Abstract
Intervertebral discs are pads of fibrocartilage that separate vertebrae in the spine. Discs in the lumbar spine are prone to injury and degeneration, and are a major cause of back pain.
The soft ‘nucleus pulposus’ region of a disc is surrounded by concentric layers of the ‘annulus fibrosus’ which act in tension to restrain the nucleus. Discs are comprised mostly of proteoglycan, water and collagen. Proteoglycan molecules bind water, enabling disc tissues to resist compression, and ensuring that they are markedly poroelastic. A reinforcing network of strong collagen fibres confers tensile properties, which increase towards the disc periphery, and with increasing specimen size (according to the theory of chopped-fibre composites). Tissue composition and strength depend on genetic inheritance, and deteriorate with age.
Compressive forces acting down the long axis of the spine arise from gravity, and from tension in the muscles of the back and abdomen. Excessive compression can cause the fluid-like nucleus to herniate vertically into an adjacent vertebra, leading to nucleus decompression and internal collapse of the annulus. On the other hand, excessive bending of the spine can stretch the outer annulus to such an extent that a pressurised nucleus can herniate horizontally through it (a ‘slipped disc’) and generate pain radiating down the leg (‘sciatica’).
An avascular adult human disc has such a low cell density that it is unable to strengthen much in response to increased mechanical loading, and if injured is incapable of true healing. Disc ‘degeneration’ is primarily an aberrant biological (i.e. cell-mediated) reaction to progressive structural failure. Degeneration can allow ingrowth of blood vessels and nerves, leading to chronic discogenic back pain.
Mike Adams,
Department of Anatomy (CCCA),
University of Bristol