Micro-porosity of the intervertebral disc and its effects on fluid transport: A scanning electron microscopy and histological study

The intervertebral disc (IVD) allows for mobility of the spine. Healthy function of the IVD relies on complex transport of nutrients, primarily through fluid mechanisms. However, current understanding of these fundamentals lacks observable experimental properties, such as porosity, which will greatly enhance both the mathematical descriptions and computational simulations of the anatomical region. This thesis establishes data for the rat IVD porosity and suggests a possible mathematical method for analyzing such data.;Research has shown that the material properties of the IVD plays an important role in healthy disc function. This thesis also examines the impact of changing porosity and on the fluid permeability of the cartilaginous endplate (CE). DDD affects most individuals in old age, and the costs of treating this disease are becoming ever more expensive as medicine continues to advance the life expectancy age. Understanding how the structures of the IVD remain intact throughout our lives will play an immensely important role in developing bioengineered materials, surgical techniques, and therapies for rehabilitation of DDD.;Porosity measurements were taken from the spines of healthy Sprague Dawley rats. Spines were dissected, dehydrated, and either prepared for SEM imaging or histological analysis. For SEM imaging, samples were processed according to different protocols and results compared. Overall porosity was Images showed a strong indication for a possible attachment mechanism between the CE and adjacent bone while additionally providing a secondary measurement system of porosity and anatomical surface features. Histological samples were fixed in polymethylmethacrylate (PMMA), sectioned across the middle of the IVD sagitally, and stained for both quantitative porosity measurements and qualitative compositional observations. Results show ventral and dorsal differences within and cranial and caudal differences between the cartilaginous endplates of the IVD. Porosity was determined to be anisotropic with a radial dependence. Visible pore fractions ranged from 10-25% in the cranial CE and from 7-20% caudally, with both peaks approximately adjacent to the nucleus pulposus. The research presented in here and also in literature show that the as-measured state of dehydrated samples is not a true measure of porosity and needs to include intrafibrillar volumes. The cranial and caudal CE pore sizes were determined to increase proximally to distally from the spinal cord ranging from 1300mum 2-8900mum2 and from 520mum2-870mum 2 respectively. The cranial CE within a vertebral body unit had markedly larger pore distribution in the distal 40% than the caudal CE that had a relatively small pore distribution across the entire diameter. Visible pore fractions ranged from 9.4% to 25% in the cranial CE and from 7.9% to 18.4% in the caudal CE, both showing a radial dependence. The CE thickness also showed radial dependence for both top and bottom endplates with values ranging from .201mm-.548mm and .182mm-.319mm respectively, exhibiting a both an axial location and radial dependence. The pore fractions were greatest above and below the Nucleus Pulposus (NP) implying maximum flow in these regions and that cranial and caudal endplates have very different porosity characteristics.