Research of Corrosion Resistance of Titanium Nickelide Rods in Spinal Pedicle Screw Devices with Various Fixation Types

In this work, the finite element method is used for numerical calculation of the stress and strain distribution on the surface of TiNi-based alloy spinal rods locked in pedicle screw constructs with various geometries of fastening elements, and an experimental study of their corrosion resistance is carried out using long–term immersion test into a 0.9% NaCl corrosion solution and fretting corrosion test by applying a cyclic load with a bending moment arising during functional movements in the lumbar spine. It has been revealed that the formation of local plastic deformation zones during initial locking and following cyclic loading, the surface area (S) and the maximum degree of deformation (ε_{plast (max)}) in which depend on the geometry of pedicle screw fastening elements, has a significant negative effect on the corrosion resistance of TiNi rods in the studied constructs. The corrosion damage of TiNi rods after long–term immersion test was revealed when using only one type of pedicle screws which causing the formation of plastic deformation zones on the rods surface at the initial stage of rods locking. It has been shown that the fretting corrosion current decreases from 5.8 ± 3 μA to 1.3 ± 0.4 μA, the area of corrosion damage decreases from 0.23 ± 0.04 to 0.015 ± 0.01 mm², and the content of Ni ions in the corrosive environment reduces by 2.9-fold when testing in screws with fastening elements leading to the formation of local plastic deformation zones with ε_{plast (max) (I)} = 62%; S_(I) = 7.7 mm² and ε_{plast (max) (II)} = 3.7%; S_(II) = 4.9 mm², respectively. It has been demonstrated that the application of the third type of pedicle screws with the fastening elements which do not lead to the formation of plastic deformation zones on TiNi rods surface at initial locking and during the subsequent application of cyclic load, completely prevents the development of the corrosion process, indicating the need to select the optimal geometry of the fastening elements to ensure high reliability and durability of medical implant constructs with components made of TiNi-based alloys.