A Minimally Invasive Cage for Spinal Fusion Surgery Utilizing Superelastic Hinges

A prototype cage implant for spinal fusion surgery has been designed and developed. Spinal fusion surgery is sometimes performed to alleviate low back pain. The cage implant is a spacer that sits in between two vertebrae to allow for bone growth and fusion, all while relieving compression of the spinal cord. The cage implant is minimally invasive in nature, utilizing embedded nitinol hinges as dual purpose actuators and assembly structural elements. The cage implant utilizes elliptical shaped nitinol hinge pins as actuators to allow the cage to be in a straightened before deployment and manipulate its shape to an oblong octagon once within the disc space. A new modeling technique was developed to aid with the design of the nitinol ellipses. The model is MATLAB based and accounts for the non Mises behavior of nitinol through a correction factor for mapping the effective stress and strain. A nitinol rod and an elliptical geometry were examined experimentally and show the robustness of the developed model. These experiments were conducted to design the nitinol hinges for the cage implant. The cage implant is made of two different materials, nitinol hinge actuators and the containing titanium structural segments. The nitinol hinge actuators are completely enclosed within the medical grade titanium segments through the use of selective laser sintering.