Monolithic Superelastic Rod with Variable Flexural Stiffness for Spinal Fixation

To reduce the risk of adjacent segment degeneration and fracture associated with rigid spinal fixation techniques, while providing adequate stability to the spine, an original concept of monolithic spinal rod with variable flexural stiffness is proposed. The concept is based on the use of Ti-Ni shape memory alloy rods subjected to different processing schedules implying local annealing, cold work, or combination of both. A feasibility study of the concurrent technological routes is performed by comparing their potential in controlling locally material microstructure and therefore material properties. To illustrate the versatility of the proposed technology, an entire specter of all the available stress-strain patterns of a monolithic Ti-Ni rod of a given composition resulted from different combinations of processing conditions is shown. Finally, a monolithic Ti-Ni (5.5 mm diameter - 450 mm long) rod containing three parts: one central and two peripheral (cervical and lumbar) is manufactured and in-vitro tested. The central part with greater stiffness behaves as a conventional material with common elastoplastic behavior and it serves for rigid spine correction; this part can be contoured permanently on the operation site. The peripheral parts behave superelastically, and have greater flexural stiffness in the lumbar region and lower stiffness in the cervical region – to provide peripheral dynamic fixation and to decrease stress concentration in the vicinity of the implant extremities.