The intervertebral disc is a heterogeneous structure that contributes to flexibility and load support in the spine. It consists of three anatomic zones: the outer anulus fibrosus. the central nucleus pulposus. and the intermediate transition zone. Under physiologic loading, the cells in these tissues experience a complex array of mechanical stimuli that are known to regulate cellular metabolism. Importantly, the response of intervertebral disc cells to mechanical stimuli differs depending on the anatomic zone of cell origin. These differences in cell response are believed to result in part from differences in the cell micromechanical environment.
In this dissertation, a finite element model was developed to predict the micromechanical environment of intervertebral disc cells. The model was capable of describing a number of important mechanical phenomena: flow-dependent viscoelasticity using the biphasic theory for soft tissues: finite deformation effects using a hyperelastic constitutive law for the solid phase: and material anisotropy by including a fiber-reinforced continuum law in the hyperelastic strain energy function. To construct accurate finite element meshes, the in situ geometry of IVD cells were measured experimentally using laser scanning confocal microscopy and three-dimensional reconstruction techniques. Material properties for the cells were estimated by measuring cell deformation in a well-controlled alginate culture system and matching these measured deformations to finite element predictions.
Model results indicated that the micromechanical environment varied dramatically between zones. Cells in the anulus fibrosus and transition zone experienced large strain concentrations and volume changes, up to 5 times higher than corresponding values in the far field. In the nucleus pulposus. cells experienced much smaller stresses and strains compared to the other zones. Cell micromechanical environment was also strongly affected by cell geometry. Specifically, identical loading conditions led to larger deformations in spherical cells compared to those that were elongated in the fiber direction.
Results of this study have applications in understanding the role of mechanics in controlling intervertebral disc behavior, since quantitative information about the mechanical stimuli experienced by cells will be necessary to elucidate precise relationships between mechanical loading and cell response.
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