NONINVASIVE EVALUATIONS OF SLENDER GRAPHITE RODS AND HUMAN THORACOLUMBAR SPINE
Mechanical properties, internal condition and fracture risk of structural components can be assessed by noninvasive techniques being preferred mainly because of their efficiency and speed. This study presents noninvasive evaluations of slender graphite rods and human thoracolumbar spine. An experimental approach for graphite rods and numerical approaches for both graphite rods and human thoracolumbar spine were developed. Internal cracks may occur in the graphite rods during the manufacturing process. In an effort to develop a nondestructive testing approach to evaluation of the graphite rods, transient elastic impact was used. Wave theory was used for solid rods. Subsequently, numerical models were developed to determine the response of rods containing cracks. Experiments on graphite rods with and without cracks were conducted and the internal condition was determined from the recorded signals. The rods were then cut lengthwise to reveal the internal condition and verify the predicted results. The knowledge gained from simulations allowed for the presence of cracks to be detected. For fracture risk assessment of vertebra, finite element (FE) models with simplified geometry, material properties or loading conditions were developed in the past. To investigate the role of these parameters, two FE models were created from CT images: an isolated L1 vertebra and a T12-L2 spinal segment with ligaments, discs and facets. Each model was examined with both homogeneous and spatially varying bone tissue properties. Stresses and strains were compared for uniform compression and flexion. Inclusion of heterogeneous bone properties and physiological loading in FE models was critical to assess vertebral fracture risk. The fracture risk of an osteoporotic thoracolumbar junction was assessed using the FE model of L2-T12 spinal segment. Osteoporosis was simulated in four stages, which included disc stiffening and stiffness losses in cancellous core and cortical cortex. Overall stiffness of the segment, and stresses and strains in two sections of L1 were computed for uniform compression and flexion at each stage. This study clearly delineated that osteoporotic bone was at high risk for fracture through not only increased bone stresses and strains with loading, but also changes in the volume and location of bone experiencing these high strains.
Graphite; Noninvasive; Spine; Finite element model; Osteoporosis; Vertebra
dissertation or thesis