Integrated Investigations of Lumbar Spine Biomechanics, Implant Fixation, and Design for Additive Manufacturing under Physiologic Loading

Abstract

Spinal implant loosening and migration are significant concerns following the surgical treatment of musculoskeletal spinal disorders because of the complex and highly variable loading experienced in the spine. During post-operative rehabilitation, the mechanical interface between the vertebral bone and metallic implant is subjected to variable, combined (multi-axial) loading modes during activities of daily living. However, most current methods for pre-clinical evaluations of spinal implants focus on application of simplified loading conditions (e.g. uniaxial toggle, static compression or pull-out tests) which do not fully represent physiologic multi-axial loading. As a result, there is a lack of biomechanical understanding in how spinal implants can fail following surgery. One of the goals of this thesis is to better understand mechanisms of clinical implant failures by improving pre-clinical spinal implant testing methodologies through the adoption of physiologically-derived loading conditions. Spinal implants are often manufactured using titanium alloys like Ti6Al4V, beneficial for their superior osteogenic qualities. However, the mismatch in mechanical moduli between titanium and bone is a known cause of problematic stress shielding and failure of the bone-implant interface. To address this challenge, laser powder bed fusion (LPBF) additive manufacturing allows for the production of unique metamaterials that can act to reduce the bulk modulus of titanium implants, bringing mechanical properties closer to that of human bone. However, there is currently a disconnect between the understanding of additive manufacturing (AM) and the mechanical properties of bone. Therefore, a second goal of this thesis was to develop design for additive manufacturing (DfAM) tools for selecting lattice properties to best match vertebral bone properties while ensuring manufacturability for LPBF. Based on the two overall goals, the thesis research was defined based on the intersection of three themes: (1) multiaxial spinal loading, (2) spinal implant evaluation, and (3) LPBF AM. Within these themes, four in vitro biomechanical human cadaver investigations were completed to develop physiologically-derived, multi-axial loading test protocols for a commercially available joint motion simulator to evaluate the impact of multi-axial loading on implant loosening and migration. The first study examined the effect of uniaxial and multiaxial cyclic loading on pedicle screw loosening in osteoporotic human bone (n=7). It was found that multiaxial loading significantly accelerated pedicle screw loosening as well as increased deformation volume at the bone-implant interface. The second (n=8) and third (n=8) biomechanical studies focused on developing six degree-of-freedom (6DOF) test protocols for spinal loading simulation. Lumbar spine stability was compared between existing pure moment test methods against physiologically-derived 6DOF loading waveforms associated with activities of daily living. It was found that the commercial joint motion simulator was reliable for applying complex, physiologically-derived lumbar spinal loads. The loads associated with in vivo spinal movements resulted in reduced range of motion compared to application of pure moments. Longitudinal gait simulation also had little impact on lumbar spine segment biomechanics. The fourth in vitro investigation (n=8) was aimed at leveraging the developed protocols to evaluate implant migration of an LPBF additively manufactured spinal implant. It was found that cage penetration into the adjacent vertebra (failure of the bone-implant interface) happened most often during simulated gait testing, while cage migration, towards the spinal cord, occurred primarily during flexion-extension. Two additional DfAM investigations were undertaken to develop tools for selecting Ti6Al4V lattices for orthopaedic applications and evaluating manufacturability of lattice designs. A comprehensive literature review was undertaken to synthesize a dataset relating lattice parameters to compressive mechanical properties. Gibson-Ashby plots were generated to relate this dataset to human bone properties for selection of lattice parameters in orthopaedic implant designs. A manufacturability evaluation was also completed to determine which lattice structures were considered manufacturable based on geometric and defect analysis. It was found that surface-based lattice structures were the most manufacturable due to the high interconnectivity of the down-skin regions. Lastly, these DfAM tools were used to develop novel lumbar interbody cage implants, which were investigated under the physiologically-derived loading (described above). Collectively, the research completed in this thesis provides new biomechanical understanding of the lumbar spine and spinal implants under multi-axial cyclic loading. With improved knowledge of how spinal implant failures can occur under multi-axial loading, pre-clinical testing standards can now be improved to help avoid these failure mechanisms, ultimately leading to the hope of improved clinical outcomes following spinal surgery. Further, new DfAM tools were created and evaluated providing new avenues for the use of emergent LPBF AM techniques for orthopaedic applications.