Abstract

Implants used in lumbar spinal fusion and lumbar total disc replacement procedures have improved substantially over the years, however, the opportunity remains to improve either the adjustability, durability, motion, or minimally invasive characteristics which could aid in achieving optimal surgical outcomes. It is hypothesized that the inherent advantages of compliant mechanisms specifically Deployable Euler Spiral Connectors (DESCs) and a new mechanism known as the interior contact aided rolling element (I-CORE) are well suited for each of these applications and will be explored and utilized as the basis of these proposed designs. By addressing these challenges, the research endeavors to improve patient outcomes in both lumbar fusion and lumbar disc replacement procedures and advance the field of lumbar spinal surgery. Expandable cages, ideal for minimally invasive lumbar fusion, can be implanted in a compact form and then expanded. Differences in shape, size, material, and adjustability of cages are often overlooked. A systematic review revealed that design variations, such as rectangular titanium cages with medial-lateral and vertical expansion, and those with adjustable lordosis, are linked to better patient outcomes. Deployable Euler Spiral Connectors were leveraged to create an expandable fusion cage while also including recommended features from the systematic review. Finite element analysis, benchtop mechanical testing, and validation via in vitro device insertion were performed. The design highlights the potential of compliant mechanism devices for advancing minimally invasive lumbar fusion. A new compliant mechanism called the Interior Contact-Aided Rolling Element (I-CORE) is described and modeled using the pseudo-rigid-body method. The new mechanism included two degrees of rotational freedom at a tailorable stiffness combined with tailorable vertical compressibility and a mobile center of rotation. The model is outlined and validated using FEA and benchtop mechanical testing. The model is shown to be sufficiently accurate for initial design work. A novel motion preserving spinal implant using the I-CORE mechanism is described. Prototypes were manufactured from Ti6Al4V and validated using benchtop mechanical and in vitro biomechanical testing. Properties including compressive, shear, and torsional stiffness were quantified. The device was tested for expulsion and subsidence forces, showing results comparable to current devices. In vitro testing with human cadaveric lumbar segments demonstrated that the compliant interbody device maintained robust compressive, shear, and torsional strength, closely replicating the motion quality of intact segments when properly placed.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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