corrosion compared with traditional Ti6Al4V or CoCr rods. Untypical corrosion damage including erosions combined with the build-up of titanium oxide as a corrosion product on surface of Nitinol rods in areas where it was in contact with titanium pedicle screw was observed after fretting and immersion tests. This damage was similar to that found previously on few explanted rods. It was revealed that the pedicle screw design affects the amount of corrosion damage observed after the tests dramatically. It was also found that the creation of defectless oxide layer on the surface of Ti6Al4V screws by low voltage microacr oxidation may prevent the development of corrosion damage on Nitinol rods.