Residual Stresses in Titanium Spinal Rods: Effects of Two Contouring Methods and Material Plastic Properties

Posterior spinal fixation based on long spinal rods is the clinical gold standard for the treatment of severe deformities. Rods need to be contoured prior to implantation to fit the natural curvature of the spine. The contouring processes is known to introduce residual stresses and strains which affect the static and fatigue mechanical response of the implant, as determined through time- and cost-consuming experimental tests. Finite element (FE) models promise to provide an immediate understanding on residual stresses and strains within a contoured spinal rods and a further insight on their complex distribution. This study aims at investigating two rod contouring strategies, French bender (FB) contouring (clinical gold standard), and uniform contouring, through validated FE models. A careful characterization of the elastoplastic material response of commercial implants is led. Compared to uniform contouring, FB induces highly localized plasticizations in compression under the contouring pin with extensive lateral sections undergoing tensile residual stresses. The sensitivity analysis highlighted that the assumed postyielding properties significantly affect the numerical predictions; therefore, an accurate material characterization is recommended.