Pedicle screw instrumentation is one of the most common fixation methods in spinal surgery used to immobilize the destabilized spine and facilitate bone incorporation. Failure of the system, associated with pedicle screw loosening or breakage or the bone-screw interface problems, prevents bone fusion and probably requires additional surgeries or removal of the system. The purpose of this research was, thus, to understand underlying mechanisms behind the failure of the bone-pedicle screw interface.

The first part of the research focused on the deformation characteristics of the pedicle cortex during screw insertion. Fourteen human cadaveric vertebrae with linear strain gauges attached at the pedicle were implanted with commercially available pedicle screws and strain changes were observed at the cortex of pedicle. Secondly, the effects of the viscoelastic properties of the interface on the holding power of the screws were investigated using bovine and human models. Thirty-nine calf vertebrae and 14 human cadaveric vertebrae were instrumented with pedicle screws and each screw was withdrawn with one of the two pullout models:​ Standard pullout model and stress relaxation pullout model, a custom pullout protocol that allowed stress relaxation steps during pullout test. Variety of pedicle screw designs and loading rates were utilized for the experiments. Third, the micro-structure of the cortical bone and architecture of the trabecular bone at the pedicle were studied.

The results showed that the cortex of the pedicle underwent plastic deformation during screw placement although the screws did not contact the cortex. The amount of plastic deformation could also be minimized by changing the screw placement technique. It was shown that the stress relaxation had deleterious effects on the strength and stiffness of the bone-screw interface. These effects were dependent of the screw design. The analyses on the structure of the pedicle showed that the trabeculae within the pedicle seemed to be isotropic and plate-like. The pedicle cortex did not exhibit lam inar bone that envelopes the osteons at the periosteum. This helped to explain the fragility of the pedicle cortex to expansion during screw implantation.