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Degree

Doctor of Philosophy, Case Western Reserve University, Biomedical Engineering, 1990.

Abstract

The hierarchical organization of collagen fibrils in the intervertebral disc is characterized by structural gradients. In the annulus fibrosus, the thickness of lamellae increases abruptly 2 mm inward from the edge of the disc, dividing the annulus into peripheral and transitional regions. Lamellae in the lateral and posterior annulus have a broad distribution of lamellae thicknesses. In alternate lamellae, fibrils are inclined with respect to the vertical axis of the spine in a layup structure. From the edge of the disc inward to the nucleus, this interlamellar angle decreases from 62 to 45 degrees. Within lamellae, collagen fibrils exhibit a planar crimped morphology. The plane of the waveform is inclined with respect to the spinal axis by the interlamellar angle. From the edge of the disc inward, the crimp angle increases from 20 to 45 degrees and the crimp period decreases from 26 to 20 um. A hierarchical model of the disc is presented that incorporates these morphological gradients. Mechanical testing in load-deflection, stress relaxation, and creep modes reveals the response at each level of the hierarchy to compression. The stress-strain curve of the disc in compression contains toe, linear elastic, and yield regions similar to other collagenous tissues in tension. This demonstrat es that while the disc is loaded in compression, the fibrils of the annulus are loaded in tension. The role water transport plays in determining the mechanical properties of the disc is also established. During constant compressive strain stress relaxation experiments, the volume of the disc decreases with time by an amount equal to the macroscopic strain. A model is developed which hypothesizes that the relaxation and creep responses are due to transport of water out of the disc as the result of the pressure gradient across the cartilage endplates caused by an externally applied stress. The model considers cases analogous to the 3- and 4-parameter viscoelastic models, but contains elements which have a physical significance.

Keywords

Hierarchical structure mechanical properties collagen intervertebral disc.

Advisor

Janie Fouke

Pages

134p.

Document number: case1054650676
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=case1054650676

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