Intervertebral disc degeneration is a prevalent problem and is a key factor directly or indirectly related to low back pain. The mechanical environment of the cells in a degenerated disc is substantially altered, and likely influences the metabolic behavior of the cells. A more detailed and quantitative description of the stress-strain environment in and around the cells would be valuable to better understand the process by which the cells sense and react to the mechanical stimulus. Recent studies have greatly contributed to the pool of material property data describing the extracellular matrix and the intervertebral disc cells. An d with the advent of laser scanning confocal microscopy, the ability to validate the in situ deformation of cells under externally applied loads is now possible. Therefore, the objective of this study has been to develop a finite element model of cells in the intervertebral disc, and to investigate the in situ mechanical environment of the cells under physiologic loading.

Several three-dimensional finite element models were developed:​ 1) a lumbar motion segment model including two adjacent vertebrae, ligaments, and the intervertebral disc;​ 2) a tissue model to duplicate specimens used in bi-axial tests to validate material property formulations and model predictions;​ and 3) a micro model with sufficient flexibility to model various cell shapes including spheroids, ovoids, and more general shapes from reconstructed microscope images. Selected elements in the larger scale models were used to "drive" the micro models. Material properties of the disc were based on a fibre reinforced poro-hyperelastic material model. The cells were modeled as linear elastic.

The motion segment model was used to simulate a known dangerous movement consisting of combined axial compression, lateral bending, twisting and forward flexion. Elements in the posterolateral and anterior regions of the disc were then used to drive micro models of ovoid and spherical cells in the outer and inner annulus respectively. Significant differences in cell volume changes and deformation were observed between the different regions. Such differences were primarily between the anterior and posterolateral regions. Volume decreases and maximum cellular deformation were found to occur in the posterolateral region. Conversely, cell volumes increased in the anterior regions. These results are significant in conjunction with epidemiological observations which have shown the posterolateral region of the disc to be associated with accelerated degeneration and higher incidences of herniation.

Using the tissue model in bi-axial loading, many aspects of cell shape effects and multi cell arrangements were explored. It was found that ovoid cells were more resistant to volume changes than spherical cells. Furthermore, ovoid cells were found to become more like spheres whereas spheres became more like ovoids when subjected to bi-axial loading. This behavior was consistent when matrix conditions were orthotropic as well as isotropic. However, under uni-axial loading these observations were different in that both ovoids and spheres became more elongated. This suggests that loading is more significant to cell signaling than the matrix conditions. It is known that in vivo strains at the periphery of the disc are bi-axial, however, much of the material testing to date has been uni-axial. There is also further significance in the differing response between the ovoid and sphere shaped cells. It is commonly accepted that cells within tissues normally under tensile loads (i.e. ligaments) are elongated or ovoid in shape;​ whereas, in tissues normally under compressive loads (i.e. cartilage) the cells are more spheroid in shape. These factors suggest that cell shape may be inherent in the load sensing mechanism of the cell. Developing a better understanding of this mechanism will be a significant step towards solving the mysteries of intervertebral disc degeneration and low back pain.