The Anatomy and Microcirculation of the Intervertebral Disc

Abstract

Low back pain is a costly financial and loss of productivity societal issue. Although its aetiology is unclear, it has been associated with the intervertebral disc and its degeneration which has been thought to be caused in part by poor nutrition. In this thesis the caudal disc of skeletally mature equines is utilised as an animal model. Techniques such as x-ray, light imaging, histology and magnetic resonance imaging were employed to investigate the vascular and structural anatomy of the disc and its surroundings and the uptake of tracers within the disc tissue. A detailed study of the anatomy revealed similarities with the human disc. The equine disc consists of two distinct structural areas; a nucleus and annulus. The surrounding vascularisation is similar; a main anteriorly positioned artery, the median caudal artery splits and encircles the centre of the vertebral body providing nutrition to the vertebral body. Smaller vessels anastomose over the surface of the vertebral body. Within the vertebral body the vessels end in capillary terminations at the edge of the vertebral cartilage endplate. As in humans these terminations were seen to vary along the endplate with shape and density; the capillaries are densest and larger in the area next to the nucleus. The cartilage endplate itself was found to have a variable width; of between 0.16 mm and 0.33 mm being widest at the nucleus. The annulus was seen to consist of lamellar rings which had high collagen content. A marked difference between equines and humans found was the number and width of lamellae present; equines were found to have on average 5 lamellae with a width range of 140 10 1110 microns. The shape of the discs was also found to be different with equines having almost circular coccygeal discs which have a concave superior and inferior surface. The nucleus of the disc, unlike current literature was found to have local order. An important contribution to knowledge which this thesis has made is data collected on diffusion time and partition coefficient on many regions within the disc. It was found that to reach equilibrium it took up to 22 hours in the outer anterior annulus but only 5 ½ hours in the central nucleus. Diffusion was found to be fastest with the neutral ring-shaped molecule Gadovist and slowest in the positive ion, manganese. Partition coefficients between the tracers were also found to vary. The highest partition coefficient was 6 in the central nucleus with manganese and the lowest was 0.5 with Magnevist at 0.5 in the nucleus area. This information will be useful in aiding drug delivery clinically and performing contrast enhanced imaging for pathology detection.