The vertebral endplate: its structure and function
Date: 21 October 2019
University of Exeter
PhD in Physics
The cause of degenerative changes to the intervertebral disc that can produce back pain are still indefinable. One newer avenue of investigation includes the analysis of vertebral endplates, which lie between vertebral bodies and their adjacent discs. This thesis investigates the structure and function of the vertebral endplate. ...
The cause of degenerative changes to the intervertebral disc that can produce back pain are still indefinable. One newer avenue of investigation includes the analysis of vertebral endplates, which lie between vertebral bodies and their adjacent discs. This thesis investigates the structure and function of the vertebral endplate. Multiphoton microscopy was employed as the main imaging method as it has previously shown its ability to differentiate between tissue in osteochondral regions, and its ability to image label free, thick samples reduces sample processing. Animal endplate models were utilised as a healthy model, alongside degenerate human samples from surgery. This thesis aimed to investigate the structure of the vertebral endplate as well as its function. The structure of the endplate was confirmed to contain three distinct regions; the bony endplate, mineralised cartilage and non-mineralised cartilage. The thickness of the cartilage endplate was shown to be thicker in the cranial endplate (p = 0.008), and the mineralised region thicker than the non-mineralised (p = 0.005). Additionally, this study confirmed how anchorage of the intervertebral disc is achieved. Microstructurally, annular bundles penetrate through both the mineralised and non-mineralised regions of the cartilage endplate. Within these layers, the bundles sub-divide in a branched structure as an integration mechanism. Energy disperse spectroscopy showed a higher calcium to phosphate ratio in young adult ovine spines (p<0.05) compared with mature ovine spines, and age differences in the structure of the bony endplate were also noted. Additionally, Raman microscopy on bovine sections showed higher mineralisation in the endplate under the nucleus than annulus using the mineral to matrix ratio (peak ratios of 3.15 and 2.35 respectively). This work highlights the specificity of each of the tissues in the endplate, and how they change based on their location (nucleus/annulus, caudal/cranial) and with age. Degenerative human samples showed higher two photon fluorescence (TPF) signal than the bovine model, and also showed a highly aligned collagen region, parallel to the tidemark which agrees with previous literature. An important function of the endplate is its role as a nutritional pathway for the diffusion of solutes through the endplate and the convective transport of water. Vessels within the endplate were imaged using multiphoton microscopy. They were measured to be as small as 5 ¹m in diameter and within close proximity to the disc (< 70 μm), though never crossing the tide mark. Additionally, the use of a fluorescent tracer confirmed the diffusion from the disc into vessels in the endplate. Through the use of real-time imaging of a fluorescent tracer diffusing from the disc through the endplate, this study showed the tidemark as a barrier, initially holding the tracer up. The study additionally confirmed the presence of small pores in the mineralised cartilage post tidemark. Further relating to the function of the endplate, a series of compressive loading experiments were carried out in order to provide novel insight into the micromechanics of the regions of the endplate. The disc displayed the highest amount of micro-strain with measurements reaching as high as - 40 %, though initial strain appears primarily due to fibre realignment. This work highlights the importance of the cartilage components of the endplate in the micromechanics of the disc. Previously, the vertebral endplate has been underappreciated within the literature and clinical understanding of its role. This works highlights the intimacy between the intervertebral disc and vertebral endplate reinforcing the necessity for further investigation of this unique region.
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