Ms. Heather Anne Lynch Guerin
Advisor: Dr. Dawn M. Elliott
Department of Mechanical Engineering
McKay Laboratory of Orthopaedic Research
University of Pennsylvania

Abstract

The intervertebral discs permit motion and distribute the large and complex load of the spine. The annulus fibrosus of the intervertebral disc is comprised of a ring of layered, angled fibers embedded in a hydrated matrix of aggregated proteoglycans, with smaller amounts of minor collagens, elastin, and small proteoglycans. This structure and composition enables the intervertebral disc to withstand loads in tension, torsion, compression, shear and bending, and results in inhomogeneous, anisotropic, and nonlinear mechanical behaviors. The specific contributions of structure to mechanical function remains unclear. Therefore, the objective of this study was to characterize the relationship between annulus fibrosus structures and mechanical function using a combination of experimental testing and mathematical modeling.

Annulus fibrosus samples were tested in 3 orientations in usiaxil tension. A multi-dimensional stress-strain dataset was created and mechanical properties were measured. Additionally, load-induced fiber-reorientation was measured using a novel application of Fast Fourier Transform. Reorientation was found to be affine.

A structurally motivated, finite deformational, fiber-reinforced, multi-dimensional hyperelastic model was developed for orthotropic annulus fibrosus. Terms to describe fibers, matrix, and interactions (shear and perpendicular to the fiber directions) between annulus fibrosus structures were explicitly included in the model. The effect of these terms on the simultaneous fit to multi-dimensional experimental data was determined by comparing R2 goodness-of-fit, model parameters, and Pearson correlation coefficients. The contribution of structural terms to overall tissue stress was calculated. Both shear and perpendicular interaction terms were necessary to accurately model multi-dimensional behavior. Fiber stretch and shear interactions dominated contributions to circumferential direction stress, while perpendicular and shear interactions dominated axial stress. The results reported here suggest that interactions between fibers and matrix play an important role in annulus fibrosus mechanical behaviors like nonlinearity and contribute to overall tissue load-bearing.

Tuesday, October 18, 2005
2 PM, 337 Towne Bldg.