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- Biochemical changes in the intervertebral disc begin at about the age of 18 years. What are these changes and what is their overall result as far as disc function is concerned?
- List the changes that occur in the anulus and nucleus of the disc during degeneration.
- In degeneration, the disc height actually increases, what is the mechanism behind this increase?
- How is disc degradation different from disc degeneration?
- In order for the nucleus of the disc to become externalized (prolapse or extrude) what factors must be present, what changes must occur in the nucleus and what speculations have been made as to how these changes come about?
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There is decrease in the hyaluronic acid and muccopolysaccharide content in the disc leading to a decrease in the hydrophilic properties of the disc so that it cannot imbibe or retain water optimally
The water content decreases from about 90% to about 70% most of which decrease occurs by middle age, the anular lamelli begin separating from each other in random circumferential clefts. The nucleus become firmer and more viscous and in extreme age may break down into rubble. The disc height increases.
Increase in disc height results from selective absorption of the horizontal trabeculae in the vertebral body. This causes a loss of lateral bracing for the vertical trabelculae which collapse especially over the area of greatest stress, the central portions of the body. The pressure from the disc then bends the end plate into the body increasing the height of the disc by about 10% over adult life.
Disc degeneration is a universal phenomenon whereas degradation is pathological. The differences include, the formation of radial tears and migration of the nuclear material from its central location outwards often to the point of externalization. Disc height is lost as resorption occurs.
With degeneration, the nucleus is too well bound to the anulus and too viscous for it to migrate. In order for migration to occur, a pathway must be present so either radial tearing or end plate fracturing must be present. The nucleus must be liquified and forces must be exerted that cause it to migrate. Speculations on why radial tearing occurs centers mainly around zygopophyseal joint asymmetry causing habitual aysmmetrical forces to be exerted on the disc. It is believed that the unbinding of the nucleus from the anulus may be caused by an autoimmune reaction as the nucleus becomes exposed to the vascular system either through the radial tear or via traumatic vertical prolapsing.
Bogduck, N. Twomey, LT. Clinical anatomy of the lumbar Spine, 2nd Edition, Chruchill Livingston 1991
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Twomey, LT. Taylor, J. Furniss, B. Age changes in the bone density and structure of the lumbar vertebral column.
Twomey, LT. and Taylor, J. Age changes in lumbar intervertebral discs. Acta Orthop Scan 56:496-499 1986
Taylor, JTKF. Little, K. Intracellular matrix of the intervertebral disc in ageing and in prolapse. Nature 208:384-386 1965
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