Intervertebral Disc Structure and Mechanical Function Under Physiological Loading Quantified Non-invasively Utilizing MRI and Image Registration

The intervertebral discs (IVD) functions to permit motion, distribute load, and dissipate energy in the spine. It performs these functions through its heterogeneous structural organization and biochemical composition consisting of several tissue substructures: the central gelatinous nucleus pulposus (NP), the surrounding fiber reinforced layered annulus fibrosus (AF), and the cartilaginous endplates (CEP) that are positioned between the NP and vertebral endplates. Each tissue contributes individually to overall disc mechanics and by interacting with adjacent tissues. Disruption of the disc\u27s tissues through aging, degeneration, or tear will not only alter the affected tissue mechanical properties, but also the mechanical behavior of adjacent tissues and, ultimately, overall disc segment function. Thus, there is a need to measure disc tissue and segment mechanics in the intact disc so that interactions between substructures are not disrupted. Such measurements would be valuable to study mechanisms of disc function and degeneration, and develop and evaluate surgical procedures and therapeutic implants. The objectives of this study were to develop, validate, and apply methods to visualize and quantify IVD substructure geometry and track internal deformations for intact human discs under axial compression. The CEP and AF were visualized through MRI parameter mapping and image sequence optimization for ideal contrast. High-resolution images enabled geometric measurements. Axial compression was performed using a custom-built loading device that permitted long relaxation times outside of the MRI, 300 m isotropic resolution images were acquired, and image registration methods applied to measure 3D internal strain. In conclusion, new methods to visualize and quantify CEP thickness, annular tear detection and geometric quantification, and non-invasively measure 3D internal disc strains were established. No correlation was found between CEP thickness and disc level; however the periphery was significantly thicker compared to central locations. Clear distinction of adjacent AF lamellae enabled annular tear detection and detailed geometric quantification. Annular tears demonstrated non-classic geometry through interconnecting radial, circumferential, and perinuclear formations. Regional strain inhomogeneity was observed qualitatively and quantitatively. Variation in strain magnitudes might be explained by geometry in axial and circumferential strain while peak radial strain in the posterior AF may have important implications for disc herniation