Tiges monolithiques aux propriétés variables pour la stabilisation de la colonne vertébrale : fabrication et étude biomécanique

Abstract

Metallic rods used for spinal arthrodesis are stiff and solid to provide the stability needed for fusion and to prevent implant failure. However, the risk of adjacent (noninstrumented) segment disease and fracture also increases with the use of rigid constructs. Dynamic stabilization systems (DSS) have been tested to prevent adjacent segment degeneration. Concerns with such dynamic stabilization systems include possible mechanical failure and degeneration within the stabilized segments. The main objective of this project is to develop a new kind of spinal rod with variable flexural stiffness. Such implant should ideally combine static and dynamic stabilization capabilities with greater stiffness in zones where stability from rigid fixation is critical and lower stiffness in zones where dynamic properties and load-sharing abilities are important. First, the manufacturing method to build spinal rods of variable flexural stiffness was developed. Local cold working and local Joule-effect annealing were applied on 2 mm Ø Ti-Ni shape memory alloy wires. In particular, it could be demonstrated that an annealing duration of 10 minutes is sufficient to change the material behaviour from elastoplastic to superelastic. It was also observed that a mechanical property gradient is created between the heated and not heated zones. During fatigue testing, the heterogeneous samples were comparable to homogeneous wire considering their number of cycles to failure. Second, Joule-effect annealing was applied to Ti-Ni rods having a 5.5mm diameter which is comparable to commercial implants. A numerical model was developed and validated to optimize the Joule heating procedure and the flexural stiffness of the rod. The model simulates the interaction between the annealing temperature and the mechanical properties. In the future the rod model should be included in a spine segment to further optimize the flexural stiffness profile of the rod. Third, in-vitro biomechanical tests were conducted on porcine spine segments to assess the stabilization capability of the Ti-Ni spinal rods with variable flexural stiffness. Spécimens were instrumented using three kinds of rods (Titanium, Ti-Ni superelastic and Ti-Ni with variable stiffness). Two anchor configurations were tested: all pedicle screws and pedicle screws with a transverse hook at the upper instrumented level (UIV). The results indicate that a combination of transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff − half superelastic) rods provides more motion at the UIV level and applies less force on the anchors, potentially improving the load sharing capability of the instrumentation. Scientific contributions Scientific contributions of this work can include: • An original manufacturing technique to produce Ti-Ni rods with variable flexural stiffness was proposed and validated. Joule-effect annealing appeared to be an easy, reliable and fast process to manufacture such rods; • An original finite element model capable of simulating the impact of local annealing on the bending stiffness of spinal rods was developed and validated; • An experimental methodology for in-vitro biomechanical testing of porcine spine segments was developed and validated. During these tests, vertebral rotations, specimen stiffness, forces on anchors and intradiscal pressures were recorded; • The results of in-vitro testing can be used for the development and validation of an instrumented numerical spine model.

Date	10 Nov 2015
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Vladimir Brailovski (Supervisor), Yvan Petit (Co-supervisor) & Jean Marc Mac-Thiong (Co-supervisor)