TiNbSn alloy spine rods resistant to fatigue fracture and flattening

The critical issues of fatigue fracture and flattening in the Ti-6Al-4V and CoCrMo spine rods for spine deformity correction surgery remain unresolved. To address them we propose a hypothesis that fatigue fracture stems from crystal defects and residual stresses, and flattening originates from the Bauschinger effect. To test the hypothesis, TiNbSn alloy spine rods with an apex radius of 120 mm (R120) were prepared by heating an as-swaged TiNbSn rod at 400°C for 5 h ("warm-contoured") using a rod fixing device designed to shape the rod while minimizing plastic deformation to reduce crystal defects. The warm-contoured TiNbSn spine rod exhibited a compressive residual stress of approximately 150 MPa at both concave and convex apices, while Ti-6Al-4V spine rod contoured to R120 using a bender demonstrated a tensile residual stress of around 400 MPa at the concave apex. Three point bending tests revealed that Ti-6Al-4V, CoCrMo and TiNbSn (heat-treated at 400°C for 5 h) spine rods contoured to R120 using a bender indicated significantly low yielding load (less than 200 N) under reverse loading, while the TiNbSn spine rod that was warm-contoured to R120 without using a bender demonstrated an extremely high yielding load (around 1000 N) when subjected to reverse loading. It is concluded from the obtained results on the residual stress measurements and three point bending properties that the warm-contoured TiNbSn rods are resistant to fatigue fracture and flattening.