X-ray Micro-Tomography and Volumetric Strain Measurement in the Intervertebral Disc

• Main Author: Disney, Catherine M
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2018
• Subjects: Biomedical engineering; Physiology
• ISBN: 1658493613; 9781658493611

The majority of the population suffers from low back pain at some point in their life. Of which, around 40% of cases are caused by intervertebral degeneration (IVDD). The degeneration process is complex, with mechanics and biology interrelated where mechanical overloading causes a cell response resulting in degeneration of extracellular matrix. However, the native IVD micro- structural and mechanical environment is poorly understood. This thesis presents a method to visualise and quantify microstructural deformation of the intact native IVD under load using X-ray micro-computed tomography (microCT). X-ray transmission contrast in soft tissues is weak but can be enhanced using heavy metal stains or in-line phase contrast imaging. The first objective was to image IVD microstructure using microCT with either 'laboratory' (microfocus tube) or synchrotron X-ray sources. Staining of discs improved contrast but there was uneven penetration and it relied on chemical fixation which influenced tissue structure. Native IVD microstructure was successfully resolved in tissue segments using in-line phase contrast synchrotron microCT. This imaging modality was then used to image an intact native rat IVD under sequential compression (four cumulative 2% strain steps). Deformation was then measured using digital volume correlation and mapped as strain. The main components (endplates, nucleus pulposus and annulus fibrosus) were resolved with details including individual outer lamellae and collagen-fibre bundles for the first volumetric imaging of intact native IVD at this resolution. Collagen bundles could be traced and their orientation defined before and after compression. Maximum principle strain showed no slipping at lamella boundaries, local strain patterns were at a similar scale and distribution to the elastic network with some heterogeneous areas and maximum strain direction aligned with bundle orientation suggesting bundle stretching and sliding. This method has the potential to bridge the gap between measures of macro-mechanical properties and the local 3D micro-mechanical environment experienced by cells. This is the first evaluation of strain at the micro scale level in the intact IVD. However, analysis was restricted to volumes of interest due to streaking artefacts. Image quality was improved by employing careful sample preparation and alignment of the sample in the beam. Resolved structures are now extended to the full disc with all lamellae visible, cells throughout the disc have been identified and imaging remained consistent in a series of scans where the disc was sequentially compressed. The method developed here provides a quantitative framework for future work including investigating the micro mechanical environment during IVDD and the effects of needle injury required for injecting tissue engineered constructs.