Screw Insertion Damage Effects on Nitinol Corrosion Resistance in Spinal Fixation Plates

Nitinol is commonly used as a retention clip to prevent backout of fixation screws in spinal anterior cervical plates (ACP). During implantation, the screw damages the retention clip surface due to metallic contact during insertion into the vertebra. The effects of this surface damage on corrosion resistance of the Nitinol clip have not been thoroughly investigated and may be dependent on the level of screw-clip interference in the construct design. Therefore, the goal of this study is to characterize the effects on the Nitinol corrosion resistance using varying degrees of screw-clip interference in a modified ACP assembly.

Generic Nitinol retention clips were manufactured with three different surface finishes - dark oxide, mechanical polish, and chemically etched with interference levels of 0.9 mm, 0.5 mm, 0.1 mm, and -0.2 mm, from high to no interference, respectively. An assembly procedure was developed to simulate the clip damage upon screw insertion with a clinically relevant torque of 7 N-m applied. Corrosion testing was conducted per ASTM F2129-19a with a sample size of N = 6. Initial results on dark oxide clips indicated a significant decrease in breakdown potential between 0.9 mm (Eb=211mV_SCE) and -0.2 mm (Eb=282mV_SCE) interference levels (p<0.05). Scanning electron microscopy of the dark oxide surface for the highest interference showed the most damage. The surface oxide of the clips will be characterized using electrochemical impedance spectroscopy and correlated to the F2129-19a test results. The findings of this study will provide insight into corrosion susceptibility of mechanically damaged Nitinol in orthopedic applications.