Constitutive Modeling and Damage Mechanics of the Annulus Fibrosus

by Nathaniel T. Hollingsworth Annulus fibrosus damage has been shown to come from combinations of bending, lifting, and twisting. Although the these motions can induce annulus fibrosus damage, the stress-strain state of the annulus during these motions is not known. The objective of this project was to develop more complete constitutive models and to investigate annulus fibrosus damage mechanics. Constitutive modeling of the annulus fibrosus has neglected to explicitly model shear loading in the past. In an effort to improve the AF constitutive models during shear loading, four nonlinear, hyperelastic constitutive models were evaluated for their ability to model annulus fibrosus stress-strain relationships for uniaxial, biaxial, and simple shear experimental data. The best model indicated that the collagen crosslinks stored a large fraction of the applied strain energy. To investigate the annulus fibrosus damage mechanics, the annulus fibrosus stress-strain state was measured for an intact motion segment loaded in flexion combined with compression and the residual stress-strain state was measured for unloaded motion segments. In an unloaded motion segment, the annulus fibrosus is in a state of equibiaxial tension. In flexion, the axial direction stress increases to approximately 5 times the circumferential stress; the strain was equal-magnitude with axial tensile strain and circumferential compressive strain. Applying the flexion strain to the constitutive model hinted Nathaniel T. Hollingsworth that this deformation preferentially loads the collagen crosslinks. Cyclically loading planar annulus fibrosus samples with stress limits 160% of the stress measured in flexion combined with compression indicated that collagen crosslinks accumulate the initial damage of the annulus fibrosus.