Characterization of the three-dimensional kinematics and failure of human spinal segments
Liebschner, Michael A. K.
Doctor of Philosophy
Spine disorders are one of the most prevalent and costly problems facing modern medicine, with an estimated annual cost of over $95 billion. Improved understanding of the biomechanical properties of the normal and pathological spine is essential for prevention of such disorders and treatment of traumatic injury, disc degeneration, or other ailments. However, the complex structure of the spine makes experimental testing that is relevant to physiological behavior difficult. Hence, new testing methods are needed to provide more accurate descriptions of in vivo behavior. Therefore, three principal aims are pursued in this work. The purpose of the first aim was to investigate the biomechanical performance of the human cadaver spine by applying a pure bending moment in conjunction with a range of compressive axial loads. This study showed that a minimum of 500 N compressive axial preload along the spinal curvature produces more comparable results to in-vivo studies, providing an important guideline for experiments investigating range of motion and spinal stability. The purpose of the second aim was to determine the motion response of the spinal joint to pure bending in any plane around its circumference. Towards that goal, the three-dimensional motion envelope of two-level spinal segments was analyzed. Since the posterior elements were intact, restriction in motion caused by zygapophyseal joints resulted a smaller displacement for extension than in flexion. Characterization of the motion envelope can provide a better understanding of the complex joint kinematics and assist in the design of new implants with the goal of restoring joint function. The purpose of the third aim was to investigate the strength of a single vertebral body during compression fracture by following the path of least resistance. In vivo, the spinal column continuously maintains equilibrium and minimizes stress via a complex system of muscles, tendons and ligaments such that each vertebra experiences a predominantly axial load. In a typical experiment, compressive loads are also applied axially but unwanted shear forces and moments are generated as well. Using a novel testing approach, identification of the weakest region of a vertral body, and following this path of least resistance, a true measure of vertebral strength, we were able to demonstrate the effect of current experimental limitations in overpredicting bone strength.
Biomedical engineering; Biophysics